GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the
remainder. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The "overall options" allow you to stop this
process at an intermediate stage. For example, the -c option says not to
run the linker. Then the output consists of object files output by the
assembler.
Other options are passed on to one or more stages of processing. Some
options control the preprocessor and others the compiler itself. Yet
other options control the assembler and linker; most of these are not
documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful for
C programs; when an option is only useful with another language (usually
C++), the explanation says so explicitly. If the description for a
particular option does not mention a source language, you can use that
option with all supported languages.
The usual way to run GCC is to run the executable called gcc, or
machine-gcc when cross-compiling, or machine-gcc-version to run a
specific version of GCC. When you compile C++ programs, you should invoke
GCC as g++ instead.
The gcc program accepts options and file names as operands. Many options
have multi-letter names; therefore multiple single-letter options may not
be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order
you use doesn't matter. Order does matter when you use several options
of the same kind; for example, if you specify -L more than once, the
directories are searched in the order specified. Also, the placement of
the -l option is significant.
Many options have long names starting with -f or with -W---for example,
-fmove-loop-invariants, -Wformat and so on. Most of these have both
positive and negative forms; the negative form of -ffoo is -fno-foo.
This manual documents only one of these two forms, whichever one is not
the default.
Some options take one or more arguments typically separated either by a
space or by the equals sign (=) from the option name. Unless documented
otherwise, an argument can be either numeric or a string. Numeric
arguments must typically be small unsigned decimal or hexadecimal
integers. Hexadecimal arguments must begin with the 0x prefix.
Arguments to options that specify a size threshold of some sort may be
arbitrarily large decimal or hexadecimal integers followed by a byte size
suffix designating a multiple of bytes such as "kB" and "KiB" for
kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and
mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such
arguments are designated by byte-size in the following text. Refer to
the NIST, IEC, and other relevant national and international standards
for the full listing and explanation of the binary and decimal byte size
prefixes.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are
in the following sections.
Overall Options
-c -S -E -o file -dumpbase dumpbase -dumpbase-ext auxdropsuf
-dumpdir dumppfx -x language -v -### --help[=class[,...]]
--target-help --version -pass-exit-codes -pipe -specs=file
-wrapper @file -ffile-prefix-map=old=new -fcanon-prefix-map
-fplugin=file -fplugin-arg-name=arg -fdump-ada-spec[-slim]
-fada-spec-parent=unit -fdump-go-spec=file
C Language Options
-ansi -std=standard -aux-info filename -fno-asm -fno-builtin
-fno-builtin-function -fcond-mismatch -ffreestanding -fgimple
-fgnu-tm -fgnu89-inline -fhosted -flax-vector-conversions
-fms-extensions -fpermitted-flt-eval-methods=standard
-fplan9-extensions -fsigned-bitfields -funsigned-bitfields
-fsigned-char -funsigned-char -fstrict-flex-arrays[=n]
-fsso-struct=endianness
C++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n
-fno-assume-sane-operators-new-delete -fchar8_t -fcheck-new
-fconcepts -fconstexpr-depth=n -fconstexpr-cache-depth=n
-fconstexpr-loop-limit=n -fconstexpr-ops-limit=n
-fno-elide-constructors -fno-enforce-eh-specs -fno-gnu-keywords
-fno-immediate-escalation -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fmodule-header[=kind] -fmodule-only -fmodules
-fmodule-implicit-inline -fno-module-lazy
-fmodule-mapper=specification -fmodule-version-ignore -fms-extensions
-fnew-inheriting-ctors -fnew-ttp-matching -fno-nonansi-builtins
-fnothrow-opt -fno-operator-names -fno-optional-diags
-fno-pretty-templates -frange-for-ext-temps -fno-rtti
-fsized-deallocation -fstrong-eval-order[=kind]
-ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat
-fext-numeric-literals -flang-info-include-translate[=header]
-flang-info-include-translate-not -flang-info-module-cmi[=module]
-stdlib=libstdc++,libc++ -Wabi-tag -Wcatch-value -Wcatch-value=n
-Wno-class-conversion -Wclass-memaccess -Wcomma-subscript
-Wconditionally-supported -Wno-conversion-null
-Wctad-maybe-unsupported -Wctor-dtor-privacy -Wdangling-reference
-Wno-defaulted-function-deleted -Wno-delete-incomplete
-Wdelete-non-virtual-dtor -Wno-deprecated-array-compare
-Wdeprecated-copy -Wdeprecated-copy-dtor
-Wno-deprecated-enum-enum-conversion
-Wno-deprecated-enum-float-conversion -Weffc++
-Wno-elaborated-enum-base -Wno-exceptions -Wextra-semi
-Wno-global-module -Wno-inaccessible-base
-Wno-inherited-variadic-ctor -Wno-init-list-lifetime
-Winvalid-constexpr -Winvalid-imported-macros -Wno-invalid-offsetof
-Wno-literal-suffix -Wmismatched-new-delete -Wmismatched-tags
-Wmultiple-inheritance -Wnamespaces -Wnarrowing -Wnoexcept
-Wnoexcept-type -Wnon-virtual-dtor -Wpessimizing-move
-Wno-placement-new -Wplacement-new=n -Wrange-loop-construct
-Wredundant-move -Wredundant-tags -Wreorder -Wregister
-Wstrict-null-sentinel -Wno-subobject-linkage -Wtemplates
-Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wself-move -Wsign-promo -Wsized-deallocation
-Wsuggest-final-methods -Wsuggest-final-types -Wsuggest-override
-Wno-template-body -Wno-template-id-cdtor -Wtemplate-names-tu-local
-Wno-terminate -Wno-vexing-parse -Wvirtual-inheritance
-Wno-virtual-move-assign -Wvolatile
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck
-fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept
-Wno-property-assign-default -Wno-protocol -Wobjc-root-class
-Wselector -Wstrict-selector-match -Wundeclared-selector
OpenMP and OpenACC Options
-foffload=arg -foffload-options=arg -fopenacc -fopenacc-dim=geom
-fopenmp -fopenmp-simd -fopenmp-target-simd-clone[=device-type]
Diagnostic Message Formatting Options
-fmessage-length=n -fdiagnostics-plain-output
-fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fdiagnostics-urls=[auto|never|always]
-fdiagnostics-format=[text|sarif-stderr|sarif-file|json|json-
stderr|json-file] -fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC
-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC
-fno-diagnostics-json-formatting -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-event-links
-fno-diagnostics-show-labels -fno-diagnostics-show-line-numbers
-fno-diagnostics-show-cwe -fno-diagnostics-show-rules
-fno-diagnostics-show-highlight-colors
-fdiagnostics-minimum-margin-width=width
-fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
-fdiagnostics-show-template-tree -fno-elide-type
-fdiagnostics-path-format=[none|separate-events|inline-events]
-fdiagnostics-show-path-depths -fno-show-column
-fdiagnostics-column-unit=[display|byte]
-fdiagnostics-column-origin=origin
-fdiagnostics-escape-format=[unicode|bytes]
-fdiagnostics-text-art-charset=[none|ascii|unicode|emoji]
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors
-fpermissive -w -Wextra -Wall -Wabi=n -Waddress
-Wno-address-of-packed-member -Waggregate-return -Walloc-size
-Walloc-size-larger-than=byte-size -Walloc-zero -Walloca
-Walloca-larger-than=byte-size -Wno-aggressive-loop-optimizations
-Warith-conversion -Warray-bounds -Warray-bounds=n -Warray-compare
-Warray-parameter -Warray-parameter=n -Wno-attributes
-Wattribute-alias=n -Wno-attribute-alias -Wno-attribute-warning
-Wbidi-chars=[none|unpaired|any|ucn] -Wbool-compare -Wbool-operation
-Wno-builtin-declaration-mismatch -Wno-builtin-macro-redefined
-Wc90-c99-compat -Wc99-c11-compat -Wc11-c23-compat -Wc23-c2y-compat
-Wc++-compat -Wc++11-compat -Wc++14-compat -Wc++17-compat
-Wc++20-compat -Wno-c++11-extensions -Wno-c++14-extensions
-Wno-c++17-extensions -Wno-c++20-extensions -Wno-c++23-extensions
-Wcalloc-transposed-args -Wcast-align -Wcast-align=strict
-Wcast-function-type -Wcast-qual -Wchar-subscripts -Wclobbered
-Wcomment -Wcompare-distinct-pointer-types -Wno-complain-wrong-lang
-Wconversion -Wno-coverage-mismatch -Wno-cpp -Wdangling-else
-Wdangling-pointer -Wdangling-pointer=n -Wdate-time -Wno-deprecated
-Wno-deprecated-declarations -Wno-designated-init
-Wdisabled-optimization -Wno-discarded-array-qualifiers
-Wno-discarded-qualifiers -Wno-div-by-zero -Wdouble-promotion
-Wduplicated-branches -Wduplicated-cond -Wempty-body
-Wno-endif-labels -Wenum-compare -Wenum-conversion
-Wenum-int-mismatch -Werror -Werror=* -Wexpansion-to-defined
-Wfatal-errors -Wflex-array-member-not-at-end -Wfloat-conversion
-Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul
-Wno-format-extra-args -Wformat-nonliteral -Wformat-overflow=n
-Wformat-security -Wformat-signedness -Wformat-truncation=n
-Wformat-y2k -Wframe-address -Wframe-larger-than=byte-size
-Wno-free-nonheap-object -Wheader-guard -Wno-if-not-aligned
-Wno-ignored-attributes -Wignored-qualifiers
-Wno-incompatible-pointer-types -Whardened -Wimplicit
-Wimplicit-fallthrough -Wimplicit-fallthrough=n
-Wno-implicit-function-declaration -Wno-implicit-int
-Winfinite-recursion -Winit-self -Winline -Wno-int-conversion
-Wint-in-bool-context -Wno-int-to-pointer-cast
-Wno-invalid-memory-model -Winvalid-pch -Winvalid-utf8 -Wno-unicode
-Wjump-misses-init -Wlarger-than=byte-size -Wleading-whitespace=kind
-Wlogical-not-parentheses -Wlogical-op -Wlong-long
-Wno-lto-type-mismatch -Wmain -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation
-Wmissing-attributes -Wmissing-braces -Wmissing-field-initializers
-Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn
-Wmusttail-local-addr -Wmaybe-musttail-local-addr
-Wno-missing-profile -Wno-multichar -Wmultistatement-macros
-Wnonnull -Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc]
-Wnull-dereference -Wno-odr -Wopenacc-parallelism -Wopenmp
-Wopenmp-simd -Wno-overflow -Woverlength-strings
-Wno-override-init-side-effects -Wpacked -Wno-packed-bitfield-compat
-Wpacked-not-aligned -Wpadded -Wparentheses -Wno-pedantic-ms-format
-Wpointer-arith -Wno-pointer-compare -Wno-pointer-to-int-cast
-Wno-pragmas -Wno-pragma-once-outside-header -Wno-prio-ctor-dtor
-Wno-psabi -Wredundant-decls -Wrestrict -Wno-return-local-addr
-Wreturn-type -Wno-scalar-storage-order -Wsequence-point -Wshadow
-Wshadow=global -Wshadow=local -Wshadow=compatible-local
-Wno-shadow-ivar -Wno-shift-count-negative -Wno-shift-count-overflow
-Wshift-negative-value -Wno-shift-overflow -Wshift-overflow=n
-Wsign-compare -Wsign-conversion -Wno-sizeof-array-argument
-Wsizeof-array-div -Wsizeof-pointer-div -Wsizeof-pointer-memaccess
-Wstack-protector -Wstack-usage=byte-size -Wstrict-aliasing
-Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n
-Wstring-compare -Wno-stringop-overflow -Wno-stringop-overread
-Wno-stringop-truncation -Wstrict-flex-arrays
-Wsuggest-attribute=attribute-name -Wswitch -Wno-switch-bool
-Wswitch-default -Wswitch-enum -Wno-switch-outside-range
-Wno-switch-unreachable -Wsync-nand -Wsystem-headers
-Wtautological-compare -Wtrailing-whitespace
-Wtrailing-whitespace=kind -Wtrampolines -Wtrigraphs
-Wtrivial-auto-var-init -Wno-tsan -Wtype-limits -Wundef
-Wuninitialized -Wunknown-pragmas -Wunsuffixed-float-constants
-Wunterminated-string-initialization -Wunused
-Wunused-but-set-parameter -Wunused-but-set-variable
-Wunused-const-variable -Wunused-const-variable=n -Wunused-function
-Wunused-label -Wunused-local-typedefs -Wunused-macros
-Wunused-parameter -Wno-unused-result -Wunused-value
-Wunused-variable -Wuse-after-free -Wuse-after-free=n
-Wuseless-cast -Wno-varargs -Wvariadic-macros
-Wvector-operation-performance -Wvla -Wvla-larger-than=byte-size
-Wno-vla-larger-than -Wvolatile-register-var -Wwrite-strings
-Wno-xor-used-as-pow -Wzero-as-null-pointer-constant
-Wzero-length-bounds
Static Analyzer Options
-fanalyzer -fanalyzer-call-summaries -fanalyzer-checker=name
-fno-analyzer-feasibility -fanalyzer-fine-grained
-fanalyzer-show-events-in-system-headers -fno-analyzer-state-merge
-fno-analyzer-state-purge -fno-analyzer-suppress-followups
-fanalyzer-transitivity -fno-analyzer-undo-inlining
-fanalyzer-verbose-edges -fanalyzer-verbose-state-changes
-fanalyzer-verbosity=level -fdump-analyzer -fdump-analyzer-callgraph
-fdump-analyzer-exploded-graph -fdump-analyzer-exploded-nodes
-fdump-analyzer-exploded-nodes-2 -fdump-analyzer-exploded-nodes-3
-fdump-analyzer-exploded-paths -fdump-analyzer-feasibility
-fdump-analyzer-infinite-loop -fdump-analyzer-json
-fdump-analyzer-state-purge -fdump-analyzer-stderr
-fdump-analyzer-supergraph -fdump-analyzer-untracked
-Wno-analyzer-double-fclose -Wno-analyzer-double-free
-Wno-analyzer-exposure-through-output-file
-Wno-analyzer-exposure-through-uninit-copy
-Wno-analyzer-fd-access-mode-mismatch -Wno-analyzer-fd-double-close
-Wno-analyzer-fd-leak -Wno-analyzer-fd-phase-mismatch
-Wno-analyzer-fd-type-mismatch -Wno-analyzer-fd-use-after-close
-Wno-analyzer-fd-use-without-check -Wno-analyzer-file-leak
-Wno-analyzer-free-of-non-heap -Wno-analyzer-imprecise-fp-arithmetic
-Wno-analyzer-infinite-loop -Wno-analyzer-infinite-recursion
-Wno-analyzer-jump-through-null -Wno-analyzer-malloc-leak
-Wno-analyzer-mismatching-deallocation -Wno-analyzer-null-argument
-Wno-analyzer-null-dereference -Wno-analyzer-out-of-bounds
-Wno-analyzer-overlapping-buffers
-Wno-analyzer-possible-null-argument
-Wno-analyzer-possible-null-dereference
-Wno-analyzer-putenv-of-auto-var -Wno-analyzer-shift-count-negative
-Wno-analyzer-shift-count-overflow -Wno-analyzer-stale-setjmp-buffer
-Wno-analyzer-tainted-allocation-size -Wno-analyzer-tainted-assertion
-Wno-analyzer-tainted-array-index -Wno-analyzer-tainted-divisor
-Wno-analyzer-tainted-offset -Wno-analyzer-tainted-size
-Wanalyzer-symbol-too-complex -Wanalyzer-too-complex
-Wno-analyzer-undefined-behavior-ptrdiff
-Wno-analyzer-undefined-behavior-strtok
-Wno-analyzer-unsafe-call-within-signal-handler
-Wno-analyzer-use-after-free
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
-Wno-analyzer-use-of-uninitialized-value
-Wno-analyzer-va-arg-type-mismatch -Wno-analyzer-va-list-exhausted
-Wno-analyzer-va-list-leak -Wno-analyzer-va-list-use-after-va-end
-Wno-analyzer-write-to-const -Wno-analyzer-write-to-string-literal
C and Objective-C-only Warning Options
-Wbad-function-cast -Wdeprecated-non-prototype -Wfree-labels
-Wmissing-declarations -Wmissing-parameter-name
-Wmissing-parameter-type -Wdeclaration-missing-parameter-type
-Wmissing-prototypes -Wmissing-variable-declarations -Wnested-externs
-Wold-style-declaration -Wold-style-definition -Wstrict-prototypes
-Wtraditional -Wtraditional-conversion -Wdeclaration-after-statement
-Wpointer-sign
Debugging Options
-g -glevel -gdwarf -gdwarf-version -gbtf -gctf -gctflevel
-gprune-btf -gno-prune-btf -ggdb -grecord-gcc-switches
-gno-record-gcc-switches -gstrict-dwarf -gno-strict-dwarf
-gas-loc-support -gno-as-loc-support -gas-locview-support
-gno-as-locview-support -gcodeview -gcolumn-info -gno-column-info
-gdwarf32 -gdwarf64 -gstatement-frontiers -gno-statement-frontiers
-gvariable-location-views -gno-variable-location-views
-ginternal-reset-location-views -gno-internal-reset-location-views
-ginline-points -gno-inline-points -gvms -gz[=type] -gsplit-dwarf
-gdescribe-dies -gno-describe-dies -fdebug-prefix-map=old=new
-fdebug-types-section -fno-eliminate-unused-debug-types
-femit-struct-debug-baseonly -femit-struct-debug-reduced
-femit-struct-debug-detailed[=spec-list]
-fno-eliminate-unused-debug-symbols -femit-class-debug-always
-fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking
-fvar-tracking-assignments
Optimization Options
-faggressive-loop-optimizations -falign-functions[=n[:m:[n2[:m2]]]]
-falign-jumps[=n[:m:[n2[:m2]]]] -falign-labels[=n[:m:[n2[:m2]]]]
-falign-loops[=n[:m:[n2[:m2]]]] -fmin-function-alignment=[n]
-fno-allocation-dce -fallow-store-data-races -fassociative-math
-fauto-profile -fauto-profile[=path] -fauto-inc-dec
-fbranch-probabilities -fcaller-saves -fcombine-stack-adjustments
-fconserve-stack -ffold-mem-offsets -fcompare-elim -fcprop-registers
-fcrossjumping -fcse-follow-jumps -fcse-skip-blocks
-fcx-fortran-rules -fcx-limited-range -fdata-sections -fdce
-fdelayed-branch -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse
-fearly-inlining -fipa-sra -fexpensive-optimizations
-ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store
-fexcess-precision=style -ffinite-loops -fforward-propagate
-ffp-contract=style -ffunction-sections -fgcse -fgcse-after-reload
-fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm
-fhoist-adjacent-loads -fif-conversion -fif-conversion2
-findirect-inlining -finline-stringops[=fn] -finline-functions
-finline-functions-called-once -finline-limit=n
-finline-small-functions -fipa-modref -fipa-cp -fipa-cp-clone
-fipa-bit-cp -fipa-vrp -fipa-pta -fipa-profile -fipa-pure-const
-fipa-reference -fipa-reference-addressable
-fipa-reorder-for-locality -fipa-stack-alignment -fipa-icf
-fira-algorithm=algorithm -flate-combine-instructions -flifetime-dse
-flive-patching=level -fira-region=region -fira-hoist-pressure
-fira-loop-pressure -fno-ira-share-save-slots
-fno-ira-share-spill-slots -fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts -fkeep-inline-functions
-fkeep-static-functions -fkeep-static-consts
-flimit-function-alignment -flive-range-shrinkage -floop-block
-floop-interchange -floop-strip-mine -floop-unroll-and-jam
-floop-nest-optimize -floop-parallelize-all -flra-remat -flto
-flto-compression-level -flto-partition=alg -flto-incremental=path
-flto-incremental-cache-size=n -fmalloc-dce -fmerge-all-constants
-fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves
-fmove-loop-invariants -fmove-loop-stores -fno-branch-count-reg
-fno-defer-pop -fno-fp-int-builtin-inexact -fno-function-cse
-fno-guess-branch-probability -fno-inline -fno-math-errno
-fno-peephole -fno-peephole2 -fno-printf-return-value
-fno-sched-interblock -fno-sched-spec -fno-signed-zeros
-fno-toplevel-reorder -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-crc
-foptimize-sibling-calls -fpartial-inlining -fpeel-loops
-fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction
-fprofile-use -fprofile-use=path -fprofile-partial-training
-fprofile-values -fprofile-reorder-functions -freciprocal-math -free
-frename-registers -freorder-blocks
-freorder-blocks-algorithm=algorithm -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math
-fsave-optimization-record -fsched2-use-superblocks -fsched-pressure
-fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n]
-fsched-group-heuristic -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic -fsched-rank-heuristic
-fsched-last-insn-heuristic -fsched-dep-count-heuristic
-fschedule-fusion -fschedule-insns -fschedule-insns2
-fsection-anchors -fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap -fshrink-wrap-separate
-fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller
-fsplit-loops -fsplit-paths -fsplit-wide-types
-fsplit-wide-types-early -fssa-backprop -fssa-phiopt -fstdarg-opt
-fstore-merging -fstrict-aliasing -fipa-strict-aliasing
-fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-cselim -ftree-dce -ftree-dominator-opts -ftree-dse
-ftree-forwprop -ftree-fre -fcode-hoisting -ftree-loop-if-convert
-ftree-loop-im -ftree-phiprop -ftree-loop-distribution
-ftree-loop-distribute-patterns -ftree-loop-ivcanon
-ftree-loop-linear -ftree-loop-optimize -ftree-loop-vectorize
-ftree-parallelize-loops=n -ftree-pre -ftree-partial-pre
-ftree-pta -ftree-reassoc -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge
-ftree-ter -ftree-vectorize -ftree-vrp -ftrivial-auto-var-init
-funconstrained-commons -funit-at-a-time -funroll-all-loops
-funroll-loops -funsafe-math-optimizations -funswitch-loops -fipa-ra
-fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb
-fwhole-program -fwpa -fuse-linker-plugin -fzero-call-used-regs
--param name=value -O -O0 -O1 -O2 -O3 -Os -Ofast -Og -Oz
Program Instrumentation Options
-p -pg -fprofile-arcs --coverage -ftest-coverage
-fcondition-coverage -fpath-coverage -fprofile-abs-path
-fprofile-dir=path -fprofile-generate -fprofile-generate=path
-fprofile-info-section -fprofile-info-section=name
-fprofile-note=path -fprofile-prefix-path=path
-fprofile-update=method -fprofile-filter-files=regex
-fprofile-exclude-files=regex
-fprofile-reproducible=[multithreaded|parallel-runs|serial]
-fsanitize=style -fsanitize-recover -fsanitize-recover=style
-fsanitize-trap -fsanitize-trap=style -fasan-shadow-offset=number
-fsanitize-sections=s1,s2,... -fsanitize-undefined-trap-on-error
-fbounds-check -fcf-protection
-fcf-protection=[full|branch|return|none|check] -fharden-compares
-fharden-conditional-branches -fhardened
-fharden-control-flow-redundancy -fhardcfr-skip-leaf
-fhardcfr-check-exceptions -fhardcfr-check-returning-calls
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]
-fstack-protector -fstack-protector-all -fstack-protector-strong
-fstack-protector-explicit -fstack-check -fstack-limit-register=reg
-fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack
-fstrub=disable -fstrub=strict -fstrub=relaxed -fstrub=all
-fstrub=at-calls -fstrub=internal -fvtable-verify=[std|preinit|none]
-fvtv-counts -fvtv-debug -finstrument-functions
-finstrument-functions-once
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
-fprofile-prefix-map=old=new -fpatchable-function-entry=N[,M]
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -dD
-dI -dM -dN -dU -fdebug-cpp -fdirectives-only
-fdollars-in-identifiers -fexec-charset=charset
-fextended-identifiers -finput-charset=charset
-fmacro-prefix-map=old=new -fmax-include-depth=depth
-fno-canonical-system-headers -fpch-deps -fpch-preprocess
-fpreprocessed -ftabstop=width -ftrack-macro-expansion
-fwide-exec-charset=charset -fworking-directory -H -imacros file
-include file -M -MD -MF -MG -MM -MMD -MP -MQ -MT
-Mno-modules -no-integrated-cpp -P -pthread -remap -traditional
-traditional-cpp -trigraphs -Umacro -undef -Wp,option
-Xpreprocessor option
Assembler Options
-Wa,option -Xassembler option
Linker Options
object-file-name -fuse-ld=linker -llibrary -nostartfiles
-nodefaultlibs -nolibc -nostdlib -nostdlib++ -e entry
--entry=entry -pie -pthread -r -rdynamic -s -static -static-pie
-static-libgcc -static-libstdc++ -static-libasan -static-libtsan
-static-liblsan -static-libubsan -shared -shared-libgcc -symbolic
-T script -Wl,option -Xlinker option -u symbol -z keyword
Directory Options
-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir
-iplugindir=dir -iprefix file -iquote dir -isysroot dir -isystem
dir -iwithprefix dir -iwithprefixbefore dir --embed-dir=dir -Ldir
-no-canonical-prefixes --no-sysroot-suffix -nostdinc -nostdinc++
--sysroot=dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables
-fasynchronous-unwind-tables -fno-gnu-unique -finhibit-size-directive
-fcommon -fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE
-fno-plt -fno-jump-tables -fno-bit-tests -frecord-gcc-switches
-freg-struct-return -fshort-enums -fshort-wchar -fverbose-asm
-fpack-struct[=n] -fleading-underscore -ftls-model=model
-fstack-reuse=reuse_level -fstack-use-cumulative-args -ftrampolines
-ftrampoline-impl=[stack|heap] -ftrapv -fwrapv
-fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
-fzero-init-padding-bits=value
Developer Options
-dletters -dumpspecs -dumpmachine -dumpversion -dumpfullversion
-fcallgraph-info[=su,da] -fchecking -fchecking=n -fdbg-cnt-list
-fdbg-cnt=counter-value-list -fdisable-ipa-pass_name
-fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-list
-fdisable-tree-pass_name -fdisable-tree-pass-name=range-list
-fdump-debug -fdump-earlydebug -fdump-noaddr -fdump-unnumbered
-fdump-unnumbered-links -fdump-final-insns[=file] -fdump-ipa-all
-fdump-ipa-cgraph -fdump-ipa-inline -fdump-lang-all
-fdump-lang-switch -fdump-lang-switch-options
-fdump-lang-switch-options=filename -fdump-passes -fdump-rtl-pass
-fdump-rtl-pass=filename -fdump-statistics -fdump-tree-all
-fdump-tree-switch -fdump-tree-switch-options
-fdump-tree-switch-options=filename -fcompare-debug[=opts]
-fcompare-debug-second -fenable-kind-pass -fenable-kind-pass=range-
list -fira-verbose=n -flto-report -flto-report-wpa -fmem-report-wpa
-fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report -fopt-info
-fopt-info-options[=file] -fmultiflags -fprofile-report
-frandom-seed=string -fsched-verbose=n -fsel-sched-verbose
-fsel-sched-dump-cfg -fsel-sched-pipelining-verbose -fstats
-fstack-usage -ftime-report -ftime-report-details
-fvar-tracking-assignments-toggle -gtoggle -print-file-name=library
-print-libgcc-file-name -print-multi-directory -print-multi-lib
-print-multi-os-directory -print-prog-name=program
-print-search-dirs -Q -print-sysroot -print-sysroot-headers-suffix
-save-temps -save-temps=cwd -save-temps=obj -time[=file]
Machine-Dependent Options
AArch64 Options (AArch64 Options) -mabi=name -mbig-endian
-mlittle-endian -mgeneral-regs-only -mcmodel=tiny -mcmodel=small
-mcmodel=large -mstrict-align -mno-strict-align
-momit-leaf-frame-pointer -mtls-dialect=desc
-mtls-dialect=traditional -mtls-size=size -mfix-cortex-a53-835769
-mfix-cortex-a53-843419 -mlow-precision-recip-sqrt
-mlow-precision-sqrt -mlow-precision-div -mpc-relative-literal-loads
-msign-return-address=scope -mbranch-protection=none|standard|pac-
ret[+leaf +b-key]|bti|gcs -mharden-sls=opts -march=name -mcpu=name
-mtune=name -moverride=string -mverbose-cost-dump
-mstack-protector-guard=guard -mstack-protector-guard-reg=sysreg
-mstack-protector-guard-offset=offset -mtrack-speculation
-moutline-atomics -mearly-ldp-fusion -mlate-ldp-fusion
-Wexperimental-fmv-target
Adapteva Epiphany Options (Adapteva Epiphany Options) -mhalf-reg-file
-mprefer-short-insn-regs -mbranch-cost=num -mcmove -mnops=num
-msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify
-mstack-offset=num -mround-nearest -mlong-calls -mshort-calls
-msmall16 -mfp-mode=mode -mvect-double -max-vect-align=num
-msplit-vecmove-early -m1reg-reg
AMD GCN Options (AMD GCN Options) -march=gpu -mtune=gpu
-mstack-size=bytes
ARC Options (ARC Options) -mbarrel-shifter -mjli-always -mcpu=cpu
-mA6 -mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast
-mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -matomic -mnorm
-mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap
-mcrc -mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc
-mswape -mtelephony -mxy -misize -mannotate-align -marclinux
-marclinux_prof -mlong-calls -mmedium-calls -msdata
-mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num
-mvolatile-cache -mtp-regno=regno -malign-call -mauto-modify-reg
-mbbit-peephole -mno-brcc -mcase-vector-pcrel -mcompact-casesi
-mno-cond-exec -mearly-cbranchsi -mexpand-adddi -mindexed-loads
-mlra -mlra-priority-none -mlra-priority-compact
-mlra-priority-noncompact -mmillicode -mmixed-code -mq-class -mRcq
-mRcw -msize-level=level -mtune=cpu -mmultcost=num
-mcode-density-frame -munalign-prob-threshold=probability
-mmpy-option=multo -mdiv-rem -mcode-density -mll64 -mfpu=fpu
-mrf16 -mbranch-index
ARM Options (ARM Options) -mapcs-frame -mno-apcs-frame -mabi=name
-mapcs-stack-check -mno-apcs-stack-check -mapcs-reentrant
-mno-apcs-reentrant -mgeneral-regs-only -msched-prolog
-mno-sched-prolog -mlittle-endian -mbig-endian -mbe8 -mbe32
-mfloat-abi=name -mfp16-format=name -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name -mtune=name
-mprint-tune-info -mstructure-size-boundary=n -mabort-on-noreturn
-mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base
-mpic-register=reg -mnop-fun-dllimport -mpoke-function-name -mthumb
-marm -mflip-thumb -mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking -mtp=name
-mtls-dialect=dialect -mword-relocations -mfix-cortex-m3-ldrd
-mfix-cortex-a57-aes-1742098 -mfix-cortex-a72-aes-1655431
-munaligned-access -mneon-for-64bits -mslow-flash-data
-masm-syntax-unified -mrestrict-it -mverbose-cost-dump -mpure-code
-mcmse -mfix-cmse-cve-2021-35465 -mstack-protector-guard=guard
-mstack-protector-guard-offset=offset -mfdpic
-mbranch-protection=none|standard|pac-ret[+leaf] [+bti]|bti[+pac-
ret[+leaf]]
AVR Options (AVR Options) -mmcu=mcu -mabsdata -maccumulate-args
-mcvt -mbranch-cost=cost -mfuse-add=level -mfuse-move=level
-mfuse-move2 -mcall-prologues -mgas-isr-prologues -mint8 -mflmap
-mdouble=bits -mlong-double=bits -mno-call-main -mn_flash=size
-mfract-convert-truncate -mno-interrupts -mmain-is-OS_task -mrelax
-mrmw -mstrict-X -mtiny-stack -mrodata-in-ram -msplit-bit-shift
-msplit-ldst -mshort-calls -mskip-bug -muse-nonzero-bits
-nodevicelib -nodevicespecs -Waddr-space-convert -Wmisspelled-isr
Blackfin Options (Blackfin Options) -mcpu=cpu[-sirevision] -msim
-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly
-mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1
-mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data
-mno-sep-data -mlong-calls -mno-long-calls -mfast-fp -minline-plt
-mmulticore -mcorea -mcoreb -msdram -micplb
C6X Options (C6X Options) -mbig-endian -mlittle-endian -march=cpu
-msim -msdata=sdata-type
CRIS Options (CRIS Options) -mcpu=cpu -march=cpu -mtune=cpu
-mmax-stack-frame=n -metrax4 -metrax100 -mpdebug -mcc-init
-mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit
-m16-bit -m8-bit -mno-prologue-epilogue -melf -maout -sim -sim2
-mmul-bug-workaround -mno-mul-bug-workaround
C-SKY Options (C-SKY Options) -march=arch -mcpu=cpu -mbig-endian
-EB -mlittle-endian -EL -mhard-float -msoft-float -mfpu=fpu
-mdouble-float -mfdivdu -mfloat-abi=name -melrw -mistack -mmp
-mcp -mcache -msecurity -mtrust -mdsp -medsp -mvdsp -mdiv
-msmart -mhigh-registers -manchor -mpushpop -mmultiple-stld
-mconstpool -mstack-size -mccrt -mbranch-cost=n -mcse-cc
-msched-prolog -msim
Cygwin and MinGW Options (Cygwin and MinGW Options) -mconsole
-mcrtdll=library -mdll -mnop-fun-dllimport -mthread -municode
-mwin32 -mwindows -fno-set-stack-executable
Darwin Options (Darwin Options) -all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle -bundle_loader
-client_name -compatibility_version -current_version -dead_strip
-dependency-file -dylib_file -dylinker_install_name -dynamic
-dynamiclib -exported_symbols_list -filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -iframework -image_base -init
-install_name -keep_private_externs -multi_module -multiply_defined
-multiply_defined_unused -noall_load -no_dead_strip_inits_and_terms
-nodefaultrpaths -nofixprebinding -nomultidefs -noprebind
-noseglinkedit -pagezero_size -prebind
-prebind_all_twolevel_modules -private_bundle -read_only_relocs
-sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate
-sectobjectsymbols -sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table -seg_addr_table_filename
-seglinkedit -segprot -segs_read_only_addr -segs_read_write_addr
-single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined -unexported_symbols_list
-weak_reference_mismatches -whatsloaded -F -gused -gfull
-mmacosx-version-min=version -mkernel -mone-byte-bool
-munreachable-traps
DEC Alpha Options (DEC Alpha Options) -mno-fp-regs -msoft-float
-mieee -mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode
-mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants
-mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix -msafe-bwa
-msafe-partial -mfloat-vax -mfloat-ieee -mexplicit-relocs
-msmall-data -mlarge-data -msmall-text -mlarge-text
-mmemory-latency=time
eBPF Options (eBPF Options) -mbig-endian -mlittle-endian
-mframe-limit=bytes -mxbpf -mco-re -mno-co-re -mjmpext -mjmp32
-malu32 -mv3-atomics -mbswap -msdiv -msmov -mcpu=version
-masm=dialect -minline-memops-threshold=bytes
FR30 Options (FR30 Options) -msmall-model -mno-lsim
FT32 Options (FT32 Options) -msim -mlra -mnodiv -mft32b
-mcompress -mnopm
FRV Options (FRV Options) -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64
-mhard-float -msoft-float -malloc-cc -mfixed-cc -mdword
-mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd
-mno-muladd -mfdpic -minline-plt -mgprel-ro -multilib-library-pic
-mlinked-fp -mlong-calls -malign-labels -mlibrary-pic -macc-4
-macc-8 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move
-moptimize-membar -mno-optimize-membar -mscc -mno-scc -mcond-exec
-mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec
-mno-multi-cond-exec -mnested-cond-exec -mno-nested-cond-exec
-mtomcat-stats -mTLS -mtls -mcpu=cpu
GNU/Linux Options (GNU/Linux Options) -mglibc -muclibc -mmusl
-mbionic -mandroid -tno-android-cc -tno-android-ld
H8/300 Options (H8/300 Options) -mrelax -mh -ms -mn -mexr
-mno-exr -mint32 -malign-300
HPPA Options (HPPA Options) -march=architecture-type
-matomic-libcalls -mbig-switch -mcaller-copies -mdisable-fpregs
-mdisable-indexing -mordered -mfast-indirect-calls -mgas -mgnu-ld
-mhp-ld -mfixed-range=register-range -mcoherent-ldcw -mjump-in-delay
-mlinker-opt -mlong-calls -mlong-load-store -mno-atomic-libcalls
-mno-disable-fpregs -mno-disable-indexing -mno-fast-indirect-calls
-mno-gas -mno-jump-in-delay -mno-long-load-store
-mno-portable-runtime -mno-soft-float -mno-space-regs -msoft-float
-mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime
-mschedule=cpu-type -mspace-regs -msoft-mult -msio -mwsio
-munix=unix-std -nolibdld -static -threads
IA-64 Options (IA-64 Options) -mbig-endian -mlittle-endian -mgnu-as
-mgnu-ld -mno-pic -mvolatile-asm-stop -mregister-names -msdata
-mno-sdata -mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency
-minline-float-divide-max-throughput -mno-inline-float-divide
-minline-int-divide-min-latency -minline-int-divide-max-throughput
-mno-inline-int-divide -minline-sqrt-min-latency
-minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm
-mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size
-mtune=cpu-type -milp32 -mlp64 -msched-br-data-spec
-msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc
-msched-spec-control-ldc -msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-
insns
LM32 Options (LM32 Options) -mbarrel-shift-enabled -mdivide-enabled
-mmultiply-enabled -msign-extend-enabled -muser-enabled
LoongArch Options (LoongArch Options) -march=arch-type -mtune=tune-
type -mabi=base-abi-type -mfpu=fpu-type -msimd=simd-type -msoft-float
-msingle-float -mdouble-float -mlsx -mno-lsx -mlasx -mno-lasx
-mbranch-cost=n -maddr-reg-reg-cost=n -mcheck-zero-division
-mno-check-zero-division -mcond-move-int -mno-cond-move-int
-mcond-move-float -mno-cond-move-float -memcpy -mno-memcpy
-mstrict-align -mno-strict-align -G num -mmax-inline-memcpy-size=n
-mexplicit-relocs=style -mexplicit-relocs -mno-explicit-relocs
-mdirect-extern-access -mno-direct-extern-access -mcmodel=code-model
-mrelax -mpass-mrelax-to-as -mrecip -mrecip=opt -mfrecipe
-mno-frecipe -mdiv32 -mno-div32 -mlam-bh -mno-lam-bh -mlamcas
-mno-lamcas -mld-seq-sa -mno-ld-seq-sa -mtls-dialect=opt
-mannotate-tablejump -mno-annotate-tablejump
M32C Options (M32C Options) -mcpu=cpu -msim -memregs=number
M32R/D Options (M32R/D Options) -m32r2 -m32rx -m32r -mdebug
-malign-loops -mno-align-loops -missue-rate=number
-mbranch-cost=number -mmodel=code-size-model-type -msdata=sdata-type
-mno-flush-func -mflush-func=name -mno-flush-trap
-mflush-trap=number -G num
M680x0 Options (M680x0 Options) -march=arch -mcpu=cpu -mtune=tune
-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060
-mcpu32 -m5200 -m5206e -m528x -m5307 -m5407 -mcfv4e -mbitfield
-mno-bitfield -mc68000 -mc68020 -mnobitfield -mrtd -mno-rtd
-mdiv -mno-div -mshort -mno-short -mhard-float -m68881
-msoft-float -mpcrel -malign-int -mstrict-align -msep-data
-mno-sep-data -mshared-library-id=n -mid-shared-library
-mno-id-shared-library -mxgot -mno-xgot -mlong-jump-table-offsets
MCore Options (MCore Options) -mhardlit -mno-hardlit -mdiv
-mno-div -mrelax-immediates -mno-relax-immediates -mwide-bitfields
-mno-wide-bitfields -m4byte-functions -mno-4byte-functions
-mcallgraph-data -mno-callgraph-data -mslow-bytes -mno-slow-bytes
-mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MicroBlaze Options (MicroBlaze Options) -msoft-float -mhard-float
-msmall-divides -mcpu=cpu -mmemcpy -mxl-soft-mul -mxl-soft-div
-mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check -mxl-gp-opt
-mno-clearbss -mxl-multiply-high -mxl-float-convert -mxl-float-sqrt
-mbig-endian -mlittle-endian -mxl-reorder -mxl-mode-app-model
-mpic-data-is-text-relative
MIPS Options (MIPS Options) -EL -EB -march=arch -mtune=arch -mips1
-mips2 -mips3 -mips4 -mips32 -mips32r2 -mips32r3 -mips32r5
-mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6
-mips16 -mno-mips16 -mflip-mips16 -minterlink-compressed
-mno-interlink-compressed -minterlink-mips16 -mno-interlink-mips16
-mabi=abi -mabicalls -mno-abicalls -mshared -mno-shared -mplt
-mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfpxx -mfp64
-mhard-float -msoft-float -mno-float -msingle-float -mdouble-float
-modd-spreg -mno-odd-spreg -mabs=mode -mnan=encoding -mdsp
-mno-dsp -mdspr2 -mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva -mvirt
-mno-virt -mxpa -mno-xpa -mcrc -mno-crc -mginv -mno-ginv
-mmicromips -mno-micromips -mmsa -mno-msa -mloongson-mmi
-mno-loongson-mmi -mloongson-ext -mno-loongson-ext -mloongson-ext2
-mno-loongson-ext2 -mfpu=fpu-type -msmartmips -mno-smartmips
-mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d
-mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64 -mlong32
-msym32 -mno-sym32 -Gnum -mlocal-sdata -mno-local-sdata
-mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt -membedded-data
-mno-embedded-data -muninit-const-in-rodata
-mno-uninit-const-in-rodata -mcode-readable=setting -msplit-addresses
-mno-split-addresses -mexplicit-relocs -mno-explicit-relocs
-mexplicit-relocs=release -mcheck-zero-division
-mno-check-zero-division -mdivide-traps -mdivide-breaks
-mload-store-pairs -mno-load-store-pairs -mstrict-align
-mno-strict-align -mno-unaligned-access -munaligned-access -mmemcpy
-mno-memcpy -mlong-calls -mno-long-calls -mmad -mno-mad -mimadd
-mno-imadd -mfused-madd -mno-fused-madd -nocpp -mfix-24k
-mno-fix-24k -mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400
-mfix-r5900 -mno-fix-r5900 -mfix-r10000 -mno-fix-r10000
-mfix-rm7000 -mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120
-mfix-vr4130 -mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1
-mflush-func=func -mno-flush-func -mbranch-cost=num -mbranch-likely
-mno-branch-likely -mcompact-branches=policy -mfp-exceptions
-mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci
-mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4
-mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address
-mframe-header-opt -mno-frame-header-opt
MMIX Options (MMIX Options) -mlibfuncs -mno-libfuncs -mepsilon
-mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv
-mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict
-mbase-addresses -mno-base-addresses -msingle-exit -mno-single-exit
MN10300 Options (MN10300 Options) -mmult-bug -mno-mult-bug -mno-am33
-mam33 -mam33-2 -mam34 -mtune=cpu-type -mreturn-pointer-on-d0
-mno-crt0 -mrelax -mliw -msetlb
Moxie Options (Moxie Options) -meb -mel -mmul.x -mno-crt0
MSP430 Options (MSP430 Options) -msim -masm-hex -mmcu= -mcpu=
-mlarge -msmall -mrelax -mwarn-mcu -mcode-region= -mdata-region=
-msilicon-errata= -msilicon-errata-warn= -mhwmult= -minrt
-mtiny-printf -mmax-inline-shift=
NDS32 Options (NDS32 Options) -mbig-endian -mlittle-endian
-mreduced-regs -mfull-regs -mcmov -mno-cmov -mext-perf
-mno-ext-perf -mext-perf2 -mno-ext-perf2 -mext-string
-mno-ext-string -mv3push -mno-v3push -m16bit -mno-16bit
-misr-vector-size=num -mcache-block-size=num -march=arch
-mcmodel=code-model -mctor-dtor -mrelax
Nvidia PTX Options (Nvidia PTX Options) -m64 -mmainkernel
-moptimize
OpenRISC Options (OpenRISC Options) -mboard=name -mnewlib
-mhard-mul -mhard-div -msoft-mul -msoft-div -msoft-float
-mhard-float -mdouble-float -munordered-float -mcmov -mror -mrori
-msext -msfimm -mshftimm -mcmodel=code-model
PDP-11 Options (PDP-11 Options) -mfpu -msoft-float -mac0 -mno-ac0
-m40 -m45 -m10 -mint32 -mno-int16 -mint16 -mno-int32 -msplit
-munix-asm -mdec-asm -mgnu-asm -mlra
PowerPC Options See RS/6000 and PowerPC Options.
PRU Options (PRU Options) -mmcu=mcu -minrt -mno-relax -mloop
-mabi=variant
RISC-V Options (RISC-V Options) -mbranch-cost=N-instruction -mplt
-mno-plt -mabi=ABI-string -mfdiv -mno-fdiv -mfence-tso
-mno-fence-tso -mdiv -mno-div -misa-spec=ISA-spec-string -march=ISA-
string -mtune=processor-string -mpreferred-stack-boundary=num
-msmall-data-limit=N-bytes -msave-restore -mno-save-restore
-mshorten-memrefs -mno-shorten-memrefs -mstrict-align
-mno-strict-align -mcmodel=medlow -mcmodel=medany -mcmodel=large
-mexplicit-relocs -mno-explicit-relocs -mrelax -mno-relax
-mriscv-attribute -mno-riscv-attribute -malign-data=type
-mbig-endian -mlittle-endian -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset
-mcsr-check -mno-csr-check -mmovcc -mno-movcc -minline-atomics
-mno-inline-atomics -minline-strlen -mno-inline-strlen
-minline-strcmp -mno-inline-strcmp -minline-strncmp
-mno-inline-strncmp -mtls-dialect=desc -mtls-dialect=trad
RL78 Options (RL78 Options) -msim -mmul=none -mmul=g13 -mmul=g14
-mallregs -mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13 -mg14
-m64bit-doubles -m32bit-doubles -msave-mduc-in-interrupts
RS/6000 and PowerPC Options (RS/6000 and PowerPC Options) -mcpu=cpu-
type -mtune=cpu-type -mcmodel=code-model -mpowerpc64 -maltivec
-mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb
-mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb
-mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc -mno-fp-in-toc
-mno-sum-in-toc -m64 -m32 -mxl-compat -mno-xl-compat -mpe
-malign-power -malign-natural -msoft-float -mhard-float -mmultiple
-mno-multiple -mupdate -mno-update -mavoid-indexed-addresses
-mno-avoid-indexed-addresses -mfused-madd -mno-fused-madd
-mbit-align -mno-bit-align -mstrict-align -mno-strict-align
-mrelocatable -mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian
-mbig -mbig-endian -mdynamic-no-pic -mswdiv -msingle-pic-base
-mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux
-mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi
-mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-msplit-patch-nops -mlongcall -mno-longcall -mpltseq -mno-pltseq
-mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mno-block-ops-unaligned-vsx
-mstring-compare-inline-limit=num -misel -mno-isel -mvrsave
-mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mprototype
-mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata
-msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip
-mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision
-mveclibabi=type -mfriz -mno-friz -mpointers-to-nested-functions
-mno-pointers-to-nested-functions -msave-toc-indirect
-mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion -mcrypto
-mno-crypto -mhtm -mno-htm -mquad-memory -mno-quad-memory
-mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm
-mno-compat-align-parm -mfloat128 -mno-float128 -mfloat128-hardware
-mno-float128-hardware -mgnu-attribute -mno-gnu-attribute
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mprefixed -mno-prefixed
-mpcrel -mno-pcrel -mmma -mno-mmma -mrop-protect -mno-rop-protect
-mprivileged -mno-privileged
RX Options (RX Options) -m64bit-doubles -m32bit-doubles -fpu
-nofpu -mcpu= -mbig-endian-data -mlittle-endian-data -msmall-data
-msim -mno-sim -mas100-syntax -mno-as100-syntax -mrelax
-mmax-constant-size= -mint-register= -mpid -mallow-string-insns
-mno-allow-string-insns -mjsr -mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts
S/390 and zSeries Options (S/390 and zSeries Options) -mtune=cpu-type
-march=cpu-type -mhard-float -msoft-float -mhard-dfp -mno-hard-dfp
-mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain
-mpacked-stack -mno-packed-stack -msmall-exec -mno-small-exec
-mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch
-mhtm -mvx -mzvector -mtpf-trace -mno-tpf-trace -mtpf-trace-skip
-mno-tpf-trace-skip -mfused-madd -mno-fused-madd -mwarn-framesize
-mwarn-dynamicstack -mstack-size -mstack-guard
-mhotpatch=halfwords,halfwords
SH Options (SH Options) -m1 -m2 -m2e -m2a-nofpu -m2a-single-only
-m2a-single -m2a -m3 -m3e -m4-nofpu -m4-single-only -m4-single
-m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -mb -ml
-mdalign -mrelax -mbigtable -mfmovd -mrenesas -mno-renesas
-mnomacsave -mieee -mno-ieee -mbitops -misize
-minline-ic_invalidate -mpadstruct -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name
-mfixed-range=register-range -maccumulate-outgoing-args
-matomic-model=atomic-model -mbranch-cost=num -mzdcbranch
-mno-zdcbranch -mcbranch-force-delay-slot -mfused-madd
-mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra
-mpretend-cmove -mtas
Solaris 2 Options (Solaris 2 Options) -mclear-hwcap -mno-clear-hwcap
-mimpure-text -mno-impure-text -pthreads
SPARC Options (SPARC Options) -mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -mmemory-model=mem-model -m32 -m64 -mapp-regs
-mno-app-regs -mfaster-structs -mno-faster-structs -mflat
-mno-flat -mfpu -mno-fpu -mhard-float -msoft-float
-mhard-quad-float -msoft-quad-float -mstack-bias -mno-stack-bias
-mstd-struct-return -mno-std-struct-return -munaligned-doubles
-mno-unaligned-doubles -muser-mode -mno-user-mode -mv8plus
-mno-v8plus -mvis -mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3
-mvis3b -mno-vis3b -mvis4 -mno-vis4 -mvis4b -mno-vis4b -mcbcond
-mno-cbcond -mfmaf -mno-fmaf -mfsmuld -mno-fsmuld -mpopc
-mno-popc -msubxc -mno-subxc -mfix-at697f -mfix-ut699 -mfix-ut700
-mfix-gr712rc
System V Options (System V Options) -Qy -Qn -YP,paths -Ym,dir
V850 Options (V850 Options) -mlong-calls -mno-long-calls -mep
-mno-ep -mprolog-function -mno-prolog-function -mspace -mtda=n
-msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt
-mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e
-mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float
-mhard-float -mgcc-abi -mrh850-abi -mbig-switch
VAX Options (VAX Options) -munix -mgnu -md -md-float -mg
-mg-float -mlra
Visium Options (Visium Options) -mdebug -msim -mfpu -mno-fpu
-mhard-float -msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode
-muser-mode
VMS Options (VMS Options) -mvms-return-codes -mdebug-main=prefix
-mmalloc64 -mpointer-size=size
VxWorks Options (VxWorks Options) -mrtp -msmp -non-static -Bstatic
-Bdynamic -Xbind-lazy -Xbind-now
x86 Options (x86 Options) -mtune=cpu-type -march=cpu-type
-mtune-ctrl=feature-list -mdump-tune-features -mno-default
-mfpmath=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387
-m80387 -mhard-float -msoft-float -mno-wide-multiply -mrtd
-malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32
-mmwait -mrecip -mrecip=opt -mvzeroupper -mprefer-avx128
-mprefer-vector-width=opt -mpartial-vector-fp-math -mmove-max=bits
-mstore-max=bits -mnoreturn-no-callee-saved-registers -mmmx -msse
-msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2
-mavx512f -mavx512cd -mavx512vl -mavx512bw -mavx512dq
-mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mfsgsbase
-mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mptwrite -mclflushopt
-mclwb -mxsavec -mxsaves -msse4a -m3dnow -m3dnowa -mpopcnt
-mabm -mbmi -mtbm -mfma4 -mxop -madx -mlzcnt -mbmi2 -mfxsr
-mxsave -mxsaveopt -mrtm -mhle -mlwp -mmwaitx -mclzero -mpku
-mthreads -mgfni -mvaes -mwaitpkg -mshstk -mmanual-endbr
-mcet-switch -mforce-indirect-call -mavx512vbmi2 -mavx512bf16
-menqcmd -mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b
-mavx512vpopcntdq -mavx512vnni -mprfchw -mrdpid -mrdseed -msgx
-mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8
-mamx-bf16 -muintr -mhreset -mavxvnni -mamx-fp8 -mavx512fp16
-mavxifma -mavxvnniint8 -mavxneconvert -mcmpccxadd -mamx-fp16
-mprefetchi -mraoint -mamx-complex -mavxvnniint16 -msm3 -msha512
-msm4 -mapxf -musermsr -mavx10.1 -mavx10.1-256 -mavx10.1-512
-mevex512 -mavx10.2 -mamx-avx512 -mamx-tf32 -mamx-transpose -mmovrs
-mamx-movrs -mcldemote -mms-bitfields -mno-align-stringops
-minline-all-stringops -minline-stringops-dynamically
-mstringop-strategy=alg -mkl -mwidekl -mmemcpy-strategy=strategy
-mmemset-strategy=strategy -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mlong-double-64
-mlong-double-80 -mlong-double-128 -mregparm=num -msseregparm
-mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64 -mpc80
-mdaz-ftz -mstackrealign -momit-leaf-frame-pointer -mno-red-zone
-mno-tls-direct-seg-refs -mcmodel=code-model -mabi=name
-maddress-mode=mode -m32 -m64 -mx32 -m16 -miamcu
-mlarge-data-threshold=num -msse2avx -mfentry -mrecord-mcount
-mnop-mcount -m8bit-idiv -minstrument-return=type -mfentry-name=name
-mfentry-section=name -mavx256-split-unaligned-load
-mavx256-split-unaligned-store -malign-data=type
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol -mgeneral-regs-only
-mcall-ms2sysv-xlogues -mrelax-cmpxchg-loop -mindirect-branch=choice
-mfunction-return=choice -mindirect-branch-register
-mharden-sls=choice -mindirect-branch-cs-prefix -mneeded
-mno-direct-extern-access -munroll-only-small-loops -mlam=choice
x86 Windows Options See Cygwin and MinGW Options.
Xstormy16 Options (Xstormy16 Options) -msim
Xtensa Options (Xtensa Options) -mconst16 -mno-const16 -mfused-madd
-mno-fused-madd -mforce-no-pic -mserialize-volatile
-mno-serialize-volatile -mtext-section-literals
-mno-text-section-literals -mauto-litpools -mno-auto-litpools
-mtarget-align -mno-target-align -mlongcalls -mno-longcalls
-mabi=abi-type -mextra-l32r-costs=cycles -mstrict-align
-mno-strict-align
zSeries Options See S/390 and zSeries Options.
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order. GCC is capable of
preprocessing and compiling several files either into several assembler
input files, or into one assembler input file; then each assembler input
file produces an object file, and linking combines all the object files
(those newly compiled, and those specified as input) into an executable
file.
For any given input file, the file name suffix determines what kind of
compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc
library to make an Objective-C program work.
file.mi
Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the libobjc
library to make an Objective-C++ program work. Note that .M refers
to a literal capital M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a
precompiled header (default), or C, C++ header file to be turned into
an Ada spec (via the -fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in .cxx, the
last two letters must both be literally x. Likewise, .C refers to a
literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header or Ada spec.
file.f
file.for
file.ftn
file.fi
Fixed form Fortran source code that should not be preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed (with the
traditional preprocessor).
file.f90
file.f95
file.f03
file.f08
file.fii
Free form Fortran source code that should not be preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with the
traditional preprocessor).
file.cob
file.COB
file.cbl
file.CBL
COBOL source code.
file.go
Go source code.
file.d
D source code.
file.di
D interface file.
file.dd
D documentation code (Ddoc).
file.ads
Ada source code file that contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also
called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or
package body). Such files are also called bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file name with
no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather
than letting the compiler choose a default based on the file name
suffix). This option applies to all following input files until the
next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-system-header c++-user-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output objc-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
objc++-cpp-output
assembler assembler-with-cpp
ada adascil adawhy
cobol
d
f77 f77-cpp-input f95 f95-cpp-input
go
modula-2 modula-2-cpp-output
rust
lto
Note that -x does not imply a particular language standard. For
example -x f77 may also require -std=legacy for some older source
codes.
-x none
Turn off any specification of a language, so that subsequent files
are handled according to their file name suffixes (as if -x has not
been used at all).
If you only want some of the stages of compilation, you can use -x (or
filename suffixes) to tell gcc where to start, and one of the options -c,
-S, or -E to say where gcc is to stop. Note that some combinations (for
example, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are
ignored.
-S Stop after the stage of compilation proper; do not assemble. The
output is in the form of an assembler code file for each non-
assembler input file specified.
By default, the assembler file name for a source file is made by
replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper.
The output is in the form of preprocessed source code, which is sent
to the standard output.
Input files that don't require preprocessing are ignored.
-o file
Place the primary output in file file. This applies to whatever sort
of output is being produced, whether it be an executable file, an
object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in
a.out, the object file for source.suffix in source.o, its assembler
file in source.s, a precompiled header file in source.suffix.gch, and
all preprocessed C source on standard output.
Though -o names only the primary output, it also affects the naming
of auxiliary and dump outputs. See the examples below. Unless
overridden, both auxiliary outputs and dump outputs are placed in the
same directory as the primary output. In auxiliary outputs, the
suffix of the input file is replaced with that of the auxiliary
output file type; in dump outputs, the suffix of the dump file is
appended to the input file suffix. In compilation commands, the base
name of both auxiliary and dump outputs is that of the primary
output; in compile and link commands, the primary output name, minus
the executable suffix, is combined with the input file name. If both
share the same base name, disregarding the suffix, the result of the
combination is that base name, otherwise, they are concatenated,
separated by a dash.
gcc -c foo.c ...
will use foo.o as the primary output, and place aux outputs and dumps
next to it, e.g., aux file foo.dwo for -gsplit-dwarf, and dump file
foo.c.???r.final for -fdump-rtl-final.
If a non-linker output file is explicitly specified, aux and dump
files by default take the same base name:
gcc -c foo.c -o dir/foobar.o ...
will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.
A linker output will instead prefix aux and dump outputs:
gcc foo.c bar.c -o dir/foobar ...
will generally name aux outputs dir/foobar-foo.* and
dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and
dir/foobar-bar.c.*.
The one exception to the above is when the executable shares the base
name with the single input:
gcc foo.c -o dir/foo ...
in which case aux outputs are named dir/foo.* and dump outputs named
dir/foo.c.*.
The location and the names of auxiliary and dump outputs can be
adjusted by the options -dumpbase, -dumpbase-ext, -dumpdir,
-save-temps=cwd, and -save-temps=obj.
-dumpbase dumpbase
This option sets the base name for auxiliary and dump output files.
It does not affect the name of the primary output file. Intermediate
outputs, when preserved, are not regarded as primary outputs, but as
auxiliary outputs:
gcc -save-temps -S foo.c
saves the (no longer) temporary preprocessed file in foo.i, and then
compiles to the (implied) output file foo.s, whereas:
gcc -save-temps -dumpbase save-foo -c foo.c
preprocesses to in save-foo.i, compiles to save-foo.s (now an
intermediate, thus auxiliary output), and then assembles to the
(implied) output file foo.o.
Absent this option, dump and aux files take their names from the
input file, or from the (non-linker) output file, if one is
explicitly specified: dump output files (e.g. those requested by
-fdump-* options) with the input name suffix, and aux output files
(those requested by other non-dump options, e.g. "-save-temps",
"-gsplit-dwarf", "-fcallgraph-info") without it.
Similar suffix differentiation of dump and aux outputs can be
attained for explicitly-given -dumpbase basename.suf by also
specifying -dumpbase-ext .suf.
If dumpbase is explicitly specified with any directory component, any
dumppfx specification (e.g. -dumpdir or -save-temps=*) is ignored,
and instead of appending to it, dumpbase fully overrides it:
gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
-dumpdir pfx- -save-temps=cwd ...
creates auxiliary and dump outputs named alt/foo.*, disregarding dir/
in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in
-dumpdir.
When -dumpbase is specified in a command that compiles multiple
inputs, or that compiles and then links, it may be combined with
dumppfx, as specified under -dumpdir. Then, each input file is
compiled using the combined dumppfx, and default values for dumpbase
and auxdropsuf are computed for each input file:
gcc foo.c bar.c -c -dumpbase main ...
creates foo.o and bar.o as primary outputs, and avoids overwriting
the auxiliary and dump outputs by using the dumpbase as a prefix,
creating auxiliary and dump outputs named main-foo.* and main-bar.*.
An empty string specified as dumpbase avoids the influence of the
output basename in the naming of auxiliary and dump outputs during
compilation, computing default values :
gcc -c foo.c -o dir/foobar.o -dumpbase " ...
will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note
how their basenames are taken from the input name, but the directory
still defaults to that of the output.
The empty-string dumpbase does not prevent the use of the output
basename for outputs during linking:
gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...
The compilation of the source files will name auxiliary outputs
dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and
dir/bar.c.*. LTO recompilation during linking will use dir/foobar.
as the prefix for dumps and auxiliary files.
-dumpbase-ext auxdropsuf
When forming the name of an auxiliary (but not a dump) output file,
drop trailing auxdropsuf from dumpbase before appending any suffixes.
If not specified, this option defaults to the suffix of a default
dumpbase, i.e., the suffix of the input file when -dumpbase is not
present in the command line, or dumpbase is combined with dumppfx.
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...
creates dir/foo.o as the main output, and generates auxiliary outputs
in dir/x-foo.*, taking the location of the primary output, and
dropping the .c suffix from the dumpbase. Dump outputs retain the
suffix: dir/x-foo.c.*.
This option is disregarded if it does not match the suffix of a
specified dumpbase, except as an alternative to the executable suffix
when appending the linker output base name to dumppfx, as specified
below:
gcc foo.c bar.c -o main.out -dumpbase-ext .out ...
creates main.out as the primary output, and avoids overwriting the
auxiliary and dump outputs by using the executable name minus
auxdropsuf as a prefix, creating auxiliary outputs named main-foo.*
and main-bar.* and dump outputs named main-foo.c.* and main-bar.c.*.
-dumpdir dumppfx
When forming the name of an auxiliary or dump output file, use
dumppfx as a prefix:
gcc -dumpdir pfx- -c foo.c ...
creates foo.o as the primary output, and auxiliary outputs named
pfx-foo.*, combining the given dumppfx with the default dumpbase
derived from the default primary output, derived in turn from the
input name. Dump outputs also take the input name suffix:
pfx-foo.c.*.
If dumppfx is to be used as a directory name, it must end with a
directory separator:
gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...
creates obj/bar.o as the primary output, and auxiliary outputs named
dir/bar.*, combining the given dumppfx with the default dumpbase
derived from the primary output name. Dump outputs also take the
input name suffix: dir/bar.c.*.
It defaults to the location of the output file, unless the output
file is a special file like "/dev/null". Options -save-temps=cwd and
-save-temps=obj override this default, just like an explicit -dumpdir
option. In case multiple such options are given, the last one
prevails:
gcc -dumpdir pfx- -c foo.c -save-temps=obj ...
outputs foo.o, with auxiliary outputs named foo.* because
-save-temps=* overrides the dumppfx given by the earlier -dumpdir
option. It does not matter that =obj is the default for -save-temps,
nor that the output directory is implicitly the current directory.
Dump outputs are named foo.c.*.
When compiling from multiple input files, if -dumpbase is specified,
dumpbase, minus a auxdropsuf suffix, and a dash are appended to (or
override, if containing any directory components) an explicit or
defaulted dumppfx, so that each of the multiple compilations gets
differently-named aux and dump outputs.
gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...
outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*,
appending dumpbase- to dumppfx. Dump outputs retain the input file
suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively.
Contrast with the single-input compilation:
gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...
that, applying -dumpbase to a single source, does not compute and
append a separate dumpbase per input file. Its auxiliary and dump
outputs go in dir/pfx-main.*.
When compiling and then linking from multiple input files, a
defaulted or explicitly specified dumppfx also undergoes the
dumpbase- transformation above (e.g. the compilation of foo.c and
bar.c above, but without -c). If neither -dumpdir nor -dumpbase are
given, the linker output base name, minus auxdropsuf, if specified,
or the executable suffix otherwise, plus a dash is appended to the
default dumppfx instead. Note, however, that unlike earlier cases of
linking:
gcc foo.c bar.c -dumpdir dir/pfx- -o main ...
does not append the output name main to dumppfx, because -dumpdir is
explicitly specified. The goal is that the explicitly-specified
dumppfx may contain the specified output name as part of the prefix,
if desired; only an explicitly-specified -dumpbase would be combined
with it, in order to avoid simply discarding a meaningful option.
When compiling and then linking from a single input file, the linker
output base name will only be appended to the default dumppfx as
above if it does not share the base name with the single input file
name. This has been covered in single-input linking cases above, but
not with an explicit -dumpdir that inhibits the combination, even if
overridden by -save-temps=*:
gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...
Auxiliary outputs are named foo.*, and dump outputs foo.c.*, in the
current working directory as ultimately requested by -save-temps=cwd.
Summing it all up for an intuitive though slightly imprecise data
flow: the primary output name is broken into a directory part and a
basename part; dumppfx is set to the former, unless overridden by
-dumpdir or -save-temps=*, and dumpbase is set to the latter, unless
overriden by -dumpbase. If there are multiple inputs or linking,
this dumpbase may be combined with dumppfx and taken from each input
file. Auxiliary output names for each input are formed by combining
dumppfx, dumpbase minus suffix, and the auxiliary output suffix; dump
output names are only different in that the suffix from dumpbase is
retained.
When it comes to auxiliary and dump outputs created during LTO
recompilation, a combination of dumppfx and dumpbase, as given or as
derived from the linker output name but not from inputs, even in
cases in which this combination would not otherwise be used as such,
is passed down with a trailing period replacing the compiler-added
dash, if any, as a -dumpdir option to lto-wrapper; being involved in
linking, this program does not normally get any -dumpbase and
-dumpbase-ext, and it ignores them.
When running sub-compilers, lto-wrapper appends LTO stage names to
the received dumppfx, ensures it contains a directory component so
that it overrides any -dumpdir, and passes that as -dumpbase to sub-
compilers.
-v Print (on standard error output) the commands executed to run the
stages of compilation. Also print the version number of the compiler
driver program and of the preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and arguments are quoted
unless they contain only alphanumeric characters or "./-_". This is
useful for shell scripts to capture the driver-generated command
lines.
--help
Print (on the standard output) a description of the command-line
options understood by gcc. If the -v option is also specified then
--help is also passed on to the various processes invoked by gcc, so
that they can display the command-line options they accept. If the
-Wextra option has also been specified (prior to the --help option),
then command-line options that have no documentation associated with
them are also displayed.
--target-help
Print (on the standard output) a description of target-specific
command-line options for each tool. For some targets extra target-
specific information may also be printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-line
options understood by the compiler that fit into all specified
classes and qualifiers. These are the supported classes:
optimizers
Display all of the optimization options supported by the
compiler.
warnings
Display all of the options controlling warning messages produced
by the compiler.
target
Display target-specific options. Unlike the --target-help option
however, target-specific options of the linker and assembler are
not displayed. This is because those tools do not currently
support the extended --help= syntax.
params
Display the values recognized by the --param option.
language
Display the options supported for language, where language is the
name of one of the languages supported in this version of GCC.
If an option is supported by all languages, one needs to select
common class.
common
Display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an equal
sign in the same continuous piece of text, such as:
--help=target.
separate
Display options taking an argument that appears as a separate
word following the original option, such as: -o output-file.
Thus for example to display all the undocumented target-specific
switches supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^
character, so for example to display all binary warning options
(i.e., ones that are either on or off and that do not take an
argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted
qualifiers.
Combining several classes is possible, although this usually
restricts the output so much that there is nothing to display. One
case where it does work, however, is when one of the classes is
target. For example, to display all the target-specific optimization
options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each
successive use displays its requested class of options, skipping
those that have already been displayed. If --help is also specified
anywhere on the command line then this takes precedence over any
--help= option.
If the -Q option appears on the command line before the --help=
option, then the descriptive text displayed by --help= is changed.
Instead of describing the displayed options, an indication is given
as to whether the option is enabled, disabled or set to a specific
value (assuming that the compiler knows this at the point where the
--help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line
options, so for example it is possible to find out which
optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are enabled
by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
--version
Display the version number and copyrights of the invoked GCC.
-pass-exit-codes
Normally the gcc program exits with the code of 1 if any phase of the
compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program instead returns with the
numerically highest error produced by any phase returning an error
indication. The C, C++, and Fortran front ends return 4 if an
internal compiler error is encountered.
-pipe
Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems
where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
-specs=file
Process file after the compiler reads in the standard specs file, in
order to override the defaults which the gcc driver program uses when
determining what switches to pass to cc1, cc1plus, as, ld, etc. More
than one -specs=file can be specified on the command line, and they
are processed in order, from left to right.
-wrapper
Invoke all subcommands under a wrapper program. The name of the
wrapper program and its parameters are passed as a comma separated
list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus the
invocation of cc1 is gdb --args cc1 ....
-ffile-prefix-map=old=new
When compiling files residing in directory old, record any references
to them in the result of the compilation as if the files resided in
directory new instead. Specifying this option is equivalent to
specifying all the individual -f*-prefix-map options. This can be
used to make reproducible builds that are location independent.
Directories referenced by directives are not affected by these
options. See also -fmacro-prefix-map, -fdebug-prefix-map,
-fprofile-prefix-map and -fcanon-prefix-map.
-fcanon-prefix-map
For the -f*-prefix-map options normally comparison of old prefix
against the filename that would be normally referenced in the result
of the compilation is done using textual comparison of the prefixes,
or ignoring character case for case insensitive filesystems and
considering slashes and backslashes as equal on DOS based
filesystems. The -fcanon-prefix-map causes such comparisons to be
done on canonicalized paths of old and the referenced filename.
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared object
to be dlopen'd by the compiler. The base name of the shared object
file is used to identify the plugin for the purposes of argument
parsing (See -fplugin-arg-name-key=value below). Each plugin should
define the callback functions specified in the Plugins API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin
called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada
specs.
-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs
as child units of parent unit.
-fdump-go-spec=file
For input files in any language, generate corresponding Go
declarations in file. This generates Go "const", "type", "var", and
"func" declarations which may be a useful way to start writing a Go
interface to code written in some other language.
@file
Read command-line options from file. The options read are inserted
in place of the original @file option. If file does not exist, or
cannot be read, then the option will be treated literally, and not
removed.
Options in file are separated by whitespace. A whitespace character
may be included in an option by surrounding the entire option in
either single or double quotes. Any character (including a
backslash) may be included by prefixing the character to be included
with a backslash. The file may itself contain additional @file
options; any such options will be processed recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp,
.CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or
(for shared template code) .tcc; preprocessed C++ files use the suffix
.ii; and C++20 module interface units sometimes use .ixx, .cppm, .cxxm,
.c++m, or .ccm.
GCC recognizes files with these names and compiles them as C++ programs
even if you call the compiler the same way as for compiling C programs
(usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program
that calls GCC and automatically specifies linking against the C++
library. It treats .c, .h and .i files as C++ source files instead of C
source files unless -x is used. This program is also useful when
precompiling a C header file with a .h extension for use in C++
compilations. On many systems, g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-
line options that you use for compiling programs in any language; or
command-line options meaningful for C and related languages; or options
that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from
C, such as C++, Objective-C and Objective-C++) that the compiler accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is
equivalent to -std=c++98.
-std=
Determine the language standard. This option is currently only
supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or c++98,
and GNU dialects of those standards, such as gnu90 or gnu++98. When
a base standard is specified, the compiler accepts all programs
following that standard plus those using GNU extensions that do not
contradict it. For example, -std=c90 turns off certain features of
GCC that are incompatible with ISO C90, such as the "asm" and
"typeof" keywords, but not other GNU extensions that do not have a
meaning in ISO C90, such as omitting the middle term of a "?:"
expression. On the other hand, when a GNU dialect of a standard is
specified, all features supported by the compiler are enabled, even
when those features change the meaning of the base standard. As a
result, some strict-conforming programs may be rejected. The
particular standard is used by -Wpedantic to identify which features
are GNU extensions given that version of the standard. For example
-std=gnu90 -Wpedantic warns about C++ style // comments, while
-std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that
conflict with ISO C90 are disabled). Same as -ansi for C code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. This standard is substantially completely supported,
modulo bugs and floating-point issues (mainly but not entirely
relating to optional C99 features from Annexes F and G). See
<https://gcc.gnu.org/c99status.html> for more information. The
names c9x and iso9899:199x are deprecated.
c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard. This standard
is substantially completely supported, modulo bugs, floating-
point issues (mainly but not entirely relating to optional C11
features from Annexes F and G) and the optional Annexes K
(Bounds-checking interfaces) and L (Analyzability). The name c1x
is deprecated.
c17
c18
iso9899:2017
iso9899:2018
ISO C17, the 2017 revision of the ISO C standard (published in
2018). This standard is same as C11 except for corrections of
defects (all of which are also applied with -std=c11) and a new
value of "__STDC_VERSION__", and so is supported to the same
extent as C11.
c23
c2x
iso9899:2024
ISO C23, the 2023 revision of the ISO C standard (published in
2024). The name c2x is deprecated.
c2y The next version of the ISO C standard, still under development.
The support for this version is experimental and incomplete.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features).
gnu99
gnu9x
GNU dialect of ISO C99. The name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. The name gnu1x is deprecated.
gnu17
gnu18
GNU dialect of ISO C17.
gnu23
gnu2x
GNU dialect of ISO C23. This is the default for C code. The
name gnu2x is deprecated.
gnu2y
The next version of the ISO C standard, still under development,
plus GNU extensions. The support for this version is
experimental and incomplete. The name gnu2x is deprecated.
c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical corrigendum and
some additional defect reports. Same as -ansi for C++ code.
gnu++98
gnu++03
GNU dialect of -std=c++98.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. The name c++0x is
deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. The name gnu++0x is deprecated.
c++14
c++1y
The 2014 ISO C++ standard plus amendments. The name c++1y is
deprecated.
gnu++14
gnu++1y
GNU dialect of -std=c++14. The name gnu++1y is deprecated.
c++17
c++1z
The 2017 ISO C++ standard plus amendments. The name c++1z is
deprecated.
gnu++17
gnu++1z
GNU dialect of -std=c++17. This is the default for C++ code.
The name gnu++1z is deprecated.
c++20
c++2a
The 2020 ISO C++ standard plus amendments. Support is
experimental, and could change in incompatible ways in future
releases. The name c++2a is deprecated.
gnu++20
gnu++2a
GNU dialect of -std=c++20. Support is experimental, and could
change in incompatible ways in future releases. The name gnu++2a
is deprecated.
c++23
c++2b
The 2023 ISO C++ standard plus amendments (published in 2024).
Support is experimental, and could change in incompatible ways in
future releases. The name c++2b is deprecated.
gnu++23
gnu++2b
GNU dialect of -std=c++23. Support is experimental, and could
change in incompatible ways in future releases. The name gnu++2b
is deprecated.
c++2c
c++26
The next revision of the ISO C++ standard, planned for 2026.
Support is highly experimental, and will almost certainly change
in incompatible ways in future releases.
gnu++2c
gnu++26
GNU dialect of -std=c++2c. Support is highly experimental, and
will almost certainly change in incompatible ways in future
releases.
-aux-info filename
Output to the given filename prototyped declarations for all
functions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any
language other than C.
Besides declarations, the file indicates, in comments, the origin of
each declaration (source file and line), whether the declaration was
implicit, prototyped or unprototyped (I, N for new or O for old,
respectively, in the first character after the line number and the
colon), and whether it came from a declaration or a definition (C or
F, respectively, in the following character). In the case of
function definitions, a K&R-style list of arguments followed by their
declarations is also provided, inside comments, after the
declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that
code can use these words as identifiers. You can use the keywords
"__asm__", "__inline__" and "__typeof__" instead. In C, -ansi
implies -fno-asm.
In C++, "inline" is a standard keyword and is not affected by this
switch. You may want to use the -fno-gnu-keywords flag instead,
which disables "typeof" but not "asm" and "inline". In C99 mode
(-std=c99 or -std=gnu99), this switch only affects the "asm" and
"typeof" keywords, since "inline" is a standard keyword in ISO C99.
In C23 mode (-std=c23 or -std=gnu23), this switch only affects the
"asm" keyword, since "typeof" is a standard keyword in ISO C23.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with __builtin_
as prefix.
GCC normally generates special code to handle certain built-in
functions more efficiently; for instance, calls to "alloca" may
become single instructions which adjust the stack directly, and calls
to "memcpy" may become inline copy loops. The resulting code is
often both smaller and faster, but since the function calls no longer
appear as such, you cannot set a breakpoint on those calls, nor can
you change the behavior of the functions by linking with a different
library. In addition, when a function is recognized as a built-in
function, GCC may use information about that function to warn about
problems with calls to that function, or to generate more efficient
code, even if the resulting code still contains calls to that
function. For example, warnings are given with -Wformat for bad
calls to "printf" when "printf" is built in and "strlen" is known not
to modify global memory.
With the -fno-builtin-function option only the built-in function
function is disabled. function must not begin with __builtin_. If a
function is named that is not built-in in this version of GCC, this
option is ignored. There is no corresponding -fbuiltin-function
option; if you wish to enable built-in functions selectively when
using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and
third arguments. The value of such an expression is void. This
option is not supported for C++.
-ffreestanding
Assert that compilation targets a freestanding environment. This
implies -fno-builtin. A freestanding environment is one in which the
standard library may not exist, and program startup may not
necessarily be at "main". The most obvious example is an OS kernel.
This is equivalent to -fno-hosted.
-fgimple
Enable parsing of function definitions marked with "__GIMPLE". This
is an experimental feature that allows unit testing of GIMPLE passes.
-fgnu-tm
When the option -fgnu-tm is specified, the compiler generates code
for the Linux variant of Intel's current Transactional Memory ABI
specification document (Revision 1.1, May 6 2009). This is an
experimental feature whose interface may change in future versions of
GCC, as the official specification changes. Please note that not all
architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-
call exceptions (-fnon-call-exceptions).
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU
semantics for "inline" functions when in C99 mode.
Using this option is roughly equivalent to adding the "gnu_inline"
function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99
semantics for "inline" when in C99 or gnu99 mode (i.e., it specifies
the default behavior). This option is not supported in -std=c90 or
-std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which semantics are in
effect for "inline" functions.
-fhosted
Assert that compilation targets a hosted environment. This implies
-fbuiltin. A hosted environment is one in which the entire standard
library is available, and in which "main" has a return type of "int".
Examples are nearly everything except a kernel. This is equivalent
to -fno-freestanding.
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of
elements and/or incompatible element types. This option should not
be used for new code.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to
previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only
accepted with this option.
Note that this option is off for all targets except for x86 targets
using ms-abi.
-fpermitted-flt-eval-methods=style
ISO/IEC TS 18661-3 defines new permissible values for
"FLT_EVAL_METHOD" that indicate that operations and constants with a
semantic type that is an interchange or extended format should be
evaluated to the precision and range of that type. These new values
are a superset of those permitted under C99/C11, which does not
specify the meaning of other positive values of "FLT_EVAL_METHOD".
As such, code conforming to C11 may not have been written expecting
the possibility of the new values.
-fpermitted-flt-eval-methods specifies whether the compiler should
allow only the values of "FLT_EVAL_METHOD" specified in C99/C11, or
the extended set of values specified in ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or similar)
is -fpermitted-flt-eval-methods=c11. The default when in a GNU
dialect (-std=gnu11 or similar) is
-fpermitted-flt-eval-methods=ts-18661-3.
The -fdeps-* options are used to extract structured dependency
information for a source. This involves determining what resources
provided by other source files will be required to compile the source
as well as what resources are provided by the source. This
information can be used to add required dependencies between
compilation rules of dependent sources based on their contents rather
than requiring such information be reflected within the build tools
as well.
-fdeps-file=file
Where to write structured dependency information.
-fdeps-format=format
The format to use for structured dependency information. p1689r5 is
the only supported format right now. Note that when this argument is
specified, the output of -MF is stripped of some information (namely
C++ modules) so that it does not use extended makefile syntax not
understood by most tools.
-fdeps-target=file
Analogous to -MT but for structured dependency information. This
indicates the target which will ultimately need any required
resources and provide any resources extracted from the source that
may be required by other sources.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures
with anonymous fields to functions that expect pointers to elements
of the type of the field, and permits referring to anonymous fields
declared using a typedef. This is only supported for C, not C++.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when
the declaration does not use either "signed" or "unsigned". By
default, such a bit-field is signed, because this is consistent: the
basic integer types such as "int" are signed types.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the
negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is
either like "unsigned char" by default or like "signed char" by
default.
Ideally, a portable program should always use "signed char" or
"unsigned char" when it depends on the signedness of an object. But
many programs have been written to use plain "char" and expect it to
be signed, or expect it to be unsigned, depending on the machines
they were written for. This option, and its inverse, let you make
such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char"
or "unsigned char", even though its behavior is always just like one
of those two.
-fstrict-flex-arrays (C and C++ only)
-fstrict-flex-arrays=level (C and C++ only)
Control when to treat the trailing array of a structure as a flexible
array member for the purpose of accessing the elements of such an
array. The value of level controls the level of strictness.
-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which
is the strictest; a trailing array is treated as a flexible array
member only when it is declared as a flexible array member per C99
standard onwards.
The negative form -fno-strict-flex-arrays is equivalent to
-fstrict-flex-arrays=0, which is the least strict. In this case all
trailing arrays of structures are treated as flexible array members.
There are two more levels in between 0 and 3, which are provided to
support older code that uses the GCC zero-length array extension
([0]) or one-element array as flexible array members ([1]). When
level is 1, the trailing array is treated as a flexible array member
when it is declared as either [], [0], or [1]. When level is 2, the
trailing array is treated as a flexible array member when it is
declared as either [], or [0].
You can control this behavior for a specific trailing array field of
a structure by using the variable attribute "strict_flex_array"
attribute.
The -fstrict_flex_arrays option interacts with the
-Wstrict-flex-arrays option.
-fsso-struct=endianness
Set the default scalar storage order of structures and unions to the
specified endianness. The accepted values are big-endian, little-
endian and native for the native endianness of the target (the
default). This option is not supported for C++.
Warning: the -fsso-struct switch causes GCC to generate code that is
not binary compatible with code generated without it if the specified
endianness is not the native endianness of the target.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful
for C++ programs. You can also use most of the GNU compiler options
regardless of what language your program is in. For example, you might
compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for C++
programs; you can use the other options with any language supported by
GCC.
Some options for compiling C programs, such as -std, are also relevant
for C++ programs.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to the C++
ABI specification. Therefore, the ABI obtained using version 0 will
change in different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in G++
3.2.
Version 2 is the version of the C++ ABI that first appeared in G++
3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a
template argument.
Version 4, which first appeared in G++ 4.5, implements a standard
mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of
attribute const/volatile on function pointer types, decltype of a
plain decl, and use of a function parameter in the declaration of
another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion
behavior of C++11 scoped enums and the mangling of template argument
packs, const/static_cast, prefix ++ and --, and a class scope
function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as
a builtin type and corrects the mangling of lambdas in default
argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution
behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of
"nullptr_t".
Version 10, which first appeared in G++ 6.1, adds mangling of
attributes that affect type identity, such as ia32 calling convention
attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of
sizeof... expressions and operator names. For multiple entities with
the same name within a function, that are declared in different
scopes, the mangling now changes starting with the twelfth
occurrence. It also implies -fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the calling
conventions for empty classes on the x86_64 target and for classes
with only deleted copy/move constructors. It accidentally changes
the calling convention for classes with a deleted copy constructor
and a trivial move constructor.
Version 13, which first appeared in G++ 8.2, fixes the accidental
change in version 12.
Version 14, which first appeared in G++ 10, corrects the mangling of
the nullptr expression.
Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag
regression.
Version 16, which first appeared in G++ 11, changes the mangling of
"__alignof__" to be distinct from that of "alignof", and dependent
operator names.
Version 17, which first appeared in G++ 12, fixes layout of classes
that inherit from aggregate classes with default member initializers
in C++14 and up.
Version 18, which first appeared in G++ 13, fixes manglings of
lambdas that have additional context.
Version 19, which first appeared in G++ 14, fixes manglings of
structured bindings to include ABI tags.
Version 20, which first appeared in G++ 15, fixes manglings of
lambdas in static data member initializers.
See also -Wabi.
-fabi-compat-version=n
On targets that support strong aliases, G++ works around mangling
changes by creating an alias with the correct mangled name when
defining a symbol with an incorrect mangled name. This switch
specifies which ABI version to use for the alias.
With -fabi-version=0 (the default), this defaults to 13 (GCC 8.2
compatibility). If another ABI version is explicitly selected, this
defaults to 0. For compatibility with GCC versions 3.2 through 4.9,
use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used
for compatibility aliases. If this option is provided along with
-Wabi (without the version), the version from this option is used for
the warning.
-fno-access-control
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
-faligned-new
Enable support for C++17 "new" of types that require more alignment
than "void* ::operator new(std::size_t)" provides. A numeric
argument such as "-faligned-new=32" can be used to specify how much
alignment (in bytes) is provided by that function, but few users will
need to override the default of "alignof(std::max_align_t)".
This flag is enabled by default for -std=c++17.
-fno-assume-sane-operators-new
The C++ standard allows replacing the global "new", "new[]", "delete"
and "delete[]" operators, though a lot of C++ programs don't replace
them and just use the implementation provided version. Furthermore,
the C++ standard allows omitting those calls if they are made from
new or delete expressions (and by extension the same is assumed if
"__builtin_operator_new" or "__builtin_operator_delete" functions are
used). This option allows control over some optimizations around
calls to those operators. With "-fassume-sane-operators-new-delete"
option GCC may assume that calls to the replaceable global operators
from new or delete expressions or from "__builtin_operator_new" or
"__builtin_operator_delete" calls don't read or modify any global
variables or variables whose address could escape to the operators
(global state; except for "errno" for the "new" and "new[]"
operators). This allows most optimizations across those calls and is
something that the implementation provided operators satisfy unless
"malloc" implementation details are observable in the code or unless
"malloc" hooks are used, but might not be satisfied if a program
replaces those operators. This behavior is enabled by default. With
"-fno-assume-sane-operators-new-delete" option GCC must assume all
these calls (whether from new or delete expressions or called
directly) may read and write global state unless proven otherwise
(e.g. when GCC compiles their implementation). Use this option if
those operators are or may be replaced and code needs to expect such
behavior.
-fchar8_t
-fno-char8_t
Enable support for "char8_t" as adopted for C++20. This includes the
addition of a new "char8_t" fundamental type, changes to the types of
UTF-8 string and character literals, new signatures for user-defined
literals, associated standard library updates, and new
"__cpp_char8_t" and "__cpp_lib_char8_t" feature test macros.
This option enables functions to be overloaded for ordinary and UTF-8
strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2
and introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;
The change to the types of UTF-8 string and character literals
introduces incompatibilities with ISO C++11 and later standards. For
example, the following code is well-formed under ISO C++11, but is
ill-formed when -fchar8_t is specified.
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' requested
-fcheck-new
Check that the pointer returned by "operator new" is non-null before
attempting to modify the storage allocated. This check is normally
unnecessary because the C++ standard specifies that "operator new"
only returns 0 if it is declared "throw()", in which case the
compiler always checks the return value even without this option. In
all other cases, when "operator new" has a non-empty exception
specification, memory exhaustion is signalled by throwing
"std::bad_alloc". See also new (nothrow).
-fconcepts
Enable support for the C++ Concepts feature for constraining template
arguments. With -std=c++20 and above, Concepts are part of the
language standard, so -fconcepts defaults to on.
Some constructs that were allowed by the earlier C++ Extensions for
Concepts Technical Specification, ISO 19217 (2015), but didn't make
it into the standard, could additionally be enabled by -fconcepts-ts.
The option -fconcepts-ts was deprecated in GCC 14 and removed in GCC
15; users are expected to convert their code to C++20 concepts.
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr functions
to n. A limit is needed to detect endless recursion during constant
expression evaluation. The minimum specified by the standard is 512.
-fconstexpr-cache-depth=n
Set the maximum level of nested evaluation depth for C++11 constexpr
functions that will be cached to n. This is a heuristic that trades
off compilation speed (when the cache avoids repeated calculations)
against memory consumption (when the cache grows very large from
highly recursive evaluations). The default is 8. Very few users are
likely to want to adjust it, but if your code does heavy constexpr
calculations you might want to experiment to find which value works
best for you.
-fconstexpr-fp-except
Annex F of the C standard specifies that IEC559 floating point
exceptions encountered at compile time should not stop compilation.
C++ compilers have historically not followed this guidance, instead
treating floating point division by zero as non-constant even though
it has a well defined value. This flag tells the compiler to give
Annex F priority over other rules saying that a particular operation
is undefined.
constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except
-fconstexpr-loop-limit=n
Set the maximum number of iterations for a loop in C++14 constexpr
functions to n. A limit is needed to detect infinite loops during
constant expression evaluation. The default is 262144 (1<<18).
-fconstexpr-ops-limit=n
Set the maximum number of operations during a single constexpr
evaluation. Even when number of iterations of a single loop is
limited with the above limit, if there are several nested loops and
each of them has many iterations but still smaller than the above
limit, or if in a body of some loop or even outside of a loop too
many expressions need to be evaluated, the resulting constexpr
evaluation might take too long. The default is 33554432 (1<<25).
-fcontracts
Enable experimental support for the C++ Contracts feature, as briefly
added to and then removed from the C++20 working paper (N4820). The
implementation also includes proposed enhancements from papers P1290,
P1332, and P1429. This functionality is intended mostly for those
interested in experimentation towards refining the feature to get it
into shape for a future C++ standard.
On violation of a checked contract, the violation handler is called.
Users can replace the violation handler by defining
void
handle_contract_violation (const std::experimental::contract_violation&);
There are different sets of additional flags that can be used
together to specify which contracts will be checked and how, for
N4820 contracts, P1332 contracts, or P1429 contracts; these sets
cannot be used together.
-fcontract-mode=[on|off]
Control whether any contracts have any semantics at all.
Defaults to on.
-fcontract-assumption-mode=[on|off]
[N4820] Control whether contracts with level axiom should have
the assume semantic. Defaults to on.
-fcontract-build-level=[off|default|audit]
[N4820] Specify which level of contracts to generate checks for.
Defaults to default.
-fcontract-continuation-mode=[on|off]
[N4820] Control whether to allow the program to continue
executing after a contract violation. That is, do checked
contracts have the maybe semantic described below rather than the
never semantic. Defaults to off.
-fcontract-role=<name>:<default>,<audit>,<axiom>
[P1332] Specify the concrete semantics for each contract level of
a particular contract role.
-fcontract-semantic=[default|audit|axiom]:<semantic>
[P1429] Specify the concrete semantic for a particular contract
level.
-fcontract-strict-declarations=[on|off]
Control whether to reject adding contracts to a function after
its first declaration. Defaults to off.
The possible concrete semantics for that can be specified with
-fcontract-role or -fcontract-semantic are:
"ignore"
This contract has no effect.
"assume"
This contract is treated like C++23 "[[assume]]".
"check_never_continue"
"never"
"abort"
This contract is checked. If it fails, the violation handler is
called. If the handler returns, "std::terminate" is called.
"check_maybe_continue"
"maybe"
This contract is checked. If it fails, the violation handler is
called. If the handler returns, execution continues normally.
-fcoroutines
Enable support for the C++ coroutines extension (experimental).
-fdiagnostics-all-candidates
Permit the C++ front end to note all candidates during overload
resolution failure, including when a deleted function is selected.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a
temporary that is only used to initialize another object of the same
type. Specifying this option disables that optimization, and forces
G++ to call the copy constructor in all cases. This option also
causes G++ to call trivial member functions which otherwise would be
expanded inline.
In C++17, the compiler is required to omit these temporaries, but
this option still affects trivial member functions.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception
specifications at run time. This option violates the C++ standard,
but may be useful for reducing code size in production builds, much
like defining "NDEBUG". This does not give user code permission to
throw exceptions in violation of the exception specifications; the
compiler still optimizes based on the specifications, so throwing an
unexpected exception results in undefined behavior at run time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow "thread_local" and
"threadprivate" variables to have dynamic (runtime) initialization.
To support this, any use of such a variable goes through a wrapper
function that performs any necessary initialization. When the use
and definition of the variable are in the same translation unit, this
overhead can be optimized away, but when the use is in a different
translation unit there is significant overhead even if the variable
doesn't actually need dynamic initialization. If the programmer can
be sure that no use of the variable in a non-defining TU needs to
trigger dynamic initialization (either because the variable is
statically initialized, or a use of the variable in the defining TU
will be executed before any uses in another TU), they can avoid this
overhead with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is
-fextern-tls-init. On targets that do not support symbol aliases,
the default is -fno-extern-tls-init.
-ffold-simple-inlines
-fno-fold-simple-inlines
Permit the C++ frontend to fold calls to "std::move", "std::forward",
"std::addressof" and "std::as_const". In contrast to inlining, this
means no debug information will be generated for such calls. Since
these functions are rarely interesting to debug, this flag is enabled
by default unless -fno-inline is active.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__" instead.
This option is implied by the strict ISO C++ dialects: -ansi,
-std=c++98, -std=c++11, etc.
-fno-immediate-escalation
Do not enable immediate function escalation whereby certain functions
can be promoted to consteval, as specified in P2564R3. For example:
consteval int id(int i) { return i; }
constexpr int f(auto t)
{
return t + id(t); // id causes f<int> to be promoted to consteval
}
void g(int i)
{
f (3);
}
compiles in C++20: "f" is an immediate-escalating function (due to
the "auto" it is a function template and is declared "constexpr") and
id(t) is an immediate-escalating expression, so "f" is promoted to
"consteval". Consequently, the call to id(t) is in an immediate
context, so doesn't have to produce a constant (that is the mechanism
allowing consteval function composition). However, with
-fno-immediate-escalation, "f" is not promoted to "consteval", and
since the call to consteval function id(t) is not a constant
expression, the compiler rejects the code.
This option is turned on by default; it is only effective in C++20
mode or later.
-fimplicit-constexpr
Make inline functions implicitly constexpr, if they satisfy the
requirements for a constexpr function. This option can be used in
C++14 mode or later. This can result in initialization changing from
dynamic to static and other optimizations.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated
implicitly (i.e. by use); only emit code for explicit instantiations.
If you use this option, you must take care to structure your code to
include all the necessary explicit instantiations to avoid getting
undefined symbols at link time.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that
compiles with and without optimization need the same set of explicit
instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
these functions are not inlined everywhere they are called.
-fmodules
-fno-modules
Enable support for C++20 modules. The -fno-modules is usually not
needed, as that is the default. Even though this is a C++20 feature,
it is not currently implicitly enabled by selecting that standard
version.
-fmodule-header
-fmodule-header=user
-fmodule-header=system
Compile a header file to create an importable header unit.
-fmodule-implicit-inline
Member functions defined in their class definitions are not
implicitly inline for modular code. This is different to traditional
C++ behavior, for good reasons. However, it may result in a
difficulty during code porting. This option makes such function
definitions implicitly inline. It does however generate an ABI
incompatibility, so you must use it everywhere or nowhere. (Such
definitions outside of a named module remain implicitly inline,
regardless.)
-fno-module-lazy
Disable lazy module importing and module mapper creation.
-fmodule-mapper=[hostname]:port[?ident]
-fmodule-mapper=|program[?ident] args...
-fmodule-mapper==socket[?ident]
-fmodule-mapper=<>[inout][?ident]
-fmodule-mapper=<in>out[?ident]
-fmodule-mapper=file[?ident]
An oracle to query for module name to filename mappings. If
unspecified the CXX_MODULE_MAPPER environment variable is used, and
if that is unset, an in-process default is provided.
-fmodule-only
Only emit the Compiled Module Interface, inhibiting any object file.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such as
implicit int and getting a pointer to member function via non-
standard syntax.
-fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C++11 constructor
inheritance. This is part of C++17 but also considered to be a
Defect Report against C++11 and C++14. This flag is enabled by
default unless -fabi-version=10 or lower is specified.
-fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template template
parameters and default arguments: this allows a template with default
template arguments as an argument for a template template parameter
with fewer template parameters. This flag is enabled by default for
-std=c++17.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include "ffs", "alloca", "_exit", "index",
"bzero", "conjf", and other related functions.
-fnothrow-opt
Treat a "throw()" exception specification as if it were a "noexcept"
specification to reduce or eliminate the text size overhead relative
to a function with no exception specification. If the function has
local variables of types with non-trivial destructors, the exception
specification actually makes the function smaller because the EH
cleanups for those variables can be optimized away. The semantic
effect is that an exception thrown out of a function with such an
exception specification results in a call to "terminate" rather than
"unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
"compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need
to issue. Currently, the only such diagnostic issued by G++ is the
one for a name having multiple meanings within a class.
-fno-pretty-templates
When an error message refers to a specialization of a function
template, the compiler normally prints the signature of the template
followed by the template arguments and any typedefs or typenames in
the signature (e.g. "void f(T) [with T = int]" rather than "void
f(int)") so that it's clear which template is involved. When an
error message refers to a specialization of a class template, the
compiler omits any template arguments that match the default template
arguments for that template. If either of these behaviors make it
harder to understand the error message rather than easier, you can
use -fno-pretty-templates to disable them.
-frange-for-ext-temps
Enable lifetime extension of C++ range based for temporaries. With
-std=c++23 and above this is part of the language standard, so
lifetime of the temporaries is extended until the end of the loop by
default. This option allows enabling that behavior also in earlier
versions of the standard.
-fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ run-time type identification features
("dynamic_cast" and "typeid"). If you don't use those parts of the
language, you can save some space by using this flag. Note that
exception handling uses the same information, but G++ generates it as
needed. The "dynamic_cast" operator can still be used for casts that
do not require run-time type information, i.e. casts to "void *" or
to unambiguous base classes.
Mixing code compiled with -frtti with that compiled with -fno-rtti
may not work. For example, programs may fail to link if a class
compiled with -fno-rtti is used as a base for a class compiled with
-frtti.
-fsized-deallocation
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined replacement
deallocation functions that, for example, use the size of the object
to make deallocation faster. Enabled by default under -std=c++14 and
above. The flag -Wsized-deallocation warns about places that might
want to add a definition.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of
enumerated type can only be one of the values of the enumeration (as
defined in the C++ standard; basically, a value that can be
represented in the minimum number of bits needed to represent all the
enumerators). This assumption may not be valid if the program uses a
cast to convert an arbitrary integer value to the enumerated type.
This option has no effect for an enumeration type with a fixed
underlying type.
-fstrong-eval-order
-fstrong-eval-order=kind
Evaluate member access, array subscripting, and shift expressions in
left-to-right order, and evaluate assignment in right-to-left order,
as adopted for C++17. -fstrong-eval-order is equivalent to
-fstrong-eval-order=all, and is enabled by default with -std=c++17 or
later.
-fstrong-eval-order=some enables just the ordering of member access
and shift expressions, and is the default for C++ dialects prior to
C++17.
-fstrong-eval-order=none is equivalent to -fno-strong-eval-order.
-ftemplate-backtrace-limit=n
Set the maximum number of template instantiation notes for a single
warning or error to n. The default value is 10.
-ftemplate-depth=n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect endless
recursions during template class instantiation. ANSI/ISO C++
conforming programs must not rely on a maximum depth greater than 17
(changed to 1024 in C++11). The default value is 900, as the
compiler can run out of stack space before hitting 1024 in some
situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++
ABI for thread-safe initialization of local statics. You can use
this option to reduce code size slightly in code that doesn't need to
be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with
the "__cxa_atexit" function rather than the "atexit" function. This
option is required for fully standards-compliant handling of static
destructors, but only works if your C library supports
"__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This causes
"std::uncaught_exception" to be incorrect, but is necessary if the
runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare
pointers to inline functions or methods where the addresses of the
two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods
with "__attribute__ ((visibility ("hidden")))" so that they do not
appear in the export table of a DSO and do not require a PLT
indirection when used within the DSO. Enabling this option can have
a dramatic effect on load and link times of a DSO as it massively
reduces the size of the dynamic export table when the library makes
heavy use of templates.
The behavior of this switch is not quite the same as marking the
methods as hidden directly, because it does not affect static
variables local to the function or cause the compiler to deduce that
the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the
effect of the switch for that method. For example, if you do want to
compare pointers to a particular inline method, you might mark it as
having default visibility. Marking the enclosing class with explicit
visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option
as their linkage might otherwise cross a shared library boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++
linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like
-fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit
visibility specifications that are defined in more than one
shared object: those declarations are permitted if they are
permitted when this option is not used.
In new code it is better to use -fvisibility=hidden and export those
classes that are intended to be externally visible. Unfortunately it
is possible for code to rely, perhaps accidentally, on the Visual
Studio behavior.
Among the consequences of these changes are that static data members
of the same type with the same name but defined in different shared
objects are different, so changing one does not change the other; and
that pointers to function members defined in different shared objects
may not compare equal. When this flag is given, it is a violation of
the ODR to define types with the same name differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker.
By default, G++ uses weak symbols if they are available. This option
exists only for testing, and should not be used by end-users; it
results in inferior code and has no benefits. This option may be
removed in a future release of G++.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal number
suffixes as GNU extensions. When this option is turned off these
suffixes are treated as C++11 user-defined literal numeric suffixes.
This is on by default for all pre-C++11 dialects and all GNU
dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This
option is off by default for ISO C++11 onwards (-std=c++11, ...).
-nostdinc++
Do not search for header files in the standard directories specific
to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)
-flang-info-include-translate
-flang-info-include-translate-not
-flang-info-include-translate=header
Inform of include translation events. The first will note accepted
include translations, the second will note declined include
translations. The header form will inform of include translations
relating to that specific header. If header is of the form "user" or
"<system>" it will be resolved to a specific user or system header
using the include path.
-flang-info-module-cmi
-flang-info-module-cmi=module
Inform of Compiled Module Interface pathnames. The first will note
all read CMI pathnames. The module form will not reading a specific
module's CMI. module may be a named module or a header-unit (the
latter indicated by either being a pathname containing directory
separators or enclosed in "<>" or "").
-stdlib=libstdc++,libc++
When G++ is configured to support this option, it allows
specification of alternate C++ runtime libraries. Two options are
available: libstdc++ (the default, native C++ runtime for G++) and
libc++ which is the C++ runtime installed on some operating systems
(e.g. Darwin versions from Darwin11 onwards). The option switches
G++ to use the headers from the specified library and to emit
"-lstdc++" or "-lc++" respectively, when a C++ runtime is required
for linking.
In addition, these warning options have meanings only for C++ programs:
-Wabi-tag (C++ and Objective-C++ only)
Warn when a type with an ABI tag is used in a context that does not
have that ABI tag. See C++ Attributes for more information about ABI
tags.
-Wcomma-subscript (C++ and Objective-C++ only)
Warn about uses of a comma expression within a subscripting
expression. This usage was deprecated in C++20 and is going to be
removed in C++23. However, a comma expression wrapped in "( )" is
not deprecated. Example:
void f(int *a, int b, int c) {
a[b,c]; // deprecated in C++20, invalid in C++23
a[(b,c)]; // OK
}
In C++23 it is valid to have comma separated expressions in a
subscript when an overloaded subscript operator is found and supports
the right number and types of arguments. G++ will accept the
formerly valid syntax for code that is not valid in C++23 but used to
be valid but deprecated in C++20 with a pedantic warning that can be
disabled with -Wno-comma-subscript.
Enabled by default with -std=c++20 unless -Wno-deprecated, and after
-std=c++23 regardless of -Wno-deprecated. Before -std=c++20, enabled
with explicit -Wdeprecated.
This warning is upgraded to an error by -pedantic-errors in C++23
mode or later.
-Wctad-maybe-unsupported (C++ and Objective-C++ only)
Warn when performing class template argument deduction (CTAD) on a
type with no explicitly written deduction guides. This warning will
point out cases where CTAD succeeded only because the compiler
synthesized the implicit deduction guides, which might not be what
the programmer intended. Certain style guides allow CTAD only on
types that specifically "opt-in"; i.e., on types that are designed to
support CTAD. This warning can be suppressed with the following
pattern:
struct allow_ctad_t; // any name works
template <typename T> struct S {
S(T) { }
};
// Guide with incomplete parameter type will never be considered.
S(allow_ctad_t) -> S<void>;
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither friends nor
public static member functions. Also warn if there are no non-
private methods, and there's at least one private member function
that isn't a constructor or destructor.
-Wdangling-reference (C++ and Objective-C++ only)
Warn when a reference is bound to a temporary whose lifetime has
ended. For example:
int n = 1;
const int& r = std::max(n - 1, n + 1); // r is dangling
In the example above, two temporaries are created, one for each
argument, and a reference to one of the temporaries is returned.
However, both temporaries are destroyed at the end of the full
expression, so the reference "r" is dangling. This warning also
detects dangling references in member initializer lists:
const int& f(const int& i) { return i; }
struct S {
const int &r; // r is dangling
S() : r(f(10)) { }
};
Member functions are checked as well, but only their object argument:
struct S {
const S& self () { return *this; }
};
const S& s = S().self(); // s is dangling
Certain functions are safe in this respect, for example
"std::use_facet": they take and return a reference, but they don't
return one of its arguments, which can fool the warning. Such
functions can be excluded from the warning by wrapping them in a
const T& foo (const T&) { ... }
will be disabled for all the member functions.
-Wdangling-reference also warns about code like
auto p = std::minmax(1, 2);
where "std::minmax" returns "std::pair<const int&, const int&>", and
both references dangle after the end of the full expression that
contains the call to "std::minmax".
The warning does not warn for "std::span"-like classes. We consider
classes of the form:
template<typename T>
struct Span {
T* data_;
std::size len_;
};
as "std::span"-like; that is, the class is a non-union class that has
a pointer data member and a trivial destructor.
The warning can be disabled by using the "gnu::no_dangling"
attribute.
This warning is enabled by -Wextra.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when "delete" is used to destroy an instance of a class that has
virtual functions and non-virtual destructor. It is unsafe to delete
an instance of a derived class through a pointer to a base class if
the base class does not have a virtual destructor. This warning is
enabled by -Wall.
-Wdeprecated-copy (C++ and Objective-C++ only)
Warn that the implicit declaration of a copy constructor or copy
assignment operator is deprecated if the class has a user-provided
copy constructor or copy assignment operator, in C++11 and up. This
warning is enabled by -Wextra. With -Wdeprecated-copy-dtor, also
deprecate if the class has a user-provided destructor.
-Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only)
Disable the warning about the case when the usual arithmetic
conversions are applied on operands where one is of enumeration type
and the other is of a different enumeration type. This conversion
was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
int k = f - e;
-Wdeprecated-enum-enum-conversion is enabled by default with
-std=c++20. In pre-C++20 dialects, this warning can be enabled by
-Wenum-conversion or -Wdeprecated.
-Wno-deprecated-enum-float-conversion (C++ and Objective-C++ only)
Disable the warning about the case when the usual arithmetic
conversions are applied on operands where one is of enumeration type
and the other is of a floating-point type. This conversion was
deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;
-Wdeprecated-enum-float-conversion is enabled by default with
-std=c++20. In pre-C++20 dialects, this warning can be enabled by
-Wenum-conversion or -Wdeprecated.
-Wdeprecated-literal-operator (C++ and Objective-C++ only)
Warn that the declaration of a user-defined literal operator with a
space before the suffix is deprecated. This warning is enabled by
default in C++23, or with explicit -Wdeprecated.
string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred
-Wdeprecated-variadic-comma-omission (C++ and Objective-C++ only)
Warn that omitting a comma before the varargs "..." at the end of a
function parameter list is deprecated. This warning is enabled by
default in C++26, or with explicit -Wdeprecated.
void f1(int...); // deprecated
void f1(int, ...); // preferred
template <typename ...T>
void f2(T...); // ok
template <typename ...T>
void f3(T......); // deprecated
-Wno-elaborated-enum-base
For C++11 and above, warn if an (invalid) additional enum-base is
used in an elaborated-type-specifier. That is, if an enum with given
underlying type and no enumerator list is used in a declaration other
than just a standalone declaration of the enum. Enabled by default.
This warning is upgraded to an error with -pedantic-errors.
-Wno-init-list-lifetime (C++ and Objective-C++ only)
Do not warn about uses of "std::initializer_list" that are likely to
result in dangling pointers. Since the underlying array for an
"initializer_list" is handled like a normal C++ temporary object, it
is easy to inadvertently keep a pointer to the array past the end of
the array's lifetime. For example:
* If a function returns a temporary "initializer_list", or a local
"initializer_list" variable, the array's lifetime ends at the end
of the return statement, so the value returned has a dangling
pointer.
* If a new-expression creates an "initializer_list", the array only
lives until the end of the enclosing full-expression, so the
"initializer_list" in the heap has a dangling pointer.
* When an "initializer_list" variable is assigned from a brace-
enclosed initializer list, the temporary array created for the
right side of the assignment only lives until the end of the
full-expression, so at the next statement the "initializer_list"
variable has a dangling pointer.
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior
* When a list constructor stores the "begin" pointer from the
"initializer_list" argument, this doesn't extend the lifetime of
the array, so if a class variable is constructed from a temporary
"initializer_list", the pointer is left dangling by the end of
the variable declaration statement.
-Winvalid-constexpr
Warn when a function never produces a constant expression. In C++20
and earlier, for every "constexpr" function and function template,
there must be at least one set of function arguments in at least one
instantiation such that an invocation of the function or constructor
could be an evaluated subexpression of a core constant expression.
C++23 removed this restriction, so it's possible to have a function
or a function template marked "constexpr" for which no invocation
satisfies the requirements of a core constant expression.
This warning is enabled as a pedantic warning by default in C++20 and
earlier. In C++23, -Winvalid-constexpr can be turned on, in which
case it will be an ordinary warning. For example:
void f (int& i);
constexpr void
g (int& i)
{
// Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
f(i);
}
-Winvalid-imported-macros
Verify all imported macro definitions are valid at the end of
compilation. This is not enabled by default, as it requires
additional processing to determine. It may be useful when preparing
sets of header-units to ensure consistent macros.
-Wno-literal-suffix (C++ and Objective-C++ only)
Do not warn when a string or character literal is followed by a ud-
suffix which does not begin with an underscore. As a conforming
extension, GCC treats such suffixes as separate preprocessing tokens
in order to maintain backwards compatibility with code that uses
formatting macros from "<inttypes.h>". For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing token.
This option also controls warnings when a user-defined literal
operator is declared with a literal suffix identifier that doesn't
begin with an underscore. Literal suffix identifiers that don't begin
with an underscore are reserved for future standardization.
These warnings are enabled by default.
-Wno-narrowing (C++ and Objective-C++ only)
For C++11 and later standards, narrowing conversions are diagnosed by
default, as required by the standard. A narrowing conversion from a
constant produces an error, and a narrowing conversion from a non-
constant produces a warning, but -Wno-narrowing suppresses the
diagnostic. Note that this does not affect the meaning of well-
formed code; narrowing conversions are still considered ill-formed in
SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion
prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of a call
to a function that does not have a non-throwing exception
specification (i.e. "throw()" or "noexcept") but is known by the
compiler to never throw an exception.
-Wnoexcept-type (C++ and Objective-C++ only)
Warn if the C++17 feature making "noexcept" part of a function type
changes the mangled name of a symbol relative to C++14. Enabled by
-Wabi and -Wc++17-compat.
As an example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }
In C++14, "f" calls "f<void(*)()>", but in C++17 it calls
"f<void(*)()noexcept>".
-Wclass-memaccess (C++ and Objective-C++ only)
Warn when the destination of a call to a raw memory function such as
"memset" or "memcpy" is an object of class type, and when writing
into such an object might bypass the class non-trivial or deleted
constructor or copy assignment, violate const-correctness or
encapsulation, or corrupt virtual table pointers. Modifying the
representation of such objects may violate invariants maintained by
member functions of the class. For example, the call to "memset"
below is undefined because it modifies a non-trivial class object and
is, therefore, diagnosed. The safe way to either initialize or clear
the storage of objects of such types is by using the appropriate
constructor or assignment operator, if one is available.
std::string str = "abc";
memset (&str, 0, sizeof str);
The -Wclass-memaccess option is enabled by -Wall. Explicitly casting
the pointer to the class object to "void *" or to a type that can be
safely accessed by the raw memory function suppresses the warning.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible non-virtual
destructor itself or in an accessible polymorphic base class, in
which case it is possible but unsafe to delete an instance of a
derived class through a pointer to the class itself or base class.
This warning is automatically enabled if -Weffc++ is specified. The
-Wdelete-non-virtual-dtor option (enabled by -Wall) should be
preferred because it warns about the unsafe cases without false
positives.
-Wregister (C++ and Objective-C++ only)
Warn on uses of the "register" storage class specifier, except when
it is part of the GNU Explicit Register Variables extension. The use
of the "register" keyword as storage class specifier has been
deprecated in C++11 and removed in C++17. Enabled by default with
-std=c++17.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code does not
match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i" and "j" to
match the declaration order of the members, emitting a warning to
that effect. This warning is enabled by -Wall.
-Wno-pessimizing-move (C++ and Objective-C++ only)
This warning warns when a call to "std::move" prevents copy elision.
A typical scenario when copy elision can occur is when returning in a
function with a class return type, when the expression being returned
is the name of a non-volatile automatic object, and is not a function
parameter, and has the same type as the function return type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}
But in this example, the "std::move" call prevents copy elision.
This warning is enabled by -Wall.
-Wno-redundant-move (C++ and Objective-C++ only)
This warning warns about redundant calls to "std::move"; that is,
when a move operation would have been performed even without the
"std::move" call. This happens because the compiler is forced to
treat the object as if it were an rvalue in certain situations such
as returning a local variable, where copy elision isn't applicable.
Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}
Here, the "std::move" call is redundant. Because G++ implements Core
Issue 1579, another example is:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}
In this example, copy elision isn't applicable because the type of
the expression being returned and the function return type differ,
yet G++ treats the return value as if it were designated by an
rvalue.
This warning is enabled by -Wextra.
-Wrange-loop-construct (C++ and Objective-C++ only)
This warning warns when a C++ range-based for-loop is creating an
unnecessary copy. This can happen when the range declaration is not
a reference, but probably should be. For example:
struct S { char arr[128]; };
void fn () {
S arr[5];
for (const auto x : arr) { ... }
}
It does not warn when the type being copied is a trivially-copyable
type whose size is less than 64 bytes.
This warning also warns when a loop variable in a range-based for-
loop is initialized with a value of a different type resulting in a
copy. For example:
void fn() {
int arr[10];
for (const double &x : arr) { ... }
}
In the example above, in every iteration of the loop a temporary
value of type "double" is created and destroyed, to which the
reference "const double &" is bound.
This warning is enabled by -Wall.
-Wredundant-tags (C++ and Objective-C++ only)
Warn about redundant class-key and enum-key in references to class
types and enumerated types in contexts where the key can be
eliminated without causing an ambiguity. For example:
struct foo;
struct foo *p; // warn that keyword struct can be eliminated
On the other hand, in this example there is no warning:
struct foo;
void foo (); // "hides" struct foo
void bar (struct foo&); // no warning, keyword struct is necessary
-Wno-subobject-linkage (C++ and Objective-C++ only)
Do not warn if a class type has a base or a field whose type uses the
anonymous namespace or depends on a type with no linkage. If a type
A depends on a type B with no or internal linkage, defining it in
multiple translation units would be an ODR violation because the
meaning of B is different in each translation unit. If A only
appears in a single translation unit, the best way to silence the
warning is to give it internal linkage by putting it in an anonymous
namespace as well. The compiler doesn't give this warning for types
defined in the main .C file, as those are unlikely to have multiple
definitions. -Wsubobject-linkage is enabled by default.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott
Meyers' Effective C++ series of books:
* Define a copy constructor and an assignment operator for classes
with dynamically-allocated memory.
* Prefer initialization to assignment in constructors.
* Have "operator=" return a reference to *this.
* Don't try to return a reference when you must return an object.
* Distinguish between prefix and postfix forms of increment and
decrement operators.
* Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also one of the
effective C++ recommendations. However, the check is extended to
warn about the lack of virtual destructor in accessible non-
polymorphic bases classes too.
When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use grep -v to filter
out those warnings.
-Wno-exceptions (C++ and Objective-C++ only)
Disable the warning about the case when an exception handler is
shadowed by another handler, which can point out a wrong ordering of
exception handlers.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn about the use of an uncasted "NULL" as sentinel. When compiling
only with GCC this is a valid sentinel, as "NULL" is defined to
"__null". Although it is a null pointer constant rather than a null
pointer, it is guaranteed to be of the same size as a pointer. But
this use is not portable across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-template friend functions are declared
within a template. In very old versions of GCC that predate
implementation of the ISO standard, declarations such as friend int
foo(int), where the name of the friend is an unqualified-id, could be
interpreted as a particular specialization of a template function;
the warning exists to diagnose compatibility problems, and is enabled
by default.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within
a C++ program. The new-style casts ("dynamic_cast", "static_cast",
"reinterpret_cast", and "const_cast") are less vulnerable to
unintended effects and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
-Woverloaded-virtual=n
Warn when a function declaration hides virtual functions from a base
class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int); // does not override
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
fails to compile.
In cases where the different signatures are not an accident, the
simplest solution is to add a using-declaration to the derived class
to un-hide the base function, e.g. add "using A::f;" to "B".
The optional level suffix controls the behavior when all the
declarations in the derived class override virtual functions in the
base class, even if not all of the base functions are overridden:
struct C {
virtual void f();
virtual void f(int);
};
struct D: public C {
void f(int); // does override
}
This pattern is less likely to be a mistake; if D is only used
virtually, the user might have decided that the base class semantics
for some of the overloads are fine.
At level 1, this case does not warn; at level 2, it does.
-Woverloaded-virtual by itself selects level 2. Level 1 is included
in -Wall.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned or
enumerated type to a signed type, over a conversion to an unsigned
type of the same size. Previous versions of G++ tried to preserve
unsignedness, but the standard mandates the current behavior.
-Wtemplates (C++ and Objective-C++ only)
Warn when a primary template declaration is encountered. Some coding
rules disallow templates, and this may be used to enforce that rule.
The warning is inactive inside a system header file, such as the STL,
so one can still use the STL. One may also instantiate or specialize
templates.
-Wmismatched-new-delete (C++ and Objective-C++ only)
Warn for mismatches between calls to "operator new" or "operator
delete" and the corresponding call to the allocation or deallocation
function. This includes invocations of C++ "operator delete" with
pointers returned from either mismatched forms of "operator new", or
from other functions that allocate objects for which the "operator
delete" isn't a suitable deallocator, as well as calls to other
deallocation functions with pointers returned from "operator new" for
which the deallocation function isn't suitable.
For example, the "delete" expression in the function below is
diagnosed because it doesn't match the array form of the "new"
expression the pointer argument was returned from. Similarly, the
call to "free" is also diagnosed.
void f ()
{
int *a = new int[n];
delete a; // warning: mismatch in array forms of expressions
char *p = new char[n];
free (p); // warning: mismatch between new and free
}
The related option -Wmismatched-dealloc diagnoses mismatches
involving allocation and deallocation functions other than "operator
new" and "operator delete".
-Wmismatched-new-delete is included in -Wall.
-Wmismatched-tags (C++ and Objective-C++ only)
Warn for declarations of structs, classes, and class templates and
their specializations with a class-key that does not match either the
definition or the first declaration if no definition is provided.
For example, the declaration of "struct Object" in the argument list
of "draw" triggers the warning. To avoid it, either remove the
redundant class-key "struct" or replace it with "class" to match its
definition.
class Object {
public:
virtual ~Object () = 0;
};
void draw (struct Object*);
It is not wrong to declare a class with the class-key "struct" as the
example above shows. The -Wmismatched-tags option is intended to
help achieve a consistent style of class declarations. In code that
is intended to be portable to Windows-based compilers the warning
helps prevent unresolved references due to the difference in the
mangling of symbols declared with different class-keys. The option
can be used either on its own or in conjunction with
-Wredundant-tags.
-Wmultiple-inheritance (C++ and Objective-C++ only)
Warn when a class is defined with multiple direct base classes. Some
coding rules disallow multiple inheritance, and this may be used to
enforce that rule. The warning is inactive inside a system header
file, such as the STL, so one can still use the STL. One may also
define classes that indirectly use multiple inheritance.
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base class. Some
coding rules disallow multiple inheritance, and this may be used to
enforce that rule. The warning is inactive inside a system header
file, such as the STL, so one can still use the STL. One may also
define classes that indirectly use virtual inheritance.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a non-
trivial C++11 move assignment operator. This is dangerous because if
the virtual base is reachable along more than one path, it is moved
multiple times, which can mean both objects end up in the moved-from
state. If the move assignment operator is written to avoid moving
from a moved-from object, this warning can be disabled.
-Wnamespaces
Warn when a namespace definition is opened. Some coding rules
disallow namespaces, and this may be used to enforce that rule. The
warning is inactive inside a system header file, such as the STL, so
one can still use the STL. One may also use using directives and
qualified names.
-Wno-template-body (C++ and Objective-C++ only)
Disable diagnosing errors when parsing a template, and instead issue
an error only upon instantiation of the template. This flag can also
be used to downgrade such errors into warnings with Wno-error= or
-fpermissive.
-Wno-template-id-cdtor (C++ and Objective-C++ only)
Disable the warning about the use of simple-template-id as the
declarator-id of a constructor or destructor, which became invalid in
C++20 via DR 2237. For example:
template<typename T> struct S {
S<T>(); // should be S();
~S<T>(); // should be ~S();
};
-Wtemplate-id-cdtor is enabled by default with -std=c++20; it is also
enabled by -Wc++20-compat.
-Wtemplate-names-tu-local
Warn when a template body hides an exposure of a translation-unit-
local entity. In most cases, referring to a translation-unit-local
entity (such as an internal linkage declaration) within an entity
that is emitted into a module's CMI is an error. However, within the
initializer of a variable, or in the body of a non-inline function,
this is not an exposure and no error is emitted.
This can cause variable or function templates to accidentally become
unusable if they reference such an entity, because other translation
units that import the template will never be able to instantiate it.
This warning attempts to detect cases where this might occur. The
presence of an explicit instantiation silences the warning.
This flag is enabled by -Wextra.
-Wno-terminate (C++ and Objective-C++ only)
Disable the warning about a throw-expression that will immediately
result in a call to "terminate".
-Wno-vexing-parse (C++ and Objective-C++ only)
Warn about the most vexing parse syntactic ambiguity. This warns
about the cases when a declaration looks like a variable definition,
but the C++ language requires it to be interpreted as a function
declaration. For instance:
void f(double a) {
int i(); // extern int i (void);
int n(int(a)); // extern int n (int);
}
Another example:
struct S { S(int); };
void f(double a) {
S x(int(a)); // extern struct S x (int);
S y(int()); // extern struct S y (int (*) (void));
S z(); // extern struct S z (void);
}
The warning will suggest options how to deal with such an ambiguity;
e.g., it can suggest removing the parentheses or using braces
instead.
This warning is enabled by default.
-Wno-class-conversion (C++ and Objective-C++ only)
Do not warn when a conversion function converts an object to the same
type, to a base class of that type, or to void; such a conversion
function will never be called.
-Wvolatile (C++ and Objective-C++ only)
Warn about deprecated uses of the "volatile" qualifier. This
includes postfix and prefix "++" and "--" expressions of
"volatile"-qualified types, using simple assignments where the left
operand is a "volatile"-qualified non-class type for their value,
compound assignments where the left operand is a "volatile"-qualified
non-class type, "volatile"-qualified function return type,
"volatile"-qualified parameter type, and structured bindings of a
"volatile"-qualified type. This usage was deprecated in C++20.
Enabled by default with -std=c++20. Before -std=c++20, enabled with
explicit -Wdeprecated.
-Waligned-new
Warn about a new-expression of a type that requires greater alignment
than the "alignof(std::max_align_t)" but uses an allocation function
without an explicit alignment parameter. This option is enabled by
-Wall.
Normally this only warns about global allocation functions, but
-Waligned-new=all also warns about class member allocation functions.
-Wno-placement-new
-Wplacement-new=n
Warn about placement new expressions with undefined behavior, such as
constructing an object in a buffer that is smaller than the type of
the object. For example, the placement new expression below is
diagnosed because it attempts to construct an array of 64 integers in
a buffer only 64 bytes large.
char buf [64];
new (buf) int[64];
This warning is enabled by default.
-Wplacement-new=1
This is the default warning level of -Wplacement-new. At this
level the warning is not issued for some strictly undefined
constructs that GCC allows as extensions for compatibility with
legacy code. For example, the following "new" expression is not
diagnosed at this level even though it has undefined behavior
according to the C++ standard because it writes past the end of
the one-element array.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();
-Wplacement-new=2
At this level, in addition to diagnosing all the same constructs
as at level 1, a diagnostic is also issued for placement new
expressions that construct an object in the last member of
structure whose type is an array of a single element and whose
size is less than the size of the object being constructed.
While the previous example would be diagnosed, the following
construct makes use of the flexible member array extension to
avoid the warning at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();
-Wcatch-value
-Wcatch-value=n (C++ and Objective-C++ only)
Warn about catch handlers that do not catch via reference. With
-Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic
class types that are caught by value. With -Wcatch-value=2 warn
about all class types that are caught by value. With -Wcatch-value=3
warn about all types that are not caught by reference. -Wcatch-value
is enabled by -Wall.
-Wconditionally-supported (C++ and Objective-C++ only)
Warn for conditionally-supported (C++11 [intro.defs]) constructs.
-Wno-defaulted-function-deleted (C++ and Objective-C++ only)
Warn when an explicitly defaulted function is deleted by the
compiler. That can occur when the function's declared type does not
match the type of the function that would have been implicitly
declared. This warning is enabled by default.
-Wno-delete-incomplete (C++ and Objective-C++ only)
Do not warn when deleting a pointer to incomplete type, which may
cause undefined behavior at runtime. This warning is enabled by
default.
-Wextra-semi (C++, Objective-C++ only)
Warn about redundant semicolons. There are various contexts in which
an extra semicolon can occur. One is a semicolon after in-class
function definitions, which is valid in all C++ dialects (and is
never a pedwarn):
struct S {
void foo () {};
};
Another is an extra semicolon at namespace scope, which has been
allowed since C++11 (therefore is a pedwarn in C++98):
struct S {
};
;
And yet another is an extra semicolon in class definitions, which has
been allowed since C++11 (therefore is a pedwarn in C++98):
struct S {
int a;
;
};
-Wno-global-module (C++ and Objective-C++ only)
Disable the diagnostic for when the global module fragment of a
module unit does not consist only of preprocessor directives.
-Wno-inaccessible-base (C++, Objective-C++ only)
This option controls warnings when a base class is inaccessible in a
class derived from it due to ambiguity. The warning is enabled by
default. Note that the warning for ambiguous virtual bases is
enabled by the -Wextra option.
struct A { int a; };
struct B : A { };
struct C : B, A { };
-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors when the
base class inherited from has a C variadic constructor; the warning
is on by default because the ellipsis is not inherited.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the "offsetof" macro to a non-POD
type. According to the 2014 ISO C++ standard, applying "offsetof" to
a non-standard-layout type is undefined. In existing C++
implementations, however, "offsetof" typically gives meaningful
results. This flag is for users who are aware that they are writing
nonportable code and who have deliberately chosen to ignore the
warning about it.
The restrictions on "offsetof" may be relaxed in a future version of
the C++ standard.
-Wsized-deallocation (C++ and Objective-C++ only)
Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with -fsized-deallocation.
-Wsuggest-final-types
Warn about types with virtual methods where code quality would be
improved if the type were declared with the C++11 "final" specifier,
or, if possible, declared in an anonymous namespace. This allows GCC
to more aggressively devirtualize the polymorphic calls. This warning
is more effective with link-time optimization, where the information
about the class hierarchy graph is more complete.
-Wsuggest-final-methods
Warn about virtual methods where code quality would be improved if
the method were declared with the C++11 "final" specifier, or, if
possible, its type were declared in an anonymous namespace or with
the "final" specifier. This warning is more effective with link-time
optimization, where the information about the class hierarchy graph
is more complete. It is recommended to first consider suggestions of
-Wsuggest-final-types and then rebuild with new annotations.
-Wsuggest-override
Warn about overriding virtual functions that are not marked with the
"override" keyword.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types.
-Wconversion-null is enabled by default.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++
languages themselves.
This section describes the command-line options that are only meaningful
for Objective-C and Objective-C++ programs. You can also use most of the
language-independent GNU compiler options. For example, you might
compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C
and Objective-C++ programs; you can use the other options with any
language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-
C compilations may also use options specific to the C front-end (e.g.,
-Wtraditional). Similarly, Objective-C++ compilations may use
C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and
Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each
literal string specified with the syntax "@"..."". The default class
name is "NXConstantString" if the GNU runtime is being used, and
"NSConstantString" if the NeXT runtime is being used (see below). On
Darwin / macOS platforms, the -fconstant-cfstrings option, if also
present, overrides the -fconstant-string-class setting and cause
"@"..."" literals to be laid out as constant CoreFoundation strings.
Note that -fconstant-cfstrings is an alias for the target-specific
-mconstant-cfstrings equivalent.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the
default for NeXT-based systems, including Darwin / macOS. The macro
"__NEXT_RUNTIME__" is predefined if (and only if) this option is
used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver
message:arg]") in this translation unit ensure that the receiver is
not "nil". This allows for more efficient entry points in the
runtime to be used. This option is only available in conjunction
with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime. This
option is currently supported only for the NeXT runtime. In that
case, Version 0 is the traditional (32-bit) ABI without support for
properties and other Objective-C 2.0 additions. Version 1 is the
traditional (32-bit) ABI with support for properties and other
Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If
nothing is specified, the default is Version 0 on 32-bit target
machines, and Version 2 on 64-bit target machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is
a C++ object with a non-trivial default constructor. If so,
synthesize a special "- (id) .cxx_construct" instance method which
runs non-trivial default constructors on any such instance variables,
in order, and then return "self". Similarly, check if any instance
variable is a C++ object with a non-trivial destructor, and if so,
synthesize a special "- (void) .cxx_destruct" method which runs all
such default destructors, in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods
thusly generated only operate on instance variables declared in the
current Objective-C class, and not those inherited from superclasses.
It is the responsibility of the Objective-C runtime to invoke all
such methods in an object's inheritance hierarchy. The "- (id)
.cxx_construct" methods are invoked by the runtime immediately after
a new object instance is allocated; the "- (void) .cxx_destruct"
methods are invoked immediately before the runtime deallocates an
object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later
has support for invoking the "- (id) .cxx_construct" and "- (void)
.cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++. This option is
required to use the Objective-C keywords @try, @throw, @catch,
@finally and @synchronized. This option is available with both the
GNU runtime and the NeXT runtime (but not available in conjunction
with the NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++
programs. This option is only available with the NeXT runtime; the
GNU runtime has a different garbage collection implementation that
does not require special compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil
receiver in method invocations before doing the actual method call.
This is the default and can be disabled using -fno-objc-nilcheck.
Class methods and super calls are never checked for nil in this way
no matter what this flag is set to. Currently this flag does nothing
when the GNU runtime, or an older version of the NeXT runtime ABI, is
used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the language
recognized by GCC 4.0. This only affects the Objective-C additions
to the C/C++ language; it does not affect conformance to C/C++
standards, which is controlled by the separate C/C++ dialect option
flags. When this option is used with the Objective-C or
Objective-C++ compiler, any Objective-C syntax that is not recognized
by GCC 4.0 is rejected. This is useful if you need to make sure that
your Objective-C code can be compiled with older versions of GCC.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the
resulting object file, and allow dyld(1) to load it in at run time
instead. This is used in conjunction with the Fix-and-Continue
debugging mode, where the object file in question may be recompiled
and dynamically reloaded in the course of program execution, without
the need to restart the program itself. Currently, Fix-and-Continue
functionality is only available in conjunction with the NeXT runtime
on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces
calls to "objc_getClass("...")" (when the name of the class is known
at compile time) with static class references that get initialized at
load time, which improves run-time performance. Specifying the
-fzero-link flag suppresses this behavior and causes calls to
"objc_getClass("...")" to be retained. This is useful in Zero-Link
debugging mode, since it allows for individual class implementations
to be modified during program execution. The GNU runtime currently
always retains calls to "objc_get_class("...")" regardless of
command-line options.
-fno-local-ivars
By default instance variables in Objective-C can be accessed as if
they were local variables from within the methods of the class
they're declared in. This can lead to shadowing between instance
variables and other variables declared either locally inside a class
method or globally with the same name. Specifying the
-fno-local-ivars flag disables this behavior thus avoiding variable
shadowing issues.
-fivar-visibility=[public|protected|private|package]
Set the default instance variable visibility to the specified option
so that instance variables declared outside the scope of any access
modifier directives default to the specified visibility.
-gen-decls
Dump interface declarations for all classes seen in the source file
to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.
-Wno-property-assign-default (Objective-C and Objective-C++ only)
Do not warn if a property for an Objective-C object has no assign
semantics specified.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is issued
for every method in the protocol that is not implemented by the
class. The default behavior is to issue a warning for every method
not explicitly implemented in the class, even if a method
implementation is inherited from the superclass. If you use the
-Wno-protocol option, then methods inherited from the superclass are
considered to be implemented, and no warning is issued for them.
-Wobjc-root-class (Objective-C and Objective-C++ only)
Warn if a class interface lacks a superclass. Most classes will
inherit from "NSObject" (or "Object") for example. When declaring
classes intended to be root classes, the warning can be suppressed by
marking their interfaces with "__attribute__((objc_root_class))".
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same selector are
found during compilation. The check is performed on the list of
methods in the final stage of compilation. Additionally, a check is
performed for each selector appearing in a "@selector(...)"
expression, and a corresponding method for that selector has been
found during compilation. Because these checks scan the method table
only at the end of compilation, these warnings are not produced if
the final stage of compilation is not reached, for example because an
error is found during compilation, or because the -fsyntax-only
option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or return types
are found for a given selector when attempting to send a message
using this selector to a receiver of type "id" or "Class". When this
flag is off (which is the default behavior), the compiler omits such
warnings if any differences found are confined to types that share
the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an undeclared
selector is found. A selector is considered undeclared if no method
with that name has been declared before the "@selector(...)"
expression, either explicitly in an @interface or @protocol
declaration, or implicitly in an @implementation section. This
option always performs its checks as soon as a "@selector(...)"
expression is found, while -Wselector only performs its checks in the
final stage of compilation. This also enforces the coding style
convention that methods and selectors must be declared before being
used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by
value, if any.
Options Controlling OpenMP and OpenACC
GCC supports OpenMP extensions to the C, C++, and Fortran languages with
the -fopenmp option. Similarly, OpenACC extensions are supported in all
three languages with -fopenacc.
-foffload=disable
-foffload=default
-foffload=target-list
Specify for which OpenMP and OpenACC offload targets code should be
generated. The default behavior, equivalent to -foffload=default, is
to generate code for all supported offload targets. The
-foffload=disable form generates code only for the host fallback,
while -foffload=target-list generates code only for the specified
comma-separated list of offload targets.
Offload targets are specified in GCC's internal target-triplet
format. You can run the compiler with -v to show the list of
configured offload targets under "OFFLOAD_TARGET_NAMES".
-foffload-options=options
-foffload-options=target-triplet-list=options
With -foffload-options=options, GCC passes the specified options to
the compilers for all enabled offloading targets. You can specify
options that apply only to a specific target or targets by using the
-foffload-options=target-list=options form. The target-list is a
comma-separated list in the same format as for the -foffload= option.
Typical command lines are
-foffload-options='-fno-math-errno -ffinite-math-only' \
-foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906
-fopenacc
in free-form Fortran and !$acc, c$acc and *$acc in fixed-form
Fortran. This option implies -pthread, and thus is only supported on
targets that have support for -pthread.
-fopenacc-dim=geom
Specify default compute dimensions for parallel offload regions that
do not explicitly specify them. The geom value is a triple of
:-separated sizes, in order gang, worker, and vector. A size can be
omitted, to use a target-specific default value.
-fopenmp
[[omp::directive(...)]], [[omp::decl(...)]], and
[[omp::sequence(...)]] in C/C++. In Fortran, it enables !$omp and
the conditional compilation sentinel !$. In fixed source form
Fortran, the sentinels can also start with c or *.
This option implies -pthread, and thus is only supported on targets
that have support for -pthread. -fopenmp implies -fopenmp-simd.
-fopenmp-simd
Enable handling of OpenMP's "simd", "declare simd", "declare
reduction", "assume", "ordered", "scan" and "loop" directive, and of
combined or composite directives with "simd" as constituent with
and "[[omp::decl(...)]]" in C/C++ and "!$omp" in Fortran. It
additionally enables the conditional compilation sentinel !$ in
Fortran. In fixed source form Fortran, the sentinels can also start
with c or *. Other OpenMP directives are ignored. Unless -fopenmp
is additionally specified, the "loop" region binds to the current
task region, independent of the specified "bind" clause.
-fopenmp-target-simd-clone
-fopenmp-target-simd-clone=device-type
In addition to generating SIMD clones for functions marked with the
"declare simd" directive, GCC also generates clones for functions
marked with the OpenMP "declare target" directive that are suitable
for vectorization when this option is in effect. The device-type may
be one of "none", "host", "nohost", and "any", which correspond to
keywords for the "device_type" clause of the "declare target"
directive; clones are generated for the intersection of devices
specified. -fopenmp-target-simd-clone is equivalent to
-fopenmp-target-simd-clone=any and -fno-openmp-target-simd-clone is
equivalent to -fopenmp-target-simd-clone=none.
At -O2 and higher (but not -Os or -Og) this optimization defaults to
-fopenmp-target-simd-clone=nohost; otherwise it is disabled by
default.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g. its width, ...). You can use the
options described below to control the formatting algorithm for
diagnostic messages, e.g. how many characters per line, how often source
location information should be reported. Note that some language front
ends may not honor these options.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. If n is zero, then no line-wrapping is done; each error
message appears on a single line. This is the default for all front
ends.
Note - this option also affects the display of the #error and
#warning pre-processor directives, and the deprecated
function/type/variable attribute. It does not however affect the
pragma GCC warning and pragma GCC error pragmas.
-fdiagnostics-plain-output
This option requests that diagnostic output look as plain as
possible, which may be useful when running dejagnu or other utilities
that need to parse diagnostics output and prefer that it remain more
stable over time. -fdiagnostics-plain-output is currently equivalent
to the following options: -fno-diagnostics-show-caret
-fno-diagnostics-show-line-numbers -fdiagnostics-color=never
-fdiagnostics-urls=never -fdiagnostics-path-format=separate-events
-fdiagnostics-text-art-charset=none -fno-diagnostics-show-event-links
In the future, if GCC changes the default appearance of its
diagnostics, the corresponding option to disable the new behavior
will be added to this list.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit source location information once; that is,
in case the message is too long to fit on a single physical line and
has to be wrapped, the source location won't be emitted (as prefix)
again, over and over, in subsequent continuation lines. This is the
default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking a
message which is too long to fit on a single line.
-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto. The
default depends on how the compiler has been configured, it can be
any of the above WHEN options or also never if GCC_COLORS environment
variable isn't present in the environment, and auto otherwise. auto
makes GCC use color only when the standard error is a terminal, and
when not executing in an emacs shell. The forms -fdiagnostics-color
and -fno-diagnostics-color are aliases for -fdiagnostics-color=always
and -fdiagnostics-color=never, respectively.
The colors are defined by the environment variable GCC_COLORS. Its
value is a colon-separated list of capabilities and Select Graphic
Rendition (SGR) substrings. SGR commands are interpreted by the
terminal or terminal emulator. (See the section in the documentation
of your text terminal for permitted values and their meanings as
character attributes.) These substring values are integers in
decimal representation and can be concatenated with semicolons.
Common values to concatenate include 1 for bold, 4 for underline, 5
for blink, 7 for inverse, 39 for default foreground color, 30 to 37
for foreground colors, 90 to 97 for 16-color mode foreground colors,
38;5;0 to 38;5;255 for 88-color and 256-color modes foreground
colors, 49 for default background color, 40 to 47 for background
colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to
48;5;255 for 88-color and 256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32:fnname=01;32:targs=35:valid=01;31:invalid=01;32\
highlight-a=01;32:highlight-b=01;34
where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan,
32 is green, 34 is blue, 01 is bold, and 31 is red. Setting
GCC_COLORS to the empty string disables colors. Supported
capabilities are as follows.
"error="
SGR substring for error: markers.
"warning="
SGR substring for warning: markers.
"note="
SGR substring for note: markers.
"path="
SGR substring for colorizing paths of control-flow events as
printed via -fdiagnostics-path-format=, such as the identifiers
of individual events and lines indicating interprocedural calls
and returns.
"range1="
SGR substring for first additional range.
"range2="
SGR substring for second additional range.
"locus="
SGR substring for location information, file:line or
file:line:column etc.
"quote="
SGR substring for information printed within quotes.
"fnname="
SGR substring for names of C++ functions.
"targs="
SGR substring for C++ function template parameter bindings.
"fixit-insert="
SGR substring for fix-it hints suggesting text to be inserted or
replaced.
"fixit-delete="
SGR substring for fix-it hints suggesting text to be deleted.
"diff-filename="
SGR substring for filename headers within generated patches.
"diff-hunk="
SGR substring for the starts of hunks within generated patches.
"diff-delete="
SGR substring for deleted lines within generated patches.
"diff-insert="
SGR substring for inserted lines within generated patches.
"type-diff="
SGR substring for highlighting mismatching types within template
arguments in the C++ frontend.
"valid="
SGR substring for highlighting valid elements within text art
diagrams.
"invalid="
SGR substring for highlighting invalid elements within text art
diagrams.
"highlight-a="
"highlight-b="
SGR substrings for contrasting two different things within
diagnostics, such as a pair of mismatching types. See
-fdiagnostics-show-highlight-colors.
-fdiagnostics-urls[=WHEN]
Use escape sequences to embed URLs in diagnostics. For example, when
-fdiagnostics-show-option emits text showing the command-line option
controlling a diagnostic, embed a URL for documentation of that
option.
WHEN is never, always, or auto. auto makes GCC use URL escape
sequences only when the standard error is a terminal, and when not
executing in an emacs shell or any graphical terminal which is known
to be incompatible with this feature, see below.
The default depends on how the compiler has been configured. It can
be any of the above WHEN options.
GCC can also be configured (via the
--with-diagnostics-urls=auto-if-env configure-time option) so that
the default is affected by environment variables. Under such a
configuration, GCC defaults to using auto if either GCC_URLS or
TERM_URLS environment variables are present and non-empty in the
environment of the compiler, or never if neither are.
However, even with -fdiagnostics-urls=always the behavior is
dependent on those environment variables: If GCC_URLS is set to empty
or no, do not embed URLs in diagnostics. If set to st, URLs use ST
escape sequences. If set to bel, the default, URLs use BEL escape
sequences. Any other non-empty value enables the feature. If
GCC_URLS is not set, use TERM_URLS as a fallback. Note: ST is an
ANSI escape sequence, string terminator ESC \, BEL is an ASCII
character, CTRL-G that usually sounds like a beep.
At this time GCC tries to detect also a few terminals that are known
to not implement the URL feature, and have bugs or at least had bugs
in some versions that are still in use, where the URL escapes are
likely to misbehave, i.e. print garbage on the screen. That list is
currently xfce4-terminal, certain known to be buggy gnome-terminal
versions, the linux console, and mingw. This check can be skipped
with the -fdiagnostics-urls=always.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the
command-line option that directly controls the diagnostic (if such an
option is known to the diagnostic machinery). Specifying the
-fno-diagnostics-show-option flag suppresses that behavior.
-fno-diagnostics-show-caret
By default, each diagnostic emitted includes the original source line
and a caret ^ indicating the column. This option suppresses this
information. The source line is truncated to n characters, if the
-fmessage-length=n option is given. When the output is done to the
terminal, the width is limited to the width given by the COLUMNS
environment variable or, if not set, to the terminal width.
-fno-diagnostics-show-labels
By default, when printing source code (via -fdiagnostics-show-caret),
diagnostics can label ranges of source code with pertinent
information, such as the types of expressions:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long int
This option suppresses the printing of these labels (in the example
above, the vertical bars and the "char *" and "long int" text).
-fno-diagnostics-show-event-links
By default, when printing execution paths (via
-fdiagnostics-path-format=inline-events), GCC will print lines
connecting related events, such as the line connecting events 1 and 2
in:
3 | if (p)
| ^
| |
| (1) following `false' branch (when `p' is NULL)... ->-+
| |
| |
|+------------------------------------------------------------+
4 || return 0;
5 || return *p;
|| ~
|| |
|+-------->(2) ...to here
| (3) dereference of NULL `p'
This option suppresses the printing of such connector lines.
-fno-diagnostics-show-cwe
Diagnostic messages can optionally have an associated
CWE ("https://cwe.mitre.org/index.html") identifier. GCC itself
only provides such metadata for some of the -fanalyzer diagnostics.
GCC plugins may also provide diagnostics with such metadata. By
default, if this information is present, it will be printed with the
diagnostic. This option suppresses the printing of this metadata.
-fno-diagnostics-show-rules
Diagnostic messages can optionally have rules associated with them,
such as from a coding standard, or a specification. GCC itself does
not do this for any of its diagnostics, but plugins may do so. By
default, if this information is present, it will be printed with the
diagnostic. This option suppresses the printing of this metadata.
-fno-diagnostics-show-highlight-colors
GCC can use color for emphasis and contrast when printing diagnostic
messages and quoting the user's source.
For example, in
demo.c: In function `test_bad_format_string_args':
../../src/demo.c:25:18: warning: format `%i' expects argument of type `int', but argument 2 has type `const char *' [-Wformat=]
25 | printf("hello %i", msg);
| ~^ ~~~
| | |
| int const char *
| %s
* the %i and "int" in the message and the "int" in the quoted
source are colored using "highlight-a" (bold green by default),
and
* the "const char *" in the message and in the quoted source are
both colored using "highlight-b" (bold blue by default).
The intent is to draw the reader's eyes to the relationships between
the various aspects of the diagnostic message and the source, using
color to group related elements and distinguish between mismatching
ones.
This additional colorization is enabled by default if color printing
is enabled (as per -fdiagnostics-color=), but it can be separately
disabled via -fno-diagnostics-show-highlight-colors.
-fno-diagnostics-show-line-numbers
By default, when printing source code (via -fdiagnostics-show-caret),
a left margin is printed, showing line numbers. This option
suppresses this left margin.
-fdiagnostics-minimum-margin-width=width
This option controls the minimum width of the left margin printed by
-fdiagnostics-show-line-numbers. It defaults to 6.
-fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for
consumption by IDEs. For each fix-it, a line will be printed after
the relevant diagnostic, starting with the string "fix-it:". For
example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
The location is expressed as a half-open range, expressed as a count
of bytes, starting at byte 1 for the initial column. In the above
example, bytes 3 through 20 of line 45 of "test.c" are to be replaced
with the given string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_all
The filename and replacement string escape backslash as "\\", tab as
"\t", newline as "\n", double quotes as "\"", non-printable
characters as octal (e.g. vertical tab as "\013").
An empty replacement string indicates that the given range is to be
removed. An empty range (e.g. "45:3-45:3") indicates that the string
is to be inserted at the given position.
-fdiagnostics-generate-patch
Print fix-it hints to stderr in unified diff format, after any
diagnostics are printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}
The diff may or may not be colorized, following the same rules as for
diagnostics (see -fdiagnostics-color).
-fdiagnostics-show-template-tree
In the C++ frontend, when printing diagnostics showing mismatching
template types, such as:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
the -fdiagnostics-show-template-tree flag enables printing a tree-
like structure showing the common and differing parts of the types,
such as:
map<
[...],
vector<
[double != float]>>
The parts that differ are highlighted with color ("double" and
"float" in this case).
-fno-elide-type
By default when the C++ frontend prints diagnostics showing
mismatching template types, common parts of the types are printed as
"[...]" to simplify the error message. For example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
Specifying the -fno-elide-type flag suppresses that behavior. This
flag also affects the output of the -fdiagnostics-show-template-tree
flag.
-fdiagnostics-path-format=KIND
Specify how to print paths of control-flow events for diagnostics
that have such a path associated with them.
KIND is none, separate-events, or inline-events, the default.
none means to not print diagnostic paths.
separate-events means to print a separate "note" diagnostic for each
event within the diagnostic. For example:
test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
inline-events means to print the events "inline" within the source
code. This view attempts to consolidate the events into runs of
sufficiently-close events, printing them as labelled ranges within
the source.
For example, the same events as above might be printed as:
'test': events 1-3
25 | list = PyList_New(0);
| ^~~~~~~~~~~~~
| |
| (1) when 'PyList_New' fails, returning NULL
26 |
27 | for (i = 0; i < count; i++) {
| ~~~
| |
| (2) when 'i < count'
28 | item = PyLong_FromLong(random());
29 | PyList_Append(list, item);
| ~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
Interprocedural control flow is shown by grouping the events by stack
frame, and using indentation to show how stack frames are nested,
pushed, and popped.
For example:
'test': events 1-2
|
| 133 | {
| | ^
| | |
| | (1) entering 'test'
| 134 | boxed_int *obj = make_boxed_int (i);
| | ~~~~~~~~~~~~~~~~~~
| | |
| | (2) calling 'make_boxed_int'
|
+--> 'make_boxed_int': events 3-4
|
| 120 | {
| | ^
| | |
| | (3) entering 'make_boxed_int'
| 121 | boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| | (4) calling 'wrapped_malloc'
|
+--> 'wrapped_malloc': events 5-6
|
| 7 | {
| | ^
| | |
| | (5) entering 'wrapped_malloc'
| 8 | return malloc (size);
| | ~~~~~~~~~~~~~
| | |
| | (6) calling 'malloc'
|
<-------------+
|
'test': event 7
|
| 138 | free_boxed_int (obj);
| | ^~~~~~~~~~~~~~~~~~~~
| | |
| | (7) calling 'free_boxed_int'
|
(etc)
-fdiagnostics-show-path-depths
This option provides additional information when printing control-
flow paths associated with a diagnostic.
If this is option is provided then the stack depth will be printed
for each run of events within
-fdiagnostics-path-format=inline-events. If provided with
-fdiagnostics-path-format=separate-events, then the stack depth and
function declaration will be appended when printing each event.
This is intended for use by GCC developers and plugin developers when
debugging diagnostics that report interprocedural control flow.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if
diagnostics are being scanned by a program that does not understand
the column numbers, such as dejagnu.
-fdiagnostics-column-unit=UNIT
Select the units for the column number. This affects traditional
diagnostics (in the absence of -fno-show-column), as well as JSON
format diagnostics if requested.
The default UNIT, display, considers the number of display columns
occupied by each character. This may be larger than the number of
bytes required to encode the character, in the case of tab
characters, or it may be smaller, in the case of multibyte
characters. For example, the character "GREEK SMALL LETTER PI
(U+03C0)" occupies one display column, and its UTF-8 encoding
requires two bytes; the character "SLIGHTLY SMILING FACE (U+1F642)"
occupies two display columns, and its UTF-8 encoding requires four
bytes.
Setting UNIT to byte changes the column number to the raw byte count
in all cases, as was traditionally output by GCC prior to version
11.1.0.
-fdiagnostics-column-origin=ORIGIN
Select the origin for column numbers, i.e. the column number assigned
to the first column. The default value of 1 corresponds to
traditional GCC behavior and to the GNU style guide. Some utilities
may perform better with an origin of 0; any non-negative value may be
specified.
-fdiagnostics-escape-format=FORMAT
When GCC prints pertinent source lines for a diagnostic it normally
attempts to print the source bytes directly. However, some
diagnostics relate to encoding issues in the source file, such as
malformed UTF-8, or issues with Unicode normalization. These
diagnostics are flagged so that GCC will escape bytes that are not
printable ASCII when printing their pertinent source lines.
This option controls how such bytes should be escaped.
The default FORMAT, unicode displays Unicode characters that are not
printable ASCII in the form <U+XXXX>, and bytes that do not
correspond to a Unicode character validly-encoded in UTF-8-encoded
will be displayed as hexadecimal in the form <XX>.
For example, a source line containing the string before followed by
the Unicode character U+03C0 ("GREEK SMALL LETTER PI", with UTF-8
encoding 0xCF 0x80) followed by the byte 0xBF (a stray UTF-8 trailing
byte), followed by the string after will be printed for such a
diagnostic as:
before<U+03C0><BF>after
Setting FORMAT to bytes will display all non-printable-ASCII bytes in
the form <XX>, thus showing the underlying encoding of non-ASCII
Unicode characters. For the example above, the following will be
printed:
before<CF><80><BF>after
-fdiagnostics-text-art-charset=CHARSET
Some diagnostics can contain "text art" diagrams: visualizations
created from text, intended to be viewed in a monospaced font.
This option selects which characters should be used for printing such
diagrams, if any. CHARSET is none, ascii, unicode, or emoji.
The none value suppresses the printing of such diagrams. The ascii
value will ensure that such diagrams are pure ASCII ("ASCII art").
The unicode value will allow for conservative use of unicode drawing
characters (such as box-drawing characters). The emoji value further
adds the possibility of emoji in the output (such as emitting U+26A0
WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the
emoji variant of the character).
The default is emoji, except when the environment variable LANG is
set to C, in which case the default is ascii.
-fdiagnostics-format=FORMAT
Select a different format for printing diagnostics. FORMAT is text,
sarif-stderr, sarif-file, json, json-stderr, or json-file.
Using this option replaces any additional "output sinks" added by
-fdiagnostics-add-output=, or that set by -fdiagnostics-set-output=.
The default is text.
The sarif-stderr and sarif-file formats both emit diagnostics in
SARIF Version 2.1.0 format, either to stderr, or to a file named
source.sarif, respectively.
The various json, json-stderr, and json-file values are deprecated
and refer to a legacy JSON-based output format. The json format is a
synonym for json-stderr. The json-stderr and json-file formats are
identical, apart from where the JSON is emitted to. With json-
stderr, the JSON is emitted to stderr, whereas with json-file it is
written to source.gcc.json.
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC
Add an additional "output sink" for emitting diagnostics.
DIAGNOSTICS-OUTPUT-SPEC should specify a scheme, optionally followed
by ":" and one or more KEY=VALUE pairs, in this form:
<SCHEME>
<SCHEME>:<KEY>=<VALUE>
<SCHEME>:<KEY>=<VALUE>,<KEY2>=<VALUE2>
etc.
Schemes, keys, or values with a name prefixed "experimental" may
change or be removed without notice.
SCHEME can be
text
Emit diagnostics to stderr using GCC's classic text output
format.
Supported keys are:
color=[yes|no]
Override colorization settings from -fdiagnostics-color for
this text output.
experimental-nesting=[yes|no]
Enable an experimental mode that emphasizes hierarchical
relationships within diagnostics messages, displaying
location information on separate lines.
experimental-nesting-show-locations=[yes|no]
If "experimental-nesting=yes", then by default locations are
shown; set this key to "no" to disable printing such
locations. This exists for use by GCC developers, for
writing DejaGnu test cases.
experimental-nesting-show-levels=[yes|no]
This is a debugging option for use with
"experimental-nesting=yes". Set this key to "yes" to print
explicit nesting levels in the output. This exists for use
by GCC developers.
sarif
Emit diagnostics to a file in SARIF format.
Supported keys are:
file=FILENAME
Specify the filename to write the SARIF output to,
potentially with a leading absolute or relative path. If not
specified, it defaults to source.sarif.
version=[2.1|2.2-prerelease]
Specify the version of SARIF to use for the output. If not
specified, defaults to 2.1. "2.2-prerelease" uses an
unofficial draft of the future SARIF 2.2 specification and
should only be used for experimentation in this release.
For example,
-fdiagnostics-add-output=sarif:version=2.1,file=foo.2.1.sarif
-fdiagnostics-add-output=sarif:version=2.2-prerelease,file=foo.2.2.sarif
would add a pair of outputs, each writing to a different file, using
versions 2.1 and 2.2 of the SARIF standard respectively.
In EBNF:
<diagnostics-output-specifier> = <diagnostics-output-name>
| <diagnostics-output-name>, ":", <key-value-pairs>;
<diagnostics-output-name> = "text" | "sarif";
<key-value-pairs> = <key-value-pair>
| <key-value-pair> "," <key-value-pairs>;
<key-value-pair> = <key> "=" <value>;
<key> = ? string without a '=' ? ;
<value> = ? string without a ',' ? ;
-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC
This works in a similar way to -fdiagnostics-add-output= except that
instead of adding an additional "output sink" for diagnostics, it
replaces all existing output sinks, such as from
-fdiagnostics-format=, -fdiagnostics-add-output=, or a prior call to
-fdiagnostics-set-output=.
-fno-diagnostics-json-formatting
By default, when JSON is emitted for diagnostics (via
-fdiagnostics-format=sarif-stderr, -fdiagnostics-format=sarif-file,
-fdiagnostics-format=json, -fdiagnostics-format=json-stderr,
-fdiagnostics-format=json-file), GCC will add newlines and
indentation to visually emphasize the hierarchical structure of the
JSON.
Use -fno-diagnostics-json-formatting to suppress this whitespace. It
must be passed before the option it is to affect.
This is intended for compatibility with tools that do not expect the
output to contain newlines, such as that emitted by older GCC
releases.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not
inherently erroneous but that are risky or suggest there may have been an
error.
The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which point GCC
bails out rather than attempting to continue processing the source
code. If n is 0 (the default), there is no limit on the number of
error messages produced. If -Wfatal-errors is also specified, then
-Wfatal-errors takes precedence over this option.
-w Inhibit all warning messages.
-Werror
Turn all warnings into errors.
-Werror=
Turn the specified warning into an error. The specifier for a
warning is appended; for example -Werror=switch turns the warnings
controlled by -Wswitch into errors. This switch takes a negative
form, to be used to negate -Werror for specific warnings; for example
-Wno-error=switch makes -Wswitch warnings not be errors, even when
-Werror is in effect.
The warning message for each controllable warning includes the option
that controls the warning. That option can then be used with
-Werror= and -Wno-error= as described above. (Printing of the option
in the warning message can be disabled using the
-fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo.
However, -Wno-error=foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first
error occurred rather than trying to keep going and printing further
error messages.
You can request many specific warnings with options beginning with -W,
for example -Wunused-variable to request warnings on declarations of
variables that are never used. Each of these specific warning options
also has a negative form beginning with -Wno- to turn off warnings; for
example, -Wno-unused-variable. This manual lists only one of the two
forms, whichever is not the default. For further language-specific
options also refer to C++ Dialect Options and Objective-C and
Objective-C++ Dialect Options. Additional warnings can be produced by
enabling the static analyzer;
Some options, such as -Wall and -Wextra, turn on other options, such as
-Wunused, which may turn on further options, such as -Wunused-variable.
The combined effect of positive and negative forms is that more specific
options have priority over less specific ones, independently of their
position in the command line. For options of the same specificity, the
last one takes effect. Options enabled or disabled via pragmas take
effect as if they appeared at the end of the command line.
When an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC gives an error stating that the option is not
recognized. However, if the -Wno- form is used, the behavior is slightly
different: no diagnostic is produced for -Wno-unknown-warning unless
other diagnostics are being produced. This allows the use of new -Wno-
options with old compilers, but if something goes wrong, the compiler
warns that an unrecognized option is present.
The effectiveness of some warnings depends on optimizations also being
enabled. For example, -Wsuggest-final-types is more effective with link-
time optimization. Some other warnings may not be issued at all unless
optimization is enabled. While optimization in general improves the
efficacy of warnings about control and data-flow problems, in some cases
it may also cause false positives.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; diagnose
all programs that use forbidden extensions, and some other programs
that do not follow ISO C and ISO C++. This follows the version of
the ISO C or C++ standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or
without this option (though a rare few require -ansi or a -std option
specifying the version of the standard). However, without this
option, certain GNU extensions and traditional C and C++ features are
supported as well. With this option, they are diagnosed (or rejected
with -pedantic-errors).
-Wpedantic does not cause warning messages for use of the alternate
keywords whose names begin and end with __. This alternate format
can also be used to disable warnings for non-ISO __intN types, i.e.
__intN__. Pedantic warnings are also disabled in the expression that
follows "__extension__". However, only system header files should
use these escape routes; application programs should avoid them.
Some warnings about non-conforming programs are controlled by options
other than -Wpedantic; in many cases they are implied by -Wpedantic
but can be disabled separately by their specific option, e.g.
-Wpedantic -Wno-pointer-sign.
Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu90 or gnu99, there is a corresponding base
standard, the version of ISO C on which the GNU extended dialect is
based. Warnings from -Wpedantic are given where they are required by
the base standard. (It does not make sense for such warnings to be
given only for features not in the specified GNU C dialect, since by
definition the GNU dialects of C include all features the compiler
supports with the given option, and there would be nothing to warn
about.)
-pedantic-errors
Give an error whenever the base standard (see -Wpedantic) requires a
diagnostic, in some cases where there is undefined behavior at
compile-time and in some other cases that do not prevent compilation
of programs that are valid according to the standard. This is not
equivalent to -Werror=pedantic: the latter option is unlikely to be
useful, as it only makes errors of the diagnostics that are
controlled by -Wpedantic, whereas this option also affects required
diagnostics that are always enabled or controlled by options other
than -Wpedantic.
If you want the required diagnostics that are warnings by default to
be errors instead, but don't also want to enable the -Wpedantic
diagnostics, you can specify -pedantic-errors -Wno-pedantic (or
-pedantic-errors -Wno-error=pedantic to enable them but only as
warnings).
Some required diagnostics are errors by default, but can be reduced
to warnings using -fpermissive or their specific warning option, e.g.
-Wno-error=narrowing.
Some diagnostics for non-ISO practices are controlled by specific
warning options other than -Wpedantic, but are also made errors by
-pedantic-errors. For instance:
-Wattributes (for standard attributes) -Wchanges-meaning (C++)
-Wcomma-subscript (C++23 or later) -Wdeclaration-after-statement (C90
or earlier) -Welaborated-enum-base (C++11 or later) -Wimplicit-int
(C99 or later) -Wimplicit-function-declaration (C99 or later)
-Wincompatible-pointer-types -Wint-conversion -Wlong-long (C90 or
earlier) -Wmain -Wnarrowing (C++11 or later) -Wpointer-arith
-Wpointer-sign -Wincompatible-pointer-types -Wregister (C++17 or
later) -Wvla (C90 or earlier) -Wwrite-strings (C++11 or later)
-fpermissive
Downgrade some required diagnostics about nonconformant code from
errors to warnings. Thus, using -fpermissive allows some
nonconforming code to compile. Some C++ diagnostics are controlled
only by this flag, but it also downgrades some C and C++ diagnostics
that have their own flag:
-Wdeclaration-missing-parameter-type (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only)
-Wimplicit-int (C and Objective-C only) -Wincompatible-pointer-types
(C and Objective-C only) -Wint-conversion (C and Objective-C only)
-Wnarrowing (C++ and Objective-C++ only) -Wreturn-mismatch (C and
Objective-C only) -Wtemplate-body (C++ and Objective-C++ only)
The -fpermissive option is the default for historic C language modes
(-std=c89, -std=gnu89, -std=c90, -std=gnu90).
-Wall
This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros. This also
enables some language-specific warnings described in C++ Dialect
Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Waligned-new (C++ and Objective-C++ only) -Warray-bounds=1
(only with -O2) -Warray-compare -Warray-parameter=2 -Wbool-compare
-Wbool-operation -Wc++11-compat -Wc++14-compat -Wc++17compat
-Wc++20compat -Wcatch-value (C++ and Objective-C++ only)
-Wchar-subscripts -Wclass-memaccess (C++ and Objective-C++ only)
-Wcomment -Wdangling-else -Wdangling-pointer=2
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
-Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare
(in C/ObjC; this is on by default in C++) -Wenum-int-mismatch (C and
Objective-C only) -Wformat=1 -Wformat-contains-nul -Wformat-diag
-Wformat-extra-args -Wformat-overflow=1 -Wformat-truncation=1
-Wformat-zero-length -Wframe-address -Wimplicit (C and Objective-C
only) -Wimplicit-function-declaration (C and Objective-C only)
-Wimplicit-int (C and Objective-C only) -Winfinite-recursion
-Winit-self (C++ and Objective-C++ only) -Wint-in-bool-context
-Wlogical-not-parentheses -Wmain (only for C/ObjC and unless
-ffreestanding) -Wmaybe-uninitialized -Wmemset-elt-size
-Wmemset-transposed-args -Wmisleading-indentation (only for C/C++)
-Wmismatched-dealloc -Wmismatched-new-delete (C++ and Objective-C++
only) -Wmissing-attributes -Wmissing-braces (only for C/ObjC)
-Wmultistatement-macros -Wnarrowing (C++ and Objective-C++ only)
-Wnonnull -Wnonnull-compare -Wopenmp-simd (C and C++ only)
-Woverloaded-virtual=1 (C++ and Objective-C++ only)
-Wpacked-not-aligned -Wparentheses -Wpessimizing-move (C++ and
Objective-C++ only) -Wpointer-sign (only for C/ObjC)
-Wrange-loop-construct (C++ and Objective-C++ only) -Wreorder (C++
and Objective-C++ only) -Wrestrict -Wreturn-type -Wself-move (C++ and
Objective-C++ only) -Wsequence-point -Wsign-compare (C++ and
Objective-C++ only) -Wsizeof-array-div -Wsizeof-pointer-div
-Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1
-Wswitch -Wtautological-compare -Wtrigraphs -Wuninitialized
-Wunknown-pragmas -Wunused -Wunused-but-set-variable
-Wunused-const-variable=1 (only for C/ObjC) -Wunused-function
-Wunused-label -Wunused-local-typedefs -Wunused-value
-Wunused-variable -Wuse-after-free=2 -Wvla-parameter
-Wvolatile-register-var -Wzero-length-bounds
Note that some warning flags are not implied by -Wall. Some of them
warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid
in some cases, and there is no simple way to modify the code to
suppress the warning. Some of them are enabled by -Wextra but many of
them must be enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by -Wall.
(This option used to be called -W. The older name is still
supported, but the newer name is more descriptive.)
-Wabsolute-value (only for C/ObjC) -Walloc-size
-Wcalloc-transposed-args -Wcast-function-type -Wclobbered
-Wdangling-reference (C++ only) -Wdeprecated-copy (C++ and
Objective-C++ only) -Wempty-body -Wenum-conversion (only for C/ObjC)
-Wexpansion-to-defined -Wignored-qualifiers (only for C/C++)
-Wimplicit-fallthrough=3 -Wmaybe-uninitialized
-Wmissing-field-initializers -Wmissing-parameter-name (C/ObjC only)
-Wmissing-parameter-type (C/ObjC only) -Wold-style-declaration
(C/ObjC only) -Woverride-init (C/ObjC only) -Wredundant-move (C++ and
Objective-C++ only) -Wshift-negative-value (in C++11 to C++17 and in
C99 and newer) -Wsign-compare (C++ and Objective-C++ only)
-Wsized-deallocation (C++ and Objective-C++ only) -Wstring-compare
-Wtype-limits -Wuninitialized -Wunterminated-string-initialization
-Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)
The option -Wextra also prints warning messages for the following
cases:
* A pointer is compared against integer zero with "<", "<=", ">",
or ">=".
* (C++ only) An enumerator and a non-enumerator both appear in a
conditional expression.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array that has been declared
"register".
* (C++ only) Taking the address of a variable that has been
declared "register".
* (C++ only) A base class is not initialized in the copy
constructor of a derived class.
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn about code affected by ABI changes. This includes code that may
not be compatible with the vendor-neutral C++ ABI as well as the
psABI for the particular target. The latter warnings are also
controlled separately by -Wpsabi, which is implied by -Wabi.
Since G++ now defaults to updating the ABI with each major release,
normally -Wabi warns only about C++ ABI compatibility problems if
there is a check added later in a release series for an ABI issue
discovered since the initial release. -Wabi warns about more things
if an older ABI version is selected (with -fabi-version=n).
-Wabi can also be used with an explicit version number to warn about
C++ ABI compatibility with a particular -fabi-version level, e.g.
-Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is
not specified, the version number from this option is used for
compatibility aliases. If no explicit version number is provided
with this option, but -fabi-compat-version is specified, that version
number is used for C++ ABI warnings.
Although an effort has been made to warn about all such cases, there
are probably some cases that are not warned about, even though G++ is
generating incompatible code. There may also be cases where warnings
are emitted even though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be binary
compatible with code generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default
from GCC 3.4 to 4.9) include:
* A template with a non-type template parameter of reference type
was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This was fixed in -fabi-version=3.
* SIMD vector types declared using "__attribute ((vector_size))"
were mangled in a non-standard way that does not allow for
overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
* "__attribute ((const))" and "noreturn" were mangled as type
qualifiers, and "decltype" of a plain declaration was folded
away.
These mangling issues were fixed in -fabi-version=5.
* Scoped enumerators passed as arguments to a variadic function are
promoted like unscoped enumerators, causing "va_arg" to complain.
On most targets this does not actually affect the parameter
passing ABI, as there is no way to pass an argument smaller than
"int".
Also, the ABI changed the mangling of template argument packs,
"const_cast", "static_cast", prefix increment/decrement, and a
class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
* Lambdas in default argument scope were mangled incorrectly, and
the ABI changed the mangling of "nullptr_t".
These issues were corrected in -fabi-version=7.
* When mangling a function type with function-cv-qualifiers, the
un-qualified function type was incorrectly treated as a
substitution candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
* "decltype(nullptr)" incorrectly had an alignment of 1, leading to
unaligned accesses. Note that this did not affect the ABI of a
function with a "nullptr_t" parameter, as parameters have a
minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
* Target-specific attributes that affect the identity of a type,
such as ia32 calling conventions on a function type (stdcall,
regparm, etc.), did not affect the mangled name, leading to name
collisions when function pointers were used as template
arguments.
This was fixed in -fabi-version=10, the default for GCC 6.1.
-Wpsabi (C, Objective-C, C++ and Objective-C++ only)
-Wpsabi enables warnings about processor-specific ABI changes, such
as changes in alignment requirements or how function arguments are
passed. On several targets, including AArch64, ARM, x86, MIPS,
RS6000/PowerPC, and S/390, these details have changed between
different versions of GCC and/or different versions of the C or C++
language standards in ways that affect binary compatibility of
compiled code. With -Wpsabi, GCC can detect potentially incompatible
usages and warn you about them.
-Wpsabi is enabled by default, and is also implied by -Wabi.
-Wno-changes-meaning (C++ and Objective-C++ only)
C++ requires that unqualified uses of a name within a class have the
same meaning in the complete scope of the class, so declaring the
name after using it is ill-formed:
struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaning
By default, the B1 case is only a warning because the two
declarations have the same type, while the B2 case is an error. Both
diagnostics can be disabled with -Wno-changes-meaning. Alternately,
the error case can be reduced to a warning with
-Wno-error=changes-meaning or -fpermissive.
Both diagnostics are also suppressed by -fms-extensions.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause
of error, as programmers often forget that this type is signed on
some machines. This warning is enabled by -Wall.
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the -fprofile-use
option. If a source file is changed between compiling with
-fprofile-generate and with -fprofile-use, the files with the profile
feedback can fail to match the source file and GCC cannot use the
profile feedback information. By default, this warning is enabled
and is treated as an error. -Wno-coverage-mismatch can be used to
disable the warning or -Wno-error=coverage-mismatch can be used to
disable the error. Disabling the error for this warning can result
in poorly optimized code and is useful only in the case of very minor
changes such as bug fixes to an existing code-base. Completely
disabling the warning is not recommended.
-Wno-coverage-too-many-conditions
Warn if -fcondition-coverage is used and an expression have too many
terms and GCC gives up coverage. Coverage is given up when there are
more terms in the conditional than there are bits in a
"gcov_type_unsigned". This warning is enabled by default.
-Wno-coverage-too-many-paths
Warn if -fpath-coverage is used and a function has too many paths and
GCC gives up coverage. Giving up is controlled by
-fpath-coverage-limit. This warning is enabled by default.
-Wno-coverage-invalid-line-number
Warn in case a function ends earlier than it begins due to an invalid
linenum macros. The warning is emitted only with --coverage enabled.
By default, this warning is enabled and is treated as an error.
-Wno-coverage-invalid-line-number can be used to disable the warning
or -Wno-error=coverage-invalid-line-number can be used to disable the
error.
-Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly promoted to
"double". CPUs with a 32-bit "single-precision" floating-point unit
implement "float" in hardware, but emulate "double" in software. On
such a machine, doing computations using "double" values is much more
expensive because of the overhead required for software emulation.
It is easy to accidentally do computations with "double" because
floating-point literals are implicitly of type "double". For
example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double" because
the floating-point literal is a "double".
-Wduplicate-decl-specifier (C and Objective-C only)
Warn if a declaration has duplicate "const", "volatile", "restrict"
or "_Atomic" specifier. This warning is enabled by -Wall.
-Wformat
-Wformat=n
Check calls to "printf" and "scanf", etc., to make sure that the
arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others specified
by format attributes, in the "printf", "scanf", "strftime" and
"strfmon" (an X/Open extension, not in the C standard) families (or
other target-specific families). Which functions are checked without
format attributes having been specified depends on the standard
version selected, and such checks of functions without the attribute
specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU
libc version 2.2. These include all ISO C90 and C99 features, as
well as features from the Single Unix Specification and some BSD and
GNU extensions. Other library implementations may not support all
these features; GCC does not support warning about features that go
beyond a particular library's limitations. However, if -Wpedantic is
used with -Wformat, warnings are given about format features not in
the selected standard version (but not for "strfmon" formats, since
those are not in any version of the C standard).
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and -Wno-format is
equivalent to -Wformat=0. Since -Wformat also checks for null
format arguments for several functions, -Wformat also implies
-Wnonnull. Some aspects of this level of format checking can be
disabled by the options: -Wno-format-contains-nul,
-Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is
enabled by -Wall.
-Wformat=2
Enable -Wformat plus additional format checks. Currently
equivalent to -Wformat -Wformat-nonliteral -Wformat-security
-Wformat-y2k.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings that
contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a
"printf" or "scanf" format function. The C standard specifies that
such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with $ operand number specifications, normally warnings are
still given, since the implementation could not know what type to
pass to "va_arg" to skip the unused arguments. However, in the case
of "scanf" formats, this option suppresses the warning if the unused
arguments are all pointers, since the Single Unix Specification says
that such unused arguments are allowed.
-Wformat-overflow
-Wformat-overflow=level
Warn about calls to formatted input/output functions such as
"sprintf" and "vsprintf" that might overflow the destination buffer.
When the exact number of bytes written by a format directive cannot
be determined at compile-time it is estimated based on heuristics
that depend on the level argument and on optimization. While
enabling optimization will in most cases improve the accuracy of the
warning, it may also result in false positives.
-Wformat-overflow
-Wformat-overflow=1
Level 1 of -Wformat-overflow enabled by -Wformat employs a
conservative approach that warns only about calls that most
likely overflow the buffer. At this level, numeric arguments to
format directives with unknown values are assumed to have the
value of one, and strings of unknown length to be empty. Numeric
arguments that are known to be bounded to a subrange of their
type, or string arguments whose output is bounded either by their
directive's precision or by a finite set of string literals, are
assumed to take on the value within the range that results in the
most bytes on output. For example, the call to "sprintf" below
is diagnosed because even with both a and b equal to zero, the
terminating NUL character ('\0') appended by the function to the
destination buffer will be written past its end. Increasing the
size of the buffer by a single byte is sufficient to avoid the
warning, though it may not be sufficient to avoid the overflow.
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}
-Wformat-overflow=2
Level 2 warns also about calls that might overflow the
destination buffer given an argument of sufficient length or
magnitude. At level 2, unknown numeric arguments are assumed to
have the minimum representable value for signed types with a
precision greater than 1, and the maximum representable value
otherwise. Unknown string arguments whose length cannot be
assumed to be bounded either by the directive's precision, or by
a finite set of string literals they may evaluate to, or the
character array they may point to, are assumed to be 1 character
long.
At level 2, the call in the example above is again diagnosed, but
this time because with a equal to a 32-bit "INT_MIN" the first %i
directive will write some of its digits beyond the end of the
destination buffer. To make the call safe regardless of the
values of the two variables, the size of the destination buffer
must be increased to at least 34 bytes. GCC includes the minimum
size of the buffer in an informational note following the
warning.
An alternative to increasing the size of the destination buffer
is to constrain the range of formatted values. The maximum
length of string arguments can be bounded by specifying the
precision in the format directive. When numeric arguments of
format directives can be assumed to be bounded by less than the
precision of their type, choosing an appropriate length modifier
to the format specifier will reduce the required buffer size.
For example, if a and b in the example above can be assumed to be
within the precision of the "short int" type then using either
the %hi format directive or casting the argument to "short"
reduces the maximum required size of the buffer to 24 bytes.
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The
C standard specifies that zero-length formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions
that represent possible security problems. At present, this warns
about calls to "printf" and "scanf" functions where the format string
is not a string literal and there are no format arguments, as in
"printf (foo);". This may be a security hole if the format string
came from untrusted input and contains %n. (This is currently a
subset of what -Wformat-nonliteral warns about, but in future
warnings may be added to -Wformat-security that are not included in
-Wformat-nonliteral.)
-Wformat-signedness
If -Wformat is specified, also warn if the format string requires an
unsigned argument and the argument is signed and vice versa.
-Wformat-truncation
-Wformat-truncation=level
Warn about calls to formatted input/output functions such as
"snprintf" and "vsnprintf" that might result in output truncation.
When the exact number of bytes written by a format directive cannot
be determined at compile-time it is estimated based on heuristics
that depend on the level argument and on optimization. While
enabling optimization will in most cases improve the accuracy of the
warning, it may also result in false positives. Except as noted
otherwise, the option uses the same logic -Wformat-overflow.
-Wformat-truncation
-Wformat-truncation=1
Level 1 of -Wformat-truncation enabled by -Wformat employs a
conservative approach that warns only about calls to bounded
functions whose return value is unused and that will most likely
result in output truncation.
-Wformat-truncation=2
Level 2 warns also about calls to bounded functions whose return
value is used and that might result in truncation given an
argument of sufficient length or magnitude.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats that may
yield only a two-digit year.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a
non-null value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with
the -Wno-nonnull option.
-Wnonnull-compare
Warn when comparing an argument marked with the "nonnull" function
attribute against null inside the function.
-Wnonnull-compare is included in -Wall. It can be disabled with the
-Wno-nonnull-compare option.
-Wnull-dereference
Warn if the compiler detects paths that trigger erroneous or
undefined behavior due to dereferencing a null pointer. This option
is only active when -fdelete-null-pointer-checks is active, which is
enabled by optimizations in most targets. The precision of the
warnings depends on the optimization options used.
-Wno-musttail-local-addr
Do not warn about passing a pointer (or in C++, a reference) to a
local variable or label to argument of a "musttail" call. Those
variables go out of scope before the tail call instruction.
-Wmaybe-musttail-local-addr
Warn when address of a local variable can escape to a "musttail"
call, unless it goes out of scope already before the "musttail" call.
int foo (int *);
int
bar (int *x)
{
if (x[0] == 1)
{
int a = 42;
foo (&a);
/* Without the C<musttail> attribute this call would not
be tail called, because address of the C<a> variable escapes
and the second foo call could dereference it. With the attribute
the local variables are assumed to go out of scope immediately
before the tail call instruction and the compiler warns about
this. */
[[gnu::musttail]] return foo (nullptr);
}
else
{
{
int a = 42;
foo (&a);
}
/* The C<a> variable isn't already in scope, so even when it
escaped, even without C<musttail> attribute it would be
undefined behavior to dereference it and the compiler could
turn this into a tail call. No warning is diagnosed here. */
[[gnu::musttail]] return foo (nullptr);
}
}
This warning is enabled by -Wextra.
-Wnrvo (C++ and Objective-C++ only)
Warn if the compiler does not elide the copy from a local variable to
the return value of a function in a context where it is allowed by
[class.copy.elision]. This elision is commonly known as the Named
Return Value Optimization. For instance, in the example below the
compiler cannot elide copies from both v1 and v2, so it elides
neither.
std::vector<int> f()
{
std::vector<int> v1, v2;
// ...
if (cond) return v1;
else return v2; // warning: not eliding copy
}
-Winfinite-recursion
Warn about infinitely recursive calls. The warning is effective at
all optimization levels but requires optimization in order to detect
infinite recursion in calls between two or more functions.
-Winfinite-recursion is included in -Wall.
Compare with -Wanalyzer-infinite-recursion which provides a similar
diagnostic, but is implemented in a different way (as part of
-fanalyzer).
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with
themselves. Note this option can only be used with the
-Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the following
snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
-Wno-implicit-int (C and Objective-C only)
This option controls warnings when a declaration does not specify a
type. This warning is enabled by default, as an error, in C99 and
later dialects of C, and also by -Wall. The error can be downgraded
to a warning using -fpermissive (along with certain other errors), or
for this error alone, with -Wno-error=implicit-int.
This warning is upgraded to an error by -pedantic-errors.
-Wno-implicit-function-declaration (C and Objective-C only)
This option controls warnings when a function is used before being
declared. This warning is enabled by default, as an error, in C99
and later dialects of C, and also by -Wall. The error can be
downgraded to a warning using -fpermissive (along with certain other
errors), or for this error alone, with
-Wno-error=implicit-function-declaration.
This warning is upgraded to an error by -pedantic-errors.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This
warning is enabled by -Wall.
-Whardened
Warn when -fhardened did not enable an option from its set (for which
see -fhardened). For instance, using -fhardened and
-fstack-protector at the same time on the command line causes
-Whardened to warn because -fstack-protector-strong will not be
enabled by -fhardened.
This warning is enabled by default and has effect only when
-fhardened is enabled.
-Wimplicit-fallthrough
-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and
-Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0.
-Wimplicit-fallthrough=n
Warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}
This warning does not warn when the last statement of a case cannot
fall through, e.g. when there is a return statement or a call to
function declared with the noreturn attribute.
-Wimplicit-fallthrough= also takes into account control flow
statements, such as ifs, and only warns when appropriate. E.g.
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}
Since there are occasions where a switch case fall through is
desirable, GCC provides an attribute, "__attribute__
((fallthrough))", that is to be used along with a null statement to
suppress this warning that would normally occur:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}
C++17 and C23 provide a standard way to suppress the
-Wimplicit-fallthrough warning using "[[fallthrough]];" instead of
the GNU attribute. In C++11 or C++14 users can use
"[[gnu::fallthrough]];", which is a GNU extension. Instead of these
attributes, it is also possible to add a fallthrough comment to
silence the warning. The whole body of the C or C++ style comment
should match the given regular expressions listed below. The option
argument n specifies what kind of comments are accepted:
*<-Wimplicit-fallthrough=0 disables the warning altogether.>
*<-Wimplicit-fallthrough=1 matches ".*" regular>
expression, any comment is used as fallthrough comment.
*<-Wimplicit-fallthrough=2 case insensitively matches>
".*falls?[ \t-]*thr(ough|u).*" regular expression.
*<-Wimplicit-fallthrough=3 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[
\t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s |
|-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s |
|-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<-Wimplicit-fallthrough=4 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">
*<-Wimplicit-fallthrough=5 doesn't recognize any comments as>
fallthrough comments, only attributes disable the warning.
The comment needs to be followed after optional whitespace and other
comments by "case" or "default" keywords or by a user label that
precedes some "case" or "default" label.
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}
The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.
-Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only)
Control if warnings triggered by the "warn_if_not_aligned" attribute
should be issued. These warnings are enabled by default.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier such as
"const". For ISO C such a type qualifier has no effect, since the
value returned by a function is not an lvalue. For C++, the warning
is only emitted for scalar types or "void". ISO C prohibits
qualified "void" return types on function definitions, so such return
types always receive a warning even without this option.
This warning is also enabled by -Wextra.
-Wno-ignored-attributes (C and C++ only)
This option controls warnings when an attribute is ignored. This is
different from the -Wattributes option in that it warns whenever the
compiler decides to drop an attribute, not that the attribute is
either unknown, used in a wrong place, etc. This warning is enabled
by default.
-Wmain
Warn if the type of "main" is suspicious. "main" should be a
function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types. This
warning is enabled by default in C++ and is enabled by either -Wall
or -Wpedantic.
This warning is upgraded to an error by -pedantic-errors.
-Wmisleading-indentation (C and C++ only)
Warn when the indentation of the code does not reflect the block
structure. Specifically, a warning is issued for "if", "else",
"while", and "for" clauses with a guarded statement that does not use
braces, followed by an unguarded statement with the same indentation.
In the following example, the call to "bar" is misleadingly indented
as if it were guarded by the "if" conditional.
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */
In the case of mixed tabs and spaces, the warning uses the -ftabstop=
option to determine if the statements line up (defaulting to 8).
The warning is not issued for code involving multiline preprocessor
logic such as the following example.
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);
The warning is not issued after a "#line" directive, since this
typically indicates autogenerated code, and no assumptions can be
made about the layout of the file that the directive references.
This warning is enabled by -Wall in C and C++.
-Wmissing-attributes
Warn when a declaration of a function is missing one or more
attributes that a related function is declared with and whose absence
may adversely affect the correctness or efficiency of generated code.
For example, the warning is issued for declarations of aliases that
use attributes to specify less restrictive requirements than those of
their targets. This typically represents a potential optimization
opportunity. By contrast, the -Wattribute-alias=2 option controls
warnings issued when the alias is more restrictive than the target,
which could lead to incorrect code generation. Attributes considered
include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf",
"malloc", "nonnull", "noreturn", "nothrow", "pure",
"returns_nonnull", and "returns_twice".
In C++, the warning is issued when an explicit specialization of a
primary template declared with attribute "alloc_align", "alloc_size",
"assume_aligned", "format", "format_arg", "malloc", or "nonnull" is
declared without it. Attributes "deprecated", "error", and "warning"
suppress the warning..
You can use the "copy" attribute to apply the same set of attributes
to a declaration as that on another declaration without explicitly
enumerating the attributes. This attribute can be applied to
declarations of functions, variables, or types.
-Wmissing-attributes is enabled by -Wall.
For example, since the declaration of the primary function template
below makes use of both attribute "malloc" and "alloc_size" the
declaration of the explicit specialization of the template is
diagnosed because it is missing one of the attributes.
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In
the following example, the initializer for "a" is not fully
bracketed, but that for "b" is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C, Objective-C++ and Fortran
only)
Warn if a user-supplied include directory does not exist. This option
is disabled by default for C, C++, Objective-C and Objective-C++. For
Fortran, it is partially enabled by default by warning for -I and -J,
only.
-Wno-missing-profile
This option controls warnings if feedback profiles are missing when
using the -fprofile-use option. This option diagnoses those cases
where a new function or a new file is added between compiling with
-fprofile-generate and with -fprofile-use, without regenerating the
profiles. In these cases, the profile feedback data files do not
contain any profile feedback information for the newly added function
or file respectively. Also, in the case when profile count data
(.gcda) files are removed, GCC cannot use any profile feedback
information. In all these cases, warnings are issued to inform you
that a profile generation step is due. Ignoring the warning can
result in poorly optimized code. -Wno-missing-profile can be used to
disable the warning, but this is not recommended and should be done
only when non-existent profile data is justified.
-Wmismatched-dealloc
Warn for calls to deallocation functions with pointer arguments
returned from allocation functions for which the former isn't a
suitable deallocator. A pair of functions can be associated as
matching allocators and deallocators by use of attribute "malloc".
Unless disabled by the -fno-builtin option the standard functions
"calloc", "malloc", "realloc", and "free", as well as the
corresponding forms of C++ "operator new" and "operator delete" are
implicitly associated as matching allocators and deallocators. In
the following example "mydealloc" is the deallocator for pointers
returned from "myalloc".
void mydealloc (void*);
__attribute__ ((malloc (mydealloc, 1))) void*
myalloc (size_t);
void f (void)
{
void *p = myalloc (32);
// ...use p...
free (p); // warning: not a matching deallocator for myalloc
mydealloc (p); // ok
}
In C++, the related option -Wmismatched-new-delete diagnoses
mismatches involving either "operator new" or "operator delete".
Option -Wmismatched-dealloc is included in -Wall.
-Wmultistatement-macros
Warn about unsafe multiple statement macros that appear to be guarded
by a clause such as "if", "else", "for", "switch", or "while", in
which only the first statement is actually guarded after the macro is
expanded.
For example:
#define DOIT x++; y++
if (c)
DOIT;
will increment "y" unconditionally, not just when "c" holds. The can
usually be fixed by wrapping the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;
This warning is enabled by -Wall in C and C++.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is expected,
or when operators are nested whose precedence people often get
confused about.
Also warn if a comparison like "x<=y<=z" appears; this is equivalent
to "(x<=y ? 1 : 0) <= z", which is a different interpretation from
that of ordinary mathematical notation.
Also warn for dangerous uses of the GNU extension to "?:" with
omitted middle operand. When the condition in the "?": operator is a
boolean expression, the omitted value is always 1. Often programmers
expect it to be a value computed inside the conditional expression
instead.
For C++ this also warns for some cases of unnecessary parentheses in
declarations, which can indicate an attempt at a function call
instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}
This warning is enabled by -Wall.
-Wno-self-move (C++ and Objective-C++ only)
This warning warns when a value is moved to itself with "std::move".
Such a "std::move" typically has no effect.
struct T {
...
};
void fn()
{
T t;
...
t = std::move (t);
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of
violations of sequence point rules in the C and C++ standards.
The C and C++ standards define the order in which expressions in a
C/C++ program are evaluated in terms of sequence points, which
represent a partial ordering between the execution of parts of the
program: those executed before the sequence point, and those executed
after it. These occur after the evaluation of a full expression (one
which is not part of a larger expression), after the evaluation of
the first operand of a "&&", "||", "? :" or "," (comma) operator,
before a function is called (but after the evaluation of its
arguments and the expression denoting the called function), and in
certain other places. Other than as expressed by the sequence point
rules, the order of evaluation of subexpressions of an expression is
not specified. All these rules describe only a partial order rather
than a total order, since, for example, if two functions are called
within one expression with no sequence point between them, the order
in which the functions are called is not specified. However, the
standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the
values of objects take effect. Programs whose behavior depends on
this have undefined behavior; the C and C++ standards specify that
"Between the previous and next sequence point an object shall have
its stored value modified at most once by the evaluation of an
expression. Furthermore, the prior value shall be read only to
determine the value to be stored.". If a program breaks these rules,
the results on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] =
b[n++]" and "a[i++] = i;". Some more complicated cases are not
diagnosed by this option, and it may give an occasional false
positive result, but in general it has been found fairly effective at
detecting this sort of problem in programs.
The C++17 standard will define the order of evaluation of operands in
more cases: in particular it requires that the right-hand side of an
assignment be evaluated before the left-hand side, so the above
examples are no longer undefined. But this option will still warn
about them, to help people avoid writing code that is undefined in C
and earlier revisions of C++.
The standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle cases.
Links to discussions of the problem, including proposed formal
definitions, may be found on the GCC readings page, at
<https://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr
Do not warn about returning a pointer (or in C++, a reference) to a
variable that goes out of scope after the function returns.
-Wreturn-mismatch
Warn about return statements without an expressions in functions
which do not return "void". Also warn about a "return" statement
with an expression in a function whose return type is "void", unless
the expression type is also "void". As a GNU extension, the latter
case is accepted without a warning unless -Wpedantic is used.
Attempting to use the return value of a non-"void" function other
than "main" that flows off the end by reaching the closing curly
brace that terminates the function is undefined.
This warning is specific to C and enabled by default. In C99 and
later language dialects, it is treated as an error. It can be
downgraded to a warning using -fpermissive (along with other
warnings), or for just this warning, with -Wno-error=return-mismatch.
-Wreturn-type
Warn whenever a function is defined with a return type that defaults
to "int" (unless -Wimplicit-int is active, which takes precedence).
Also warn if execution may reach the end of the function body, or if
the function does not contain any return statement at all.
Attempting to use the return value of a non-"void" function other
than "main" that flows off the end by reaching the closing curly
brace that terminates the function is undefined.
Unlike in C, in C++, flowing off the end of a non-"void" function
other than "main" results in undefined behavior even when the value
of the function is not used.
This warning is enabled by default in C++ and by -Wall otherwise.
-Wno-shift-count-negative
Controls warnings if a shift count is negative. This warning is
enabled by default.
-Wno-shift-count-overflow
Controls warnings if a shift count is greater than or equal to the
bit width of the type. This warning is enabled by default.
-Wshift-negative-value
Warn if left shifting a negative value. This warning is enabled by
-Wextra in C99 (and newer) and C++11 to C++17 modes.
-Wno-shift-overflow
-Wshift-overflow=n
These options control warnings about left shift overflows.
-Wshift-overflow=1
This is the warning level of -Wshift-overflow and is enabled by
default in C99 and C++11 modes (and newer). This warning level
does not warn about left-shifting 1 into the sign bit. (However,
in C, such an overflow is still rejected in contexts where an
integer constant expression is required.) No warning is emitted
in C++20 mode (and newer), as signed left shifts always wrap.
-Wshift-overflow=2
This warning level also warns about left-shifting 1 into the sign
bit, unless C++14 mode (or newer) is active.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. (The presence of a "default" label prevents this
warning.) "case" labels that do not correspond to enumerators also
provoke warnings when this option is used, unless the enumeration is
marked with the "flag_enum" attribute. This warning is enabled by
-Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. "case" labels that do not correspond to enumerators
also provoke warnings when this option is used, unless the
enumeration is marked with the "flag_enum" attribute. The only
difference between -Wswitch and this option is that this option gives
a warning about an omitted enumeration code even if there is a
"default" label.
-Wno-switch-bool
Do not warn when a "switch" statement has an index of boolean type
and the case values are outside the range of a boolean type. It is
possible to suppress this warning by casting the controlling
expression to a type other than "bool". For example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++ programs.
-Wno-switch-outside-range
This option controls warnings when a "switch" case has a value that
is outside of its respective type range. This warning is enabled by
default for C and C++ programs.
-Wno-switch-unreachable
Do not warn when a "switch" statement contains statements between the
controlling expression and the first case label, which will never be
executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}
-Wswitch-unreachable does not warn if the statement between the
controlling expression and the first case label is just a
declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}
This warning is enabled by default for C and C++ programs.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-
in functions are used. These functions changed semantics in GCC 4.4.
-Wtrivial-auto-var-init
Warn when "-ftrivial-auto-var-init" cannot initialize the automatic
variable. A common situation is an automatic variable that is
declared between the controlling expression and the first case label
of a "switch" statement.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise
unused (aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused
(aside from its declaration). This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a non-
inline static function is unused. This warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is
enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used. This
warning is enabled by -Wall.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its
declaration. This option is not enabled by "-Wunused" unless
"-Wextra" is also specified.
To suppress this warning use the "unused" attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute
"warn_unused_result" does not use its return value. The default is
-Wunused-result.
-Wunused-variable
Warn whenever a local or static variable is unused aside from its
declaration. This option implies -Wunused-const-variable=1 for C, but
not for C++. This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable
-Wunused-const-variable=n
Warn whenever a constant static variable is unused aside from its
declaration.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable=1
Warn about unused static const variables defined in the main
compilation unit, but not about static const variables declared
in any header included.
-Wunused-const-variable=1 is enabled by either -Wunused-variable
or -Wunused for C, but not for C++. In C this declares variable
storage, but in C++ this is not an error since const variables
take the place of "#define"s.
-Wunused-const-variable=2
This warning level also warns for unused constant static
variables in headers (excluding system headers). It is
equivalent to the short form -Wunused-const-variable. This level
must be explicitly requested in both C and C++ because it might
be hard to clean up all headers included.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not
used. To suppress this warning cast the unused expression to "void".
This includes an expression-statement or the left-hand side of a
comma expression that contains no side effects. For example, an
expression such as "x[i,j]" causes a warning, while "x[(void)i,j]"
does not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined, except those documented as
needing to be specified explicitly.
In order to get a warning about an unused function parameter, you
must either specify -Wextra -Wunused (note that -Wall implies
-Wunused), or separately specify -Wunused-parameter and/or
-Wunused-but-set-parameter.
-Wunused enables only -Wunused-const-variable=1 rather than
-Wunused-const-variable, and only for C, not C++.
-Wuse-after-free (C, Objective-C, C++ and Objective-C++ only)
-Wuse-after-free=n
Warn about uses of pointers to dynamically allocated objects that
have been rendered indeterminate by a call to a deallocation
function. The warning is enabled at all optimization levels but may
yield different results with optimization than without.
-Wuse-after-free=1
At level 1 the warning attempts to diagnose only unconditional
uses of pointers made indeterminate by a deallocation call or a
successful call to "realloc", regardless of whether or not the
call resulted in an actual reallocation of memory. This includes
double-"free" calls as well as uses in arithmetic and relational
expressions. Although undefined, uses of indeterminate pointers
in equality (or inequality) expressions are not diagnosed at this
level.
-Wuse-after-free=2
At level 2, in addition to unconditional uses, the warning also
diagnoses conditional uses of pointers made indeterminate by a
deallocation call. As at level 2, uses in equality (or
inequality) expressions are not diagnosed. For example, the
second call to "free" in the following function is diagnosed at
this level:
struct A { int refcount; void *data; };
void release (struct A *p)
{
int refcount = --p->refcount;
free (p);
if (refcount == 0)
free (p->data); // warning: p may be used after free
}
-Wuse-after-free=3
At level 3, the warning also diagnoses uses of indeterminate
pointers in equality expressions. All uses of indeterminate
pointers are undefined but equality tests sometimes appear after
calls to "realloc" as an attempt to determine whether the call
resulted in relocating the object to a different address. They
are diagnosed at a separate level to aid gradually transitioning
legacy code to safe alternatives. For example, the equality test
in the function below is diagnosed at this level:
void adjust_pointers (int**, int);
void grow (int **p, int n)
{
int **q = (int**)realloc (p, n *= 2);
if (q == p)
return;
adjust_pointers ((int**)q, n);
}
To avoid the warning at this level, store offsets into allocated
memory instead of pointers. This approach obviates needing to
adjust the stored pointers after reallocation.
-Wuse-after-free=2 is included in -Wall.
-Wuseless-cast (C, Objective-C, C++ and Objective-C++ only)
Warn when an expression is cast to its own type. This warning does
not occur when a class object is converted to a non-reference type as
that is a way to create a temporary:
struct S { };
void g (S&&);
void f (S&& arg)
{
g (S(arg)); // make arg prvalue so that it can bind to S&&
}
-Wuninitialized
Warn if an object with automatic or allocated storage duration is
used without having been initialized. In C++, also warn if a non-
static reference or non-static "const" member appears in a class
without constructors.
In addition, passing a pointer (or in C++, a reference) to an
uninitialized object to a "const"-qualified argument of a built-in
function known to read the object is also diagnosed by this warning.
(-Wmaybe-uninitialized is issued for ordinary functions.)
If you want to warn about code that uses the uninitialized value of
the variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized elements of
structure, union or array variables as well as for variables that are
uninitialized as a whole. They do not occur for variables or
elements declared "volatile". Because these warnings depend on
optimization, the exact variables or elements for which there are
warnings depend on the precise optimization options and version of
GCC used.
Note that there may be no warning about a variable that is used only
to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the warnings
are printed.
In C++, this warning also warns about using uninitialized objects in
member-initializer-lists. For example, GCC warns about "b" being
uninitialized in the following snippet:
struct A {
int a;
int b;
A() : a(b) { }
};
-Wno-invalid-memory-model
This option controls warnings for invocations of __atomic Builtins,
__sync Builtins, and the C11 atomic generic functions with a memory
consistency argument that is either invalid for the operation or
outside the range of values of the "memory_order" enumeration. For
example, since the "__atomic_store" and "__atomic_store_n" built-ins
are only defined for the relaxed, release, and sequentially
consistent memory orders the following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}
-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized
For an object with automatic or allocated storage duration, if there
exists a path from the function entry to a use of the object that is
initialized, but there exist some other paths for which the object is
not initialized, the compiler emits a warning if it cannot prove the
uninitialized paths are not executed at run time.
In addition, passing a pointer (or in C++, a reference) to an
uninitialized object to a "const"-qualified function argument is also
diagnosed by this warning. (-Wuninitialized is issued for built-in
functions known to read the object.) Annotating the function with
attribute "access (none)" indicates that the argument isn't used to
access the object and avoids the warning.
These warnings are only possible in optimizing compilation, because
otherwise GCC does not keep track of the state of variables.
These warnings are made optional because GCC may not be able to
determine when the code is correct in spite of appearing to have an
error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always
initialized, but GCC doesn't know this. To suppress the warning, you
need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might
be changed by a call to "longjmp". The compiler sees only the calls
to "setjmp". It cannot know where "longjmp" will be called; in fact,
a signal handler could call it at any point in the code. As a
result, you may get a warning even when there is in fact no problem
because "longjmp" cannot in fact be called at the place that would
cause a problem.
Some spurious warnings can be avoided if you declare all the
functions you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
by GCC. If this command-line option is used, warnings are even
issued for unknown pragmas in system header files. This is not the
case if the warnings are only enabled by the -Wall command-line
option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas. See also
-Wunknown-pragmas.
-Wno-pragma-once-outside-header
header file, such as a main file.
-Wno-prio-ctor-dtor
Do not warn if a priority from 0 to 100 is used for constructor or
destructor. The use of constructor and destructor attributes allow
you to assign a priority to the constructor/destructor to control its
order of execution before "main" is called or after it returns. The
priority values must be greater than 100 as the compiler reserves
priority values between 0--100 for the implementation.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that the
compiler is using for optimization. The warning does not catch all
cases, but does attempt to catch the more common pitfalls. It is
included in -Wall. It is equivalent to -Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that the
compiler is using for optimization. Higher levels correspond to
higher accuracy (fewer false positives). Higher levels also
correspond to more effort, similar to the way -O works.
-Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful
when higher levels do not warn but -fstrict-aliasing still breaks the
code, as it has very few false negatives. However, it has many false
positives. Warns for all pointer conversions between possibly
incompatible types, even if never dereferenced. Runs in the front
end only.
Level 2: Aggressive, quick, not too precise. May still have many
false positives (not as many as level 1 though), and few false
negatives (but possibly more than level 1). Unlike level 1, it only
warns when an address is taken. Warns about incomplete types. Runs
in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false
positives and few false negatives. Slightly slower than levels 1 or
2 when optimization is enabled. Takes care of the common
pun+dereference pattern in the front end: "*(int*)&some_float". If
optimization is enabled, it also runs in the back end, where it deals
with multiple statement cases using flow-sensitive points-to
information. Only warns when the converted pointer is dereferenced.
Does not warn about incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when signed overflow is undefined. It
warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does
not warn about all cases where the code might overflow: it only warns
about cases where the compiler implements some optimization. Thus
this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is
perfectly safe if the values of the variables involved are such that
overflow never does, in fact, occur. Therefore this warning can
easily give a false positive: a warning about code that is not
actually a problem. To help focus on important issues, several
warning levels are defined. No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a loop
requires, in particular when determining whether a loop will be
executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid.
For example the compiler simplifies "x + 1 > x" to 1. This level
of -Wstrict-overflow is enabled by -Wall; higher levels are not,
and must be explicitly requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to a
constant. For example: "abs (x) >= 0". This can only be
simplified when signed integer overflow is undefined, because
"abs (INT_MIN)" overflows to "INT_MIN", which is less than zero.
-Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified.
For example: "x + 1 > 1" is simplified to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above
cases. For example: "(x * 10) / 5" is simplified to "x * 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude of
a constant involved in a comparison. For example: "x + 2 > y" is
simplified to "x + 1 >= y". This is reported only at the highest
warning level because this simplification applies to many
comparisons, so this warning level gives a very large number of
false positives.
-Wstring-compare
Warn for calls to "strcmp" and "strncmp" whose result is determined
to be either zero or non-zero in tests for such equality owing to the
length of one argument being greater than the size of the array the
other argument is stored in (or the bound in the case of "strncmp").
Such calls could be mistakes. For example, the call to "strcmp"
below is diagnosed because its result is necessarily non-zero
irrespective of the contents of the array "a".
extern char a[4];
void f (char *d)
{
strcpy (d, "string");
...
if (0 == strcmp (a, d)) // cannot be true
puts ("a and d are the same");
}
-Wstring-compare is enabled by -Wextra.
-Wno-stringop-overflow
-Wstringop-overflow
-Wstringop-overflow=type
Warn for code that can be statically determined to cause buffer
overflows or memory overruns, such as calls to "memcpy" and "strcpy"
that overflow the destination buffer. The optional argument is one
greater than the type of Object Size Checking to perform to determine
the size of the destination. The argument is meaningful only for
string functions that operate on character arrays; raw memory
functions like "memcpy" always use type-zero Object Size Checking.
The option also warns for calls that specify a size in excess of the
largest possible object or at most "SIZE_MAX / 2" bytes.
The option produces the best results with optimization enabled but
can detect a small subset of simple buffer overflows even without
optimization in calls to the GCC built-in functions like
"__builtin_memcpy" that correspond to the standard functions. In any
case, the option warns about just a subset of buffer overflows
detected by the corresponding overflow checking built-ins, such as
"__builtin___memcpy_chk", which can perform run-time checking if the
access cannot be identified as safe at compile time.
For example, the option issues a warning for the "strcpy" call below
because it copies at least 5 characters (the string "blue" including
the terminating NUL) into the buffer of size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}
The effect of this option is not limited to string or memory
manipulation functions. In this example, a warning is diagnosed
because a 1-element array is passed to a function requiring at least
a 4-element array argument:
void f (int[static 4]);
void g (void)
{
int *p = (int *) malloc (1 * sizeof(int));
f (p); // warning here
}
Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow
-Wstringop-overflow=1
The -Wstringop-overflow=1 option uses type-zero Object Size
Checking to determine the sizes of destination objects. At this
setting the option does not warn for writes past the end of
subobjects of larger objects accessed by pointers unless the size
of the largest surrounding object is known. When the destination
may be one of several objects it is assumed to be the largest one
of them. On Linux systems, when optimization is enabled at this
setting the option warns for the same code as when the
"_FORTIFY_SOURCE" macro is defined to a non-zero value.
-Wstringop-overflow=2
The -Wstringop-overflow=2 option uses type-one Object Size
Checking to determine the sizes of destination objects. At this
setting the option warns about overflows when writing to members
of the largest complete objects whose exact size is known.
However, it does not warn for excessive writes to the same
members of unknown objects referenced by pointers since they may
point to arrays containing unknown numbers of elements. This is
the default setting of the option.
-Wstringop-overflow=3
The -Wstringop-overflow=3 option uses type-two Object Size
Checking to determine the sizes of destination objects. At this
setting the option warns about overflowing the smallest object or
data member. This is the most restrictive setting of the option
that may result in warnings for safe code.
-Wstringop-overflow=4
The -Wstringop-overflow=4 option uses type-three Object Size
Checking to determine the sizes of destination objects. At this
setting the option warns about overflowing any data members, and
when the destination is one of several objects it uses the size
of the largest of them to decide whether to issue a warning.
Similarly to -Wstringop-overflow=3 this setting of the option may
result in warnings for benign code.
-Wno-stringop-overread
Warn for calls to string manipulation functions such as "memchr", or
"strcpy" that are determined to read past the end of the source
sequence.
Option -Wstringop-overread is enabled by default.
-Wno-stringop-truncation
Do not warn for calls to bounded string manipulation functions such
as "strncat", "strncpy", and "stpncpy" that may either truncate the
copied string or leave the destination unchanged.
In the following example, the call to "strncat" specifies a bound
that is less than the length of the source string. As a result, the
copy of the source will be truncated and so the call is diagnosed.
To avoid the warning use "bufsize - strlen (buf) - 1)" as the bound.
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}
As another example, the following call to "strncpy" results in
copying to "d" just the characters preceding the terminating NUL,
without appending the NUL to the end. Assuming the result of
"strncpy" is necessarily a NUL-terminated string is a common mistake,
and so the call is diagnosed. To avoid the warning when the result
is not expected to be NUL-terminated, call "memcpy" instead.
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}
In the following example, the call to "strncpy" specifies the size of
the destination buffer as the bound. If the length of the source
string is equal to or greater than this size the result of the copy
will not be NUL-terminated. Therefore, the call is also diagnosed.
To avoid the warning, specify "sizeof buf - 1" as the bound and set
the last element of the buffer to "NUL".
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}
In situations where a character array is intended to store a sequence
of bytes with no terminating "NUL" such an array may be annotated
with attribute "nonstring" to avoid this warning. Such arrays,
however, are not suitable arguments to functions that expect
"NUL"-terminated strings. To help detect accidental misuses of such
arrays GCC issues warnings unless it can prove that the use is safe.
-Wstrict-flex-arrays (C and C++ only)
Warn about improper usages of flexible array members according to the
level of the "strict_flex_array (level)" attribute attached to the
trailing array field of a structure if it's available, otherwise
according to the level of the option -fstrict-flex-arrays=level.
"-Wstrict-flex-arrays" is effective only when level is greater than
0.
When level=1, warnings are issued for a trailing array reference of a
structure that have 2 or more elements if the trailing array is
referenced as a flexible array member.
When level=2, in addition to level=1, additional warnings are issued
for a trailing one-element array reference of a structure if the
array is referenced as a flexible array member.
When level=3, in addition to level=2, additional warnings are issued
for a trailing zero-length array reference of a structure if the
array is referenced as a flexible array member.
This option is more effective when -ftree-vrp is active (the default
for -O2 and above) but some warnings may be diagnosed even without
optimization.
-Wsuggest-attribute=attribute-name
Warn for cases where adding an attribute may be beneficial. The
attribute-names currently supported are listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
-Wmissing-noreturn
-Wsuggest-attribute=malloc
-Wsuggest-attribute=returns_nonnull
Warn about functions that might be candidates for attributes
"pure", "const", "noreturn", "malloc" or "returns_nonnull". The
compiler only warns for functions visible in other compilation
units or (in the case of "pure" and "const") if it cannot prove
that the function returns normally. A function returns normally
if it doesn't contain an infinite loop or return abnormally by
throwing, calling "abort" or trapping. This analysis requires
option -fipa-pure-const, which is enabled by default at -O and
higher. Higher optimization levels improve the accuracy of the
analysis.
-Wsuggest-attribute=format
-Wmissing-format-attribute
Warn about function pointers that might be candidates for
"format" attributes. Note these are only possible candidates,
not absolute ones. GCC guesses that function pointers with
"format" attributes that are used in assignment, initialization,
parameter passing or return statements should have a
corresponding "format" attribute in the resulting type. I.e. the
left-hand side of the assignment or initialization, the type of
the parameter variable, or the return type of the containing
function respectively should also have a "format" attribute to
avoid the warning.
GCC also warns about function definitions that might be
candidates for "format" attributes. Again, these are only
possible candidates. GCC guesses that "format" attributes might
be appropriate for any function that calls a function like
"vprintf" or "vscanf", but this might not always be the case, and
some functions for which "format" attributes are appropriate may
not be detected.
-Wsuggest-attribute=cold
Warn about functions that might be candidates for "cold"
attribute. This is based on static detection and generally only
warns about functions which always leads to a call to another
"cold" function such as wrappers of C++ "throw" or fatal error
reporting functions leading to "abort".
-Walloc-size
Warn about calls to allocation functions decorated with attribute
"alloc_size" that specify insufficient size for the target type of
the pointer the result is assigned to, including those to the built-
in forms of the functions "aligned_alloc", "alloca", "calloc",
"malloc", and "realloc".
-Walloc-zero
Warn about calls to allocation functions decorated with attribute
"alloc_size" that specify zero bytes, including those to the built-in
forms of the functions "aligned_alloc", "alloca", "calloc", "malloc",
and "realloc". Because the behavior of these functions when called
with a zero size differs among implementations (and in the case of
"realloc" has been deprecated) relying on it may result in subtle
portability bugs and should be avoided.
-Wcalloc-transposed-args
Warn about calls to allocation functions decorated with attribute
"alloc_size" with two arguments, which use "sizeof" operator as the
earlier size argument and don't use it as the later size argument.
This is a coding style warning. The first argument to "calloc" is
documented to be number of elements in array, while the second
argument is size of each element, so "calloc (n, sizeof (int))" is
preferred over "calloc (sizeof (int), n)". If "sizeof" in the
earlier argument and not the latter is intentional, the warning can
be suppressed by using "calloc (sizeof (struct S) + 0, n)" or "calloc
(1 * sizeof (struct S), 4)" or using "sizeof" in the later argument
as well.
-Walloc-size-larger-than=byte-size
Warn about calls to functions decorated with attribute "alloc_size"
that attempt to allocate objects larger than the specified number of
bytes, or where the result of the size computation in an integer type
with infinite precision would exceed the value of PTRDIFF_MAX on the
target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default.
Warnings controlled by the option can be disabled either by
specifying byte-size of SIZE_MAX or more or by
-Wno-alloc-size-larger-than.
-Wno-alloc-size-larger-than
Disable -Walloc-size-larger-than= warnings. The option is equivalent
to -Walloc-size-larger-than=SIZE_MAX or larger.
-Walloca
This option warns on all uses of "alloca" in the source.
-Walloca-larger-than=byte-size
This option warns on calls to "alloca" with an integer argument whose
value is either zero, or that is not bounded by a controlling
predicate that limits its value to at most byte-size. It also warns
for calls to "alloca" where the bound value is unknown. Arguments of
non-integer types are considered unbounded even if they appear to be
constrained to the expected range.
For example, a bounded case of "alloca" could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}
In the above example, passing "-Walloca-larger-than=1000" would not
issue a warning because the call to "alloca" is known to be at most
1000 bytes. However, if "-Walloca-larger-than=500" were passed, the
compiler would emit a warning.
Unbounded uses, on the other hand, are uses of "alloca" with no
controlling predicate constraining its integer argument. For
example:
void func ()
{
void *p = alloca (n);
f (p);
}
If "-Walloca-larger-than=500" were passed, the above would trigger a
warning, but this time because of the lack of bounds checking.
Note, that even seemingly correct code involving signed integers
could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}
In the above example, n could be negative, causing a larger than
expected argument to be implicitly cast into the "alloca" call.
This option also warns when "alloca" is used in a loop.
-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually
only effective when -ftree-vrp is active (default for -O2 and
above).
See also -Wvla-larger-than=byte-size.
-Wno-alloca-larger-than
Disable -Walloca-larger-than= warnings. The option is equivalent to
-Walloca-larger-than=SIZE_MAX or larger.
-Warith-conversion
Do warn about implicit conversions from arithmetic operations even
when conversion of the operands to the same type cannot change their
values. This affects warnings from -Wconversion, -Wfloat-conversion,
and -Wsign-conversion.
void f (char c, int i)
{
c = c + i; // warns with B<-Wconversion>
c = c + 1; // only warns with B<-Warith-conversion>
}
-Warray-bounds
-Warray-bounds=n
Warn about out of bounds subscripts or offsets into arrays. This
warning is enabled by -Wall. It is more effective when -ftree-vrp is
active (the default for -O2 and above) but a subset of instances are
issued even without optimization.
By default, the trailing array of a structure will be treated as a
flexible array member by -Warray-bounds or -Warray-bounds=n if it is
declared as either a flexible array member per C99 standard onwards
([]), a GCC zero-length array extension ([0]), or an one-element
array ([1]). As a result, out of bounds subscripts or offsets into
zero-length arrays or one-element arrays are not warned by default.
You can add the option -fstrict-flex-arrays or
-fstrict-flex-arrays=level to control how this option treat trailing
array of a structure as a flexible array member:
when level<=1, no change to the default behavior.
when level=2, additional warnings will be issued for out of bounds
subscripts or offsets into one-element arrays;
when level=3, in addition to level=2, additional warnings will be
issued for out of bounds subscripts or offsets into zero-length
arrays.
-Warray-bounds=1
This is the default warning level of -Warray-bounds and is
enabled by -Wall; higher levels are not, and must be explicitly
requested.
-Warray-bounds=2
This warning level also warns about the intermediate results of
pointer arithmetic that may yield out of bounds values. This
warning level may give a larger number of false positives and is
deactivated by default.
-Wunterminated-string-initialization (C and Objective-C only)
Warn about character arrays initialized as unterminated character
sequences with a string literal, unless the declaration being
initialized has the "nonstring" attribute. For example:
char arr[3] = "foo"; /* Warning. */
char arr2[3] __attribute__((nonstring)) = "bar"; /* No warning. */
This warning is enabled by -Wextra. If -Wc++-compat is enabled, the
warning has slightly different wording and warns even if the
declaration being initialized has the "nonstring" warning, as in C++
such initializations are an error.
-Warray-compare
Warn about equality and relational comparisons between two operands
of array type. This comparison was deprecated in C++20. For
example:
int arr1[5];
int arr2[5];
bool same = arr1 == arr2;
-Warray-compare is enabled by -Wall.
-Warray-parameter
-Warray-parameter=n
Warn about redeclarations of functions involving parameters of array
or pointer types of inconsistent kinds or forms, and enable the
detection of out-of-bounds accesses to such parameters by warnings
such as -Warray-bounds.
If the first function declaration uses the array form for a parameter
declaration, the bound specified in the array is assumed to be the
minimum number of elements expected to be provided in calls to the
function and the maximum number of elements accessed by it. Failing
to provide arguments of sufficient size or accessing more than the
maximum number of elements may be diagnosed by warnings such as
-Warray-bounds or -Wstringop-overflow. At level 1, the warning
diagnoses inconsistencies involving array parameters declared using
the "T[static N]" form.
For example, the warning triggers for the second declaration of "f"
because the first one with the keyword "static" specifies that the
array argument must have at least four elements, while the second
allows an array of any size to be passed to "f".
void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)
void g (void)
{
int *p = (int *)malloc (1 * sizeof (int));
f (p); // warning (array too small)
...
}
At level 2 the warning also triggers for redeclarations involving any
other inconsistency in array or pointer argument forms denoting array
sizes. Pointers and arrays of unspecified bound are considered
equivalent and do not trigger a warning.
void g (int*);
void g (int[]); // no warning
void g (int[8]); // warning (inconsistent array bound)
-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option
triggers warnings for similar inconsistencies involving Variable
Length Array arguments.
The short form of the option -Warray-parameter is equivalent to
-Warray-parameter=2. The negative form -Wno-array-parameter is
equivalent to -Warray-parameter=0.
-Wattribute-alias=n
-Wno-attribute-alias
Warn about declarations using the "alias" and similar attributes
whose target is incompatible with the type of the alias.
-Wattribute-alias=1
The default warning level of the -Wattribute-alias option
diagnoses incompatibilities between the type of the alias
declaration and that of its target. Such incompatibilities are
typically indicative of bugs.
-Wattribute-alias=2
At this level -Wattribute-alias also diagnoses cases where the
attributes of the alias declaration are more restrictive than the
attributes applied to its target. These mismatches can
potentially result in incorrect code generation. In other cases
they may be benign and could be resolved simply by adding the
missing attribute to the target. For comparison, see the
-Wmissing-attributes option, which controls diagnostics when the
alias declaration is less restrictive than the target, rather
than more restrictive.
Attributes considered include "alloc_align", "alloc_size",
"cold", "const", "hot", "leaf", "malloc", "nonnull", "noreturn",
"nothrow", "pure", "returns_nonnull", and "returns_twice".
-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the
default. You can disable these warnings with either
-Wno-attribute-alias or -Wattribute-alias=0.
-Wbidi-chars=[none|unpaired|any|ucn]
Warn about possibly misleading UTF-8 bidirectional control characters
in comments, string literals, character constants, and identifiers.
Such characters can change left-to-right writing direction into
right-to-left (and vice versa), which can cause confusion between the
logical order and visual order. This may be dangerous; for instance,
it may seem that a piece of code is not commented out, whereas it in
fact is.
There are three levels of warning supported by GCC. The default is
-Wbidi-chars=unpaired, which warns about improperly terminated bidi
contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any
warns about any use of bidirectional control characters.
By default, this warning does not warn about UCNs. It is, however,
possible to turn on such checking by using -Wbidi-chars=unpaired,ucn
or -Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid, and is
equivalent to -Wbidi-chars=unpaired,ucn, if no previous
-Wbidi-chars=any was specified.
-Wbool-compare
Warn about boolean expression compared with an integer value
different from "true"/"false". For instance, the following
comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
-Wbool-operation
Warn about suspicious operations on expressions of a boolean type.
For instance, bitwise negation of a boolean is very likely a bug in
the program. For C, this warning also warns about incrementing or
decrementing a boolean, which rarely makes sense. (In C++,
decrementing a boolean is always invalid. Incrementing a boolean is
invalid in C++17, and deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches
Warn when an if-else has identical branches. This warning detects
cases like
if (p != NULL)
return 0;
else
return 0;
It doesn't warn when both branches contain just a null statement.
This warning also warn for conditional operators:
int i = x ? *p : *p;
-Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain. For
instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }
-Wframe-address
Warn when the __builtin_frame_address or __builtin_return_address is
called with an argument greater than 0. Such calls may return
indeterminate values or crash the program. The warning is included
in -Wall.
-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being discarded.
Typically, the compiler warns if a "const char *" variable is passed
to a function that takes a "char *" parameter. This option can be
used to suppress such a warning.
-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are pointer targets
are being discarded. Typically, the compiler warns if a "const int
(*)[]" variable is passed to a function that takes a "int (*)[]"
parameter. This option can be used to suppress such a warning.
-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers that have
incompatible types. This warning is for cases not covered by
-Wno-pointer-sign, which warns for pointer argument passing or
assignment with different signedness.
By default, in C99 and later dialects of C, GCC treats this issue as
an error. The error can be downgraded to a warning using
-fpermissive (along with certain other errors), or for this error
alone, with -Wno-error=incompatible-pointer-types.
This warning is upgraded to an error by -pedantic-errors.
-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and pointer to
integer conversions. This warning is about implicit conversions; for
explicit conversions the warnings -Wno-int-to-pointer-cast and
-Wno-pointer-to-int-cast may be used.
By default, in C99 and later dialects of C, GCC treats this issue as
an error. The error can be downgraded to a warning using
-fpermissive (along with certain other errors), or for this error
alone, with -Wno-error=int-conversion.
This warning is upgraded to an error by -pedantic-errors.
-Wzero-as-null-pointer-constant
Warn when a literal 0 is used as null pointer constant.
-Wzero-length-bounds
Warn about accesses to elements of zero-length array members that
might overlap other members of the same object. Declaring interior
zero-length arrays is discouraged because accesses to them are
undefined.
For example, the first two stores in function "bad" are diagnosed
because the array elements overlap the subsequent members "b" and
"c". The third store is diagnosed by -Warray-bounds because it is
beyond the bounds of the enclosing object.
struct X { int a[0]; int b, c; };
struct X x;
void bad (void)
{
x.a[0] = 0; // -Wzero-length-bounds
x.a[1] = 1; // -Wzero-length-bounds
x.a[2] = 2; // -Warray-bounds
}
Option -Wzero-length-bounds is enabled by -Warray-bounds.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-
point division by zero is not warned about, as it can be a legitimate
way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the
assumption that they usually do not indicate real problems and would
only make the compiler output harder to read. Using this command-
line option tells GCC to emit warnings from system headers as if they
occurred in user code. However, note that using -Wall in conjunction
with this option does not warn about unknown pragmas in system
headers---for that, -Wunknown-pragmas must also be used.
-Wtautological-compare
Warn if a self-comparison always evaluates to true or false. This
warning detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }
This warning also warns about bitwise comparisons that always
evaluate to true or false, for instance:
if ((a & 16) == 10) { ... }
will always be false.
This warning is enabled by -Wall.
-Wtrailing-whitespace
-Wtrailing-whitespace=kind
Warn about trailing whitespace at the end of lines, including inside
of comments, but excluding trailing whitespace in raw string
literals. "-Wtrailing-whitespace" is equivalent to
"-Wtrailing-whitespace=blanks" and warns just about trailing space
and horizontal tab characters. "-Wtrailing-whitespace=any" warns
about those or trailing form feed or vertical tab characters.
"-Wno-trailing-whitespace" or "-Wtrailing-whitespace=none" disables
the warning, which is the default. This is a coding style warning.
-Wleading-whitespace=kind
Warn about style issues in leading whitespace, but not about the
amount of indentation. Some projects use coding styles where only
spaces are used for indentation, others use only tabs, others use
zero or more tabs (for multiples of "-ftabstop=n") followed by zero
or fewer than n spaces. No warning is emitted on lines which contain
solely whitespace (although "-Wtrailing-whitespace=" warning might be
emitted), no warnings are emitted inside of raw string literals.
Warnings are also emitted for leading whitespace inside of multi-line
comments. "-Wleading-whitespace=spaces" warns about leading
whitespace other than spaces for projects which want to indent just
by spaces. "-Wleading-whitespace=tabs" warns about leading
whitespace other than horizontal tabs for projects which want to
indent just by horizontal tabs. "-Wleading-whitespace=blanks" warns
about leading whitespace other than spaces and horizontal tabs, or
about horizontal tab after a space in the leading whitespace, or
about n or more consecutive spaces in leading whitespace (where n is
argument of "-ftabstop=n", 8 by default).
"-Wleading-whitespace=none" disables the warning, which is the
default. This is a coding style warning.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions. A
trampoline is a small piece of data or code that is created at run
time on the stack when the address of a nested function is taken, and
is used to call the nested function indirectly. For some targets, it
is made up of data only and thus requires no special treatment. But,
for most targets, it is made up of code and thus requires the stack
to be made executable in order for the program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you
need to compute (by analyzing the code, or in some other way) the
maximum or likely maximum error that the computation introduces, and
allow for it when performing comparisons (and when producing output,
but that's a different problem). In particular, instead of testing
for equality, you should check to see whether the two values have
ranges that overlap; and this is done with the relational operators,
so equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in traditional
and ISO C. Also warn about ISO C constructs that have no traditional
C equivalent, and/or problematic constructs that should be avoided.
* Macro parameters that appear within string literals in the macro
body. In traditional C macro replacement takes place within
string literals, but in ISO C it does not.
* In traditional C, some preprocessor directives did not exist.
Traditional preprocessors only considered a line to be a
directive if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C
understands but ignores because the # does not appear as the
first character on the line. It also suggests you hide
indenting them. Some traditional implementations do not
recognize "#elif", so this option suggests avoiding it
altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating-point
constant suffixes. (Traditional C does support the L suffix on
integer constants.) Note, these suffixes appear in macros
defined in the system headers of most modern systems, e.g. the
_MIN/_MAX macros in "<limits.h>". Use of these macros in user
code might normally lead to spurious warnings, however GCC's
integrated preprocessor has enough context to avoid warning in
these cases.
* A function declared external in one block and then used after the
end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one. This
construct is not accepted by some traditional C compilers.
* The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only
issued if the base of the constant is ten. I.e. hexadecimal or
octal values, which typically represent bit patterns, are not
warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a separate
namespace for labels.
* Initialization of unions. If the initializer is zero, the
warning is omitted. This is done under the assumption that the
zero initializer in user code appears conditioned on e.g.
"__STDC__" to avoid missing initializer warnings and relies on
default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating-point values and
vice versa. The absence of these prototypes when compiling with
traditional C causes serious problems. This is a subset of the
possible conversion warnings; for the full set use
-Wtraditional-conversion.
* Use of ISO C style function definitions. This warning
intentionally is not issued for prototype declarations or
variadic functions because these ISO C features appear in your
code when using libiberty's traditional C compatibility macros,
"PARAMS" and "VPARAMS". This warning is also bypassed for nested
functions because that feature is already a GCC extension and
thus not relevant to traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from
what would happen to the same argument in the absence of a prototype.
This includes conversions of fixed point to floating and vice versa,
and conversions changing the width or signedness of a fixed-point
argument except when the same as the default promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block. This
construct, known from C++, was introduced with ISO C99 and is by
default allowed in GCC. It is not supported by ISO C90.
This warning is upgraded to an error by -pedantic-errors.
-Wshadow
Warn whenever a local variable or type declaration shadows another
variable, parameter, type, class member (in C++), or instance
variable (in Objective-C) or whenever a built-in function is
shadowed. Note that in C++, the compiler warns if a local variable
shadows an explicit typedef, but not if it shadows a
struct/class/enum. If this warning is enabled, it includes also all
instances of local shadowing. This means that -Wno-shadow=local and
-Wno-shadow=compatible-local are ignored when -Wshadow is used. Same
as -Wshadow=global.
-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an instance variable in
an Objective-C method.
-Wshadow=global
Warn for any shadowing. Same as -Wshadow.
-Wshadow=local
Warn when a local variable shadows another local variable or
parameter.
-Wshadow=compatible-local
Warn when a local variable shadows another local variable or
parameter whose type is compatible with that of the shadowing
variable. In C++, type compatibility here means the type of the
shadowing variable can be converted to that of the shadowed variable.
The creation of this flag (in addition to -Wshadow=local) is based on
the idea that when a local variable shadows another one of
incompatible type, it is most likely intentional, not a bug or typo,
as shown in the following example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}
Since the two variable "i" in the example above have incompatible
types, enabling only -Wshadow=compatible-local does not emit a
warning. Because their types are incompatible, if a programmer
accidentally uses one in place of the other, type checking is
expected to catch that and emit an error or warning. Use of this
flag instead of -Wshadow=local can possibly reduce the number of
warnings triggered by intentional shadowing. Note that this also
means that shadowing "const char *i" by "char *i" does not emit a
warning.
This warning is also enabled by -Wshadow=local.
-Wlarger-than=byte-size
Warn whenever an object is defined whose size exceeds byte-size.
-Wlarger-than=PTRDIFF_MAX is enabled by default. Warnings controlled
by the option can be disabled either by specifying byte-size of
SIZE_MAX or more or by -Wno-larger-than.
Also warn for calls to bounded functions such as "memchr" or
"strnlen" that specify a bound greater than the largest possible
object, which is PTRDIFF_MAX bytes by default. These warnings can
only be disabled by -Wno-larger-than.
-Wno-larger-than
Disable -Wlarger-than= warnings. The option is equivalent to
-Wlarger-than=SIZE_MAX or larger.
-Wframe-larger-than=byte-size
Warn if the size of a function frame exceeds byte-size. The
computation done to determine the stack frame size is approximate and
not conservative. The actual requirements may be somewhat greater
than byte-size even if you do not get a warning. In addition, any
space allocated via "alloca", variable-length arrays, or related
constructs is not included by the compiler when determining whether
or not to issue a warning. -Wframe-larger-than=PTRDIFF_MAX is
enabled by default. Warnings controlled by the option can be
disabled either by specifying byte-size of SIZE_MAX or more or by
-Wno-frame-larger-than.
-Wno-frame-larger-than
Disable -Wframe-larger-than= warnings. The option is equivalent to
-Wframe-larger-than=SIZE_MAX or larger.
-Wfree-nonheap-object
Warn when attempting to deallocate an object that was either not
allocated on the heap, or by using a pointer that was not returned
from a prior call to the corresponding allocation function. For
example, because the call to "stpcpy" returns a pointer to the
terminating nul character and not to the beginning of the object, the
call to "free" below is diagnosed.
void f (char *p)
{
p = stpcpy (p, "abc");
// ...
free (p); // warning
}
-Wfree-nonheap-object is included in -Wall.
-Wstack-usage=byte-size
Warn if the stack usage of a function might exceed byte-size. The
computation done to determine the stack usage is conservative. Any
space allocated via "alloca", variable-length arrays, or related
constructs is included by the compiler when determining whether or
not to issue a warning.
The message is in keeping with the output of -fstack-usage.
* If the stack usage is fully static but exceeds the specified
amount, it's:
warning: stack usage is 1120 bytes
* If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
* If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled
by the option can be disabled either by specifying byte-size of
SIZE_MAX or more or by -Wno-stack-usage.
-Wno-stack-usage
Disable -Wstack-usage= warnings. The option is equivalent to
-Wstack-usage=SIZE_MAX or larger.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler cannot
assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler makes such
assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic without GNU
extensions, this option disables the warnings about non-ISO "printf"
/ "scanf" format width specifiers "I32", "I64", and "I" used on
Windows targets, which depend on the MS runtime.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type or
of "void". GNU C assigns these types a size of 1, for convenience in
calculations with "void *" pointers and pointers to functions. In
C++, warn also when an arithmetic operation involves "NULL". This
warning is also enabled by -Wpedantic.
This warning is upgraded to an error by -pedantic-errors.
-Wno-pointer-compare
Do not warn if a pointer is compared with a zero character constant.
This usually means that the pointer was meant to be dereferenced.
For example:
const char *p = foo ();
if (p == '\0')
return 42;
Note that the code above is invalid in C++11.
This warning is enabled by default.
-Wno-tsan
Disable warnings about unsupported features in ThreadSanitizer.
ThreadSanitizer does not support "std::atomic_thread_fence" and can
report false positives.
-Wtype-limits
Warn if a comparison is always true or always false due to the
limited range of the data type, but do not warn for constant
expressions. For example, warn if an unsigned variable is compared
against zero with "<" or ">=". This warning is also enabled by
-Wextra.
-Wabsolute-value (C and Objective-C only)
Warn for calls to standard functions that compute the absolute value
of an argument when a more appropriate standard function is
available. For example, calling "abs(3.14)" triggers the warning
because the appropriate function to call to compute the absolute
value of a double argument is "fabs". The option also triggers
warnings when the argument in a call to such a function has an
unsigned type. This warning can be suppressed with an explicit type
cast and it is also enabled by -Wextra.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or
whenever a backslash-newline appears in a // comment. This warning
is enabled by -Wall.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning
of the program. Trigraphs within comments are not warned about,
except those that would form escaped newlines.
This option is implied by -Wall. If -Wall is not given, this option
is still enabled unless trigraphs are enabled. To get trigraph
conversion without warnings, but get the other -Wall warnings, use
-trigraphs -Wall -Wno-trigraphs.
-Wundef
Warn if an undefined identifier is evaluated in an "#if" directive.
Such identifiers are replaced with zero.
-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of a macro
(including the case where the macro is expanded by an #if directive).
Such usage is not portable. This warning is also enabled by
-Wpedantic and -Wextra.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro
is used if it is expanded or tested for existence at least once. The
preprocessor also warns if the macro has not been used at the time it
is redefined or undefined.
Built-in macros, macros defined on the command line, and macros
defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped
conditional blocks, then the preprocessor reports it as unused. To
avoid the warning in such a case, you might improve the scope of the
macro's definition by, for example, moving it into the first skipped
block. Alternatively, you could provide a dummy use with something
like:
#if defined the_macro_causing_the_warning
#endif
-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are followed by text.
This sometimes happens in older programs with code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments. This warning is on
by default.
-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching type. For
example, warn if a call to a function returning an integer type is
cast to a pointer type.
-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present in ISO C99.
For instance, warn about use of variable length arrays, "long long"
type, "bool" type, compound literals, designated initializers, and so
on. This option is independent of the standards mode. Warnings are
disabled in the expression that follows "__extension__".
-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present in ISO C11.
For instance, warn about use of anonymous structures and unions,
"_Atomic" type qualifier, "_Thread_local" storage-class specifier,
"_Alignas" specifier, "Alignof" operator, "_Generic" keyword, and so
on. This option is independent of the standards mode. Warnings are
disabled in the expression that follows "__extension__".
-Wc11-c23-compat (C and Objective-C only)
-Wc11-c2x-compat (C and Objective-C only)
Warn about features not present in ISO C11, but present in ISO C23.
For instance, warn about omitting the string in "_Static_assert", use
of [[]] syntax for attributes, use of decimal floating-point types,
and so on. This option is independent of the standards mode.
Warnings are disabled in the expression that follows "__extension__".
The name -Wc11-c2x-compat is deprecated.
When not compiling in C23 mode, these warnings are upgraded to errors
by -pedantic-errors.
-Wc23-c2y-compat (C and Objective-C only)
-Wc23-c2y-compat (C and Objective-C only)
Warn about features not present in ISO C23, but present in ISO C2Y.
For instance, warn about "_Generic" selecting with a type name
instead of an expression. This option is independent of the
standards mode. Warnings are disabled in the expression that follows
"__extension__".
When not compiling in C2Y mode, these warnings are upgraded to errors
by -pedantic-errors.
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset of
ISO C and ISO C++, e.g. request for implicit conversion from "void *"
to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 1998
and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords
in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by
-Wall.
-Wc++14-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2011
and ISO C++ 2014. This warning is enabled by -Wall.
-Wc++17-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2014
and ISO C++ 2017. This warning is enabled by -Wall.
-Wc++20-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2017
and ISO C++ 2020. This warning is enabled by -Wall.
-Wno-c++11-extensions (C++ and Objective-C++ only)
Do not warn about C++11 constructs in code being compiled using an
older C++ standard. Even without this option, some C++11 constructs
will only be diagnosed if -Wpedantic is used.
-Wno-c++14-extensions (C++ and Objective-C++ only)
Do not warn about C++14 constructs in code being compiled using an
older C++ standard. Even without this option, some C++14 constructs
will only be diagnosed if -Wpedantic is used.
-Wno-c++17-extensions (C++ and Objective-C++ only)
Do not warn about C++17 constructs in code being compiled using an
older C++ standard. Even without this option, some C++17 constructs
will only be diagnosed if -Wpedantic is used.
-Wno-c++20-extensions (C++ and Objective-C++ only)
Do not warn about C++20 constructs in code being compiled using an
older C++ standard. Even without this option, some C++20 constructs
will only be diagnosed if -Wpedantic is used.
-Wno-c++23-extensions (C++ and Objective-C++ only)
Do not warn about C++23 constructs in code being compiled using an
older C++ standard. Even without this option, some C++23 constructs
will only be diagnosed if -Wpedantic is used.
-Wno-c++26-extensions (C++ and Objective-C++ only)
Do not warn about C++26 constructs in code being compiled using an
older C++ standard. Even without this option, some C++26 constructs
will only be diagnosed if -Wpedantic is used.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from
the target type. For example, warn if a "const char *" is cast to an
ordinary "char *".
Also warn when making a cast that introduces a type qualifier in an
unsafe way. For example, casting "char **" to "const char **" is
unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a "char *" is cast to
an "int *" on machines where integers can only be accessed at two- or
four-byte boundaries.
-Wcast-align=strict
Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a "char *" is cast to
an "int *" regardless of the target machine.
-Wcast-function-type
Warn when a function pointer is cast to an incompatible function
pointer. In a cast involving function types with a variable argument
list only the types of initial arguments that are provided are
considered. Any parameter of pointer-type matches any other pointer-
type. Any benign differences in integral types are ignored, like
"int" vs. "long" on ILP32 targets. Likewise type qualifiers are
ignored. The function type "void (*) (void)" is special and matches
everything, which can be used to suppress this warning. In a cast
involving pointer to member types this warning warns whenever the
type cast is changing the pointer to member type. This warning is
enabled by -Wextra.
-Wcast-user-defined
Warn when a cast to reference type does not involve a user-defined
conversion that the programmer might expect to be called.
struct A { operator const int&(); } a;
auto r = (int&)a; // warning
This warning is enabled by default.
-Wwrite-strings
When compiling C, give string constants the type "const char[length]"
so that copying the address of one into a non-"const" "char *"
pointer produces a warning. These warnings help you find at compile
time code that can try to write into a string constant, but only if
you have been very careful about using "const" in declarations and
prototypes. Otherwise, it is just a nuisance. This is why we did not
make -Wall request these warnings.
When compiling C++, warn about the deprecated conversion from string
literals to "char *". This warning is enabled by default for C++
programs.
This warning is upgraded to an error by -pedantic-errors in C++11
mode or later.
-Wclobbered
Warn for variables that might be changed by "longjmp" or "vfork".
This warning is also enabled by -Wextra.
-Wno-complain-wrong-lang
By default, language front ends complain when a command-line option
is valid, but not applicable to that front end. This may be disabled
with -Wno-complain-wrong-lang, which is mostly useful when invoking a
single compiler driver for multiple source files written in different
languages, for example:
$ g++ -fno-rtti a.cc b.f90
The driver g++ invokes the C++ front end to compile a.cc and the
Fortran front end to compile b.f90. The latter front end diagnoses
f951: Warning: command-line option '-fno-rtti' is valid for
C++/D/ObjC++ but not for Fortran, which may be disabled with
-Wno-complain-wrong-lang.
-Wcompare-distinct-pointer-types (C and Objective-C only)
Warn if pointers of distinct types are compared without a cast. This
warning is enabled by default.
-Wconversion
Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like "abs (x)" when "x" is
"double"; conversions between signed and unsigned, like "unsigned ui
= -1"; and conversions to smaller types, like "sqrtf (M_PI)". Do not
warn for explicit casts like "abs ((int) x)" and "ui = (unsigned)
-1", or if the value is not changed by the conversion like in "abs
(2.0)". Warnings about conversions between signed and unsigned
integers can be disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined
conversions; and conversions that never use a type conversion
operator: conversions to "void", the same type, a base class or a
reference to them. Warnings about conversions between signed and
unsigned integers are disabled by default in C++ unless
-Wsign-conversion is explicitly enabled.
Warnings about conversion from arithmetic on a small type back to
that type are only given with -Warith-conversion.
-Wdangling-else
Warn about constructions where there may be confusion to which "if"
statement an "else" branch belongs. Here is an example of such a
case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible "if"
statement, which in this example is "if (b)". This is often not what
the programmer expected, as illustrated in the above example by
indentation the programmer chose. When there is the potential for
this confusion, GCC issues a warning when this flag is specified. To
eliminate the warning, add explicit braces around the innermost "if"
statement so there is no way the "else" can belong to the enclosing
"if". The resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wparentheses.
-Wdangling-pointer
-Wdangling-pointer=n
Warn about uses of pointers (or C++ references) to objects with
automatic storage duration after their lifetime has ended. This
includes local variables declared in nested blocks, compound literals
and other unnamed temporary objects. In addition, warn about storing
the address of such objects in escaped pointers. The warning is
enabled at all optimization levels but may yield different results
with optimization than without.
-Wdangling-pointer=1
At level 1, the warning diagnoses only unconditional uses of
dangling pointers.
-Wdangling-pointer=2
At level 2, in addition to unconditional uses the warning also
diagnoses conditional uses of dangling pointers.
The short form -Wdangling-pointer is equivalent to
-Wdangling-pointer=2, while -Wno-dangling-pointer and
-Wdangling-pointer=0 have the same effect of disabling the warnings.
-Wdangling-pointer=2 is included in -Wall.
This example triggers the warning at level 1; the address of the
unnamed temporary is unconditionally referenced outside of its scope.
char f (char c1, char c2, char c3)
{
char *p;
{
p = (char[]) { c1, c2, c3 };
}
// warning: using dangling pointer 'p' to an unnamed temporary
return *p;
}
In the following function the store of the address of the local
variable "x" in the escaped pointer *p triggers the warning at level
1.
void g (int **p)
{
int x = 7;
// warning: storing the address of local variable 'x' in '*p'
*p = &x;
}
In this example, the array a is out of scope when the pointer s is
used. Since the code that sets "s" is conditional, the warning
triggers at level 2.
extern void frob (const char *);
void h (char *s)
{
if (!s)
{
char a[12] = "tmpname";
s = a;
}
// warning: dangling pointer 's' to 'a' may be used
frob (s);
}
-Wdate-time
Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are
encountered as they might prevent bit-wise-identical reproducible
compilations.
-Wempty-body
Warn if an empty body occurs in an "if", "else" or "do while"
statement. This warning is also enabled by -Wextra.
-Wno-endif-labels
Do not warn about stray tokens after "#else" and "#endif".
-Wenum-compare
Warn about a comparison between values of different enumerated types.
In C++ enumerated type mismatches in conditional expressions are also
diagnosed and the warning is enabled by default. In C this warning
is enabled by -Wall.
-Wenum-conversion
Warn when a value of enumerated type is implicitly converted to a
different enumerated type. This warning is enabled by -Wextra in C.
-Wenum-int-mismatch (C and Objective-C only)
Warn about mismatches between an enumerated type and an integer type
in declarations. For example:
enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);
In C, an enumerated type is compatible with "char", a signed integer
type, or an unsigned integer type. However, since the choice of the
underlying type of an enumerated type is implementation-defined, such
mismatches may cause portability issues. In C++, such mismatches are
an error. In C, this warning is enabled by -Wall and -Wc++-compat.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward
across the initialization of a variable, or jumps backward to a label
after the variable has been initialized. This only warns about
variables that are initialized when they are declared. This warning
is only supported for C and Objective-C; in C++ this sort of branch
is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled
with the -Wno-jump-misses-init option.
-Wsign-compare
Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. In C++, this warning is also enabled by -Wall. In C, it
is also enabled by -Wextra.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an integer
value, like assigning a signed integer expression to an unsigned
integer variable. An explicit cast silences the warning. In C, this
option is enabled also by -Wconversion.
-Wflex-array-member-not-at-end (C and C++ only)
Warn when a structure containing a C99 flexible array member as the
last field is not at the end of another structure. This warning
warns e.g. about
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };
-Wfloat-conversion
Warn for implicit conversions that reduce the precision of a real
value. This includes conversions from real to integer, and from
higher precision real to lower precision real values. This option is
also enabled by -Wconversion.
-Wno-scalar-storage-order
Do not warn on suspicious constructs involving reverse scalar storage
order.
-Wsizeof-array-div
Warn about divisions of two sizeof operators when the first one is
applied to an array and the divisor does not equal the size of the
array element. In such a case, the computation will not yield the
number of elements in the array, which is likely what the user
intended. This warning warns e.g. about
int fn ()
{
int arr[10];
return sizeof (arr) / sizeof (short);
}
This warning is enabled by -Wall.
-Wsizeof-pointer-div
Warn for suspicious divisions of two sizeof expressions that divide
the pointer size by the element size, which is the usual way to
compute the array size but won't work out correctly with pointers.
This warning warns e.g. about "sizeof (ptr) / sizeof (ptr[0])" if
"ptr" is not an array, but a pointer. This warning is enabled by
-Wall.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory
built-in functions if the argument uses "sizeof". This warning
triggers for example for "memset (ptr, 0, sizeof (ptr));" if "ptr" is
not an array, but a pointer, and suggests a possible fix, or about
"memcpy (&foo, ptr, sizeof (&foo));". -Wsizeof-pointer-memaccess
also warns about calls to bounded string copy functions like
"strncat" or "strncpy" that specify as the bound a "sizeof"
expression of the source array. For example, in the following
function the call to "strncat" specifies the size of the source
string as the bound. That is almost certainly a mistake and so the
call is diagnosed.
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}
The -Wsizeof-pointer-memaccess option is enabled by -Wall.
-Wno-sizeof-array-argument
Do not warn when the "sizeof" operator is applied to a parameter that
is declared as an array in a function definition. This warning is
enabled by default for C and C++ programs.
-Wmemset-elt-size
Warn for suspicious calls to the "memset" built-in function, if the
first argument references an array, and the third argument is a
number equal to the number of elements, but not equal to the size of
the array in memory. This indicates that the user has omitted a
multiplication by the element size. This warning is enabled by
-Wall.
-Wmemset-transposed-args
Warn for suspicious calls to the "memset" built-in function where the
second argument is not zero and the third argument is zero. For
example, the call "memset (buf, sizeof buf, 0)" is diagnosed because
"memset (buf, 0, sizeof buf)" was meant instead. The diagnostic is
only emitted if the third argument is a literal zero. Otherwise, if
it is an expression that is folded to zero, or a cast of zero to some
type, it is far less likely that the arguments have been mistakenly
transposed and no warning is emitted. This warning is enabled by
-Wall.
-Waddress
Warn about suspicious uses of address expressions. These include
comparing the address of a function or a declared object to the null
pointer constant such as in
void f (void);
void g (void)
{
if (!f) // warning: expression evaluates to false
abort ();
}
comparisons of a pointer to a string literal, such as in
void f (const char *x)
{
if (x == "abc") // warning: expression evaluates to false
puts ("equal");
}
and tests of the results of pointer addition or subtraction for
equality to null, such as in
void f (const int *p, int i)
{
return p + i == NULL;
}
Such uses typically indicate a programmer error: the address of most
functions and objects necessarily evaluates to true (the exception
are weak symbols), so their use in a conditional might indicate
missing parentheses in a function call or a missing dereference in an
array expression. The subset of the warning for object pointers can
be suppressed by casting the pointer operand to an integer type such
as "intptr_t" or "uintptr_t". Comparisons against string literals
result in unspecified behavior and are not portable, and suggest the
intent was to call "strcmp". The warning is suppressed if the
suspicious expression is the result of macro expansion. -Waddress
warning is enabled by -Wall.
-Wno-address-of-packed-member
Do not warn when the address of packed member of struct or union is
taken, which usually results in an unaligned pointer value. This is
enabled by default.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions. This
includes using logical operators in contexts where a bit-wise
operator is likely to be expected. Also warns when the operands of a
logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }
-Wlogical-not-parentheses
Warn about logical not used on the left hand side operand of a
comparison. This option does not warn if the right operand is
considered to be a boolean expression. Its purpose is to detect
suspicious code like the following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into
parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
-Waggregate-return
Warn if any functions that return structures or unions are defined or
called. (In languages where you can return an array, this also
elicits a warning.)
-Wno-aggressive-loop-optimizations
Do not warn if the compiler detects undefined behavior in a loop with
a constant number of iterations. -Waggressive-loop-optimizations is
enabled by default.
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This does not stop errors for incorrect use of supported
attributes.
Warnings about ill-formed uses of standard attributes are upgraded to
errors by -pedantic-errors.
Additionally, using -Wno-attributes=, it is possible to suppress
warnings about unknown scoped attributes (in C++11 and C23). For
example, -Wno-attributes=vendor::attr disables warning about the
following declaration:
[[vendor::attr]] void f();
It is also possible to disable warning about all attributes in a
namespace using -Wno-attributes=vendor:: which prevents warning about
both of these declarations:
[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();
Note that -Wno-attributes= does not imply -Wno-attributes.
-Wno-builtin-declaration-mismatch
Warn if a built-in function is declared with an incompatible
signature or as a non-function, or when a built-in function declared
with a type that does not include a prototype is called with
arguments whose promoted types do not match those expected by the
function. When -Wextra is specified, also warn when a built-in
function that takes arguments is declared without a prototype. The
-Wbuiltin-declaration-mismatch warning is enabled by default. To
avoid the warning include the appropriate header to bring the
prototypes of built-in functions into scope.
For example, the call to "memset" below is diagnosed by the warning
because the function expects a value of type "size_t" as its argument
but the type of 32 is "int". With -Wextra, the declaration of the
function is diagnosed as well.
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This
suppresses warnings for redefinition of "__TIMESTAMP__", "__TIME__",
"__DATE__", "__FILE__", and "__BASE_FILE__".
-Wfree-labels (C and Objective-C only)
Warn if a label is applied to a non-statement, or occurs at the end
of a compound statement. Such labels are allowed by C23 and later
dialects of C, and are available as a GCC extension in all other
dialects.
This warning is also enabled by -Wc11-c23-compat. It is turned into
an error if building for a C version before C23 by -pedantic-errors.
-Wheader-guard
Warn if a valid preprocessor header multiple inclusion guard has a
"#define" directive right after "#ifndef" or "#if !defined" directive
for the multiple inclusion guard, which defines a different macro
from the guard macro with a similar name, the actual multiple
inclusion guard macro isn't defined at the corresponding "#ifndef"
directive at the end of the header, and the "#define" directive
defines an object-like macro with empty definition. In such case, it
often is just a misspelled guard name, either in the "#ifndef" or
"#if !defined" directive or in the subsequent "#define" directive.
This warning is enabled by -Wall.
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration that specifies the
argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
"static" are not the first things in a declaration. This warning is
also enabled by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is given
even if there is a previous prototype. A definition using () is not
considered an old-style definition in C23 mode, because it is
equivalent to (void) in that case, but is considered an old-style
definition for older standards.
-Wdeprecated-non-prototype (C and Objective-C only)
Warn if a function declared with an empty parameter list () is called
with one or more arguments, or if a function definition with one or
more parameters is encountered after such a declaration. Both cases
are errors in C23 and later dialects of C.
This warning is also enabled by -Wc11-c23-compat.
-Wmissing-parameter-name (C and Objective-C only)
Warn if a function definition omits a parameter name, specifying only
its type. This can be used to document that a parameter is unused in
the definition. It is part of C23 and later dialects of C, and
available as a GCC extension in all other dialects.
This warning is also enabled by -Wc11-c23-compat. It is turned into
an error if building for a C version before C23 by -pedantic-errors.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wno-declaration-missing-parameter-type (C and Objective-C only)
Do not warn if a function declaration contains a parameter name
without a type. Such function declarations do not provide a function
prototype and prevent most type checking in function calls.
This warning is enabled by default. In C99 and later dialects of C,
it is treated as an error. The error can be downgraded to a warning
using -fpermissive (along with certain other errors), or for this
error alone, with -Wno-error=declaration-missing-parameter-type.
This warning is upgraded to an error by -pedantic-errors.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. Use this option to detect global functions
that do not have a matching prototype declaration in a header file.
This option is not valid for C++ because all function declarations
provide prototypes and a non-matching declaration declares an
overload rather than conflict with an earlier declaration. Use
-Wmissing-declarations to detect missing declarations in C++.
-Wmissing-variable-declarations (C and Objective-C only)
Warn if a global variable is defined without a previous declaration.
Use this option to detect global variables that do not have a
matching extern declaration in a header file.
-Wmissing-declarations
Warn if a global function is defined without a previous declaration.
Do so even if the definition itself provides a prototype. Use this
option to detect global functions that are not declared in header
files. In C, no warnings are issued for functions with previous non-
prototype declarations; use -Wmissing-prototypes to detect missing
prototypes. In C++, no warnings are issued for function templates,
or for inline functions, or for functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For
example, the following code causes such a warning, because "x.h" is
implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
In C this option does not warn about designated initializers, so the
following modification does not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C this option does not warn about the universal zero initializer {
0 }:
struct s { int f, g, h; };
struct s x = { 0 };
Likewise, in C++ this option does not warn about the empty { }
initializer, for example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other -Wextra warnings
without this one, use -Wextra -Wno-missing-field-initializers.
-Wno-missing-requires
By default, the compiler warns about a concept-id appearing as a
C++20 simple-requirement:
bool satisfied = requires { C<T> };
Here satisfied will be true if C<T> is a valid expression, which it
is for all T. Presumably the user meant to write
bool satisfied = requires { requires C<T> };
so satisfied is only true if concept C is satisfied for type T.
This warning can be disabled with -Wno-missing-requires.
-Wno-missing-template-keyword
The member access tokens ., -> and :: must be followed by the
"template" keyword if the parent object is dependent and the member
being named is a template.
template <class X>
void DoStuff (X x)
{
x.template DoSomeOtherStuff<X>(); // Good.
x.DoMoreStuff<X>(); // Warning, x is dependent.
}
In rare cases it is possible to get false positives. To silence this,
wrap the expression in parentheses. For example, the following is
treated as a template, even where m and N are integers:
void NotATemplate (my_class t)
{
int N = 5;
bool test = t.m < N > (0); // Treated as a template.
test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}
This warning can be disabled with -Wno-missing-template-keyword.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually
they indicate a typo in the user's code, as they have implementation-
defined values, and should not be used in portable code.
-Wnormalized=[none|id|nfc|nfkc]
In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters. However, sometimes when
characters outside the basic ASCII character set are used, you can
have two different character sequences that look the same. To avoid
confusion, the ISO 10646 standard sets out some normalization rules
which when applied ensure that two sequences that look the same are
turned into the same sequence. GCC can warn you if you are using
identifiers that have not been normalized; this option controls that
warning.
There are four levels of warning supported by GCC. The default is
-Wnormalized=nfc, which warns about any identifier that is not in the
ISO 10646 "C" normalized form, NFC. NFC is the recommended form for
most uses. It is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by
ISO C and ISO C++ that, when turned into NFC, are not allowed in
identifiers. That is, there's no way to use these symbols in
portable ISO C or C++ and have all your identifiers in NFC.
-Wnormalized=id suppresses the warning for these characters. It is
hoped that future versions of the standards involved will correct
this, which is why this option is not the default.
You can switch the warning off for all characters by writing
-Wnormalized=none or -Wno-normalized. You should only do this if you
are using some other normalization scheme (like "D"), because
otherwise you can easily create bugs that are literally impossible to
see.
Some characters in ISO 10646 have distinct meanings but look
identical in some fonts or display methodologies, especially once
formatting has been applied. For instance "\u207F", "SUPERSCRIPT
LATIN SMALL LETTER N", displays just like a regular "n" that has been
placed in a superscript. ISO 10646 defines the NFKC normalization
scheme to convert all these into a standard form as well, and GCC
warns if your code is not in NFKC if you use -Wnormalized=nfkc. This
warning is comparable to warning about every identifier that contains
the letter O because it might be confused with the digit 0, and so is
not the default, but may be useful as a local coding convention if
the programming environment cannot be fixed to display these
characters distinctly.
-Wno-attribute-warning
Do not warn about usage of functions declared with "warning"
attribute. By default, this warning is enabled.
-Wno-attribute-warning can be used to disable the warning or
-Wno-error=attribute-warning can be used to disable the error when
compiled with -Werror flag.
-Wno-deprecated
Do not warn about usage of deprecated features.
In C++, explicitly specifying -Wdeprecated also enables warnings
about some features that are deprecated in later language standards,
specifically -Wcomma-subscript, -Wvolatile,
-Wdeprecated-enum-float-conversion,
-Wdeprecated-enum-enum-conversion, -Wdeprecated-literal-operator, and
-Wdeprecated-variadic-comma-omission.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as
deprecated by using the "deprecated" attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Wno-odr
Warn about One Definition Rule violations during link-time
optimization. Enabled by default.
-Wopenacc-parallelism
Warn about potentially suboptimal choices related to OpenACC
parallelism.
-Wno-openmp
Warn about suspicious OpenMP code.
-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP simd directive
set by user. The -fsimd-cost-model=unlimited option can be used to
relax the cost model.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden when
using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings
without this one, use -Wextra -Wno-override-init.
-Wno-override-init-side-effects (C and Objective-C only)
Do not warn if an initialized field with side effects is overridden
when using designated initializers. This warning is enabled by
default.
-Wpacked
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure. Such
structures may be mis-aligned for little benefit. For instance, in
this code, the variable "f.x" in "struct bar" is misaligned even
though "struct bar" does not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wnopacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on
bit-fields of type "char". This was fixed in GCC 4.4 but the change
can lead to differences in the structure layout. GCC informs you
when the offset of such a field has changed in GCC 4.4. For example
there is no longer a 4-bit padding between field "a" and "b" in this
structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use -Wno-packed-bitfield-compat
to disable this warning.
-Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only)
Warn if a structure field with explicitly specified alignment in a
packed struct or union is misaligned. For example, a warning will be
issued on "struct S", like, "warning: alignment 1 of 'struct S' is
less than 8", in this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};
This warning is enabled by -Wall.
-Wpadded
Warn if padding is included in a structure, either to align an
element of the structure or to align the whole structure. Sometimes
when this happens it is possible to rearrange the fields of the
structure to reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even
in cases where multiple declaration is valid and changes nothing.
-Wrestrict
Warn when an object referenced by a "restrict"-qualified parameter
(or, in C++, a "__restrict"-qualified parameter) is aliased by
another argument, or when copies between such objects overlap. For
example, the call to the "strcpy" function below attempts to truncate
the string by replacing its initial characters with the last four.
However, because the call writes the terminating NUL into "a[4]", the
copies overlap and the call is diagnosed.
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}
The -Wrestrict option detects some instances of simple overlap even
without optimization but works best at -O2 and above. It is included
in -Wall.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
-Winline
Warn if a function that is declared as inline cannot be inlined.
Even with this option, the compiler does not warn about failures to
inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or not
to inline a function. For example, the compiler takes into account
the size of the function being inlined and the amount of inlining
that has already been done in the current function. Therefore,
seemingly insignificant changes in the source program can cause the
warnings produced by -Winline to appear or disappear.
-Winterference-size
Warn about use of C++17 "std::hardware_destructive_interference_size"
without specifying its value with --param destructive-interference-
size. Also warn about questionable values for that option.
This variable is intended to be used for controlling class layout, to
avoid false sharing in concurrent code:
struct independent_fields {
alignas(std::hardware_destructive_interference_size)
std::atomic<int> one;
alignas(std::hardware_destructive_interference_size)
std::atomic<int> two;
};
Here one and two are intended to be far enough apart that stores to
one won't require accesses to the other to reload the cache line.
By default, --param destructive-interference-size and --param
constructive-interference-size are set based on the current -mtune
option, typically to the L1 cache line size for the particular target
CPU, sometimes to a range if tuning for a generic target. So all
translation units that depend on ABI compatibility for the use of
these variables must be compiled with the same -mtune (or -mcpu).
If ABI stability is important, such as if the use is in a header for
a library, you should probably not use the hardware interference size
variables at all. Alternatively, you can force a particular value
with --param.
If you are confident that your use of the variable does not affect
ABI outside a single build of your project, you can turn off the
warning with -Wno-interference-size.
-Wint-in-bool-context
Warn for suspicious use of integer values where boolean values are
expected, such as conditional expressions (?:) using non-boolean
integer constants in boolean context, like "if (a <= b ? 2 : 3)". Or
left shifting of signed integers in boolean context, like "for (a =
0; 1 << a; a++);". Likewise for all kinds of multiplications
regardless of the data type. This warning is enabled by -Wall.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a
different size. In C++, casting to a pointer type of smaller size is
an error. Wint-to-pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a
different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but cannot
be used.
-Winvalid-utf8
Warn if an invalid UTF-8 character is found. This warning is on by
default for C++23 if -finput-charset=UTF-8 is used and turned into
error with -pedantic-errors.
-Wno-unicode
Don't diagnose invalid forms of delimited or named escape sequences
which are treated as separate tokens. Wunicode is enabled by
default.
-Wlong-long
Warn if "long long" type is used. This is enabled by either
-Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit
the warning messages, use -Wno-long-long.
This warning is upgraded to an error by -pedantic-errors.
-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the GNU
alternate syntax is used in ISO C99 mode. This is enabled by either
-Wpedantic or -Wtraditional. To inhibit the warning messages, use
-Wno-variadic-macros.
-Wno-varargs
Do not warn upon questionable usage of the macros used to handle
variable arguments like "va_start". These warnings are enabled by
default.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities of
the architecture. Mainly useful for the performance tuning. Vector
operation can be implemented "piecewise", which means that the scalar
operation is performed on every vector element; "in parallel", which
means that the vector operation is implemented using scalars of wider
type, which normally is more performance efficient; and "as a single
scalar", which means that vector fits into a scalar type.
-Wvla
Warn if a variable-length array is used in the code. -Wno-vla
prevents the -Wpedantic warning of the variable-length array.
This warning is upgraded to an error by -pedantic-errors.
-Wvla-larger-than=byte-size
If this option is used, the compiler warns for declarations of
variable-length arrays whose size is either unbounded, or bounded by
an argument that allows the array size to exceed byte-size bytes.
This is similar to how -Walloca-larger-than=byte-size works, but with
variable-length arrays.
Note that GCC may optimize small variable-length arrays of a known
value into plain arrays, so this warning may not get triggered for
such arrays.
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically
only effective when -ftree-vrp is active (default for -O2 and above).
See also -Walloca-larger-than=byte-size.
-Wno-vla-larger-than
Disable -Wvla-larger-than= warnings. The option is equivalent to
-Wvla-larger-than=SIZE_MAX or larger.
-Wvla-parameter
Warn about redeclarations of functions involving arguments of
Variable Length Array types of inconsistent kinds or forms, and
enable the detection of out-of-bounds accesses to such parameters by
warnings such as -Warray-bounds.
If the first function declaration uses the VLA form the bound
specified in the array is assumed to be the minimum number of
elements expected to be provided in calls to the function and the
maximum number of elements accessed by it. Failing to provide
arguments of sufficient size or accessing more than the maximum
number of elements may be diagnosed.
For example, the warning triggers for the following redeclarations
because the first one allows an array of any size to be passed to "f"
while the second one specifies that the array argument must have at
least "n" elements. In addition, calling "f" with the associated VLA
bound parameter in excess of the actual VLA bound triggers a warning
as well.
void f (int n, int[n]);
// warning: argument 2 previously declared as a VLA
void f (int, int[]);
void g (int n)
{
if (n > 4)
return;
int a[n];
// warning: access to a by f may be out of bounds
f (sizeof a, a);
...
}
-Wvla-parameter is included in -Wall. The -Warray-parameter option
triggers warnings for similar problems involving ordinary array
arguments.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate reads
and/or writes to register variables. This warning is enabled by
-Wall.
-Wno-xor-used-as-pow (C, C++, Objective-C and Objective-C++ only)
Disable warnings about uses of "^", the exclusive or operator, where
it appears the code meant exponentiation. Specifically, the warning
occurs when the left-hand side is the decimal constant 2 or 10 and
the right-hand side is also a decimal constant.
In C and C++, "^" means exclusive or, whereas in some other languages
(e.g. TeX and some versions of BASIC) it means exponentiation.
This warning can be silenced by converting one of the operands to
hexadecimal as well as by compiling with -Wno-xor-used-as-pow.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does
not generally indicate that there is anything wrong with your code;
it merely indicates that GCC's optimizers are unable to handle the
code effectively. Often, the problem is that your code is too big or
too complex; GCC refuses to optimize programs when the optimization
itself is likely to take inordinate amounts of time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different
signedness. This option is only supported for C and Objective-C. It
is implied by -Wall and by -Wpedantic, which can be disabled with
-Wno-pointer-sign.
This warning is upgraded to an error by -pedantic-errors.
-Wstack-protector
This option is only active when -fstack-protector is active. It
warns about functions that are not protected against stack smashing.
-Woverlength-strings
Warn about string constants that are longer than the "minimum
maximum" length specified in the C standard. Modern compilers
generally allow string constants that are much longer than the
standard's minimum limit, but very portable programs should avoid
using longer strings.
The limit applies after string constant concatenation, and does not
count the trailing NUL. In C90, the limit was 509 characters; in
C99, it was raised to 4095. C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with
-Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have a
suffix. When used together with -Wsystem-headers it warns about such
constants in system header files. This can be useful when preparing
code to use with the "FLOAT_CONST_DECIMAL64" pragma from the decimal
floating-point extension to C99.
-Wno-lto-type-mismatch
During the link-time optimization, do not warn about type mismatches
in global declarations from different compilation units. Requires
-flto to be enabled. Enabled by default.
-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used to initialize
a structure that has been marked with the "designated_init"
attribute.
Options That Control Static Analysis
-fanalyzer
This option enables an static analysis of program flow which looks
for "interesting" interprocedural paths through the code, and issues
warnings for problems found on them.
This analysis is much more expensive than other GCC warnings.
In technical terms, it performs coverage-guided symbolic execution of
the code being compiled. It is neither sound nor complete: it can
have false positives and false negatives. It is a bug-finding tool,
rather than a tool for proving program correctness.
The analyzer is only suitable for use on C code in this release.
Enabling this option effectively enables the following warnings:
-Wanalyzer-allocation-size -Wanalyzer-deref-before-check
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close
-Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion
-Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-undefined-behavior-ptrdiff
-Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value -Wanalyzer-va-arg-type-mismatch
-Wanalyzer-va-list-exhausted -Wanalyzer-va-list-leak
-Wanalyzer-va-list-use-after-va-end -Wanalyzer-write-to-const
-Wanalyzer-write-to-string-literal
This option is only available if GCC was configured with analyzer
support enabled.
-Wanalyzer-symbol-too-complex
If -fanalyzer is enabled, the analyzer uses various heuristics to
attempt to track the state of memory, but these can be defeated by
sufficiently complicated code.
By default, the analysis silently stops tracking values of
expressions if they exceed the threshold defined by --param
analyzer-max-svalue-depth=value, and falls back to an imprecise
representation for such expressions. The
-Wanalyzer-symbol-too-complex option warns if this occurs.
-Wanalyzer-too-complex
If -fanalyzer is enabled, the analyzer uses various heuristics to
attempt to explore the control flow and data flow in the program, but
these can be defeated by sufficiently complicated code.
By default, the analysis silently stops if the code is too
complicated for the analyzer to fully explore and it reaches an
internal limit. The -Wanalyzer-too-complex option warns if this
occurs.
-Wno-analyzer-allocation-size
This warning requires -fanalyzer, which enables it; to disable it,
use -Wno-analyzer-allocation-size.
This diagnostic warns for paths through the code in which a pointer
to a buffer is assigned to point at a buffer with a size that is not
a multiple of "sizeof (*pointer)".
See CWE-131: Incorrect Calculation of Buffer Size
("https://cwe.mitre.org/data/definitions/131.html").
-Wno-analyzer-deref-before-check
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-deref-before-check to disable it.
This diagnostic warns for paths through the code in which a pointer
is checked for "NULL" *after* it has already been dereferenced,
suggesting that the pointer could have been NULL. Such cases suggest
that the check for NULL is either redundant, or that it needs to be
moved to before the pointer is dereferenced.
This diagnostic also considers values passed to a function argument
marked with "__attribute__((nonnull))" as requiring a non-NULL value,
and thus will complain if such values are checked for "NULL" after
returning from such a function call.
This diagnostic is unlikely to be reported when any level of
optimization is enabled, as GCC's optimization logic will typically
consider such checks for NULL as being redundant, and optimize them
away before the analyzer "sees" them. Hence optimization should be
disabled when attempting to trigger this diagnostic.
-Wno-analyzer-double-fclose
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-double-fclose to disable it.
This diagnostic warns for paths through the code in which a "FILE *"
can have "fclose" called on it more than once.
See CWE-1341: Multiple Releases of Same Resource or Handle
("https://cwe.mitre.org/data/definitions/1341.html").
-Wno-analyzer-double-free
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-double-free to disable it.
This diagnostic warns for paths through the code in which a pointer
can have a deallocator called on it more than once, either "free", or
a deallocator referenced by attribute "malloc".
See CWE-415: Double Free
("https://cwe.mitre.org/data/definitions/415.html").
-Wno-analyzer-exposure-through-output-file
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-exposure-through-output-file to disable it.
This diagnostic warns for paths through the code in which a security-
sensitive value is written to an output file (such as writing a
password to a log file).
See CWE-532: Information Exposure Through Log Files
("https://cwe.mitre.org/data/definitions/532.html").
-Wanalyzer-exposure-through-uninit-copy
This warning requires both -fanalyzer and the use of a plugin to
specify a function that copies across a "trust boundary". Use
-Wno-analyzer-exposure-through-uninit-copy to disable it.
This diagnostic warns for "infoleaks" - paths through the code in
which uninitialized values are copied across a security boundary
(such as code within an OS kernel that copies a partially-initialized
struct on the stack to user space).
See CWE-200: Exposure of Sensitive Information to an Unauthorized
Actor ("https://cwe.mitre.org/data/definitions/200.html").
-Wno-analyzer-fd-access-mode-mismatch
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-fd-access-mode-mismatch to disable it.
This diagnostic warns for paths through code in which a "read" on a
write-only file descriptor is attempted, or vice versa.
This diagnostic also warns for code paths in a which a function with
attribute "fd_arg_read (N)" is called with a file descriptor opened
with "O_WRONLY" at referenced argument "N" or a function with
attribute "fd_arg_write (N)" is called with a file descriptor opened
with "O_RDONLY" at referenced argument N.
-Wno-analyzer-fd-double-close
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-fd-double-close to disable it.
This diagnostic warns for paths through code in which a file
descriptor can be closed more than once.
See CWE-1341: Multiple Releases of Same Resource or Handle
("https://cwe.mitre.org/data/definitions/1341.html").
-Wno-analyzer-fd-leak
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-fd-leak to disable it.
This diagnostic warns for paths through code in which an open file
descriptor is leaked.
See CWE-775: Missing Release of File Descriptor or Handle after
Effective Lifetime
("https://cwe.mitre.org/data/definitions/775.html").
-Wno-analyzer-fd-phase-mismatch
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-fd-phase-mismatch to disable it.
This diagnostic warns for paths through code in which an operation is
attempted in the wrong phase of a file descriptor's lifetime. For
example, it will warn on attempts to call "accept" on a stream socket
that has not yet had "listen" successfully called on it.
See CWE-666: Operation on Resource in Wrong Phase of Lifetime
("https://cwe.mitre.org/data/definitions/666.html").
-Wno-analyzer-fd-type-mismatch
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-fd-type-mismatch to disable it.
This diagnostic warns for paths through code in which an operation is
attempted on the wrong type of file descriptor. For example, it will
warn on attempts to use socket operations on a file descriptor
obtained via "open", or when attempting to use a stream socket
operation on a datagram socket.
-Wno-analyzer-fd-use-after-close
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-fd-use-after-close to disable it.
This diagnostic warns for paths through code in which a read or write
is called on a closed file descriptor.
This diagnostic also warns for paths through code in which a function
with attribute "fd_arg (N)" or "fd_arg_read (N)" or "fd_arg_write
(N)" is called with a closed file descriptor at referenced argument
"N".
-Wno-analyzer-fd-use-without-check
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-fd-use-without-check to disable it.
This diagnostic warns for paths through code in which a file
descriptor is used without being checked for validity.
This diagnostic also warns for paths through code in which a function
with attribute "fd_arg (N)" or "fd_arg_read (N)" or "fd_arg_write
(N)" is called with a file descriptor, at referenced argument "N",
without being checked for validity.
-Wno-analyzer-file-leak
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-file-leak to disable it.
This diagnostic warns for paths through the code in which a
"<stdio.h>" "FILE *" stream object is leaked.
See CWE-775: Missing Release of File Descriptor or Handle after
Effective Lifetime
("https://cwe.mitre.org/data/definitions/775.html").
-Wno-analyzer-free-of-non-heap
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-free-of-non-heap to disable it.
This diagnostic warns for paths through the code in which "free" is
called on a non-heap pointer (e.g. an on-stack buffer, or a global).
See CWE-590: Free of Memory not on the Heap
("https://cwe.mitre.org/data/definitions/590.html").
-Wno-analyzer-imprecise-fp-arithmetic
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-imprecise-fp-arithmetic to disable it.
This diagnostic warns for paths through the code in which floating-
point arithmetic is used in locations where precise computation is
needed. This diagnostic only warns on use of floating-point operands
inside the calculation of an allocation size at the moment.
-Wno-analyzer-infinite-loop
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-infinite-loop to disable it.
This diagnostics warns for paths through the code which appear to
lead to an infinite loop.
Specifically, the analyzer will issue this warning when it "sees" a
loop in which:
* no externally-visible work could be being done within the loop
* there is no way to escape from the loop
* the analyzer is sufficiently confident about the program state
throughout the loop to know that the above are true
One way for this warning to be emitted is when there is an execution
path through a loop for which taking the path on one iteration
implies that the same path will be taken on all subsequent
iterations.
For example, consider:
while (1)
{
char opcode = *cpu_state.pc;
switch (opcode)
{
case OPCODE_FOO:
handle_opcode_foo (&cpu_state);
break;
case OPCODE_BAR:
handle_opcode_bar (&cpu_state);
break;
}
}
The analyzer will complain for the above case because if "opcode"
ever matches none of the cases, the "switch" will follow the implicit
"default" case, making the body of the loop be a "no-op" with
"cpu_state.pc" unchanged, and thus using the same value of "opcode"
on all subseqent iterations, leading to an infinite loop.
See CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop')
("https://cwe.mitre.org/data/definitions/835.html").
-Wno-analyzer-infinite-recursion
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-infinite-recursion to disable it.
This diagnostics warns for paths through the code which appear to
lead to infinite recursion.
Specifically, when the analyzer "sees" a recursive call, it will
compare the state of memory at the entry to the new frame with that
at the entry to the previous frame of that function on the stack.
The warning is issued if nothing in memory appears to be changing;
any changes observed to parameters or globals are assumed to lead to
termination of the recursion and thus suppress the warning.
This diagnostic is likely to miss cases of infinite recursion that
are convered to iteration by the optimizer before the analyzer "sees"
them. Hence optimization should be disabled when attempting to
trigger this diagnostic.
Compare with -Winfinite-recursion, which provides a similar
diagnostic, but is implemented in a different way.
See CWE-674: Uncontrolled Recursion
("https://cwe.mitre.org/data/definitions/674.html").
-Wno-analyzer-jump-through-null
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-jump-through-null to disable it.
This diagnostic warns for paths through the code in which a "NULL"
function pointer is called.
-Wno-analyzer-malloc-leak
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-malloc-leak to disable it.
This diagnostic warns for paths through the code in which a pointer
allocated via an allocator is leaked: either "malloc", or a function
marked with attribute "malloc".
See CWE-401: Missing Release of Memory after Effective Lifetime
("https://cwe.mitre.org/data/definitions/401.html").
-Wno-analyzer-mismatching-deallocation
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-mismatching-deallocation to disable it.
This diagnostic warns for paths through the code in which the wrong
deallocation function is called on a pointer value, based on which
function was used to allocate the pointer value. The diagnostic will
warn about mismatches between "free", scalar "delete" and vector
"delete[]", and those marked as allocator/deallocator pairs using
attribute "malloc".
See CWE-762: Mismatched Memory Management Routines
("https://cwe.mitre.org/data/definitions/762.html").
-Wno-analyzer-out-of-bounds
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-out-of-bounds to disable it.
This diagnostic warns for paths through the code in which a buffer is
definitely read or written out-of-bounds. The diagnostic applies for
cases where the analyzer is able to determine a constant offset and
for accesses past the end of a buffer, also a constant capacity.
Further, the diagnostic does limited checking for accesses past the
end when the offset as well as the capacity is symbolic.
See CWE-119: Improper Restriction of Operations within the Bounds of
a Memory Buffer ("https://cwe.mitre.org/data/definitions/119.html").
For cases where the analyzer is able, it will emit a text art diagram
visualizing the spatial relationship between the memory region that
the analyzer predicts would be accessed, versus the range of memory
that is valid to access: whether they overlap, are touching, are
close or far apart; which one is before or after in memory, the
relative sizes involved, the direction of the access (read vs write),
and, in some cases, the values of data involved. This diagram can be
suppressed using -fdiagnostics-text-art-charset=none.
-Wno-analyzer-overlapping-buffers
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-overlapping-buffers to disable it.
This diagnostic warns for paths through the code in which overlapping
buffers are passed to an API for which the behavior on such buffers
is undefined.
Specifically, the diagnostic occurs on calls to the following
functions
*<"memcpy">
*<"strcat">
*<"strcpy">
for cases where the buffers are known to overlap.
-Wno-analyzer-possible-null-argument
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-possible-null-argument to disable it.
This diagnostic warns for paths through the code in which a possibly-
NULL value is passed to a function argument marked with
"__attribute__((nonnull))" as requiring a non-NULL value.
See CWE-690: Unchecked Return Value to NULL Pointer Dereference
("https://cwe.mitre.org/data/definitions/690.html").
-Wno-analyzer-possible-null-dereference
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-possible-null-dereference to disable it.
This diagnostic warns for paths through the code in which a possibly-
NULL value is dereferenced.
See CWE-690: Unchecked Return Value to NULL Pointer Dereference
("https://cwe.mitre.org/data/definitions/690.html").
-Wno-analyzer-null-argument
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-null-argument to disable it.
This diagnostic warns for paths through the code in which a value
known to be NULL is passed to a function argument marked with
"__attribute__((nonnull))" as requiring a non-NULL value.
See CWE-476: NULL Pointer Dereference
("https://cwe.mitre.org/data/definitions/476.html").
-Wno-analyzer-null-dereference
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-null-dereference to disable it.
This diagnostic warns for paths through the code in which a value
known to be NULL is dereferenced.
See CWE-476: NULL Pointer Dereference
("https://cwe.mitre.org/data/definitions/476.html").
-Wno-analyzer-putenv-of-auto-var
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-putenv-of-auto-var to disable it.
This diagnostic warns for paths through the code in which a call to
"putenv" is passed a pointer to an automatic variable or an on-stack
buffer.
See POS34-C. Do not call putenv() with a pointer to an automatic
variable as the argument
("https://wiki.sei.cmu.edu/confluence/x/6NYxBQ").
-Wno-analyzer-shift-count-negative
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-shift-count-negative to disable it.
This diagnostic warns for paths through the code in which a shift is
attempted with a negative count. It is analogous to the
-Wshift-count-negative diagnostic implemented in the C/C++ front
ends, but is implemented based on analyzing interprocedural paths,
rather than merely parsing the syntax tree. However, the analyzer
does not prioritize detection of such paths, so false negatives are
more likely relative to other warnings.
-Wno-analyzer-shift-count-overflow
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-shift-count-overflow to disable it.
This diagnostic warns for paths through the code in which a shift is
attempted with a count greater than or equal to the precision of the
operand's type. It is analogous to the -Wshift-count-overflow
diagnostic implemented in the C/C++ front ends, but is implemented
based on analyzing interprocedural paths, rather than merely parsing
the syntax tree. However, the analyzer does not prioritize detection
of such paths, so false negatives are more likely relative to other
warnings.
-Wno-analyzer-stale-setjmp-buffer
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-stale-setjmp-buffer to disable it.
This diagnostic warns for paths through the code in which "longjmp"
is called to rewind to a "jmp_buf" relating to a "setjmp" call in a
function that has returned.
When "setjmp" is called on a "jmp_buf" to record a rewind location,
it records the stack frame. The stack frame becomes invalid when the
function containing the "setjmp" call returns. Attempting to rewind
to it via "longjmp" would reference a stack frame that no longer
exists, and likely lead to a crash (or worse).
-Wno-analyzer-tainted-allocation-size
This warning requires -fanalyzer which enables it; use
-Wno-analyzer-tainted-allocation-size to disable it.
This diagnostic warns for paths through the code in which a value
that could be under an attacker's control is used as the size of an
allocation without being sanitized, so that an attacker could inject
an excessively large allocation and potentially cause a denial of
service attack.
See CWE-789: Memory Allocation with Excessive Size Value
("https://cwe.mitre.org/data/definitions/789.html").
-Wno-analyzer-tainted-assertion
This warning requires -fanalyzer which enables it; use
-Wno-analyzer-tainted-assertion to disable it.
This diagnostic warns for paths through the code in which a value
that could be under an attacker's control is used as part of a
condition without being first sanitized, and that condition guards a
call to a function marked with attribute "noreturn" (such as the
function "__builtin_unreachable"). Such functions typically indicate
abnormal termination of the program, such as for assertion failure
handlers. For example:
assert (some_tainted_value < SOME_LIMIT);
In such cases:
* when assertion-checking is enabled: an attacker could trigger a
denial of service by injecting an assertion failure
* when assertion-checking is disabled, such as by defining
"NDEBUG", an attacker could inject data that subverts the
process, since it presumably violates a precondition that is
being assumed by the code.
Note that when assertion-checking is disabled, the assertions are
typically removed by the preprocessor before the analyzer has a
chance to "see" them, so this diagnostic can only generate warnings
on builds in which assertion-checking is enabled.
For the purpose of this warning, any function marked with attribute
"noreturn" is considered as a possible assertion failure handler,
including "__builtin_unreachable". Note that these functions are
sometimes removed by the optimizer before the analyzer "sees" them.
Hence optimization should be disabled when attempting to trigger this
diagnostic.
See CWE-617: Reachable Assertion
("https://cwe.mitre.org/data/definitions/617.html").
The warning can also report problematic constructions such as
switch (some_tainted_value) {
case 0:
/* [...etc; various valid cases omitted...] */
break;
default:
__builtin_unreachable (); /* BUG: attacker can trigger this */
}
despite the above not being an assertion failure, strictly speaking.
-Wno-analyzer-tainted-array-index
This warning requires -fanalyzer which enables it; use
-Wno-analyzer-tainted-array-index to disable it.
This diagnostic warns for paths through the code in which a value
that could be under an attacker's control is used as the index of an
array access without being sanitized, so that an attacker could
inject an out-of-bounds access.
See CWE-129: Improper Validation of Array Index
("https://cwe.mitre.org/data/definitions/129.html").
-Wno-analyzer-tainted-divisor
This warning requires -fanalyzer which enables it; use
-Wno-analyzer-tainted-divisor to disable it.
This diagnostic warns for paths through the code in which a value
that could be under an attacker's control is used as the divisor in a
division or modulus operation without being sanitized, so that an
attacker could inject a division-by-zero.
See CWE-369: Divide By Zero
("https://cwe.mitre.org/data/definitions/369.html").
-Wno-analyzer-tainted-offset
This warning requires -fanalyzer which enables it; use
-Wno-analyzer-tainted-offset to disable it.
This diagnostic warns for paths through the code in which a value
that could be under an attacker's control is used as a pointer offset
without being sanitized, so that an attacker could inject an out-of-
bounds access.
See CWE-823: Use of Out-of-range Pointer Offset
("https://cwe.mitre.org/data/definitions/823.html").
-Wno-analyzer-tainted-size
This warning requires -fanalyzer which enables it; use
-Wno-analyzer-tainted-size to disable it.
This diagnostic warns for paths through the code in which a value
that could be under an attacker's control is used as the size of an
operation such as "memset" without being sanitized, so that an
attacker could inject an out-of-bounds access.
See CWE-129: Improper Validation of Array Index
("https://cwe.mitre.org/data/definitions/129.html").
-Wno-analyzer-undefined-behavior-ptrdiff
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-undefined-behavior-ptrdiff to disable it.
This diagnostic warns for paths through the code in which a pointer
subtraction occurs where the pointers refer to different chunks of
memory. Such code relies on undefined behavior, as pointer
subtraction is only defined for cases where both pointers point to
within (or just after) the same array.
See CWE-469: Use of Pointer Subtraction to Determine Size
("https://cwe.mitre.org/data/definitions/469.html").
-Wno-analyzer-undefined-behavior-strtok
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-undefined-behavior-strtok to disable it.
This diagnostic warns for paths through the code in which a call is
made to "strtok" with undefined behavior.
Specifically, passing NULL as the first parameter for the initial
call to "strtok" within a process has undefined behavior.
-Wno-analyzer-unsafe-call-within-signal-handler
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-unsafe-call-within-signal-handler to disable it.
This diagnostic warns for paths through the code in which a function
known to be async-signal-unsafe (such as "fprintf") is called from a
signal handler.
See CWE-479: Signal Handler Use of a Non-reentrant Function
("https://cwe.mitre.org/data/definitions/479.html").
-Wno-analyzer-use-after-free
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-use-after-free to disable it.
This diagnostic warns for paths through the code in which a pointer
is used after a deallocator is called on it: either "free", or a
deallocator referenced by attribute "malloc".
See CWE-416: Use After Free
("https://cwe.mitre.org/data/definitions/416.html").
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-use-of-pointer-in-stale-stack-frame to disable it.
This diagnostic warns for paths through the code in which a pointer
is dereferenced that points to a variable in a stale stack frame.
-Wno-analyzer-va-arg-type-mismatch
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-va-arg-type-mismatch to disable it.
This diagnostic warns for interprocedural paths through the code for
which the analyzer detects an attempt to use "va_arg" to extract a
value passed to a variadic call, but uses a type that does not match
that of the expression passed to the call.
See CWE-686: Function Call With Incorrect Argument Type
("https://cwe.mitre.org/data/definitions/686.html").
-Wno-analyzer-va-list-exhausted
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-va-list-exhausted to disable it.
This diagnostic warns for interprocedural paths through the code for
which the analyzer detects an attempt to use "va_arg" to access the
next value passed to a variadic call, but all of the values in the
"va_list" have already been consumed.
See CWE-685: Function Call With Incorrect Number of Arguments
("https://cwe.mitre.org/data/definitions/685.html").
-Wno-analyzer-va-list-leak
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-va-list-leak to disable it.
This diagnostic warns for interprocedural paths through the code for
which the analyzer detects that "va_start" or "va_copy" has been
called on a "va_list" without a corresponding call to "va_end".
-Wno-analyzer-va-list-use-after-va-end
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-va-list-use-after-va-end to disable it.
This diagnostic warns for interprocedural paths through the code for
which the analyzer detects an attempt to use a "va_list" after
"va_end" has been called on it. "va_list".
-Wno-analyzer-write-to-const
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-write-to-const to disable it.
This diagnostic warns for paths through the code in which the
analyzer detects an attempt to write through a pointer to a "const"
object. However, the analyzer does not prioritize detection of such
paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-write-to-string-literal
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-write-to-string-literal to disable it.
This diagnostic warns for paths through the code in which the
analyzer detects an attempt to write through a pointer to a string
literal. However, the analyzer does not prioritize detection of such
paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-use-of-uninitialized-value
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-use-of-uninitialized-value to disable it.
This diagnostic warns for paths through the code in which an
uninitialized value is used.
See CWE-457: Use of Uninitialized Variable
("https://cwe.mitre.org/data/definitions/457.html").
The analyzer has hardcoded knowledge about the behavior of the following
memory-management functions:
*<"alloca">
*<The built-in functions "__builtin_alloc",>
"__builtin_alloc_with_align", @item "__builtin_calloc",
"__builtin_free", "__builtin_malloc", "__builtin_memcpy",
"__builtin_memcpy_chk", "__builtin_memset", "__builtin_memset_chk",
"__builtin_realloc", "__builtin_stack_restore", and
"__builtin_stack_save"
*<"calloc">
*<"free">
*<"malloc">
*<"memset">
*<"operator delete">
*<"operator delete []">
*<"operator new">
*<"operator new []">
*<"realloc">
*<"strdup">
*<"strndup">
of the following functions for working with file descriptors:
*<"open">
*<"close">
*<"creat">
*<"dup", "dup2" and "dup3">
*<"isatty">
*<"pipe", and "pipe2">
*<"read">
*<"write">
*<"socket", "bind", "listen", "accept", and "connect">
of the following functions for working with "<stdio.h>" streams:
*<The built-in functions "__builtin_fprintf",>
"__builtin_fprintf_unlocked", "__builtin_fputc",
"__builtin_fputc_unlocked", "__builtin_fputs",
"__builtin_fputs_unlocked", "__builtin_fwrite",
"__builtin_fwrite_unlocked", "__builtin_printf",
"__builtin_printf_unlocked", "__builtin_putc", "__builtin_putchar",
"__builtin_putchar_unlocked", "__builtin_putc_unlocked",
"__builtin_puts", "__builtin_puts_unlocked", "__builtin_vfprintf",
and "__builtin_vprintf"
*<"fopen">
*<"fclose">
*<"ferror">
*<"fgets">
*<"fgets_unlocked">
*<"fileno">
*<"fread">
*<"getc">
*<"getchar">
*<"fprintf">
*<"printf">
*<"fwrite">
and of the following functions:
*<The built-in functions "__builtin_expect",>
"__builtin_expect_with_probability", "__builtin_strchr",
"__builtin_strcpy", "__builtin_strcpy_chk", "__builtin_strlen",
"__builtin_va_copy", and "__builtin_va_start"
*<The GNU extensions "error" and "error_at_line">
*<"getpass">
*<"longjmp">
*<"putenv">
*<"setjmp">
*<"siglongjmp">
*<"signal">
*<"sigsetjmp">
*<"strcat">
*<"strchr">
*<"strlen">
In addition, various functions with an "__analyzer_" prefix have special
meaning to the analyzer, described in the GCC Internals manual.
Pertinent parameters for controlling the exploration are:
*<--param analyzer-bb-explosion-factor=value>
*<--param analyzer-max-enodes-per-program-point=value>
*<--param analyzer-max-recursion-depth=value>
*<--param analyzer-min-snodes-for-call-summary=value>
The following options control the analyzer.
-fanalyzer-call-summaries
Simplify interprocedural analysis by computing the effect of certain
calls, rather than exploring all paths through the function from
callsite to each possible return.
If enabled, call summaries are only used for functions with more than
one call site, and that are sufficiently complicated (as per --param
analyzer-min-snodes-for-call-summary=value).
-fanalyzer-checker=name
Restrict the analyzer to run just the named checker, and enable it.
-fanalyzer-debug-text-art-headings
This option is intended for analyzer developers. If enabled, the
analyzer will add extra annotations to any diagrams it generates.
-fno-analyzer-feasibility
This option is intended for analyzer developers.
By default the analyzer verifies that there is a feasible control
flow path for each diagnostic it emits: that the conditions that hold
are not mutually exclusive. Diagnostics for which no feasible path
can be found are rejected. This filtering can be suppressed with
-fno-analyzer-feasibility, for debugging issues in this code.
-fanalyzer-fine-grained
This option is intended for analyzer developers.
Internally the analyzer builds an "exploded graph" that combines
control flow graphs with data flow information.
By default, an edge in this graph can contain the effects of a run of
multiple statements within a basic block. With
-fanalyzer-fine-grained, each statement gets its own edge.
-fanalyzer-show-duplicate-count
This option is intended for analyzer developers: if multiple
diagnostics have been detected as being duplicates of each other, it
emits a note when reporting the best diagnostic, giving the number of
additional diagnostics that were suppressed by the deduplication
logic.
-fanalyzer-show-events-in-system-headers
By default the analyzer emits simplified diagnostics paths by hiding
events fully located within a system header. With
-fanalyzer-show-events-in-system-headers such events are no longer
suppressed.
-fno-analyzer-state-merge
This option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by merging
sufficiently similar states at each program point as it builds its
"exploded graph". With -fno-analyzer-state-merge this merging can be
suppressed, for debugging state-handling issues.
-fno-analyzer-state-purge
This option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by purging
aspects of state at a program point that appear to no longer be
relevant e.g. the values of locals that aren't accessed later in the
function and which aren't relevant to leak analysis.
With -fno-analyzer-state-purge this purging of state can be
suppressed, for debugging state-handling issues.
-fno-analyzer-suppress-followups
This option is intended for analyzer developers.
By default the analyzer will stop exploring an execution path after
encountering certain diagnostics, in order to avoid potentially
issuing a cascade of follow-up diagnostics.
The diagnostics that terminate analysis along a path are:
*<-Wanalyzer-null-argument>
*<-Wanalyzer-null-dereference>
*<-Wanalyzer-use-after-free>
*<-Wanalyzer-use-of-pointer-in-stale-stack-frame>
*<-Wanalyzer-use-of-uninitialized-value>
With -fno-analyzer-suppress-followups the analyzer will continue to
explore such paths even after such diagnostics, which may be helpful
for debugging issues in the analyzer, or for microbenchmarks for
detecting undefined behavior.
-fanalyzer-transitivity
This option enables transitivity of constraints within the analyzer.
-fno-analyzer-undo-inlining
This option is intended for analyzer developers.
-fanalyzer runs relatively late compared to other code analysis
tools, and some optimizations have already been applied to the code.
In particular function inlining may have occurred, leading to the
interprocedural execution paths emitted by the analyzer containing
function frames that don't correspond to those in the original source
code.
By default the analyzer attempts to reconstruct the original function
frames, and to emit events showing the inlined calls.
With -fno-analyzer-undo-inlining this attempt to reconstruct the
original frame information can be disabled, which may be of help when
debugging issues in the analyzer.
-fanalyzer-verbose-edges
This option is intended for analyzer developers. It enables more
verbose, lower-level detail in the descriptions of control flow
within diagnostic paths.
-fanalyzer-verbose-state-changes
This option is intended for analyzer developers. It enables more
verbose, lower-level detail in the descriptions of events relating to
state machines within diagnostic paths.
-fanalyzer-verbosity=level
This option controls the complexity of the control flow paths that
are emitted for analyzer diagnostics.
The level can be one of:
0 At this level, interprocedural call and return events are
displayed, along with the most pertinent state-change events
relating to a diagnostic. For example, for a double-"free"
diagnostic, both calls to "free" will be shown.
1 As per the previous level, but also show events for the entry to
each function.
2 As per the previous level, but also show events relating to
control flow that are significant to triggering the issue (e.g.
"true path taken" at a conditional).
This level is the default.
3 As per the previous level, but show all control flow events, not
just significant ones.
4 This level is intended for analyzer developers; it adds various
other events intended for debugging the analyzer.
-fdump-analyzer
Dump internal details about what the analyzer is doing to
file.analyzer.txt. -fdump-analyzer-stderr overrides this option.
-fdump-analyzer-stderr
Dump internal details about what the analyzer is doing to stderr.
This option overrides -fdump-analyzer.
-fdump-analyzer-callgraph
Dump a representation of the call graph suitable for viewing with
GraphViz to file.callgraph.dot.
-fdump-analyzer-exploded-graph
Dump a representation of the "exploded graph" suitable for viewing
with GraphViz to file.eg.dot. Nodes are color-coded based on state-
machine states to emphasize state changes.
-fdump-analyzer-exploded-nodes
Emit diagnostics showing where nodes in the "exploded graph" are in
relation to the program source.
-fdump-analyzer-exploded-nodes-2
Dump a textual representation of the "exploded graph" to file.eg.txt.
-fdump-analyzer-exploded-nodes-3
Dump a textual representation of the "exploded graph" to one dump
file per node, to file.eg-id.txt. This is typically a large number
of dump files.
-fdump-analyzer-exploded-paths
Dump a textual representation of the "exploded path" for each
diagnostic to file.idx.kind.epath.txt.
-fdump-analyzer-feasibility
Dump internal details about the analyzer's search for feasible paths.
The details are written in a form suitable for viewing with GraphViz
to filenames of the form file.*.fg.dot, file.*.tg.dot, and
file.*.fpath.txt.
-fdump-analyzer-infinite-loop
Dump internal details about the analyzer's search for infinite loops.
The details are written in a form suitable for viewing with GraphViz
to filenames of the form file.*.infinite-loop.dot.
-fdump-analyzer-json
Dump a compressed JSON representation of analyzer internals to
file.analyzer.json.gz. The precise format is subject to change.
-fdump-analyzer-state-purge
As per -fdump-analyzer-supergraph, dump a representation of the
"supergraph" suitable for viewing with GraphViz, but annotate the
graph with information on what state will be purged at each node.
The graph is written to file.state-purge.dot.
-fdump-analyzer-supergraph
Dump representations of the "supergraph" suitable for viewing with
GraphViz to file.supergraph.dot and to file.supergraph-eg.dot. These
show all of the control flow graphs in the program, with
interprocedural edges for calls and returns. The second dump
contains annotations showing nodes in the "exploded graph" and
diagnostics associated with them.
-fdump-analyzer-untracked
Emit custom warnings with internal details intended for analyzer
developers.
Options for Debugging Your Program
To tell GCC to emit extra information for use by a debugger, in almost
all cases you need only to add -g to your other options. Some debug
formats can co-exist (like DWARF with CTF) when each of them is enabled
explicitly by adding the respective command line option to your other
options.
GCC allows you to use -g with -O. The shortcuts taken by optimized code
may occasionally be surprising: some variables you declared may not exist
at all; flow of control may briefly move where you did not expect it;
some statements may not be executed because they compute constant results
or their values are already at hand; some statements may execute in
different places because they have been moved out of loops. Nevertheless
it is possible to debug optimized output. This makes it reasonable to
use the optimizer for programs that might have bugs.
If you are not using some other optimization option, consider using -Og
with -g. With no -O option at all, some compiler passes that collect
information useful for debugging do not run at all, so that -Og may
result in a better debugging experience.
-g Produce debugging information in the operating system's native format
(stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging
information.
On most systems that use stabs format, -g enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but probably makes other debuggers
crash or refuse to read the program. If you want to control for
certain whether to generate the extra information, use -gvms (see
below).
-ggdb
Produce debugging information for use by GDB. This means to use the
most expressive format available (DWARF, stabs, or the native format
if neither of those are supported), including GDB extensions if at
all possible.
-gdwarf
-gdwarf-version
Produce debugging information in DWARF format (if that is supported).
The value of version may be either 2, 3, 4 or 5; the default version
for most targets is 5 (with the exception of VxWorks, TPF and Darwin
/ macOS, which default to version 2, and AIX, which defaults to
version 4).
Note that with DWARF Version 2, some ports require and always use
some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for
maximum benefit. Version 5 requires GDB 8.0 or higher.
GCC no longer supports DWARF Version 1, which is substantially
different than Version 2 and later. For historical reasons, some
other DWARF-related options such as -fno-dwarf2-cfi-asm) retain a
reference to DWARF Version 2 in their names, but apply to all
currently-supported versions of DWARF.
-gbtf
Request BTF debug information. BTF is the default debugging format
for the eBPF target. On other targets, like x86, BTF debug
information can be generated along with DWARF debug information when
both of the debug formats are enabled explicitly via their respective
command line options.
-gprune-btf
-gno-prune-btf
Prune BTF information before emission. When pruning, only type
information for types used by global variables and file-scope
functions will be emitted. If compiling for the BPF target with BPF
CO-RE enabled, type information will also be emitted for types used
in BPF CO-RE relocations. In addition, struct and union types which
are only referred to via pointers from members of other struct or
union types shall be pruned and replaced with BTF_KIND_FWD, as though
those types were only present in the input as forward declarations.
This option substantially reduces the size of produced BTF
information, but at significant loss in the amount of detailed type
information. It is primarily useful when compiling for the BPF
target, to minimize the size of the resulting object, and to
eliminate BTF information which is not immediately relevant to the
BPF program loading process.
This option is enabled by default for the BPF target when generating
BTF information.
-gctf
-gctflevel
Request CTF debug information and use level to specify how much CTF
debug information should be produced. If -gctf is specified without
a value for level, the default level of CTF debug information is 2.
CTF debug information can be generated along with DWARF debug
information when both of the debug formats are enabled explicitly via
their respective command line options.
Level 0 produces no CTF debug information at all. Thus, -gctf0
negates -gctf.
Level 1 produces CTF information for tracebacks only. This includes
callsite information, but does not include type information.
Level 2 produces type information for entities (functions, data
objects etc.) at file-scope or global-scope only.
-gvms
Produce debugging information in Alpha/VMS debug format (if that is
supported). This is the format used by DEBUG on Alpha/VMS systems.
-gcodeview
Produce debugging information in CodeView debug format (if that is
supported). This is the format used by Microsoft Visual C++ on
Windows.
-glevel
-ggdblevel
-gvmslevel
Request debugging information and also use level to specify how much
information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates -g.
Level 1 produces minimal information, enough for making backtraces in
parts of the program that you don't plan to debug. This includes
descriptions of functions and external variables, and line number
tables, but no information about local variables.
Level 3 includes extra information, such as all the macro definitions
present in the program. Some debuggers support macro expansion when
you use -g3.
If you use multiple -g options, with or without level numbers, the
last such option is the one that is effective.
-gdwarf does not accept a concatenated debug level, to avoid
confusion with -gdwarf-level. Instead use an additional -glevel
option to change the debug level for DWARF.
-fno-eliminate-unused-debug-symbols
By default, no debug information is produced for symbols that are not
actually used. Use this option if you want debug information for all
symbols.
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only one
object file, emit it in all object files using the class. This
option should be used only with debuggers that are unable to handle
the way GCC normally emits debugging information for classes because
using this option increases the size of debugging information by as
much as a factor of two.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging
information that are identical in different object files. Merging is
not supported by all assemblers or linkers. Merging decreases the
size of the debug information in the output file at the cost of
increasing link processing time. Merging is enabled by default.
-fdebug-prefix-map=old=new
When compiling files residing in directory old, record debugging
information describing them as if the files resided in directory new
instead. This can be used to replace a build-time path with an
install-time path in the debug info. It can also be used to change
an absolute path to a relative path by using . for new. This can
give more reproducible builds, which are location independent, but
may require an extra command to tell GDB where to find the source
files. See also -ffile-prefix-map and -fcanon-prefix-map.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored
at each position in code. Better debugging information is then
generated (if the debugging information format supports this
information).
It is enabled by default when compiling with optimization (-Os, -O,
-O2, ...), debugging information (-g) and the debug info format
supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and
attempt to carry the annotations over throughout the compilation all
the way to the end, in an attempt to improve debug information while
optimizing. Use of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case
annotations are created and maintained, but discarded at the end. By
default, this flag is enabled together with -fvar-tracking, except
when selective scheduling is enabled.
-gsplit-dwarf
If DWARF debugging information is enabled, separate as much debugging
information as possible into a separate output file with the
extension .dwo. This option allows the build system to avoid linking
files with debug information. To be useful, this option requires a
debugger capable of reading .dwo files.
-gdwarf32
-gdwarf64
If DWARF debugging information is enabled, the -gdwarf32 selects the
32-bit DWARF format and the -gdwarf64 selects the 64-bit DWARF
format. The default is target specific, on most targets it is
-gdwarf32 though. The 32-bit DWARF format is smaller, but can't
support more than 2GiB of debug information in any of the DWARF debug
information sections. The 64-bit DWARF format allows larger debug
information and might not be well supported by all consumers yet.
-gdescribe-dies
Add description attributes to some DWARF DIEs that have no name
attribute, such as artificial variables, external references and call
site parameter DIEs.
-gpubnames
Generate DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
-ggnu-pubnames
Generate ".debug_pubnames" and ".debug_pubtypes" sections in a format
suitable for conversion into a GDB index. This option is only useful
with a linker that can produce GDB index version 7.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into their
own ".debug_types" section instead of making them part of the
".debug_info" section. It is more efficient to put them in a
separate comdat section since the linker can then remove duplicates.
But not all DWARF consumers support ".debug_types" sections yet and
on some objects ".debug_types" produces larger instead of smaller
debugging information.
-grecord-gcc-switches
-gno-record-gcc-switches
This switch causes the command-line options used to invoke the
compiler that may affect code generation to be appended to the
DW_AT_producer attribute in DWARF debugging information. The options
are concatenated with spaces separating them from each other and from
the compiler version. It is enabled by default. See also
-frecord-gcc-switches for another way of storing compiler options
into the object file.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than
selected with -gdwarf-version. On most targets using non-conflicting
DWARF extensions from later standard versions is allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than selected
with -gdwarf-version.
-gas-loc-support
Inform the compiler that the assembler supports ".loc" directives.
It may then use them for the assembler to generate DWARF2+ line
number tables.
This is generally desirable, because assembler-generated line-number
tables are a lot more compact than those the compiler can generate
itself.
This option will be enabled by default if, at GCC configure time, the
assembler was found to support such directives.
-gno-as-loc-support
Force GCC to generate DWARF2+ line number tables internally, if
DWARF2+ line number tables are to be generated.
-gas-locview-support
Inform the compiler that the assembler supports "view" assignment and
reset assertion checking in ".loc" directives.
This option will be enabled by default if, at GCC configure time, the
assembler was found to support them.
-gno-as-locview-support
Force GCC to assign view numbers internally, if
-gvariable-location-views are explicitly requested.
-gcolumn-info
-gno-column-info
Emit location column information into DWARF debugging information,
rather than just file and line. This option is enabled by default.
-gstatement-frontiers
-gno-statement-frontiers
This option causes GCC to create markers in the internal
representation at the beginning of statements, and to keep them
roughly in place throughout compilation, using them to guide the
output of "is_stmt" markers in the line number table. This is
enabled by default when compiling with optimization (-Os, -O1, -O2,
...), and outputting DWARF 2 debug information at the normal level.
-gvariable-location-views
-gvariable-location-views=incompat5
-gno-variable-location-views
Augment variable location lists with progressive view numbers implied
from the line number table. This enables debug information consumers
to inspect state at certain points of the program, even if no
instructions associated with the corresponding source locations are
present at that point. If the assembler lacks support for view
numbers in line number tables, this will cause the compiler to emit
the line number table, which generally makes them somewhat less
compact. The augmented line number tables and location lists are
fully backward-compatible, so they can be consumed by debug
information consumers that are not aware of these augmentations, but
they won't derive any benefit from them either.
This is enabled by default when outputting DWARF 2 debug information
at the normal level, as long as there is assembler support,
-fvar-tracking-assignments is enabled and -gstrict-dwarf is not.
When assembler support is not available, this may still be enabled,
but it will force GCC to output internal line number tables, and if
-ginternal-reset-location-views is not enabled, that will most
certainly lead to silently mismatching location views.
There is a proposed representation for view numbers that is not
backward compatible with the location list format introduced in DWARF
5, that can be enabled with -gvariable-location-views=incompat5.
This option may be removed in the future, is only provided as a
reference implementation of the proposed representation. Debug
information consumers are not expected to support this extended
format, and they would be rendered unable to decode location lists
using it.
-ginternal-reset-location-views
-gno-internal-reset-location-views
Attempt to determine location views that can be omitted from location
view lists. This requires the compiler to have very accurate insn
length estimates, which isn't always the case, and it may cause
incorrect view lists to be generated silently when using an assembler
that does not support location view lists. The GNU assembler will
flag any such error as a "view number mismatch". This is only
enabled on ports that define a reliable estimation function.
-ginline-points
-gno-inline-points
Generate extended debug information for inlined functions. Location
view tracking markers are inserted at inlined entry points, so that
address and view numbers can be computed and output in debug
information. This can be enabled independently of location views, in
which case the view numbers won't be output, but it can only be
enabled along with statement frontiers, and it is only enabled by
default if location views are enabled.
-gz[=type]
Produce compressed debug sections in DWARF format, if that is
supported. If type is not given, the default type depends on the
capabilities of the assembler and linker used. type may be one of
none (don't compress debug sections), or zlib (use zlib compression
in ELF gABI format). If the linker doesn't support writing
compressed debug sections, the option is rejected. Otherwise, if the
assembler does not support them, -gz is silently ignored when
producing object files.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base name
of the compilation source file matches the base name of file in which
the struct is defined.
This option substantially reduces the size of debugging information,
but at significant potential loss in type information to the
debugger. See -femit-struct-debug-reduced for a less aggressive
option. See -femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base name
of the compilation source file matches the base name of file in which
the type is defined, unless the struct is a template or defined in a
system header.
This option significantly reduces the size of debugging information,
with some potential loss in type information to the debugger. See
-femit-struct-debug-baseonly for a more aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler generates debug
information. The intent is to reduce duplicate struct debug
information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced and
-femit-struct-debug-baseonly, which serves for most needs.
A specification has the
syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that are
used directly (dir:) or used indirectly (ind:). A struct type is
used directly when it is the type of a variable, member. Indirect
uses arise through pointers to structs. That is, when use of an
incomplete struct is valid, the use is indirect. An example is
struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary structs
(ord:) or generic structs (gen:). Generic structs are a bit
complicated to explain. For C++, these are non-explicit
specializations of template classes, or non-template classes within
the above. Other programming languages have generics, but
-femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for which
the compiler should emit debug information. The values none and any
have the normal meaning. The value base means that the base of name
of the file in which the type declaration appears must match the base
of the name of the main compilation file. In practice, this means
that when compiling foo.c, debug information is generated for types
declared in that file and foo.h, but not other header files. The
value sys means those types satisfying base or declared in system or
compiler headers.
You may need to experiment to determine the best settings for your
application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated ".eh_frame" section
instead of using GAS ".cfi_*" directives.
-fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids producing debug
symbol output for types that are nowhere used in the source file
being compiled. Sometimes it is useful to have GCC emit debugging
information for all types declared in a compilation unit, regardless
of whether or not they are actually used in that compilation unit,
for example if, in the debugger, you want to cast a value to a type
that is not actually used in your program (but is declared). More
often, however, this results in a significant amount of wasted space.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the
cost of compilation and to make debugging produce the expected results.
Statements are independent: if you stop the program with a breakpoint
between statements, you can then assign a new value to any variable or
change the program counter to any other statement in the function and get
exactly the results you expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the
performance and/or code size at the expense of compilation time and
possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the
program. Compiling multiple files at once to a single output file mode
allows the compiler to use information gained from all of the files when
compiling each of them.
Not all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed in this section.
Most optimizations are completely disabled at -O0 or if an -O level is
not set on the command line, even if individual optimization flags are
specified. Similarly, -Og suppresses many optimization passes.
Depending on the target and how GCC was configured, a slightly different
set of optimizations may be enabled at each -O level than those listed
here. You can invoke GCC with -Q --help=optimizers to find out the exact
set of optimizations that are enabled at each level.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot
more memory for a large function.
With -O, the compiler tries to reduce code size and execution time,
without performing any optimizations that take a great deal of
compilation time.
-O is the recommended optimization level for large machine-generated
code as a sensible balance between time taken to compile and memory
use: higher optimization levels perform optimizations with greater
algorithmic complexity than at -O.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-modref
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fivopts -fmerge-constants
-fmove-loop-invariants -fmove-loop-stores -fomit-frame-pointer
-freorder-blocks -fshrink-wrap -fshrink-wrap-separate
-fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink -ftree-slsr
-ftree-sra -ftree-ter -funit-at-a-time
-O2 Optimize even more. GCC performs nearly all supported optimizations
that do not involve a space-speed tradeoff. As compared to -O, this
option increases both compilation time and the performance of the
generated code.
-O2 turns on all optimization flags specified by -O1. It also turns
on the following optimization flags:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations -ffinite-loops
-fgcse -fgcse-lm -fhoist-adjacent-loads -finline-functions
-finline-small-functions -findirect-inlining -fipa-bit-cp -fipa-cp
-fipa-icf -fipa-ra -fipa-sra -fipa-vrp
-fisolate-erroneous-paths-dereference -flra-remat -foptimize-crc
-foptimize-sibling-calls -foptimize-strlen -fpartial-inlining
-fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec -fstore-merging -fstrict-aliasing
-fthread-jumps -ftree-builtin-call-dce -ftree-loop-vectorize
-ftree-pre -ftree-slp-vectorize -ftree-switch-conversion
-ftree-tail-merge -ftree-vrp -fvect-cost-model=very-cheap
Please note the warning under -fgcse about invoking -O2 on programs
that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2
and also turns on the following optimization flags:
-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides
-O0 Reduce compilation time and make debugging produce the expected
results. This is the default.
At -O0, GCC completely disables most optimization passes; they are
not run even if you explicitly enable them on the command line, or
are listed by -Q --help=optimizers as being enabled by default. Many
optimizations performed by GCC depend on code analysis or
canonicalization passes that are enabled by -O, and it would not be
useful to run individual optimization passes in isolation.
-Os Optimize for size. -Os enables all -O2 optimizations except those
that often increase code size:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stc
It also enables -finline-functions, causes the compiler to tune for
code size rather than execution speed, and performs further
optimizations designed to reduce code size.
-Ofast
Disregard strict standards compliance. -Ofast enables all -O3
optimizations. It also enables optimizations that are not valid for
all standard-compliant programs. It turns on -ffast-math,
-fallow-store-data-races and the Fortran-specific -fstack-arrays,
unless -fmax-stack-var-size is specified, and -fno-protect-parens.
It turns off -fsemantic-interposition.
-Og Optimize debugging experience. -Og should be the optimization level
of choice for the standard edit-compile-debug cycle, offering a
reasonable level of optimization while maintaining fast compilation
and a good debugging experience. It is a better choice than -O0 for
producing debuggable code because some compiler passes that collect
debug information are disabled at -O0.
Like -O0, -Og completely disables a number of optimization passes so
that individual options controlling them have no effect. Otherwise
-Og enables all -O1 optimization flags except for those that may
interfere with debugging:
-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fmove-loop-stores -fssa-phiopt
-ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra
-Oz Optimize aggressively for size rather than speed. This may increase
the number of instructions executed if those instructions require
fewer bytes to encode. -Oz behaves similarly to -Os including
enabling most -O2 optimizations.
If you use multiple -O options, with or without level numbers, the last
such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most flags
have both positive and negative forms; the negative form of -ffoo is
-fno-foo. In the table below, only one of the forms is listed---the one
you typically use. You can figure out the other form by either removing
no- or adding it.
The following options control specific optimizations. They are either
activated by -O options or are related to ones that are. You can use the
following flags in the rare cases when "fine-tuning" of optimizations to
be performed is desired.
-fno-defer-pop
For machines that must pop arguments after a function call, always
pop the arguments as soon as each function returns. At levels -O1
and higher, -fdefer-pop is the default; this allows the compiler to
let arguments accumulate on the stack for several function calls and
pop them all at once.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to combine
two instructions and checks if the result can be simplified. If loop
unrolling is active, two passes are performed and the second is
scheduled after loop unrolling.
This option is enabled by default at optimization levels -O1, -O2,
-O3, -Os.
-favoid-store-forwarding
-fno-avoid-store-forwarding
Many CPUs will stall for many cycles when a load partially depends on
previous smaller stores. This pass tries to detect such cases and
avoid the penalty by changing the order of the load and store and
then fixing up the loaded value.
Disabled by default.
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction.
-ffp-contract=fast enables floating-point expression contraction such
as forming of fused multiply-add operations if the target has native
support for them. -ffp-contract=on enables floating-point expression
contraction if allowed by the language standard. This is implemented
for C and C++, where it enables contraction within one expression,
but not across different statements.
The default is -ffp-contract=off for C in a standards compliant mode
(-std=c11 or similar), -ffp-contract=fast otherwise.
-fomit-frame-pointer
Omit the frame pointer in functions that don't need one. This avoids
the instructions to save, set up and restore the frame pointer; on
many targets it also makes an extra register available.
On some targets this flag has no effect because the standard calling
sequence always uses a frame pointer, so it cannot be omitted.
Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer
is used in all functions. Several targets always omit the frame
pointer in leaf functions.
Enabled by default at -O1 and higher.
-foptimize-crc
Detect loops calculating CRC (performing polynomial long division)
and replace them with a faster implementation. Detect 8, 16, 32, and
64 bit CRC, with a constant polynomial without the leading 1 bit, for
both bit-forward and bit-reversed cases. If the target supports a
CRC instruction and the polynomial used in the source code matches
the polynomial used in the CRC instruction, generate that CRC
instruction. Otherwise, if the target supports a carry-less-
multiplication instruction, generate CRC using it; otherwise generate
table-based CRC.
Enabled by default at -O2 and higher.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-foptimize-strlen
Optimize various standard C string functions (e.g. "strlen", "strchr"
or "strcpy") and their "_FORTIFY_SOURCE" counterparts into faster
alternatives.
Enabled at levels -O2, -O3.
-finline-stringops[=fn]
Expand memory and string operations (for now, only "memset") inline,
even when the length is variable or big enough as to require looping.
This is most useful along with -ffreestanding and -fno-builtin.
In some circumstances, it enables the compiler to generate code that
takes advantage of known alignment and length multipliers, but even
then it may be less efficient than optimized runtime implementations,
and grow code size so much that even a less performant but shared
implementation runs faster due to better use of code caches. This
option is disabled by default.
-fno-inline
Do not expand any functions inline apart from those marked with the
"always_inline" attribute. This is the default when not optimizing.
Single functions can be exempted from inlining by marking them with
the "noinline" attribute.
-finline-small-functions
Integrate functions into their callers when their body is smaller
than expected function call code (so overall size of program gets
smaller). The compiler heuristically decides which functions are
simple enough to be worth integrating in this way. This inlining
applies to all functions, even those not declared inline.
Enabled at levels -O2, -O3, -Os.
-findirect-inlining
Inline also indirect calls that are discovered to be known at compile
time thanks to previous inlining. This option has any effect only
when inlining itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
-finline-functions
Consider all functions for inlining, even if they are not declared
inline. The compiler heuristically decides which functions are worth
integrating in this way.
If all calls to a given function are integrated, and the function is
declared "static", then the function is normally not output as
assembler code in its own right.
Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and
-fauto-profile.
-finline-functions-called-once
Consider all "static" functions called once for inlining into their
caller even if they are not marked "inline". If a call to a given
function is integrated, then the function is not output as assembler
code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body
seems smaller than the function call overhead early before doing
-fprofile-generate instrumentation and real inlining pass. Doing so
makes profiling significantly cheaper and usually inlining faster on
programs having large chains of nested wrapper functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal of
unused parameters and replacement of parameters passed by reference
by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined.
This flag allows coarse control of this limit. n is the size of
functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may
be specified individually by using --param name=value. The
-finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
is set to n/2.
See below for a documentation of the individual parameters
controlling inlining and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results in default
behavior.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way does it represent
a count of assembly instructions and as such its exact meaning might
change from one release to an another.
-fno-keep-inline-dllexport
This is a more fine-grained version of -fkeep-inline-functions, which
applies only to functions that are declared using the "dllexport"
attribute or declspec.
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the
object file, even if the function has been inlined into all of its
callers. This switch does not affect functions using the "extern
inline" extension in GNU C90. In C++, emit any and all inline
functions into the object file.
-fkeep-static-functions
Emit "static" functions into the object file, even if the function is
never used.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't turned
on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the
compiler to check if a variable is referenced, regardless of whether
or not optimization is turned on, use the -fno-keep-static-consts
option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating-
point constants) across compilation units.
This option is the default for optimized compilation if the assembler
and linker support it. Use -fno-merge-constants to inhibit this
behavior.
Enabled at levels -O1, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating-
point types. Languages like C or C++ require each variable,
including multiple instances of the same variable in recursive calls,
to have distinct locations, so using this option results in non-
conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first
scheduling pass. This pass looks at innermost loops and reorders
their instructions by overlapping different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with register
moves allowed. By setting this flag certain anti-dependences edges
are deleted, which triggers the generation of reg-moves based on the
life-range analysis. This option is effective only with
-fmodulo-sched enabled.
-fno-branch-count-reg
Disable the optimization pass that scans for opportunities to use
"decrement and branch" instructions on a count register instead of
instruction sequences that decrement a register, compare it against
zero, and then branch based upon the result. This option is only
meaningful on architectures that support such instructions, which
include x86, PowerPC, IA-64 and S/390. Note that the
-fno-branch-count-reg option doesn't remove the decrement and branch
instructions from the generated instruction stream introduced by
other optimization passes.
The default is -fbranch-count-reg at -O1 and higher, except for -Og.
-fno-function-cse
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the optimizations
performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables
that are initialized to zero into BSS. This can save space in the
resulting code.
This option turns off this behavior because some programs explicitly
rely on variables going to the data section---e.g., so that the
resulting executable can find the beginning of that section and/or
make assumptions based on that.
The default is -fzero-initialized-in-bss except in Ada.
-fthread-jumps
Perform optimizations that check to see if a jump branches to a
location where another comparison subsumed by the first is found. If
so, the first branch is redirected to either the destination of the
second branch or a point immediately following it, depending on
whether the condition is known to be true or false.
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types
When using a type that occupies multiple registers, such as "long
long" on a 32-bit system, split the registers apart and allocate them
independently. This normally generates better code for those types,
but may make debugging more difficult.
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types-early
Fully split wide types early, instead of very late. This option has
no effect unless -fsplit-wide-types is turned on.
This is the default on some targets.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump
instructions when the target of the jump is not reached by any other
path. For example, when CSE encounters an "if" statement with an
"else" clause, CSE follows the jump when the condition tested is
false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps
that conditionally skip over blocks. When CSE encounters a simple
"if" statement with no else clause, -fcse-skip-blocks causes CSE to
follow the jump around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations are
performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass
also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC extension,
you may get better run-time performance if you disable the global
common subexpression elimination pass by adding -fno-gcse to the
command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination
attempts to move loads that are only killed by stores into
themselves. This allows a loop containing a load/store sequence to
be changed to a load outside the loop, and a copy/store within the
loop.
Enabled by default when -fgcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global
common subexpression elimination. This pass attempts to move stores
out of loops. When used in conjunction with -fgcse-lm, loops
containing a load/store sequence can be changed to a load before the
loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after stores to
the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination
pass is performed after reload. The purpose of this pass is to clean
up redundant spilling.
Enabled by -O3, -fprofile-use and -fauto-profile.
-faggressive-loop-optimizations
This option tells the loop optimizer to use language constraints to
derive bounds for the number of iterations of a loop. This assumes
that loop code does not invoke undefined behavior by for example
causing signed integer overflows or out-of-bound array accesses. The
bounds for the number of iterations of a loop are used to guide loop
unrolling and peeling and loop exit test optimizations. This option
is enabled by default.
-funconstrained-commons
This option tells the compiler that variables declared in common
blocks (e.g. Fortran) may later be overridden with longer trailing
arrays. This prevents certain optimizations that depend on knowing
the array bounds.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies
equivalent code and saves code size. The resulting code may or may
not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses.
This pass is always skipped on architectures that do not have
instructions to support this. Enabled by default at -O1 and higher
on architectures that support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default at -O1
and higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default at
-O1 and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents.
This includes use of conditional moves, min, max, set flags and abs
instructions, and some tricks doable by standard arithmetics. The
use of conditional execution on chips where it is available is
controlled by -fif-conversion2.
Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.
-fif-conversion2
Use conditional execution (where available) to transform conditional
jumps into branch-less equivalents.
Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.
-fdeclone-ctor-dtor
The C++ ABI requires multiple entry points for constructors and
destructors: one for a base subobject, one for a complete object, and
one for a virtual destructor that calls operator delete afterwards.
For a hierarchy with virtual bases, the base and complete variants
are clones, which means two copies of the function. With this
option, the base and complete variants are changed to be thunks that
call a common implementation.
Enabled by -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and
that no code or data element resides at address zero. This option
enables simple constant folding optimizations at all optimization
levels. In addition, other optimization passes in GCC use this flag
to control global dataflow analyses that eliminate useless checks for
null pointers; these assume that a memory access to address zero
always results in a trap, so that if a pointer is checked after it
has already been dereferenced, it cannot be null.
Note however that in some environments this assumption is not true.
Use -fno-delete-null-pointer-checks to disable this optimization for
programs that depend on that behavior.
This option is enabled by default on most targets. On AVR and
MSP430, this option is completely disabled.
Passes that use the dataflow information are enabled independently at
different optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls. This
is done both within a procedure and interprocedurally as part of
indirect inlining (-findirect-inlining) and interprocedural constant
propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
-fdevirtualize-speculatively
Attempt to convert calls to virtual functions to speculative direct
calls. Based on the analysis of the type inheritance graph,
determine for a given call the set of likely targets. If the set is
small, preferably of size 1, change the call into a conditional
deciding between direct and indirect calls. The speculative calls
enable more optimizations, such as inlining. When they seem useless
after further optimization, they are converted back into original
form.
-fdevirtualize-at-ltrans
Stream extra information needed for aggressive devirtualization when
running the link-time optimizer in local transformation mode. This
option enables more devirtualization but significantly increases the
size of streamed data. For this reason it is disabled by default.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively
expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions. This is
especially helpful for the x86-64 architecture, which implicitly
zero-extends in 64-bit registers after writing to their lower 32-bit
half.
Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000
and x86 at levels -O2, -O3, -Os.
-fno-lifetime-dse
In C++ the value of an object is only affected by changes within its
lifetime: when the constructor begins, the object has an
indeterminate value, and any changes during the lifetime of the
object are dead when the object is destroyed. Normally dead store
elimination will take advantage of this; if your code relies on the
value of the object storage persisting beyond the lifetime of the
object, you can use this flag to disable this optimization. To
preserve stores before the constructor starts (e.g. because your
operator new clears the object storage) but still treat the object as
dead after the destructor, you can use -flifetime-dse=1. The default
behavior can be explicitly selected with -flifetime-dse=2.
-flifetime-dse=0 is equivalent to -fno-lifetime-dse.
-flive-range-shrinkage
Attempt to decrease register pressure through register live range
shrinkage. This is helpful for fast processors with small or
moderate size register sets.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register
allocator. The algorithm argument can be priority, which specifies
Chow's priority coloring, or CB, which specifies Chaitin-Briggs
coloring. Chaitin-Briggs coloring is not implemented for all
architectures, but for those targets that do support it, it is the
default because it generates better code.
-fira-region=region
Use specified regions for the integrated register allocator. The
region argument should be one of the following:
all Use all loops as register allocation regions. This can give the
best results for machines with a small and/or irregular register
set.
mixed
Use all loops except for loops with small register pressure as
the regions. This value usually gives the best results in most
cases and for most architectures, and is enabled by default when
compiling with optimization for speed (-O, -O2, ...).
one Use all functions as a single region. This typically results in
the smallest code size, and is enabled by default for -Os or -O0.
-fira-hoist-pressure
Use IRA to evaluate register pressure in the code hoisting pass for
decisions to hoist expressions. This option usually results in
smaller code, but it can slow the compiler down.
This option is enabled at level -Os for all targets.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to move
loop invariants. This option usually results in generation of faster
and smaller code on machines with large register files (>= 32
registers), but it can slow the compiler down.
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard
registers living through a call. Each hard register gets a separate
stack slot, and as a result function stack frames are larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers. Each
pseudo-register that does not get a hard register gets a separate
stack slot, and as a result function stack frames are larger.
-flra-remat
Enable CFG-sensitive rematerialization in LRA. Instead of loading
values of spilled pseudos, LRA tries to rematerialize (recalculate)
values if it is profitable.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions
to exploit instruction slots available after delayed branch
instructions.
Enabled at levels -O1, -O2, -O3, -Os, but not at -Og.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions
to eliminate execution stalls due to required data being unavailable.
This helps machines that have slow floating point or memory load
instructions by allowing other instructions to be issued until the
result of the load or floating-point instruction is required.
Conventionally enabled at optimization levels -O2 and -O3. However,
many targets override this behavior. For example, on x86, it is
disabled at all levels, while on AArch64, it is enabled only at -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of
instruction scheduling after register allocation has been done. This
is especially useful on machines with a relatively small number of
registers and where memory load instructions take more than one
cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Disable instruction scheduling across basic blocks, which is normally
enabled when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fno-sched-spec
Disable speculative motion of non-load instructions, which is
normally enabled when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before register
allocation. This only makes sense when scheduling before register
allocation is enabled, i.e. with -fschedule-insns or at -O2 or
higher. Usage of this option can improve the generated code and
decrease its size by preventing register pressure increase above the
number of available hard registers and subsequent spills in register
allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes
sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes
sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the
queue of stalled insns into the ready list during the second
scheduling pass. -fno-sched-stalled-insns means that no insns are
moved prematurely, -fsched-stalled-insns=0 means there is no limit on
how many queued insns can be moved prematurely.
-fsched-stalled-insns without a value is equivalent to
-fsched-stalled-insns=1.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) are examined for a dependency on
a stalled insn that is a candidate for premature removal from the
queue of stalled insns. This has an effect only during the second
scheduling pass, and only if -fsched-stalled-insns is used.
-fno-sched-stalled-insns-dep is equivalent to
-fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a
value is equivalent to -fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, use superblock scheduling.
This allows motion across basic block boundaries, resulting in faster
schedules. This option is experimental, as not all machine
descriptions used by GCC model the CPU closely enough to avoid
unreliable results from the algorithm.
This only makes sense when scheduling after register allocation, i.e.
with -fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors
the instruction that belongs to a schedule group. This is enabled by
default when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This heuristic
favors instructions on the critical path. This is enabled by default
when scheduling is enabled, i.e. with -fschedule-insns or
-fschedule-insns2 or at -O2 or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler. This
heuristic favors speculative instructions with greater dependency
weakness. This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors
the instruction belonging to a basic block with greater size or
frequency. This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This
heuristic favors the instruction that is less dependent on the last
instruction scheduled. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This
heuristic favors the instruction that has more instructions depending
on it. This is enabled by default when scheduling is enabled, i.e.
with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling. If a
loop is modulo scheduled, later scheduling passes may change its
schedule. Use this option to control that behavior.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the first scheduler pass.
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the second scheduler pass.
-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective
scheduling. This option has no effect unless one of
-fselective-scheduling or -fselective-scheduling2 is turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline
outer loops. This option has no effect unless -fsel-sched-pipelining
is turned on.
-fsemantic-interposition
Some object formats, like ELF, allow interposing of symbols by the
dynamic linker. This means that for symbols exported from the DSO,
the compiler cannot perform interprocedural propagation, inlining and
other optimizations in anticipation that the function or variable in
question may change. While this feature is useful, for example, to
rewrite memory allocation functions by a debugging implementation, it
is expensive in the terms of code quality. With
-fno-semantic-interposition the compiler assumes that if
interposition happens for functions the overwriting function will
have precisely the same semantics (and side effects). Similarly if
interposition happens for variables, the constructor of the variable
will be the same. The flag has no effect for functions explicitly
declared inline (where it is never allowed for interposition to
change semantics) and for symbols explicitly declared weak.
-fshrink-wrap
Emit function prologues only before parts of the function that need
it, rather than at the top of the function. This flag is enabled by
default at -O and higher.
-fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue separately,
so that those parts are only executed when needed. This option is on
by default, but has no effect unless -fshrink-wrap is also turned on
and the target supports this.
-fcaller-saves
Enable allocation of values to registers that are clobbered by
function calls, by emitting extra instructions to save and restore
the registers around such calls. Such allocation is done only when
it seems to result in better code.
This option is always enabled by default on certain machines, usually
those which have no call-preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory
references and then tries to find ways to combine them.
Enabled by default at -O1 and higher.
-fipa-ra
Use caller save registers for allocation if those registers are not
used by any called function. In that case it is not necessary to
save and restore them around calls. This is only possible if called
functions are part of same compilation unit as current function and
they are compiled before it.
Enabled at levels -O2, -O3, -Os, however the option is disabled if
generated code will be instrumented for profiling (-p, or -pg) or if
callee's register usage cannot be known exactly (this happens on
targets that do not expose prologues and epilogues in RTL).
-fconserve-stack
Attempt to minimize stack usage. The compiler attempts to use less
stack space, even if that makes the program slower. This option
implies setting the large-stack-frame parameter to 100 and the large-
stack-frame-growth parameter to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at
-O1 and higher.
-fcode-hoisting
Perform code hoisting. Code hoisting tries to move the evaluation of
expressions executed on all paths to the function exit as early as
possible. This is especially useful as a code size optimization, but
it often helps for code speed as well. This flag is enabled by
default at -O2 and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag is
enabled by default at -O2 and -O3.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive. This flag
is enabled by default at -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by
default at -O1 and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference
between FRE and PRE is that FRE only considers expressions that are
computed on all paths leading to the redundant computation. This
analysis is faster than PRE, though it exposes fewer redundancies.
This flag is enabled by default at -O1 and higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This
pass is enabled by default at -O1 and higher.
-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-else if
the loads are from adjacent locations in the same structure and the
target architecture has a conditional move instruction. This flag is
enabled by default at -O2 and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary
copy operations. This flag is enabled by default at -O1 and higher.
-fipa-pure-const
Discover which functions are pure or constant. Enabled by default at
-O1 and higher.
-fipa-reference
Discover which static variables do not escape the compilation unit.
Enabled by default at -O1 and higher.
-fipa-reference-addressable
Discover read-only, write-only and non-addressable static variables.
Enabled by default at -O1 and higher.
-fipa-reorder-for-locality
Group call chains close together in the binary layout to improve code
locality and minimize jump distances between frequently called
functions. Unlike -freorder-functions this pass considers the call
chains between functions and groups them together, rather than
grouping all hot/normal/cold/never-executed functions into separate
sections. Unlike -fprofile-reorder-functions it aims to improve code
locality throughout the runtime of the program rather than focusing
on program startup. This option is incompatible with an explicit
-flto-partition= option since it enforces a custom partitioning
scheme. If using this option it is recommended to also use profile
feedback, but this option is not enabled by default otherwise.
-fipa-stack-alignment
Reduce stack alignment on call sites if possible. Enabled by
default.
-fipa-pta
Perform interprocedural pointer analysis and interprocedural
modification and reference analysis. This option can cause excessive
memory and compile-time usage on large compilation units. It is not
enabled by default at any optimization level.
-fipa-profile
Perform interprocedural profile propagation. The functions called
only from cold functions are marked as cold. Also functions executed
once (such as "cold", "noreturn", static constructors or destructors)
are identified. Cold functions and loop less parts of functions
executed once are then optimized for size. Enabled by default at -O1
and higher.
-fipa-modref
Perform interprocedural mod/ref analysis. This optimization analyzes
the side effects of functions (memory locations that are modified or
referenced) and enables better optimization across the function call
boundary. This flag is enabled by default at -O1 and higher.
-fipa-cp
Perform interprocedural constant propagation. This optimization
analyzes the program to determine when values passed to functions are
constants and then optimizes accordingly. This optimization can
substantially increase performance if the application has constants
passed to functions. This flag is enabled by default at -O2, -Os and
-O3. It is also enabled by -fprofile-use and -fauto-profile.
-fipa-cp-clone
Perform function cloning to make interprocedural constant propagation
stronger. When enabled, interprocedural constant propagation
performs function cloning when externally visible function can be
called with constant arguments. Because this optimization can create
multiple copies of functions, it may significantly increase code size
(see --param ipa-cp-unit-growth=value). This flag is enabled by
default at -O3. It is also enabled by -fprofile-use and
-fauto-profile.
-fipa-bit-cp
When enabled, perform interprocedural bitwise constant propagation.
This flag is enabled by default at -O2 and by -fprofile-use and
-fauto-profile. It requires that -fipa-cp is enabled.
-fipa-vrp
When enabled, perform interprocedural propagation of value ranges.
This flag is enabled by default at -O2. It requires that -fipa-cp is
enabled.
-fipa-icf
Perform Identical Code Folding for functions and read-only variables.
The optimization reduces code size and may disturb unwind stacks by
replacing a function by equivalent one with a different name. The
optimization works more effectively with link-time optimization
enabled.
Although the behavior is similar to the Gold Linker's ICF
optimization, GCC ICF works on different levels and thus the
optimizations are not same - there are equivalences that are found
only by GCC and equivalences found only by Gold.
This flag is enabled by default at -O2 and -Os.
-flate-combine-instructions
Enable two instruction combination passes that run relatively late in
the compilation process. One of the passes runs before register
allocation and the other after register allocation. The main aim of
the passes is to substitute definitions into all uses.
Most targets enable this flag by default at -O2 and -Os.
-flive-patching=level
Control GCC's optimizations to produce output suitable for live-
patching.
If the compiler's optimization uses a function's body or information
extracted from its body to optimize/change another function, the
latter is called an impacted function of the former. If a function
is patched, its impacted functions should be patched too.
The impacted functions are determined by the compiler's
interprocedural optimizations. For example, a caller is impacted
when inlining a function into its caller, cloning a function and
changing its caller to call this new clone, or extracting a
function's pureness/constness information to optimize its direct or
indirect callers, etc.
Usually, the more IPA optimizations enabled, the larger the number of
impacted functions for each function. In order to control the number
of impacted functions and more easily compute the list of impacted
function, IPA optimizations can be partially enabled at two different
levels.
The level argument should be one of the following:
inline-clone
Only enable inlining and cloning optimizations, which includes
inlining, cloning, interprocedural scalar replacement of
aggregates and partial inlining. As a result, when patching a
function, all its callers and its clones' callers are impacted,
therefore need to be patched as well.
-flive-patching=inline-clone disables the following optimization
flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra
-fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp
-fipa-vrp -fipa-pure-const -fipa-reference-addressable
-fipa-stack-alignment -fipa-modref
inline-only-static
Only enable inlining of static functions. As a result, when
patching a static function, all its callers are impacted and so
need to be patched as well.
In addition to all the flags that -flive-patching=inline-clone
disables, -flive-patching=inline-only-static disables the
following additional optimization flags: -fipa-cp-clone
-fipa-sra -fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the default
value is inline-clone.
This flag is disabled by default.
Note that -flive-patching is not supported with link-time
optimization (-flto).
-fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined behavior due to
dereferencing a null pointer. Isolate those paths from the main
control flow and turn the statement with erroneous or undefined
behavior into a trap. This flag is enabled by default at -O2 and
higher and depends on -fdelete-null-pointer-checks also being
enabled.
-fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined behavior due to a
null value being used in a way forbidden by a "returns_nonnull" or
"nonnull" attribute. Isolate those paths from the main control flow
and turn the statement with erroneous or undefined behavior into a
trap. This is not currently enabled, but may be enabled by -O2 in
the future.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by
default at -O1 and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and
propagate pointer alignment information. This pass only operates on
local scalar variables and is enabled by default at -O1 and higher,
except for -Og. It requires that -ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This
pass only operates on local scalar variables and is enabled by
default at -O1 and higher.
-fssa-backprop
Propagate information about uses of a value up the definition chain
in order to simplify the definitions. For example, this pass strips
sign operations if the sign of a value never matters. The flag is
enabled by default at -O1 and higher.
-fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize conditional
code. This pass is enabled by default at -O1 and higher, except for
-Og.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to
initializations from a scalar array. This flag is enabled by default
at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace one with a
jump to the other. This optimization is known as tail merging or
cross jumping. This flag is enabled by default at -O2 and higher.
The compilation time in this pass can be limited using max-tail-
merge-comparisons parameter and max-tail-merge-iterations parameter.
-ftree-cselim
Perform conditional store elimination on trees. This flag is enabled
by default at -O1 and higher on targets that have conditional move
instructions.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled
by default at -O1 and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to built-in
functions that may set "errno" but are otherwise free of side
effects. This flag is enabled by default at -O2 and higher if -Os is
not also specified.
-ffinite-loops
Assume that a loop with an exit will eventually take the exit and not
loop indefinitely. This allows the compiler to remove loops that
otherwise have no side-effects, not considering eventual endless
looping as such.
This option is enabled by default at -O2 for C++ with -std=c++11 or
higher.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and expression
simplification) based on a dominator tree traversal. This also
performs jump threading (to reduce jumps to jumps). This flag is
enabled by default at -O1 and higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a
store into a memory location that is later overwritten by another
store without any intervening loads. In this case the earlier store
can be deleted. This flag is enabled by default at -O1 and higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since it
increases effectiveness of code motion optimizations. It also saves
one jump. This flag is enabled by default at -O1 and higher. It is
not enabled for -Os, since it usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default
at -O1 and higher.
-ftree-loop-linear
-floop-strip-mine
-floop-block
Perform loop nest optimizations. Same as -floop-nest-optimize. To
use this code transformation, GCC has to be configured with
--with-isl to enable the Graphite loop transformation infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we
generate the polyhedral representation and transform it back to
gimple. Using -fgraphite-identity we can check the costs or benefits
of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal
optimizations are also performed by the code generator isl, like
index splitting and dead code elimination in loops.
-floop-nest-optimize
Enable the isl based loop nest optimizer. This is a generic loop
nest optimizer based on the Pluto optimization algorithms. It
calculates a loop structure optimized for data-locality and
parallelism. This option is experimental.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that can
be parallelized. Parallelize all the loops that can be analyzed to
not contain loop carried dependences without checking that it is
profitable to parallelize the loops.
-ftree-coalesce-vars
While transforming the program out of the SSA representation, attempt
to reduce copying by coalescing versions of different user-defined
variables, instead of just compiler temporaries. This may severely
limit the ability to debug an optimized program compiled with
-fno-var-tracking-assignments. In the negated form, this flag
prevents SSA coalescing of user variables. This option is enabled by
default if optimization is enabled, and it does very little
otherwise.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to
branch-less equivalents. The intent is to remove control-flow from
the innermost loops in order to improve the ability of the
vectorization pass to handle these loops. This is enabled by default
if vectorization is enabled.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache performance
on big loop bodies and allow further loop optimizations, like
parallelization or vectorization, to take place. For example, the
loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
This flag is enabled by default at -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated with
calls to a library. This flag is enabled by default at -O2 and
higher, and by -fprofile-use and -fauto-profile.
This pass distributes the initialization loops and generates a call
to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset
zero.
-floop-interchange
Perform loop interchange outside of graphite. This flag can improve
cache performance on loop nest and allow further loop optimizations,
like vectorization, to take place. For example, the loop
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
is transformed to
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
This flag is enabled by default at -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-floop-unroll-and-jam
Apply unroll and jam transformations on feasible loops. In a loop
nest this unrolls the outer loop by some factor and fuses the
resulting multiple inner loops. This flag is enabled by default at
-O3. It is also enabled by -fprofile-use and -fauto-profile.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only
invariants that are hard to handle at RTL level (function calls,
operations that expand to nontrivial sequences of insns). With
-funswitch-loops it also moves operands of conditions that are
invariant out of the loop, so that we can use just trivial
invariantness analysis in loop unswitching. The pass also includes
store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for
which determining number of iterations requires complicated analysis.
Later optimizations then may determine the number easily. Useful
especially in connection with unrolling.
-ftree-scev-cprop
Perform final value replacement. If a variable is modified in a loop
in such a way that its value when exiting the loop can be determined
using only its initial value and the number of loop iterations,
replace uses of the final value by such a computation, provided it is
sufficiently cheap. This reduces data dependencies and may allow
further simplifications. Enabled by default at -O1 and higher.
-fivopts
Perform induction variable optimizations (strength reduction,
induction variable merging and induction variable elimination) on
trees. Enabled by default at -O1 and higher.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n
threads. This is only possible for loops whose iterations are
independent and can be arbitrarily reordered. The optimization is
only profitable on multiprocessor machines, for loops that are CPU-
intensive, rather than constrained e.g. by memory bandwidth. This
option implies -pthread, and thus is only supported on targets that
have support for -pthread.
-ftree-pta
Perform function-local points-to analysis on trees. This flag is
enabled by default at -O1 and higher, except for -Og.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces
structure references with scalars to prevent committing structures to
memory too early. This flag is enabled by default at -O1 and higher,
except for -Og.
-fstore-merging
Perform merging of narrow stores to consecutive memory addresses.
This pass merges contiguous stores of immediate values narrower than
a word into fewer wider stores to reduce the number of instructions.
This is enabled by default at -O2 and higher as well as -Os.
-ftree-ter
Perform temporary expression replacement during the SSA->normal
phase. Single use/single def temporaries are replaced at their use
location with their defining expression. This results in non-GIMPLE
code, but gives the expanders much more complex trees to work on
resulting in better RTL generation. This is enabled by default at
-O1 and higher.
-ftree-slsr
Perform straight-line strength reduction on trees. This recognizes
related expressions involving multiplications and replaces them by
less expensive calculations when possible. This is enabled by
default at -O1 and higher.
-ftree-vectorize
Perform vectorization on trees. This flag enables
-ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly
specified.
-ftree-loop-vectorize
Perform loop vectorization on trees. This flag is enabled by default
at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by
default at -O2 and by -ftree-vectorize, -fprofile-use, and
-fauto-profile.
-ftrivial-auto-var-init=choice
Initialize automatic variables with either a pattern or with zeroes
to increase the security and predictability of a program by
preventing uninitialized memory disclosure and use. GCC still
considers an automatic variable that doesn't have an explicit
initializer as uninitialized, -Wuninitialized and
-Wanalyzer-use-of-uninitialized-value will still report warning
messages on such automatic variables and the compiler will perform
optimization as if the variable were uninitialized. With this
option, GCC will also initialize any padding of automatic variables
that have structure or union types to zeroes. However, the current
implementation cannot initialize automatic variables that are
declared between the controlling expression and the first case of a
"switch" statement. Using -Wtrivial-auto-var-init to report all such
cases.
The three values of choice are:
* uninitialized doesn't initialize any automatic variables. This
is C and C++'s default.
* pattern Initialize automatic variables with values which will
likely transform logic bugs into crashes down the line, are
easily recognized in a crash dump and without being values that
programmers can rely on for useful program semantics. The
current value is byte-repeatable pattern with byte "0xFE". The
values used for pattern initialization might be changed in the
future.
* zero Initialize automatic variables with zeroes.
The default is uninitialized.
Note that the initializer values, whether zero or pattern, refer to
data representation (in memory or machine registers), rather than to
their interpretation as numerical values. This distinction may be
important in languages that support types with biases or implicit
multipliers, and with such extensions as hardbool. For example, a
variable that uses 8 bits to represent (biased) quantities in the
"range 160..400" will be initialized with the bit patterns 0x00 or
0xFE, depending on choice, whether or not these representations stand
for values in that range, and even if they do, the interpretation of
the value held by the variable will depend on the bias. A hardbool
variable that uses say "0X5A" and 0xA5 for "false" and "true",
respectively, will trap with either choice of trivial initializer,
i.e., zero initialization will not convert to the representation for
"false", even if it would for a "static" variable of the same type.
This means the initializer pattern doesn't generally depend on the
type of the initialized variable. One notable exception is that
(non-hardened) boolean variables that fit in registers are
initialized with "false" (zero), even when pattern is requested.
You can control this behavior for a specific variable by using the
variable attribute "uninitialized".
-fvect-cost-model=model
Alter the cost model used for vectorization. The model argument
should be one of unlimited, dynamic, cheap or very-cheap. With the
unlimited model the vectorized code-path is assumed to be profitable
while with the dynamic model a runtime check guards the vectorized
code-path to enable it only for iteration counts that will likely
execute faster than when executing the original scalar loop. The
cheap model disables vectorization of loops where doing so would be
cost prohibitive for example due to required runtime checks for data
dependence or alignment but otherwise is equal to the dynamic model.
The very-cheap model disables vectorization of loops when any runtime
check for data dependence or alignment is required, it also disables
vectorization of epilogue loops but otherwise is equal to the cheap
model.
The default cost model depends on other optimization flags and is
either dynamic or cheap.
-fsimd-cost-model=model
Alter the cost model used for vectorization of loops marked with the
OpenMP simd directive. The model argument should be one of
unlimited, dynamic, cheap. All values of model have the same meaning
as described in -fvect-cost-model and by default a cost model defined
with -fvect-cost-model is used.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the
constant propagation pass, but instead of values, ranges of values
are propagated. This allows the optimizers to remove unnecessary
range checks like array bound checks and null pointer checks. This
is enabled by default at -O2 and higher. Null pointer check
elimination is only done if -fdelete-null-pointer-checks is enabled.
-fsplit-paths
Split paths leading to loop backedges. This can improve dead code
elimination and common subexpression elimination. This is enabled by
default at -O3 and above.
-fsplit-ivs-in-unroller
Enables expression of values of induction variables in later
iterations of the unrolled loop using the value in the first
iteration. This breaks long dependency chains, thus improving
efficiency of the scheduling passes.
A combination of -fweb and CSE is often sufficient to obtain the same
effect. However, that is not reliable in cases where the loop body
is more complicated than a single basic block. It also does not work
at all on some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some local
variables when unrolling a loop, which can result in superior code.
This optimization is enabled by default for PowerPC targets, but
disabled by default otherwise.
-fpartial-inlining
Inline parts of functions. This option has any effect only when
inlining itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations
(especially memory loads and stores) performed in previous iterations
of loops.
This option is enabled at level -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch
memory to improve the performance of loops that access large arrays.
This option may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
Disabled at level -Os.
-fno-printf-return-value
Do not substitute constants for known return value of formatted
output functions such as "sprintf", "snprintf", "vsprintf", and
"vsnprintf" (but not "printf" of "fprintf"). This transformation
allows GCC to optimize or even eliminate branches based on the known
return value of these functions called with arguments that are either
constant, or whose values are known to be in a range that makes
determining the exact return value possible. For example, when
-fprintf-return-value is in effect, both the branch and the body of
the "if" statement (but not the call to "snprint") can be optimized
away when "i" is a 32-bit or smaller integer because the return value
is guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...
The -fprintf-return-value option relies on other optimizations and
yields best results with -O2 and above. It works in tandem with the
-Wformat-overflow and -Wformat-truncation options. The
-fprintf-return-value option is enabled by default.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference
between -fno-peephole and -fno-peephole2 is in how they are
implemented in the compiler; some targets use one, some use the
other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2,
-O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not
provided by profiling feedback (-fprofile-arcs). These heuristics
are based on the control flow graph. If some branch probabilities
are specified by "__builtin_expect", then the heuristics are used to
guess branch probabilities for the rest of the control flow graph,
taking the "__builtin_expect" info into account. The interactions
between the heuristics and "__builtin_expect" can be complex, and in
some cases, it may be useful to disable the heuristics so that the
effects of "__builtin_expect" are easier to understand.
It is also possible to specify expected probability of the expression
with "__builtin_expect_with_probability" built-in function.
The default is -fguess-branch-probability at levels -O, -O2, -O3,
-Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce
number of taken branches and improve code locality.
Enabled at levels -O1, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm
Use the specified algorithm for basic block reordering. The
algorithm argument can be simple, which does not increase code size
(except sometimes due to secondary effects like alignment), or stc,
the "software trace cache" algorithm, which tries to put all often
executed code together, minimizing the number of branches executed by
making extra copies of code.
The default is simple at levels -O1, -Os, and stc at levels -O2, -O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in
order to reduce number of taken branches, partitions hot and cold
basic blocks into separate sections of the assembly and .o files, to
improve paging and cache locality performance.
This optimization is automatically turned off in the presence of
exception handling or unwind tables (on targets using
setjump/longjump or target specific scheme), for linkonce sections,
for functions with a user-defined section attribute and on any
architecture that does not support named sections. When
-fsplit-stack is used this option is not enabled by default (to avoid
linker errors), but may be enabled explicitly (if using a working
linker).
Enabled for x86 at levels -O2, -O3, -Os.
-freorder-functions
Reorder functions in the object file in order to improve code
locality. Unlike -fipa-reorder-for-locality this option prioritises
grouping all functions within a category
(hot/normal/cold/never-executed) together. This is implemented by
using special subsections ".text.hot" for most frequently executed
functions and ".text.unlikely" for unlikely executed functions.
Reordering is done by the linker so object file format must support
named sections and linker must place them in a reasonable way.
This option isn't effective unless you either provide profile
feedback (see -fprofile-arcs for details) or manually annotate
functions with "hot" or "cold" attributes.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules applicable
to the language being compiled. For C (and C++), this activates
optimizations based on the type of expressions. In particular, an
object of one type is assumed never to reside at the same address as
an object of a different type, unless the types are almost the same.
For example, an "unsigned int" can alias an "int", but not a "void*"
or a "double". A character type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one
most recently written to (called "type-punning") is common. Even
with -fstrict-aliasing, type-punning is allowed, provided the memory
is accessed through the union type. So, the code above works as
expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting
pointer and dereferencing the result has undefined behavior, even if
the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-fipa-strict-aliasing
Controls whether rules of -fstrict-aliasing are applied across
function boundaries. Note that if multiple functions gets inlined
into a single function the memory accesses are no longer considered
to be crossing a function boundary.
The -fipa-strict-aliasing option is enabled by default and is
effective only in combination with -fstrict-aliasing.
-falign-functions
-falign-functions=n
-falign-functions=n:m
-falign-functions=n:m:n2
-falign-functions=n:m:n2:m2
Align the start of functions to the next power-of-two greater than or
equal to n, skipping up to m-1 bytes. This ensures that at least the
first m bytes of the function can be fetched by the CPU without
crossing an n-byte alignment boundary. This is an optimization of
code performance and alignment is ignored for functions considered
cold. If alignment is required for all functions, use
-fmin-function-alignment.
If m is not specified, it defaults to n.
Examples: -falign-functions=32 aligns functions to the next 32-byte
boundary, -falign-functions=24 aligns to the next 32-byte boundary
only if this can be done by skipping 23 bytes or less,
-falign-functions=32:7 aligns to the next 32-byte boundary only if
this can be done by skipping 6 bytes or less.
The second pair of n2:m2 values allows you to specify a secondary
alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte
boundary if this can be done by skipping 6 bytes or less, otherwise
aligns to the next 32-byte boundary if this can be done by skipping 2
bytes or less. If m2 is not specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two; in
that case, it is rounded up.
-fno-align-functions and -falign-functions=1 are equivalent and mean
that functions are not aligned.
If n is not specified or is zero, use a machine-dependent default.
The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-flimit-function-alignment
If this option is enabled, the compiler tries to avoid unnecessarily
overaligning functions. It attempts to instruct the assembler to
align by the amount specified by -falign-functions, but not to skip
more bytes than the size of the function.
-falign-labels
-falign-labels=n
-falign-labels=n:m
-falign-labels=n:m:n2
-falign-labels=n:m:n2:m2
Align all branch targets to a power-of-two boundary.
Parameters of this option are analogous to the -falign-functions
option. -fno-align-labels and -falign-labels=1 are equivalent and
mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than
this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default
which is very likely to be 1, meaning no alignment. The maximum
allowed n option value is 65536.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
-falign-loops=n:m
-falign-loops=n:m:n2
-falign-loops=n:m:n2:m2
Align loops to a power-of-two boundary. If the loops are executed
many times, this makes up for any execution of the dummy padding
instructions. This is an optimization of code performance and
alignment is ignored for loops considered cold.
If -falign-labels is greater than this value, then its value is used
instead.
Parameters of this option are analogous to the -falign-functions
option. -fno-align-loops and -falign-loops=1 are equivalent and mean
that loops are not aligned. The maximum allowed n option value is
65536.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
-falign-jumps=n:m
-falign-jumps=n:m:n2
-falign-jumps=n:m:n2:m2
Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping. In this case, no
dummy operations need be executed. This is an optimization of code
performance and alignment is ignored for jumps considered cold.
If -falign-labels is greater than this value, then its value is used
instead.
Parameters of this option are analogous to the -falign-functions
option. -fno-align-jumps and -falign-jumps=1 are equivalent and mean
that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default.
The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-fmin-function-alignment
Specify minimal alignment of functions to the next power-of-two
greater than or equal to n. Unlike -falign-functions this alignment
is applied also to all functions (even those considered cold). The
alignment is also not affected by -flimit-function-alignment
-fno-allocation-dce
Do not remove unused C++ allocations (using operator "new" and
operator "delete") in dead code elimination.
See also -fmalloc-dce.
-fallow-store-data-races
Allow the compiler to perform optimizations that may introduce new
data races on stores, without proving that the variable cannot be
concurrently accessed by other threads. Does not affect optimization
of local data. It is safe to use this option if it is known that
global data will not be accessed by multiple threads.
Examples of optimizations enabled by -fallow-store-data-races include
hoisting or if-conversions that may cause a value that was already in
memory to be re-written with that same value. Such re-writing is
safe in a single threaded context but may be unsafe in a multi-
threaded context. Note that on some processors, if-conversions may
be required in order to enable vectorization.
Enabled at level -Ofast.
-funit-at-a-time
This option is left for compatibility reasons. -funit-at-a-time has
no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder
and -fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm" statements.
Output them in the same order that they appear in the input file.
When this option is used, unreferenced static variables are not
removed. This option is intended to support existing code that
relies on a particular ordering. For new code, it is better to use
attributes when possible.
-ftoplevel-reorder is the default at -O1 and higher, and also at -O0
if -fsection-anchors is explicitly requested. Additionally
-fno-toplevel-reorder implies -fno-section-anchors.
-funreachable-traps
With this option, the compiler turns calls to "__builtin_unreachable"
into traps, instead of using them for optimization. This also
affects any such calls implicitly generated by the compiler.
This option has the same effect as -fsanitize=unreachable
-fsanitize-trap=unreachable, but does not affect the values of those
options. If -fsanitize=unreachable is enabled, that option takes
priority over this one.
This option is enabled by default at -O0 and -Og.
-fweb
Constructs webs as commonly used for register allocation purposes and
assign each web individual pseudo register. This allows the register
allocation pass to operate on pseudos directly, but also strengthens
several other optimization passes, such as CSE, loop optimizer and
trivial dead code remover. It can, however, make debugging
impossible, since variables no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the whole program
being compiled. All public functions and variables with the
exception of "main" and those merged by attribute
"externally_visible" become static functions and in effect are
optimized more aggressively by interprocedural optimizers.
With -flto this option has a limited use. In most cases the precise
list of symbols used or exported from the binary is known the
resolution info passed to the link-time optimizer by the linker
plugin. It is still useful if no linker plugin is used or during
incremental link step when final code is produced (with -flto
-flinker-output=nolto-rel).
-flto[=n]
This option runs the standard link-time optimizer. When invoked with
source code, it generates GIMPLE (one of GCC's internal
representations) and writes it to special ELF sections in the object
file. When the object files are linked together, all the function
bodies are read from these ELF sections and instantiated as if they
had been part of the same translation unit.
To use the link-time optimizer, -flto and optimization options should
be specified at compile time and during the final link. It is
recommended that you compile all the files participating in the same
link with the same options and also specify those options at link
time. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of
GIMPLE into special ELF sections inside foo.o and bar.o. The final
invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the
two files into a single internal image, and compiles the result as
usual. Since both foo.o and bar.o are merged into a single image,
this causes all the interprocedural analyses and optimizations in GCC
to work across the two files as if they were a single one. This
means, for example, that the inliner is able to inline functions in
bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them
together into a single GIMPLE representation and optimizes them as
usual to produce myprog.
The important thing to keep in mind is that to enable link-time
optimizations you need to use the GCC driver to perform the link
step. GCC automatically performs link-time optimization if any of
the objects involved were compiled with the -flto command-line
option. You can always override the automatic decision to do link-
time optimization by passing -fno-lto to the link command.
To make whole program optimization effective, it is necessary to make
certain whole program assumptions. The compiler needs to know what
functions and variables can be accessed by libraries and runtime
outside of the link-time optimized unit. When supported by the
linker, the linker plugin (see -fuse-linker-plugin) passes
information to the compiler about used and externally visible
symbols. When the linker plugin is not available, -fwhole-program
should be used to allow the compiler to make these assumptions, which
leads to more aggressive optimization decisions.
When a file is compiled with -flto without -fuse-linker-plugin, the
generated object file is larger than a regular object file because it
contains GIMPLE bytecodes and the usual final code (see
-ffat-lto-objects). This means that object files with LTO
information can be linked as normal object files; if -fno-lto is
passed to the linker, no interprocedural optimizations are applied.
Note that when -fno-fat-lto-objects is enabled the compile stage is
faster but you cannot perform a regular, non-LTO link on them.
When producing the final binary, GCC only applies link-time
optimizations to those files that contain bytecode. Therefore, you
can mix and match object files and libraries with GIMPLE bytecodes
and final object code. GCC automatically selects which files to
optimize in LTO mode and which files to link without further
processing.
Generally, options specified at link time override those specified at
compile time, although in some cases GCC attempts to infer link-time
options from the settings used to compile the input files.
If you do not specify an optimization level option -O at link time,
then GCC uses the highest optimization level used when compiling the
object files. Note that it is generally ineffective to specify an
optimization level option only at link time and not at compile time,
for two reasons. First, compiling without optimization suppresses
compiler passes that gather information needed for effective
optimization at link time. Second, some early optimization passes
can be performed only at compile time and not at link time.
There are some code generation flags preserved by GCC when generating
bytecodes, as they need to be used during the final link. Currently,
the following options and their settings are taken from the first
object file that explicitly specifies them: -fcommon, -fexceptions,
-fnon-call-exceptions, -fgnu-tm and all the -m target flags.
The following options -fPIC, -fpic, -fpie and -fPIE are combined
based on the following scheme:
B<-fPIC> + B<-fpic> = B<-fpic>
B<-fPIC> + B<-fno-pic> = B<-fno-pic>
B<-fpic/-fPIC> + (no option) = (no option)
B<-fPIC> + B<-fPIE> = B<-fPIE>
B<-fpic> + B<-fPIE> = B<-fpie>
B<-fPIC/-fpic> + B<-fpie> = B<-fpie>
Certain ABI-changing flags are required to match in all compilation
units, and trying to override this at link time with a conflicting
value is ignored. This includes options such as -freg-struct-return
and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv,
-fno-trapv or -fno-strict-aliasing are passed through to the link
stage and merged conservatively for conflicting translation units.
Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take
precedence; and for example -ffp-contract=off takes precedence over
-ffp-contract=fast. You can override them at link time.
Diagnostic options such as -Wstringop-overflow are passed through to
the link stage and their setting matches that of the compile-step at
function granularity. Note that this matters only for diagnostics
emitted during optimization. Note that code transforms such as
inlining can lead to warnings being enabled or disabled for regions
if code not consistent with the setting at compile time.
When you need to pass options to the assembler via -Wa or -Xassembler
make sure to either compile such translation units with -fno-lto or
consistently use the same assembler options on all translation units.
You can alternatively also specify assembler options at LTO link
time.
To enable debug info generation you need to supply -g at compile
time. If any of the input files at link time were built with debug
info generation enabled the link will enable debug info generation as
well. Any elaborate debug info settings like the dwarf level
-gdwarf-5 need to be explicitly repeated at the linker command line
and mixing different settings in different translation units is
discouraged.
If LTO encounters objects with C linkage declared with incompatible
types in separate translation units to be linked together (undefined
behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be
issued. The behavior is still undefined at run time. Similar
diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply
interprocedural optimizations on files written in different
languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime
libraries and -lgfortran is added to get the Fortran runtime
libraries. In general, when mixing languages in LTO mode, you should
use the same link command options as when mixing languages in a
regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library
archive, say libfoo.a, it is possible to extract and use them in an
LTO link if you are using a linker with plugin support. To create
static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead
of ar and ranlib; to show the symbols of object files with GIMPLE
bytecode, use gcc-nm. Those commands require that ar, ranlib and nm
have been compiled with plugin support. At link time, use the flag
-fuse-linker-plugin to ensure that the library participates in the
LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE
files from libfoo.a and passes them on to the running GCC to make
them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not
enable the linker plugin, then the objects inside libfoo.a are
extracted and linked as usual, but they do not participate in the LTO
optimization process. In order to make a static library suitable for
both LTO optimization and usual linkage, compile its object files
with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole
program to operate. If the program does not require any symbols to
be exported, it is possible to combine -flto and -fwhole-program to
allow the interprocedural optimizers to use more aggressive
assumptions which may lead to improved optimization opportunities.
Use of -fwhole-program is not needed when linker plugin is active
(see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate
bytecode that is portable between different types of hosts. The
bytecode files are versioned and there is a strict version check, so
bytecode files generated in one version of GCC do not work with an
older or newer version of GCC.
Link-time optimization does not work well with generation of
debugging information on systems other than those using a combination
of ELF and DWARF.
If you specify the optional n, the optimization and code generation
done at link time is executed in parallel using n parallel jobs by
utilizing an installed make program. The environment variable MAKE
may be used to override the program used.
You can also specify -flto=jobserver to use GNU make's job server
mode to determine the number of parallel jobs. This is useful when
the Makefile calling GCC is already executing in parallel. You must
prepend a + to the command recipe in the parent Makefile for this to
work. This option likely only works if MAKE is GNU make. Even
without the option value, GCC tries to automatically detect a running
GNU make's job server.
Use -flto=auto to use GNU make's job server, if available, or
otherwise fall back to autodetection of the number of CPU threads
present in your system.
-flto-partition=alg
Specify the partitioning algorithm used by the link-time optimizer.
The value is either 1to1 to specify a partitioning mirroring the
original source files or balanced to specify partitioning into
equally sized chunks (whenever possible) or max to create new
partition for every symbol where possible or cache to balance chunk
sizes while keeping related symbols together for better caching in
incremental LTO. Specifying none as an algorithm disables
partitioning and streaming completely. The default value is
balanced. While 1to1 can be used as an workaround for various code
ordering issues, the max partitioning is intended for internal
testing only. The value one specifies that exactly one partition
should be used while the value none bypasses partitioning and
executes the link-time optimization step directly from the WPA phase.
-flto-incremental=path
Enable incremental LTO, with its cache in given existing directory.
Can significantly shorten edit-compile cycles with LTO.
When used with LTO (-flto), the output of translation units inside
LTO is cached. Cached translation units are likely to be encountered
again when recompiling with small code changes, leading to recompile
time reduction.
Multiple GCC instances can use the same cache in parallel.
-flto-incremental-cache-size=n
Specifies number of cache entries in incremental LTO after which to
prune old entries. This is a soft limit, temporarily there may be
more entries.
-flto-compression-level=n
This option specifies the level of compression used for intermediate
language written to LTO object files, and is only meaningful in
conjunction with LTO mode (-flto). GCC currently supports two LTO
compression algorithms. For zstd, valid values are 0 (no compression)
to 19 (maximum compression), while zlib supports values from 0 to 9.
Values outside this range are clamped to either minimum or maximum of
the supported values. If the option is not given, a default balanced
compression setting is used.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization.
This option relies on plugin support in the linker, which is
available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE
bytecode out of library archives. This improves the quality of
optimization by exposing more code to the link-time optimizer. This
information specifies what symbols can be accessed externally (by
non-LTO object or during dynamic linking). Resulting code quality
improvements on binaries (and shared libraries that use hidden
visibility) are similar to -fwhole-program. See -flto for a
description of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled
and GCC was configured for use with a linker supporting plugins (GNU
ld 2.21 or newer or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate
language and the object code. This makes them usable for both LTO
linking and normal linking. This option is effective only when
compiling with -flto and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but
requires the complete toolchain to be aware of LTO. It requires a
linker with linker plugin support for basic functionality.
Additionally, nm, ar and ranlib need to support linker plugins to
allow a full-featured build environment (capable of building static
libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers
to pass the right options to these tools. With non fat LTO makefiles
need to be modified to use them.
Note that modern binutils provide plugin auto-load mechanism.
Installing the linker plugin into $libdir/bfd-plugins has the same
effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-
ranlib).
The default is -fno-fat-lto-objects on targets with linker plugin
support.
-fcompare-elim
After register allocation and post-register allocation instruction
splitting, identify arithmetic instructions that compute processor
flags similar to a comparison operation based on that arithmetic. If
possible, eliminate the explicit comparison operation.
This pass only applies to certain targets that cannot explicitly
represent the comparison operation before register allocation is
complete.
Enabled at levels -O1, -O2, -O3, -Os.
-ffold-mem-offsets
-fno-fold-mem-offsets
Try to eliminate add instructions by folding them in memory
loads/stores.
Enabled at levels -O2, -O3.
-fcprop-registers
After register allocation and post-register allocation instruction
splitting, perform a copy-propagation pass to try to reduce
scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O1, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded
programs may be inconsistent due to missed counter updates. When this
option is specified, GCC uses heuristics to correct or smooth out
such inconsistencies. By default, GCC emits an error message when an
inconsistent profile is detected.
This option is enabled by -fauto-profile.
-fprofile-partial-training
With "-fprofile-use" all portions of programs not executed during
train run are optimized aggressively for size rather than speed. In
some cases it is not practical to train all possible hot paths in the
program. (For example, program may contain functions specific for a
given hardware and training may not cover all hardware configurations
program is run on.) With "-fprofile-partial-training" profile
feedback is ignored for all functions not executed during the train
run, leading them to be optimized as if they were compiled without
profile feedback. This leads to better performance when train run is
not representative but also leads to significantly bigger code.
-fprofile-use
-fprofile-use=path
Enable profile feedback-directed optimizations, and the following
optimizations, many of which are generally profitable only with
profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops
-ftracer -fvpt -finline-functions -fipa-cp -fipa-cp-clone
-fipa-bit-cp -fpredictive-commoning -fsplit-loops -funswitch-loops
-fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize
-fvect-cost-model=dynamic -ftree-loop-distribute-patterns
-fprofile-reorder-functions
Before you can use this option, you must first generate profiling
information.
By default, GCC emits an error message if the feedback profiles do
not match the source code. This error can be turned into a warning
by using -Wno-error=coverage-mismatch. Note this may result in
poorly optimized code. Additionally, by default, GCC also emits a
warning message if the feedback profiles do not exist (see
-Wmissing-profile).
If path is specified, GCC looks at the path to find the profile
feedback data files. See -fprofile-dir.
-fauto-profile
-fauto-profile=path
Enable sampling-based feedback-directed optimizations, and the
following optimizations, many of which are generally profitable only
with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops
-ftracer -fvpt -finline-functions -fipa-cp -fipa-cp-clone
-fipa-bit-cp -fpredictive-commoning -fsplit-loops -funswitch-loops
-fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize
-fvect-cost-model=dynamic -ftree-loop-distribute-patterns
-fprofile-correction
path is the name of a file containing AutoFDO profile information.
If omitted, it defaults to fbdata.afdo in the current directory.
Producing an AutoFDO profile data file requires running your program
with the perf utility on a supported GNU/Linux target system. For
more information, see <https://perf.wiki.kernel.org/>.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data to a
format that can be used by GCC. You must also supply the unstripped
binary for your program to this tool. See
<https://github.com/google/autofdo>.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding floating-point
arithmetic. These options trade off between speed and correctness. All
must be specifically enabled.
-fexcess-precision=style
This option allows control over excess precision on machines where
floating-point operations occur in a format with more precision or
range than the IEEE standard and interchange floating-point types.
An example of such a target is x87 floating point on x86 processors,
which uses an 80-bit representation internally instead of the 64-bit
IEEE format. For most programs, the excess precision is harmless,
but some programs may rely on the requirements of the C or C++
language standards for handling IEEE values.
By default, -fexcess-precision=fast is in effect; this means that
operations may be carried out in a wider precision than the types
specified in the source if that would result in faster code, and it
is unpredictable when rounding to the types specified in the source
code takes place. When compiling C or C++, if
-fexcess-precision=standard is specified then excess precision
follows the rules specified in ISO C99 or C++; in particular, both
casts and assignments cause values to be rounded to their semantic
types (whereas -ffloat-store only affects assignments). This option
is enabled by default for C or C++ if a strict conformance option
such as -std=c99 or -std=c++17 is used. -ffast-math enables
-fexcess-precision=fast by default regardless of whether a strict
conformance option is used. If -fexcess-precision=16 is specified,
constants and the results of expressions with types "_Float16" and
"__bf16" are computed without excess precision.
-fexcess-precision=standard is not implemented for languages other
than C or C++. On the x86, it has no effect if -mfpmath=sse or
-mfpmath=sse+387 is specified; in the former case, IEEE semantics
apply without excess precision, and in the latter, rounding is
unpredictable.
-ffloat-store
Do not store floating-point variables in registers, and inhibit other
options that might change whether a floating-point value is taken
from a register or memory. This option has generally been subsumed
by -fexcess-precision=standard, which is more general. If you do use
-ffloat-store, you may need to modify your program to explicitly
store intermediate computations in temporary variables since
-ffloat-store handles rounding to IEEE format only on assignments and
not casts as -fexcess-precision=standard does.
-ffast-math
Sets the options -fno-math-errno, -funsafe-math-optimizations,
-ffinite-math-only, -fno-rounding-math, -fno-signaling-nans,
-fcx-limited-range and -fexcess-precision=fast.
This option causes the preprocessor macro "__FAST_MATH__" to be
defined.
This option is not turned on by any -O option besides -Ofast since it
can result in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
-fno-math-errno
Do not set "errno" after calling math functions that are executed
with a single instruction, e.g., "sqrt". A program that relies on
IEEE exceptions for math error handling may want to use this flag for
speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option besides -Ofast since
it can result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is
therefore no reason for the compiler to consider the possibility that
it might, and -fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or ANSI
standards. When used at link time, it may include libraries or
startup files that change the default FPU control word or other
similar optimizations.
This option is not turned on by any -O option besides -Ofast since it
can result in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications. Enables
-fno-signed-zeros, -fno-trapping-math, -fassociative-math and
-freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point
operations. This violates the ISO C and C++ language standard by
possibly changing computation result. NOTE: re-ordering may change
the sign of zero as well as ignore NaNs and inhibit or create
underflow or overflow (and thus cannot be used on code that relies on
rounding behavior like "(x + 2**52) - 2**52". May also reorder
floating-point comparisons and thus may not be used when ordered
comparisons are required. This option requires that both
-fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it
doesn't make much sense with -frounding-math. For Fortran the option
is automatically enabled when both -fno-signed-zeros and
-fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by the
value if this enables optimizations. For example "x / y" can be
replaced with "x * (1/y)", which is useful if "(1/y)" is subject to
common subexpression elimination. Note that this loses precision and
increases the number of flops operating on the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that
arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option besides -Ofast since it
can result in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the behavior of
distinct +0.0 and -0.0 values, which then prohibits simplification of
expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only).
This option implies that the sign of a zero result isn't significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate
user-visible traps. These traps include division by zero, overflow,
underflow, inexact result and invalid operation. This option
requires that -fno-signaling-nans be in effect. Setting this option
may allow faster code if one relies on "non-stop" IEEE arithmetic,
for example.
This option is not turned on by any -O option besides -Ofast since it
can result in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions.
The default is -ftrapping-math.
Future versions of GCC may provide finer control of this setting
using C99's "FENV_ACCESS" pragma. This command-line option will be
used along with -frounding-math to specify the default state for
"FENV_ACCESS".
-frounding-math
Disable transformations and optimizations that assume default
floating-point rounding behavior (round-to-nearest). This option
should be specified for programs that change the FP rounding mode
dynamically, or that may be executed with a non-default rounding
mode. This option disables constant folding of floating-point
expressions at compile time (which may be affected by rounding mode)
and arithmetic transformations that are unsafe in the presence of
sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected by rounding mode.
Future versions of GCC may provide finer control of this setting
using C99's "FENV_ACCESS" pragma. This command-line option will be
used along with -ftrapping-math to specify the default state for
"FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-
visible traps during floating-point operations. Setting this option
disables optimizations that may change the number of exceptions
visible with signaling NaNs. This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be
defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that affect signaling NaN behavior.
-fno-fp-int-builtin-inexact
Do not allow the built-in functions "ceil", "floor", "round" and
"trunc", and their "float" and "long double" variants, to generate
code that raises the "inexact" floating-point exception for
noninteger arguments. ISO C99 and C11 allow these functions to raise
the "inexact" exception, but ISO/IEC TS 18661-1:2014, the C bindings
to IEEE 754-2008, as integrated into ISO C23, does not allow these
functions to do so.
The default is -ffp-int-builtin-inexact, allowing the exception to be
raised, unless C23 or a later C standard is selected. This option
does nothing unless -ftrapping-math is in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions
generate a call to a library function then the "inexact" exception
may be raised if the library implementation does not follow TS 18661.
-fsingle-precision-constant
Treat floating-point constants as single precision instead of
implicitly converting them to double-precision constants.
-fcx-limited-range
When enabled, this option states that a range reduction step is not
needed when performing complex division. Also, there is no checking
whether the result of a complex multiplication or division is "NaN +
I*NaN", with an attempt to rescue the situation in that case. The
default is -fno-cx-limited-range, but is enabled by -ffast-math.
This option controls the default setting of the ISO C99
"CX_LIMITED_RANGE" pragma. Nevertheless, the option applies to all
languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range
reduction is done as part of complex division, but there is no
checking whether the result of a complex multiplication or division
is "NaN + I*NaN", with an attempt to rescue the situation in that
case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance,
but are not enabled by any -O options. This section includes
experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile
it a second time using -fbranch-probabilities, to improve
optimizations based on the number of times each branch was taken.
When a program compiled with -fprofile-arcs exits, it saves arc
execution counts to a file called sourcename.gcda for each source
file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source code
and the same optimization options for both compilations. See details
about the file naming in -fprofile-arcs.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each
JUMP_INSN and CALL_INSN. These can be used to improve optimization.
Currently, they are only used in one place: in reorg.cc, instead of
guessing which path a branch is most likely to take, the REG_BR_PROB
values are used to exactly determine which path is taken more often.
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data about
values of expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered from
profiling values of expressions for usage in optimizations.
Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.
-fprofile-reorder-functions
Function reordering based on profile instrumentation collects first
time of execution of a function and orders these functions in
ascending order, aiming to optimize program startup through more
efficient loading of text segments.
Enabled with -fprofile-use.
-fvpt
If combined with -fprofile-arcs, this option instructs the compiler
to add code to gather information about values of expressions.
With -fbranch-probabilities, it reads back the data gathered and
actually performs the optimizations based on them. Currently the
optimizations include specialization of division operations using the
knowledge about the value of the denominator.
Enabled with -fprofile-use and -fauto-profile.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This optimization
most benefits processors with lots of registers. Depending on the
debug information format adopted by the target, however, it can make
debugging impossible, since variables no longer stay in a "home
register".
Enabled by default with -funroll-loops.
-fschedule-fusion
Performs a target dependent pass over the instruction stream to
schedule instructions of same type together because target machine
can execute them more efficiently if they are adjacent to each other
in the instruction flow.
Enabled at levels -O2, -O3, -Os.
-ftracer
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function allowing
other optimizations to do a better job.
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile
time or upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop, -fweb and -frename-registers. It also turns
on complete loop peeling (i.e. complete removal of loops with a small
constant number of iterations). This option makes code larger, and
may or may not make it run faster.
Enabled by -fprofile-use and -fauto-profile.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain
when the loop is entered. This usually makes programs run more
slowly. -funroll-all-loops implies the same options as
-funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that they do not
roll much (from profile feedback or static analysis). It also turns
on complete loop peeling (i.e. complete removal of loops with small
constant number of iterations).
Enabled by -O3, -fprofile-use, and -fauto-profile.
-fmalloc-dce
Control whether "malloc" (and its variants such as "calloc" or
"strdup"), can be optimized away provided its return value is only
used as a parameter of "free" call or compared with "NULL". If
-fmalloc-dce=1 is used, only calls to "free" are allowed while with
-fmalloc-dce=2 also comparisons with "NULL" pointer are considered
safe to remove.
The default is -fmalloc-dce=2. See also -fallocation-dce.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer.
Enabled at level -O1 and higher, except for -Og.
-fmove-loop-stores
Enables the loop store motion pass in the GIMPLE loop optimizer.
This moves invariant stores to after the end of the loop in exchange
for carrying the stored value in a register across the iteration.
Note for this option to have an effect -ftree-loop-im has to be
enabled as well. Enabled at level -O1 and higher, except for -Og.
-fsplit-loops
Split a loop into two if it contains a condition that's always true
for one side of the iteration space and false for the other.
Enabled by -fprofile-use and -fauto-profile.
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with
duplicates of the loop on both branches (modified according to result
of the condition).
Enabled by -fprofile-use and -fauto-profile.
-fversion-loops-for-strides
If a loop iterates over an array with a variable stride, create
another version of the loop that assumes the stride is always one.
For example:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;
becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;
This is particularly useful for assumed-shape arrays in Fortran where
(for example) it allows better vectorization assuming contiguous
accesses. This flag is enabled by default at -O3. It is also
enabled by -fprofile-use and -fauto-profile.
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output
file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name
in the output file.
Use these options on systems where the linker can perform
optimizations to improve locality of reference in the instruction
space. Most systems using the ELF object format have linkers with
such optimizations. On AIX, the linker rearranges sections (CSECTs)
based on the call graph. The performance impact varies.
Together with a linker garbage collection (linker --gc-sections
option) these options may lead to smaller statically-linked
executables (after stripping).
On ELF/DWARF systems these options do not degenerate the quality of
the debug information. There could be issues with other object
files/debug info formats.
Only use these options when there are significant benefits from doing
so. When you specify these options, the assembler and linker create
larger object and executable files and are also slower. These
options affect code generation. They prevent optimizations by the
compiler and assembler using relative locations inside a translation
unit since the locations are unknown until link time. An example of
such an optimization is relaxing calls to short call instructions.
-fstdarg-opt
Optimize the prologue of variadic argument functions with respect to
usage of those arguments.
-fsection-anchors
Try to reduce the number of symbolic address calculations by using
shared "anchor" symbols to address nearby objects. This
transformation can help to reduce the number of GOT entries and GOT
accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if you
compile it with -fsection-anchors, it accesses the variables from a
common anchor point instead. The effect is similar to the following
pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
-fzero-call-used-regs=choice
Zero call-used registers at function return to increase program
security by either mitigating Return-Oriented Programming (ROP)
attacks or preventing information leakage through registers.
The possible values of choice are the same as for the
"zero_call_used_regs" attribute. The default is skip.
You can control this behavior for a specific function by using the
function attribute "zero_call_used_regs".
--param name=value
In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC does not inline
functions that contain more than a certain number of instructions.
You can control some of these constants on the command line using the
--param option.
The names of specific parameters, and the meaning of the values, are
tied to the internals of the compiler, and are subject to change
without notice in future releases.
In order to get the minimal, maximal and default values of a
parameter, use the --help=param -Q options.
In each case, the value is an integer. The following choices of name
are recognized for all targets:
phiopt-factor-max-stmts-live
When factoring statements out of if/then/else, this is the max #
of statements after the defining statement to be allow to extend
the lifetime of a name
predictable-branch-outcome
When branch is predicted to be taken with probability lower than
this threshold (in percent), then it is considered well
predictable.
max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional branches around a
block and replace them with conditionally executed instructions.
This parameter gives the maximum number of instructions in a
block which should be considered for if-conversion. The compiler
will also use other heuristics to decide whether if-conversion is
likely to be profitable.
file-cache-files
Max number of files in the file cache. The file cache is used to
print source lines in diagnostics and do some source checks like
-Wmisleading-indentation.
file-cache-lines
Max number of lines to index into file cache. When 0 this is
automatically sized. The file cache is used to print source
lines in diagnostics and do some source checks like
-Wmisleading-indentation.
max-rtl-if-conversion-predictable-cost
RTL if-conversion will try to remove conditional branches around
a block and replace them with conditionally executed
instructions. These parameters give the maximum permissible cost
for the sequence that would be generated by if-conversion
depending on whether the branch is statically determined to be
predictable or not. The units for this parameter are the same as
those for the GCC internal seq_cost metric. The compiler will
try to provide a reasonable default for this parameter using the
BRANCH_COST target macro.
max-crossjump-edges
The maximum number of incoming edges to consider for cross-
jumping. The algorithm used by -fcrossjumping is O(N^2) in the
number of edges incoming to each block. Increasing values mean
more aggressive optimization, making the compilation time
increase with probably small improvement in executable size.
min-crossjump-insns
The minimum number of instructions that must be matched at the
end of two blocks before cross-jumping is performed on them.
This value is ignored in the case where all instructions in the
block being cross-jumped from are matched.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks
instead of jumping. The expansion is relative to a jump
instruction.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that
jumps to a computed goto. To avoid O(N^2) behavior in a number
of passes, GCC factors computed gotos early in the compilation
process, and unfactors them as late as possible. Only computed
jumps at the end of a basic blocks with no more than max-goto-
duplication-insns are unfactored.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for
an instruction to fill a delay slot. If more than this arbitrary
number of instructions are searched, the time savings from
filling the delay slot are minimal, so stop searching.
Increasing values mean more aggressive optimization, making the
compilation time increase with probably small improvement in
execution time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of
instructions to consider when searching for a block with valid
live register information. Increasing this arbitrarily chosen
value means more aggressive optimization, increasing the
compilation time. This parameter should be removed when the
delay slot code is rewritten to maintain the control-flow graph.
max-gcse-memory
The approximate maximum amount of memory in "kB" that can be
allocated in order to perform the global common subexpression
elimination optimization. If more memory than specified is
required, the optimization is not done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger than
this value for any expression, then RTL PRE inserts or removes
the expression and thus leaves partially redundant computations
in the instruction stream.
max-pending-list-length
The maximum number of pending dependencies scheduling allows
before flushing the current state and starting over. Large
functions with few branches or calls can create excessively large
lists which needlessly consume memory and resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should
make when modulo scheduling a loop. Larger values can
exponentially increase compilation time.
max-inline-functions-called-once-loop-depth
Maximal loop depth of a call considered by inline heuristics that
tries to inline all functions called once.
max-inline-functions-called-once-insns
Maximal estimated size of functions produced while inlining
functions called once.
max-inline-insns-single
Several parameters control the tree inliner used in GCC. This
number sets the maximum number of instructions (counted in GCC's
internal representation) in a single function that the tree
inliner considers for inlining. This only affects functions
declared inline and methods implemented in a class declaration
(C++).
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of
functions that would otherwise not be considered for inlining by
the compiler are investigated. To those functions, a different
(more restrictive) limit compared to functions declared inline
can be applied (--param max-inline-insns-auto).
max-inline-insns-small
This is the bound applied to calls that are considered relevant
with -finline-small-functions.
max-inline-insns-size
This is the bound applied to calls that are optimized for size.
Small growth may be desirable to anticipate optimization
opportunities exposed by inlining.
uninlined-function-insns
Number of instructions accounted by inliner for function overhead
such as function prologue and epilogue.
uninlined-function-time
Extra time accounted by inliner for function overhead such as
time needed to execute function prologue and epilogue.
inline-heuristics-hint-percent
The scale (in percents) applied to inline-insns-single,
inline-insns-single-O2, inline-insns-auto when inline heuristics
hints that inlining is very profitable (will enable later
optimizations).
uninlined-thunk-insns
uninlined-thunk-time
Same as --param uninlined-function-insns and --param uninlined-
function-time but applied to function thunks.
inline-min-speedup
When estimated performance improvement of caller + callee runtime
exceeds this threshold (in percent), the function can be inlined
regardless of the limit on --param max-inline-insns-single and
--param max-inline-insns-auto.
large-function-insns
The limit specifying really large functions. For functions
larger than this limit after inlining, inlining is constrained by
--param large-function-growth. This parameter is useful
primarily to avoid extreme compilation time caused by non-linear
algorithms used by the back end.
large-function-growth
Specifies maximal growth of large functions caused by inlining in
percents. For example, parameter value 100 limits large function
growth to 2.0 times the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused by
inlining of units larger than this limit is limited by --param
inline-unit-growth. For small units this might be too tight.
For example, consider a unit consisting of function A that is
inline and B that just calls A three times. If B is small
relative to A, the growth of unit is 300\% and yet such inlining
is very sane. For very large units consisting of small
inlineable functions, however, the overall unit growth limit is
needed to avoid exponential explosion of code size. Thus for
smaller units, the size is increased to --param large-unit-insns
before applying --param inline-unit-growth.
lazy-modules
Maximum number of concurrently open C++ module files when lazy
loading.
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused
by inlining. For example, parameter value 20 limits unit growth
to 1.2 times the original size. Cold functions (either marked
cold via an attribute or by profile feedback) are not accounted
into the unit size.
ipa-cp-unit-growth
Specifies maximal overall growth of the compilation unit caused
by interprocedural constant propagation. For example, parameter
value 10 limits unit growth to 1.1 times the original size.
ipa-cp-large-unit-insns
The size of translation unit that IPA-CP pass considers large.
large-stack-frame
The limit specifying large stack frames. While inlining the
algorithm is trying to not grow past this limit too much.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by inlining
in percents. For example, parameter value 1000 limits large
stack frame growth to 11 times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-line copy
of a self-recursive inline function can grow into by performing
recursive inlining.
--param max-inline-insns-recursive applies to functions declared
inline. For functions not declared inline, recursive inlining
happens only when -finline-functions (included in -O3) is
enabled; --param max-inline-insns-recursive-auto applies instead.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive
inlining.
--param max-inline-recursive-depth applies to functions declared
inline. For functions not declared inline, recursive inlining
happens only when -finline-functions (included in -O3) is
enabled; --param max-inline-recursive-depth-auto applies instead.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep
recursion in average and can hurt for function having little
recursion depth by increasing the prologue size or complexity of
function body to other optimizers.
When profile feedback is available (see -fprofile-generate) the
actual recursion depth can be guessed from the probability that
function recurses via a given call expression. This parameter
limits inlining only to call expressions whose probability
exceeds the given threshold (in percents).
early-inlining-insns
Specify growth that the early inliner can make. In effect it
increases the amount of inlining for code having a large
abstraction penalty.
max-early-inliner-iterations
Limit of iterations of the early inliner. This basically bounds
the number of nested indirect calls the early inliner can
resolve. Deeper chains are still handled by late inlining.
comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat
visibility are shared across multiple compilation units.
modref-max-bases
modref-max-refs
modref-max-accesses
Specifies the maximal number of base pointers, references and
accesses stored for a single function by mod/ref analysis.
modref-max-tests
Specifies the maxmal number of tests alias oracle can perform to
disambiguate memory locations using the mod/ref information.
This parameter ought to be bigger than --param modref-max-bases
and --param modref-max-refs.
modref-max-depth
Specifies the maximum depth of DFS walk used by modref escape
analysis. Setting to 0 disables the analysis completely.
modref-max-escape-points
Specifies the maximum number of escape points tracked by modref
per SSA-name.
modref-max-adjustments
Specifies the maximum number the access range is enlarged during
modref dataflow analysis.
profile-func-internal-id
A parameter to control whether to use function internal id in
profile database lookup. If the value is 0, the compiler uses an
id that is based on function assembler name and filename, which
makes old profile data more tolerant to source changes such as
function reordering etc.
min-vect-loop-bound
The minimum number of iterations under which loops are not
vectorized when -ftree-vectorize is used. The number of
iterations after vectorization needs to be greater than the value
specified by this option to allow vectorization.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression
can be moved by GCSE optimizations. This is currently supported
only in the code hoisting pass. The bigger the ratio, the more
aggressive code hoisting is with simple expressions, i.e., the
expressions that have cost less than gcse-unrestricted-cost.
Specifying 0 disables hoisting of simple expressions.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical machine
instruction, at which GCSE optimizations do not constrain the
distance an expression can travel. This is currently supported
only in the code hoisting pass. The lesser the cost, the more
aggressive code hoisting is. Specifying 0 allows all expressions
to travel unrestricted distances.
max-hoist-depth
The depth of search in the dominator tree for expressions to
hoist. This is used to avoid quadratic behavior in hoisting
algorithm. The value of 0 does not limit on the search, but may
slow down compilation of huge functions.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This is
used to avoid quadratic behavior in tree tail merging.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the function.
This is used to limit compilation time in tree tail merging.
store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned stores if it
is legal to do so.
max-stores-to-merge
The maximum number of stores to attempt to merge into wider
stores in the store merging pass.
max-store-chains-to-track
The maximum number of store chains to track at the same time in
the attempt to merge them into wider stores in the store merging
pass.
max-stores-to-track
The maximum number of stores to track at the same time in the
attemt to to merge them into wider stores in the store merging
pass.
max-unrolled-insns
The maximum number of instructions that a loop may have to be
unrolled. If a loop is unrolled, this parameter also determines
how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of
their execution that a loop may have to be unrolled. If a loop
is unrolled, this parameter also determines how many times the
loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop may have to be
peeled. If a loop is peeled, this parameter also determines how
many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-peel-branches
The maximum number of branches on the hot path through the peeled
sequence.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for
complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-depth
The maximum depth of a loop nest to be unswitched.
lim-expensive
The minimum cost of an expensive expression in the loop invariant
motion.
min-loop-cond-split-prob
When FDO profile information is available, min-loop-cond-split-
prob specifies minimum threshold for probability of semi-
invariant condition statement to trigger loop split.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables, below
which all candidates are considered for each use in induction
variable optimizations. If there are more candidates than this,
only the most relevant ones are considered to avoid quadratic
time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that
contain more induction variable uses.
iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller than this
value, always try to remove unnecessary ivs from the set when
adding a new one.
avg-loop-niter
Average number of iterations of a loop.
dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise by dead store
elimination. Larger values may result in larger compilation
times.
dse-max-alias-queries-per-store
Maximum number of queries into the alias oracle per store.
Larger values result in larger compilation times and may result
in more removed dead stores.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions
analyzer. Large expressions slow the analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar
evolutions analyzer. Complex expressions slow the analyzer.
max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by TREE if
conversion unless the loop is marked with simd pragma.
vect-max-layout-candidates
The maximum number of possible vector layouts (such as
permutations) to consider when optimizing to-be-vectorized code.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed when
doing loop versioning for alignment in the vectorizer.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed when
doing loop versioning for alias in the vectorizer.
vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access alignment for
vectorizer. Value -1 means no limit.
max-iterations-to-track
The maximum number of iterations of a loop the brute-force
algorithm for analysis of the number of iterations of the loop
tries to evaluate.
hot-bb-count-fraction
The denominator n of fraction 1/n of the maximal execution count
of a basic block in the entire program that a basic block needs
to at least have in order to be considered hot. The default is
10000, which means that a basic block is considered hot if its
execution count is greater than 1/10000 of the maximal execution
count. 0 means that it is never considered hot. Used in non-LTO
mode.
hot-bb-count-ws-permille
The number of most executed permilles, ranging from 0 to 1000, of
the profiled execution of the entire program to which the
execution count of a basic block must be part of in order to be
considered hot. The default is 990, which means that a basic
block is considered hot if its execution count contributes to the
upper 990 permilles, or 99.0%, of the profiled execution of the
entire program. 0 means that it is never considered hot. Used
in LTO mode.
hot-bb-frequency-fraction
The denominator n of fraction 1/n of the execution frequency of
the entry block of a function that a basic block of this function
needs to at least have in order to be considered hot. The
default is 1000, which means that a basic block is considered hot
in a function if it is executed more frequently than 1/1000 of
the frequency of the entry block of the function. 0 means that
it is never considered hot.
unlikely-bb-count-fraction
The denominator n of fraction 1/n of the number of profiled runs
of the entire program below which the execution count of a basic
block must be in order for the basic block to be considered
unlikely executed. The default is 20, which means that a basic
block is considered unlikely executed if it is executed in fewer
than 1/20, or 5%, of the runs of the program. 0 means that it is
always considered unlikely executed.
max-predicted-iterations
The maximum number of loop iterations we predict statically.
This is useful in cases where a function contains a single loop
with known bound and another loop with unknown bound. The known
number of iterations is predicted correctly, while the unknown
number of iterations average to roughly 10. This means that the
loop without bounds appears artificially cold relative to the
other one.
builtin-expect-probability
Control the probability of the expression having the specified
value. This parameter takes a percentage (i.e. 0 ... 100) as
input.
builtin-string-cmp-inline-length
The maximum length of a constant string for a builtin string cmp
call eligible for inlining.
align-threshold
Select fraction of the maximal frequency of executions of a basic
block in a function to align the basic block.
align-loop-iterations
A loop expected to iterate at least the selected number of
iterations is aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given
percentage of executed instructions is covered. This limits
unnecessary code size expansion.
The tracer-dynamic-coverage-feedback parameter is used only when
profile feedback is available. The real profiles (as opposed to
statically estimated ones) are much less balanced allowing the
threshold to be larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given
percentage. This is a rather artificial limit, as most of the
duplicates are eliminated later in cross jumping, so it may be
set to much higher values than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is
less than this threshold (in percent).
tracer-min-branch-probability
tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability lower than
this threshold.
Similarly to tracer-dynamic-coverage two parameters are provided.
tracer-min-branch-probability-feedback is used for compilation
with profile feedback and tracer-min-branch-probability
compilation without. The value for compilation with profile
feedback needs to be more conservative (higher) in order to make
tracer effective.
stack-clash-protection-guard-size
Specify the size of the operating system provided stack guard as
2 raised to num bytes. Higher values may reduce the number of
explicit probes, but a value larger than the operating system
provided guard will leave code vulnerable to stack clash style
attacks.
stack-clash-protection-probe-interval
Stack clash protection involves probing stack space as it is
allocated. This param controls the maximum distance between
probes into the stack as 2 raised to num bytes. Higher values
may reduce the number of explicit probes, but a value larger than
the operating system provided guard will leave code vulnerable to
stack clash style attacks.
max-cse-path-length
The maximum number of basic blocks on path that CSE considers.
max-cse-insns
The maximum number of instructions CSE processes before flushing.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation.
This parameter specifies the minimum percentage by which the
garbage collector's heap should be allowed to expand between
collections. Tuning this may improve compilation speed; it has
no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100%
when RAM >= 1GB. If "getrlimit" is available, the notion of
"RAM" is the smallest of actual RAM and "RLIMIT_DATA" or
"RLIMIT_AS". If GCC is not able to calculate RAM on a particular
platform, the lower bound of 30% is used. Setting this parameter
and ggc-min-heapsize to zero causes a full collection to occur at
every opportunity. This is extremely slow, but can be useful for
debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins
bothering to collect garbage. The first collection occurs after
the heap expands by ggc-min-expand% beyond ggc-min-heapsize.
Again, tuning this may improve compilation speed, and has no
effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that
tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded,
but with a lower bound of 4096 (four megabytes) and an upper
bound of 131072 (128 megabytes). If GCC is not able to calculate
RAM on a particular platform, the lower bound is used. Setting
this parameter very large effectively disables garbage
collection. Setting this parameter and ggc-min-expand to zero
causes a full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward for
equivalent register. Increasing values mean more aggressive
optimization, making the compilation time increase with probably
slightly better performance.
max-cselib-memory-locations
The maximum number of memory locations cselib should take into
account. Increasing values mean more aggressive optimization,
making the compilation time increase with probably slightly
better performance.
max-sched-ready-insns
The maximum number of instructions ready to be issued the
scheduler should consider at any given time during the first
scheduling pass. Increasing values mean more thorough searches,
making the compilation time increase with probably little
benefit.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for
interblock scheduling.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for
pipelining in the selective scheduler.
max-sched-region-insns
The maximum number of insns in a region to be considered for
interblock scheduling.
max-pipeline-region-insns
The maximum number of insns in a region to be considered for
pipelining in the selective scheduler.
min-spec-prob
The minimum probability (in percents) of reaching a source block
for interblock speculative scheduling.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions.
A value of 0 disables region extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for
speculative motion.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so
that speculative insns are scheduled.
sched-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to
save its state across it.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load targeting
same memory locations.
selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling.
It is a depth of search for available instructions.
selsched-max-sched-times
The maximum number of times that an instruction is scheduled
during selective scheduling. This is the limit on the number of
iterations through which the instruction may be pipelined.
selsched-insns-to-rename
The maximum number of best instructions in the ready list that
are considered for renaming in the selective scheduler.
sms-min-sc
The minimum value of stage count that swing modulo scheduler
generates.
max-last-value-rtl
The maximum size measured as number of RTLs that can be recorded
in an expression in combiner for a pseudo register as last known
value of that register.
max-combine-insns
The maximum number of instructions the RTL combiner tries to
combine.
max-combine-search-insns
The maximum number of instructions that the RTL combiner searches
in order to find the next use of a given register definition. If
this limit is reached without finding such a use, the combiner
will stop trying to optimize the definition.
Currently this limit only applies after certain successful
combination attempts, but it could be extended to other cases in
future.
integer-share-limit
Small integer constants can use a shared data structure, reducing
the compiler's memory usage and increasing its speed. This sets
the maximum value of a shared integer constant.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive stack
smashing protection when -fstack-protector is used.
min-size-for-stack-sharing
The minimum size of variables taking part in stack slot sharing
when not optimizing.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be
duplicated when threading jumps.
max-jump-thread-paths
The maximum number of paths to consider when searching for jump
threading opportunities. When arriving at a block, incoming
edges are only considered if the number of paths to be searched
so far multiplied by the number of incoming edges does not
exhaust the specified maximum number of paths to consider.
max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a field
sensitive manner during pointer analysis.
prefetch-latency
Estimate on average number of instructions that are executed
before prefetch finishes. The distance prefetched ahead is
proportional to this constant. Increasing this number may also
lead to less streams being prefetched (see simultaneous-
prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the same time.
l1-cache-line-size
The size of cache line in L1 data cache, in bytes.
l1-cache-size
The size of L1 data cache, in kilobytes.
l2-cache-size
The size of L2 data cache, in kilobytes.
prefetch-dynamic-strides
Whether the loop array prefetch pass should issue software
prefetch hints for strides that are non-constant. In some cases
this may be beneficial, though the fact the stride is non-
constant may make it hard to predict when there is clear benefit
to issuing these hints.
Set to 1 if the prefetch hints should be issued for non-constant
strides. Set to 0 if prefetch hints should be issued only for
strides that are known to be constant and below prefetch-minimum-
stride.
prefetch-minimum-stride
Minimum constant stride, in bytes, to start using prefetch hints
for. If the stride is less than this threshold, prefetch hints
will not be issued.
This setting is useful for processors that have hardware
prefetchers, in which case there may be conflicts between the
hardware prefetchers and the software prefetchers. If the
hardware prefetchers have a maximum stride they can handle, it
should be used here to improve the use of software prefetchers.
A value of -1 means we don't have a threshold and therefore
prefetch hints can be issued for any constant stride.
This setting is only useful for strides that are known and
constant.
destructive-interference-size
constructive-interference-size
The values for the C++17 variables
"std::hardware_destructive_interference_size" and
"std::hardware_constructive_interference_size". The destructive
interference size is the minimum recommended offset between two
independent concurrently-accessed objects; the constructive
interference size is the maximum recommended size of contiguous
memory accessed together. Typically both will be the size of an
L1 cache line for the target, in bytes. For a generic target
covering a range of L1 cache line sizes, typically the
constructive interference size will be the small end of the range
and the destructive size will be the large end.
The destructive interference size is intended to be used for
layout, and thus has ABI impact. The default value is not
expected to be stable, and on some targets varies with -mtune, so
use of this variable in a context where ABI stability is
important, such as the public interface of a library, is strongly
discouraged; if it is used in that context, users can stabilize
the value using this option.
The constructive interference size is less sensitive, as it is
typically only used in a static_assert to make sure that a type
fits within a cache line.
See also -Winterference-size.
loop-interchange-max-num-stmts
The maximum number of stmts in a loop to be interchanged.
loop-interchange-stride-ratio
The minimum ratio between stride of two loops for interchange to
be profitable.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the
number of prefetches to enable prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the
number of memory references to enable prefetching in a loop.
use-canonical-types
Whether the compiler should use the "canonical" type system.
Should always be 1, which uses a more efficient internal
mechanism for comparing types in C++ and Objective-C++. However,
if bugs in the canonical type system are causing compilation
failures, set this value to 0 to disable canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays that
are bigger than switch-conversion-max-branch-ratio times the
number of branches in the switch.
switch-lower-slow-alg-max-cases
Maximum number of cases for slow switch lowering algorithms to be
used.
max-partial-antic-length
Maximum length of the partial antic set computed during the tree
partial redundancy elimination optimization (-ftree-pre) when
optimizing at -O3 and above. For some sorts of source code the
enhanced partial redundancy elimination optimization can run
away, consuming all of the memory available on the host machine.
This parameter sets a limit on the length of the sets that are
computed, which prevents the runaway behavior. Setting a value
of 0 for this parameter allows an unlimited set length.
rpo-vn-max-loop-depth
Maximum loop depth that is value-numbered optimistically. When
the limit hits the innermost rpo-vn-max-loop-depth loops and the
outermost loop in the loop nest are value-numbered optimistically
and the remaining ones not.
sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when looking
for redundancies for loads and stores. If this limit is hit the
search is aborted and the load or store is not considered
redundant. The number of queries is algorithmically limited to
the number of stores on all paths from the load to the function
entry.
ira-max-loops-num
IRA uses regional register allocation by default. If a function
contains more loops than the number given by this parameter, only
at most the given number of the most frequently-executed loops
form regions for regional register allocation.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the
conflict table, the table can still require excessive amounts of
memory for huge functions. If the conflict table for a function
could be more than the size in MB given by this parameter, the
register allocator instead uses a faster, simpler, and lower-
quality algorithm that does not require building a pseudo-
register conflict table.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in
loops for decisions to move loop invariants (see -O3). The
number of available registers reserved for some other purposes is
given by this parameter. Default of the parameter is the best
found from numerous experiments.
ira-consider-dup-in-all-alts
Make IRA to consider matching constraint (duplicated operand
number) heavily in all available alternatives for preferred
register class. If it is set as zero, it means IRA only respects
the matching constraint when it's in the only available
alternative with an appropriate register class. Otherwise, it
means IRA will check all available alternatives for preferred
register class even if it has found some choice with an
appropriate register class and respect the found qualified
matching constraint.
ira-simple-lra-insn-threshold
Approximate function insn number in 1K units triggering simple
local RA.
lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in subsequent
insns. This optimization is called inheritance. EBB is used as
a region to do this optimization. The parameter defines a
minimal fall-through edge probability in percentage used to add
BB to inheritance EBB in LRA. The default value was chosen from
numerous runs of SPEC2000 on x86-64.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in compilation
time and in amount of needed compile-time memory, with very large
loops. Loops with more basic blocks than this parameter won't
have loop invariant motion optimization performed on them.
loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large loops.
This parameter limits the number of data references in loops that
are considered for data dependence analysis. These large loops
are no handled by the optimizations using loop data dependencies.
max-vartrack-size
Sets a maximum number of hash table slots to use during variable
tracking dataflow analysis of any function. If this limit is
exceeded with variable tracking at assignments enabled, analysis
for that function is retried without it, after removing all debug
insns from the function. If the limit is exceeded even without
debug insns, var tracking analysis is completely disabled for the
function. Setting the parameter to zero makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to map
variable names or debug temporaries to value expressions. This
trades compilation time for more complete debug information. If
this is set too low, value expressions that are available and
could be represented in debug information may end up not being
used; setting this higher may enable the compiler to find more
complex debug expressions, but compile time and memory use may
grow.
max-debug-marker-count
Sets a threshold on the number of debug markers (e.g. begin stmt
markers) to avoid complexity explosion at inlining or expanding
to RTL. If a function has more such gimple stmts than the set
limit, such stmts will be dropped from the inlined copy of a
function, and from its RTL expansion.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The
range below the parameter is reserved exclusively for debug insns
created by -fvar-tracking-assignments, but debug insns may get
(non-overlapping) uids above it if the reserved range is
exhausted.
ipa-sra-deref-prob-threshold
IPA-SRA replaces a pointer which is known not be NULL with one or
more new parameters only when the probability (in percent,
relative to function entry) of it being dereferenced is higher
than this parameter.
ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more new
parameters only when their cumulative size is less or equal to
ipa-sra-ptr-growth-factor times the size of the original pointer
parameter.
ipa-sra-ptrwrap-growth-factor
Additional maximum allowed growth of total size of new parameters
that ipa-sra replaces a pointer to an aggregate with, if it
points to a local variable that the caller only writes to and
passes it as an argument to other functions.
ipa-sra-max-replacements
Maximum pieces of an aggregate that IPA-SRA tracks. As a
consequence, it is also the maximum number of replacements of a
formal parameter.
sra-max-scalarization-size-Ospeed
sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA)
aim to replace scalar parts of aggregates with uses of
independent scalar variables. These parameters control the
maximum size, in storage units, of aggregate which is considered
for replacement when compiling for speed (sra-max-scalarization-
size-Ospeed) or size (sra-max-scalarization-size-Osize)
respectively.
sra-max-propagations
The maximum number of artificial accesses that Scalar Replacement
of Aggregates (SRA) will track, per one local variable, in order
to facilitate copy propagation.
tm-max-aggregate-size
When making copies of thread-local variables in a transaction,
this parameter specifies the size in bytes after which variables
are saved with the logging functions as opposed to save/restore
code sequence pairs. This option only applies when using
-fgnu-tm.
graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms, the
number of parameters in a Static Control Part (SCoP) is bounded.
A value of zero can be used to lift the bound. A variable whose
value is unknown at compilation time and defined outside a SCoP
is a parameter of the SCoP.
hardcfr-max-blocks
Disable -fharden-control-flow-redundancy for functions with a
larger number of blocks than the specified value. Zero removes
any limit.
hardcfr-max-inline-blocks
Force -fharden-control-flow-redundancy to use out-of-line
checking for functions with a larger number of basic blocks than
the specified value.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with
-floop-block or -floop-strip-mine, strip mine each loop in the
loop nest by a given number of iterations. The strip length can
be changed using the loop-block-tile-size parameter.
ipa-jump-function-lookups
Specifies number of statements visited during jump function
offset discovery.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed to
a function's parameter in order to propagate them and perform
devirtualization. ipa-cp-value-list-size is the maximum number
of values and types it stores per one formal parameter of a
function.
ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability
heuristics and performs those cloning opportunities with scores
that exceed ipa-cp-eval-threshold.
ipa-cp-max-recursive-depth
Maximum depth of recursive cloning for self-recursive function.
ipa-cp-min-recursive-probability
Recursive cloning only when the probability of call being
executed exceeds the parameter.
ipa-cp-profile-count-base
When using -fprofile-use option, IPA-CP will consider the
measured execution count of a call graph edge at this percentage
position in their histogram as the basis for its heuristics
calculation.
ipa-cp-recursive-freq-factor
The number of times interprocedural copy propagation expects
recursive functions to call themselves.
ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive when they
are evaluated for cloning.
ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to another
function will receive when they are evaluated for cloning.
ipa-max-agg-items
IPA-CP is also capable to propagate a number of scalar values
passed in an aggregate. ipa-max-agg-items controls the maximum
number of such values per one parameter.
ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would make the
number of iterations of a loop known, it adds a bonus of ipa-cp-
loop-hint-bonus to the profitability score of the candidate.
ipa-max-loop-predicates
The maximum number of different predicates IPA will use to
describe when loops in a function have known properties.
ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs alias
analysis in order to track values pointed to by function
parameters. In order not spend too much time analyzing huge
functions, it gives up and consider all memory clobbered after
examining ipa-max-aa-steps statements modifying memory.
ipa-max-switch-predicate-bounds
Maximal number of boundary endpoints of case ranges of switch
statement. For switch exceeding this limit, IPA-CP will not
construct cloning cost predicate, which is used to estimate
cloning benefit, for default case of the switch statement.
ipa-max-param-expr-ops
IPA-CP will analyze conditional statement that references some
function parameter to estimate benefit for cloning upon certain
constant value. But if number of operations in a parameter
expression exceeds ipa-max-param-expr-ops, the expression is
treated as complicated one, and is not handled by IPA analysis.
lto-partitions
Specify desired number of partitions produced during WHOPR
compilation. The number of partitions should exceed the number
of CPUs used for compilation.
lto-min-partition
Size of minimal partition for WHOPR (in estimated instructions).
This prevents expenses of splitting very small programs into too
many partitions.
lto-max-partition
Size of max partition for WHOPR (in estimated instructions). to
provide an upper bound for individual size of partition. Meant
to be used only with balanced partitioning.
lto-partition-locality-frequency-cutoff
The denominator n of fraction 1/n of the execution frequency of
callee to be cloned for a particular caller. Special value of 0
dictates to always clone without a cut-off.
lto-partition-locality-size-cutoff
Size cut-off for callee including inlined calls to be cloned for
a particular caller.
lto-max-locality-partition
Maximal size of a locality partition for LTO (in estimated
instructions). Value of 0 results in default value being used.
lto-max-streaming-parallelism
Maximal number of parallel processes used for LTO streaming.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when
C++ name lookup fails for an identifier.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of the
target block relative to a statement's original block to allow
statement sinking of a statement. Larger numbers result in more
aggressive statement sinking. A small positive adjustment is
applied for statements with memory operands as those are even
more profitable so sink.
max-stores-to-sink
The maximum number of conditional store pairs that can be sunk.
Set to 0 if either vectorization (-ftree-vectorize) or if-
conversion (-ftree-loop-if-convert) is disabled.
case-values-threshold
The smallest number of different values for which it is best to
use a jump-table instead of a tree of conditional branches. If
the value is 0, use the default for the machine.
jump-table-max-growth-ratio-for-size
The maximum code size growth ratio when expanding into a jump
table (in percent). The parameter is used when optimizing for
size.
jump-table-max-growth-ratio-for-speed
The maximum code size growth ratio when expanding into a jump
table (in percent). The parameter is used when optimizing for
speed.
tree-reassoc-width
Set the maximum number of instructions executed in parallel in
reassociated tree. This parameter overrides target dependent
heuristics used by default if has non zero value.
sched-pressure-algorithm
Choose between the two available implementations of
-fsched-pressure. Algorithm 1 is the original implementation and
is the more likely to prevent instructions from being reordered.
Algorithm 2 was designed to be a compromise between the
relatively conservative approach taken by algorithm 1 and the
rather aggressive approach taken by the default scheduler. It
relies more heavily on having a regular register file and
accurate register pressure classes. See haifa-sched.cc in the
GCC sources for more details.
The default choice depends on the target.
max-slsr-cand-scan
Set the maximum number of existing candidates that are considered
when seeking a basis for a new straight-line strength reduction
candidate.
asan-globals
Enable buffer overflow detection for global objects. This kind
of protection is enabled by default if you are using
-fsanitize=address option. To disable global objects protection
use --param asan-globals=0.
asan-stack
Enable buffer overflow detection for stack objects. This kind of
protection is enabled by default when using -fsanitize=address.
To disable stack protection use --param asan-stack=0 option.
asan-instrument-reads
Enable buffer overflow detection for memory reads. This kind of
protection is enabled by default when using -fsanitize=address.
To disable memory reads protection use --param
asan-instrument-reads=0.
asan-instrument-writes
Enable buffer overflow detection for memory writes. This kind of
protection is enabled by default when using -fsanitize=address.
To disable memory writes protection use --param
asan-instrument-writes=0 option.
asan-memintrin
Enable detection for built-in functions. This kind of protection
is enabled by default when using -fsanitize=address. To disable
built-in functions protection use --param asan-memintrin=0.
asan-use-after-return
Enable detection of use-after-return. This kind of protection is
enabled by default when using the -fsanitize=address option. To
disable it use --param asan-use-after-return=0.
Note: By default the check is disabled at run time. To enable
it, add "detect_stack_use_after_return=1" to the environment
variable ASAN_OPTIONS.
asan-instrumentation-with-call-threshold
If number of memory accesses in function being instrumented is
greater or equal to this number, use callbacks instead of inline
checks. E.g. to disable inline code use --param
asan-instrumentation-with-call-threshold=0.
asan-kernel-mem-intrinsic-prefix
If nonzero, prefix calls to "memcpy", "memset" and "memmove" with
__asan_ or __hwasan_ for -fsanitize=kernel-address or
-fsanitize=kernel-hwaddress, respectively.
hwasan-instrument-stack
Enable hwasan instrumentation of statically sized stack-allocated
variables. This kind of instrumentation is enabled by default
when using -fsanitize=hwaddress and disabled by default when
using -fsanitize=kernel-hwaddress. To disable stack
instrumentation use --param hwasan-instrument-stack=0, and to
enable it use --param hwasan-instrument-stack=1.
hwasan-random-frame-tag
When using stack instrumentation, decide tags for stack variables
using a deterministic sequence beginning at a random tag for each
frame. With this parameter unset tags are chosen using the same
sequence but beginning from 1. This is enabled by default for
-fsanitize=hwaddress and unavailable for
-fsanitize=kernel-hwaddress. To disable it use --param
hwasan-random-frame-tag=0.
hwasan-instrument-allocas
Enable hwasan instrumentation of dynamically sized stack-
allocated variables. This kind of instrumentation is enabled by
default when using -fsanitize=hwaddress and disabled by default
when using -fsanitize=kernel-hwaddress. To disable
instrumentation of such variables use --param
hwasan-instrument-allocas=0, and to enable it use --param
hwasan-instrument-allocas=1.
hwasan-instrument-reads
Enable hwasan checks on memory reads. Instrumentation of reads
is enabled by default for both -fsanitize=hwaddress and
-fsanitize=kernel-hwaddress. To disable checking memory reads
use --param hwasan-instrument-reads=0.
hwasan-instrument-writes
Enable hwasan checks on memory writes. Instrumentation of writes
is enabled by default for both -fsanitize=hwaddress and
-fsanitize=kernel-hwaddress. To disable checking memory writes
use --param hwasan-instrument-writes=0.
hwasan-instrument-mem-intrinsics
Enable hwasan instrumentation of builtin functions.
Instrumentation of these builtin functions is enabled by default
for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress.
To disable instrumentation of builtin functions use --param
hwasan-instrument-mem-intrinsics=0.
use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller or equal to
this number, directly poison (or unpoison) shadow memory instead
of using run-time callbacks.
tsan-distinguish-volatile
Emit special instrumentation for accesses to volatiles.
tsan-instrument-func-entry-exit
Emit instrumentation calls to __tsan_func_entry() and
__tsan_func_exit().
max-fsm-thread-path-insns
Maximum number of instructions to copy when duplicating blocks on
a finite state automaton jump thread path.
threader-debug
threader-debug=[none|all] Enables verbose dumping of the threader
solver.
parloops-chunk-size
Chunk size of omp schedule for loops parallelized by parloops.
parloops-schedule
Schedule type of omp schedule for loops parallelized by parloops
(static, dynamic, guided, auto, runtime).
parloops-min-per-thread
The minimum number of iterations per thread of an innermost
parallelized loop for which the parallelized variant is preferred
over the single threaded one. Note that for a parallelized loop
nest the minimum number of iterations of the outermost loop per
thread is two.
max-ssa-name-query-depth
Maximum depth of recursion when querying properties of SSA names
in things like fold routines. One level of recursion corresponds
to following a use-def chain.
max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when looking for a
must-def specifying the dynamic type of an object that invokes a
virtual call we may be able to devirtualize speculatively.
ranger-debug
Specifies the type of debug output to be issued for ranges.
unroll-jam-min-percent
The minimum percentage of memory references that must be
optimized away for the unroll-and-jam transformation to be
considered profitable.
unroll-jam-max-unroll
The maximum number of times the outer loop should be unrolled by
the unroll-and-jam transformation.
max-rtl-if-conversion-unpredictable-cost
Maximum permissible cost for the sequence that would be generated
by the RTL if-conversion pass for a branch that is considered
unpredictable.
max-variable-expansions-in-unroller
If -fvariable-expansion-in-unroller is used, the maximum number
of times that an individual variable will be expanded during loop
unrolling.
partial-inlining-entry-probability
Maximum probability of the entry BB of split region (in percent
relative to entry BB of the function) to make partial inlining
happen.
max-tracked-strlens
Maximum number of strings for which strlen optimization pass will
track string lengths.
gcse-after-reload-partial-fraction
The threshold ratio for performing partial redundancy elimination
after reload.
gcse-after-reload-critical-fraction
The threshold ratio of critical edges execution count that permit
performing redundancy elimination after reload.
max-loop-header-insns
The maximum number of insns in loop header duplicated by the copy
loop headers pass.
vect-epilogues-nomask
Enable loop epilogue vectorization using smaller vector size.
vect-partial-vector-usage
Controls when the loop vectorizer considers using partial vector
loads and stores as an alternative to falling back to scalar
code. 0 stops the vectorizer from ever using partial vector
loads and stores. 1 allows partial vector loads and stores if
vectorization removes the need for the code to iterate. 2 allows
partial vector loads and stores in all loops. The parameter only
has an effect on targets that support partial vector loads and
stores.
vect-inner-loop-cost-factor
The maximum factor which the loop vectorizer applies to the cost
of statements in an inner loop relative to the loop being
vectorized. The factor applied is the maximum of the estimated
number of iterations of the inner loop and this parameter. The
default value of this parameter is 50.
vect-induction-float
Enable loop vectorization of floating point inductions.
vect-force-slp
Force the use of SLP when vectorizing, fail if not possible.
vrp-block-limit
Maximum number of basic blocks before VRP switches to a lower
memory algorithm.
vrp-sparse-threshold
Maximum number of basic blocks before VRP uses a sparse bitmap
cache.
vrp-switch-limit
Maximum number of outgoing edges in a switch before VRP will not
process it.
vrp-vector-threshold
Maximum number of basic blocks for VRP to use a basic cache
vector.
avoid-fma-max-bits
Maximum number of bits for which we avoid creating FMAs.
fully-pipelined-fma
Whether the target fully pipelines FMA instructions. If non-
zero, reassociation considers the benefit of parallelizing FMA's
multiplication part and addition part, assuming FMUL and FMA use
the same units that can also do FADD.
sms-loop-average-count-threshold
A threshold on the average loop count considered by the swing
modulo scheduler.
sms-dfa-history
The number of cycles the swing modulo scheduler considers when
checking conflicts using DFA.
graphite-allow-codegen-errors
Whether codegen errors should be ICEs when -fchecking.
sms-max-ii-factor
A factor for tuning the upper bound that swing modulo scheduler
uses for scheduling a loop.
lra-max-considered-reload-pseudos
The max number of reload pseudos which are considered during
spilling a non-reload pseudo.
max-pow-sqrt-depth
Maximum depth of sqrt chains to use when synthesizing
exponentiation by a real constant.
max-dse-active-local-stores
Maximum number of active local stores in RTL dead store
elimination.
asan-instrument-allocas
Enable asan allocas/VLAs protection.
max-iterations-computation-cost
Bound on the cost of an expression to compute the number of
iterations.
max-isl-operations
Maximum number of isl operations, 0 means unlimited.
graphite-max-arrays-per-scop
Maximum number of arrays per scop.
max-vartrack-reverse-op-size
Max. size of loc list for which reverse ops should be added.
fsm-scale-path-stmts
Scale factor to apply to the number of statements in a threading
path crossing a loop backedge when comparing to
--param=max-jump-thread-duplication-stmts.
uninit-control-dep-attempts
Maximum number of nested calls to search for control dependencies
during uninitialized variable analysis.
uninit-max-chain-len
Maximum number of predicates anded for each predicate ored in the
normalized predicate chain.
uninit-max-num-chains
Maximum number of predicates ored in the normalized predicate
chain.
uninit-max-prune-work
Maximum amount of work done to prune paths where the variable is
always initialized.
sched-autopref-queue-depth
Hardware autoprefetcher scheduler model control flag. Number of
lookahead cycles the model looks into; at ' ' only enable
instruction sorting heuristic.
loop-versioning-max-inner-insns
The maximum number of instructions that an inner loop can have
before the loop versioning pass considers it too big to copy.
loop-versioning-max-outer-insns
The maximum number of instructions that an outer loop can have
before the loop versioning pass considers it too big to copy,
discounting any instructions in inner loops that directly benefit
from versioning.
ssa-name-def-chain-limit
The maximum number of SSA_NAME assignments to follow in
determining a property of a variable such as its value. This
limits the number of iterations or recursive calls GCC performs
when optimizing certain statements or when determining their
validity prior to issuing diagnostics.
store-merging-max-size
Maximum size of a single store merging region in bytes.
store-forwarding-max-distance
Maximum number of instruction distance that a small store
forwarded to a larger load may stall. Value '0' disables the cost
checks for the avoid-store-forwarding pass.
hash-table-verification-limit
The number of elements for which hash table verification is done
for each searched element.
max-find-base-term-values
Maximum number of VALUEs handled during a single find_base_term
call.
analyzer-max-enodes-per-program-point
The maximum number of exploded nodes per program point within the
analyzer, before terminating analysis of that point.
analyzer-max-constraints
The maximum number of constraints per state.
analyzer-min-snodes-for-call-summary
The minimum number of supernodes within a function for the
analyzer to consider summarizing its effects at call sites.
analyzer-max-enodes-for-full-dump
The maximum depth of exploded nodes that should appear in a dot
dump before switching to a less verbose format.
analyzer-max-recursion-depth
The maximum number of times a callsite can appear in a call stack
within the analyzer, before terminating analysis of a call that
would recurse deeper.
analyzer-max-svalue-depth
The maximum depth of a symbolic value, before approximating the
value as unknown.
analyzer-max-infeasible-edges
The maximum number of infeasible edges to reject before declaring
a diagnostic as infeasible.
gimple-fe-computed-hot-bb-threshold
The number of executions of a basic block which is considered
hot. The parameter is used only in GIMPLE FE.
analyzer-bb-explosion-factor
The maximum number of 'after supernode' exploded nodes within the
analyzer per supernode, before terminating analysis.
analyzer-text-art-string-ellipsis-threshold
The number of bytes at which to ellipsize string literals in
analyzer text art diagrams.
analyzer-text-art-ideal-canvas-width
The ideal width in characters of text art diagrams generated by
the analyzer.
analyzer-text-art-string-ellipsis-head-len
The number of literal bytes to show at the head of a string
literal in text art when ellipsizing it.
analyzer-text-art-string-ellipsis-tail-len
The number of literal bytes to show at the tail of a string
literal in text art when ellipsizing it.
ranger-logical-depth
Maximum depth of logical expression evaluation ranger will look
through when evaluating outgoing edge ranges.
ranger-recompute-depth
Maximum depth of instruction chains to consider for recomputation
in the outgoing range calculator.
relation-block-limit
Maximum number of relations the oracle will register in a basic
block.
transitive-relations-work-bound
Work bound when discovering transitive relations from existing
relations.
min-pagesize
Minimum page size for warning and early break vectorization
purposes.
openacc-kernels
Specify mode of OpenACC `kernels' constructs handling. With
--param=openacc-kernels=decompose, OpenACC `kernels' constructs
are decomposed into parts, a sequence of compute constructs, each
then handled individually. This is work in progress. With
--param=openacc-kernels=parloops, OpenACC `kernels' constructs
are handled by the parloops pass, en bloc. This is the current
default.
openacc-privatization
Control whether the -fopt-info-omp-note and applicable
-fdump-tree-*-details options emit OpenACC privatization
diagnostics. With --param=openacc-privatization=quiet, don't
diagnose. This is the current default. With
--param=openacc-privatization=noisy, do diagnose.
cycle-accurate-model
Specifies whether GCC should assume that the scheduling
description is mostly a cycle-accurate model of the target
processor the code is intended to run on, in the absence of cache
misses. Nonzero means that the selected scheduling model is
accurate and likely describes an in-order processor, and that
scheduling should aggressively spill to try and fill any pipeline
bubbles. This is the current default. Zero means the scheduling
description might not be available/accurate or perhaps not
applicable at all, such as for modern out-of-order processors.
The following choices of name are available on AArch64 targets:
aarch64-vect-compare-costs
When vectorizing, consider using multiple different approaches
and use the cost model to choose the cheapest one. This
includes:
* Trying both SVE and Advanced SIMD, when SVE is available.
* Trying to use 64-bit Advanced SIMD vectors for the smallest
data elements, rather than using 128-bit vectors for
everything.
* Trying to use "unpacked" SVE vectors for smaller elements.
This includes storing smaller elements in larger containers
and accessing elements with extending loads and truncating
stores.
aarch64-float-recp-precision
The number of Newton iterations for calculating the reciprocal
for float type. The precision of division is proportional to
this param when division approximation is enabled. The default
value is 1.
aarch64-double-recp-precision
The number of Newton iterations for calculating the reciprocal
for double type. The precision of division is proportional to
this param when division approximation is enabled. The default
value is 2.
aarch64-autovec-preference
Force an ISA selection strategy for auto-vectorization.
default
Use the default heuristics.
asimd-only
Use only Advanced SIMD for auto-vectorization.
sve-only
Use only SVE for auto-vectorization.
prefer-asimd
Use both Advanced SIMD and SVE. Prefer Advanced SIMD when
the costs are deemed equal.
prefer-sve
Use both Advanced SIMD and SVE. Prefer SVE when the costs
are deemed equal.
aarch64-ldp-policy
Fine-grained policy for load pairs. With
--param=aarch64-ldp-policy=default, use the policy of the tuning
structure. This is the current default. With
--param=aarch64-ldp-policy=always, emit ldp regardless of
alignment. With --param=aarch64-ldp-policy=never, do not emit
ldp. With --param=aarch64-ldp-policy=aligned, emit ldp only if
the source pointer is aligned to at least double the alignment of
the type.
aarch64-stp-policy
Fine-grained policy for store pairs. With
--param=aarch64-stp-policy=default, use the policy of the tuning
structure. This is the current default. With
--param=aarch64-stp-policy=always, emit stp regardless of
alignment. With --param=aarch64-stp-policy=never, do not emit
stp. With --param=aarch64-stp-policy=aligned, emit stp only if
the source pointer is aligned to at least double the alignment of
the type.
aarch64-ldp-alias-check-limit
Limit on the number of alias checks performed by the AArch64
load/store pair fusion pass when attempting to form an ldp/stp.
Higher values make the pass more aggressive at re-ordering loads
over stores, at the expense of increased compile time.
aarch64-ldp-writeback
Param to control which writeback opportunities we try to handle
in the AArch64 load/store pair fusion pass. A value of zero
disables writeback handling. One means we try to form pairs
involving one or more existing individual writeback accesses
where possible. A value of two means we also try to
opportunistically form writeback opportunities by folding in
trailing destructive updates of the base register used by a pair.
aarch64-loop-vect-issue-rate-niters
The tuning for some AArch64 CPUs tries to take both latencies and
issue rates into account when deciding whether a loop should be
vectorized using SVE, vectorized using Advanced SIMD, or not
vectorized at all. If this parameter is set to n, GCC will not
use this heuristic for loops that are known to execute in fewer
than n Advanced SIMD iterations.
aarch64-vect-unroll-limit
The vectorizer will use available tuning information to determine
whether it would be beneficial to unroll the main vectorized loop
and by how much. This parameter set's the upper bound of how
much the vectorizer will unroll the main loop. The default value
is four.
The following choices of name are available on GCN targets:
gcn-preferred-vectorization-factor
Preferred vectorization factor: default, 32, 64.
The following choices of name are available on i386 and x86_64
targets:
x86-stlf-window-ninsns
Instructions number above which STFL stall penalty can be
compensated.
x86-stv-max-visits
The maximum number of use and def visits when discovering a STV
chain before the discovery is aborted.
Program Instrumentation Options
GCC supports a number of command-line options that control adding run-
time instrumentation to the code it normally generates. For example, one
purpose of instrumentation is collect profiling statistics for use in
finding program hot spots, code coverage analysis, or profile-guided
optimizations. Another class of program instrumentation is adding run-
time checking to detect programming errors like invalid pointer
dereferences or out-of-bounds array accesses, as well as deliberately
hostile attacks such as stack smashing or C++ vtable hijacking. There is
also a general hook which can be used to implement other forms of tracing
or function-level instrumentation for debug or program analysis purposes.
-p
-pg Generate extra code to write profile information suitable for the
analysis program prof (for -p) or gprof (for -pg). You must use this
option when compiling the source files you want data about, and you
must also use it when linking.
You can use the function attribute "no_instrument_function" to
suppress profiling of individual functions when compiling with these
options.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During
execution the program records how many times each branch and call is
executed and how many times it is taken or returns. On targets that
support constructors with priority support, profiling properly
handles constructors, destructors and C++ constructors (and
destructors) of classes which are used as a type of a global
variable.
When the compiled program exits it saves this data to a file called
auxname.gcda for each source file. The data may be used for profile-
directed optimizations (-fbranch-probabilities), or for test coverage
analysis (-ftest-coverage). Each object file's auxname is generated
from the name of the output file, if explicitly specified and it is
not the final executable, otherwise it is the basename of the source
file. In both cases any suffix is removed (e.g. foo.gcda for input
file dir/foo.c, or dir/foo.gcda for output file specified as -o
dir/foo.o).
Note that if a command line directly links source files, the
corresponding .gcda files will be prefixed with the unsuffixed name
of the output file. E.g. "gcc a.c b.c -o binary" would generate
binary-a.gcda and binary-b.gcda files.
-fcondition-coverage
Add code so that program conditions are instrumented. During
execution the program records what terms in a conditional contributes
to a decision, which can be used to verify that all terms in a
Boolean function are tested and have an independent effect on the
outcome of a decision. The result can be read with "gcov
--conditions".
-fpath-coverage
Add code so that the paths taken are tracked. During execution the
program records the prime paths taken. The number of paths grows
very fast with complexity, and to avoid exploding compile times GCC
will give up instrumentation if the approximate number of paths
exceeds the limit controlled by -fpath-coverage-limit. The result
can be read with "gcov --prime-paths --prime-paths-lines
--prime-paths-source",
-fpath-coverage-limit=limit
The threshold at which point -fpath-coverage gives up on
instrumenting a function. This limit is approximate and
conservative, as GCC uses a pessimistic heuristic which slightly
overcounts the running number of paths, and gives up if the threshold
is reached before finding all the paths. This option is not for fine
grained control over which functions to instrument - rather it is
intended to limit the effect of path explosion and keep compile times
reasonable. The default is 250000.
--coverage
This option is used to compile and link code instrumented for
coverage analysis. The option is a synonym for -fprofile-arcs
-ftest-coverage (when compiling) and -lgcov (when linking). See the
documentation for those options for more details.
* Compile the source files with -fprofile-arcs plus optimization
and code generation options. For test coverage analysis, use the
additional -ftest-coverage option. You do not need to profile
every source file in a program.
* Compile the source files additionally with -fprofile-abs-path to
create absolute path names in the .gcno files. This allows gcov
to find the correct sources in projects where compilations occur
with different working directories.
* Link your object files with -lgcov or -fprofile-arcs (the latter
implies the former).
* Run the program on a representative workload to generate the arc
profile information. This may be repeated any number of times.
You can run concurrent instances of your program, and provided
that the file system supports locking, the data files will be
correctly updated. Unless a strict ISO C dialect option is in
effect, "fork" calls are detected and correctly handled without
double counting.
Moreover, an object file can be recompiled multiple times and the
corresponding .gcda file merges as long as the source file and
the compiler options are unchanged.
* For profile-directed optimizations, compile the source files
again with the same optimization and code generation options plus
-fbranch-probabilities.
* For test coverage analysis, use gcov to produce human readable
information from the .gcno and .gcda files. Refer to the gcov
documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a
program flow graph, then finds a spanning tree for the graph. Only
arcs that are not on the spanning tree have to be instrumented: the
compiler adds code to count the number of times that these arcs are
executed. When an arc is the only exit or only entrance to a block,
the instrumentation code can be added to the block; otherwise, a new
basic block must be created to hold the instrumentation code.
With -fcondition-coverage, for each conditional in your program GCC
creates a bitset and records the exercised boolean values that have
an independent effect on the outcome of that expression.
With -fpath-coverage, GCC finds and enumerates and records the taken
prime paths of each function, unless the number of paths would exceed
the limit controlled by -fpath-coverage-limit. If the limit is
exceeded the function is not instrumented as if -fpath-coverage was
not used. A prime path is the longest sequence of unique blocks,
except possibly the first and last, which is not a subpath of any
other path.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to
show program coverage. Each source file's note file is called
auxname.gcno. Refer to the -fprofile-arcs option above for a
description of auxname and instructions on how to generate test
coverage data. Coverage data matches the source files more closely
if you do not optimize.
-fprofile-abs-path
Automatically convert relative source file names to absolute path
names in the .gcno files. This allows gcov to find the correct
sources in projects where compilations occur with different working
directories.
-fprofile-dir=path
Set the directory to search for the profile data files in to path.
This option affects only the profile data generated by
-fprofile-generate, -ftest-coverage, -fprofile-arcs and used by
-fprofile-use and -fbranch-probabilities and its related options.
Both absolute and relative paths can be used. By default, GCC uses
the current directory as path, thus the profile data file appears in
the same directory as the object file. In order to prevent the file
name clashing, if the object file name is not an absolute path, we
mangle the absolute path of the sourcename.gcda file and use it as
the file name of a .gcda file. See details about the file naming in
-fprofile-arcs. See similar option -fprofile-note.
When an executable is run in a massive parallel environment, it is
recommended to save profile to different folders. That can be done
with variables in path that are exported during run-time:
%p process ID.
%q{VAR}
value of environment variable VAR
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to produce
profile useful for later recompilation with profile feedback based
optimization. You must use -fprofile-generate both when compiling
and when linking your program.
The following options are enabled: -fprofile-arcs, -fprofile-values,
-finline-functions, and -fipa-bit-cp.
If path is specified, GCC looks at the path to find the profile
feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile information,
use -fprofile-use.
-fprofile-info-section
-fprofile-info-section=name
Register the profile information in the specified section instead of
using a constructor/destructor. The section name is name if it is
specified, otherwise the section name defaults to ".gcov_info". A
pointer to the profile information generated by -fprofile-arcs is
placed in the specified section for each translation unit. This
option disables the profile information registration through a
constructor and it disables the profile information processing
through a destructor. This option is not intended to be used in
hosted environments such as GNU/Linux. It targets freestanding
environments (for example embedded systems) with limited resources
which do not support constructors/destructors or the C library file
I/O.
The linker could collect the input sections in a continuous memory
block and define start and end symbols. A GNU linker script example
which defines a linker output section follows:
.gcov_info :
{
PROVIDE (__gcov_info_start = .);
KEEP (*(.gcov_info))
PROVIDE (__gcov_info_end = .);
}
The program could dump the profiling information registered in this
linker set for example like this:
#include <gcov.h>
#include <stdio.h>
#include <stdlib.h>
extern const struct gcov_info *const __gcov_info_start[];
extern const struct gcov_info *const __gcov_info_end[];
static void
dump (const void *d, unsigned n, void *arg)
{
const unsigned char *c = d;
for (unsigned i = 0; i < n; ++i)
printf ("%02x", c[i]);
}
static void
filename (const char *f, void *arg)
{
__gcov_filename_to_gcfn (f, dump, arg );
}
static void *
allocate (unsigned length, void *arg)
{
return malloc (length);
}
static void
dump_gcov_info (void)
{
const struct gcov_info *const *info = __gcov_info_start;
const struct gcov_info *const *end = __gcov_info_end;
/* Obfuscate variable to prevent compiler optimizations. */
__asm__ ("" : "+r" (info));
while (info != end)
{
void *arg = NULL;
__gcov_info_to_gcda (*info, filename, dump, allocate, arg);
putchar ('\n');
++info;
}
}
int
main (void)
{
dump_gcov_info ();
return 0;
}
The merge-stream subcommand of gcov-tool may be used to deserialize
the data stream generated by the "__gcov_filename_to_gcfn" and
"__gcov_info_to_gcda" functions and merge the profile information
into .gcda files on the host filesystem.
-fprofile-note=path
If path is specified, GCC saves .gcno file into path location. If
you combine the option with multiple source files, the .gcno file
will be overwritten.
-fprofile-prefix-path=path
This option can be used in combination with
profile-generate=profile_dir and profile-use=profile_dir to inform
GCC where is the base directory of built source tree. By default
profile_dir will contain files with mangled absolute paths of all
object files in the built project. This is not desirable when
directory used to build the instrumented binary differs from the
directory used to build the binary optimized with profile feedback
because the profile data will not be found during the optimized
build. In such setups -fprofile-prefix-path=path with path pointing
to the base directory of the build can be used to strip the
irrelevant part of the path and keep all file names relative to the
main build directory.
-fprofile-prefix-map=old=new
When compiling files residing in directory old, record profiling
information (with --coverage) describing them as if the files resided
in directory new instead. See also -ffile-prefix-map and
-fcanon-prefix-map.
-fprofile-update=method
Alter the update method for an application instrumented for profile
feedback based optimization. The method argument should be one of
single, atomic or prefer-atomic. The first one is useful for single-
threaded applications, while the second one prevents profile
corruption by emitting thread-safe code.
Warning: When an application does not properly join all threads (or
creates an detached thread), a profile file can be still corrupted.
Using prefer-atomic would be transformed either to atomic, when
supported by a target, or to single otherwise. The GCC driver
automatically selects prefer-atomic when -pthread is present in the
command line, otherwise the default method is single.
If atomic is selected, then the profile information is updated using
atomic operations on a best-effort basis. Ideally, the profile
information is updated through atomic operations in hardware. If the
target platform does not support the required atomic operations in
hardware, however, libatomic is available, then the profile
information is updated through calls to libatomic. If the target
platform neither supports the required atomic operations in hardware
nor libatomic, then the profile information is not atomically updated
and a warning is issued. In this case, the obtained profiling
information may be corrupt for multi-threaded applications.
For performance reasons, if 64-bit counters are used for the
profiling information and the target platform only supports 32-bit
atomic operations in hardware, then the performance critical
profiling updates are done using two 32-bit atomic operations for
each counter update. If a signal interrupts these two operations
updating a counter, then the profiling information may be in an
inconsistent state.
-fprofile-filter-files=regex
Instrument only functions from files whose name matches any of the
regular expressions (separated by semi-colons).
For example, -fprofile-filter-files=main\.c;module.*\.c will
instrument only main.c and all C files starting with 'module'.
-fprofile-exclude-files=regex
Instrument only functions from files whose name does not match any of
the regular expressions (separated by semi-colons).
For example, -fprofile-exclude-files=/usr/.* will prevent
instrumentation of all files that are located in the /usr/ folder.
-fprofile-reproducible=[multithreaded|parallel-runs|serial]
Control level of reproducibility of profile gathered by
"-fprofile-generate". This makes it possible to rebuild program with
same outcome which is useful, for example, for distribution packages.
With -fprofile-reproducible=serial the profile gathered by
-fprofile-generate is reproducible provided the trained program
behaves the same at each invocation of the train run, it is not
multi-threaded and profile data streaming is always done in the same
order. Note that profile streaming happens at the end of program run
but also before "fork" function is invoked.
Note that it is quite common that execution counts of some part of
programs depends, for example, on length of temporary file names or
memory space randomization (that may affect hash-table collision
rate). Such non-reproducible part of programs may be annotated by
"no_instrument_function" function attribute. gcov-dump with -l can be
used to dump gathered data and verify that they are indeed
reproducible.
With -fprofile-reproducible=parallel-runs collected profile stays
reproducible regardless the order of streaming of the data into gcda
files. This setting makes it possible to run multiple instances of
instrumented program in parallel (such as with "make -j"). This
reduces quality of gathered data, in particular of indirect call
profiling.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector. Memory access
instructions are instrumented to detect out-of-bounds and use-after-
free bugs. The option enables -fsanitize-address-use-after-scope.
See <https://github.com/google/sanitizers/wiki/AddressSanitizer> for
more details. The run-time behavior can be influenced using the
ASAN_OPTIONS environment variable. When set to "help=1", the
available options are shown at startup of the instrumented program.
See
<https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>
for a list of supported options. The option cannot be combined with
-fsanitize=thread or -fsanitize=hwaddress. Note that the only
targets -fsanitize=hwaddress is currently supported on are x86-64
(only with "-mlam=u48" or "-mlam=u57" options) and AArch64, in both
cases only in ABIs with 64-bit pointers.
When compiling with -fsanitize=address, you should also use -g to
produce more meaningful output. To get more accurate stack traces,
it is possible to use options such as -O0, -O1, or -Og (which, for
instance, prevent most function inlining),
-fno-optimize-sibling-calls (which prevents optimizing sibling and
tail recursive calls; this option is implicit for -O0, -O1, or -Og),
or -fno-ipa-icf (which disables Identical Code Folding for
functions). Using -fno-omit-frame-pointer also improves stack
traces. Since multiple runs of the program may yield backtraces with
different addresses due to ASLR (Address Space Layout Randomization),
it may be desirable to turn ASLR off. On Linux, this can be achieved
with setarch `uname -m` -R ./prog.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See
<https://github.com/google/kernel-sanitizers> for more details.
-fsanitize=hwaddress
Enable Hardware-assisted AddressSanitizer, which uses a hardware
ability to ignore the top byte of a pointer to allow the detection of
memory errors with a low memory overhead. Memory access instructions
are instrumented to detect out-of-bounds and use-after-free bugs.
The option enables -fsanitize-address-use-after-scope. See
<https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html>
for more details. The run-time behavior can be influenced using the
HWASAN_OPTIONS environment variable. When set to "help=1", the
available options are shown at startup of the instrumented program.
The option cannot be combined with -fsanitize=thread or
-fsanitize=address, and is currently only available on AArch64.
-fsanitize=kernel-hwaddress
Enable Hardware-assisted AddressSanitizer for compilation of the
Linux kernel. Similar to -fsanitize=kernel-address but using an
alternate instrumentation method, and similar to -fsanitize=hwaddress
but with instrumentation differences necessary for compiling the
Linux kernel. These differences are to avoid hwasan library
initialization calls and to account for the stack pointer having a
different value in its top byte.
Note: This option has different defaults to the -fsanitize=hwaddress.
Instrumenting the stack and alloca calls are not on by default but
are still possible by specifying the command-line options --param
hwasan-instrument-stack=1 and --param hwasan-instrument-allocas=1
respectively. Using a random frame tag is not implemented for kernel
instrumentation.
-fsanitize=pointer-compare
Instrument comparison operation (<, <=, >, >=) with pointer operands.
The option must be combined with either -fsanitize=kernel-address or
-fsanitize=address The option cannot be combined with
-fsanitize=thread. Note: By default the check is disabled at run
time. To enable it, add "detect_invalid_pointer_pairs=2" to the
environment variable ASAN_OPTIONS. Using
"detect_invalid_pointer_pairs=1" detects invalid operation only when
both pointers are non-null.
-fsanitize=pointer-subtract
Instrument subtraction with pointer operands. The option must be
combined with either -fsanitize=kernel-address or -fsanitize=address
The option cannot be combined with -fsanitize=thread. Note: By
default the check is disabled at run time. To enable it, add
"detect_invalid_pointer_pairs=2" to the environment variable
ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid
operation only when both pointers are non-null.
-fsanitize=shadow-call-stack
Enable ShadowCallStack, a security enhancement mechanism used to
protect programs against return address overwrites (e.g. stack buffer
overflows.) It works by saving a function's return address to a
separately allocated shadow call stack in the function prologue and
restoring the return address from the shadow call stack in the
function epilogue. Instrumentation only occurs in functions that
need to save the return address to the stack.
Currently it only supports the aarch64 platform. It is specifically
designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK
option. For the user space programs, runtime support is not
currently provided in libc and libgcc. Users who want to use this
feature in user space need to provide their own support for the
runtime. It should be noted that this may cause the ABI rules to be
broken.
On aarch64, the instrumentation makes use of the platform register
"x18". This generally means that any code that may run on the same
thread as code compiled with ShadowCallStack must be compiled with
the flag -ffixed-x18, otherwise functions compiled without
-ffixed-x18 might clobber "x18" and so corrupt the shadow stack
pointer.
Also, because there is no userspace runtime support, code compiled
with ShadowCallStack cannot use exception handling. Use
-fno-exceptions to turn off exceptions.
See <https://clang.llvm.org/docs/ShadowCallStack.html> for more
details.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access
instructions are instrumented to detect data race bugs. See
<https://github.com/google/sanitizers/wiki#threadsanitizer> for more
details. The run-time behavior can be influenced using the
TSAN_OPTIONS environment variable; see
<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for
a list of supported options. The option cannot be combined with
-fsanitize=address, -fsanitize=leak.
When compiling with -fsanitize=thread, you should also use -g to
produce more meaningful output.
Note that sanitized atomic builtins cannot throw exceptions when
operating on invalid memory addresses with non-call exceptions
(-fnon-call-exceptions).
-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This option only
matters for linking of executables. The executable is linked against
a library that overrides "malloc" and other allocator functions. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer>
for more details. The run-time behavior can be influenced using the
LSAN_OPTIONS environment variable. The option cannot be combined
with -fsanitize=thread.
-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior
detector. Various computations are instrumented to detect undefined
behavior at runtime. See
<https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html> for
more details. The run-time behavior can be influenced using the
UBSAN_OPTIONS environment variable. Current suboptions are:
-fsanitize=shift
This option enables checking that the result of a shift operation
is not undefined. Note that what exactly is considered undefined
differs slightly between C and C++, as well as between ISO C90
and C99, etc. This option has two suboptions,
-fsanitize=shift-base and -fsanitize=shift-exponent.
-fsanitize=shift-exponent
This option enables checking that the second argument of a shift
operation is not negative and is smaller than the precision of
the promoted first argument.
-fsanitize=shift-base
If the second argument of a shift operation is within range,
check that the result of a shift operation is not undefined.
Note that what exactly is considered undefined differs slightly
between C and C++, as well as between ISO C90 and C99, etc.
-fsanitize=integer-divide-by-zero
Detect integer division by zero.
-fsanitize=unreachable
With this option, the compiler turns the "__builtin_unreachable"
call into a diagnostics message call instead. When reaching the
"__builtin_unreachable" call, the behavior is undefined.
-fsanitize=vla-bound
This option instructs the compiler to check that the size of a
variable length array is positive.
-fsanitize=null
This option enables pointer checking. Particularly, the
application built with this option turned on will issue an error
message when it tries to dereference a NULL pointer, or if a
reference (possibly an rvalue reference) is bound to a NULL
pointer, or if a method is invoked on an object pointed by a NULL
pointer.
-fsanitize=return
This option enables return statement checking. Programs built
with this option turned on will issue an error message when the
end of a non-void function is reached without actually returning
a value. This option works in C++ only.
-fsanitize=signed-integer-overflow
This option enables signed integer overflow checking. We check
that the result of "+", "*", and both unary and binary "-" does
not overflow in the signed arithmetics. This also detects
"INT_MIN / -1" signed division. Note, integer promotion rules
must be taken into account. That is, the following is not an
overflow:
signed char a = SCHAR_MAX;
a++;
-fsanitize=bounds
This option enables instrumentation of array bounds. Various out
of bounds accesses are detected. Flexible array members,
flexible array member-like arrays, and initializers of variables
with static storage are not instrumented, with the exception of
flexible array member-like arrays for which
"-fstrict-flex-arrays" or "-fstrict-flex-arrays=" options or
"strict_flex_array" attributes say they shouldn't be treated like
flexible array member-like arrays.
-fsanitize=bounds-strict
This option enables strict instrumentation of array bounds. Most
out of bounds accesses are detected, including flexible array
member-like arrays. Initializers of variables with static
storage are not instrumented.
-fsanitize=alignment
This option enables checking of alignment of pointers when they
are dereferenced, or when a reference is bound to insufficiently
aligned target, or when a method or constructor is invoked on
insufficiently aligned object.
-fsanitize=object-size
This option enables instrumentation of memory references using
the "__builtin_dynamic_object_size" function. Various out of
bounds pointer accesses are detected.
-fsanitize=float-divide-by-zero
Detect floating-point division by zero. Unlike other similar
options, -fsanitize=float-divide-by-zero is not enabled by
-fsanitize=undefined, since floating-point division by zero can
be a legitimate way of obtaining infinities and NaNs.
-fsanitize=float-cast-overflow
This option enables floating-point type to integer conversion
checking. We check that the result of the conversion does not
overflow. Unlike other similar options,
-fsanitize=float-cast-overflow is not enabled by
-fsanitize=undefined. This option does not work well with
"FE_INVALID" exceptions enabled.
-fsanitize=nonnull-attribute
This option enables instrumentation of calls, checking whether
null values are not passed to arguments marked as requiring a
non-null value by the "nonnull" function attribute.
-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return statements in
functions marked with "returns_nonnull" function attribute, to
detect returning of null values from such functions.
-fsanitize=bool
This option enables instrumentation of loads from bool. If a
value other than 0/1 is loaded, a run-time error is issued.
-fsanitize=enum
This option enables instrumentation of loads from an enum type.
If a value outside the range of values for the enum type is
loaded, a run-time error is issued.
-fsanitize=vptr
This option enables instrumentation of C++ member function calls,
member accesses and some conversions between pointers to base and
derived classes, to verify the referenced object has the correct
dynamic type.
-fsanitize=pointer-overflow
This option enables instrumentation of pointer arithmetics. If
the pointer arithmetics overflows, a run-time error is issued.
-fsanitize=builtin
This option enables instrumentation of arguments to selected
builtin functions. If an invalid value is passed to such
arguments, a run-time error is issued. E.g. passing 0 as the
argument to "__builtin_ctz" or "__builtin_clz" invokes undefined
behavior and is diagnosed by this option.
Note that sanitizers tend to increase the rate of false positive
warnings, most notably those around -Wmaybe-uninitialized. We
recommend against combining -Werror and [the use of] sanitizers.
While -ftrapv causes traps for signed overflows to be emitted,
-fsanitize=undefined gives a diagnostic message. This currently
works only for the C family of languages.
-fno-sanitize=all
This option disables all previously enabled sanitizers.
-fsanitize=all is not allowed, as some sanitizers cannot be used
together.
-fasan-shadow-offset=number
This option forces GCC to use custom shadow offset in
AddressSanitizer checks. It is useful for experimenting with
different shadow memory layouts in Kernel AddressSanitizer.
-fsanitize-sections=s1,s2,...
Sanitize global variables in selected user-defined sections. si may
contain wildcards.
-fsanitize-recover[=opts]
-fsanitize-recover= controls error recovery mode for sanitizers
mentioned in comma-separated list of opts. Enabling this option for
a sanitizer component causes it to attempt to continue running the
program as if no error happened. This means multiple runtime errors
can be reported in a single program run, and the exit code of the
program may indicate success even when errors have been reported.
The -fno-sanitize-recover= option can be used to alter this behavior:
only the first detected error is reported and program then exits with
a non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its
suboptions except for -fsanitize=unreachable and -fsanitize=return),
-fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero,
-fsanitize=bounds-strict, -fsanitize=kernel-address and
-fsanitize=address. For these sanitizers error recovery is turned on
by default, except -fsanitize=address, for which this feature is
experimental. -fsanitize-recover=all and -fno-sanitize-recover=all
is also accepted, the former enables recovery for all sanitizers that
support it, the latter disables recovery for all sanitizers that
support it.
Even if a recovery mode is turned on the compiler side, it needs to
be also enabled on the runtime library side, otherwise the failures
are still fatal. The runtime library defaults to "halt_on_error=0"
for ThreadSanitizer and UndefinedBehaviorSanitizer, while default
value for AddressSanitizer is "halt_on_error=1". This can be
overridden through setting the "halt_on_error" flag in the
corresponding environment variable.
Syntax without an explicit opts parameter is deprecated. It is
equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
-fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-after-scope
bugs. The option sets -fstack-reuse to none.
-fsanitize-trap[=opts]
The -fsanitize-trap= option instructs the compiler to report for
sanitizers mentioned in comma-separated list of opts undefined
behavior using "__builtin_trap" rather than a "libubsan" library
routine. If this option is enabled for certain sanitizer, it takes
precedence over the -fsanitizer-recover= for that sanitizer,
"__builtin_trap" will be emitted and be fatal regardless of whether
recovery is enabled or disabled using -fsanitize-recover=.
The advantage of this is that the "libubsan" library is not needed
and is not linked in, so this is usable even in freestanding
environments.
Currently this feature works with -fsanitize=undefined (and its
suboptions except for -fsanitize=vptr),
-fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and
-fsanitize=bounds-strict. "-fsanitize-trap=all" can be also
specified, which enables it for "undefined" suboptions,
-fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and
-fsanitize=bounds-strict. If "-fsanitize-trap=undefined" or
"-fsanitize-trap=all" is used and "-fsanitize=vptr" is enabled on the
command line, the instrumentation is silently ignored as the
instrumentation always needs "libubsan" support, -fsanitize-trap=vptr
is not allowed.
-fsanitize-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option is deprecated
equivalent of -fsanitize-trap=all.
-fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation. Inserts a call
to "__sanitizer_cov_trace_pc" into every basic block.
-fsanitize-coverage=trace-cmp
Enable dataflow guided fuzzing code instrumentation. Inserts a call
to "__sanitizer_cov_trace_cmp1", "__sanitizer_cov_trace_cmp2",
"__sanitizer_cov_trace_cmp4" or "__sanitizer_cov_trace_cmp8" for
integral comparison with both operands variable or
"__sanitizer_cov_trace_const_cmp1",
"__sanitizer_cov_trace_const_cmp2",
"__sanitizer_cov_trace_const_cmp4" or
"__sanitizer_cov_trace_const_cmp8" for integral comparison with one
operand constant, "__sanitizer_cov_trace_cmpf" or
"__sanitizer_cov_trace_cmpd" for float or double comparisons and
"__sanitizer_cov_trace_switch" for switch statements.
-fcf-protection=[full|branch|return|none|check]
-fcf-protection
Enable code instrumentation to increase program security by checking
that target addresses of control-flow transfer instructions (such as
indirect function call, function return, indirect jump) are valid.
This prevents diverting the flow of control to an unexpected target.
This is intended to protect against such threats as Return-oriented
Programming (ROP), and similarly call/jmp-oriented programming
(COP/JOP).
The -fcf-protection= keywords are interpreted as follows.
The value "branch" tells the compiler to implement checking of
validity of control-flow transfer at the point of indirect branch
instructions, i.e. call/jmp instructions.
The value "return" implements checking of validity at the point of
returning from a function.
The value "full" is an alias for specifying both "branch" and
"return".
The value "check" is used for the final link with link-time
optimization (LTO). An error is issued if LTO object files are
compiled with different -fcf-protection values. The value "check" is
ignored at the compile time.
The value "none" turns off instrumentation.
-fcf-protection is an alias for -fcf-protection=full. To override a
previous -fcf-protection option on the command line, add
-fcf-protection=none and then -fcf-protection=kind.
The macro "__CET__" is defined when -fcf-protection is used. The
first bit of "__CET__" is set to 1 for the value "branch" and the
second bit of "__CET__" is set to 1 for the "return".
You can also use the "nocf_check" attribute to identify which
functions and calls should be skipped from instrumentation.
Currently the x86 GNU/Linux target provides an implementation based
on Intel Control-flow Enforcement Technology (CET) which works for
i686 processor or newer.
-fharden-compares
For every logical test that survives gimple optimizations and is not
the condition in a conditional branch (for example, conditions tested
for conditional moves, or to store in boolean variables), emit extra
code to compute and verify the reversed condition, and to call
"__builtin_trap" if the results do not match. Use with
-fharden-conditional-branches to cover all conditionals.
-fharden-conditional-branches
For every non-vectorized conditional branch that survives gimple
optimizations, emit extra code to compute and verify the reversed
condition, and to call "__builtin_trap" if the result is unexpected.
Use with -fharden-compares to cover all conditionals.
-fharden-control-flow-redundancy
Emit extra code to set booleans when entering basic blocks, and to
verify and trap, at function exits, when the booleans do not form an
execution path that is compatible with the control flow graph.
Verification takes place before returns, before mandatory tail calls
(see below) and, optionally, before escaping exceptions with
-fhardcfr-check-exceptions, before returning calls with
-fhardcfr-check-returning-calls, and before noreturn calls with
-fhardcfr-check-noreturn-calls). Tuning options --param hardcfr-max-
blocks and --param hardcfr-max-inline-blocks are available.
Tail call optimization takes place too late to affect control flow
redundancy, but calls annotated as mandatory tail calls by language
front-ends, and any calls marked early enough as potential tail calls
would also have verification issued before the call, but these
possibilities are merely theoretical, as these conditions can only be
met when using custom compiler plugins.
-fhardcfr-skip-leaf
Disable -fharden-control-flow-redundancy in leaf functions.
-fhardcfr-check-exceptions
When -fharden-control-flow-redundancy is active, check the recorded
execution path against the control flow graph at exception escape
points, as if the function body was wrapped with a cleanup handler
that performed the check and reraised. This option is enabled by
default; use -fno-hardcfr-check-exceptions to disable it.
-fhardcfr-check-returning-calls
When -fharden-control-flow-redundancy is active, check the recorded
execution path against the control flow graph before any function
call immediately followed by a return of its result, if any, so as to
not prevent tail-call optimization, whether or not it is ultimately
optimized to a tail call.
This option is enabled by default whenever sibling call optimizations
are enabled (see -foptimize-sibling-calls), but it can be enabled (or
disabled, using its negated form) explicitly, regardless of the
optimizations.
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]
When -fharden-control-flow-redundancy is active, check the recorded
execution path against the control flow graph before "noreturn"
calls, either all of them (always), those that aren't expected to
return control to the caller through an exception (no-xthrow, the
default), those that may not return control to the caller through an
exception either (nothrow), or none of them (never).
Checking before a "noreturn" function that may return control to the
caller through an exception may cause checking to be performed more
than once, if the exception is caught in the caller, whether by a
handler or a cleanup. When -fhardcfr-check-exceptions is also
enabled, the compiler will avoid associating a "noreturn" call with
the implicitly-added cleanup handler, since it would be redundant
with the check performed before the call, but other handlers or
cleanups in the function, if activated, will modify the recorded
execution path and check it again when another checkpoint is hit.
The checkpoint may even be another "noreturn" call, so checking may
end up performed multiple times.
Various optimizers may cause calls to be marked as "noreturn" and/or
"nothrow", even in the absence of the corresponding attributes, which
may affect the placement of checks before calls, as well as the
addition of implicit cleanup handlers for them. This
unpredictability, and the fact that raising and reraising exceptions
frequently amounts to implicitly calling "noreturn" functions, have
made no-xthrow the default setting for this option: it excludes from
the "noreturn" treatment only internal functions used to (re)raise
exceptions, that are not affected by these optimizations.
-fhardened
Enable a set of flags for C and C++ that improve the security of the
generated code without affecting its ABI. The precise flags enabled
may change between major releases of GCC, but are currently:
-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE -pie -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)
The list of options enabled by -fhardened can be generated using the
--help=hardened option.
When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 is used
instead.
This option is intended to be used in production builds, not merely
in debug builds.
Currently, -fhardened is only supported on GNU/Linux targets.
-fhardened only enables a particular option if it wasn't already
specified anywhere on the command line. For instance, -fhardened
-fstack-protector will only enable -fstack-protector, but not
-fstack-protector-strong.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing
attacks. This is done by adding a guard variable to functions with
vulnerable objects. This includes functions that call "alloca", and
functions with buffers larger than or equal to 8 bytes. The guards
are initialized when a function is entered and then checked when the
function exits. If a guard check fails, an error message is printed
and the program exits. Only variables that are actually allocated on
the stack are considered, optimized away variables or variables
allocated in registers don't count.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
-fstack-protector-strong
Like -fstack-protector but includes additional functions to be
protected --- those that have local array definitions, or have
references to local frame addresses. Only variables that are
actually allocated on the stack are considered, optimized away
variables or variables allocated in registers don't count.
-fstack-protector-explicit
Like -fstack-protector but only protects those functions which have
the "stack_protect" attribute.
-fstack-check
Generate code to verify that you do not go beyond the boundary of the
stack. You should specify this flag if you are running in an
environment with multiple threads, but you only rarely need to
specify it in a single-threaded environment since stack overflow is
automatically detected on nearly all systems if there is only one
stack.
Note that this switch does not actually cause checking to be done;
the operating system or the language runtime must do that. The
switch causes generation of code to ensure that they see the stack
being extended.
You can additionally specify a string parameter: no means no
checking, generic means force the use of old-style checking, specific
means use the best checking method and is equivalent to bare
-fstack-check.
Old-style checking is a generic mechanism that requires no specific
target support in the compiler but comes with the following
drawbacks:
1. Modified allocation strategy for large objects: they are always
allocated dynamically if their size exceeds a fixed threshold.
Note this may change the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when it
is topped by a particular function, stack checking is not
reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy
and the generic implementation, code performance is hampered.
Note that old-style stack checking is also the fallback method for
specific if no target support has been added in the compiler.
-fstack-check= is designed for Ada's needs to detect infinite
recursion and stack overflows. specific is an excellent choice when
compiling Ada code. It is not generally sufficient to protect
against stack-clash attacks. To protect against those you want
-fstack-clash-protection.
-fstack-clash-protection
Generate code to prevent stack clash style attacks. When this option
is enabled, the compiler will only allocate one page of stack space
at a time and each page is accessed immediately after allocation.
Thus, it prevents allocations from jumping over any stack guard page
provided by the operating system.
Most targets do not fully support stack clash protection. However,
on those targets -fstack-clash-protection will protect dynamic stack
allocations. -fstack-clash-protection may also provide limited
protection for static stack allocations if the target supports
-fstack-check=specific.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain
value, either the value of a register or the address of a symbol. If
a larger stack is required, a signal is raised at run time. For most
targets, the signal is raised before the stack overruns the boundary,
so it is possible to catch the signal without taking special
precautions.
For instance, if the stack starts at absolute address 0x80000000 and
grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of
128KB. Note that this may only work with the GNU linker.
You can locally override stack limit checking by using the
"no_stack_limit" function attribute.
-fsplit-stack
Generate code to automatically split the stack before it overflows.
The resulting program has a discontiguous stack which can only
overflow if the program is unable to allocate any more memory. This
is most useful when running threaded programs, as it is no longer
necessary to calculate a good stack size to use for each thread.
This is currently only implemented for the x86 targets running
GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without
-fsplit-stack, there may not be much stack space available for the
latter code to run. If compiling all code, including library code,
with -fsplit-stack is not an option, then the linker can fix up these
calls so that the code compiled without -fsplit-stack always has a
large stack. Support for this is implemented in the gold linker in
GNU binutils release 2.21 and later.
-fstrub=disable
Disable stack scrubbing entirely, ignoring any "strub" attributes.
See
-fstrub=strict
Functions default to "strub" mode "disabled", and apply strictly the
restriction that only functions associated with "strub"-"callable"
modes ("at-calls", "callable" and "always_inline" "internal") are
"callable" by functions with "strub"-enabled modes ("at-calls" and
"internal").
-fstrub=relaxed
Restore the default stack scrub ("strub") setting, namely, "strub" is
only enabled as required by "strub" attributes associated with
function and data types. "Relaxed" means that strub contexts are
only prevented from calling functions explicitly associated with
"strub" mode "disabled". This option is only useful to override
other -fstrub=* options that precede it in the command line.
-fstrub=at-calls
Enable "at-calls" "strub" mode where viable. The primary use of this
option is for testing. It exercises the "strub" machinery in
scenarios strictly local to a translation unit. This "strub" mode
modifies function interfaces, so any function that is visible to
other translation units, or that has its address taken, will not be
affected by this option. Optimization options may also affect
viability. See the "strub" attribute documentation for details on
viability and eligibility requirements.
-fstrub=internal
Enable "internal" "strub" mode where viable. The primary use of this
option is for testing. This option is intended to exercise
thoroughly parts of the "strub" machinery that implement the less
efficient, but interface-preserving "strub" mode. Functions that
would not be affected by this option are quite uncommon.
-fstrub=all
Enable some "strub" mode where viable. When both strub modes are
viable, "at-calls" is preferred. -fdump-ipa-strubm adds function
attributes that tell which mode was selected for each function. The
primary use of this option is for testing, to exercise thoroughly the
"strub" machinery.
-fvtable-verify=[std|preinit|none]
This option is only available when compiling C++ code. It turns on
(or off, if using -fvtable-verify=none) the security feature that
verifies at run time, for every virtual call, that the vtable pointer
through which the call is made is valid for the type of the object,
and has not been corrupted or overwritten. If an invalid vtable
pointer is detected at run time, an error is reported and execution
of the program is immediately halted.
This option causes run-time data structures to be built at program
startup, which are used for verifying the vtable pointers. The
options std and preinit control the timing of when these data
structures are built. In both cases the data structures are built
before execution reaches "main". Using -fvtable-verify=std causes
the data structures to be built after shared libraries have been
loaded and initialized. -fvtable-verify=preinit causes them to be
built before shared libraries have been loaded and initialized.
If this option appears multiple times in the command line with
different values specified, none takes highest priority over both std
and preinit; preinit takes priority over std.
-fvtv-debug
When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, causes debug versions of the runtime
functions for the vtable verification feature to be called. This
flag also causes the compiler to log information about which vtable
pointers it finds for each class. This information is written to a
file named vtv_set_ptr_data.log in the directory named by the
environment variable VTV_LOGS_DIR if that is defined or the current
working directory otherwise.
Note: This feature appends data to the log file. If you want a fresh
log file, be sure to delete any existing one.
-fvtv-counts
This is a debugging flag. When used in conjunction with
-fvtable-verify=std or -fvtable-verify=preinit, this causes the
compiler to keep track of the total number of virtual calls it
encounters and the number of verifications it inserts. It also
counts the number of calls to certain run-time library functions that
it inserts and logs this information for each compilation unit. The
compiler writes this information to a file named vtv_count_data.log
in the directory named by the environment variable VTV_LOGS_DIR if
that is defined or the current working directory otherwise. It also
counts the size of the vtable pointer sets for each class, and writes
this information to vtv_class_set_sizes.log in the same directory.
Note: This feature appends data to the log files. To get fresh log
files, be sure to delete any existing ones.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just
after function entry and just before function exit, the following
profiling functions are called with the address of the current
function and its call site. (On some platforms,
"__builtin_return_address" does not work beyond the current function,
so the call site information may not be available to the profiling
functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current
function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in
other functions. The profiling calls indicate where, conceptually,
the inline function is entered and exited. This means that
addressable versions of such functions must be available. If all
your uses of a function are expanded inline, this may mean an
additional expansion of code size. If you use "extern inline" in
your C code, an addressable version of such functions must be
provided. (This is normally the case anyway, but if you get lucky
and the optimizer always expands the functions inline, you might have
gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in
which case this instrumentation is not done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling
functions cannot safely be called (perhaps signal handlers, if the
profiling routines generate output or allocate memory).
-finstrument-functions-once
This is similar to -finstrument-functions, but the profiling
functions are called only once per instrumented function, i.e. the
first profiling function is called after the first entry into the
instrumented function and the second profiling function is called
before the exit corresponding to this first entry.
The definition of "once" for the purpose of this option is a little
vague because the implementation is not protected against data races.
As a result, the implementation only guarantees that the profiling
functions are called at least once per process and at most once per
thread, but the calls are always paired, that is to say, if a thread
calls the first function, then it will call the second function,
unless it never reaches the exit of the instrumented function.
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation (see
the description of -finstrument-functions). If the file that
contains a function definition matches with one of file, then that
function is not instrumented. The match is done on substrings: if
the file parameter is a substring of the file name, it is considered
to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames contain
/bits/stl or include/sys.
If, for some reason, you want to include letter , in one of sym,
write ,. For example,
-finstrument-functions-exclude-file-list=',,tmp' (note the single
quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to -finstrument-functions-exclude-file-list, but this
option sets the list of function names to be excluded from
instrumentation. The function name to be matched is its user-visible
name, such as "vector<int> blah(const vector<int> &)", not the
internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The match
is done on substrings: if the sym parameter is a substring of the
function name, it is considered to be a match. For C99 and C++
extended identifiers, the function name must be given in UTF-8, not
using universal character names.
-fpatchable-function-entry=N[,M]
Generate N NOPs right at the beginning of each function, with the
function entry point before the Mth NOP. If M is omitted, it defaults
to 0 so the function entry points to the address just at the first
NOP. The NOP instructions reserve extra space which can be used to
patch in any desired instrumentation at run time, provided that the
code segment is writable. The amount of space is controllable
indirectly via the number of NOPs; the NOP instruction used
corresponds to the instruction emitted by the internal GCC back-end
interface "gen_nop". This behavior is target-specific and may also
depend on the architecture variant and/or other compilation options.
For run-time identification, the starting addresses of these areas,
which correspond to their respective function entries minus M, are
additionally collected in the "__patchable_function_entries" section
of the resulting binary.
Note that the value of "__attribute__ ((patchable_function_entry
(N,M)))" takes precedence over command-line option
-fpatchable-function-entry=N,M. This can be used to increase the
area size or to remove it completely on a single function. If "N=0",
no pad location is recorded.
The NOP instructions are inserted at---and maybe before, depending on
M---the function entry address, even before the prologue. On PowerPC
with the ELFv2 ABI, for a function with dual entry points, the local
entry point is this function entry address by default. See the
-msplit-patch-nops option to change this.
The maximum value of N and M is 65535. On PowerPC with the ELFv2
ABI, for a function with dual entry points, the supported values for
M are 0, 2, 6 and 14 when not using -msplit-patch-nops.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source
file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of
these options make sense only together with -E because they cause the
preprocessor output to be unsuitable for actual compilation.
In addition to the options listed here, there are a number of options to
control search paths for include files documented in Directory Options.
Options to control preprocessor diagnostics are listed in Warning
Options.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they
appeared during translation phase three in a #define directive. In
particular, the definition is truncated by embedded newline
characters.
If you are invoking the preprocessor from a shell or shell-like
program you may need to use the shell's quoting syntax to protect
characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line,
write its argument list with surrounding parentheses before the
equals sign (if any). Parentheses are meaningful to most shells, so
you should quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the
command line. All -imacros file and -include file options are
processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided
with a -D option.
-include file
Process file as if "#include "file"" appeared as the first line of
the primary source file. However, the first directory searched for
file is the preprocessor's working directory instead of the directory
containing the main source file. If not found there, it is searched
for in the remainder of the "#include "..."" search chain as normal.
If multiple -include options are given, the files are included in the
order they appear on the command line.
-imacros file
Exactly like -include, except that any output produced by scanning
file is thrown away. Macros it defines remain defined. This allows
you to acquire all the macros from a header without also processing
its declarations.
All files specified by -imacros are processed before all files
specified by -include.
-undef
Do not predefine any system-specific or GCC-specific macros. The
standard predefined macros remain defined.
-pthread
Define additional macros required for using the POSIX threads
library. You should use this option consistently for both
compilation and linking. This option is supported on GNU/Linux
targets, most other Unix derivatives, and also on x86 Cygwin and
MinGW targets.
-M Instead of outputting the result of preprocessing, output a rule
suitable for make describing the dependencies of the main source
file. The preprocessor outputs one make rule containing the object
file name for that source file, a colon, and the names of all the
included files, including those coming from -include or -imacros
command-line options.
Unless specified explicitly (with -MT or -MQ), the object file name
consists of the name of the source file with any suffix replaced with
object file suffix and with any leading directory parts removed. If
there are many included files then the rule is split into several
lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such
as -dM. To avoid mixing such debug output with the dependency rules
you should explicitly specify the dependency output file with -MF, or
use an environment variable like DEPENDENCIES_OUTPUT. Debug output
is still sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with an
implicit -w.
-MM Like -M but do not mention header files that are found in system
header directories, nor header files that are included, directly or
indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an
#include directive does not in itself determine whether that header
appears in -MM dependency output.
-MF file
When used with -M or -MM, specifies a file to write the dependencies
to. If no -MF switch is given the preprocessor sends the rules to
the same place it would send preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the
default dependency output file.
If file is -, then the dependencies are written to stdout.
-MG In conjunction with an option such as -M requesting dependency
generation, -MG assumes missing header files are generated files and
adds them to the dependency list without raising an error. The
dependency filename is taken directly from the "#include" directive
without prepending any path. -MG also suppresses preprocessed
output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-Mno-modules
Disable dependency generation for compiled module interfaces.
-MP This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing. These
dummy rules work around errors make gives if you remove header files
without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, deletes any
directory components and any file suffix such as .c, and appends the
platform's usual object suffix. The result is the target.
An -MT option sets the target to be exactly the string you specify.
If you want multiple targets, you can specify them as a single
argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make.
-MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with
-MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied. The
driver determines file based on whether an -o option is given. If it
is, the driver uses its argument but with a suffix of .d, otherwise
it takes the name of the input file, removes any directory components
and suffix, and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to
specify the dependency output file, but if used without -E, each -o
is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency
output file as a side effect of the compilation process.
-MMD
Like -MD except mention only user header files, not system header
files.
-fpreprocessed
Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion, trigraph
conversion, escaped newline splicing, and processing of most
directives. The preprocessor still recognizes and removes comments,
so that you can pass a file preprocessed with -C to the compiler
without problems. In this mode the integrated preprocessor is little
more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the
extensions .i, .ii or .mi. These are the extensions that GCC uses
for preprocessed files created by -save-temps.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives such
as "#define", "#ifdef", and "#error". Other preprocessor operations,
such as macro expansion and trigraph conversion are not performed.
In addition, the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin
macros is disabled. Macros such as "__LINE__", which are
contextually dependent, are handled normally. This enables
compilation of files previously preprocessed with "-E
-fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take
precedence. This enables full preprocessing of files previously
preprocessed with "-E -fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names and extended characters in
identifiers. This option is enabled by default for C99 (and later C
standard versions) and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with
canonicalization.
-fmax-include-depth=depth
Set the maximum depth of the nested #include. The default is 200.
-fsearch-include-path[=kind]
Look for input files on the #include path, not just the current
directory. This is particularly useful with C++20 modules, for which
both header units and module interface units need to be compiled
directly:
g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cc
kind defaults to user, which looks on the "#include "..."" search
path; you can also explicitly specify system for the "#include <...>"
search path.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor
report correct column numbers in warnings or errors, even if tabs
appear on the line. If the value is less than 1 or greater than 100,
the option is ignored. The default is 8.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the
compiler to emit diagnostic about the current macro expansion stack
when a compilation error occurs in a macro expansion. Using this
option makes the preprocessor and the compiler consume more memory.
The level parameter can be used to choose the level of precision of
token location tracking thus decreasing the memory consumption if
necessary. Value 0 of level de-activates this option. Value 1 tracks
tokens locations in a degraded mode for the sake of minimal memory
overhead. In this mode all tokens resulting from the expansion of an
argument of a function-like macro have the same location. Value 2
tracks tokens locations completely. This value is the most memory
hungry. When this option is given no argument, the default parameter
value is 2.
Note that "-ftrack-macro-expansion=2" is activated by default.
-fmacro-prefix-map=old=new
When preprocessing files residing in directory old, expand the
"__FILE__" and "__BASE_FILE__" macros as if the files resided in
directory new instead. This can be used to change an absolute path
to a relative path by using . for new which can result in more
reproducible builds that are location independent. This option also
affects "__builtin_FILE()" during compilation. See also
-ffile-prefix-map and -fcanon-prefix-map.
-fexec-charset=charset
Set the execution character set, used for string and character
constants. The default is UTF-8. charset can be any encoding
supported by the system's "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and
character constants. The default is one of UTF-32BE, UTF-32LE,
UTF-16BE, or UTF-16LE, whichever corresponds to the width of
"wchar_t" and the big-endian or little-endian byte order being used
for code generation. As with -fexec-charset, charset can be any
encoding supported by the system's "iconv" library routine; however,
you will have problems with encodings that do not fit exactly in
"wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the character
set of the input file to the source character set used by GCC. If
the locale does not specify, or GCC cannot get this information from
the locale, the default is UTF-8. This can be overridden by either
the locale or this command-line option. Currently the command-line
option takes precedence if there's a conflict. charset can be any
encoding supported by the system's "iconv" library routine.
-fpch-deps
When using precompiled headers, this flag causes the dependency-
output flags to also list the files from the precompiled header's
dependencies. If not specified, only the precompiled header are
listed and not the files that were used to create it, because those
files are not consulted when a precompiled header is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E. It
in the output to mark the place where the precompiled header was
found, and its filename. When -fpreprocessed is in use, GCC
This option is off by default, because the resulting preprocessed
output is only really suitable as input to GCC. It is switched on by
-save-temps.
to edit the filename if the PCH file is available in a different
location. The filename may be absolute or it may be relative to
GCC's current directory.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that let
the compiler know the current working directory at the time of
preprocessing. When this option is enabled, the preprocessor emits,
after the initial linemarker, a second linemarker with the current
working directory followed by two slashes. GCC uses this directory,
when it's present in the preprocessed input, as the directory emitted
as the current working directory in some debugging information
formats. This option is implicitly enabled if debugging information
is enabled, but this can be inhibited with the negated form
-fno-working-directory. If the -P flag is present in the command
line, this option has no effect, since no "#line" directives are
emitted whatsoever.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer.
This form is preferred to the older form -A predicate(answer), which
is still supported, because it does not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-C Do not discard comments. All comments are passed through to the
output file, except for comments in processed directives, which are
deleted along with the directive.
You should be prepared for side effects when using -C; it causes the
preprocessor to treat comments as tokens in their own right. For
example, comments appearing at the start of what would be a directive
line have the effect of turning that line into an ordinary source
line, since the first token on the line is no longer a #.
-CC Do not discard comments, including during macro expansion. This is
like -C, except that comments contained within macros are also passed
through to the output file where the macro is expanded.
In addition to the side effects of the -C option, the -CC option
causes all C++-style comments inside a macro to be converted to
C-style comments. This is to prevent later use of that macro from
inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-P Inhibit generation of linemarkers in the output from the
preprocessor. This might be useful when running the preprocessor on
something that is not C code, and will be sent to a program which
might be confused by the linemarkers.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C preprocessors, as
opposed to ISO C preprocessors. See the GNU CPP manual for details.
Note that GCC does not otherwise attempt to emulate a pre-standard C
compiler, and these options are only supported with the -E switch, or
when invoking CPP explicitly.
-trigraphs
Support ISO C trigraphs. These are three-character sequences, all
starting with ??, that are defined by ISO C to stand for single
characters. For example, ??/ stands for \, so '??/n' is a character
constant for a newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
By default, GCC ignores trigraphs, but in standard-conforming modes
it converts them. See the -std and -ansi options.
-remap
Enable special code to work around file systems which only permit
very short file names, such as MS-DOS.
-H Print the name of each header file used, in addition to other normal
activities. Each name is indented to show how deep in the #include
stack it is. Precompiled header files are also printed, even if they
are found to be invalid; an invalid precompiled header file is
printed with ...x and a valid one with ...! .
-dletters
Says to make debugging dumps during compilation as specified by
letters. The flags documented here are those relevant to the
preprocessor. Other letters are interpreted by the compiler proper,
or reserved for future versions of GCC, and so are silently ignored.
If you specify letters whose behavior conflicts, the result is
undefined.
-dM Instead of the normal output, generate a list of #define
directives for all the macros defined during the execution of the
preprocessor, including predefined macros. This gives you a way
of finding out what is predefined in your version of the
preprocessor. Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a
synonym for -fdump-rtl-mach.
-dD Like -dM except that it outputs both the #define directives and
the result of preprocessing. Both kinds of output go to the
standard output file.
-dN Like -dD, but emit only the macro names, not their expansions.
-dI Output #include directives in addition to the result of
preprocessing.
-dU Like -dD except that only macros that are expanded, or whose
definedness is tested in preprocessor directives, are output; the
output is delayed until the use or test of the macro; and #undef
directives are also output for macros tested but undefined at the
time.
-fdebug-cpp
This option is only useful for debugging GCC. When used from CPP or
with -E, it dumps debugging information about location maps. Every
token in the output is preceded by the dump of the map its location
belongs to.
When used from GCC without -E, this option has no effect.
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass option
directly through to the preprocessor. If option contains commas, it
is split into multiple options at the commas. However, many options
are modified, translated or interpreted by the compiler driver before
being passed to the preprocessor, and -Wp forcibly bypasses this
phase. The preprocessor's direct interface is undocumented and
subject to change, so whenever possible you should avoid using -Wp
and let the driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to
supply system-specific preprocessor options that GCC does not
recognize.
If you want to pass an option that takes an argument, you must use
-Xpreprocessor twice, once for the option and once for the argument.
-no-integrated-cpp
Perform preprocessing as a separate pass before compilation. By
default, GCC performs preprocessing as an integrated part of input
tokenization and parsing. If this option is provided, the
appropriate language front end (cc1, cc1plus, or cc1obj for C, C++,
and Objective-C, respectively) is instead invoked twice, once for
preprocessing only and once for actual compilation of the
preprocessed input. This option may be useful in conjunction with
the -B or -wrapper options to specify an alternate preprocessor or
perform additional processing of the program source between normal
preprocessing and compilation.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains
commas, it is split into multiple options at the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to
supply system-specific assembler options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use
-Xassembler twice, once for the option and once for the argument.
Options for Linking
These options come into play when the compiler links object files into an
executable output file. They are meaningless if the compiler is not
doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as input
to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.
-flinker-output=type
This option controls code generation of the link-time optimizer. By
default the linker output is automatically determined by the linker
plugin. For debugging the compiler and if incremental linking with a
non-LTO object file is desired, it may be useful to control the type
manually.
If type is exec, code generation produces a static binary. In this
case -fpic and -fpie are both disabled.
If type is dyn, code generation produces a shared library. In this
case -fpic or -fPIC is preserved, but not enabled automatically.
This allows to build shared libraries without position-independent
code on architectures where this is possible, i.e. on x86.
If type is pie, code generation produces an -fpie executable. This
results in similar optimizations as exec except that -fpie is not
disabled if specified at compilation time.
If type is rel, the compiler assumes that incremental linking is
done. The sections containing intermediate code for link-time
optimization are merged, pre-optimized, and output to the resulting
object file. In addition, if -ffat-lto-objects is specified, binary
code is produced for future non-LTO linking. The object file produced
by incremental linking is smaller than a static library produced from
the same object files. At link time the result of incremental
linking also loads faster than a static library assuming that the
majority of objects in the library are used.
Finally nolto-rel configures the compiler for incremental linking
where code generation is forced, a final binary is produced, and the
intermediate code for later link-time optimization is stripped. When
multiple object files are linked together the resulting code is
better optimized than with link-time optimizations disabled (for
example, cross-module inlining happens), but most of benefits of
whole program optimizations are lost.
During the incremental link (by -r) the linker plugin defaults to
rel. With current interfaces to GNU Binutils it is however not
possible to incrementally link LTO objects and non-LTO objects into a
single mixed object file. If any of object files in incremental link
cannot be used for link-time optimization, the linker plugin issues a
warning and uses nolto-rel. To maintain whole program optimization,
it is recommended to link such objects into static library instead.
Alternatively it is possible to use H.J. Lu's binutils with support
for mixed objects.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
-fuse-ld=lld
Use the LLVM lld linker instead of the default linker.
-fuse-ld=mold
Use the Modern Linker (mold) instead of the default linker.
-llibrary
-l library
Search the library named library when linking. (The second
alternative with the library as a separate argument is only for POSIX
compliance and is not recommended.)
The -l option is passed directly to the linker by GCC. Refer to your
linker documentation for exact details. The general description
below applies to the GNU linker.
The linker searches a standard list of directories for the library.
The directories searched include several standard system directories
plus any that you specify with -L.
Static libraries are archives of object files, and have file names
like liblibrary.a. Some targets also support shared libraries, which
typically have names like liblibrary.so. If both static and shared
libraries are found, the linker gives preference to linking with the
shared library unless the -static option is used.
It makes a difference where in the command you write this option; the
linker searches and processes libraries and object files in the order
they are specified. Thus, foo.o -lz bar.o searches library z after
file foo.o but before bar.o. If bar.o refers to functions in z,
those functions may not be loaded.
-lobjc
You need this special case of the -l option in order to link an
Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The
standard system libraries are used normally, unless -nostdlib,
-nolibc, or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the
libraries you specify are passed to the linker, and options
specifying linkage of the system libraries, such as -static-libgcc or
-shared-libgcc, are ignored. The standard startup files are used
normally, unless -nostartfiles is used.
The compiler may generate calls to "memcmp", "memset", "memcpy" and
"memmove". These entries are usually resolved by entries in libc.
These entry points should be supplied through some other mechanism
when this option is specified.
-nolibc
Do not use the C library or system libraries tightly coupled with it
when linking. Still link with the startup files, libgcc or toolchain
provided language support libraries such as libgnat, libgfortran or
libstdc++ unless options preventing their inclusion are used as well.
This typically removes -lc from the link command line, as well as
system libraries that normally go with it and become meaningless when
absence of a C library is assumed, for example -lpthread or -lm in
some configurations. This is intended for bare-board targets when
there is indeed no C library available.
-nostdlib
Do not use the standard system startup files or libraries when
linking. No startup files and only the libraries you specify are
passed to the linker, and options specifying linkage of the system
libraries, such as -static-libgcc or -shared-libgcc, are ignored.
The compiler may generate calls to "memcmp", "memset", "memcpy" and
"memmove". These entries are usually resolved by entries in libc.
These entry points should be supplied through some other mechanism
when this option is specified.
One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal subroutines which
GCC uses to overcome shortcomings of particular machines, or special
needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other
standard libraries. In other words, when you specify -nostdlib or
-nodefaultlibs you should usually specify -lgcc as well. This
ensures that you have no unresolved references to internal GCC
library subroutines. (An example of such an internal subroutine is
"__main", used to ensure C++ constructors are called.)
-nostdlib++
Do not implicitly link with standard C++ libraries.
-e entry
--entry=entry
Specify that the program entry point is entry. The argument is
interpreted by the linker; the GNU linker accepts either a symbol
name or an address.
-pie
Produce a dynamically linked position independent executable on
targets that support it. For predictable results, you must also
specify the same set of options used for compilation (-fpie, -fPIE,
or model suboptions) when you specify this linker option.
-no-pie
Don't produce a dynamically linked position independent executable.
-static-pie
Produce a static position independent executable on targets that
support it. A static position independent executable is similar to a
static executable, but can be loaded at any address without a dynamic
linker. For predictable results, you must also specify the same set
of options used for compilation (-fpie, -fPIE, or model suboptions)
when you specify this linker option.
-pthread
Link with the POSIX threads library. This option is supported on
GNU/Linux targets, most other Unix derivatives, and also on x86
Cygwin and MinGW targets. On some targets this option also sets
flags for the preprocessor, so it should be used consistently for
both compilation and linking.
-r Produce a relocatable object as output. This is also known as
partial linking.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that
support it. This instructs the linker to add all symbols, not only
used ones, to the dynamic symbol table. This option is needed for
some uses of "dlopen" or to allow obtaining backtraces from within a
program.
-s Remove all symbol table and relocation information from the
executable.
-static
On systems that support dynamic linking, this overrides -pie and
prevents linking with the shared libraries. On other systems, this
option has no effect.
-shared
Produce a shared object which can then be linked with other objects
to form an executable. Not all systems support this option. For
predictable results, you must also specify the same set of options
used for compilation (-fpic, -fPIC, or model suboptions) when you
specify this linker option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options
force the use of either the shared or static version, respectively.
If no shared version of libgcc was built when the compiler was
configured, these options have no effect.
There are several situations in which an application should use the
shared libgcc instead of the static version. The most common of
these is when the application wishes to throw and catch exceptions
across different shared libraries. In that case, each of the
libraries as well as the application itself should use the shared
libgcc.
Therefore, the G++ driver automatically adds -shared-libgcc whenever
you build a shared library or a main executable, because C++ programs
typically use exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you
may find that they are not always linked with the shared libgcc. If
GCC finds, at its configuration time, that you have a non-GNU linker
or a GNU linker that does not support option --eh-frame-hdr, it links
the shared version of libgcc into shared libraries by default.
Otherwise, it takes advantage of the linker and optimizes away the
linking with the shared version of libgcc, linking with the static
version of libgcc by default. This allows exceptions to propagate
through such shared libraries, without incurring relocation costs at
library load time.
However, if a library or main executable is supposed to throw or
catch exceptions, you must link it using the G++ driver, or using the
option -shared-libgcc, such that it is linked with the shared libgcc.
-static-libasan
When the -fsanitize=address option is used to link a program, the GCC
driver automatically links against libasan. If libasan is available
as a shared library, and the -static option is not used, then this
links against the shared version of libasan. The -static-libasan
option directs the GCC driver to link libasan statically, without
necessarily linking other libraries statically.
-static-libtsan
When the -fsanitize=thread option is used to link a program, the GCC
driver automatically links against libtsan. If libtsan is available
as a shared library, and the -static option is not used, then this
links against the shared version of libtsan. The -static-libtsan
option directs the GCC driver to link libtsan statically, without
necessarily linking other libraries statically.
-static-liblsan
When the -fsanitize=leak option is used to link a program, the GCC
driver automatically links against liblsan. If liblsan is available
as a shared library, and the -static option is not used, then this
links against the shared version of liblsan. The -static-liblsan
option directs the GCC driver to link liblsan statically, without
necessarily linking other libraries statically.
-static-libubsan
When the -fsanitize=undefined option is used to link a program, the
GCC driver automatically links against libubsan. If libubsan is
available as a shared library, and the -static option is not used,
then this links against the shared version of libubsan. The
-static-libubsan option directs the GCC driver to link libubsan
statically, without necessarily linking other libraries statically.
-static-libstdc++
When the g++ program is used to link a C++ program, it normally
automatically links against libstdc++. If libstdc++ is available as
a shared library, and the -static option is not used, then this links
against the shared version of libstdc++. That is normally fine.
However, it is sometimes useful to freeze the version of libstdc++
used by the program without going all the way to a fully static link.
The -static-libstdc++ option directs the g++ driver to link libstdc++
statically, without necessarily linking other libraries statically.
-symbolic
Bind references to global symbols when building a shared object.
Warn about any unresolved references (unless overridden by the link
editor option -Xlinker -z -Xlinker defs). Only a few systems support
this option.
-T script
Use script as the linker script. This option is supported by most
systems using the GNU linker. On some targets, such as bare-board
targets without an operating system, the -T option may be required
when linking to avoid references to undefined symbols.
-Xlinker option
Pass option as an option to the linker. You can use this to supply
system-specific linker options that GCC does not recognize.
If you want to pass an option that takes a separate argument, you
must use -Xlinker twice, once for the option and once for the
argument. For example, to pass -assert definitions, you must write
-Xlinker -assert -Xlinker definitions. It does not work to write
-Xlinker "-assert definitions", because this passes the entire string
as a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass
arguments to linker options using the option=value syntax than as
separate arguments. For example, you can specify -Xlinker
-Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other
linkers may not support this syntax for command-line options.
-Wl,option
Pass option as an option to the linker. If option contains commas,
it is split into multiple options at the commas. You can use this
syntax to pass an argument to the option. For example,
-Wl,-Map,output.map passes -Map output.map to the linker. When using
the GNU linker, you can also get the same effect with
-Wl,-Map=output.map.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library
modules to define it. You can use -u multiple times with different
symbols to force loading of additional library modules.
-z keyword
-z is passed directly on to the linker along with the keyword
keyword. See the section in the documentation of your linker for
permitted values and their meanings.
Options for Directory Search
These options specify directories to search for header files, for
libraries and for parts of the compiler:
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be searched for
header files during preprocessing. If dir begins with = or $SYSROOT,
then the = or $SYSROOT is replaced by the sysroot prefix; see
--sysroot and -isysroot.
Directories specified with -iquote apply only to the quote form of
the directive, "#include "file"". Directories specified with -I,
-isystem, or -idirafter apply to lookup for both the
"#include "file"" and "#include <file>" directives.
You can specify any number or combination of these options on the
command line to search for header files in several directories. The
lookup order is as follows:
1. For the quote form of the include directive, the directory of the
current file is searched first.
2. For the quote form of the include directive, the directories
specified by -iquote options are searched in left-to-right order,
as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-
right order.
4. Directories specified with -isystem options are scanned in left-
to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in
left-to-right order.
You can use -I to override a system header file, substituting your
own version, since these directories are searched before the standard
system header file directories. However, you should not use this
option to add directories that contain vendor-supplied system header
files; use -isystem for that.
The -isystem and -idirafter options also mark the directory as a
system directory, so that it gets the same special treatment that is
applied to the standard system directories.
If a standard system include directory, or a directory specified with
-isystem, is also specified with -I, the -I option is ignored. The
directory is still searched but as a system directory at its normal
position in the system include chain. This is to ensure that GCC's
procedure to fix buggy system headers and the ordering for the
"#include_next" directive are not inadvertently changed. If you
really need to change the search order for system directories, use
the -nostdinc and/or -isystem options.
-I- Split the include path. This option has been deprecated. Please use
-iquote instead for -I directories before the -I- and remove the -I-
option.
Any directories specified with -I options before -I- are searched
only for headers requested with "#include "file""; they are not
searched for "#include <file>". If additional directories are
specified with -I options after the -I-, those directories are
searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for "#include "file"".
There is no way to override this effect of -I-.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If
the prefix represents a directory, you should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add
the resulting directory to the include search path.
-iwithprefixbefore puts it in the same place -I would; -iwithprefix
puts it where -idirafter would.
-isysroot dir
This option is like the --sysroot option, but applies only to header
files (except for Darwin targets, where it applies to both header
files and libraries). See the --sysroot option for more information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-specific
C++ headers.
-nostdinc
Do not search the standard system directories for header files. Only
the directories explicitly specified with -I, -iquote, -isystem,
and/or -idirafter options (and the directory of the current file, if
appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard
directories, but do still search the other standard directories.
(This option is used when building the C++ library.)
--embed-dir=dir
Append dir directory to the list of searched directories for "#embed"
preprocessing directive or "__has_embed" macro. There are no default
directories for "#embed".
If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced
by the sysroot prefix; see --sysroot and -isysroot.
-iplugindir=dir
Set the directory to search for plugins that are passed by
-fplugin=name instead of -fplugin=path/name.so. This option is not
meant to be used by the user, but only passed by the driver.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries,
include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms cpp,
cc1, as and ld. It tries prefix as a prefix for each program it
tries to run, both with and without machine/version/ for the
corresponding target machine and compiler version.
For each subprogram to be run, the compiler driver first tries the -B
prefix, if any. If that name is not found, or if -B is not
specified, the driver tries two standard prefixes, /usr/lib/gcc/ and
/usr/local/lib/gcc/. If neither of those results in a file name that
is found, the unmodified program name is searched for using the
directories specified in your PATH environment variable.
The compiler checks to see if the path provided by -B refers to a
directory, and if necessary it adds a directory separator character
at the end of the path.
-B prefixes that effectively specify directory names also apply to
libraries in the linker, because the compiler translates these
options into -L options for the linker. They also apply to include
files in the preprocessor, because the compiler translates these
options into -isystem options for the preprocessor. In this case,
the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the
-B prefix, if needed. If it is not found there, the two standard
prefixes above are tried, and that is all. The file is left out of
the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use the
environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/,
where N is a number in the range 0 to 9, then it is replaced by
[dir/]include. This is to help with boot-strapping the compiler.
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or /./,
or make the path absolute when generating a relative prefix.
--sysroot=dir
Use dir as the logical root directory for headers and libraries. For
example, if the compiler normally searches for headers in
/usr/include and libraries in /usr/lib, it instead searches
dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the
--sysroot option applies to libraries, but the -isysroot option
applies to header files.
The GNU linker (beginning with version 2.16) has the necessary
support for this option. If your linker does not support this
option, the header file aspect of --sysroot still works, but the
library aspect does not.
--no-sysroot-suffix
For some targets, a suffix is added to the root directory specified
with --sysroot, depending on the other options used, so that headers
may for example be found in dir/suffix/usr/include instead of
dir/usr/include. This option disables the addition of such a suffix.
Options for Code Generation Conventions
These machine-independent options control the interface conventions used
in code generation.
Most of them have both positive and negative forms; the negative form of
-ffoo is -fno-foo. In the table below, only one of the forms is
listed---the one that is not the default. You can figure out the other
form by either removing no- or adding it.
-fstack-reuse=reuse-level
This option controls stack space reuse for user declared local/auto
variables and compiler generated temporaries. reuse_level can be
all, named_vars, or none. all enables stack reuse for all local
variables and temporaries, named_vars enables the reuse only for user
defined local variables with names, and none disables stack reuse
completely. The default value is all. The option is needed when the
program extends the lifetime of a scoped local variable or a compiler
generated temporary beyond the end point defined by the language.
When a lifetime of a variable ends, and if the variable lives in
memory, the optimizing compiler has the freedom to reuse its stack
space with other temporaries or scoped local variables whose live
range does not overlap with it. Legacy code extending local lifetime
is likely to break with the stack reuse optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well defined by the
C++ standard. When a lifetime of a temporary ends, and if the
temporary lives in memory, the optimizing compiler has the freedom to
reuse its stack space with other temporaries or scoped local
variables whose live range does not overlap with it. However some of
the legacy code relies on the behavior of older compilers in which
temporaries' stack space is not reused, the aggressive stack reuse
can lead to runtime errors. This option is used to control the
temporary stack reuse optimization.
-fstack-use-cumulative-args
This option instructs the compiler to use the
"cumulative_args_t"-based stack layout target hooks,
"TARGET_FUNCTION_ARG_BOUNDARY_CA" and
"TARGET_FUNCTION_ARG_ROUND_BOUNDARY_CA". If a given target does not
define these hooks, the default behaviour is to fallback to using the
standard non-"_CA" variants instead. Certain targets (such as AArch64
Darwin) require using the more advanced "_CA"-based hooks: For these
targets this option should be enabled by default.
-ftrapv
This option generates traps for signed overflow on addition,
subtraction, multiplication operations. The options -ftrapv and
-fwrapv override each other, so using -ftrapv -fwrapv on the command-
line results in -fwrapv being effective. Note that only active
options override, so using -ftrapv -fwrapv -fno-wrapv on the command-
line results in -ftrapv being effective.
-fwrapv
This option instructs the compiler to assume that signed arithmetic
overflow of addition, subtraction and multiplication wraps around
using twos-complement representation. This flag enables some
optimizations and disables others. The options -ftrapv and -fwrapv
override each other, so using -ftrapv -fwrapv on the command-line
results in -fwrapv being effective. Note that only active options
override, so using -ftrapv -fwrapv -fno-wrapv on the command-line
results in -ftrapv being effective.
-fwrapv-pointer
This option instructs the compiler to assume that pointer arithmetic
overflow on addition and subtraction wraps around using twos-
complement representation. This flag disables some optimizations
which assume pointer overflow is invalid.
-fstrict-overflow
This option implies -fno-wrapv -fno-wrapv-pointer and when negated
implies -fwrapv -fwrapv-pointer.
-fexceptions
Enable exception handling. Generates extra code needed to propagate
exceptions. For some targets, this implies GCC generates frame
unwind information for all functions, which can produce significant
data size overhead, although it does not affect execution. If you do
not specify this option, GCC enables it by default for languages like
C++ that normally require exception handling, and disables it for
languages like C that do not normally require it. However, you may
need to enable this option when compiling C code that needs to
interoperate properly with exception handlers written in C++. You
may also wish to disable this option if you are compiling older C++
programs that don't use exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions.
Note that this requires platform-specific runtime support that does
not exist everywhere. Moreover, it only allows trapping instructions
to throw exceptions, i.e. memory references or floating-point
instructions. It does not allow exceptions to be thrown from
arbitrary signal handlers such as "SIGALRM". This enables
-fexceptions.
-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but don't
otherwise contribute to the execution of the program can be optimized
away. This does not affect calls to functions except those with the
"pure" or "const" attributes. This option is enabled by default for
the Ada and C++ compilers, as permitted by the language
specifications. Optimization passes that cause dead exceptions to be
removed are enabled independently at different optimization levels.
-funwind-tables
Similar to -fexceptions, except that it just generates any needed
static data, but does not affect the generated code in any other way.
You normally do not need to enable this option; instead, a language
processor that needs this handling enables it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in DWARF format, if supported by target
machine. The table is exact at each instruction boundary, so it can
be used for stack unwinding from asynchronous events (such as
debugger or garbage collector).
-fno-gnu-unique
On systems with recent GNU assembler and C library, the C++ compiler
uses the "STB_GNU_UNIQUE" binding to make sure that definitions of
template static data members and static local variables in inline
functions are unique even in the presence of "RTLD_LOCAL"; this is
necessary to avoid problems with a library used by two different
"RTLD_LOCAL" plugins depending on a definition in one of them and
therefore disagreeing with the other one about the binding of the
symbol. But this causes "dlclose" to be ignored for affected DSOs;
if your program relies on reinitialization of a DSO via "dlclose" and
"dlopen", you can use -fno-gnu-unique.
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer
ones, rather than in registers. This convention is less efficient,
but it has the advantage of allowing intercallability between GCC-
compiled files and files compiled with other compilers, particularly
the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on
the target configuration macros.
Short structures and unions are those whose size and alignment match
that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not
binary compatible with code compiled with the -freg-struct-return
switch. Use it to conform to a non-default application binary
interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible. This
is more efficient for small structures than -fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return,
GCC defaults to whichever convention is standard for the target. If
there is no standard convention, GCC defaults to -fpcc-struct-return,
except on targets where GCC is the principal compiler. In those
cases, we can choose the standard, and we chose the more efficient
register return alternative.
Warning: code compiled with the -freg-struct-return switch is not
binary compatible with code compiled with the -fpcc-struct-return
switch. Use it to conform to a non-default application binary
interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the
declared range of possible values. Specifically, the "enum" type is
equivalent to the smallest integer type that has enough room. This
option has no effect for an enumeration type with a fixed underlying
type.
Warning: the -fshort-enums switch causes GCC to generate code that is
not binary compatible with code generated without that switch. Use
it to conform to a non-default application binary interface.
-fshort-wchar
Override the underlying type for "wchar_t" to be "short unsigned int"
instead of the default for the target. This option is useful for
building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is
not binary compatible with code generated without that switch. Use
it to conform to a non-default application binary interface.
-fcommon
In C code, this option controls the placement of global variables
defined without an initializer, known as tentative definitions in the
C standard. Tentative definitions are distinct from declarations of
a variable with the "extern" keyword, which do not allocate storage.
The default is -fno-common, which specifies that the compiler places
uninitialized global variables in the BSS section of the object file.
This inhibits the merging of tentative definitions by the linker so
you get a multiple-definition error if the same variable is
accidentally defined in more than one compilation unit.
The -fcommon places uninitialized global variables in a common block.
This allows the linker to resolve all tentative definitions of the
same variable in different compilation units to the same object, or
to a non-tentative definition. This behavior is inconsistent with
C++, and on many targets implies a speed and code size penalty on
global variable references. It is mainly useful to enable legacy
code to link without errors.
-fno-ident
Ignore the "#ident" directive.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This option
is used when compiling crtstuff.c; you should not need to use it for
anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to
make it more readable. This option is generally only of use to those
who actually need to read the generated assembly code (perhaps while
debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be
omitted and is useful when comparing two assembler files.
The added comments include:
* information on the compiler version and command-line options,
* the source code lines associated with the assembly instructions,
in the form FILENAME:LINENUMBER:CONTENT OF LINE,
* hints on which high-level expressions correspond to the various
assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}
compiling to (x86_64) assembly via -S and emitting the result direct
to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -
gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbits
The comments are intended for humans rather than machines and hence
the precise format of the comments is subject to change.
-frecord-gcc-switches
This switch causes the command line used to invoke the compiler to be
recorded into the object file that is being created. This switch is
only implemented on some targets and the exact format of the
recording is target and binary file format dependent, but it usually
takes the form of a section containing ASCII text. This switch is
related to the -fverbose-asm switch, but that switch only records
information in the assembler output file as comments, so it never
reaches the object file. See also -grecord-gcc-switches for another
way of storing compiler options into the object file.
-fpic
Generate position-independent code (PIC) suitable for use in a shared
library, if supported for the target machine. Such code accesses all
constant addresses through a global offset table (GOT). The dynamic
loader resolves the GOT entries when the program starts (the dynamic
loader is not part of GCC; it is part of the operating system). If
the GOT size for the linked executable exceeds a machine-specific
maximum size, you get an error message from the linker indicating
that -fpic does not work; in that case, recompile with -fPIC instead.
(These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the
m68k and RS/6000. The x86 has no such limit.)
Position-independent code requires special support, and therefore
works only on certain machines. For the x86, GCC supports PIC for
System V but not for the Sun 386i. Code generated for the IBM
RS/6000 is always position-independent.
When this flag is set, the macros "__pic__" and "__PIC__" are defined
to 1.
-fPIC
If supported for the target machine, emit position-independent code,
suitable for dynamic linking and avoiding any limit on the size of
the global offset table. This option makes a difference on AArch64,
m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore
works only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are defined
to 2.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but the generated
position-independent code can be only linked into executables.
Usually these options are used to compile code that will be linked
using the -pie GCC option.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The
macros have the value 1 for -fpie and 2 for -fPIE.
-fno-plt
Do not use the PLT for external function calls in position-
independent code. Instead, load the callee address at call sites
from the GOT and branch to it. This leads to more efficient code by
eliminating PLT stubs and exposing GOT loads to optimizations. On
architectures such as 32-bit x86 where PLT stubs expect the GOT
pointer in a specific register, this gives more register allocation
freedom to the compiler. Lazy binding requires use of the PLT; with
-fno-plt all external symbols are resolved at load time.
Alternatively, the function attribute "noplt" can be used to avoid
calls through the PLT for specific external functions.
In position-dependent code, a few targets also convert calls to
functions that are marked to not use the PLT to use the GOT instead.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be
more efficient than other code generation strategies. This option is
of use in conjunction with -fpic or -fPIC for building code that
forms part of a dynamic linker and cannot reference the address of a
jump table. On some targets, jump tables do not require a GOT and
this option is not needed.
-fno-bit-tests
Do not use bit tests for switch statements even where it would be
more efficient than other code generation strategies.
-ffixed-reg
Treat the register named reg as a fixed register; generated code
should never refer to it (except perhaps as a stack pointer, frame
pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are
machine-specific and are defined in the "REGISTER_NAMES" macro in the
machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is
clobbered by function calls. It may be allocated for temporaries or
variables that do not live across a call. Functions compiled this
way do not save and restore the register reg.
It is an error to use this flag with the frame pointer or stack
pointer. Use of this flag for other registers that have fixed
pervasive roles in the machine's execution model produces disastrous
results.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by
functions. It may be allocated even for temporaries or variables
that live across a call. Functions compiled this way save and
restore the register reg if they use it.
It is an error to use this flag with the frame pointer or stack
pointer. Use of this flag for other registers that have fixed
pervasive roles in the machine's execution model produces disastrous
results.
A different sort of disaster results from the use of this flag for a
register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together
without holes. When a value is specified (which must be a small
power of two), pack structure members according to this value,
representing the maximum alignment (that is, objects with default
alignment requirements larger than this are output potentially
unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is
not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimal. Use it to conform to a
non-default application binary interface.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly
change the way C symbols are represented in the object file. One use
is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code
that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary
interface. Not all targets provide complete support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model argument
should be one of global-dynamic, local-dynamic, initial-exec or
local-exec. Note that the choice is subject to optimization: the
compiler may use a more efficient model for symbols not visible
outside of the translation unit, or if -fpic is not given on the
command line.
The default without -fpic is initial-exec; with -fpic the default is
global-dynamic.
-ftrampolines
For targets that normally need trampolines for nested functions,
always generate them instead of using descriptors. Otherwise, for
targets that do not need them, like for example HP-PA or IA-64, do
nothing.
A trampoline is a small piece of code that is created at run time on
the stack when the address of a nested function is taken, and is used
to call the nested function indirectly. Therefore, it requires the
stack to be made executable in order for the program to work
properly.
-fno-trampolines is enabled by default on a language by language
basis to let the compiler avoid generating them, if it computes that
this is safe, and replace them with descriptors. Descriptors are
made up of data only, but the generated code must be prepared to deal
with them. As of this writing, -fno-trampolines is enabled by
default only for Ada.
Moreover, code compiled with -ftrampolines and code compiled with
-fno-trampolines are not binary compatible if nested functions are
present. This option must therefore be used on a program-wide basis
and be manipulated with extreme care.
For languages other than Ada, the "-ftrampolines" and
"-fno-trampolines" options currently have no effect, and trampolines
are always generated on platforms that need them for nested
functions.
-ftrampoline-impl=[stack|heap]
By default, trampolines are generated on stack. However, certain
platforms (such as the Apple M1) do not permit an executable stack.
Compiling with -ftrampoline-impl=heap generate calls to
"__gcc_nested_func_ptr_created" and "__gcc_nested_func_ptr_deleted"
in order to allocate and deallocate trampoline space on the
executable heap. These functions are implemented in libgcc, and will
only be provided on specific targets: x86_64 Darwin, x86_64 and
aarch64 Linux. PLEASE NOTE: Heap trampolines are not guaranteed to
be correctly deallocated if you "setjmp", instantiate nested
functions, and then "longjmp" back to a state prior to having
allocated those nested functions.
-fvisibility=[default|internal|hidden|protected]
Set the default ELF image symbol visibility to the specified
option---all symbols are marked with this unless overridden within
the code. Using this feature can very substantially improve linking
and load times of shared object libraries, produce more optimized
code, provide near-perfect API export and prevent symbol clashes. It
is strongly recommended that you use this in any shared objects you
distribute.
Despite the nomenclature, default always means public; i.e.,
available to be linked against from outside the shared object.
protected and internal are pretty useless in real-world usage so the
only other commonly used option is hidden. The default if
-fvisibility isn't specified is default, i.e., make every symbol
public.
A good explanation of the benefits offered by ensuring ELF symbols
have the correct visibility is given by "How To Write Shared
Libraries" by Ulrich Drepper (which can be found at
<https://www.akkadia.org/drepper/>)---however a superior solution
made possible by this option to marking things hidden when the
default is public is to make the default hidden and mark things
public. This is the norm with DLLs on Windows and with
-fvisibility=hidden and "__attribute__ ((visibility("default")))"
instead of "__declspec(dllexport)" you get almost identical semantics
with identical syntax. This is a great boon to those working with
cross-platform projects.
For those adding visibility support to existing code, you may find
declarations you wish to set visibility for with (for example)
pop". Bear in mind that symbol visibility should be viewed as part
of the API interface contract and thus all new code should always
specify visibility when it is not the default; i.e., declarations
only for use within the local DSO should always be marked explicitly
as hidden as so to avoid PLT indirection overheads---making this
abundantly clear also aids readability and self-documentation of the
code. Note that due to ISO C++ specification requirements, "operator
new" and "operator delete" must always be of default visibility.
Be aware that headers from outside your project, in particular system
headers and headers from any other library you use, may not be
expecting to be compiled with visibility other than the default. You
before including any such headers.
"extern" declarations are not affected by -fvisibility, so a lot of
code can be recompiled with -fvisibility=hidden with no
modifications. However, this means that calls to "extern" functions
with no explicit visibility use the PLT, so it is more effective to
tell the compiler which "extern" declarations should be treated as
hidden.
Note that -fvisibility does affect C++ vague linkage entities. This
means that, for instance, an exception class that is be thrown
between DSOs must be explicitly marked with default visibility so
that the type_info nodes are unified between the DSOs.
An overview of these techniques, their benefits and how to use them
is at <https://gcc.gnu.org/wiki/Visibility>.
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or
other structure fields, although the compiler usually honors those
types anyway) should use a single access of the width of the field's
type, aligned to a natural alignment if possible. For example,
targets with memory-mapped peripheral registers might require all
such accesses to be 16 bits wide; with this flag you can declare all
peripheral bit-fields as "unsigned short" (assuming short is 16 bits
on these targets) to force GCC to use 16-bit accesses instead of,
perhaps, a more efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient
instruction. In the previous example, that might be a 32-bit load
instruction, even though that accesses bytes that do not contain any
portion of the bit-field, or memory-mapped registers unrelated to the
one being updated.
In some cases, such as when the "packed" attribute is applied to a
structure field, it may not be possible to access the field with a
single read or write that is correctly aligned for the target
machine. In this case GCC falls back to generating multiple accesses
rather than code that will fault or truncate the result at run time.
Note: Due to restrictions of the C/C++11 memory model, write
accesses are not allowed to touch non bit-field members. It is
therefore recommended to define all bits of the field's type as bit-
field members.
The default value of this option is determined by the application
binary interface for the target processor.
-fsync-libcalls
This option controls whether any out-of-line instance of the "__sync"
family of functions may be used to implement the C++11 "__atomic"
family of functions.
The default value of this option is enabled, thus the only useful
form of the option is -fno-sync-libcalls. This option is used in the
implementation of the libatomic runtime library.
-fzero-init-padding-bits=value
Guarantee zero initialization of padding bits in automatic variable
initializers. Certain languages guarantee zero initialization of
padding bits in certain cases, e.g. C23 when using empty initializers
("{}"), or C++ when using zero-initialization or C guarantees that
fields not specified in an initializer have their padding bits zero
initialized. This option allows to change when padding bits in
initializers are guaranteed to be zero initialized. The default is
"-fzero-init-padding-bits=standard", which makes no further
guarantees than the corresponding standard. E.g.
struct A { char a; unsigned long long b; char c; };
union B { char a; unsigned long long b; };
struct A a = {}; // C23 guarantees padding bits are zero.
struct A b = { 1, 2, 3 }; // No guarantees.
union B c = {}; // C23 guarantees padding bits are zero.
union B d = { 1 }; // No guarantees.
"-fzero-init-padding-bits=unions" guarantees zero initialization of
padding bits in unions on top of what the standards guarantee, if the
initializer of an union is empty (then all bits of the union are zero
initialized) or if the initialized member of the union is smaller
than the size of the union (in that case guarantees padding bits
outside of the initialized member to be zero initialized). This was
the GCC behavior before GCC 15 and in the above example guarantees
zero initialization of last "sizeof (unsigned long long) - 1" bytes
in the union.
"-fzero-init-padding-bits=all" guarantees additionally zero
initialization of padding bits of other aggregates, so the padding in
between "b.a" and "b.b" (if any) and tail padding in the structure
(if any).
GCC Developer Options
This section describes command-line options that are primarily of
interest to GCC developers, including options to support compiler testing
and investigation of compiler bugs and compile-time performance problems.
This includes options that produce debug dumps at various points in the
compilation; that print statistics such as memory use and execution time;
and that print information about GCC's configuration, such as where it
searches for libraries. You should rarely need to use any of these
options for ordinary compilation and linking tasks.
Many developer options that cause GCC to dump output to a file take an
optional =filename suffix. You can specify stdout or - to dump to
standard output, and stderr for standard error.
If =filename is omitted, a default dump file name is constructed by
concatenating the base dump file name, a pass number, phase letter, and
pass name. The base dump file name is the name of output file produced
by the compiler if explicitly specified and not an executable; otherwise
it is the source file name. The pass number is determined by the order
passes are registered with the compiler's pass manager. This is
generally the same as the order of execution, but passes registered by
plugins, target-specific passes, or passes that are otherwise registered
late are numbered higher than the pass named final, even if they are
executed earlier. The phase letter is one of i (inter-procedural
analysis), l (language-specific), r (RTL), or t (tree). The files are
created in the directory of the output file.
-fcallgraph-info
-fcallgraph-info=MARKERS
Makes the compiler output callgraph information for the program, on a
per-object-file basis. The information is generated in the common
VCG format. It can be decorated with additional, per-node and/or
per-edge information, if a list of comma-separated markers is
additionally specified. When the "su" marker is specified, the
callgraph is decorated with stack usage information; it is equivalent
to -fstack-usage. When the "da" marker is specified, the callgraph
is decorated with information about dynamically allocated objects.
When compiling with -flto, no callgraph information is output along
with the object file. At LTO link time, -fcallgraph-info may
generate multiple callgraph information files next to intermediate
LTO output files.
-dletters
-fdump-rtl-pass
-fdump-rtl-pass=filename
Says to make debugging dumps during compilation at times specified by
letters. This is used for debugging the RTL-based passes of the
compiler.
Some -dletters switches have different meaning when -E is used for
preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option
letters. Here are the possible letters for use in pass and letters,
and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out
constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on
architectures that have auto inc or auto dec instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two
branch target load optimization passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping
after the three if conversion passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two
common subexpression elimination passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two
dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after
the two forward propagation passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global
common subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop optimization
passes.
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass,
if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the
basic block scheduling passes.
-fdump-rtl-ree
Dump after sign/zero extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
These options enable dumping after five rounds of instruction
splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some
architectures.
-fdump-rtl-stack
Dump after conversion from GCC's "flat register file" registers
to the x87's stack-like registers. This pass is only run on x86
variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after
the two subreg expansion passes.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous debugging
information.
-dD Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment indicating which
pattern and alternative is used. The length and cost of each
instruction are also printed.
-dP Dump the RTL in the assembler output as a comment before each
instruction. Also turns on -dp annotation.
-dx Just generate RTL for a function instead of compiling it.
Usually used with -fdump-rtl-expand.
-fdump-debug
Dump debugging information generated during the debug generation
phase.
-fdump-earlydebug
Dump debugging information generated during the early debug
generation phase.
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it
more feasible to use diff on debugging dumps for compiler invocations
with different compiler binaries and/or different text / bss / data /
heap / stack / dso start locations.
-freport-bug
Collect and dump debug information into a temporary file if an
internal compiler error (ICE) occurs.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and address
output. This makes it more feasible to use diff on debugging dumps
for compiler invocations with different options, in particular with
and without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress
instruction numbers for the links to the previous and next
instructions in a sequence.
-fdump-ipa-switch
-fdump-ipa-switch-options
Control the dumping at various stages of inter-procedural analysis
language tree to a file. The file name is generated by appending a
switch specific suffix to the source file name, and the file is
created in the same directory as the output file. The following
dumps are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused function
removal, and inlining decisions.
inline
Dump after function inlining.
strubm
Dump after selecting "strub" modes, and recording the selections
as function attributes.
strub
Dump "strub" transformations: interface changes, function
wrapping, and insertion of builtin calls for stack scrubbing and
watermarking.
Additionally, the options -optimized, -missed, -note, and -all can be
provided, with the same meaning as for -fopt-info, defaulting to
-optimized.
For example, -fdump-ipa-inline-optimized-missed will emit information
on callsites that were inlined, along with callsites that were not
inlined.
By default, the dump will contain messages about successful
optimizations (equivalent to -optimized) together with low-level
details about the analysis.
-fdump-ipa-clones
Create a dump file containing information about creation of call
graph node clones and removals of call graph nodes during inter-
procedural optimizations and transformations. Its main intended use
is that tools that create live-patches can determine the set of
functions that need to be live-patched to completely replace a
particular function (see -flive-patching). The file name is
generated by appending suffix "ipa-clones" to the source file name,
and the file is created in the same directory as the output file.
Each entry in the file is on a separate line containing semicolon
separated fields.
In the case of call graph clone creation, the individual fields are:
1. String "Callgraph clone".
2. Name of the function being cloned as it is presented to the
assembler.
3. A number that uniquely represents the function being cloned in
the call graph. Note that the number is unique only within a
compilation unit or within whole-program analysis but is likely
to be different in the two phases.
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.
7. Name of the new function clone as it is presented to the
assembler.
8. A number that uniquely represents the new function clone in the
call graph. Note that the number is unique only within a
compilation unit or within whole-program analysis but is likely
to be different in the two phases.
9. The file name of the source file where the source code location
of the new clone points to.
10. The line to which the source code location of the new clone
points to.
11. The column to which the source code location of the new clone
points to.
12. A string that determines the reason for cloning.
In the case of call graph clone removal, the individual fields are:
1. String "Callgraph removal".
2. Name of the function being removed as it would be presented to
the assembler.
3. A number that uniquely represents the function being cloned in
the call graph. Note that the number is unique only within a
compilation unit or within whole-program analysis but is likely
to be different in the two phases.
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.
-fdump-lang
Dump language-specific information. The file name is made by
appending .lang to the source file name.
-fdump-lang-all
-fdump-lang-switch
-fdump-lang-switch-options
-fdump-lang-switch-options=filename
Control the dumping of language-specific information. The options
and filename portions behave as described in the -fdump-tree option.
The following switch values are accepted:
all Enable all language-specific dumps.
class
Dump class hierarchy information. Virtual table information is
emitted unless 'slim' is specified. This option is applicable to
C++ only.
module
Dump module information. Options lineno (locations), graph
(reachability), blocks (clusters), uid (serialization), alias
(mergeable), asmname (Elrond), eh (mapper) & vops (macros) may
provide additional information. This option is applicable to C++
only.
raw Dump the raw internal tree data. This option is applicable to
C++ only.
-fdump-passes
Print on stderr the list of optimization passes that are turned on
and off by the current command-line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file.
The file name is generated by appending a suffix ending in
.statistics to the source file name, and the file is created in the
same directory as the output file. If the -option form is used,
-stats causes counters to be summed over the whole compilation unit
while -details dumps every event as the passes generate them. The
default with no option is to sum counters for each function compiled.
-fdump-tree-all
-fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
Control the dumping at various stages of processing the intermediate
language tree to a file. If the -options form is used, options is a
list of - separated options which control the details of the dump.
Not all options are applicable to all dumps; those that are not
meaningful are ignored. The following options are available
address
Print the address of each node. Usually this is not meaningful
as it changes according to the environment and source file. Its
primary use is for tying up a dump file with a debug environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that
in the dump instead of "DECL_NAME". Its primary use is ease of
use working backward from mangled names in the assembly file.
slim
When dumping front-end intermediate representations, inhibit
dumping of members of a scope or body of a function merely
because that scope has been reached. Only dump such items when
they are directly reachable by some other path.
When dumping pretty-printed trees, this option inhibits dumping
the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form instead
of the default LISP-like representation.
raw Print a raw representation of the tree. By default, trees are
pretty-printed into a C-like representation.
details
Enable more detailed dumps (not honored by every dump option).
Also include information from the optimization passes.
stats
Enable dumping various statistics about the pass (not honored by
every dump option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
graph
For each of the other indicated dump files (-fdump-rtl-pass),
dump a representation of the control flow graph suitable for
viewing with GraphViz to file.passid.pass.dot. Each function in
the file is pretty-printed as a subgraph, so that GraphViz can
render them all in a single plot.
This option currently only works for RTL dumps, and the RTL is
always dumped in slim form.
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each statement.
scev
Enable showing scalar evolution analysis details.
optimized
Enable showing optimization information (only available in
certain passes).
missed
Enable showing missed optimization information (only available in
certain passes).
note
Enable other detailed optimization information (only available in
certain passes).
all Turn on all options, except raw, slim, verbose and lineno.
optall
Turn on all optimization options, i.e., optimized, missed, and
note.
To determine what tree dumps are available or find the dump for a
pass of interest follow the steps below.
1. Invoke GCC with -fdump-passes and in the stderr output look for a
code that corresponds to the pass you are interested in. For
example, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2"
correspond to the three Value Range Propagation passes. The
number at the end distinguishes distinct invocations of the same
pass.
2. To enable the creation of the dump file, append the pass code to
the -fdump- option prefix and invoke GCC with it. For example,
to enable the dump from the Early Value Range Propagation pass,
invoke GCC with the -fdump-tree-evrp option. Optionally, you may
specify the name of the dump file. If you don't specify one, GCC
creates as described below.
3. Find the pass dump in a file whose name is composed of three
components separated by a period: the name of the source file GCC
was invoked to compile, a numeric suffix indicating the pass
number followed by the letter t for tree passes (and the letter r
for RTL passes), and finally the pass code. For example, the
Early VRP pass dump might be in a file named myfile.c.038t.evrp
in the current working directory. Note that the numeric codes
are not stable and may change from one version of GCC to another.
-fopt-info
-fopt-info-options
-fopt-info-options=filename
Controls optimization dumps from various optimization passes. If the
-options form is used, options is a list of - separated option
keywords to select the dump details and optimizations.
The options can be divided into three groups:
1. options describing what kinds of messages should be emitted,
2. options describing the verbosity of the dump, and
3. options describing which optimizations should be included.
The options from each group can be freely mixed as they are non-
overlapping. However, in case of any conflicts, the later options
override the earlier options on the command line.
The following options control which kinds of messages should be
emitted:
optimized
Print information when an optimization is successfully applied.
It is up to a pass to decide which information is relevant. For
example, the vectorizer passes print the source location of loops
which are successfully vectorized.
missed
Print information about missed optimizations. Individual passes
control which information to include in the output.
note
Print verbose information about optimizations, such as certain
transformations, more detailed messages about decisions etc.
all Print detailed optimization information. This includes optimized,
missed, and note.
The following option controls the dump verbosity:
internals
By default, only "high-level" messages are emitted. This option
enables additional, more detailed, messages, which are likely to
only be of interest to GCC developers.
One or more of the following option keywords can be used to describe
a group of optimizations:
ipa Enable dumps from all interprocedural optimizations.
loop
Enable dumps from all loop optimizations.
inline
Enable dumps from all inlining optimizations.
omp Enable dumps from all OMP (Offloading and Multi Processing)
optimizations.
vec Enable dumps from all vectorization optimizations.
optall
Enable dumps from all optimizations. This is a superset of the
optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which means
to dump messages about successful optimizations from all the passes,
omitting messages that are treated as "internals".
If the filename is provided, then the dumps from all the applicable
optimizations are concatenated into the filename. Otherwise the dump
is output onto stderr. Though multiple -fopt-info options are
accepted, only one of them can include a filename. If other filenames
are provided then all but the first such option are ignored.
Note that the output filename is overwritten in case of multiple
translation units. If a combined output from multiple translation
units is desired, stderr should be used instead.
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into
missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities from
vectorization passes on stderr. Note that -fopt-info-vec-missed is
equivalent to -fopt-info-missed-vec. The order of the optimization
group names and message types listed after -fopt-info does not
matter.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as optimized
locations from all the inlining passes into inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are in conflict
since only one output file is allowed. In this case, only the first
option takes effect and the subsequent options are ignored. Thus only
vec.miss is produced which contains dumps from the vectorizer about
missed opportunities.
-fsave-optimization-record
Write a SRCFILE.opt-record.json.gz file detailing what optimizations
were performed, for those optimizations that support -fopt-info.
This option is experimental and the format of the data within the
compressed JSON file is subject to change.
It is roughly equivalent to a machine-readable version of
-fopt-info-all, as a collection of messages with source file, line
number and column number, with the following additional data for each
message:
* the execution count of the code being optimized, along with
metadata about whether this was from actual profile data, or just
an estimate, allowing consumers to prioritize messages by code
hotness,
* the function name of the code being optimized, where applicable,
* the "inlining chain" for the code being optimized, so that when a
function is inlined into several different places (which might
themselves be inlined), the reader can distinguish between the
copies,
* objects identifying those parts of the message that refer to
expressions, statements or symbol-table nodes, which of these
categories they are, and, when available, their source code
location,
* the GCC pass that emitted the message, and
* the location in GCC's own code from which the message was emitted
Additionally, some messages are logically nested within other
messages, reflecting implementation details of the optimization
passes.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the
amount of debugging output the scheduler prints to the dump files.
For n greater than zero, -fsched-verbose outputs the same information
as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one,
it also output basic block probabilities, detailed ready list
information and unit/insn info. For n greater than two, it includes
RTL at abort point, control-flow and regions info. And for n over
four, -fsched-verbose also includes dependence info.
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of options that are used to explicitly disable/enable
optimization passes. These options are intended for use for
debugging GCC. Compiler users should use regular options for
enabling/disabling passes instead.
-fdisable-ipa-pass
Disable IPA pass pass. pass is the pass name. If the same pass
is statically invoked in the compiler multiple times, the pass
name should be appended with a sequential number starting from 1.
-fdisable-rtl-pass
-fdisable-rtl-pass=range-list
Disable RTL pass pass. pass is the pass name. If the same pass
is statically invoked in the compiler multiple times, the pass
name should be appended with a sequential number starting from 1.
range-list is a comma-separated list of function ranges or
assembler names. Each range is a number pair separated by a
colon. The range is inclusive in both ends. If the range is
trivial, the number pair can be simplified as a single number.
If the function's call graph node's uid falls within one of the
specified ranges, the pass is disabled for that function. The
uid is shown in the function header of a dump file, and the pass
names can be dumped by using option -fdump-passes.
-fdisable-tree-pass
-fdisable-tree-pass=range-list
Disable tree pass pass. See -fdisable-rtl for the description of
option arguments.
-fenable-ipa-pass
Enable IPA pass pass. pass is the pass name. If the same pass
is statically invoked in the compiler multiple times, the pass
name should be appended with a sequential number starting from 1.
-fenable-rtl-pass
-fenable-rtl-pass=range-list
Enable RTL pass pass. See -fdisable-rtl for option argument
description and examples.
-fenable-tree-pass
-fenable-tree-pass=range-list
Enable tree pass pass. See -fdisable-rtl for the description of
option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-fchecking
-fchecking=n
Enable internal consistency checking. The default depends on the
compiler configuration. -fchecking=2 enables further internal
consistency checking that might affect code generation.
-frandom-seed=string
This option provides a seed that GCC uses in place of random numbers
in generating certain symbol names that have to be different in every
compiled file. It is also used to place unique stamps in coverage
data files and the object files that produce them. You can use the
-frandom-seed option to produce reproducibly identical object files.
The string can either be a number (decimal, octal or hex) or an
arbitrary string (in which case it's converted to a number by
computing CRC32).
The string should be different for every file you compile.
-save-temps
Store the usual "temporary" intermediate files permanently; name them
as auxiliary output files, as specified described under -dumpbase and
-dumpdir.
When used in combination with the -x command-line option, -save-temps
is sensible enough to avoid overwriting an input source file with the
same extension as an intermediate file. The corresponding
intermediate file may be obtained by renaming the source file before
using -save-temps.
-save-temps=cwd
Equivalent to -save-temps -dumpdir ./.
-save-temps=obj
Equivalent to -save-temps -dumpdir oouuttddiirr//, where outdir/ is the
directory of the output file specified after the -o option, including
any directory separators. If the -o option is not used, the
-save-temps=obj switch behaves like -save-temps=cwd.
-time[=file]
Report the CPU time taken by each subprocess in the compilation
sequence. For C source files, this is the compiler proper and
assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like
this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time spent
executing the program itself. The second number is "system time",
time spent executing operating system routines on behalf of the
program. Both numbers are in seconds.
With the specification of an output file, the output is appended to
the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before the program
name, and the options passed to the program are displayed, so that
one can later tell what file was being compiled, and with which
options.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the
optional argument is omitted (or if file is "."), the name of the
dump file is determined by appending ".gkd" to the dump base name,
see -dumpbase.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second
time, adding opts and -fcompare-debug-second to the arguments passed
to the second compilation. Dump the final internal representation in
both compilations, and print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and
nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is
defined to a string starting with a dash, then it is used for opts,
otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent
to -fno-compare-debug, which disables the dumping of the final
representation and the second compilation, preventing even
GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set
GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC
rejects as an invalid option in any actual compilation (rather than
preprocessing, assembly or linking). To get just a warning, setting
GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the second
compilation requested by -fcompare-debug, along with options to
silence warnings, and omitting other options that would cause the
compiler to produce output to files or to standard output as a side
effect. Dump files and preserved temporary files are renamed so as
to contain the ".gk" additional extension during the second
compilation, to avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the
first compilation to be skipped, which makes it useful for little
other than debugging the compiler proper.
-gtoggle
Turn off generation of debug info, if leaving out this option
generates it, or turn it on at level 2 otherwise. The position of
this argument in the command line does not matter; it takes effect
after all other options are processed, and it does so only once, no
matter how many times it is given. This is mainly intended to be
used with -fcompare-debug.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle
toggles -g.
-Q When used on the command line prior to --help=, -Q acts as a modifier
to the help output.
Otherwise, this option makes the compiler print out each function
name as it is compiled, and print some statistics about each pass
when it finishes.
-ftime-report
Makes the compiler print some statistics to stderr about the time
consumed by each pass when it finishes.
If SARIF output of diagnostics was requested via
-fdiagnostics-format=sarif-file or -fdiagnostics-format=sarif-stderr
then the -ftime-report information is instead emitted in JSON form as
part of SARIF output. The precise format of this JSON data is
subject to change, and the values may not exactly match those emitted
to stderr due to being written out at a slightly different place
within the compiler.
-ftime-report-details
Record the time consumed by infrastructure parts separately for each
pass.
-fira-verbose=n
Control the verbosity of the dump file for the integrated register
allocator. The default value is 5. If the value n is greater or
equal to 10, the dump output is sent to stderr using the same format
as n minus 10.
-flto-report
Prints a report with internal details on the workings of the link-
time optimizer. The contents of this report vary from version to
version. It is meant to be useful to GCC developers when processing
object files in LTO mode (via -flto).
Disabled by default.
-flto-report-wpa
Like -flto-report, but only print for the WPA phase of link-time
optimization.
-fmem-report
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent memory
allocation for the WPA phase only.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory
allocation before or after interprocedural optimization.
-fmultiflags
This option enables multilib-aware "TFLAGS" to be used to build
target libraries with options different from those the compiler is
configured to use by default, through the use of specs set up by
compiler internals, by the target, or by builders at configure time.
Like "TFLAGS", this allows the target libraries to be built for
portable baseline environments, while the compiler defaults to more
demanding ones. That's useful because users can easily override the
defaults the compiler is configured to use to build their own
programs, if the defaults are not ideal for their target environment,
whereas rebuilding the runtime libraries is usually not as easy or
desirable.
Unlike "TFLAGS", the use of specs enables different flags to be
selected for different multilibs. The way to accomplish that is to
build with make TFLAGS=-fmultiflags, after configuring
--with-specs=%{fmultiflags:...}.
This option is discarded by the driver once it's done processing
driver self spec.
It is also useful to check that "TFLAGS" are being used to build all
target libraries, by configuring a non-bootstrap compiler
--with-specs='%{!fmultiflags:%emissing TFLAGS}' and building the
compiler and target libraries.
-fprofile-report
Makes the compiler print some statistics about consistency of the
(estimated) profile and effect of individual passes.
-fstack-usage
Makes the compiler output stack usage information for the program, on
a per-function basis. The filename for the dump is made by appending
.su to the auxname. auxname is generated from the name of the output
file, if explicitly specified and it is not an executable, otherwise
it is the basename of the source file. An entry is made up of three
fields:
* The name of the function.
* A number of bytes.
* One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates the stack
statically: a fixed number of bytes are allocated for the frame on
function entry and released on function exit; no stack adjustments
are otherwise made in the function. The second field is this fixed
number of bytes.
The qualifier "dynamic" means that the function manipulates the stack
dynamically: in addition to the static allocation described above,
stack adjustments are made in the body of the function, for example
to push/pop arguments around function calls. If the qualifier
"bounded" is also present, the amount of these adjustments is bounded
at compile time and the second field is an upper bound of the total
amount of stack used by the function. If it is not present, the
amount of these adjustments is not bounded at compile time and the
second field only represents the bounded part.
-fstats
Emit statistics about front-end processing at the end of the
compilation. This option is supported only by the C++ front end, and
the information is generally only useful to the G++ development team.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter lower and upper bound. counter-value-
list is a comma-separated list of name:lower_bound1-upper_bound1
[:lower_bound2-upper_bound2...] tuples which sets the name of the
counter and list of closed intervals. The lower_bound is optional
and is zero initialized if not set. For example, with
-fdbg-cnt=dce:2-4:10-11,tail_call:10, "dbg_cnt(dce)" returns true
only for second, third, fourth, tenth and eleventh invocation. For
"dbg_cnt(tail_call)" true is returned for first 10 invocations.
-print-file-name=library
Print the full absolute name of the library file library that would
be used when linking---and don't do anything else. With this option,
GCC does not compile or link anything; it just prints the file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by
any other switches present in the command line. This directory is
supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler switches
that enable them. The directory name is separated from the switches
by ;, and each switch starts with an @ instead of the -, without
spaces between multiple switches. This is supposed to ease shell
processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to
some lib subdirectory. If OS libraries are present in the lib
subdirectory and no multilibs are used, this is usually just ., if OS
libraries are present in libsuffix sibling directories this prints
e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in
lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
-print-multiarch
Print the path to OS libraries for the selected multiarch, relative
to some lib subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do
want to link with libgcc.a. You can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list of
program and library directories gcc searches---and don't do anything
else.
This is useful when gcc prints the error message installation
problem, cannot exec cpp0: No such file or directory. To resolve
this you either need to put cpp0 and the other compiler components
where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing /.
-print-sysroot
Print the target sysroot directory that is used during compilation.
This is the target sysroot specified either at configure time or
using the --sysroot option, possibly with an extra suffix that
depends on compilation options. If no target sysroot is specified,
the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for
headers, or give an error if the compiler is not configured with such
a suffix---and don't do anything else.
-dumpmachine
Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0, 6.3.0 or 7)---and don't
do anything else. This is the compiler version used in filesystem
paths and specs. Depending on how the compiler has been configured it
can be just a single number (major version), two numbers separated by
a dot (major and minor version) or three numbers separated by dots
(major, minor and patchlevel version).
-dumpfullversion
Print the full compiler version---and don't do anything else. The
output is always three numbers separated by dots, major, minor and
patchlevel version.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else.
(This is used when GCC itself is being built.)
Machine-Dependent Options
Each target machine supported by GCC can have its own options---for
example, to allow you to compile for a particular processor variant or
ABI, or to control optimizations specific to that machine. By
convention, the names of machine-specific options start with -m.
Some configurations of the compiler also support additional target-
specific options, usually for compatibility with other compilers on the
same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-mabi=name
Generate code for the specified data model. Permissible values are
ilp32 for SysV-like data model where int, long int and pointers are
32 bits, and lp64 for SysV-like data model where int is 32 bits, but
long int and pointers are 64 bits.
The default depends on the specific target configuration. Note that
the LP64 and ILP32 ABIs are not link-compatible; you must compile
your entire program with the same ABI, and link with a compatible set
of libraries.
The ilp32 model is deprecated.
-mbig-endian
Generate big-endian code. This is the default when GCC is configured
for an aarch64_be-*-* target.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This
will prevent the compiler from using floating-point and Advanced SIMD
registers but will not impose any restrictions on the assembler.
-mlittle-endian
Generate little-endian code. This is the default when GCC is
configured for an aarch64-*-* but not an aarch64_be-*-* target.
-mcmodel=tiny
Generate code for the tiny code model. The program and its
statically defined symbols must be within 1MB of each other.
Programs can be statically or dynamically linked.
-mcmodel=small
Generate code for the small code model. The program and its
statically defined symbols must be within 4GB of each other.
Programs can be statically or dynamically linked. This is the
default code model.
-mcmodel=large
Generate code for the large code model. This makes no assumptions
about addresses and sizes of sections. Programs can be statically
linked only. The -mcmodel=large option is incompatible with
-mabi=ilp32, -fpic and -fPIC.
-mtp=name
Specify the system register to use as a thread pointer. The valid
values are tpidr_el0, tpidrro_el0, tpidr_el1, tpidr_el2, tpidr_el3.
For backwards compatibility the aliases el0, el1, el2, el3 are also
accepted. The default setting is tpidr_el0. It is recommended to
compile all code intended to interoperate with the same value of this
option to avoid accessing a different thread pointer from the wrong
exception level.
-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned on
a natural object boundary as described in the architecture
specification.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former
behavior is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported
locations are global for a global canary or sysreg for a canary in an
appropriate system register.
With the latter choice the options -mstack-protector-guard-reg=reg
and -mstack-protector-guard-offset=offset furthermore specify which
system register to use as base register for reading the canary, and
from what offset from that base register. There is no default
register or offset as this is entirely for use within the Linux
kernel.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for dynamic
accesses of TLS variables. This is the default.
-mtls-dialect=traditional
Use traditional TLS as the thread-local storage mechanism for dynamic
accesses of TLS variables.
-mtls-size=size
Specify bit size of immediate TLS offsets. Valid values are 12, 24,
32, 48. This option requires binutils 2.26 or newer.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53 erratum
number 835769. This involves inserting a NOP instruction between
memory instructions and 64-bit integer multiply-accumulate
instructions. This flag will be ignored if an architecture or cpu is
specified on the command line which does not need the workaround.
-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
Enable or disable the workaround for the ARM Cortex-A53 erratum
number 843419. This erratum workaround is made at link time and this
will only pass the corresponding flag to the linker. This flag will
be ignored if an architecture or cpu is specified on the command line
which does not need the workaround.
-mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt
Enable or disable the reciprocal square root approximation. This
option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this reduces
precision of reciprocal square root results to about 16 bits for
single precision and to 32 bits for double precision.
-mlow-precision-sqrt
-mno-low-precision-sqrt
Enable or disable the square root approximation. This option only
has an effect if -ffast-math or -funsafe-math-optimizations is used
as well. Enabling this reduces precision of square root results to
about 16 bits for single precision and to 32 bits for double
precision. If enabled, it implies -mlow-precision-recip-sqrt.
-mlow-precision-div
-mno-low-precision-div
Enable or disable the division approximation. This option only has
an effect if -ffast-math or -funsafe-math-optimizations is used as
well. Enabling this reduces precision of division results to about
16 bits for single precision and to 32 bits for double precision.
-mtrack-speculation
-mno-track-speculation
Enable or disable generation of additional code to track speculative
execution through conditional branches. The tracking state can then
be used by the compiler when expanding calls to
"__builtin_speculation_safe_copy" to permit a more efficient code
sequence to be generated.
-moutline-atomics
-mno-outline-atomics
Enable or disable calls to out-of-line helpers to implement atomic
operations. These helpers will, at runtime, determine if the LSE
instructions from ARMv8.1-A can be used; if not, they will use the
load/store-exclusive instructions that are present in the base
ARMv8.0 ISA.
This option is only applicable when compiling for the base ARMv8.0
instruction set. If using a later revision, e.g. -march=armv8.1-a or
-march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used
directly. The same applies when using -mcpu= when the selected cpu
supports the lse feature. This option is on by default.
-march=name
Specify the name of the target architecture and, optionally, one or
more feature modifiers. This option has the form
-march=arch{+[no]feature}*.
The table below summarizes the permissible values for arch and the
features that they enable by default:
arch value : Architecture : Includes by default
armv8-a : Armv8-A : +fp, +simd
armv8.1-a : Armv8.1-A : armv8-a, +crc, +lse, +rdma
armv8.2-a : Armv8.2-A : armv8.1-a
armv8.3-a : Armv8.3-A : armv8.2-a, +pauth, +fcma, +jscvt
armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod, +rcpc2
armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres, +frintts,
+flagm2
armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
armv8.7-a : Armv8.7-A : armv8.6-a, +wfxt, +xs
armv8.8-a : Armv8.8-a : armv8.7-a, +mops
armv8.9-a : Armv8.9-a : armv8.8-a
armv9-a : Armv9-A : armv8.5-a, +sve, +sve2
armv9.1-a : Armv9.1-A : armv9-a, +bf16, +i8mm
armv9.2-a : Armv9.2-A : armv9.1-a, +wfxt, +xs
armv9.3-a : Armv9.3-A : armv9.2-a, +mops
armv9.4-a : Armv9.4-A : armv9.3-a, +sve2p1
armv9.5-a : Armv9.5-A : armv9.4-a, cpa, +faminmax, +lut
armv8-r : Armv8-R : armv8-r
The value native is available on native AArch64 GNU/Linux and causes
the compiler to pick the architecture of the host system. This
option has no effect if the compiler is unable to recognize the
architecture of the host system. When -march=native is given and no
other -mcpu or -mtune is given then GCC will pick the host CPU as the
CPU to tune for as well as select the architecture features from.
That is, -march=native is treated as -mcpu=native.
The permissible values for feature are listed in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where
conflicting feature modifiers are specified, the right-most feature
is used.
GCC uses name to determine what kind of instructions it can emit when
generating assembly code. If -march is specified without either of
-mtune or -mcpu also being specified, the code is tuned to perform
well across a range of target processors implementing the target
architecture.
-mtune=name
Specify the name of the target processor for which GCC should tune
the performance of the code. Permissible values for this option are:
generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72,
cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77,
cortex-a65, cortex-a65ae, cortex-a34, cortex-a78, cortex-a78ae,
cortex-a78c, ares, exynos-m1, emag, falkor, oryon-1, neoverse-512tvb,
neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1, neoverse-v2,
grace, neoverse-v3, neoverse-v3ae, neoverse-n3, olympus, cortex-a725,
cortex-x925, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx,
octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96
octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx,
fujitsu-monaka, thunderx, thunderxt88, thunderxt88p1, thunderxt81,
tsv110, hip12, thunderxt83, thunderx2t99, thunderx3t110, zeus,
cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35,
cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55,
cortex-r82, cortex-r82ae, cortex-x1, cortex-x1c, cortex-x2,
cortex-x3, cortex-x4, cortex-a510, cortex-a520, cortex-a520ae,
cortex-a710, cortex-a715, cortex-a720, cortex-a720ae, ampere1,
ampere1a, ampere1b, cobalt-100, apple-m1, apple-m2, apple-m3,
apple-m4 and native.
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55,
cortex-a76.cortex-a55, apple-m1, apple-m2, apple-m3, gb10 specify
that GCC should tune for a big.LITTLE system.
The value neoverse-512tvb specifies that GCC should tune for Neoverse
cores that (a) implement SVE and (b) have a total vector bandwidth of
512 bits per cycle. In other words, the option tells GCC to tune for
Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic
instructions a cycle and that can execute an equivalent number of SVE
arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit
SVE). This is more general than tuning for a specific core like
Neoverse V1 but is more specific than the default tuning described
below.
Additionally on native AArch64 GNU/Linux systems the value native
tunes performance to the host system. This option has no effect if
the compiler is unable to recognize the processor of the host system.
Where none of -mtune=, -mcpu= or -march= are specified, the code is
tuned to perform well across a range of target processors.
This option cannot be suffixed by feature modifiers.
-mcpu=name
Specify the name of the target processor, optionally suffixed by one
or more feature modifiers. This option has the form
-mcpu=cpu{+[no]feature}*, where the permissible values for cpu are
the same as those available for -mtune. The permissible values for
feature are documented in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where
conflicting feature modifiers are specified, the right-most feature
is used.
GCC uses name to determine what kind of instructions it can emit when
generating assembly code (as if by -march) and to determine the
target processor for which to tune for performance (as if by -mtune).
Where this option is used in conjunction with -march or -mtune, those
options take precedence over the appropriate part of this option.
-mcpu=neoverse-512tvb is special in that it does not refer to a
specific core, but instead refers to all Neoverse cores that (a)
implement SVE and (b) have a total vector bandwidth of 512 bits a
cycle. Unless overridden by -march, -mcpu=neoverse-512tvb generates
code that can run on a Neoverse V1 core, since Neoverse V1 is the
first Neoverse core with these properties. Unless overridden by
-mtune, -mcpu=neoverse-512tvb tunes code in the same way as for
-mtune=neoverse-512tvb.
-moverride=string
Override tuning decisions made by the back-end in response to a
-mtune= switch. The syntax, semantics, and accepted values for
string in this option are not guaranteed to be consistent across
releases.
This option is only intended to be useful when developing GCC.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This
option is provided for use in debugging the compiler.
-mpc-relative-literal-loads
-mno-pc-relative-literal-loads
Enable or disable PC-relative literal loads. With this option
literal pools are accessed using a single instruction and emitted
after each function. This limits the maximum size of functions to
1MB. This is enabled by default for -mcmodel=tiny.
-msign-return-address=scope
Select the function scope on which return address signing will be
applied. Permissible values are none, which disables return address
signing, non-leaf, which enables pointer signing for functions which
are not leaf functions, and all, which enables pointer signing for
all functions. The default value is none. This option has been
deprecated by -mbranch-protection.
-mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti|gcs
Select the branch protection features to use. none is the default
and turns off all types of branch protection. standard turns on all
types of branch protection features. If a feature has additional
tuning options, then standard sets it to its standard level.
pac-ret[+leaf] turns on return address signing to its standard level:
signing functions that save the return address to memory (non-leaf
functions will practically always do this) using the a-key. The
optional argument leaf can be used to extend the signing to include
leaf functions. The optional argument b-key can be used to sign the
functions with the B-key instead of the A-key. bti turns on branch
target identification mechanism. gcs turns on guarded control stack
compatible code generation.
-mharden-sls=opts
Enable compiler hardening against straight line speculation (SLS).
opts is a comma-separated list of the following options:
retbr
blr
In addition, -mharden-sls=all enables all SLS hardening while
-mharden-sls=none disables all SLS hardening.
-mearly-ra=scope
Determine when to enable an early register allocation pass. This
pass runs before instruction scheduling and tries to find a spill-
free allocation of floating-point and vector code. It also tries to
make use of strided multi-register instructions, such as SME2's
strided LD1 and ST1.
The possible values of scope are: all, which runs the pass on all
functions; strided, which runs the pass on functions that have access
to strided multi-register instructions; and none, which disables the
pass.
-mearly-ra=all is the default for -O2 and above, and for -Os.
-mearly-ra=none is the default otherwise.
-mearly-ldp-fusion
Enable the copy of the AArch64 load/store pair fusion pass that runs
before register allocation. Enabled by default at -O and above.
-mlate-ldp-fusion
Enable the copy of the AArch64 load/store pair fusion pass that runs
after register allocation. Enabled by default at -O and above.
-msve-vector-bits=bits
Specify the number of bits in an SVE vector register. This option
only has an effect when SVE is enabled.
GCC supports two forms of SVE code generation: "vector-length
agnostic" output that works with any size of vector register and
"vector-length specific" output that allows GCC to make assumptions
about the vector length when it is useful for optimization reasons.
The possible values of bits are: scalable, 128, 256, 512, 1024 and
2048. Specifying scalable selects vector-length agnostic output. At
present -msve-vector-bits=128 also generates vector-length agnostic
output for big-endian targets. All other values generate vector-
length specific code. The behavior of these values may change in
future releases and no value except scalable should be relied on for
producing code that is portable across different hardware SVE vector
lengths.
The default is -msve-vector-bits=scalable, which produces vector-
length agnostic code.
-Wexperimental-fmv-target
Warn about use of experimental Function Multi Versioning. The Arm C
Language Extension specification for Function Multi Versioning is
beta and subject to change. Any usage of FMV is caveated that future
behavior change and incompatibility is likely.
-march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be any of the following
and their inverses nofeature:
crc Enable CRC extension. This is on by default for -march=armv8.1-a.
crypto
Enable Crypto extension. This also enables Advanced SIMD and
floating-point instructions.
fp Enable floating-point instructions. This is on by default for all
possible values for options -march and -mcpu.
simd
Enable Advanced SIMD instructions. This also enables floating-point
instructions. This is on by default for all possible values for
options -march and -mcpu.
sve Enable Scalable Vector Extension instructions. This also enables
Advanced SIMD and floating-point instructions.
lse Enable Large System Extension instructions. This is on by default
for -march=armv8.1-a.
rdma
Enable Round Double Multiply Accumulate instructions. This is on by
default for -march=armv8.1-a.
fp16
Enable FP16 extension. This also enables floating-point
instructions.
fp16fml
Enable FP16 fmla extension. This also enables FP16 extensions and
floating-point instructions. This option is enabled by default for
-march=armv8.4-a. Use of this option with architectures prior to
Armv8.2-A is not supported.
rcpc
Enable the RCpc extension. This enables the use of the LDAPR
instructions for load-acquire atomic semantics, and passes it on to
the assembler, enabling inline asm statements to use instructions
from the RCpc extension.
dotprod
Enable the Dot Product extension. This also enables Advanced SIMD
instructions.
aes Enable the Armv8-a aes and pmull crypto extension. This also enables
Advanced SIMD instructions.
sha2
Enable the Armv8-a sha2 crypto extension. This also enables Advanced
SIMD instructions.
sha3
Enable the sha512 and sha3 crypto extension. This also enables
Advanced SIMD instructions. Use of this option with architectures
prior to Armv8.2-A is not supported.
sm4 Enable the sm3 and sm4 crypto extension. This also enables Advanced
SIMD instructions. Use of this option with architectures prior to
Armv8.2-A is not supported.
profile
Enable the Statistical Profiling extension. This option is only to
enable the extension at the assembler level and does not affect code
generation.
rng Enable the Armv8.5-a Random Number instructions. This option is only
to enable the extension at the assembler level and does not affect
code generation.
memtag
Enable the Armv8.5-a Memory Tagging Extensions. Use of this option
with architectures prior to Armv8.5-A is not supported.
sb Enable the Armv8-a Speculation Barrier instruction. This option is
only to enable the extension at the assembler level and does not
affect code generation. This option is enabled by default for
-march=armv8.5-a.
ssbs
Enable the Armv8-a Speculative Store Bypass Safe instruction. This
option is only to enable the extension at the assembler level and
does not affect code generation. This option is enabled by default
for -march=armv8.5-a.
predres
Enable the Armv8-a Execution and Data Prediction Restriction
instructions. This option is only to enable the extension at the
assembler level and does not affect code generation. This option is
enabled by default for -march=armv8.5-a.
sve2
Enable the Armv8-a Scalable Vector Extension 2. This also enables
SVE instructions.
sve2-bitperm
Enable SVE2 bitperm instructions. This also enables SVE2
instructions.
sve2-sm4
Enable SVE2 sm4 instructions. This also enables SVE2 instructions.
sve2-aes
Enable SVE2 aes instructions. This also enables SVE2 instructions.
sve2-sha3
Enable SVE2 sha3 instructions. This also enables SVE2 instructions.
sve2p1
Enable SVE2.1 instructions. This also enables SVE2 instructions.
tme Enable the Transactional Memory Extension.
i8mm
Enable 8-bit Integer Matrix Multiply instructions. This also enables
Advanced SIMD and floating-point instructions. This option is
enabled by default for -march=armv8.6-a. Use of this option with
architectures prior to Armv8.2-A is not supported.
f32mm
Enable 32-bit Floating point Matrix Multiply instructions. This also
enables SVE instructions. Use of this option with architectures
prior to Armv8.2-A is not supported.
f64mm
Enable 64-bit Floating point Matrix Multiply instructions. This also
enables SVE instructions. Use of this option with architectures
prior to Armv8.2-A is not supported.
bf16
Enable brain half-precision floating-point instructions. This also
enables Advanced SIMD and floating-point instructions. This option
is enabled by default for -march=armv8.6-a. Use of this option with
architectures prior to Armv8.2-A is not supported.
ls64
Enable the 64-byte atomic load and store instructions for
accelerators.
mops
Enable the instructions to accelerate memory operations like
"memcpy", "memmove", "memset". This option is enabled by default for
-march=armv8.8-a
flagm
Enable the Flag Manipulation instructions Extension.
flagm2
Enable the FlagM2 flag conversion instructions.
pauth
Enable the Pointer Authentication Extension.
cssc
Enable the Common Short Sequence Compression instructions.
sme Enable the Scalable Matrix Extension. This is only supported when
SVE2 is also enabled.
sme-i16i64
Enable the FEAT_SME_I16I64 extension to SME. This also enables SME
instructions.
sme-f64f64
Enable the FEAT_SME_F64F64 extension to SME. This also enables SME
instructions.
sme2
Enable the Scalable Matrix Extension 2. This also enables SME
instructions.
sme-b16b16
Enable the FEAT_SME_B16B16 extension to SME. This also enables SME2
and SVE_B16B16 instructions.
sme-f16f16
Enable the FEAT_SME_F16F16 extension to SME. This also enables SME2
instructions.
sme2p1
Enable the Scalable Matrix Extension version 2.1. This also enables
SME2 instructions.
fcma
Enable the complex number SIMD extensions.
jscvt
Enable the "fjcvtzs" JavaScript conversion instruction.
frintts
Enable floating-point round to integral value instructions.
wfxt
Enable "wfet" and "wfit" instructions.
xs Enable the XS memory attribute extension.
lse128
Enable the LSE128 128-bit atomic instructions extension. This also
enables LSE instructions.
d128
Enable support for 128-bit system register read/write instructions.
This also enables the LSE128 extension.
gcs Enable support for Armv9.4-a Guarded Control Stack extension.
the Enable support for Armv8.9-a/9.4-a translation hardening extension.
rcpc2
Enable the RCpc2 extension.
rcpc3
Enable the RCpc3 (Release Consistency) extension.
fp8 Enable the fp8 (8-bit floating point) extension.
fp8fma
Enable the fp8 (8-bit floating point) multiply accumulate extension.
ssve-fp8fma
Enable the fp8 (8-bit floating point) multiply accumulate extension
in streaming mode.
fp8dot4
Enable the fp8 (8-bit floating point) to single-precision 4-way dot
product extension.
ssve-fp8dot4
Enable the fp8 (8-bit floating point) to single-precision 4-way dot
product extension in streaming mode.
fp8dot2
Enable the fp8 (8-bit floating point) to half-precision 2-way dot
product extension.
ssve-fp8dot2
Enable the fp8 (8-bit floating point) to half-precision 2-way dot
product extension in streaming mode.
faminmax
Enable the Floating Point Absolute Maximum/Minimum extension.
lut Enable the Lookup Table extension.
cpa Enable the Checked Pointer Arithmetic instructions.
sve-b16b16
Enable the SVE non-widening brain floating-point ("bf16") extension.
This only has an effect when "sve2" or "sme2" are also enabled.
Feature crypto implies aes, sha2, and simd, which implies fp.
Conversely, nofp implies nosimd, which implies nocrypto, noaes and
nosha2.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That allows
code to run on hardware variants that lack these registers.
-mprefer-short-insn-regs
Preferentially allocate registers that allow short instruction
generation. This can result in increased instruction count, so this
may either reduce or increase overall code size.
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This
cost is only a heuristic and is not guaranteed to produce consistent
results across releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num NOPs before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an "fsub"
instruction and test the flags. This is faster than a software
comparison, but can get incorrect results in the presence of NaNs, or
when two different small numbers are compared such that their
difference is calculated as zero. The default is -msoft-cmpsf, which
uses slower, but IEEE-compliant, software comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the stack pointer.
E.g., a value of 8 means that the eight bytes in the range
"sp+0...sp+7" can be used by leaf functions without stack allocation.
Values other than 8 or 16 are untested and unlikely to work. Note
also that this option changes the ABI; compiling a program with a
different stack offset than the libraries have been compiled with
generally does not work. This option can be useful if you want to
evaluate if a different stack offset would give you better code, but
to actually use a different stack offset to build working programs,
it is recommended to configure the toolchain with the appropriate
--with-stack-offset=num option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to
truncating. The default is -mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all calls might be
beyond the offset range of the "b" / "bl" instructions, and therefore
load the function address into a register before performing a
(otherwise direct) call. This is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct calls
are in the range of the "b" / "bl" instructions, so use these
instructions for direct calls. The default is -mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This does
not apply to function addresses for which -mlong-calls semantics are
in effect.
-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This determines
the floating-point mode that is provided and expected at function
call and return time. Making this mode match the mode you
predominantly need at function start can make your programs smaller
and faster by avoiding unnecessary mode switches.
mode can be set to one the following values:
caller
Any mode at function entry is valid, and retained or restored
when the function returns, and when it calls other functions.
This mode is useful for compiling libraries or other compilation
units you might want to incorporate into different programs with
different prevailing FPU modes, and the convenience of being able
to use a single object file outweighs the size and speed overhead
for any extra mode switching that might be needed, compared with
what would be needed with a more specific choice of prevailing
FPU mode.
truncate
This is the mode used for floating-point calculations with
truncating (i.e. round towards zero) rounding mode. That
includes conversion from floating point to integer.
round-nearest
This is the mode used for floating-point calculations with round-
to-nearest-or-even rounding mode.
int This is the mode used to perform integer calculations in the FPU,
e.g. integer multiply, or integer multiply-and-accumulate.
The default is -mfp-mode=caller
-mno-split-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of
32-bit loads, generation of post-increment addresses, and generation
of post-modify addresses. The defaults are msplit-lohi, -mpost-inc,
and -mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is
-mvect-double, which uses DImode as preferred SIMD mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4 or 8.
The default is 8. Note that this is an ABI change, even though many
library function interfaces are unaffected if they don't use SIMD
vector modes in places that affect size and/or alignment of relevant
types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory
this can give better register allocation, but so far the reverse
seems to be generally the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes loading small
negative constants and certain bitmasks faster. Allowable values for
reg are r43 and r63, which specify use of that register as a fixed
register, and none, which means that no register is used for this
purpose. The default is -m1reg-none.
AMD GCN Options
These options are defined specifically for the AMD GCN port.
-march=gpu
-mtune=gpu
Set architecture type or tuning for gpu. Supported values for gpu are
gfx900
Compile for GCN5 Vega 10 devices (gfx900).
gfx902
Compile for GCN5 Vega gfx902 devices. (Experimental)
gfx904
Compile for GCN5 Vega gfx904 devices. (Experimental)
gfx906
Compile for GCN5 Vega 20 devices (gfx906).
gfx908
Compile for CDNA1 Instinct MI100 series devices (gfx908).
gfx909
Compile for GCN5 Vega gfx909 devices. (Experimental)
gfx90a
Compile for CDNA2 Instinct MI200 series devices (gfx90a).
gfx90c
Compile for GCN5 Vega 7 devices (gfx90c).
gfx9-generic
Compile generic code for Vega devices, executable on the
following subset of GFX9 devices: gfx900, gfx902, gfx904, gfx906,
gfx909 and gfx90c. (Experimental)
gfx1030
Compile for RDNA2 gfx1030 devices (GFX10 series).
gfx1031
Compile for RDNA2 gfx1031 devices (GFX10 series). (Experimental)
gfx1032
Compile for RDNA2 gfx1032 devices (GFX10 series). (Experimental)
gfx1033
Compile for RDNA2 gfx1033 devices (GFX10 series). (Experimental)
gfx1034
Compile for RDNA2 gfx1034 devices (GFX10 series). (Experimental)
gfx1035
Compile for RDNA2 gfx1035 devices (GFX10 series). (Experimental)
gfx1036
Compile for RDNA2 gfx1036 devices (GFX10 series).
gfx10-3-generic
Compile generic code for GFX10-3 devices, executable on gfx1030,
gfx1031, gfx1032, gfx1033, gfx1034, gfx1035, and gfx1036.
(Experimental)
gfx1100
Compile for RDNA3 gfx1100 devices (GFX11 series).
gfx1101
Compile for RDNA3 gfx1101 devices (GFX11 series). (Experimental)
gfx1102
Compile for RDNA3 gfx1102 devices (GFX11 series). (Experimental)
gfx1103
Compile for RDNA3 gfx1103 devices (GFX11 series).
gfx1150
Compile for RDNA3 gfx1150 devices (GFX11 series). (Experimental)
gfx1151
Compile for RDNA3 gfx1151 devices (GFX11 series). (Experimental)
gfx1152
Compile for RDNA3 gfx1152 devices (GFX11 series). (Experimental)
gfx1153
Compile for RDNA3 gfx1153 devices (GFX11 series). (Experimental)
gfx11-generic
Compile generic code for GFX11 devices, executable on gfx1100,
gfx1101, gfx1102, gfx1103, gfx1150, gfx1151, gfx1152, and
gfx1153. (Experimental)
-msram-ecc=on
-msram-ecc=off
-msram-ecc=any
Compile binaries suitable for devices with the SRAM-ECC feature
enabled, disabled, or either mode. This feature can be enabled per-
process on some devices. The compiled code must match the device
mode. The default is any, for devices that support it.
-mstack-size=bytes
Specify how many bytes of stack space will be requested for each GPU
thread (wave-front). Beware that there may be many threads and
limited memory available. The size of the stack allocation may also
have an impact on run-time performance. The default is 32KB when
using OpenACC or OpenMP, and 1MB otherwise.
-mxnack=on
-mxnack=off
-mxnack=any
Compile binaries suitable for devices with the XNACK feature enabled,
disabled, or either mode. Some devices always require XNACK and some
allow the user to configure XNACK. The compiled code must match the
device mode. The default is -mxnack=any on devices that support
Unified Shared Memory, and -mxnack=no otherwise.
ARC Options
The following options control the architecture variant for which code is
being compiled:
-mbarrel-shifter
Generate instructions supported by barrel shifter. This is the
default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
-mjli-always
Force to call a function using jli_s instruction. This option is
valid only for ARCv2 architecture.
-mcpu=cpu
Set architecture type, register usage, and instruction scheduling
parameters for cpu. There are also shortcut alias options available
for backward compatibility and convenience. Supported values for cpu
are
arc600
Compile for ARC600. Aliases: -mA6, -mARC600.
arc601
Compile for ARC601. Alias: -mARC601.
arc700
Compile for ARC700. Aliases: -mA7, -mARC700. This is the
default when configured with --with-cpu=arc700.
arcem
Compile for ARC EM.
archs
Compile for ARC HS.
em Compile for ARC EM CPU with no hardware extensions.
em4 Compile for ARC EM4 CPU.
em4_dmips
Compile for ARC EM4 DMIPS CPU.
em4_fpus
Compile for ARC EM4 DMIPS CPU with the single-precision floating-
point extension.
em4_fpuda
Compile for ARC EM4 DMIPS CPU with single-precision floating-
point and double assist instructions.
hs Compile for ARC HS CPU with no hardware extensions except the
atomic instructions.
hs34
Compile for ARC HS34 CPU.
hs38
Compile for ARC HS38 CPU.
hs38_linux
Compile for ARC HS38 CPU with all hardware extensions on.
hs4x
Compile for ARC HS4x CPU.
hs4xd
Compile for ARC HS4xD CPU.
hs4x_rel31
Compile for ARC HS4x CPU release 3.10a.
arc600_norm
Compile for ARC 600 CPU with "norm" instructions enabled.
arc600_mul32x16
Compile for ARC 600 CPU with "norm" and 32x16-bit multiply
instructions enabled.
arc600_mul64
Compile for ARC 600 CPU with "norm" and "mul64"-family
instructions enabled.
arc601_norm
Compile for ARC 601 CPU with "norm" instructions enabled.
arc601_mul32x16
Compile for ARC 601 CPU with "norm" and 32x16-bit multiply
instructions enabled.
arc601_mul64
Compile for ARC 601 CPU with "norm" and "mul64"-family
instructions enabled.
nps400
Compile for ARC 700 on NPS400 chip.
em_mini
Compile for ARC EM minimalist configuration featuring reduced
register set.
-mdpfp
-mdpfp-compact
Generate double-precision FPX instructions, tuned for the compact
implementation.
-mdpfp-fast
Generate double-precision FPX instructions, tuned for the fast
implementation.
-mno-dpfp-lrsr
Disable "lr" and "sr" instructions from using FPX extension aux
registers.
-mea
Generate extended arithmetic instructions. Currently only "divaw",
"adds", "subs", and "sat16" are supported. Only valid for
-mcpu=ARC700.
-mno-mpy
Do not generate "mpy"-family instructions for ARC700. This option is
deprecated.
-mmul32x16
Generate 32x16-bit multiply and multiply-accumulate instructions.
-mmul64
Generate "mul64" and "mulu64" instructions. Only valid for
-mcpu=ARC600.
-mnorm
Generate "norm" instructions. This is the default if -mcpu=ARC700 is
in effect.
-mspfp
-mspfp-compact
Generate single-precision FPX instructions, tuned for the compact
implementation.
-mspfp-fast
Generate single-precision FPX instructions, tuned for the fast
implementation.
-msimd
Enable generation of ARC SIMD instructions via target-specific
builtins. Only valid for -mcpu=ARC700.
-msoft-float
This option ignored; it is provided for compatibility purposes only.
Software floating-point code is emitted by default, and this default
can overridden by FPX options; -mspfp, -mspfp-compact, or -mspfp-fast
for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for
double precision.
-mswap
Generate "swap" instructions.
-matomic
This enables use of the locked load/store conditional extension to
implement atomic memory built-in functions. Not available for ARC
6xx or ARC EM cores.
-mdiv-rem
Enable "div" and "rem" instructions for ARCv2 cores.
-mcode-density
Enable code density instructions for ARC EM. This option is on by
default for ARC HS.
-mll64
Enable double load/store operations for ARC HS cores.
-mtp-regno=regno
Specify thread pointer register number.
-mmpy-option=multo
Compile ARCv2 code with a multiplier design option. You can specify
the option using either a string or numeric value for multo. wlh1 is
the default value. The recognized values are:
0
none
No multiplier available.
1
w 16x16 multiplier, fully pipelined. The following instructions
are enabled: "mpyw" and "mpyuw".
2
wlh1
32x32 multiplier, fully pipelined (1 stage). The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
3
wlh2
32x32 multiplier, fully pipelined (2 stages). The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
4
wlh3
Two 16x16 multipliers, blocking, sequential. The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
5
wlh4
One 16x16 multiplier, blocking, sequential. The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
6
wlh5
One 32x4 multiplier, blocking, sequential. The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
7
plus_dmpy
ARC HS SIMD support.
8
plus_macd
ARC HS SIMD support.
9
plus_qmacw
ARC HS SIMD support.
This option is only available for ARCv2 cores.
-mfpu=fpu
Enables support for specific floating-point hardware extensions for
ARCv2 cores. Supported values for fpu are:
fpus
Enables support for single-precision floating-point hardware
extensions.
fpud
Enables support for double-precision floating-point hardware
extensions. The single-precision floating-point extension is
also enabled. Not available for ARC EM.
fpuda
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. The
single-precision floating-point extension is also enabled. This
option is only available for ARC EM.
fpuda_div
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. The
single-precision floating-point, square-root, and divide
extensions are also enabled. This option is only available for
ARC EM.
fpuda_fma
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. The
single-precision floating-point and fused multiply and add
hardware extensions are also enabled. This option is only
available for ARC EM.
fpuda_all
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. All
single-precision floating-point hardware extensions are also
enabled. This option is only available for ARC EM.
fpus_div
Enables support for single-precision floating-point, square-root
and divide hardware extensions.
fpud_div
Enables support for double-precision floating-point, square-root
and divide hardware extensions. This option includes option
fpus_div. Not available for ARC EM.
fpus_fma
Enables support for single-precision floating-point and fused
multiply and add hardware extensions.
fpud_fma
Enables support for double-precision floating-point and fused
multiply and add hardware extensions. This option includes
option fpus_fma. Not available for ARC EM.
fpus_all
Enables support for all single-precision floating-point hardware
extensions.
fpud_all
Enables support for all single- and double-precision floating-
point hardware extensions. Not available for ARC EM.
-mirq-ctrl-saved=register-range, blink, lp_count
Specifies general-purposes registers that the processor automatically
saves/restores on interrupt entry and exit. register-range is
specified as two registers separated by a dash. The register range
always starts with "r0", the upper limit is "fp" register. blink and
lp_count are optional. This option is only valid for ARC EM and ARC
HS cores.
-mrgf-banked-regs=number
Specifies the number of registers replicated in second register bank
on entry to fast interrupt. Fast interrupts are interrupts with the
highest priority level P0. These interrupts save only PC and
STATUS32 registers to avoid memory transactions during interrupt
entry and exit sequences. Use this option when you are using fast
interrupts in an ARC V2 family processor. Permitted values are 4, 8,
16, and 32.
-mlpc-width=width
Specify the width of the "lp_count" register. Valid values for width
are 8, 16, 20, 24, 28 and 32 bits. The default width is fixed to 32
bits. If the width is less than 32, the compiler does not attempt to
transform loops in your program to use the zero-delay loop mechanism
unless it is known that the "lp_count" register can hold the required
loop-counter value. Depending on the width specified, the compiler
and run-time library might continue to use the loop mechanism for
various needs. This option defines macro "__ARC_LPC_WIDTH__" with
the value of width.
-mrf16
This option instructs the compiler to generate code for a 16-entry
register file. This option defines the "__ARC_RF16__" preprocessor
macro.
-mbranch-index
Enable use of "bi" or "bih" instructions to implement jump tables.
The following options are passed through to the assembler, and also
define preprocessor macro symbols.
-mdsp-packa
Passed down to the assembler to enable the DSP Pack A extensions.
Also sets the preprocessor symbol "__Xdsp_packa". This option is
deprecated.
-mdvbf
Passed down to the assembler to enable the dual Viterbi butterfly
extension. Also sets the preprocessor symbol "__Xdvbf". This option
is deprecated.
-mlock
Passed down to the assembler to enable the locked load/store
conditional extension. Also sets the preprocessor symbol "__Xlock".
-mmac-d16
Passed down to the assembler. Also sets the preprocessor symbol
"__Xxmac_d16". This option is deprecated.
-mmac-24
Passed down to the assembler. Also sets the preprocessor symbol
"__Xxmac_24". This option is deprecated.
-mrtsc
Passed down to the assembler to enable the 64-bit time-stamp counter
extension instruction. Also sets the preprocessor symbol "__Xrtsc".
This option is deprecated.
-mswape
Passed down to the assembler to enable the swap byte ordering
extension instruction. Also sets the preprocessor symbol "__Xswape".
-mtelephony
Passed down to the assembler to enable dual- and single-operand
instructions for telephony. Also sets the preprocessor symbol
"__Xtelephony". This option is deprecated.
-mxy
Passed down to the assembler to enable the XY memory extension. Also
sets the preprocessor symbol "__Xxy".
The following options control how the assembly code is annotated:
-misize
Annotate assembler instructions with estimated addresses.
-mannotate-align
Does nothing. Preserved for backward compatibility.
The following options are passed through to the linker:
-marclinux
Passed through to the linker, to specify use of the "arclinux"
emulation. This option is enabled by default in tool chains built
for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when
profiling is not requested.
-marclinux_prof
Passed through to the linker, to specify use of the "arclinux_prof"
emulation. This option is enabled by default in tool chains built
for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when
profiling is requested.
The following options control the semantics of generated code:
-mlong-calls
Generate calls as register indirect calls, thus providing access to
the full 32-bit address range.
-mmedium-calls
Don't use less than 25-bit addressing range for calls, which is the
offset available for an unconditional branch-and-link instruction.
Conditional execution of function calls is suppressed, to allow use
of the 25-bit range, rather than the 21-bit range with conditional
branch-and-link. This is the default for tool chains built for
"arc-linux-uclibc" and "arceb-linux-uclibc" targets.
-G num
Put definitions of externally-visible data in a small data section if
that data is no bigger than num bytes. The default value of num is 4
for any ARC configuration, or 8 when we have double load/store
operations.
-mno-sdata
Do not generate sdata references. This is the default for tool
chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets.
-mvolatile-cache
Use ordinarily cached memory accesses for volatile references. This
is the default.
-mno-volatile-cache
Enable cache bypass for volatile references.
The following options fine tune code generation:
-malign-call
Does nothing. Preserved for backward compatibility.
-mauto-modify-reg
Enable the use of pre/post modify with register displacement.
-mbbit-peephole
Does nothing. Preserved for backward compatibility.
-mno-brcc
This option disables a target-specific pass in arc_reorg to generate
compare-and-branch ("brcc") instructions. It has no effect on
generation of these instructions driven by the combiner pass.
-mcase-vector-pcrel
Use PC-relative switch case tables to enable case table shortening.
This is the default for -Os.
-mcompact-casesi
Enable compact "casesi" pattern. This is the default for -Os, and
only available for ARCv1 cores. This option is deprecated.
-mno-cond-exec
Disable the ARCompact-specific pass to generate conditional execution
instructions.
Due to delay slot scheduling and interactions between operand
numbers, literal sizes, instruction lengths, and the support for
conditional execution, the target-independent pass to generate
conditional execution is often lacking, so the ARC port has kept a
special pass around that tries to find more conditional execution
generation opportunities after register allocation, branch
shortening, and delay slot scheduling have been done. This pass
generally, but not always, improves performance and code size, at the
cost of extra compilation time, which is why there is an option to
switch it off. If you have a problem with call instructions
exceeding their allowable offset range because they are
conditionalized, you should consider using -mmedium-calls instead.
-mearly-cbranchsi
Enable pre-reload use of the "cbranchsi" pattern.
-mexpand-adddi
Expand "adddi3" and "subdi3" at RTL generation time into "add.f",
"adc" etc. This option is deprecated.
-mindexed-loads
Enable the use of indexed loads. This can be problematic because
some optimizers then assume that indexed stores exist, which is not
the case.
-mlra
Does nothing. Preserved for backward compatibility.
-mlra-priority-none
Don't indicate any priority for target registers.
-mlra-priority-compact
Indicate target register priority for r0..r3 / r12..r15.
-mlra-priority-noncompact
Reduce target register priority for r0..r3 / r12..r15.
-mmillicode
When optimizing for size (using -Os), prologues and epilogues that
have to save or restore a large number of registers are often
shortened by using call to a special function in libgcc; this is
referred to as a millicode call. As these calls can pose performance
issues, and/or cause linking issues when linking in a nonstandard
way, this option is provided to turn on or off millicode call
generation.
-mcode-density-frame
This option enable the compiler to emit "enter" and "leave"
instructions. These instructions are only valid for CPUs with code-
density feature.
-mmixed-code
Does nothing. Preserved for backward compatibility.
-mq-class
Ths option is deprecated. Enable q instruction alternatives. This
is the default for -Os.
-mRcq
Does nothing. Preserved for backward compatibility.
-mRcw
Does nothing. Preserved for backward compatibility.
-msize-level=level
Fine-tune size optimization with regards to instruction lengths and
alignment. The recognized values for level are:
0 No size optimization. This level is deprecated and treated like
1.
1 Short instructions are used opportunistically.
2 In addition, alignment of loops and of code after barriers are
dropped.
3 In addition, optional data alignment is dropped, and the option
Os is enabled.
This defaults to 3 when -Os is in effect. Otherwise, the behavior
when this is not set is equivalent to level 1.
-mtune=cpu
Set instruction scheduling parameters for cpu, overriding any implied
by -mcpu=.
Supported values for cpu are
ARC600
Tune for ARC600 CPU.
ARC601
Tune for ARC601 CPU.
ARC700
Tune for ARC700 CPU with standard multiplier block.
ARC700-xmac
Tune for ARC700 CPU with XMAC block.
ARC725D
Tune for ARC725D CPU.
ARC750D
Tune for ARC750D CPU.
core3
Tune for ARCv2 core3 type CPU. This option enable usage of
"dbnz" instruction.
release31a
Tune for ARC4x release 3.10a.
-mmultcost=num
Cost to assume for a multiply instruction, with 4 being equal to a
normal instruction.
-munalign-prob-threshold=probability
Does nothing. Preserved for backward compatibility.
The following options are maintained for backward compatibility, but are
now deprecated and will be removed in a future release:
-margonaut
Obsolete FPX.
-mbig-endian
-EB Compile code for big-endian targets. Use of these options is now
deprecated. Big-endian code is supported by configuring GCC to build
"arceb-elf32" and "arceb-linux-uclibc" targets, for which big endian
is the default.
-mlittle-endian
-EL Compile code for little-endian targets. Use of these options is now
deprecated. Little-endian code is supported by configuring GCC to
build "arc-elf32" and "arc-linux-uclibc" targets, for which little
endian is the default.
-mbarrel_shifter
Replaced by -mbarrel-shifter.
-mdpfp_compact
Replaced by -mdpfp-compact.
-mdpfp_fast
Replaced by -mdpfp-fast.
-mdsp_packa
Replaced by -mdsp-packa.
-mEA
Replaced by -mea.
-mmac_24
Replaced by -mmac-24.
-mmac_d16
Replaced by -mmac-d16.
-mspfp_compact
Replaced by -mspfp-compact.
-mspfp_fast
Replaced by -mspfp-fast.
-mtune=cpu
Values arc600, arc601, arc700 and arc700-xmac for cpu are replaced by
ARC600, ARC601, ARC700 and ARC700-xmac respectively.
-multcost=num
Replaced by -mmultcost.
ARM Options
These -m options are defined for the ARM port:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-
gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call
Standard for all functions, even if this is not strictly necessary
for correct execution of the code. Specifying -fomit-frame-pointer
with this option causes the stack frames not to be generated for leaf
functions. The default is -mno-apcs-frame. This option is
deprecated.
-mapcs
This is a synonym for -mapcs-frame and is deprecated.
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb
instruction sets. Without this option, on pre-v5 architectures, the
two instruction sets cannot be reliably used inside one program. The
default is -mno-thumb-interwork, since slightly larger code is
generated when -mthumb-interwork is specified. In AAPCS
configurations this option is meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function prologue, or
the merging of those instruction with the instructions in the
function's body. This means that all functions start with a
recognizable set of instructions (or in fact one of a choice from a
small set of different function prologues), and this information can
be used to locate the start of functions inside an executable piece
of code. The default is -msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are:
soft, softfp and hard.
Specifying soft causes GCC to generate output containing library
calls for floating-point operations. softfp allows the generation of
code using hardware floating-point instructions, but still uses the
soft-float calling conventions. hard allows generation of floating-
point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that
the hard-float and soft-float ABIs are not link-compatible; you must
compile your entire program with the same ABI, and link with a
compatible set of libraries.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This
will prevent the compiler from using floating-point and Advanced SIMD
registers but will not impose any restrictions on the assembler.
-mlittle-endian
Generate code for a processor running in little-endian mode. This is
the default for all standard configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the default
is to compile code for a little-endian processor.
-mbe8
-mbe32
When linking a big-endian image select between BE8 and BE32 formats.
The option has no effect for little-endian images and is ignored.
The default is dependent on the selected target architecture. For
ARMv6 and later architectures the default is BE8, for older
architectures the default is BE32. BE32 format has been deprecated
by ARM.
-march=name[+extension...]
This specifies the name of the target ARM architecture. GCC uses
this name to determine what kind of instructions it can emit when
generating assembly code. This option can be used in conjunction
with or instead of the -mcpu= option.
Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j,
armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve,
armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a,
armv8.6-a, armv9-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m,
armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, iwmmxt and
iwmmxt2.
Additionally, the following architectures, which lack support for the
Thumb execution state, are recognized but support is deprecated:
armv4.
Many of the architectures support extensions. These can be added by
appending +extension to the architecture name. Extension options are
processed in order and capabilities accumulate. An extension will
also enable any necessary base extensions upon which it depends. For
example, the +crypto extension will always enable the +simd
extension. The exception to the additive construction is for
extensions that are prefixed with +no...: these extensions disable
the specified option and any other extensions that may depend on the
presence of that extension.
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing
-march=armv7-a+vfpv4 since the +simd option is entirely disabled by
the +nofp option that follows it.
Most extension names are generically named, but have an effect that
is dependent upon the architecture to which it is applied. For
example, the +simd option can be applied to both armv7-a and armv8-a
architectures, but will enable the original ARMv7-A Advanced SIMD
(Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a.
The table below lists the supported extensions for each architecture.
Architectures not mentioned do not support any extensions.
armv5te
armv6
armv6j
armv6k
armv6kz
armv6t2
armv6z
armv6zk
+fp The VFPv2 floating-point instructions. The extension +vfpv2
can be used as an alias for this extension.
+nofp
Disable the floating-point instructions.
armv7
The common subset of the ARMv7-A, ARMv7-R and ARMv7-M
architectures.
+fp The VFPv3 floating-point instructions, with 16 double-
precision registers. The extension +vfpv3-d16 can be used as
an alias for this extension. Note that floating-point is not
supported by the base ARMv7-M architecture, but is compatible
with both the ARMv7-A and ARMv7-R architectures.
+nofp
Disable the floating-point instructions.
armv7-a
+mp The multiprocessing extension.
+sec
The security extension.
+fp The VFPv3 floating-point instructions, with 16 double-
precision registers. The extension +vfpv3-d16 can be used as
an alias for this extension.
+simd
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point
instructions. The extensions +neon and +neon-vfpv3 can be
used as aliases for this extension.
+vfpv3
The VFPv3 floating-point instructions, with 32 double-
precision registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-
precision registers and the half-precision floating-point
conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-
precision registers and the half-precision floating-point
conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-
precision registers.
+vfpv4
The VFPv4 floating-point instructions, with 32 double-
precision registers.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point
instructions, with the half-precision floating-point
conversion operations.
+neon-vfpv4
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point
instructions.
+nosimd
Disable the Advanced SIMD instructions (does not disable
floating point).
+nofp
Disable the floating-point and Advanced SIMD instructions.
armv7ve
The extended version of the ARMv7-A architecture with support for
virtualization.
+fp The VFPv4 floating-point instructions, with 16 double-
precision registers. The extension +vfpv4-d16 can be used as
an alias for this extension.
+simd
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point
instructions. The extension +neon-vfpv4 can be used as an
alias for this extension.
+vfpv3-d16
The VFPv3 floating-point instructions, with 16 double-
precision registers.
+vfpv3
The VFPv3 floating-point instructions, with 32 double-
precision registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-
precision registers and the half-precision floating-point
conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-
precision registers and the half-precision floating-point
conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-
precision registers.
+vfpv4
The VFPv4 floating-point instructions, with 32 double-
precision registers.
+neon
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point
instructions. The extension +neon-vfpv3 can be used as an
alias for this extension.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point
instructions, with the half-precision floating-point
conversion operations.
+nosimd
Disable the Advanced SIMD instructions (does not disable
floating point).
+nofp
Disable the floating-point and Advanced SIMD instructions.
armv8-a
+crc
The Cyclic Redundancy Check (CRC) instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
armv8.1-a
+simd
The ARMv8.1-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
armv8.2-a
armv8.3-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD and
floating-point instructions.
+fp16fml
The half-precision floating-point fmla extension. This also
enables the half-precision floating-point extension and
Advanced SIMD and floating-point instructions.
+simd
The ARMv8.1-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions.
+dotprod
Enable the Dot Product extension. This also enables Advanced
SIMD instructions.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also
enables Advanced SIMD and floating-point instructions.
+bf16
Brain half-precision floating-point instructions. This also
enables Advanced SIMD and floating-point instructions.
armv8.4-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product
extension and the half-precision floating-point fmla
extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions
as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions as well as the
Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also
enables Advanced SIMD and floating-point instructions.
+bf16
Brain half-precision floating-point instructions. This also
enables Advanced SIMD and floating-point instructions.
armv8.5-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product
extension and the half-precision floating-point fmla
extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions
as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions as well as the
Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also
enables Advanced SIMD and floating-point instructions.
+bf16
Brain half-precision floating-point instructions. This also
enables Advanced SIMD and floating-point instructions.
armv8.6-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product
extension and the half-precision floating-point fmla
extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions
as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions as well as the
Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also
enables Advanced SIMD and floating-point instructions.
+bf16
Brain half-precision floating-point instructions. This also
enables Advanced SIMD and floating-point instructions.
armv7-r
+fp.sp
The single-precision VFPv3 floating-point instructions. The
extension +vfpvxd can be used as an alias for this
extension.
+fp The VFPv3 floating-point instructions with 16 double-
precision registers. The extension +vfpv3-d16 can be used as
an alias for this extension.
+vfpvxd-d16-fp16
The single-precision VFPv3 floating-point instructions with
16 double-precision registers and the half-precision
floating-point conversion operations.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions with 16 double-
precision registers and the half-precision floating-point
conversion operations.
+nofp
Disable the floating-point extension.
+idiv
The ARM-state integer division instructions.
+noidiv
Disable the ARM-state integer division extension.
armv7e-m
+fp The single-precision VFPv4 floating-point instructions.
+fpv5
The single-precision FPv5 floating-point instructions.
+fp.dp
The single- and double-precision FPv5 floating-point
instructions.
+nofp
Disable the floating-point extensions.
armv8.1-m.main
+dsp
The DSP instructions.
+mve
The M-Profile Vector Extension (MVE) integer instructions.
+mve.fp
The M-Profile Vector Extension (MVE) integer and single
precision floating-point instructions.
+fp The single-precision floating-point instructions.
+fp.dp
The single- and double-precision floating-point instructions.
+nofp
Disable the floating-point extension.
+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected
coprocessors according to the numbers given in the options in
the range 0 to 7.
+pacbti
Enable the Pointer Authentication and Branch Target
Identification Extension.
armv8-m.main
+dsp
The DSP instructions.
+nodsp
Disable the DSP extension.
+fp The single-precision floating-point instructions.
+fp.dp
The single- and double-precision floating-point instructions.
+nofp
Disable the floating-point extension.
+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected
coprocessors according to the numbers given in the options in
the range 0 to 7.
armv8-r
+crc
The Cyclic Redundancy Check (CRC) instructions.
+fp.sp
The single-precision FPv5 floating-point instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
-march=native causes the compiler to auto-detect the architecture of
the build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the auto-
detect is unsuccessful the option has no effect.
-march=unset causes the compiler to ignore any -march=... options
that appear earlier on the command line and behave as if the option
was never passed. This is useful to avoid warnings about conflicting
CPU and architecture options when the two produce different
architecture specifications.
-mtune=name
This option specifies the name of the target ARM processor for which
GCC should tune the performance of the code. For some ARM
implementations better performance can be obtained by using this
option. Permissible names are: arm7tdmi, arm7tdmi-s, arm710t,
arm720t, arm740t, strongarm, strongarm110, strongarm1100,
strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t,
arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi,
arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e,
arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s,
arm1156t2f-s, arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5,
cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17,
cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57,
cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae,
cortex-a77, cortex-a78, cortex-a78ae, cortex-a78c, cortex-a710, ares,
cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52,
cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3,
cortex-m4, cortex-m7, cortex-m23, cortex-m33, cortex-m35p,
cortex-m52, cortex-m55, cortex-m85, cortex-x1, cortex-x1c,
cortex-m1.small-multiply, cortex-m0.small-multiply,
cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1,
neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526,
fa626, fa606te, fa626te, fmp626, fa726te, star-mc1, xgene1, apple-m1,
apple-m2, apple-m3.
Additionally, this option can specify that GCC should tune the
performance of the code for a big.LITTLE system. Permissible names
are: cortex-a15.cortex-a7, cortex-a17.cortex-a7,
cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35,
cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.
-mtune=generic-arch specifies that GCC should tune the performance
for a blend of processors within architecture arch. The aim is to
generate code that run well on the current most popular processors,
balancing between optimizations that benefit some CPUs in the range,
and avoiding performance pitfalls of other CPUs. The effects of this
option may change in future GCC versions as CPU models come and go.
-mtune permits the same extension options as -mcpu, but the extension
options do not affect the tuning of the generated code.
-mtune=native causes the compiler to auto-detect the CPU of the build
computer. At present, this feature is only supported on GNU/Linux,
and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
-mcpu=name[+extension...]
This specifies the name of the target ARM processor. GCC uses this
name to derive the name of the target ARM architecture (as if
specified by -march) and the ARM processor type for which to tune for
performance (as if specified by -mtune). Where this option is used
in conjunction with -march or -mtune, those options take precedence
over the appropriate part of this option.
Many of the supported CPUs implement optional architectural
extensions. Where this is so the architectural extensions are
normally enabled by default. If implementations that lack the
extension exist, then the extension syntax can be used to disable
those extensions that have been omitted. For floating-point and
Advanced SIMD (Neon) instructions, the settings of the options
-mfloat-abi and -mfpu must also be considered: floating-point and
Advanced SIMD instructions will only be used if -mfloat-abi is not
set to soft; and any setting of -mfpu other than auto will override
the available floating-point and SIMD extension instructions.
For example, cortex-a9 can be found in three major configurations:
integer only, with just a floating-point unit or with floating-point
and Advanced SIMD. The default is to enable all the instructions,
but the extensions +nosimd and +nofp can be used to disable just the
SIMD or both the SIMD and floating-point instructions respectively.
Permissible names for this option are the same as those for -mtune.
The following extension options are common to the listed CPUs:
+nodsp
Disable the DSP instructions on cortex-m33, cortex-m35p,
cortex-m52, cortex-m55 and cortex-m85. Also disable the
M-Profile Vector Extension (MVE) integer and single precision
floating-point instructions on cortex-m52, cortex-m55 and
cortex-m85.
+nopacbti
Disable the Pointer Authentication and Branch Target
Identification Extension on cortex-m52 and cortex-m85.
+nomve
Disable the M-Profile Vector Extension (MVE) integer and single
precision floating-point instructions on cortex-m52, cortex-m55
and cortex-m85.
+nomve.fp
Disable the M-Profile Vector Extension (MVE) single precision
floating-point instructions on cortex-m52, cortex-m55 and
cortex-m85.
+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected
coprocessors according to the numbers given in the options in the
range 0 to 7 on cortex-m52, cortex-m55 and star-mc1.
+nofp
Disables the floating-point instructions on arm9e, arm946e-s,
arm966e-s, arm968e-s, arm10e, arm1020e, arm1022e, arm926ej-s,
arm1026ej-s, cortex-r5, cortex-r7, cortex-r8, cortex-m4,
cortex-m7, cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and
cortex-m85. Disables the floating-point and SIMD instructions on
generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a12, cortex-a15, cortex-a17, cortex-a15.cortex-a7,
cortex-a17.cortex-a7, cortex-a32, cortex-a35, cortex-a53 and
cortex-a55.
+nofp.dp
Disables the double-precision component of the floating-point
instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52,
cortex-r52plus and cortex-m7.
+nosimd
Disables the SIMD (but not floating-point) instructions on
generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
+crypto
Enables the cryptographic instructions on cortex-a32, cortex-a35,
cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73,
cortex-a75, exynos-m1, xgene1, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a73.cortex-a35,
cortex-a73.cortex-a53 and cortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to VFPv3 with
16 double-precision registers. It supports the following extension
options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16,
vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The
meanings are the same as for the extensions to -march=armv7-a.
-mcpu=generic-arch is also permissible, and is equivalent to
-march=arch -mtune=generic-arch. See -mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build
computer. At present, this feature is only supported on GNU/Linux,
and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
-mcpu=unset causes the compiler to ignore any -mcpu=... options that
appear earlier on the command line and behave as if the option was
never passed. This is useful to avoid warnings about conflicting CPU
and architecture options when the two produce different architecture
specifications.
-mfpu=name
This specifies what floating-point hardware (or hardware emulation)
is available on the target. Permissible names are: auto, vfpv2,
vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpvxd, vfpvxd-fp16,
neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4,
fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and
crypto-neon-fp-armv8. Note that neon is an alias for neon-vfpv3 and
vfp is an alias for vfpv2.
The setting auto is the default and is special. It causes the
compiler to select the floating-point and Advanced SIMD instructions
based on the settings of -mcpu and -march.
If the selected floating-point hardware includes the NEON extension
(e.g. -mfpu=neon), note that floating-point operations are not
generated by GCC's auto-vectorization pass unless
-funsafe-math-optimizations is also specified. This is because NEON
hardware does not fully implement the IEEE 754 standard for floating-
point arithmetic (in particular denormal values are treated as zero),
so the use of NEON instructions may lead to a loss of precision.
You can also set the fpu name at function level by using the
"target("fpu=")" function attributes or pragmas.
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point
type. Permissible names are none, ieee, and alternative; the default
is none, in which case the "__fp16" type is not defined.
-mstructure-size-boundary=n
The sizes of all structures and unions are rounded up to a multiple
of the number of bits set by this option. Permissible values are 8,
32 and 64. The default value varies for different toolchains. For
the COFF targeted toolchain the default value is 8. A value of 64 is
only allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code,
but can also increase the size of the program. Different values are
potentially incompatible. Code compiled with one value cannot
necessarily expect to work with code or libraries compiled with
another value, if they exchange information using structures or
unions.
This option is deprecated.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn"
function. It is executed if the function tries to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the
target function lies outside of the 64-megabyte addressing range of
the offset-based version of subroutine call instruction.
Even if this switch is enabled, not all function calls are turned
into long calls. The heuristic is that static functions, functions
that have the "short_call" attribute, functions that are inside the
definitions have already been compiled within the current compilation
unit are not turned into long calls. The exceptions to this rule are
that weak function definitions, functions with the "long_call"
attribute or the "section" attribute, and functions that are within
long calls.
This feature is not enabled by default. Specifying -mno-long-calls
restores the default behavior, as does placing the function calls
switches have no effect on how the compiler generates code to handle
function calls via function pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than
loading it in the prologue for each function. The runtime system is
responsible for initializing this register with an appropriate value
before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. For standard PIC
base case, the default is any suitable register determined by
compiler. For single PIC base case, the default is R9 if target is
EABI based or stack-checking is enabled, otherwise the default is
R10.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is
fixed at static link time. This permits using PC-relative addressing
operations to access data known to be in the data segment. For non-
VxWorks RTP targets, this option is enabled by default. When
disabled on such targets, it will enable -msingle-pic-base by
default.
-mpoke-function-name
Write the name of each function into the text section, directly
preceding the function prologue. The generated code is similar to
this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of "pc"
stored at "fp + 0". If the trace function then looks at location "pc
- 12" and the top 8 bits are set, then we know that there is a
function name embedded immediately preceding this location and has
length "((pc[-3]) & 0xff000000)".
-mthumb
-marm
Select between generating code that executes in ARM and Thumb states.
The default for most configurations is to generate code that executes
in ARM state, but the default can be changed by configuring GCC with
the --with-mode=state configure option.
You can also override the ARM and Thumb mode for each function by
using the "target("thumb")" and "target("arm")" function attributes
or pragmas.
-mflip-thumb
Switch ARM/Thumb modes on alternating functions. This option is
provided for regression testing of mixed Thumb/ARM code generation,
and is not intended for ordinary use in compiling code.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure
Call Standard for all non-leaf functions. (A leaf function is one
that does not call any other functions.) The default is
-mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure
Call Standard for all leaf functions. (A leaf function is one that
does not call any other functions.) The default is
-mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an
ARM instruction set header which switches to Thumb mode before
executing the rest of the function. This allows these functions to
be called from non-interworking code. This option is not valid in
AAPCS configurations because interworking is enabled by default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to
execute correctly regardless of whether the target code has been
compiled for interworking or not. There is a small overhead in the
cost of executing a function pointer if this option is enabled. This
option is not valid in AAPCS configurations because interworking is
enabled by default.
-mtp=name
Specify the access model for the thread local storage pointer. The
model soft generates calls to "__aeabi_read_tp". Other accepted
models are tpidrurw, tpidruro and tpidrprw which fetch the thread
pointer from the corresponding system register directly (supported
from the arm6k architecture and later). These system registers are
accessed through the CP15 co-processor interface and the argument
cp15 is also accepted as a convenience alias of tpidruro. The
argument auto uses the best available method for the selected
processor. The default setting is auto.
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local storage. Two
dialects are supported---gnu and gnu2. The gnu dialect selects the
original GNU scheme for supporting local and global dynamic TLS
models. The gnu2 dialect selects the GNU descriptor scheme, which
provides better performance for shared libraries. The GNU descriptor
scheme is compatible with the original scheme, but does require new
assembler, linker and library support. Initial and local exec TLS
models are unaffected by this option and always use the original
scheme.
-mword-relocations
Only generate absolute relocations on word-sized values (i.e.
R_ARM_ABS32). This is enabled by default on targets (uClinux,
SymbianOS) where the runtime loader imposes this restriction, and
when -fpic or -fPIC is specified. This option conflicts with
-mslow-flash-data.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd"
instructions with overlapping destination and base registers are
used. This option avoids generating these instructions. This option
is enabled by default when -mcpu=cortex-m3 is specified.
-mfix-cortex-a57-aes-1742098
-mno-fix-cortex-a57-aes-1742098
-mfix-cortex-a72-aes-1655431
-mno-fix-cortex-a72-aes-1655431
Enable (disable) mitigation for an erratum on Cortex-A57 and
Cortex-A72 that affects the AES cryptographic instructions. This
option is enabled by default when either -mcpu=cortex-a57 or
-mcpu=cortex-a72 is specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit values
from addresses that are not 16- or 32- bit aligned. By default
unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for
ARMv8-M Baseline architectures, and enabled for all other
architectures. If unaligned access is not enabled then words in
packed data structures are accessed a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in the generated
object file to either true or false, depending upon the setting of
this option. If unaligned access is enabled then the preprocessor
symbol "__ARM_FEATURE_UNALIGNED" is also defined.
-mneon-for-64bits
This option is deprecated and has no effect.
-mslow-flash-data
Assume loading data from flash is slower than fetching instruction.
Therefore literal load is minimized for better performance. This
option is only supported when compiling for ARMv7 M-profile and off
by default. It conflicts with -mword-relocations.
-masm-syntax-unified
Assume inline assembler is using unified asm syntax. The default is
currently off which implies divided syntax. This option has no
impact on Thumb2. However, this may change in future releases of GCC.
Divided syntax should be considered deprecated.
-mrestrict-it
Restricts generation of IT blocks to conform to the rules of ARMv8-A.
IT blocks can only contain a single 16-bit instruction from a select
set of instructions. This option is on by default for ARMv8-A Thumb
mode.
-mprint-tune-info
Print CPU tuning information as comment in assembler file. This is
an option used only for regression testing of the compiler and not
intended for ordinary use in compiling code. This option is disabled
by default.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This
option is provided for use in debugging the compiler.
-mpure-code
Do not allow constant data to be placed in code sections.
Additionally, when compiling for ELF object format give all text
sections the ELF processor-specific section attribute
"SHF_ARM_PURECODE". This option is only available when generating
non-pic code for M-profile targets.
-mcmse
Generate secure code as per the "ARMv8-M Security Extensions:
Requirements on Development Tools Engineering Specification", which
can be found on
<https://developer.arm.com/documentation/ecm0359818/latest/>.
-mfix-cmse-cve-2021-35465
Mitigate against a potential security issue with the "VLLDM"
instruction in some M-profile devices when using CMSE
(CVE-2021-365465). This option is enabled by default when the option
-mcpu= is used with "cortex-m33", "cortex-m35p", "cortex-m52",
"cortex-m55", "cortex-m85" or "star-mc1". The option
-mno-fix-cmse-cve-2021-35465 can be used to disable the mitigation.
-mstack-protector-guard=guard
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported
locations are global for a global canary or tls for a canary
accessible via the TLS register. The option
-mstack-protector-guard-offset= is for use with
-fstack-protector-guard=tls and not for use in user-land code.
-mfdpic
-mno-fdpic
Select the FDPIC ABI, which uses 64-bit function descriptors to
represent pointers to functions. When the compiler is configured for
"arm-*-uclinuxfdpiceabi" targets, this option is on by default and
implies -fPIE if none of the PIC/PIE-related options is provided. On
other targets, it only enables the FDPIC-specific code generation
features, and the user should explicitly provide the PIC/PIE-related
options as needed.
Note that static linking is not supported because it would still
involve the dynamic linker when the program self-relocates. If such
behavior is acceptable, use -static and -Wl,-dynamic-linker options.
The opposite -mno-fdpic option is useful (and required) to build the
Linux kernel using the same ("arm-*-uclinuxfdpiceabi") toolchain as
the one used to build the userland programs.
-mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]]
Enable branch protection features (armv8.1-m.main only). none
generate code without branch protection or return address signing.
standard[+leaf] generate code with all branch protection features
enabled at their standard level. pac-ret[+leaf] generate code with
return address signing set to its standard level, which is to sign
all functions that save the return address to memory. leaf When
return address signing is enabled, also sign leaf functions even if
they do not write the return address to memory. +bti Add landing-pad
instructions at the permitted targets of indirect branch
instructions.
If the +pacbti architecture extension is not enabled, then all branch
protection and return address signing operations are constrained to
use only the instructions defined in the architectural-NOP space. The
generated code will remain backwards-compatible with earlier versions
of the architecture, but the additional security can be enabled at
run time on processors that support the PACBTI extension.
Branch target enforcement using BTI can only be enabled at runtime if
all code in the application has been compiled with at least
-mbranch-protection=bti.
Any setting other than none is supported only on armv8-m.main or
later.
The default is to generate code without branch protection or return
address signing.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify the AVR instruction set architecture (ISA) or device type.
The default for this option is "avr2".
The following AVR devices and ISAs are supported. Note: A complete
device support consists of startup code "crtmcu.o", a device header
"avr/io*.h", a device library "libmcu.a" and a device-specs
("https://gcc.gnu.org/wiki/avr-gcc#spec-files") file "specs-mcu".
Only the latter is provided by the compiler according the supported
"mcu"s below. The rest is supported by AVR-LibC
("https://github.com/avrdudes/avr-libc/"), or by means of "atpack"
("https://gcc.gnu.org/wiki/avr-gcc#atpack") files from the hardware
manufacturer.
"avr2"
"Classic" devices with up to 8 KiB of program memory. mcu =
"attiny22", "attiny26", "at90s2313", "at90s2323", "at90s2333",
"at90s2343", "at90s4414", "at90s4433", "at90s4434", "at90c8534",
"at90s8515", "at90s8535".
"avr25"
"Classic" devices with up to 8 KiB of program memory and with the
"MOVW" instruction. mcu = "attiny13", "attiny13a", "attiny24",
"attiny24a", "attiny25", "attiny261", "attiny261a", "attiny2313",
"attiny2313a", "attiny43u", "attiny44", "attiny44a", "attiny45",
"attiny48", "attiny441", "attiny461", "attiny461a", "attiny4313",
"attiny84", "attiny84a", "attiny85", "attiny87", "attiny88",
"attiny828", "attiny841", "attiny861", "attiny861a", "ata5272",
"ata6616c", "at86rf401".
"avr3"
"Classic" devices with 16 KiB up to 64 KiB of program memory.
mcu = "at76c711", "at43usb355".
"avr31"
"Classic" devices with 128 KiB of program memory. mcu =
"atmega103", "at43usb320".
"avr35"
"Classic" devices with 16 KiB up to 64 KiB of program memory and
with the "MOVW" instruction. mcu = "attiny167", "attiny1634",
"atmega8u2", "atmega16u2", "atmega32u2", "ata5505", "ata6617c",
"ata664251", "at90usb82", "at90usb162".
"avr4"
"Enhanced" devices with up to 8 KiB of program memory. mcu =
"atmega48", "atmega48a", "atmega48p", "atmega48pa", "atmega48pb",
"atmega8", "atmega8a", "atmega8hva", "atmega88", "atmega88a",
"atmega88p", "atmega88pa", "atmega88pb", "atmega8515",
"atmega8535", "ata5795", "ata6285", "ata6286", "ata6289",
"ata6612c", "at90pwm1", "at90pwm2", "at90pwm2b", "at90pwm3",
"at90pwm3b", "at90pwm81".
"avr5"
"Enhanced" devices with 16 KiB up to 64 KiB of program memory.
mcu = "atmega16", "atmega16a", "atmega16hva", "atmega16hva2",
"atmega16hvb", "atmega16hvbrevb", "atmega16m1", "atmega16u4",
"atmega161", "atmega162", "atmega163", "atmega164a",
"atmega164p", "atmega164pa", "atmega165", "atmega165a",
"atmega165p", "atmega165pa", "atmega168", "atmega168a",
"atmega168p", "atmega168pa", "atmega168pb", "atmega169",
"atmega169a", "atmega169p", "atmega169pa", "atmega32",
"atmega32a", "atmega32c1", "atmega32hvb", "atmega32hvbrevb",
"atmega32m1", "atmega32u4", "atmega32u6", "atmega323",
"atmega324a", "atmega324p", "atmega324pa", "atmega324pb",
"atmega325", "atmega325a", "atmega325p", "atmega325pa",
"atmega328", "atmega328p", "atmega328pb", "atmega329",
"atmega329a", "atmega329p", "atmega329pa", "atmega3250",
"atmega3250a", "atmega3250p", "atmega3250pa", "atmega3290",
"atmega3290a", "atmega3290p", "atmega3290pa", "atmega406",
"atmega64", "atmega64a", "atmega64c1", "atmega64hve",
"atmega64hve2", "atmega64m1", "atmega64rfr2", "atmega640",
"atmega644", "atmega644a", "atmega644p", "atmega644pa",
"atmega644rfr2", "atmega645", "atmega645a", "atmega645p",
"atmega649", "atmega649a", "atmega649p", "atmega6450",
"atmega6450a", "atmega6450p", "atmega6490", "atmega6490a",
"atmega6490p", "ata5790", "ata5790n", "ata5791", "ata6613c",
"ata6614q", "ata5782", "ata5831", "ata8210", "ata8510",
"ata5787", "ata5835", "ata5700m322", "ata5702m322", "at90pwm161",
"at90pwm216", "at90pwm316", "at90can32", "at90can64",
"at90scr100", "at90usb646", "at90usb647", "at94k", "m3000".
"avr51"
"Enhanced" devices with 128 KiB of program memory. mcu =
"atmega128", "atmega128a", "atmega128rfa1", "atmega128rfr2",
"atmega1280", "atmega1281", "atmega1284", "atmega1284p",
"atmega1284rfr2", "at90can128", "at90usb1286", "at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of
program memory. mcu = "atmega256rfr2", "atmega2560",
"atmega2561", "atmega2564rfr2".
"avrxmega2"
"XMEGA" devices with more than 8 KiB and up to 64 KiB of program
memory. mcu = "atxmega8e5", "atxmega16a4", "atxmega16a4u",
"atxmega16c4", "atxmega16d4", "atxmega16e5", "atxmega32a4",
"atxmega32a4u", "atxmega32c3", "atxmega32c4", "atxmega32d3",
"atxmega32d4", "atxmega32e5", "avr64da28", "avr64da28s",
"avr64da32", "avr64da32s", "avr64da48", "avr64da48s",
"avr64da64", "avr64da64s", "avr64db28", "avr64db32", "avr64db48",
"avr64db64", "avr64dd14", "avr64dd20", "avr64dd28", "avr64dd32",
"avr64du28", "avr64du32", "avr64ea28", "avr64ea32", "avr64ea48",
"avr64sd28", "avr64sd32", "avr64sd48".
"avrxmega3"
"XMEGA" devices with up to 64 KiB of combined program memory and
RAM, and with program memory visible in the RAM address space.
mcu = "attiny202", "attiny204", "attiny212", "attiny214",
"attiny402", "attiny404", "attiny406", "attiny412", "attiny414",
"attiny416", "attiny416auto", "attiny417", "attiny424",
"attiny426", "attiny427", "attiny804", "attiny806", "attiny807",
"attiny814", "attiny816", "attiny817", "attiny824", "attiny826",
"attiny827", "attiny1604", "attiny1606", "attiny1607",
"attiny1614", "attiny1616", "attiny1617", "attiny1624",
"attiny1626", "attiny1627", "attiny3214", "attiny3216",
"attiny3217", "attiny3224", "attiny3226", "attiny3227",
"atmega808", "atmega809", "atmega1608", "atmega1609",
"atmega3208", "atmega3209", "atmega4808", "atmega4809",
"avr16dd14", "avr16dd20", "avr16dd28", "avr16dd32", "avr16du14",
"avr16du20", "avr16du28", "avr16du32", "avr16ea28", "avr16ea32",
"avr16ea48", "avr16eb14", "avr16eb20", "avr16eb28", "avr16eb32",
"avr16la14", "avr16la20", "avr16la28", "avr16la32", "avr32da28",
"avr32da28s", "avr32da32", "avr32da32s", "avr32da48",
"avr32da48s", "avr32db28", "avr32db32", "avr32db48", "avr32dd14",
"avr32dd20", "avr32dd28", "avr32dd32", "avr32du14", "avr32du20",
"avr32du28", "avr32du32", "avr32ea28", "avr32ea32", "avr32ea48",
"avr32eb14", "avr32eb20", "avr32eb28", "avr32eb32", "avr32la14",
"avr32la20", "avr32la28", "avr32la32", "avr32sd20", "avr32sd28",
"avr32sd32".
"avrxmega4"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of
program memory. mcu = "atxmega64a3", "atxmega64a3u",
"atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3",
"atxmega64d3", "atxmega64d4", "avr128da28", "avr128da28s",
"avr128da32", "avr128da32s", "avr128da48", "avr128da48s",
"avr128da64", "avr128da64s", "avr128db28", "avr128db32",
"avr128db48", "avr128db64".
"avrxmega5"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of
program memory and more than 64 KiB of RAM. mcu = "atxmega64a1",
"atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128 KiB of program memory. mcu =
"atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3",
"atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3",
"atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3",
"atxmega256a3b", "atxmega256a3bu", "atxmega256a3u",
"atxmega256c3", "atxmega256d3", "atxmega384c3", "atxmega384d3".
"avrxmega7"
"XMEGA" devices with more than 128 KiB of program memory and more
than 64 KiB of RAM. mcu = "atxmega128a1", "atxmega128a1u",
"atxmega128a4u".
"avrtiny"
"Reduced Tiny" Tiny core devices with only 16 general purpose
registers and 512 B up to 4 KiB of program memory. mcu =
"attiny4", "attiny5", "attiny9", "attiny10", "attiny102",
"attiny104", "attiny20", "attiny40".
"avr1"
This ISA is implemented by the minimal AVR core and supported for
assembler only. mcu = "attiny11", "attiny12", "attiny15",
"attiny28", "at90s1200".
-mabsdata
Assume that all data in static storage can be accessed by LDS / STS
instructions. This option has only an effect on reduced Tiny devices
like ATtiny40. See also the "absdata" AVR Variable
Attributes,variable attribute.
-mcvt
Use a compact vector table. Some devices support a CVT with only
four entries: 0=Reset, 1=NMI, 2=Prio1 IRQ, 3=Prio0 IRQs. This option
will link startup code from "crtmcu-cvt.o" instead of the usual
"crtmcu.o". Apart from providing a compact vector table, the startup
code will set bit "CPUINT_CTRLA.CPUINT_CVT" which enables the CVT on
the device.
When you do not want the startup code to set
"CPUINT_CTRLA.CPUINT_CVT", then you can satisfy symbol "__init_cvt"
so that the respective code is no more pulled in from "libmcu.a".
For example, you can link with "-Wl,--defsym,__init_cvt=0".
The CVT startup code is available since AVR-LibC v2.3
("https://github.com/avrdudes/avr-libc/issues/1010").
-mdouble=bits
-mlong-double=bits
Set the size (in bits) of the "double" or "long double" type,
respectively. Possible values for bits are 32 and 64. Whether or
not a specific value for bits is allowed depends on the
"--with-double=" and "--with-long-double=" configure options
("https://gcc.gnu.org/install/configure.html#avr"), and the same
applies for the default values of the options.
-mgas-isr-prologues
Interrupt service routines (ISRs) may use the "__gcc_isr" pseudo
instruction supported by GNU Binutils. If this option is on, the
feature can still be disabled for individual ISRs by means of the AVR
Function Attributes,,"no_gccisr" function attribute. This feature is
activated per default if optimization is on (but not with -Og,
@pxref{Optimize Options}), and if GNU Binutils support PR21683
("https://sourceware.org/PR21683").
-mint8
Assume "int" to be 8-bit integer. This affects the sizes of all
types: a "char" is 1 byte, an "int" is 1 byte, a "long" is 2 bytes,
and "long long" is 4 bytes. Please note that this option does not
conform to the C standards, but it results in smaller code size.
-mmain-is-OS_task
Do not save registers in "main". The effect is the same like
attaching attribute AVR Function Attributes,,"OS_task" to "main". It
is activated per default if optimization is on.
-mno-call-main
Don't run "main" by means of
XCALL main
XJMP exit
Instead, put "main" in section ".init9"
("https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init")
so that no call is required. By setting this option the user asserts
that "main" will not return.
This option can be used for devices with very limited resources in
order to save a few bytes of code and stack space. It will work as
expected since AVR-LibC v2.3
("https://github.com/avrdudes/avr-libc/issues/1012"). With older
versions, there will be no performance gain.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size
is smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter "RCALL"
resp. "RJMP" instruction if applicable. Setting -mrelax just adds
the --mlink-relax option to the assembler's command line and the
--relax option to the linker's command line.
Jump relaxing is performed by the linker because jump offsets are not
known before code is located. Therefore, the assembler code generated
by the compiler is the same, but the instructions in the executable
may differ from instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the
section on "EIND" and linker stubs below.
-mrodata-in-ram
-mno-rodata-in-ram
Locate the ".rodata" sections for read-only data in RAM resp. in
program memory. For most devices, there is no choice and this option
acts rather like an assertion.
Since v14 and for the AVR64* and AVR128* devices, ".rodata" is
located in flash memory per default, provided the required GNU
Binutils support (PR31124 ("https://sourceware.org/PR31124")) is
available. In that case, -mrodata-in-ram can be used to return to
the old layout with ".rodata" in RAM.
-mtiny-stack
Only change the lower 8 bits of the stack pointer.
-mfract-convert-truncate
Allow to use truncation instead of rounding towards zero for
fractional fixed-point types.
-nodevicelib
Don't link against AVR-LibC's device specific library "libmcu.a".
Notice that since AVR-LibC v2.3, that library contains code that is
essential for the correct functioning of a program. In particular,
it contains parts of the startup code like: "__init_sp"
("https://github.com/avrdudes/avr-libc/issues/1011") to initialize
the stack pointer with symbol "__stack", "__init_cvt"
("https://github.com/avrdudes/avr-libc/issues/1010") to set up the
hardware to use a compact vector table with -mcvt, "__call_main"
("https://github.com/avrdudes/avr-libc/issues/1012") to call "main"
and "exit", and "__do_flmap_init"
("https://github.com/avrdudes/avr-libc/issues/931") to set up FLMAP
according to symbol "__flmap".
-nodevicespecs
Don't add -specs=device-specs/specs-mcu to the compiler driver's
command line. The user takes responsibility for supplying the sub-
processes like compiler proper, assembler and linker with appropriate
command line options. This means that the user has to supply her
private device specs file by means of -specs=path-to-specs-file.
There is no more need for option -mmcu=mcu.
This option can also serve as a replacement for the older way of
specifying custom device-specs files that needed -B some-path to
point to a directory which contains a folder named "device-specs"
which contains a specs file named "specs-mcu", where mcu was
specified by -mmcu=mcu.
-Waddr-space-convert
Warn about conversions between address spaces in the case where the
resulting address space is not contained in the incoming address
space.
-Wmisspelled-isr
Warn if the ISR is misspelled, i.e. without __vector prefix. Enabled
by default.
AVR Optimization Options
The following options are pure optimization options. Options
-mgas-isr-prologues, -mmain-is-OS_task, -mno-call-main and -mrelax from
above are only almost optimization options, since there are rare
occasions where their different code generation matters.
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the needed
stack space for outgoing function arguments once in function
prologue/epilogue. Without this option, outgoing arguments are
pushed before calling a function and popped afterwards. See also the
-fdefer-pop Optimize Options,,optimization option.
Popping the arguments after the function call can be expensive on AVR
so that accumulating the stack space might lead to smaller
executables because arguments need not be removed from the stack
after such a function call.
This option can lead to reduced code size for functions that perform
several calls to functions that get their arguments on the stack like
calls to printf-like functions.
-mbranch-cost=cost
Set the branch costs for conditional branch instructions to cost.
Reasonable values for cost are small, non-negative integers. The
default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to appropriate
subroutines. Code size is smaller.
-mfuse-add
-mno-fuse-add
-mfuse-add=level
Optimize indirect memory accesses on reduced Tiny devices. The
default uses "level=1" for optimizations -Og and -O1, and "level=2"
for higher optimizations. Valid values for level are 0, 1 and 2.
-mfuse-move
-mno-fuse-move
-mfuse-move=level
Run a post reload optimization pass that tries to fuse move
instructions and to split multi-byte instructions into 8-bit
operations. The default uses "level=3" for optimization -O1, and
"level=23" for higher optimizations. Valid values for level are in
the range 0 ... 23 which is a 3:2:2:2 mixed radix value. Each digit
controls some aspect of the optimization.
-mfuse-move2
Run a post combine optimization pass that tries to fuse move
instructions.
-mstrict-X
Use address register "X" in a way proposed by the hardware. This
means that "X" is only used in indirect, post-increment or pre-
decrement addressing.
Without this option, the "X" register may be used in the same way as
"Y" or "Z" which then is emulated by additional instructions. For
example, loading a value with "X+const" addressing with a small non-
negative "const < 64" to a register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-msplit-bit-shift
Split multi-byte shifts with a constant offset into a shift with a
byte offset and a residual shift with a non-byte offset. This
optimization is turned on per default for -O2 and higher, including
-Os but excluding -Oz. Splitting of shifts with a constant offset
that is a multiple of 8 is controlled by -mfuse-move.
-msplit-ldst
Split multi-byte loads and stores into several byte loads and stores.
This optimization is turned on per default for -O2 and higher.
-muse-nonzero-bits
Enable optimizations that are only possible when some bits in a
register are always zero. This optimization is turned on per default
for -O2 and higher.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16 bits wide. The address of a
function or label is represented as word address so that indirect jumps
and calls can target any code address in the range of 64 Ki words.
In order to facilitate indirect jump on devices with more than 128 Ki
bytes of program memory space, there is a special function register
called "EIND" that serves as most significant part of the target address
when "EICALL" or "EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as follows by the
compiler and are subject to some limitations:
* The compiler never sets "EIND".
* The compiler uses "EIND" implicitly in "EICALL"/"EIJMP" instructions
or might read "EIND" directly in order to emulate an indirect
call/jump by means of a "RET" instruction.
* The compiler assumes that "EIND" never changes during the startup
code or during the application. In particular, "EIND" is not
saved/restored in function or interrupt service routine
prologue/epilogue.
* For indirect calls to functions and computed goto, the linker
generates stubs. Stubs are jump pads sometimes also called
trampolines. Thus, the indirect call/jump jumps to such a stub. The
stub contains a direct jump to the desired address.
* Linker relaxation must be turned on so that the linker generates the
stubs correctly in all situations. See the compiler option -mrelax
and the linker option --relax. There are corner cases where the
linker is supposed to generate stubs but aborts without relaxation
and without a helpful error message.
* The default linker script is arranged for code with "EIND = 0". If
code is supposed to work for a setup with "EIND != 0", a custom
linker script has to be used in order to place the sections whose
name start with ".trampolines" into the segment where "EIND" points
to.
* The startup code from libgcc never sets "EIND". Notice that startup
code is a blend of code from libgcc and AVR-LibC. For the impact of
AVR-LibC on "EIND", see the AVR-LibC user manual
("https://avrdudes.github.io/avr-libc/avr-libc-user-manual/").
* It is legitimate for user-specific startup code to set up "EIND"
early, for example by means of initialization code located in section
".init3". Such code runs prior to general startup code that
initializes RAM and calls constructors, but after the bit of startup
code from AVR-LibC that sets "EIND" to the segment where the vector
table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker script.
* Stubs are generated automatically by the linker if the following two
conditions are met:
-<The address of a label is taken by means of the "gs" modifier>
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The final location of that label is in a code segment>
outside the segment where the stubs are located.
* The compiler emits such "gs" modifiers for code labels in the
following situations:
-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see -mcall-prologues>
command-line option.
-<Switch/case dispatch tables. If you do not want such dispatch>
tables you can specify the -fno-jump-tables command-line option.
-<C and C++ constructors/destructors called during startup/shutdown.>
-<If the tools hit a "gs()" modifier explained above.>
* Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be called
through a symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with -Wl,--defsym,func_4=0x4.
Alternatively, "func_4" can be defined in the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function
Registers
Some AVR devices support memories larger than the 64 KiB range that can
be accessed with 16-bit pointers. To access memory locations outside
this 64 KiB range, the content of a "RAMP" register is used as high part
of the address: The "X", "Y", "Z" address register is concatenated with
the "RAMPX", "RAMPY", "RAMPZ" special function register, respectively, to
get a wide address. Similarly, "RAMPD" is used together with direct
addressing.
* The startup code initializes the "RAMP" special function registers
with zero.
* If a AVR Named Address Spaces,named address space other than generic
or "__flash" is used, then "RAMPZ" is set as needed before the
operation.
* If the device supports RAM larger than 64 KiB and the compiler needs
to change "RAMPZ" to accomplish an operation, "RAMPZ" is reset to
zero after the operation.
* If the device comes with a specific "RAMP" register, the ISR
prologue/epilogue saves/restores that SFR and initializes it with
zero in case the ISR code might (implicitly) use it.
* RAM larger than 64 KiB is not supported by GCC for AVR targets. If
you use inline assembler to read from locations outside the 16-bit
address range and change one of the "RAMP" registers, you must reset
it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for
the presence or absence of features. Almost any of the following built-
in macros are deduced from device capabilities and thus triggered by the
-mmcu= command-line option.
For even more AVR-specific built-in macros see AVR Named Address Spaces
and AVR Built-in Functions.
"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that identifies the
architecture and depends on the -mmcu=mcu option. Possible values
are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5",
"avr51", "avr6",
respectively and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4",
"avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu
specifies a device, this built-in macro is set accordingly. For
example, with -mmcu=atmega8 the macro is defined to 4.
"__AVR_Device__"
Setting -mmcu=device defines this built-in macro which reflects the
device's name. For example, -mmcu=atmega8 defines the built-in macro
"__AVR_ATmega8__", -mmcu=attiny261a defines "__AVR_ATtiny261A__",
etc.
The built-in macros' names follow the scheme "__AVR_Device__" where
Device is the device name as from the AVR user manual. The difference
between Device in the built-in macro and device in -mmcu=device is
that the latter is always lowercase.
If device is not a device but only a core architecture like avr51,
this macro is not defined.
"__AVR_DEVICE_NAME__"
Setting -mmcu=device defines this built-in macro to the device's
name. For example, with -mmcu=atmega8 the macro is defined to
"atmega8".
If device is not a device but only a core architecture like avr51,
this macro is not defined.
"__AVR_CVT__"
The code is being compiled with option "-mcvt" to use a compact
vector table.
"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of devices.
"__AVR_HAVE_ADIW__"
The device has the "ADIW" and "SBIW" instructions.
"__AVR_HAVE_ELPM__"
The device has the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-
register moves.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is the case
for devices with more than 8 KiB of program memory.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This is the
case for devices with more than 128 KiB of program memory. This also
means that the program counter (PC) is 3 bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2 bytes wide. This is the case for
devices with up to 128 KiB of program memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit respectively
16-bit register by the compiler. The definition of these macros is
affected by -mtiny-stack.
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special function
register or has an 8-bit stack pointer, respectively. The definition
of these macros is affected by -mmcu= and in the cases of -mmcu=avr2
and -mmcu=avr25 also by -msp8.
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special
function register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the -mno-interrupts command-line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit
instructions because of a hardware erratum. Skip instructions are
"SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The second macro is only
defined if "__AVR_HAVE_JMP_CALL__" is also set.
"__AVR_ISA_RMW__"
The device has Read-Modify-Write instructions (XCH, LAC, LAS and
LAT).
"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function registers directly
like "IN", "OUT", "SBI", etc. may use a different address as if
addressed by an instruction to access RAM like "LD" or "STS". This
offset depends on the device architecture and has to be subtracted
from the RAM address in order to get the respective I/O address.
"__AVR_SHORT_CALLS__"
The -mshort-calls command line option is set.
"__AVR_PM_BASE_ADDRESS__=addr"
Some devices support reading from flash memory by means of "LD*"
instructions. The flash memory is seen in the data address space at
an offset of "__AVR_PM_BASE_ADDRESS__". If this macro is not
defined, this feature is not available. If defined, the address
space is linear and there is no need to put ".rodata" into RAM. This
is handled by the default linker description file, and is currently
available for "avrtiny" and "avrxmega3". Even more convenient, there
is no need to use address spaces like "__flash" or features like
attribute "progmem" and "pgm_read_*".
"__AVR_HAVE_FLMAP__"
This macro is defined provided the following conditions are met:
*<The device has the "NVMCTRL_CTRLB.FLMAP" bitfield.>
This applies to the AVR64* and AVR128* devices.
*<It's not known at assembler-time which emulation will be used.>
This implies the compiler was configured with GNU Binutils that
implement PR31124 ("https://sourceware.org/PR31124").
"__AVR_RODATA_IN_RAM__"
This macro is undefined when the code is compiled for a core
architecture.
When the code is compiled for a device, the macro is defined to 1
when the ".rodata" sections for read-only data is located in RAM; and
defined to 0, otherwise.
"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-LibC. See
the --with-avrlibc configure option.
"__HAVE_SIGNAL_N__"
The compiler supports the "signal(num)" and "interrupt(num)" AVR
Function Attributes,,function attributes with an argument num that
specifies the number of the interrupt service routine.
"__HAVE_DOUBLE_MULTILIB__"
Defined if -mdouble= acts as a multilib option.
"__HAVE_DOUBLE32__"
"__HAVE_DOUBLE64__"
Defined if the compiler supports 32-bit double resp. 64-bit double.
The actual layout is specified by option -mdouble=.
"__DEFAULT_DOUBLE__"
The size in bits of "double" if -mdouble= is not set. To test the
layout of "double" in a program, use the built-in macro
"__SIZEOF_DOUBLE__".
"__HAVE_LONG_DOUBLE32__"
"__HAVE_LONG_DOUBLE64__"
"__HAVE_LONG_DOUBLE_MULTILIB__"
"__DEFAULT_LONG_DOUBLE__"
Same as above, but for "long double" instead of "double".
"__WITH_DOUBLE_COMPARISON__"
Reflects the "--with-double-comparison={tristate|bool|libf7}"
configure option ("https://gcc.gnu.org/install/configure.html#avr")
and is defined to 2 or 3.
"__WITH_LIBF7_LIBGCC__"
"__WITH_LIBF7_MATH__"
"__WITH_LIBF7_MATH_SYMBOLS__"
Reflects the "--with-libf7={libgcc|math|math-symbols}"
configure option ("https://gcc.gnu.org/install/configure.html#avr").
AVR Internal Options
The following options are used internally by the compiler and to
communicate between device specs files and the compiler proper. You don't
need to set these options by hand, in particular they are not
optimization options. Using these options in the wrong way may lead to
sub-optimal or wrong code. They are documented for completeness, and in
order to get a better understanding of device specs
("https://gcc.gnu.org/wiki/avr-gcc#spec-files") files.
-mn-flash=num
Assume that the flash memory has a size of num times 64 KiB. This
determines which "__flashN" address spaces are available.
-mflmap
The device has the "FLMAP" bit field located in special function
register "NVMCTRL_CTRLB".
-mrmw
Assume that the device supports the Read-Modify-Write instructions
"XCH", "LAC", "LAS" and "LAT".
-mshort-calls
Assume that "RJMP" and "RCALL" can target the whole program memory.
This option is used for multilib generation and selection for the
devices from architecture "avrxmega3".
-mskip-bug
Generate code without skips ("CPSE", "SBRS", "SBRC", "SBIS", "SBIC")
over 32-bit instructions.
-msp8
Treat the stack pointer register as an 8-bit register, i.e. assume
the high byte of the stack pointer is zero. This option is used by
the compiler to select and build multilibs for architectures "avr2"
and "avr25". These architectures mix devices with and without "SPH".
Blackfin Options
-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor. Currently, cpu
can be one of bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525,
bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539,
bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m,
bf549m, bf561, bf592.
The optional sirevision specifies the silicon revision of the target
Blackfin processor. Any workarounds available for the targeted
silicon revision are enabled. If sirevision is none, no workarounds
are enabled. If sirevision is any, all workarounds for the targeted
processor are enabled. The "__SILICON_REVISION__" macro is defined
to two hexadecimal digits representing the major and minor numbers in
the silicon revision. If sirevision is none, the
"__SILICON_REVISION__" is not defined. If sirevision is any, the
"__SILICON_REVISION__" is defined to be 0xffff. If this optional
sirevision is not used, GCC assumes the latest known silicon revision
of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the
bfin-elf toolchain, this option causes the hardware BSP provided by
libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Note that support for bf561 is incomplete. For bf561, only the
preprocessor macro is defined.
-msim
Specifies that the program will be run on the simulator. This causes
the simulator BSP provided by libgloss to be linked in. This option
has effect only for bfin-elf toolchain. Certain other options, such
as -mid-shared-library and -mfdpic, imply -msim.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This
avoids the instructions to save, set up and restore frame pointers
and makes an extra register available in leaf functions.
-mspecld-anomaly
When enabled, the compiler ensures that the generated code does not
contain speculative loads after jump instructions. If this option is
used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from
occurring.
-mcsync-anomaly
When enabled, the compiler ensures that the generated code does not
contain CSYNC or SSYNC instructions too soon after conditional
branches. If this option is used, "__WORKAROUND_SPECULATIVE_SYNCS"
is defined.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions from
occurring too soon after a conditional branch.
-mlow64k
When enabled, the compiler is free to take advantage of the knowledge
that the entire program fits into the low 64k of memory.
-mno-low64k
Assume that the program is arbitrarily large. This is the default.
-mstack-check-l1
Do stack checking using information placed into L1 scratchpad memory
by the uClinux kernel.
-mid-shared-library
Generate code that supports shared libraries via the library ID
method. This allows for execute in place and shared libraries in an
environment without virtual memory management. This option implies
-fPIC. With a bfin-elf target, this option implies -msim.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being
used. This is the default.
-mleaf-id-shared-library
Generate code that supports shared libraries via the library ID
method, but assumes that this library or executable won't link
against any other ID shared libraries. That allows the compiler to
use faster code for jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against any ID
shared libraries. Slower code is generated for jump and call insns.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library
being compiled. Specifying a value of 0 generates more compact code;
specifying other values forces the allocation of that number to the
current library but is no more space- or time-efficient than omitting
this option.
-msep-data
Generate code that allows the data segment to be located in a
different area of memory from the text segment. This allows for
execute in place in an environment without virtual memory management
by eliminating relocations against the text section.
-mno-sep-data
Generate code that assumes that the data segment follows the text
segment. This is the default.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the
target function lies outside of the 24-bit addressing range of the
offset-based version of subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls
restores the default behavior. Note these switches have no effect on
how the compiler generates code to handle function calls via function
pointers.
-mfast-fp
Link with the fast floating-point library. This library relaxes some
of the IEEE floating-point standard's rules for checking inputs
against Not-a-Number (NAN), in the interest of performance.
-minline-plt
Enable inlining of PLT entries in function calls to functions that
are not known to bind locally. It has no effect without -mfdpic.
-mmulticore
Build a standalone application for multicore Blackfin processors.
This option causes proper start files and link scripts supporting
multicore to be used, and defines the macro "__BFIN_MULTICORE". It
can only be used with -mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which selects the
one-application-per-core programming model. Without -mcorea or
-mcoreb, the single-application/dual-core programming model is used.
In this model, the main function of Core B should be named as
"coreb_main".
If this option is not used, the single-core application programming
model is used.
-mcorea
Build a standalone application for Core A of BF561 when using the
one-application-per-core programming model. Proper start files and
link scripts are used to support Core A, and the macro "__BFIN_COREA"
is defined. This option can only be used in conjunction with
-mmulticore.
-mcoreb
Build a standalone application for Core B of BF561 when using the
one-application-per-core programming model. Proper start files and
link scripts are used to support Core B, and the macro "__BFIN_COREB"
is defined. When this option is used, "coreb_main" should be used
instead of "main". This option can only be used in conjunction with
-mmulticore.
-msdram
Build a standalone application for SDRAM. Proper start files and link
scripts are used to put the application into SDRAM, and the macro
"__BFIN_SDRAM" is defined. The loader should initialize SDRAM before
loading the application.
-micplb
Assume that ICPLBs are enabled at run time. This has an effect on
certain anomaly workarounds. For Linux targets, the default is to
assume ICPLBs are enabled; for standalone applications the default is
off.
C6X Options
-march=name
This specifies the name of the target architecture. GCC uses this
name to determine what kind of instructions it can emit when
generating assembly code. Permissible names are: c62x, c64x, c64x+,
c67x, c67x+, c674x.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-msim
Choose startup files and linker script suitable for the simulator.
-msdata=default
Put small global and static data in the ".neardata" section, which is
pointed to by register "B14". Put small uninitialized global and
static data in the ".bss" section, which is adjacent to the
".neardata" section. Put small read-only data into the ".rodata"
section. The corresponding sections used for large pieces of data
are ".fardata", ".far" and ".const".
-msdata=all
Put all data, not just small objects, into the sections reserved for
small data, and use addressing relative to the "B14" register to
access them.
-msdata=none
Make no use of the sections reserved for small data, and use absolute
addresses to access all data. Put all initialized global and static
data in the ".fardata" section, and all uninitialized data in the
".far" section. Put all constant data into the ".const" section.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for
architecture-type are v3, v8 and v10 for respectively ETRAX 4,
ETRAX 100, and ETRAX 100 LX. Default is v0.
-mtune=architecture-type
Tune to architecture-type everything applicable about the generated
code, except for the ABI and the set of available instructions. The
choices for architecture-type are the same as for
-march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and
-march=v8 respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU
models where it applies. This option is disabled by default.
-mpdebug
Enable CRIS-specific verbose debug-related information in the
assembly code. This option also has the effect of turning off the
#NO_APP formatted-code indicator to the assembler at the beginning of
the assembly file.
-mcc-init
Do not use condition-code results from previous instruction; always
emit compare and test instructions before use of condition codes.
-mno-side-effects
Do not emit instructions with side effects in addressing modes other
than post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no- options) arrange (eliminate arrangements) for the
stack frame, individual data and constants to be aligned for the
maximum single data access size for the chosen CPU model. The
default is to arrange for 32-bit alignment. ABI details such as
structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these
options arrange for stack frame, writable data and constants to all
be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and
epilogue which set up the stack frame are omitted and no return
instructions or return sequences are generated in the code. Use this
option only together with visual inspection of the compiled code: no
warnings or errors are generated when call-saved registers must be
saved, or storage for local variables needs to be allocated.
-melf
Legacy no-op option.
-sim
This option arranges to link with input-output functions from a
simulator library. Code, initialized data and zero-initialized data
are allocated consecutively.
-sim2
Like -sim, but pass linker options to locate initialized data at
0x40000000 and zero-initialized data at 0x80000000.
C-SKY Options
GCC supports these options when compiling for C-SKY V2 processors.
-march=arch
Specify the C-SKY target architecture. Valid values for arch are:
ck801, ck802, ck803, ck807, and ck810. The default is ck810.
-mcpu=cpu
Specify the C-SKY target processor. Valid values for cpu are: ck801,
ck801t, ck802, ck802t, ck802j, ck803, ck803h, ck803t, ck803ht,
ck803f, ck803fh, ck803e, ck803eh, ck803et, ck803eht, ck803ef,
ck803efh, ck803ft, ck803eft, ck803efht, ck803r1, ck803hr1, ck803tr1,
ck803htr1, ck803fr1, ck803fhr1, ck803er1, ck803ehr1, ck803etr1,
ck803ehtr1, ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1,
ck803efhtr1, ck803s, ck803st, ck803se, ck803sf, ck803sef, ck803seft,
ck807e, ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef, ck810eft,
ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft, and
ck810ftv.
-mbig-endian
-EB
-mlittle-endian
-EL Select big- or little-endian code. The default is little-endian.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are:
soft, softfp and hard.
Specifying soft causes GCC to generate output containing library
calls for floating-point operations. softfp allows the generation of
code using hardware floating-point instructions, but still uses the
soft-float calling conventions. hard allows generation of floating-
point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that
the hard-float and soft-float ABIs are not link-compatible; you must
compile your entire program with the same ABI, and link with a
compatible set of libraries.
-mhard-float
-msoft-float
Select hardware or software floating-point implementations. The
default is soft float.
-mdouble-float
-mno-double-float
When -mhard-float is in effect, enable generation of double-precision
float instructions. This is the default except when compiling for
CK803.
-mfdivdu
-mno-fdivdu
When -mhard-float is in effect, enable generation of "frecipd",
"fsqrtd", and "fdivd" instructions. This is the default except when
compiling for CK803.
-mfpu=fpu
Select the floating-point processor. This option can only be used
with -mhard-float. Values for fpu are fpv2_sf (equivalent to
-mno-double-float -mno-fdivdu), fpv2 (-mdouble-float -mno-divdu), and
fpv2_divd (-mdouble-float -mdivdu).
-melrw
-mno-elrw
Enable the extended "lrw" instruction. This option defaults to on
for CK801 and off otherwise.
-mistack
-mno-istack
Enable interrupt stack instructions; the default is off.
The -mistack option is required to handle the "interrupt" and "isr"
function attributes.
-mmp
Enable multiprocessor instructions; the default is off.
-mcp
Enable coprocessor instructions; the default is off.
-mcache
Enable coprocessor instructions; the default is off.
-msecurity
Enable C-SKY security instructions; the default is off.
-mtrust
Enable C-SKY trust instructions; the default is off.
-mdsp
-medsp
-mvdsp
Enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions,
respectively. All of these options default to off.
-mdiv
-mno-div
Generate divide instructions. Default is off.
-msmart
-mno-smart
Generate code for Smart Mode, using only registers numbered 0-7 to
allow use of 16-bit instructions. This option is ignored for CK801
where this is the required behavior, and it defaults to on for CK802.
For other targets, the default is off.
-mhigh-registers
-mno-high-registers
Generate code using the high registers numbered 16-31. This option
is not supported on CK801, CK802, or CK803, and is enabled by default
for other processors.
-manchor
-mno-anchor
Generate code using global anchor symbol addresses.
-mpushpop
-mno-pushpop
Generate code using "push" and "pop" instructions. This option
defaults to on.
-mmultiple-stld
-mstm
-mno-multiple-stld
-mno-stm
Generate code using "stm" and "ldm" instructions. This option isn't
supported on CK801 but is enabled by default on other processors.
-mconstpool
-mno-constpool
Create constant pools in the compiler instead of deferring it to the
assembler. This option is the default and required for correct code
generation on CK801 and CK802, and is optional on other processors.
-mstack-size
-mno-stack-size
Emit ".stack_size" directives for each function in the assembly
output. This option defaults to off.
-mccrt
-mno-ccrt
Generate code for the C-SKY compiler runtime instead of libgcc. This
option defaults to off.
-mbranch-cost=n
Set the branch costs to roughly "n" instructions. The default is 1.
-msched-prolog
-mno-sched-prolog
Permit scheduling of function prologue and epilogue sequences. Using
this option can result in code that is not compliant with the C-SKY
V2 ABI prologue requirements and that cannot be debugged or
backtraced. It is disabled by default.
-msim
Links the library libsemi.a which is in compatible with simulator.
Applicable to ELF compiler only.
Cygwin and MinGW Options
These additional options are available for Microsoft Windows targets:
-mconsole
This option specifies that a console application is to be generated,
by instructing the linker to set the PE header subsystem type
required for console applications. This option is available for
Cygwin and MinGW targets and is enabled by default on those targets.
-mcrtdll=library
Preprocess, compile or link with specified C RunTime DLL library.
This option adjust predefined macros "__CRTDLL__", "__MSVCRT__",
"_UCRT" and "__MSVCRT_VERSION__" for specified CRT library, choose
start file for CRT library and link with CRT library. Recognized CRT
library names for proprocessor are: "crtdll*", "msvcrt10*",
"msvcrt20*", "msvcrt40*", "msvcr40*", "msvcrtd*", "msvcrt-os*",
"msvcr70*", "msvcr71*", "msvcr80*", "msvcr90*", "msvcr100*",
"msvcr110*", "msvcr120*" and "ucrt*". If this options is not
specified then the default MinGW import library "msvcrt" is used for
linking and no other adjustment for preprocessor is done. MinGW
import library "msvcrt" is just a symlink to (or a copy of) another
MinGW CRT import library chosen during MinGW compilation. MinGW
import library "msvcrt-os" is for Windows system CRT DLL library
"msvcrt.dll" and in most cases is the default MinGW import library.
Generally speaking, changing the CRT DLL requires recompiling the
entire MinGW CRT. This option is for experimental and testing
purposes only. This option is available for MinGW targets.
-mdll
This option is available for Cygwin and MinGW targets. It specifies
that a DLL---a dynamic link library---is to be generated, enabling
the selection of the required runtime startup object and entry point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It specifies
that the "dllimport" attribute should be ignored.
-mthreads
This option is available for MinGW targets. It specifies that MinGW-
specific thread support is to be used.
-municode
This option is available for MinGW-w64 targets. It causes the
"UNICODE" preprocessor macro to be predefined, and chooses Unicode-
capable runtime startup code.
-mwin32
This option is available for Cygwin and MinGW targets. It specifies
that the typical Microsoft Windows predefined macros are to be set in
the pre-processor, but does not influence the choice of runtime
library/startup code.
-mwindows
This option is available for Cygwin and MinGW targets. It specifies
that a GUI application is to be generated by instructing the linker
to set the PE header subsystem type appropriately.
-fno-set-stack-executable
This option is available for MinGW targets. It specifies that the
executable flag for the stack used by nested functions isn't set.
This is necessary for binaries running in kernel mode of Microsoft
Windows, as there the User32 API, which is used to set executable
privileges, isn't available.
-fwritable-relocated-rdata
This option is available for MinGW and Cygwin targets. It specifies
that relocated-data in read-only section is put into the ".data"
section. This is a necessary for older runtimes not supporting
modification of ".rdata" sections for pseudo-relocation.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It specifies
that the GNU extension to the PE file format that permits the correct
alignment of COMMON variables should be used when generating code.
It is enabled by default if GCC detects that the target assembler
found during configuration supports the feature.
See also under x86 Options for standard options.
Darwin Options
These options are defined for all architectures running the Darwin
operating system.
FSF GCC on Darwin does not create "fat" object files; it creates an
object file for the single architecture that GCC was built to target.
Apple's GCC on Darwin does create "fat" files if multiple -arch options
are used; it does so by running the compiler or linker multiple times and
joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is
determined by the flags that specify the ISA that GCC is targeting, like
-mcpu or -march. The -force_cpusubtype_ALL option can be used to
override this.
The Darwin tools vary in their behavior when presented with an ISA
mismatch. The assembler, as, only permits instructions to be used that
are valid for the subtype of the file it is generating, so you cannot put
64-bit instructions in a ppc750 object file. The linker for shared
libraries, /usr/bin/libtool, fails and prints an error if asked to create
a shared library with a less restrictive subtype than its input files
(for instance, trying to put a ppc970 object file in a ppc7400 library).
The linker for executables, ld, quietly gives the executable the most
restrictive subtype of any of its input files.
-Fdir
Add the framework directory dir to the head of the list of
directories to be searched for header files. These directories are
interleaved with those specified by -I options and are scanned in a
left-to-right order.
A framework directory is a directory with frameworks in it. A
framework is a directory with a Headers and/or PrivateHeaders
directory contained directly in it that ends in .framework. The name
of a framework is the name of this directory excluding the
.framework. Headers associated with the framework are found in one
of those two directories, with Headers being searched first. A
subframework is a framework directory that is in a framework's
Frameworks directory. Includes of subframework headers can only
appear in a header of a framework that contains the subframework, or
in a sibling subframework header. Two subframeworks are siblings if
they occur in the same framework. A subframework should not have the
same name as a framework; a warning is issued if this is violated.
Currently a subframework cannot have subframeworks; in the future,
the mechanism may be extended to support this. The standard
frameworks can be found in /System/Library/Frameworks and
/Library/Frameworks. An example include looks like "#include
<Framework/header.h>", where Framework denotes the name of the
framework and header.h is found in the PrivateHeaders or Headers
directory.
-iframeworkdir
Like -F except the directory is a treated as a system directory. The
main difference between this -iframework and -F is that with
-iframework the compiler does not warn about constructs contained
within header files found via dir. This option is valid only for the
C family of languages.
-gused
Emit debugging information for symbols that are used. For stabs
debugging format, this enables -feliminate-unused-debug-symbols.
This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-fconstant-cfstrings
The -fconstant-cfstrings is an alias for -mconstant-cfstrings.
-mconstant-cfstrings
When the NeXT runtime is being used (the default on these systems),
override any -fconstant-string-class setting and cause "@"...""
literals to be laid out as constant CoreFoundation strings.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is
version. Typical values supported for version include 12, 10.12, and
10.5.8.
If the compiler was built to use the system's headers by default,
then the default for this option is the system version on which the
compiler is running, otherwise the default is to make choices that
are compatible with as many systems and code bases as possible.
-mkernel
Enable kernel development mode. The -mkernel option sets -static,
-fno-common, -fno-use-cxa-atexit, -fno-exceptions,
-fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where
applicable. This mode also sets -mno-altivec, -msoft-float,
-fno-builtin and -mlong-branch for PowerPC targets.
-mone-byte-bool
Override the defaults for "bool" so that "sizeof(bool)==1". By
default "sizeof(bool)" is 4 when compiling for Darwin/PowerPC and 1
when compiling for Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Using this switch may require recompiling all other modules in a
program, including system libraries. Use this switch to conform to a
non-default data model.
-munreachable-traps
Causes "__builtin_unreachable" to be rendered as a trap. This is the
default for all Darwin architectures.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turnaround development, such as to
allow GDB to dynamically load .o files into already-running programs.
-findirect-data and -ffix-and-continue are provided for backwards
compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for
more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong
architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will
bind all undefined references when the file is loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more
information.
-bundle_loader executable
This option specifies the executable that will load the build output
file being linked. See man ld(1) for more information.
-dynamiclib
When passed this option, GCC produces a dynamic library instead of an
executable when linking, using the Darwin libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of one
controlled by the -mcpu or -march option.
-nodefaultrpaths
Do not add default run paths for the compiler library directories to
executables, modules or dynamic libraries. On macOS 10.5 and later,
the embedded runpath is added by default unless the user adds
-nodefaultrpaths to the link line. Run paths are needed (and
therefore enforced) to build on macOS version 10.11 or later.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man
page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for
floating-point operations. When -msoft-float is specified, functions
in libgcc.a are used to perform floating-point operations. Unless
they are replaced by routines that emulate the floating-point
operations, or compiled in such a way as to call such emulations
routines, these routines issue floating-point operations. If you
are compiling for an Alpha without floating-point operations, you
must ensure that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are
required to have floating-point registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register
set. -mno-fp-regs implies -msoft-float. If the floating-point
register set is not used, floating-point operands are passed in
integer registers as if they were integers and floating-point results
are passed in $0 instead of $f0. This is a non-standard calling
sequence, so any function with a floating-point argument or return
value called by code compiled with -mno-fp-regs must also be compiled
with that option.
A typical use of this option is building a kernel that does not use,
and hence need not save and restore, any floating-point registers.
-mieee
The Alpha architecture implements floating-point hardware optimized
for maximum performance. It is mostly compliant with the IEEE
floating-point standard. However, for full compliance, software
assistance is required. This option generates code fully IEEE-
compliant code except that the inexact-flag is not maintained (see
below). If this option is turned on, the preprocessor macro
"_IEEE_FP" is defined during compilation. The resulting code is less
efficient but is able to correctly support denormalized numbers and
exceptional IEEE values such as not-a-number and plus/minus infinity.
Other Alpha compilers call this option -ieee_with_no_inexact.
-mieee-with-inexact
This is like -mieee except the generated code also maintains the IEEE
inexact-flag. Turning on this option causes the generated code to
implement fully-compliant IEEE math. In addition to "_IEEE_FP",
"_IEEE_FP_EXACT" is defined as a preprocessor macro. On some Alpha
implementations the resulting code may execute significantly slower
than the code generated by default. Since there is very little code
that depends on the inexact-flag, you should normally not specify
this option. Other Alpha compilers call this option
-ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled.
Other Alpha compilers call this option -fptm trap-mode. The trap
mode can be set to one of four values:
n This is the default (normal) setting. The only traps that are
enabled are the ones that cannot be disabled in software (e.g.,
division by zero trap).
u In addition to the traps enabled by n, underflow traps are
enabled as well.
su Like u, but the instructions are marked to be safe for software
completion (see Alpha architecture manual for details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this
option -fprm rounding-mode. The rounding-mode can be one of:
n Normal IEEE rounding mode. Floating-point numbers are rounded
towards the nearest machine number or towards the even machine
number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating-point numbers are rounded
towards zero.
d Dynamic rounding mode. A field in the floating-point control
register (fpcr, see Alpha architecture reference manual) controls
the rounding mode in effect. The C library initializes this
register for rounding towards plus infinity. Thus, unless your
program modifies the fpcr, d corresponds to round towards plus
infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are imprecise. This
means without software assistance it is impossible to recover from a
floating trap and program execution normally needs to be terminated.
GCC can generate code that can assist operating system trap handlers
in determining the exact location that caused a floating-point trap.
Depending on the requirements of an application, different levels of
precisions can be selected:
p Program precision. This option is the default and means a trap
handler can only identify which program caused a floating-point
exception.
f Function precision. The trap handler can determine the function
that caused a floating-point exception.
i Instruction precision. The trap handler can determine the exact
instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called
-scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must
not use this option unless you also specify -mtrap-precision=i and
either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is
to emit the line .eflag 48 in the function prologue of the generated
assembly file.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it
can construct it from smaller constants in two or three instructions.
If it cannot, it outputs the constant as a literal and generates code
to load it from the data segment at run time.
Use this option to require GCC to construct all integer constants
using code, even if it takes more instructions (the maximum is six).
You typically use this option to build a shared library dynamic
loader. Itself a shared library, it must relocate itself in memory
before it can find the variables and constants in its own data
segment.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX,
CIX, FIX and MAX instruction sets. The default is to use the
instruction sets supported by the CPU type specified via -mcpu=
option or that of the CPU on which GCC was built if none is
specified.
-msafe-bwa
-mno-safe-bwa
Indicate whether in the absence of the optional BWX instruction set
GCC should generate multi-thread and async-signal safe code for byte
and aligned word memory accesses.
-msafe-partial
-mno-safe-partial
Indicate whether GCC should generate multi-thread and async-signal
safe code for partial memory accesses, including piecemeal accesses
to unaligned data as well as block accesses to leading and trailing
parts of aggregate types or other objects in memory that do not
respectively start and end on an aligned 64-bit data boundary.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-point
arithmetic instead of IEEE single and double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations
except via assembler macros. Use of these macros does not allow
optimal instruction scheduling. GNU binutils as of version 2.12
supports a new syntax that allows the compiler to explicitly mark
which relocations should apply to which instructions. This option is
mostly useful for debugging, as GCC detects the capabilities of the
assembler when it is built and sets the default accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via gp-
relative relocations. When -msmall-data is used, objects 8 bytes
long or smaller are placed in a small data area (the ".sdata" and
".sbss" sections) and are accessed via 16-bit relocations off of the
$gp register. This limits the size of the small data area to 64KB,
but allows the variables to be directly accessed via a single
instruction.
The default is -mlarge-data. With this option the data area is
limited to just below 2GB. Programs that require more than 2GB of
data must use "malloc" or "mmap" to allocate the data in the heap
instead of in the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data
and -fPIC implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of the
entire program (or shared library) fits in 4MB, and is thus reachable
with a branch instruction. When -msmall-data is used, the compiler
can assume that all local symbols share the same $gp value, and thus
reduce the number of instructions required for a function call from 4
to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters for
machine type cpu_type. You can specify either the EV style name or
the corresponding chip number. GCC supports scheduling parameters
for the EV4, EV5 and EV6 family of processors and chooses the default
values for the instruction set from the processor you specify. If
you do not specify a processor type, GCC defaults to the processor on
which the compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set extensions.
ev5
21164
Schedules as an EV5 and has no instruction set extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX
extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX
extensions.
Native toolchains also support the value native, which selects the
best architecture option for the host processor. -mcpu=native has no
effect if GCC does not recognize the processor.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type
cpu_type. The instruction set is not changed.
Native toolchains also support the value native, which selects the
best architecture option for the host processor. -mtune=native has
no effect if GCC does not recognize the processor.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory
references as seen by the application. This number is highly
dependent on the memory access patterns used by the application and
the size of the external cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock cycles for
"typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also
called Dcache, Scache, and Bcache), as well as to main memory.
Note that L3 is only valid for EV5.
eBPF Options
-mframe-limit=bytes
This specifies the hard limit for frame sizes, in bytes. Currently,
the value that can be specified should be less than or equal to
32767. Defaults to whatever limit is imposed by the version of the
Linux kernel targeted.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-mjmpext
-mno-jmpext
Enable or disable generation of extra conditional-branch
instructions. Enabled for CPU v2 and above.
-mjmp32
-mno-jmp32
Enable or disable generation of 32-bit jump instructions. Enabled
for CPU v3 and above.
-malu32
-mno-alu32
Enable or disable generation of 32-bit ALU instructions. Enabled for
CPU v3 and above.
-mv3-atomics
-mno-v3-atomics
Enable or disable instructions for general atomic operations
introduced in CPU v3. Enabled for CPU v3 and above.
-mbswap
-mno-bswap
Enable or disable byte swap instructions. Enabled for CPU v4 and
above.
-msdiv
-mno-sdiv
Enable or disable signed division and modulus instructions. Enabled
for CPU v4 and above.
-msmov
-mno-smov
Enable or disable sign-extending move and memory load instructions.
Enabled for CPU v4 and above.
-mcpu=version
This specifies which version of the eBPF ISA to target. Newer
versions may not be supported by all kernels. The default is v4.
Supported values for version are:
v1 The first stable eBPF ISA with no special features or extensions.
v2 Supports the jump extensions, as in -mjmpext.
v3 All features of v2, plus:
-<32-bit jump operations, as in -mjmp32>
-<32-bit ALU operations, as in -malu32>
-<general atomic operations, as in -mv3-atomics>
v4 All features of v3, plus:
-<Byte swap instructions, as in -mbswap>
-<Signed division and modulus instructions, as in -msdiv>
-<Sign-extending move and memory load instructions, as in -msmov>
-mco-re
Enable BPF Compile Once - Run Everywhere (CO-RE) support. Requires
and is implied by -gbtf.
-mno-co-re
Disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE
support is enabled by default when generating BTF debug information
for the BPF target.
-mxbpf
Generate code for an expanded version of BPF, which relaxes some of
the restrictions imposed by the BPF architecture:
-<Save and restore callee-saved registers at function entry and>
exit, respectively.
-masm=dialect
Outputs assembly instructions using eBPF selected dialect. The
default is pseudoc.
Supported values for dialect are:
normal
Outputs normal assembly dialect.
pseudoc
Outputs pseudo-c assembly dialect.
-minline-memops-threshold=bytes
Specifies a size threshold in bytes at or below which memmove, memcpy
and memset shall always be expanded inline. Operations dealing with
sizes larger than this threshold would have to be implemented using a
library call instead of being expanded inline, but since BPF doesn't
allow libcalls, exceeding this threshold results in a compile-time
error. The default is 1024 bytes.
FR30 Options
These options are defined specifically for the FR30 port.
-msmall-model
Use the small address space model. This can produce smaller code,
but it does assume that all symbolic values and addresses fit into a
20-bit range.
-mno-lsim
Assume that runtime support has been provided and so there is no need
to include the simulator library (libsim.a) on the linker command
line.
FT32 Options
These options are defined specifically for the FT32 port.
-msim
Specifies that the program will be run on the simulator. This causes
an alternate runtime startup and library to be linked. You must not
use this option when generating programs that will run on real
hardware; you must provide your own runtime library for whatever I/O
functions are needed.
-mlra
Does nothing. Preserved for backward compatibility.
-mnodiv
Do not use div and mod instructions.
-mft32b
Enable use of the extended instructions of the FT32B processor.
-mcompress
Compress all code using the Ft32B code compression scheme.
-mnopm
Do not generate code that reads program memory.
FRV Options
-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only use
"icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to represent
pointers to functions. Without any PIC/PIE-related options, it
implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small
data are within a 12-bit range from the GOT base address; with -fPIC
or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf
target, this option implies -msim.
-minline-plt
Enable inlining of PLT entries in function calls to functions that
are not known to bind locally. It has no effect without -mfdpic.
It's enabled by default if optimizing for speed and compiling for
shared libraries (i.e., -fPIC or -fpic), or when an optimization
option such as -O3 or above is present in the command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
-mtls
Do not assume a large TLS segment when generating thread-local code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data that
is known to be in read-only sections. It's enabled by default,
except for -fpic or -fpie: even though it may help make the global
offset table smaller, it trades 1 instruction for 4. With -fPIC or
-fPIE, it trades 3 instructions for 4, one of which may be shared by
multiple symbols, and it avoids the need for a GOT entry for the
referenced symbol, so it's more likely to be a win. If it is not,
-mno-gprel-ro can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied by
-mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You
should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer
whenever a stack frame is allocated. This option is enabled by
default and can be disabled with -mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current
compilation unit. This allows the functions to be placed anywhere
within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting NOPs into the
previous packet. This option only has an effect when VLIW packing is
enabled. It doesn't create new packets; it merely adds NOPs to
existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution
(default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be
removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the
compiler-generated code. It is enabled by default.
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar"
instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code. Possible
values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and
simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
-mglibc
Use the GNU C library. This is the default except on
*-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
-muclibc
Use uClibc C library. This is the default on *-*-linux-*uclibc*
targets.
-mmusl
Use the musl C library. This is the default on *-*-linux-*musl*
targets.
-mbionic
Use Bionic C library. This is the default on *-*-linux-*android*
targets.
-mandroid
Compile code compatible with Android platform. This is the default
on *-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions
and -fno-rtti by default. When linking, this option makes the GCC
driver pass Android-specific options to the linker. Finally, this
option causes the preprocessor macro "__ANDROID__" to be defined.
-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable
-mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard Linux
linking options to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses the
linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This
switch must be used either with -mh or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.
-mexr
Extended registers are stored on stack before execution of function
with monitor attribute. Default option is -mexr. This option is
valid only for H8S targets.
-mno-exr
Extended registers are not stored on stack before execution of
function with monitor attribute. Default option is -mno-exr. This
option is valid only for H8S targets.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the
H8/300. The default for the H8/300H and H8S is to align longs and
floats on 4-byte boundaries. -malign-300 causes them to be aligned
on 2-byte boundaries. This option has no effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices for
architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA
2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to
determine the proper architecture option for your machine. Code
compiled for lower numbered architectures runs on higher numbered
architectures, but not the other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-matomic-libcalls
Generate libcalls for atomic loads and stores when sync libcalls are
disabled. This option is enabled by default. It only affects the
generation of atomic libcalls by the HPPA backend.
Both the sync and libatomic libcall implementations use locking. As
a result, processor stores are not atomic with respect to other
atomic operations. Processor loads up to DImode are atomic with
respect to other atomic operations provided they are implemented as a
single access.
The PA-RISC architecture does not support any atomic operations in
hardware except for the "ldcw" instruction. Thus, all atomic support
is implemented using sync and atomic libcalls. Sync libcall support
is in libgcc.a. Atomic libcall support is in libatomic.
This option generates "__atomic_exchange" calls for atomic stores.
It also provides special handling for atomic DImode accesses on
32-bit targets.
-mbig-switch
Does nothing. Preserved for backward compatibility.
-mcaller-copies
The caller copies function arguments passed by hidden reference.
This option should be used with care as it is not compatible with the
default 32-bit runtime. However, only aggregates larger than eight
bytes are passed by hidden reference and the option provides better
compatibility with OpenMP.
-mcoherent-ldcw
Use ldcw/ldcd coherent cache-control hint.
-mdisable-fpregs
Disable floating-point registers. Equivalent to "-msoft-float".
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids
some rather obscure problems when compiling MIG generated code under
MACH.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This
allows GCC to emit code that performs faster indirect calls.
This option does not work in the presence of shared libraries or
nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator cannot use. This
is useful when compiling kernel code. A register range is specified
as two registers separated by a dash. Multiple register ranges can
be specified separated by a comma.
-mgas
Enable the use of assembler directives only GAS understands.
-mgnu-ld
Use options specific to GNU ld. This passes -shared to ld when
building a shared library. It is the default when GCC is configured,
explicitly or implicitly, with the GNU linker. This option does not
affect which ld is called; it only changes what parameters are passed
to that ld. The ld that is called is determined by the --with-ld
configure option, GCC's program search path, and finally by the
user's PATH. The linker used by GCC can be printed using which `gcc
-print-prog-name=ld`. This option is only available on the 64-bit
HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
-mhp-ld
Use options specific to HP ld. This passes -b to ld when building a
shared library and passes +Accept TypeMismatch to ld on all links.
It is the default when GCC is configured, explicitly or implicitly,
with the HP linker. This option does not affect which ld is called;
it only changes what parameters are passed to that ld. The ld that
is called is determined by the --with-ld configure option, GCC's
program search path, and finally by the user's PATH. The linker used
by GCC can be printed using which `gcc -print-prog-name=ld`. This
option is only available on the 64-bit HP-UX GCC, i.e. configured
with hppa*64*-*-hpux*.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes
symbolic debugging impossible.
-mlong-calls
Generate code that uses long call sequences. This ensures that a
call is always able to reach linker generated stubs. The default is
to generate long calls only when the distance from the call site to
the beginning of the function or translation unit, as the case may
be, exceeds a predefined limit set by the branch type being used.
The limits for normal calls are 7,600,000 and 240,000 bytes,
respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are
always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the
-ffunction-sections option, or when using the -mgas and
-mno-portable-runtime options together under HP-UX with the SOM
linker.
It is normally not desirable to use this option as it degrades
performance. However, it may be useful in large applications,
particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the
assembler and linker, and the type of code being generated. The
impact on systems that support long absolute calls, and long pic
symbol-difference or pc-relative calls should be relatively small.
However, an indirect call is used on 32-bit ELF systems in pic code
and it is quite long.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required
by the HP-UX 10 linker. This is equivalent to the +k option to the
HP compilers.
-mjump-in-delay
This option is ignored and provided for compatibility purposes only.
-mno-space-regs
Generate code that assumes the target has no space registers. This
allows GCC to generate faster indirect calls and use unscaled index
address modes.
Such code is suitable for level 0 PA systems and kernels.
-mordered
Assume memory references are ordered and barriers are not needed.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type cpu-
type. The choices for cpu-type are 700 7100, 7100LC, 7200, 7300 and
8000. Refer to /usr/lib/sched.models on an HP-UX system to determine
the proper scheduling option for your machine. The default
scheduling is 8000.
-msio
Generate the predefine, "_SIO", for server IO. The default is
-mwsio. This generates the predefines, "__hp9000s700",
"__hp9000s700__" and "_WSIO", for workstation IO. These options are
available under HP-UX and HI-UX.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all HPPA
targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation. You
must make your own arrangements to provide suitable library functions
for cross-compilation.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.
-msoft-mult
Use software integer multiplication.
This disables the use of the "xmpyu" instruction.
-munix=unix-std
Generate compiler predefines and select a startfile for the specified
UNIX standard. The choices for unix-std are 93, 95 and 98. 93 is
supported on all HP-UX versions. 95 is available on HP-UX 10.10 and
later. 98 is available on HP-UX 11.11 and later. The default values
are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though to 11.00, and 98
for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95
provides additional predefines for "XOPEN_UNIX" and
"_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98
provides additional predefines for "_XOPEN_UNIX",
"_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and
"_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It is important to note that this option changes the interfaces for
various library routines. It also affects the operational behavior
of the C library. Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX
standard must test, set and restore the variable
"__xpg4_extended_mask" as appropriate. Most GNU software doesn't
provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when the
-static option is specified on HP-UX 10 and later.
-static
The HP-UX implementation of setlocale in libc has a dependency on
libdld.sl. There isn't an archive version of libdld.sl. Thus, when
the -static option is specified, special link options are needed to
resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to
link with libdld.sl when the -static option is specified. This
causes the resulting binary to be dynamic. On the 64-bit port, the
linkers generate dynamic binaries by default in any case. The
-nolibdld option can be used to prevent the GCC driver from adding
these link options.
-threads
Add support for multithreading with the dce thread library under HP-
UX. This option sets flags for both the preprocessor and linker.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big-endian target. This is the default for HP-
UX.
-mlittle-endian
Generate code for a little-endian target. This is the default for
AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the default.
-mno-pic
Generate code that does not use a global pointer register. The
result is not position independent code, and violates the IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after volatile
asm statements.
-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for the stacked
registers. This may make assembler output more readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section.
This may be useful for working around optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value. This
is useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies -mconstant-gp.
This is useful when compiling firmware code.
-minline-float-divide-min-latency
Generate code for inline divides of floating-point values using the
minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating-point values using the
maximum throughput algorithm.
-mno-inline-float-divide
Do not generate inline code for divides of floating-point values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum
latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum
throughput algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency
algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput
algorithm.
-mno-inline-sqrt
Do not generate inline code for "sqrt".
-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or
multiply/subtract instructions. The default is to use these
instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF line number
debugging info. This may be useful when not using the GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the
instruction that triggered the stop bit. This can improve
instruction scheduling, but does not always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator cannot use. This
is useful when compiling kernel code. A register range is specified
as two registers separated by a dash. Multiple register ranges can
be specified separated by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22,
and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values
are itanium, itanium1, merced, itanium2, and mckinley.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
These are HP-UX specific flags.
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This results
in generation of "ld.a" instructions and the corresponding check
instructions ("ld.c" / "chk.a"). The default setting is disabled.
-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This results
in generation of "ld.a" instructions and the corresponding check
instructions ("ld.c" / "chk.a"). The default setting is enabled.
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is
available only during region scheduling (i.e. before reload). This
results in generation of the "ld.s" instructions and the
corresponding check instructions "chk.s". The default setting is
disabled.
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are
dependent on the data speculative loads before reload. This is
effective only with -msched-br-data-spec enabled. The default
setting is enabled.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are
dependent on the data speculative loads after reload. This is
effective only with -msched-ar-data-spec enabled. The default
setting is enabled.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are
dependent on the control speculative loads. This is effective only
with -msched-control-spec enabled. The default setting is enabled.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data-speculative instructions are chosen for schedule
only if there are no other choices at the moment. This makes the use
of the data speculation much more conservative. The default setting
is disabled.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control-speculative instructions are chosen for schedule
only if there are no other choices at the moment. This makes the use
of the control speculation much more conservative. The default
setting is disabled.
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies are considered during
computation of the instructions priorities. This makes the use of
the speculation a bit more conservative. The default setting is
disabled.
-msched-spec-ldc
Use a simple data speculation check. This option is on by default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by
default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option is
on by default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to cause a
conflict when placed into the same instruction group. This option is
disabled by default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling.
This flag is disabled by default.
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group, giving
lower priority to subsequent memory insns attempting to schedule in
the same instruction group. Frequently useful to prevent cache bank
conflicts. The default value is 1.
-msched-max-memory-insns-hard-limit
Makes the limit specified by msched-max-memory-insns a hard limit,
disallowing more than that number in an instruction group.
Otherwise, the limit is "soft", meaning that non-memory operations
are preferred when the limit is reached, but memory operations may
still be scheduled.
LM32 Options
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled
Enable barrel-shift instructions.
-mdivide-enabled
Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
LoongArch Options
These command-line options are defined for LoongArch targets:
-march=arch-type
Generate instructions for the machine type arch-type. -march=arch-
type allows GCC to generate code that may not run at all on
processors other than the one indicated.
The choices for arch-type are:
native
Local processor type detected by the native compiler.
loongarch64
Generic LoongArch 64-bit processor.
la464
LoongArch LA464-based processor with LSX, LASX.
la664
LoongArch LA664-based processor with LSX, LASX and all LoongArch
v1.1 instructions.
la64v1.0
LoongArch64 ISA version 1.0.
la64v1.1
LoongArch64 ISA version 1.1.
More information about LoongArch ISA versions can be found at
<https://github.com/loongson/la-toolchain-conventions>.
-mtune=tune-type
Optimize the generated code for the given processor target.
The choices for tune-type are:
native
Local processor type detected by the native compiler.
generic
Generic LoongArch processor.
loongarch64
Generic LoongArch 64-bit processor.
la464
LoongArch LA464 core.
la664
LoongArch LA664 core.
-mabi=base-abi-type
Generate code for the specified calling convention. base-abi-type
can be one of:
lp64d
Uses 64-bit general purpose registers and 32/64-bit floating-
point registers for parameter passing. Data model is LP64, where
int is 32 bits, while long int and pointers are 64 bits.
lp64f
Uses 64-bit general purpose registers and 32-bit floating-point
registers for parameter passing. Data model is LP64, where int
is 32 bits, while long int and pointers are 64 bits.
lp64s
Uses 64-bit general purpose registers and no floating-point
registers for parameter passing. Data model is LP64, where int
is 32 bits, while long int and pointers are 64 bits.
-mfpu=fpu-type
Generate code for the specified FPU type, which can be one of:
64 Allow the use of hardware floating-point instructions for 32-bit
and 64-bit operations.
32 Allow the use of hardware floating-point instructions for 32-bit
operations.
none
0 Prevent the use of hardware floating-point instructions.
-msimd=simd-type
Enable generation of LoongArch SIMD instructions for vectorization
and via builtin functions. The value can be one of:
lasx
Enable generating instructions from the 256-bit LoongArch
Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD
Extension (LSX).
lsx Enable generating instructions from the 128-bit LoongArch SIMD
Extension (LSX).
none
No LoongArch SIMD instruction may be generated.
-msoft-float
Force -mfpu=none and prevents the use of floating-point registers for
parameter passing. This option may change the target ABI.
-msingle-float
Force -mfpu=32 and allow the use of 32-bit floating-point registers
for parameter passing. This option may change the target ABI.
-mdouble-float
Force -mfpu=64 and allow the use of 32/64-bit floating-point
registers for parameter passing. This option may change the target
ABI.
-mlasx
-mno-lasx
-mlsx
-mno-lsx
Incrementally adjust the scope of the SIMD extensions (none / LSX /
LASX) that can be used by the compiler for code generation. Enabling
LASX with mlasx automatically enables LSX, and diabling LSX with mno-
lsx automatically disables LASX. These driver-only options act upon
the final msimd configuration state and make incremental changes in
the order they appear on the GCC driver's command line, deriving the
final / canonicalized msimd option that is passed to the compiler
proper.
-mbranch-cost=n
Set the cost of branches to roughly n instructions.
-maddr-reg-reg-cost=n
Set the cost of ADDRESS_REG_REG to the value calculated by n.
-mcheck-zero-division
-mno-check-zero-divison
Trap (do not trap) on integer division by zero. The default is
-mcheck-zero-division for -O0 or -Og, and -mno-check-zero-division
for other optimization levels.
-mcond-move-int
-mno-cond-move-int
Conditional moves for integral data in general-purpose registers are
enabled (disabled). The default is -mcond-move-int.
-mcond-move-float
-mno-cond-move-float
Conditional moves for floating-point registers are enabled
(disabled). The default is -mcond-move-float.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial block moves.
The default is -mno-memcpy, which allows GCC to inline most constant-
sized copies. Setting optimization level to -Os also forces the use
of "memcpy", but -mno-memcpy may override this behavior if explicitly
specified, regardless of the order these options on the command line.
-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned on
a natural object boundary as described in the architecture
specification. The default is -mno-strict-align.
-G num
Put global and static data smaller than num bytes into a small data
section. The default value is 0.
-mmax-inline-memcpy-size=n
Inline all block moves (such as calls to "memcpy" or structure
copies) less than or equal to n bytes. The default value of n is
1024.
-mcmodel=code-model
Set the code model to one of:
tiny-static (Not implemented yet)
tiny (Not implemented yet)
normal
The text segment must be within 128MB addressing space. The data
segment must be within 2GB addressing space.
medium
The text segment and data segment must be within 2GB addressing
space.
large (Not implemented yet)
extreme
This mode does not limit the size of the code segment and data
segment. The -mcmodel=extreme option is incompatible with -fplt
and/or -mexplicit-relocs=none.
The default code model is "normal".
-mexplicit-relocs=style
Set when to use assembler relocation operators when dealing with
symbolic addresses. The alternative is to use assembler macros
instead, which may limit instruction scheduling but allow linker
relaxation. with -mexplicit-relocs=none the assembler macros are
always used, with -mexplicit-relocs=always the assembler relocation
operators are always used, with -mexplicit-relocs=auto the compiler
will use the relocation operators where the linker relaxation is
impossible to improve the code quality, and macros elsewhere. The
default value for the option is determined with the assembler
capability detected during GCC build-time and the setting of -mrelax:
-mexplicit-relocs=none if the assembler does not support relocation
operators at all, -mexplicit-relocs=always if the assembler supports
relocation operators but -mrelax is not enabled,
-mexplicit-relocs=auto if the assembler supports relocation operators
and -mrelax is enabled.
-mexplicit-relocs
An alias of -mexplicit-relocs=always for backward compatibility.
-mno-explicit-relocs
An alias of -mexplicit-relocs=none for backward compatibility.
-mdirect-extern-access
-mno-direct-extern-access
Do not use or use GOT to access external symbols. The default is
-mno-direct-extern-access: GOT is used for external symbols with
default visibility, but not used for other external symbols.
With -mdirect-extern-access, GOT is not used and all external symbols
are PC-relatively addressed. It is only suitable for environments
where no dynamic link is performed, like firmwares, OS kernels,
executables linked with -static or -static-pie.
-mdirect-extern-access is not compatible with -fPIC or -fpic.
-mrelax
-mno-relax
Take (do not take) advantage of linker relaxations. If
-mpass-mrelax-to-as is enabled, this option is also passed to the
assembler. The default is determined during GCC build-time by
detecting corresponding assembler support: -mrelax if the assembler
supports both the -mrelax option and the conditional branch
relaxation (it's required or the ".align" directives and conditional
branch instructions in the assembly code outputted by GCC may be
rejected by the assembler because of a relocation overflow),
-mno-relax otherwise.
-mpass-mrelax-to-as
-mno-pass-mrelax-to-as
Pass (do not pass) the -mrelax or -mno-relax option to the assembler.
The default is determined during GCC build-time by detecting
corresponding assembler support: -mpass-mrelax-to-as if the assembler
supports the -mrelax option, -mno-pass-mrelax-to-as otherwise. This
option is mostly useful for debugging, or interoperation with
assemblers different from the build-time one.
-mrecip
This option enables use of the reciprocal estimate and reciprocal
square root estimate instructions with additional Newton-Raphson
steps to increase precision instead of doing a divide or square root
and divide for floating-point arguments. These instructions are
generated only when -funsafe-math-optimizations is enabled together
with -ffinite-math-only and -fno-trapping-math. This option is off
by default. Before you can use this option, you must sure the target
CPU supports frecipe and frsqrte instructions. Note that while the
throughput of the sequence is higher than the throughput of the non-
reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
-mrecip=opt
This option controls which reciprocal estimate instructions may be
used. opt is a comma-separated list of options, which may be
preceded by a ! to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the approximation for scalar division.
vec-div
Enable the approximation for vectorized division.
sqrt
Enable the approximation for scalar square root.
vec-sqrt
Enable the approximation for vectorized square root.
rsqrt
Enable the approximation for scalar reciprocal square root.
vec-rsqrt
Enable the approximation for vectorized reciprocal square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal
approximations, except for scalar square root.
-mfrecipe
-mno-frecipe
Use (do not use) "frecipe.{s/d}" and "frsqrte.{s/d}" instructions.
When build with -march=la664, it is enabled by default. The default
is -mno-frecipe.
-mdiv32
-mno-div32
Use (do not use) "div.w[u]" and "mod.w[u]" instructions with input
not sign-extended. When build with -march=la664, it is enabled by
default. The default is -mno-div32.
-mlam-bh
-mno-lam-bh
Use (do not use) "am{swap/add}[_db].{b/h}" instructions. When build
with -march=la664, it is enabled by default. The default is
-mno-lam-bh.
-mlamcas
-mno-lamcas
Use (do not use) "amcas[_db].{b/h/w/d}" instructions. When build
with -march=la664, it is enabled by default. The default is
-mno-lamcas.
-mld-seq-sa
-mno-ld-seq-sa
Whether a same-address load-load barrier ("dbar 0x700") is needed.
When build with -march=la664, it is enabled by default. The default
is -mno-ld-seq-sa, the load-load barrier is needed.
-mtls-dialect=opt
This option controls which tls dialect may be used for general
dynamic and local dynamic TLS models.
trad
Use traditional TLS. This is the default.
desc
Use TLS descriptors.
-mannotate-tablejump
-mno-annotate-tablejump
Create an annotation section ".discard.tablejump_annotate" to
correlate the "jirl" instruction and the jump table when a jump table
is used to optimize the "switch" statement. Some external tools, for
example objtool of the Linux kernel building system, need the
annotation to analysis the control flow. The default is
-mno-annotate-tablejump.
--param loongarch-vect-unroll-limit=n
The vectorizer will use available tuning information to determine
whether it would be beneficial to unroll the main vectorized loop and
by how much. This parameter set's the upper bound of how much the
vectorizer will unroll the main loop. The default value is six.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be one of r8c
for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm
for the M16C/80 series, or m32c for the M32C/80 series.
-msim
Specifies that the program will be run on the simulator. This causes
an alternate runtime library to be linked in which supports, for
example, file I/O. You must not use this option when generating
programs that will run on real hardware; you must provide your own
runtime library for whatever I/O functions are needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC uses during
code generation. These pseudo-registers are used like real
registers, so there is a tradeoff between GCC's ability to fit the
code into available registers, and the performance penalty of using
memory instead of registers. Note that all modules in a program must
be compiled with the same value for this option. Because of that,
you must not use this option with GCC's default runtime libraries.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their
addresses can be loaded with the "ld24" instruction), and assume all
subroutines are reachable with the "bl" instruction. This is the
default.
The addressability of a particular object can be set with the "model"
attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the
compiler generates "seth/add3" instructions to load their addresses),
and assume all subroutines are reachable with the "bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the
compiler generates "seth/add3" instructions to load their addresses),
and assume subroutines may not be reachable with the "bl" instruction
(the compiler generates the much slower "seth/add3/jl" instruction
sequence).
-msdata=none
Disable use of the small data area. Variables are put into one of
".data", ".bss", or ".rodata" (unless the "section" attribute has
been specified). This is the default.
The small data area consists of sections ".sdata" and ".sbss".
Objects may be explicitly put in the small data area with the
"section" attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not
generate special code to reference them.
-msdata=use
Put small global and static data in the small data area, and generate
special instructions to reference them.
-G num
Put global and static objects less than or equal to num bytes into
the small data or BSS sections instead of the normal data or BSS
sections. The default value of num is 8. The -msdata option must be
set to one of sdata or use for this option to have any effect.
All modules should be compiled with the same -G num value. Compiling
with different values of num may or may not work; if it doesn't the
linker gives an error message---incorrect code is not generated.
-mdebug
Makes the M32R-specific code in the compiler display some statistics
that might help in debugging programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches are preferred
over conditional code, if it is 2, then the opposite applies.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default is
12. Valid numbers are between 0 and 15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call to flush
the cache. The default is _flush_cache, but a function call is only
used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire processors. The
default settings depend on which architecture was selected when the
compiler was configured; the defaults for the most common choices are
given below.
-march=arch
Generate code for a specific M680x0 or ColdFire instruction set
architecture. Permissible values of arch for M680x0 architectures
are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire
architectures are selected according to Freescale's ISA
classification and the permissible values are: isaa, isaaplus, isab
and isac.
GCC defines a macro "__mcfarch__" whenever it is generating code for
a ColdFire target. The arch in this macro is one of the -march
arguments given above.
When used together, -march and -mtune select code that runs on a
family of similar processors but that is optimized for a particular
microarchitecture.
-mcpu=cpu
Generate code for a specific M680x0 or ColdFire processor. The
M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302,
68332 and cpu32. The ColdFire cpus are given by the table below,
which also classifies the CPUs into families:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 52x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484
5485
-mcpu=cpu overrides -march=arch if arch is compatible with cpu.
Other combinations of -mcpu and -march are rejected.
GCC defines the macro "__mcf_cpu_cpu" when ColdFire target cpu is
selected. It also defines "__mcf_family_family", where the value of
family is given by the table above.
-mtune=tune
Tune the code for a particular microarchitecture within the
constraints set by -march and -mcpu. The M680x0 microarchitectures
are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The
ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to run
relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60
is similar but includes 68060 targets as well. These two options
select the same tuning decisions as -m68020-40 and -m68020-60
respectively.
GCC defines the macros "__mcarch" and "__mcarch__" when tuning for
680x0 architecture arch. It also defines "mcarch" unless either
-ansi or a non-GNU -std option is used. If GCC is tuning for a range
of architectures, as selected by -mtune=68020-40 or -mtune=68020-60,
it defines the macros for every architecture in the range.
GCC also defines the macro "__muarch__" when tuning for ColdFire
microarchitecture uarch, where uarch is one of the arguments given
above.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler
is configured for 68000-based systems. It is equivalent to
-march=68000.
Use this option for microcontrollers with a 68000 or EC000 core,
including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68010
Generate output for a 68010. This is the default when the compiler
is configured for 68010-based systems. It is equivalent to
-march=68010.
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler
is configured for 68020-based systems. It is equivalent to
-march=68020.
-m68030
Generate output for a 68030. This is the default when the compiler
is configured for 68030-based systems. It is equivalent to
-march=68030.
-m68040
Generate output for a 68040. This is the default when the compiler
is configured for 68040-based systems. It is equivalent to
-march=68040.
This option inhibits the use of 68881/68882 instructions that have to
be emulated by software on the 68040. Use this option if your 68040
does not have code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when the compiler
is configured for 68060-based systems. It is equivalent to
-march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions
that have to be emulated by software on the 68060. Use this option
if your 68060 does not have code to emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when the compiler
is configured for CPU32-based systems. It is equivalent to
-march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+ core,
including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341,
68349 and 68360.
-m5200
Generate output for a 520X ColdFire CPU. This is the default when
the compiler is configured for 520X-based systems. It is equivalent
to -mcpu=5206, and is now deprecated in favor of that option.
Use this option for microcontroller with a 5200 core, including the
MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e
Generate output for a 5206e ColdFire CPU. The option is now
deprecated in favor of the equivalent -mcpu=5206e.
-m528x
Generate output for a member of the ColdFire 528X family. The option
is now deprecated in favor of the equivalent -mcpu=528x.
-m5307
Generate output for a ColdFire 5307 CPU. The option is now
deprecated in favor of the equivalent -mcpu=5307.
-m5407
Generate output for a ColdFire 5407 CPU. The option is now
deprecated in favor of the equivalent -mcpu=5407.
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This
includes use of hardware floating-point instructions. The option is
equivalent to -mcpu=547x, and is now deprecated in favor of that
option.
-m68020-40
Generate output for a 68040, without using any of the new
instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040. The
generated code does use the 68881 instructions that are emulated on
the 68040.
The option is equivalent to -march=68020 -mtune=68020-40.
-m68020-60
Generate output for a 68060, without using any of the new
instructions. This results in code that can run relatively
efficiently on either a 68020/68881 or a 68030 or a 68040. The
generated code does use the 68881 instructions that are emulated on
the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
-mhard-float
-m68881
Generate floating-point instructions. This is the default for 68020
and above, and for ColdFire devices that have an FPU. It defines the
macro "__HAVE_68881__" on M680x0 targets and "__mcffpu__" on ColdFire
targets.
-msoft-float
Do not generate floating-point instructions; use library calls
instead. This is the default for 68000, 68010, and 68832 targets.
It is also the default for ColdFire devices that have no FPU.
-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and remainder
instructions. If -march is used without -mcpu, the default is "on"
for ColdFire architectures and "off" for M680x0 architectures.
Otherwise, the default is taken from the target CPU (either the
default CPU, or the one specified by -mcpu). For example, the
default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.
GCC defines the macro "__mcfhwdiv__" when this option is enabled.
-mshort
Consider type "int" to be 16 bits wide, like "short int".
Additionally, parameters passed on the stack are also aligned to a
16-bit boundary even on targets whose API mandates promotion to
32-bit.
-mno-short
Do not consider type "int" to be 16 bits wide. This is the default.
-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and
-m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies
-mbitfield. This is the default if you use a configuration designed
for a 68020.
-mrtd
Use a different function-calling convention, in which functions that
take a fixed number of arguments return with the "rtd" instruction,
which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments
there.
This calling convention is incompatible with the one normally used on
Unix, so you cannot use it if you need to call libraries compiled
with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including "printf"); otherwise
incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function
with too many arguments. (Normally, extra arguments are harmlessly
ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040,
68060 and CPU32 processors, but not by the 68000 or 5200.
The default is -mno-rtd.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float",
"double", and "long double" variables on a 32-bit boundary
(-malign-int) or a 16-bit boundary (-mno-align-int). Aligning
variables on 32-bit boundaries produces code that runs somewhat
faster on processors with 32-bit busses at the expense of more
memory.
Warning: if you use the -malign-int switch, GCC aligns structures
containing the above types differently than most published
application binary interface specifications for the m68k.
Use the pc-relative addressing mode of the 68000 directly, instead of
using a global offset table. At present, this option implies -fpic,
allowing at most a 16-bit offset for pc-relative addressing. -fPIC
is not presently supported with -mpcrel, though this could be
supported for 68020 and higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references are handled by
the system.
-msep-data
Generate code that allows the data segment to be located in a
different area of memory from the text segment. This allows for
execute-in-place in an environment without virtual memory management.
This option implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the text
segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID
method. This allows for execute-in-place and shared libraries in an
environment without virtual memory management. This option implies
-fPIC.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being
used. This is the default.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library
being compiled. Specifying a value of 0 generates more compact code;
specifying other values forces the allocation of that number to the
current library, but is no more space- or time-efficient than
omitting this option.
-mxgot
-mno-xgot
When generating position-independent code for ColdFire, generate code
that works if the GOT has more than 8192 entries. This code is
larger and slower than code generated without this option. On M680x0
processors, this option is not needed; -fPIC suffices.
GCC normally uses a single instruction to load values from the GOT.
While this is relatively efficient, it only works if the GOT is
smaller than about 64k. Anything larger causes the linker to report
an error such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot. It
should then work with very large GOTs. However, code generated with
-mxgot is less efficient, since it takes 4 instructions to fetch the
value of a global symbol.
Note that some linkers, including newer versions of the GNU linker,
can create multiple GOTs and sort GOT entries. If you have such a
linker, you should only need to use -mxgot when compiling a single
object file that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-
independent code.
-mlong-jump-table-offsets
Use 32-bit offsets in "switch" tables. The default is to use 16-bit
offsets.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two
instructions or less.
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary-sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as "int"-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little-endian target.
-m210
-m340
Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so omit the
simulator library (libsim.a) from the linker command line.
-mstack-increment=size
Set the maximum amount for a single stack increment operation. Large
values can increase the speed of programs that contain functions that
need a large amount of stack space, but they can also trigger a
segmentation fault if the stack is extended too much. The default
value is 0x1000.
MicroBlaze Options
-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy
Do not optimize block moves, use "memcpy".
-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss instead.
-mcpu=cpu-type
Use features of, and schedule code for, the given CPU. Supported
values are in the format vX.YY.Z, where X is a major version, YY is
the minor version, and Z is compatibility code. Example values are
v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer divisions.
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
-mxl-gp-opt
Use GP-relative ".sdata"/".sbss" sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target.
-mxl-reorder
Use reorder instructions (swap and byte reversed load/store).
-mxl-mode-app-model
Select application model app-model. Valid models are
executable
normal executable (default), uses startup code crt0.o.
xmdstub
for use with Xilinx Microprocessor Debugger (XMD) based software
intrusive debug agent called xmdstub. This uses startup file
crt1.o and sets the start address of the program to 0x800.
bootstrap
for applications that are loaded using a bootloader. This model
uses startup file crt2.o which does not contain a processor reset
vector handler. This is suitable for transferring control on a
processor reset to the bootloader rather than the application.
novectors
for applications that do not require any of the MicroBlaze
vectors. This option may be useful for applications running
within a monitoring application. This model uses crt3.o as a
startup file.
Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-
model.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is
fixed at static link time. This allows data to be referenced by
offset from start of text address instead of GOT since PC-relative
addressing is not supported.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-*
configurations.
-march=arch
Generate code that runs on arch, which can be the name of a generic
MIPS ISA, or the name of a particular processor. The ISA names are:
mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5,
mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The
processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd,
5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1,
34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2,
1004kc, 1004kf2_1, 1004kf1_1, i6400, i6500, interaptiv, loongson2e,
loongson2f, loongson3a, gs464, gs464e, gs264e, m4k, m14k, m14kc,
m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2, octeon3,
orion, p5600, p6600, r2000, r3000, r3900, r4000, r4400, r4600, r4650,
r4700, r5900, r6000, r8000, rm7000, rm9000, r10000, r12000, r14000,
r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000,
vr5400, vr5500, xlr and xlp. The special value from-abi selects the
most compatible architecture for the selected ABI (that is, mips1 for
32-bit ABIs and mips3 for 64-bit ABIs).
The native Linux/GNU toolchain also supports the value native, which
selects the best architecture option for the host processor.
-march=native has no effect if GCC does not recognize the processor.
In processor names, a final 000 can be abbreviated as k (for example,
-march=r2k). Prefixes are optional, and vr may be written r.
Names of the form nf2_1 refer to processors with FPUs clocked at half
the rate of the core, names of the form nf1_1 refer to processors
with FPUs clocked at the same rate as the core, and names of the form
nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with
respect to the core. For compatibility reasons, nf is accepted as a
synonym for nf2_1 while nx and bfx are accepted as synonyms for
nf1_1.
GCC defines two macros based on the value of this option. The first
is "_MIPS_ARCH", which gives the name of target architecture, as a
string. The second has the form "_MIPS_ARCH_foo", where foo is the
capitalized value of "_MIPS_ARCH". For example, -march=r2000 sets
"_MIPS_ARCH" to "r2000" and defines the macro "_MIPS_ARCH_R2000".
Note that the "_MIPS_ARCH" macro uses the processor names given
above. In other words, it has the full prefix and does not
abbreviate 000 as k. In the case of from-abi, the macro names the
resolved architecture (either "mips1" or "mips3"). It names the
default architecture when no -march option is given.
-mtune=arch
Optimize for arch. Among other things, this option controls the way
instructions are scheduled, and the perceived cost of arithmetic
operations. The list of arch values is the same as for -march.
When this option is not used, GCC optimizes for the processor
specified by -march. By using -march and -mtune together, it is
possible to generate code that runs on a family of processors, but
optimize the code for one particular member of that family.
-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo", which
work in the same way as the -march ones described above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r3
Equivalent to -march=mips32r3.
-mips32r5
Equivalent to -march=mips32r5.
-mips32r6
Equivalent to -march=mips32r6.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-mips64r3
Equivalent to -march=mips64r3.
-mips64r5
Equivalent to -march=mips64r5.
-mips64r6
Equivalent to -march=mips64r6.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32
or MIPS64 architecture, it makes use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis
by means of "mips16" and "nomips16" attributes.
-mmips16e2
-mno-mips16e2
Use (do not use) the MIPS16e2 ASE. This option modifies the behavior
of the -mips16 option such that it targets the MIPS16e2 ASE.
-mflip-mips16
Generate MIPS16 code on alternating functions. This option is
provided for regression testing of mixed MIPS16/non-MIPS16 code
generation, and is not intended for ordinary use in compiling user
code.
-minterlink-compressed
-mno-interlink-compressed
Require (do not require) that code using the standard (uncompressed)
MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice
versa.
For example, code using the standard ISA encoding cannot jump
directly to MIPS16 or microMIPS code; it must either use a call or an
indirect jump. -minterlink-compressed therefore disables direct
jumps unless GCC knows that the target of the jump is not compressed.
-minterlink-mips16
-mno-interlink-mips16
Aliases of -minterlink-compressed and -mno-interlink-compressed.
These options predate the microMIPS ASE and are retained for
backwards compatibility.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally
generates 64-bit code when you select a 64-bit architecture, but you
can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see
<https://gcc.gnu.org/projects/mipso64-abi.html>.
GCC supports a variant of the o32 ABI in which floating-point
registers are 64 rather than 32 bits wide. You can select this
combination with -mabi=32 -mfp64. This ABI relies on the "mthc1" and
"mfhc1" instructions and is therefore only supported for MIPS32R2,
MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values remain the
same, but each scalar value is passed in a single 64-bit register
rather than a pair of 32-bit registers. For example, scalar
floating-point values are returned in $f0 only, not a $f0/$f1 pair.
The set of call-saved registers also remains the same in that the
even-numbered double-precision registers are saved.
Two additional variants of the o32 ABI are supported to enable a
transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx)
and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that
all code must execute correctly when run using 32-bit or 64-bit
registers. The code can be interlinked with either FP32 or FP64, but
not both. The FP64A extension is similar to the FP64 extension but
forbids the use of odd-numbered single-precision registers. This can
be used in conjunction with the "FRE" mode of FPUs in MIPS32R5
processors and allows both FP32 and FP64A code to interlink and run
in the same process without changing FPU modes.
-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for SVR4-style
dynamic objects. -mabicalls is the default for SVR4-based systems.
-mshared
-mno-shared
Generate (do not generate) code that is fully position-independent,
and that can therefore be linked into shared libraries. This option
only affects -mabicalls.
All -mabicalls code has traditionally been position-independent,
regardless of options like -fPIC and -fpic. However, as an
extension, the GNU toolchain allows executables to use absolute
accesses for locally-binding symbols. It can also use shorter GP
initialization sequences and generate direct calls to locally-defined
functions. This mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates objects
that can only be linked by the GNU linker. However, the option does
not affect the ABI of the final executable; it only affects the ABI
of relocatable objects. Using -mno-shared generally makes
executables both smaller and quicker.
-mshared is the default.
-mplt
-mno-plt
Assume (do not assume) that the static and dynamic linkers support
PLTs and copy relocations. This option only affects -mno-shared
-mabicalls. For the n64 ABI, this option has no effect without
-msym32.
You can make -mplt the default by configuring GCC with
--with-mips-plt. The default is -mno-plt otherwise.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global
offset table.
GCC normally uses a single instruction to load values from the GOT.
While this is relatively efficient, it only works if the GOT is
smaller than about 64k. Anything larger causes the linker to report
an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot. This
works with very large GOTs, although the code is also less efficient,
since it takes three instructions to fetch the value of a global
symbol.
Note that some linkers can create multiple GOTs. If you have such a
linker, you should only need to use -mxgot when a single object file
accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position
independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mfpxx
Do not assume the width of floating-point registers.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement
floating-point calculations using library calls instead.
-mno-float
Equivalent to -msoft-float, but additionally asserts that the program
being compiled does not perform any floating-point operations. This
option is presently supported only by some bare-metal MIPS
configurations, where it may select a special set of libraries that
lack all floating-point support (including, for example, the
floating-point "printf" formats). If code compiled with -mno-float
accidentally contains floating-point operations, it is likely to
suffer a link-time or run-time failure.
-msingle-float
Assume that the floating-point coprocessor only supports single-
precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-precision
operations. This is the default.
-modd-spreg
-mno-odd-spreg
Enable the use of odd-numbered single-precision floating-point
registers for the o32 ABI. This is the default for processors that
are known to support these registers. When using the o32 FPXX ABI,
-mno-odd-spreg is set by default.
-mabs=2008
-mabs=legacy
These options control the treatment of the special not-a-number (NaN)
IEEE 754 floating-point data with the "abs.fmt" and "neg.fmt" machine
instructions.
By default or when -mabs=legacy is used the legacy treatment is
selected. In this case these instructions are considered arithmetic
and avoided where correct operation is required and the input operand
might be a NaN. A longer sequence of instructions that manipulate
the sign bit of floating-point datum manually is used instead unless
the -ffinite-math-only option has also been specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment. In this
case these instructions are considered non-arithmetic and therefore
operating correctly in all cases, including in particular where the
input operand is a NaN. These instructions are therefore always used
for the respective operations.
-mnan=2008
-mnan=legacy
These options control the encoding of the special not-a-number (NaN)
IEEE 754 floating-point data.
The -mnan=legacy option selects the legacy encoding. In this case
quiet NaNs (qNaNs) are denoted by the first bit of their trailing
significand field being 0, whereas signaling NaNs (sNaNs) are denoted
by the first bit of their trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In this
case qNaNs are denoted by the first bit of their trailing significand
field being 1, whereas sNaNs are denoted by the first bit of their
trailing significand field being 0.
The default is -mnan=legacy unless GCC has been configured with
--with-nan=2008.
-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement atomic
memory built-in functions. When neither option is specified, GCC
uses the instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the
instructions and -mno-llsc can be useful when compiling for
nonstandard ISAs. You can make either option the default by
configuring GCC with --with-llsc and --without-llsc respectively.
--with-llsc is the default for some configurations; see the
installation documentation for details.
-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro "__mips_dsp". It also
defines "__mips_dsp_rev" to 1.
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros "__mips_dsp" and
"__mips_dspr2". It also defines "__mips_dsp_rev" to 2.
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This
option can only be used when generating 64-bit code and requires
hardware floating-point support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies
-mpaired-single.
-mmicromips
-mno-micromips
Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function
basis by means of "micromips" and "nomicromips" attributes.
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
-mmcu
-mno-mcu
Use (do not use) the MIPS MCU ASE instructions.
-meva
-mno-eva
Use (do not use) the MIPS Enhanced Virtual Addressing instructions.
-mvirt
-mno-virt
Use (do not use) the MIPS Virtualization (VZ) instructions.
-mxpa
-mno-xpa
Use (do not use) the MIPS eXtended Physical Address (XPA)
instructions.
-mcrc
-mno-crc
Use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.
-mginv
-mno-ginv
Use (do not use) the MIPS Global INValidate (GINV) instructions.
-mloongson-mmi
-mno-loongson-mmi
Use (do not use) the MIPS Loongson MultiMedia extensions Instructions
(MMI).
-mloongson-ext
-mno-loongson-ext
Use (do not use) the MIPS Loongson EXTensions (EXT) instructions.
-mloongson-ext2
-mno-loongson-ext2
Use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an
explanation of the default and the way that the pointer size is
determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the ABI.
All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit
"long"s, as does the 64-bit EABI; the others use 32-bit "long"s.
Pointers are the same size as "long"s, or the same size as integer
registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values,
regardless of the selected ABI. This option is useful in combination
with -mabi=64 and -mno-abicalls because it allows GCC to generate
shorter and faster references to symbolic addresses.
-G num
Put definitions of externally-visible data in a small data section if
that data is no bigger than num bytes. GCC can then generate more
efficient accesses to the data; see -mgpopt for details.
The default -G option depends on the configuration.
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data too, such as to
static variables in C. -mlocal-sdata is the default for all
configurations.
If the linker complains that an application is using too much small
data, you might want to try rebuilding the less performance-critical
parts with -mno-local-sdata. You might also want to build large
libraries with -mno-local-sdata, so that the libraries leave more
room for the main program.
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data is in a small
data section if the size of that data is within the -G limit.
-mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata -G num -mgpopt, and
Mod references a variable Var that is no bigger than num bytes, you
must make sure that Var is placed in a small data section. If Var is
defined by another module, you must either compile that module with a
high-enough -G setting or attach a "section" attribute to Var's
definition. If Var is common, you must link the application with a
high-enough -G setting.
The easiest way of satisfying these restrictions is to compile and
link every module with the same -G option. However, you may wish to
build a library that supports several different small data limits.
You can do this by compiling the library with the highest supported
-G setting and additionally using -mno-extern-sdata to stop the
library from making assumptions about externally-defined data.
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known to
be in a small data section; see -G, -mlocal-sdata and -mextern-sdata.
-mgpopt is the default for all configurations.
-mno-gpopt is useful for cases where the $gp register might not hold
the value of "_gp". For example, if the code is part of a library
that might be used in a boot monitor, programs that call boot monitor
routines pass an unknown value in $gp. (In such situations, the boot
monitor itself is usually compiled with -G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible,
then next in the small data section if possible, otherwise in data.
This gives slightly slower code than the default, but reduces the
amount of RAM required when executing, and thus may be preferred for
some embedded systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data section.
This option is only meaningful in conjunction with -membedded-data.
-mcode-readable=setting
Specify whether GCC may generate code that reads from executable
sections. There are three possible settings:
-mcode-readable=yes
Instructions may freely access executable sections. This is the
default setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable
sections, but other instructions must not do so. This option is
useful on 4KSc and 4KSd processors when the code TLBs have the
Read Inhibit bit set. It is also useful on processors that can
be configured to have a dual instruction/data SRAM interface and
that, like the M4K, automatically redirect PC-relative loads to
the instruction RAM.
-mcode-readable=no
Instructions must not access executable sections. This option
can be useful on targets that are configured to have a dual
instruction/data SRAM interface but that (unlike the M4K) do not
automatically redirect PC-relative loads to the instruction RAM.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the "%hi()" and "%lo()" assembler relocation
operators. This option has been superseded by -mexplicit-relocs but
is retained for backwards compatibility.
-mexplicit-relocs=none
-mexplicit-relocs=base
-mexplicit-relocs=pcrel
-mexplicit-relocs
-mno-explicit-relocs
These options control whether explicit relocs (such as %gp_rel) are
used. The default value depends on the version of GAS when GCC
itself was built.
The "base" explicit-relocs support was introduced into GAS in 2001.
The "pcrel" explicit-relocs support was introduced into GAS in 2014,
which supports %pcrel_hi and %pcrel_lo.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a
conditional trap or a break instruction. Using traps results in
smaller code, but is only supported on MIPS II and later. Also, some
versions of the Linux kernel have a bug that prevents trap from
generating the proper signal ("SIGFPE"). Use -mdivide-traps to allow
conditional traps on architectures that support them and
-mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden at
configure time using --with-divide=breaks. Divide-by-zero checks can
be completely disabled using -mno-check-zero-division.
-mload-store-pairs
-mno-load-store-pairs
Enable (disable) an optimization that pairs consecutive load or store
instructions to enable load/store bonding. This option is enabled by
default but only takes effect when the selected architecture is known
to support bonding.
-mstrict-align
-mno-strict-align
-munaligned-access
-mno-unaligned-access
Disable (enable) direct unaligned access for MIPS Release 6. MIPSr6
requires load/store unaligned-access support, by hardware or
trap&emulate. So -mstrict-align may be needed by kernel. The
options -munaligned-access and -mno-unaligned-access are obsoleted,
and only for backward-compatible.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial block moves.
The default is -mno-memcpy, which allows GCC to inline most constant-
sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling
functions using "jal" is more efficient but requires the caller and
callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is
-mno-long-calls.
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions, as
provided by the R4650 ISA.
-mimadd
-mno-imadd
Enable (disable) use of the "madd" and "msub" integer instructions.
The default is -mimadd on architectures that support "madd" and
"msub" except for the 74k architecture where it was found to generate
slower code.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating-point multiply-accumulate
instructions, when they are available. The default is -mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are used, the
intermediate product is calculated to infinite precision and is not
subject to the FCSR Flush to Zero bit. This may be undesirable in
some circumstances. On other processors the result is numerically
identical to the equivalent computation using separate multiply, add,
subtract and negate instructions.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user
assembler files (with a .s suffix) when assembling them.
-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill) errata.
The workarounds are implemented by the assembler rather than by GCC.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect result if
executed immediately after starting an integer division.
- A double-word or a variable shift may give an incorrect result if
executed while an integer multiplication is in progress.
- An integer division may give an incorrect result if started in a
delay slot of a taken branch or a jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect result if
executed immediately after starting an integer division.
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on revisions prior
to 3.0. They may deadlock on revisions 2.6 and earlier.
This option can only be used if the target architecture supports
branch-likely instructions. -mfix-r10000 is the default when
-march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-r5900
-mno-fix-r5900
Do not attempt to schedule the preceding instruction into the delay
slot of a branch instruction placed at the end of a short loop of six
instructions or fewer and always schedule a "nop" instruction there
instead. The short loop bug under certain conditions causes loops to
execute only once or twice, due to a hardware bug in the R5900 chip.
The workaround is implemented by the assembler rather than by GCC.
-mfix-rm7000
-mno-fix-rm7000
Work around the RM7000 "dmult"/"dmultu" errata. The workarounds are
implemented by the assembler rather than by GCC.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if one
of the operands is negative.
The workarounds for the division errata rely on special functions in
libgcc.a. At present, these functions are only provided by the
"mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between certain
pairs of instructions. These errata are handled by the assembler,
not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are
implemented by the assembler rather than by GCC, although GCC avoids
using "mflo" and "mfhi" if the VR4130 "macc", "macchi", "dmacc" and
"dmacchi" instructions are available instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently works
around the SB-1 revision 2 "F1" and "F2" floating-point errata.)
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the side
effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the outcome
of a conditional branch and speculatively executes instructions from
the "taken" branch. It later aborts these instructions if the
predicted outcome is wrong. However, on the R10K, even aborted
instructions can have side effects.
This problem only affects kernel stores and, depending on the system,
kernel loads. As an example, a speculatively-executed store may load
the target memory into cache and mark the cache line as dirty, even
if the store itself is later aborted. If a DMA operation writes to
the same area of memory before the "dirty" line is flushed, the
cached data overwrites the DMA-ed data. See the R10K processor
manual for a full description, including other potential problems.
One workaround is to insert cache barrier instructions before every
memory access that might be speculatively executed and that might
have side effects even if aborted. -mr10k-cache-barrier=setting
controls GCC's implementation of this workaround. It assumes that
aborted accesses to any byte in the following regions does not have
side effects:
1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant
address.
It is the kernel's responsibility to ensure that speculative accesses
to these regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and "jal foo" to
be executed speculatively. GCC honors this restriction for functions
it compiles itself. It expects non-GCC functions (such as hand-
written assembly code) to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be
speculatively executed and that might have side effects even if
aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be speculatively
executed and that might have side effects even if aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default
setting.
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not
call any such function. If called, the function must take the same
arguments as the common "_flush_func", that is, the address of the
memory range for which the cache is being flushed, the size of the
memory range, and the number 3 (to flush both caches). The default
depends on the target GCC was configured for, but commonly is either
"_flush_func" or "__cpu_flush".
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This
cost is only a heuristic and is not guaranteed to produce consistent
results across releases. A zero cost redundantly selects the
default, which is based on the -mtune setting.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of
the default for the selected architecture. By default, Branch Likely
instructions may be generated if they are supported by the selected
architecture. An exception is for the MIPS32 and MIPS64
architectures and processors that implement those architectures; for
those, Branch Likely instructions are not be generated by default
because the MIPS32 and MIPS64 architectures specifically deprecate
their use.
-mcompact-branches=never
-mcompact-branches=optimal
-mcompact-branches=always
These options control which form of branches will be generated. The
default is -mcompact-branches=optimal.
The -mcompact-branches=never option ensures that compact branch
instructions will never be generated.
The -mcompact-branches=always option ensures that a compact branch
instruction will be generated if available for MIPS Release 6
onwards. If a compact branch instruction is not available (or
pre-R6), a delay slot form of the branch will be used instead.
If it is used for MIPS16/microMIPS targets, it will be just ignored
now. The behavior for MIPS16/microMIPS may change in future, since
they do have some compact branch instructions.
The -mcompact-branches=optimal option will cause a delay slot branch
to be used if one is available in the current ISA and the delay slot
is successfully filled. If the delay slot is not filled, a compact
branch will be chosen if one is available.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how FP
instructions are scheduled for some processors. The default is that
FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are
emitting 64-bit code, then we can use both FP pipes. Otherwise, we
can only use one FP pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two
instructions together if the first one is 8-byte aligned. When this
option is enabled, GCC aligns pairs of instructions that it thinks
should execute in parallel.
This option only has an effect when optimizing for the VR4130. It
normally makes code faster, but at the expense of making it bigger.
It is enabled by default at optimization level -O3.
-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on architectures
that support it. The "synci" instructions (if enabled) are generated
when "__builtin___clear_cache" is compiled.
This option defaults to -mno-synci, but the default can be overridden
by configuring GCC with --with-synci.
When compiling code for single processor systems, it is generally
safe to use "synci". However, on many multi-core (SMP) systems, it
does not invalidate the instruction caches on all cores and may lead
to undefined behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25
into direct calls. This is only possible if the linker can resolve
the destination at link time and if the destination is within range
for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an
assembler and a linker that support the ".reloc" assembly directive
and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this
optimization can be performed by the assembler and the linker alone
without help from the compiler.
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the calling
function's return address. When enabled, this option extends the
usual "_mcount" interface with a new ra-address parameter, which has
type "intptr_t *" and is passed in register $12. "_mcount" can then
modify the return address by doing both of the following:
* Returning the new address in register $31.
* Storing the new address in "*ra-address", if ra-address is
nonnull.
The default is -mno-mcount-ra-address.
-mframe-header-opt
-mno-frame-header-opt
Enable (disable) frame header optimization in the o32 ABI. When
using the o32 ABI, calling functions will allocate 16 bytes on the
stack for the called function to write out register arguments. When
enabled, this optimization will suppress the allocation of the frame
header if it can be determined that it is unused.
This optimization is off by default at all optimization levels.
-mlxc1-sxc1
-mno-lxc1-sxc1
When applicable, enable (disable) the generation of "lwxc1", "swxc1",
"ldxc1", "sdxc1" instructions. Enabled by default.
-mmadd4
-mno-madd4
When applicable, enable (disable) the generation of 4-operand
"madd.s", "madd.d" and related instructions. Enabled by default.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing
all values in registers, no matter the size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with
respect to the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that
(in the called function) are seen as registers $0 and up, as opposed
to the GNU ABI which uses global registers $231 and up.
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do
not use) zero-extending load instructions by default, rather than
sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign
as the divisor. With the default, -mno-knuthdiv, the sign of the
remainder follows the sign of the dividend. Both methods are
arithmetically valid, the latter being almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly
code can be used with the "PREFIX" assembly directive.
-melf
Generate an executable in the ELF format, rather than the default mmo
format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch
prediction indicates a probable branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using a
base address automatically generates a request (handled by the
assembler and the linker) for a constant to be set up in a global
register. The register is used for one or more base address requests
within the range 0 to 255 from the value held in the register. The
generally leads to short and fast code, but the number of different
data items that can be addressed is limited. This means that a
program that uses lots of static data may require
-mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in
each function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the
MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for
the MN10300 processors.
-mam33
Generate code using features specific to the AM33 processor.
-mno-am33
Do not generate code using features specific to the AM33 processor.
This is the default.
-mam33-2
Generate code using features specific to the AM33/2.0 processor.
-mam34
Generate code using features specific to the AM34 processor.
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when
scheduling instructions. This does not change the targeted processor
type. The CPU type must be one of mn10300, am33, am33-2 or am34.
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the pointer
in both "a0" and "d0". Otherwise, the pointer is returned only in
"a0", and attempts to call such functions without a prototype result
in errors. Note that this option is on by default; use
-mno-return-pointer-on-d0 to disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation
optimization pass to shorten branches, calls and absolute memory
addresses. This option only has an effect when used on the command
line for the final link step.
This option makes symbolic debugging impossible.
-mliw
Allow the compiler to generate Long Instruction Word instructions if
the target is the AM33 or later. This is the default. This option
defines the preprocessor macro "__LIW__".
-mno-liw
Do not allow the compiler to generate Long Instruction Word
instructions. This option defines the preprocessor macro
"__NO_LIW__".
-msetlb
Allow the compiler to generate the SETLB and Lcc instructions if the
target is the AM33 or later. This is the default. This option
defines the preprocessor macro "__SETLB__".
-mno-setlb
Do not allow the compiler to generate SETLB or Lcc instructions.
This option defines the preprocessor macro "__NO_SETLB__".
Moxie Options
-meb
Generate big-endian code. This is the default for moxie-*-*
configurations.
-mel
Generate little-endian code.
-mmul.x
Generate mul.x and umul.x instructions. This is the default for
moxiebox-*-* configurations.
-mno-crt0
Do not link in the C run-time initialization object file.
MSP430 Options
These options are defined for the MSP430:
-masm-hex
Force assembly output to always use hex constants. Normally such
constants are signed decimals, but this option is available for
testsuite and/or aesthetic purposes.
-mmcu=
Select the MCU to target. This is used to create a C preprocessor
symbol based upon the MCU name, converted to upper case and pre- and
post-fixed with __. This in turn is used by the msp430.h header file
to select an MCU-specific supplementary header file.
The option also sets the ISA to use. If the MCU name is one that is
known to only support the 430 ISA then that is selected, otherwise
the 430X ISA is selected. A generic MCU name of msp430 can also be
used to select the 430 ISA. Similarly the generic msp430x MCU name
selects the 430X ISA.
In addition an MCU-specific linker script is added to the linker
command line. The script's name is the name of the MCU with .ld
appended. Thus specifying -mmcu=xxx on the gcc command line defines
the C preprocessor symbol "__XXX__" and cause the linker to search
for a script called xxx.ld.
The ISA and hardware multiply supported for the different MCUs is
hard-coded into GCC. However, an external devices.csv file can be
used to extend device support beyond those that have been hard-coded.
GCC searches for the devices.csv file using the following methods in
the given precedence order, where the first method takes precedence
over the second which takes precedence over the third.
Include path specified with "-I" and "-L"
devices.csv is searched for in each of the directories specified
by include paths and linker library search paths.
Path specified by the environment variable MSP430_GCC_INCLUDE_DIR
Define the value of the global environment variable
MSP430_GCC_INCLUDE_DIR to the full path to the directory
containing devices.csv, and GCC will search this directory for
devices.csv. If devices.csv is found, this directory is also
registered as an include path and linker library path. Header
files and linker scripts in this directory can therefore be used
without manually specifying "-I" and "-L" on the command line.
The msp430-elf{,bare}/include/devices directory
Finally, GCC examines msp430-elf{,bare}/include/devices from the
toolchain root directory. This directory does not exist in a
default installation, but if the user has created it and copied
devices.csv there, then the MCU data is read. As above, this
directory is also registered as an include path and linker
library path.
If none of the above search methods find devices.csv, then the hard-
coded MCU data is used.
-mwarn-mcu
-mno-warn-mcu
This option enables or disables warnings about conflicts between the
MCU name specified by the -mmcu option and the ISA set by the -mcpu
option and/or the hardware multiply support set by the -mhwmult
option. It also toggles warnings about unrecognized MCU names. This
option is on by default.
-mcpu=
Specifies the ISA to use. Accepted values are msp430, msp430x and
msp430xv2. This option is deprecated. The -mmcu= option should be
used to select the ISA.
-msim
Link to the simulator runtime libraries and linker script. Overrides
any scripts that would be selected by the -mmcu= option.
-mlarge
Use large-model addressing (20-bit pointers, 20-bit "size_t").
-msmall
Use small-model addressing (16-bit pointers, 16-bit "size_t").
-mrelax
This option is passed to the assembler and linker, and allows the
linker to perform certain optimizations that cannot be done until the
final link.
mhwmult=
Describes the type of hardware multiply supported by the target.
Accepted values are none for no hardware multiply, 16bit for the
original 16-bit-only multiply supported by early MCUs. 32bit for the
16/32-bit multiply supported by later MCUs and f5series for the
16/32-bit multiply supported by F5-series MCUs. A value of auto can
also be given. This tells GCC to deduce the hardware multiply
support based upon the MCU name provided by the -mmcu option. If no
-mmcu option is specified or if the MCU name is not recognized then
no hardware multiply support is assumed. "auto" is the default
setting.
Hardware multiplies are normally performed by calling a library
routine. This saves space in the generated code. When compiling at
-O3 or higher however the hardware multiplier is invoked inline.
This makes for bigger, but faster code.
The hardware multiply routines disable interrupts whilst running and
restore the previous interrupt state when they finish. This makes
them safe to use inside interrupt handlers as well as in normal code.
-minrt
Enable the use of a minimum runtime environment - no static
initializers or constructors. This is intended for memory-
constrained devices. The compiler includes special symbols in some
objects that tell the linker and runtime which code fragments are
required.
-mtiny-printf
Enable reduced code size "printf" and "puts" library functions. The
tiny implementations of these functions are not reentrant, so must be
used with caution in multi-threaded applications.
Support for streams has been removed and the string to be printed
will always be sent to stdout via the "write" syscall. The string is
not buffered before it is sent to write.
This option requires Newlib Nano IO, so GCC must be configured with
--enable-newlib-nano-formatted-io.
-mmax-inline-shift=
This option takes an integer between 0 and 64 inclusive, and sets the
maximum number of inline shift instructions which should be emitted
to perform a shift operation by a constant amount. When this value
needs to be exceeded, an mspabi helper function is used instead. The
default value is 4.
This only affects cases where a shift by multiple positions cannot be
completed with a single instruction (e.g. all shifts >1 on the 430
ISA).
Shifts of a 32-bit value are at least twice as costly, so the value
passed for this option is divided by 2 and the resulting value used
instead.
-mcode-region=
-mdata-region=
These options tell the compiler where to place functions and data
that do not have one of the "lower", "upper", "either" or "section"
attributes. Possible values are "lower", "upper", "either" or "any".
The first three behave like the corresponding attribute. The fourth
possible value - "any" - is the default. It leaves placement
entirely up to the linker script and how it assigns the standard
sections (".text", ".data", etc) to the memory regions.
-msilicon-errata=
This option passes on a request to assembler to enable the fixes for
the named silicon errata.
-msilicon-errata-warn=
This option passes on a request to the assembler to enable warning
messages when a silicon errata might need to be applied.
-mwarn-devices-csv
-mno-warn-devices-csv
Warn if devices.csv is not found or there are problem parsing it
(default: on).
NDS32 Options
These options are defined for NDS32 implementations:
-mbig-endian
Generate code in big-endian mode.
-mlittle-endian
Generate code in little-endian mode.
-mreduced-regs
Use reduced-set registers for register allocation.
-mfull-regs
Use full-set registers for register allocation.
-mcmov
Generate conditional move instructions.
-mno-cmov
Do not generate conditional move instructions.
-mext-perf
Generate performance extension instructions.
-mno-ext-perf
Do not generate performance extension instructions.
-mext-perf2
Generate performance extension 2 instructions.
-mno-ext-perf2
Do not generate performance extension 2 instructions.
-mext-string
Generate string extension instructions.
-mno-ext-string
Do not generate string extension instructions.
-mv3push
Generate v3 push25/pop25 instructions.
-mno-v3push
Do not generate v3 push25/pop25 instructions.
-m16-bit
Generate 16-bit instructions.
-mno-16-bit
Do not generate 16-bit instructions.
-misr-vector-size=num
Specify the size of each interrupt vector, which must be 4 or 16.
-mcache-block-size=num
Specify the size of each cache block, which must be a power of 2
between 4 and 512.
-march=arch
Specify the name of the target architecture.
-mcmodel=code-model
Set the code model to one of
small
All the data and read-only data segments must be within 512KB
addressing space. The text segment must be within 16MB
addressing space.
medium
The data segment must be within 512KB while the read-only data
segment can be within 4GB addressing space. The text segment
should be still within 16MB addressing space.
large
All the text and data segments can be within 4GB addressing
space.
-mctor-dtor
Enable constructor/destructor feature.
-mrelax
Guide linker to relax instructions.
Nvidia PTX Options
These options are defined for Nvidia PTX:
-m64
Ignored, but preserved for backward compatibility. Only 64-bit ABI
is supported.
-march=architecture-string
Generate code for the specified PTX ISA target architecture. Valid
architecture strings are sm_30, sm_35, sm_37, sm_52, sm_53, sm_61,
sm_70, sm_75, sm_80, and sm_89. The default depends on how the
compiler has been configured, see --with-arch.
This option sets the value of the preprocessor macro "__PTX_SM__";
for instance, for sm_35, it has the value 350.
-misa=architecture-string
Alias of -march=.
-march-map=architecture-string
Select the closest available -march= value that is not more capable.
For instance, for -march-map=sm_50 select -march=sm_37, and for
-march-map=sm_53 select -march=sm_53.
-mptx=version-string
Generate code for the specified PTX ISA version. Valid version
strings are 3.1, 4.1, 4.2, 5.0, 6.0, 6.3, 7.0, 7.3, and 7.8. The
default PTX ISA version is the one that added support for the
selected PTX ISA target architecture, see -march=, but at least 6.3,
or 7.3 for -march=sm_52 and higher.
This option sets the values of the preprocessor macros
"__PTX_ISA_VERSION_MAJOR__" and "__PTX_ISA_VERSION_MINOR__"; for
instance, for 3.1 the macros have the values 3 and 1, respectively.
-mmainkernel
Link in code for a __main kernel. This is for stand-alone instead of
offloading execution.
-moptimize
Apply partitioned execution optimizations. This is the default when
any level of optimization is selected.
-msoft-stack
-mno-soft-stack
For -mno-soft-stack (the default, unless -mgomp has been specified),
use PTX "native" stacks, that is, generate code that uses ".local"
memory or PTX "alloca" directly for stack storage. Unless -mptx=7.3
or higher and -march=sm_52 or higher are active, variable-length
arrays and dynamically allocating memory on the stack with "alloca"
are not supported.
For -msoft-stack (implied by -mgomp), generate code that does not use
".local" memory or PTX "alloca" directly for stack storage. Instead,
a per-warp stack pointer is maintained explicitly. This enables
variable-length stack allocation (with variable-length arrays or
"alloca"), and when global memory is used for underlying storage,
makes it possible to access automatic variables from other threads,
or with atomic instructions. This code generation variant is used for
OpenMP offloading, but the option is exposed on its own for the
purpose of testing the compiler; to generate code suitable for
linking into programs using OpenMP offloading, use option -mgomp.
-muniform-simt
Switch to code generation variant that allows to execute all threads
in each warp, while maintaining memory state and side effects as if
only one thread in each warp was active outside of OpenMP SIMD
regions. All atomic operations and calls to runtime (malloc, free,
vprintf) are conditionally executed (iff current lane index equals
the master lane index), and the register being assigned is copied via
a shuffle instruction from the master lane. Outside of SIMD regions
lane 0 is the master; inside, each thread sees itself as the master.
Shared memory array "int __nvptx_uni[]" stores all-zeros or all-ones
bitmasks for each warp, indicating current mode (0 outside of SIMD
regions). Each thread can bitwise-and the bitmask at position
"tid.y" with current lane index to compute the master lane index.
-mgomp
Generate code for use in OpenMP offloading: enables -msoft-stack and
-muniform-simt options, and selects corresponding multilib variant.
OpenRISC Options
These options are defined for OpenRISC:
-mboard=name
Configure a board specific runtime. This will be passed to the
linker for newlib board library linking. The default is "or1ksim".
-mnewlib
This option is ignored; it is for compatibility purposes only. This
used to select linker and preprocessor options for use with newlib.
-msoft-div
-mhard-div
Select software or hardware divide ("l.div", "l.divu") instructions.
This default is hardware divide.
-msoft-mul
-mhard-mul
Select software or hardware multiply ("l.mul", "l.muli")
instructions. This default is hardware multiply.
-msoft-float
-mhard-float
Select software or hardware for floating point operations. The
default is software.
-mdouble-float
When -mhard-float is selected, enables generation of double-precision
floating point instructions. By default functions from libgcc are
used to perform double-precision floating point operations.
-munordered-float
When -mhard-float is selected, enables generation of unordered
floating point compare and set flag ("lf.sfun*") instructions. By
default functions from libgcc are used to perform unordered floating
point compare and set flag operations.
-mcmov
Enable generation of conditional move ("l.cmov") instructions. By
default the equivalent will be generated using set and branch.
-mror
Enable generation of rotate right ("l.ror") instructions. By default
functions from libgcc are used to perform rotate right operations.
-mrori
Enable generation of rotate right with immediate ("l.rori")
instructions. By default functions from libgcc are used to perform
rotate right with immediate operations.
-msext
Enable generation of sign extension ("l.ext*") instructions. By
default memory loads are used to perform sign extension.
-msfimm
Enable generation of compare and set flag with immediate ("l.sf*i")
instructions. By default extra instructions will be generated to
store the immediate to a register first.
-mshftimm
Enable generation of shift with immediate ("l.srai", "l.srli",
"l.slli") instructions. By default extra instructions will be
generated to store the immediate to a register first.
-mcmodel=small
Generate OpenRISC code for the small model: The GOT is limited to
64k. This is the default model.
-mcmodel=large
Generate OpenRISC code for the large model: The GOT may grow up to 4G
in size.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS floating
point on the PDP-11/40 is not supported.) Implies -m45.
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler syntax).
-mno-ac0
Return floating-point results in memory. This is the default.
-m40
Generate code for a PDP-11/40. Implies -msoft-float -mno-split.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10. Implies -msoft-float -mno-split.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-msplit
Target has split instruction and data space. Implies -m45.
-munix-asm
Use Unix assembler syntax.
-mdec-asm
Use DEC assembler syntax.
-mgnu-asm
Use GNU assembler syntax. This is the default.
-mlra
Use the new LRA register allocator. By default, the old "reload"
allocator is used.
PowerPC Options
These are listed under
PRU Options
These command-line options are defined for PRU target:
-minrt
Link with a minimum runtime environment. This can significantly
reduce the size of the final ELF binary, but some standard C runtime
features are removed.
This option disables support for static initializers and
constructors. Beware that the compiler could still generate code
with static initializers and constructors. It is up to the
programmer to ensure that the source program will not use those
features.
The minimal startup code would not pass "argc" and "argv" arguments
to "main", so the latter must be declared as "int main (void)". This
is already the norm for most firmware projects.
-mmcu=mcu
Specify the PRU hardware variant to use. A correspondingly named
spec file would be loaded, passing the memory region sizes to the
linker and defining hardware-specific C macros.
Newlib provides only the "sim" spec, intended for running regression
tests using a simulator. Specs for real hardware can be obtained by
installing the GnuPruMcu ("https://github.com/dinuxbg/gnuprumcu/")
package.
-mno-relax
Make GCC pass the --no-relax command-line option to the linker
instead of the --relax option.
-mloop
Allow (or do not allow) GCC to use the LOOP instruction.
-mabi=variant
Specify the ABI variant to output code for. -mabi=ti selects the
unmodified TI ABI while -mabi=gnu selects a GNU variant that copes
more naturally with certain GCC assumptions. These are the
differences:
Function Pointer Size
TI ABI specifies that function (code) pointers are 16-bit,
whereas GNU supports only 32-bit data and code pointers.
Optional Return Value Pointer
Function return values larger than 64 bits are passed by using a
hidden pointer as the first argument of the function. TI ABI,
though, mandates that the pointer can be NULL in case the caller
is not using the returned value. GNU always passes and expects a
valid return value pointer.
The current -mabi=ti implementation simply raises a compile error
when any of the above code constructs is detected. As a consequence
the standard C library cannot be built and it is omitted when linking
with -mabi=ti.
Relaxation is a GNU feature and for safety reasons is disabled when
using -mabi=ti. The TI toolchain does not emit relocations for QBBx
instructions, so the GNU linker cannot adjust them when shortening
adjacent LDI32 pseudo instructions.
RISC-V Options
These command-line options are defined for RISC-V targets:
-mbranch-cost=n
Set the cost of branches to roughly n instructions.
-mplt
-mno-plt
When generating PIC code, do or don't allow the use of PLTs. Ignored
for non-PIC. The default is -mplt.
-mabi=ABI-string
Specify integer and floating-point calling convention. ABI-string
contains two parts: the size of integer types and the registers used
for floating-point types. For example -march=rv64ifd -mabi=lp64d
means that long and pointers are 64-bit (implicitly defining int to
be 32-bit), and that floating-point values up to 64 bits wide are
passed in F registers. Contrast this with -march=rv64ifd
-mabi=lp64f, which still allows the compiler to generate code that
uses the F and D extensions but only allows floating-point values up
to 32 bits long to be passed in registers; or -march=rv64ifd
-mabi=lp64, in which no floating-point arguments will be passed in
registers.
The default for this argument is system dependent, users who want a
specific calling convention should specify one explicitly. The valid
calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and
lp64d. Some calling conventions are impossible to implement on some
ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the
ABI requires 64-bit values be passed in F registers, but F registers
are only 32 bits wide. There are also the ilp32e ABI that can only
be used with the rv32e architecture and the lp64e ABI that can only
be used with the rv64e. Those ABIs are not well specified at
present, and are subject to change.
-mfdiv
-mno-fdiv
Do or don't use hardware floating-point divide and square root
instructions. This requires the F or D extensions for floating-point
registers. The default is to use them if the specified architecture
has these instructions.
-mfence-tso
-mno-fence-tso
Do or don't use the fence.tso instruction, which is unimplemented on
some processors (including those from T-Head). If the fence.tso
instruction is not available then a stronger fence is used instead.
-mdiv
-mno-div
Do or don't use hardware instructions for integer division. This
requires the M extension. The default is to use them if the
specified architecture has these instructions.
-misa-spec=ISA-spec-string
Specify the version of the RISC-V Unprivileged (formerly User-Level)
ISA specification to produce code conforming to. The possibilities
for ISA-spec-string are:
2.2 Produce code conforming to version 2.2.
20190608
Produce code conforming to version 20190608.
20191213
Produce code conforming to version 20191213.
The default is -misa-spec=20191213 unless GCC has been configured
with --with-isa-spec= specifying a different default version.
-march=ISA-string
Generate code for given RISC-V ISA (e.g. rv64im). ISA strings must
be lower-case. Examples include rv64i, rv32g, rv32e, and rv32imaf.
Additionally, a special value help (-march=help) is accepted to list
all supported extensions.
The syntax of the ISA string is defined as follows:
"The string must start with rv32 or rv64, followed by"
i, e, or g, referred to as the base ISA.
"The subsequent part of the string is a list of extension names.
Extension"
names can be categorized as multi-letter (e.g. zba) and single-
letter (e.g. v). Single-letter extensions can appear
consecutively, but multi-letter extensions must be separated by
underscores.
"An underscore can appear anywhere after the base ISA. It has no
specific"
effect but is used to improve readability and can act as a
separator.
"Extension names may include an optional version number, following
the"
syntax <major>p<minor> or <major>, (e.g. m2p1 or m2).
Supported extension are listed below:
Extension Name : Supported Version : Description
i @tab 2.0, 2.1 @tab Base integer extension.
e @tab 2.0 @tab Reduced base integer extension.
g @tab - @tab General-purpose computing base extension, g will
expand to i, m, a, f, d, zicsr and zifencei.
m @tab 2.0 @tab Integer multiplication and division extension.
a @tab 2.0, 2.1 @tab Atomic extension.
f @tab 2.0, 2.2 @tab Single-precision floating-point extension.
d @tab 2.0, 2.2 @tab Double-precision floating-point extension.
c @tab 2.0 @tab Compressed extension.
h @tab 1.0 @tab Hypervisor extension.
v @tab 1.0 @tab Vector extension.
zicsr
@tab 2.0 @tab Control and status register access extension.
zifencei
@tab 2.0 @tab Instruction-fetch fence extension.
zicond
@tab 1.0 @tab Integer conditional operations extension.
za64rs
@tab 1.0 @tab Reservation set size of 64 bytes.
za128rs
@tab 1.0 @tab Reservation set size of 128 bytes.
zawrs
@tab 1.0 @tab Wait-on-reservation-set extension.
zba @tab 1.0 @tab Address calculation extension.
zbb @tab 1.0 @tab Basic bit manipulation extension.
zbc @tab 1.0 @tab Carry-less multiplication extension.
zbs @tab 1.0 @tab Single-bit operation extension.
zfinx
@tab 1.0 @tab Single-precision floating-point in integer
registers extension.
zdinx
@tab 1.0 @tab Double-precision floating-point in integer
registers extension.
zhinx
@tab 1.0 @tab Half-precision floating-point in integer registers
extension.
zhinxmin
@tab 1.0 @tab Minimal half-precision floating-point in integer
registers extension.
zbkb
@tab 1.0 @tab Cryptography bit-manipulation extension.
zbkc
@tab 1.0 @tab Cryptography carry-less multiply extension.
zbkx
@tab 1.0 @tab Cryptography crossbar permutation extension.
zkne
@tab 1.0 @tab AES Encryption extension.
zknd
@tab 1.0 @tab AES Decryption extension.
zknh
@tab 1.0 @tab Hash function extension.
zkr @tab 1.0 @tab Entropy source extension.
zksed
@tab 1.0 @tab SM4 block cipher extension.
zksh
@tab 1.0 @tab SM3 hash function extension.
zkt @tab 1.0 @tab Data independent execution latency extension.
zk @tab 1.0 @tab Standard scalar cryptography extension.
zkn @tab 1.0 @tab NIST algorithm suite extension.
zks @tab 1.0 @tab ShangMi algorithm suite extension.
zihintntl
@tab 1.0 @tab Non-temporal locality hints extension.
zihintpause
@tab 1.0 @tab Pause hint extension.
zicboz
@tab 1.0 @tab Cache-block zero extension.
zicbom
@tab 1.0 @tab Cache-block management extension.
zicbop
@tab 1.0 @tab Cache-block prefetch extension.
zic64b
@tab 1.0 @tab Cache block size isf 64 bytes.
ziccamoa
@tab 1.0 @tab Main memory supports all atomics in A.
ziccif
@tab 1.0 @tab Main memory supports instruction fetch with
atomicity requirement.
zicclsm
@tab 1.0 @tab Main memory supports misaligned loads/stores.
ziccrse
@tab 1.0 @tab Main memory supports forward progress on LR/SC
sequences.
zicntr
@tab 2.0 @tab Standard extension for base counters and timers.
zihpm
@tab 2.0 @tab Standard extension for hardware performance
counters.
ztso
@tab 1.0 @tab Total store ordering extension.
zve32x
@tab 1.0 @tab Vector extensions for embedded processors.
zve32f
@tab 1.0 @tab Vector extensions for embedded processors.
zve64x
@tab 1.0 @tab Vector extensions for embedded processors.
zve64f
@tab 1.0 @tab Vector extensions for embedded processors.
zve64d
@tab 1.0 @tab Vector extensions for embedded processors.
zvl32b
@tab 1.0 @tab Minimum vector length standard extensions
zvl64b
@tab 1.0 @tab Minimum vector length standard extensions
zvl128b
@tab 1.0 @tab Minimum vector length standard extensions
zvl256b
@tab 1.0 @tab Minimum vector length standard extensions
zvl512b
@tab 1.0 @tab Minimum vector length standard extensions
zvl1024b
@tab 1.0 @tab Minimum vector length standard extensions
zvl2048b
@tab 1.0 @tab Minimum vector length standard extensions
zvl4096b
@tab 1.0 @tab Minimum vector length standard extensions
zvbb
@tab 1.0 @tab Vector basic bit-manipulation extension.
zvbc
@tab 1.0 @tab Vector carryless multiplication extension.
zvkb
@tab 1.0 @tab Vector cryptography bit-manipulation extension.
zvkg
@tab 1.0 @tab Vector GCM/GMAC extension.
zvkned
@tab 1.0 @tab Vector AES block cipher extension.
zvknha
@tab 1.0 @tab Vector SHA-2 secure hash extension.
zvknhb
@tab 1.0 @tab Vector SHA-2 secure hash extension.
zvksed
@tab 1.0 @tab Vector SM4 Block Cipher extension.
zvksh
@tab 1.0 @tab Vector SM3 Secure Hash extension.
zvkn
@tab 1.0 @tab Vector NIST Algorithm Suite extension, zvkn will
expand to zvkned, zvknhb, zvkb and zvkt.
zvknc
@tab 1.0 @tab Vector NIST Algorithm Suite with carryless multiply
extension, zvknc will expand to zvkn and zvbc.
zvkng
@tab 1.0 @tab Vector NIST Algorithm Suite with GCM extension,
zvkng will expand to zvkn and zvkg.
zvks
@tab 1.0 @tab Vector ShangMi algorithm suite extension, zvks will
expand to zvksed, zvksh, zvkb and zvkt.
zvksc
@tab 1.0 @tab Vector ShangMi algorithm suite with carryless
multiplication extension, zvksc will expand to zvks and zvbc.
zvksg
@tab 1.0 @tab Vector ShangMi algorithm suite with GCM extension,
zvksg will expand to zvks and zvkg.
zvkt
@tab 1.0 @tab Vector data independent execution latency
extension.
zfh @tab 1.0 @tab Half-precision floating-point extension.
zfhmin
@tab 1.0 @tab Minimal half-precision floating-point extension.
zvfh
@tab 1.0 @tab Vector half-precision floating-point extension.
zvfhmin
@tab 1.0 @tab Vector minimal half-precision floating-point
extension.
zvfbfmin
@tab 1.0 @tab Vector BF16 converts extension.
zfa @tab 1.0 @tab Additional floating-point extension.
zmmul
@tab 1.0 @tab Integer multiplication extension.
zca @tab 1.0 @tab Integer compressed instruction extension.
zcf @tab 1.0 @tab Compressed single-precision floating point loads
and stores extension.
zcd @tab 1.0 @tab Compressed double-precision floating point loads
and stores extension.
zcb @tab 1.0 @tab Simple compressed instruction extension.
zce @tab 1.0 @tab Compressed instruction extensions for embedded
processors.
zcmp
@tab 1.0 @tab Compressed push pop extension.
zcmt
@tab 1.0 @tab Table jump instruction extension.
smaia
@tab 1.0 @tab Advanced interrupt architecture extension.
smepmp
@tab 1.0 @tab PMP Enhancements for memory access and execution
prevention on Machine mode.
smstateen
@tab 1.0 @tab State enable extension.
ssaia
@tab 1.0 @tab Advanced interrupt architecture extension for
supervisor-mode.
sscofpmf
@tab 1.0 @tab Count overflow & filtering extension.
ssstateen
@tab 1.0 @tab State-enable extension for supervisor-mode.
sstc
@tab 1.0 @tab Supervisor-mode timer interrupts extension.
svinval
@tab 1.0 @tab Fine-grained address-translation cache invalidation
extension.
svnapot
@tab 1.0 @tab NAPOT translation contiguity extension.
svpbmt
@tab 1.0 @tab Page-based memory types extension.
xcvmac
@tab 1.0 @tab Core-V multiply-accumulate extension.
xcvalu
@tab 1.0 @tab Core-V miscellaneous ALU extension.
xcvelw
@tab 1.0 @tab Core-V event load word extension.
xtheadba
@tab 1.0 @tab T-head address calculation extension.
xtheadbb
@tab 1.0 @tab T-head basic bit-manipulation extension.
xtheadbs
@tab 1.0 @tab T-head single-bit instructions extension.
xtheadcmo
@tab 1.0 @tab T-head cache management operations extension.
xtheadcondmov
@tab 1.0 @tab T-head conditional move extension.
xtheadfmemidx
@tab 1.0 @tab T-head indexed memory operations for floating-point
registers extension.
xtheadfmv
@tab 1.0 @tab T-head double floating-point high-bit data
transmission extension.
xtheadint
@tab 1.0 @tab T-head acceleration interruption extension.
xtheadmac
@tab 1.0 @tab T-head multiply-accumulate extension.
xtheadmemidx
@tab 1.0 @tab T-head indexed memory operation extension.
xtheadmempair
@tab 1.0 @tab T-head two-GPR memory operation extension.
xtheadsync
@tab 1.0 @tab T-head multi-core synchronization extension.
xventanacondops
@tab 1.0 @tab Ventana integer conditional operations extension.
When -march= is not specified, use the setting from -mcpu.
If both -march and -mcpu= are not specified, the default for this
argument is system dependent, users who want a specific architecture
extensions should specify one explicitly.
When the RISC-V specifications define an extension as depending on
other extensions, GCC will implicitly add the dependent extensions to
the enabled extension set if they weren't added explicitly.
-mcpu=processor-string
Use architecture of and optimize the output for the given processor,
specified by particular CPU name. Permissible values for this option
are: sifive-e20, sifive-e21, sifive-e24, sifive-e31, sifive-e34,
sifive-e76, sifive-s21, sifive-s51, sifive-s54, sifive-s76,
sifive-u54, sifive-u74, sifive-x280, sifive-xp450, sifive-x670,
thead-c906, tt-ascalon-d8, xiangshan-nanhu.
Note that -mcpu does not override -march or -mtune.
-mtune=processor-string
Optimize the output for the given processor, specified by
microarchitecture or particular CPU name. Permissible values for
this option are: generic-ooo, rocket, sifive-3-series,
sifive-5-series, sifive-7-series, size, sifive-p400-series,
sifive-p600-series, and all valid options for -mcpu=.
When -mtune= is not specified, use the setting from -mcpu, the
default is rocket if both are not specified.
The size choice is not intended for use by end-users. This is used
when -Os is specified. It overrides the instruction cost info
provided by -mtune=, but does not override the pipeline info. This
helps reduce code size while still giving good performance.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte
boundary. If -mpreferred-stack-boundary is not specified, the
default is 4 (16 bytes or 128-bits).
Warning: If you use this switch, then you must build all modules with
the same value, including any libraries. This includes the system
libraries and startup modules.
-msmall-data-limit=n
Put global and static data smaller than n bytes into a special
section (on some targets).
-msave-restore
-mno-save-restore
Do or don't use smaller but slower prologue and epilogue code that
uses library function calls. The default is to use fast inline
prologues and epilogues.
-mmovcc
-mno-movcc
Do or don't produce branchless conditional-move code sequences even
with targets that do not have specific instructions for conditional
operations. If enabled, sequences of ALU operations are produced
using base integer ISA instructions where profitable.
-minline-atomics
-mno-inline-atomics
Do or don't use smaller but slower subword atomic emulation code that
uses libatomic function calls. The default is to use fast inline
subword atomics that do not require libatomic.
-minline-strlen
-mno-inline-strlen
Do or do not attempt to inline strlen calls if possible. Inlining
will only be done if the string is properly aligned and instructions
for accelerated processing are available. The default is to not
inline strlen calls.
-minline-strcmp
-mno-inline-strcmp
Do or do not attempt to inline strcmp calls if possible. Inlining
will only be done if the strings are properly aligned and
instructions for accelerated processing are available. The default
is to not inline strcmp calls.
The --param riscv-strcmp-inline-limit=n parameter controls the
maximum number of bytes compared by the inlined code. The default
value is 64.
-minline-strncmp
-mno-inline-strncmp
Do or do not attempt to inline strncmp calls if possible. Inlining
will only be done if the strings are properly aligned and
instructions for accelerated processing are available. The default
is to not inline strncmp calls.
The --param riscv-strcmp-inline-limit=n parameter controls the
maximum number of bytes compared by the inlined code. The default
value is 64.
-mshorten-memrefs
-mno-shorten-memrefs
Do or do not attempt to make more use of compressed load/store
instructions by replacing a load/store of 'base register + large
offset' with a new load/store of 'new base + small offset'. If the
new base gets stored in a compressed register, then the new
load/store can be compressed. Currently targets 32-bit integer
load/stores only.
-mstrict-align
-mno-strict-align
Do not or do generate unaligned memory accesses. The default is set
depending on whether the processor we are optimizing for supports
fast unaligned access or not.
-mscalar-strict-align
-mno-scalar-strict-align
Do not or do generate unaligned memory accesses. The default is set
depending on whether the processor we are optimizing for supports
fast unaligned access or not. This is an alias for -mstrict-align.
-mvector-strict-align
-mno-vector-strict-align
Do not or do generate unaligned vector memory accesses. The default
is set to off unless the processor we are optimizing for explicitly
supports element-misaligned vector memory access.
-mcmodel=medlow
Generate code for the medium-low code model. The program and its
statically defined symbols must lie within a single 2 GiB address
range and must lie between absolute addresses -2 GiB and +2 GiB.
Programs can be statically or dynamically linked. This is the default
code model unless GCC has been configured with --with-cmodel=
specifying a different default code model.
-mcmodel=medany
Generate code for the medium-any code model. The program and its
statically defined symbols must be within any single 2 GiB address
range. Programs can be statically or dynamically linked.
The code generated by the medium-any code model is position-
independent, but is not guaranteed to function correctly when linked
into position-independent executables or libraries.
-mcmodel=large
Generate code for a large code model, which has no restrictions on
size or placement of symbols.
-mexplicit-relocs
-mno-exlicit-relocs
Use or do not use assembler relocation operators when dealing with
symbolic addresses. The alternative is to use assembler macros
instead, which may limit optimization.
-mrelax
-mno-relax
Take advantage of linker relaxations to reduce the number of
instructions required to materialize symbol addresses. The default is
to take advantage of linker relaxations.
-mriscv-attribute
-mno-riscv-attribute
Emit (do not emit) RISC-V attribute to record extra information into
ELF objects. This feature requires at least binutils 2.32.
-mcsr-check
-mno-csr-check
Enables or disables the CSR checking.
-malign-data=type
Control how GCC aligns variables and constants of array, structure,
or union types. Supported values for type are xlen which uses x
register width as the alignment value, and natural which uses natural
alignment. xlen is the default.
-mbig-endian
Generate big-endian code. This is the default when GCC is configured
for a riscv64be-*-* or riscv32be-*-* target.
-mlittle-endian
Generate little-endian code. This is the default when GCC is
configured for a riscv64-*-* or riscv32-*-* but not a riscv64be-*-*
or riscv32be-*-* target.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported
locations are global for a global canary or tls for per-thread canary
in the TLS block.
With the latter choice the options -mstack-protector-guard-reg=reg
and -mstack-protector-guard-offset=offset furthermore specify which
register to use as base register for reading the canary, and from
what offset from that base register. There is no default register or
offset as this is entirely for use within the Linux kernel.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for dynamic
accesses of TLS variables.
-mtls-dialect=trad
Use traditional TLS as the thread-local storage mechanism for dynamic
accesses of TLS variables. This is the default.
RL78 Options
-msim
Links in additional target libraries to support operation within a
simulator.
-mmul=none
-mmul=g10
-mmul=g13
-mmul=g14
-mmul=rl78
Specifies the type of hardware multiplication and division support to
be used. The simplest is "none", which uses software for both
multiplication and division. This is the default. The "g13" value
is for the hardware multiply/divide peripheral found on the RL78/G13
(S2 core) targets. The "g14" value selects the use of the
multiplication and division instructions supported by the RL78/G14
(S3 core) parts. The value "rl78" is an alias for "g14" and the
value "mg10" is an alias for "none".
In addition a C preprocessor macro is defined, based upon the setting
of this option. Possible values are: "__RL78_MUL_NONE__",
"__RL78_MUL_G13__" or "__RL78_MUL_G14__".
-mcpu=g10
-mcpu=g13
-mcpu=g14
-mcpu=rl78
Specifies the RL78 core to target. The default is the G14 core, also
known as an S3 core or just RL78. The G13 or S2 core does not have
multiply or divide instructions, instead it uses a hardware
peripheral for these operations. The G10 or S1 core does not have
register banks, so it uses a different calling convention.
If this option is set it also selects the type of hardware multiply
support to use, unless this is overridden by an explicit -mmul=none
option on the command line. Thus specifying -mcpu=g13 enables the
use of the G13 hardware multiply peripheral and specifying -mcpu=g10
disables the use of hardware multiplications altogether.
Note, although the RL78/G14 core is the default target, specifying
-mcpu=g14 or -mcpu=rl78 on the command line does change the behavior
of the toolchain since it also enables G14 hardware multiply support.
If these options are not specified on the command line then software
multiplication routines will be used even though the code targets the
RL78 core. This is for backwards compatibility with older toolchains
which did not have hardware multiply and divide support.
In addition a C preprocessor macro is defined, based upon the setting
of this option. Possible values are: "__RL78_G10__", "__RL78_G13__"
or "__RL78_G14__".
-mg10
-mg13
-mg14
-mrl78
These are aliases for the corresponding -mcpu= option. They are
provided for backwards compatibility.
-mallregs
Allow the compiler to use all of the available registers. By default
registers "r24..r31" are reserved for use in interrupt handlers.
With this option enabled these registers can be used in ordinary
functions as well.
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits
(-m32bit-doubles) in size. The default is -m32bit-doubles.
-msave-mduc-in-interrupts
-mno-save-mduc-in-interrupts
Specifies that interrupt handler functions should preserve the MDUC
registers. This is only necessary if normal code might use the MDUC
registers, for example because it performs multiplication and
division operations. The default is to ignore the MDUC registers as
this makes the interrupt handlers faster. The target option -mg13
needs to be passed for this to work as this feature is only available
on the G13 target (S2 core). The MDUC registers will only be saved
if the interrupt handler performs a multiplication or division
operation or it calls another function.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mhard-dfp
-mno-hard-dfp
You use these options to specify which instructions are available on
the processor you are using. The default value of these options is
determined when configuring GCC. Specifying the -mcpu=cpu_type
overrides the specification of these options. We recommend you use
the -mcpu=cpu_type option rather than the options listed above.
Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC
architecture instructions in the General Purpose group, including
floating-point square root. Specifying -mpowerpc-gfxopt allows GCC
to use the optional PowerPC architecture instructions in the Graphics
group, including floating-point select.
The -mmfcrf option allows GCC to generate the move from condition
register field instruction implemented on the POWER4 processor and
other processors that support the PowerPC V2.01 architecture. The
-mpopcntb option allows GCC to generate the popcount and double-
precision FP reciprocal estimate instruction implemented on the
POWER5 processor and other processors that support the PowerPC V2.02
architecture. The -mpopcntd option allows GCC to generate the
popcount instruction implemented on the POWER7 processor and other
processors that support the PowerPC V2.06 architecture. The -mfprnd
option allows GCC to generate the FP round to integer instructions
implemented on the POWER5+ processor and other processors that
support the PowerPC V2.03 architecture. The -mcmpb option allows GCC
to generate the compare bytes instruction implemented on the POWER6
processor and other processors that support the PowerPC V2.05
architecture. The -mhard-dfp option allows GCC to generate the
decimal floating-point instructions implemented on some POWER
processors.
The -mpowerpc64 option allows GCC to generate the additional 64-bit
instructions that are found in the full PowerPC64 architecture and to
treat GPRs as 64-bit, doubleword quantities. GCC defaults to
-mno-powerpc64.
-mcpu=cpu_type
Set architecture type, register usage, and instruction scheduling
parameters for machine type cpu_type. Supported values for cpu_type
are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505,
601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801,
821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64,
e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5,
power5+, power6, power6x, power7, power8, power9, power10, power11,
powerpc, powerpc64, powerpc64le, rs64, and native.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure
32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit
little endian PowerPC architecture machine types, with an
appropriate, generic processor model assumed for scheduling purposes.
Specifying native as cpu type detects and selects the architecture
option that corresponds to the host processor of the system
performing the compilation. -mcpu=native has no effect if GCC does
not recognize the processor.
The other options specify a specific processor. Code generated under
those options runs best on that processor, and may not run at all on
others.
The -mcpu options automatically enable or disable the following
options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb
-mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw
-mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion
-mquad-memory -mquad-memory-atomic -mfloat128 -mfloat128-hardware
-mprefixed -mpcrel -mmma -mrop-protect
The particular options set for any particular CPU varies between
compiler versions, depending on what setting seems to produce optimal
code for that CPU; it doesn't necessarily reflect the actual
hardware's capabilities. If you wish to set an individual option to
a particular value, you may specify it after the -mcpu option, like
-mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or
disabled by the -mcpu option at present because AIX does not have
full support for these options. You may still enable or disable them
individually if you're sure it'll work in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type,
but do not set the architecture type or register usage, as
-mcpu=cpu_type does. The same values for cpu_type are used for
-mtune as for -mcpu. If both are specified, the code generated uses
the architecture and registers set by -mcpu, but the scheduling
parameters set by -mtune.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited to
64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other
static data may be up to a total of 4G in size. This is the default
for 64-bit Linux.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to 4G
in size. Other data and code is only limited by the 64-bit address
space.
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and also
enable the use of built-in functions that allow more direct access to
the AltiVec instruction set. You may also need to set -mabi=altivec
to adjust the current ABI with AltiVec ABI enhancements.
When -maltivec is used, the element order for AltiVec intrinsics such
as "vec_splat", "vec_extract", and "vec_insert" match array element
order corresponding to the endianness of the target. That is,
element zero identifies the leftmost element in a vector register
when targeting a big-endian platform, and identifies the rightmost
element in a vector register when targeting a little-endian platform.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so to build executables and
shared libraries with non-executable ".plt" and ".got" sections.
This is a PowerPC 32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS ".plt" section that ld.so fills in, and
requires ".plt" and ".got" sections that are both writable and
executable. This is a PowerPC 32-bit SYSV ABI option.
-msplit-patch-nops
When adding NOPs for a patchable area via the
-fpatchable-function-entry option emit the "before" NOPs in front of
the global entry point and the "after" NOPs after the local entry
point. This makes the sequence of NOPs not consecutive when a global
entry point is generated. Without this option the NOPs are emitted
directly before and after the local entry point, making them
consecutive but moving global and local entry point further apart.
If only a single entry point is generated this option has no effect.
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
-mvsx
-mno-vsx
Generate code that uses (does not use) vector/scalar (VSX)
instructions, and also enable the use of built-in functions that
allow more direct access to the VSX instruction set.
-mcrypto
-mno-crypto
Enable the use (disable) of the built-in functions that allow direct
access to the cryptographic instructions that were added in version
2.07 of the PowerPC ISA.
-mhtm
-mno-htm
Enable (disable) the use of the built-in functions that allow direct
access to the Hardware Transactional Memory (HTM) instructions that
were added in version 2.07 of the PowerPC ISA.
-mpower8-fusion
-mno-power8-fusion
Generate code that keeps (does not keeps) some integer operations
adjacent so that the instructions can be fused together on power8 and
later processors.
-mquad-memory
-mno-quad-memory
Generate code that uses (does not use) the non-atomic quad word
memory instructions. The -mquad-memory option requires use of 64-bit
mode.
-mquad-memory-atomic
-mno-quad-memory-atomic
Generate code that uses (does not use) the atomic quad word memory
instructions. The -mquad-memory-atomic option requires use of 64-bit
mode.
-mfloat128
-mno-float128
Enable/disable the __float128 keyword for IEEE 128-bit floating point
and use either software emulation for IEEE 128-bit floating point or
hardware instructions.
The VSX instruction set (-mvsx) must be enabled to use the IEEE
128-bit floating point support. The IEEE 128-bit floating point is
only supported on Linux.
The default for -mfloat128 is enabled on PowerPC Linux systems using
the VSX instruction set, and disabled on other systems.
If you use the ISA 3.0 instruction set (-mcpu=power9) on a 64-bit
system, the IEEE 128-bit floating point support will also enable the
generation of ISA 3.0 IEEE 128-bit floating point instructions.
Otherwise, if you do not specify to generate ISA 3.0 instructions or
you are targeting a 32-bit big endian system, IEEE 128-bit floating
point will be done with software emulation.
-mfloat128-hardware
-mno-float128-hardware
Enable/disable using ISA 3.0 hardware instructions to support the
__float128 data type.
The default for -mfloat128-hardware is enabled on PowerPC Linux
systems using the ISA 3.0 instruction set, and disabled on other
systems.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4
targets (including GNU/Linux). The 32-bit environment sets int, long
and pointer to 32 bits and generates code that runs on any PowerPC
variant. The 64-bit environment sets int to 32 bits and long and
pointer to 64 bits, and generates code for PowerPC64, as for
-mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created
for every executable file. The -mfull-toc option is selected by
default. In that case, GCC allocates at least one TOC entry for each
unique non-automatic variable reference in your program. GCC also
places floating-point constants in the TOC. However, only 16,384
entries are available in the TOC.
If you receive a linker error message that saying you have overflowed
the available TOC space, you can reduce the amount of TOC space used
with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc
prevents GCC from putting floating-point constants in the TOC and
-mno-sum-in-toc forces GCC to generate code to calculate the sum of
an address and a constant at run time instead of putting that sum
into the TOC. You may specify one or both of these options. Each
causes GCC to produce very slightly slower and larger code at the
expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both
of these options, specify -mminimal-toc instead. This option causes
GCC to make only one TOC entry for every file. When you specify this
option, GCC produces code that is slower and larger but which uses
extremely little TOC space. You may wish to use this option only on
files that contain less frequently-executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit
"long" type, and the infrastructure needed to support them.
Specifying -maix64 implies -mpowerpc64, while -maix32 disables the
64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics
when using AIX-compatible ABI. Pass floating-point arguments to
prototyped functions beyond the register save area (RSA) on the stack
in addition to argument FPRs. Do not assume that most significant
double in 128-bit long double value is properly rounded when
comparing values and converting to double. Use XL symbol names for
long double support routines.
The AIX calling convention was extended but not initially documented
to handle an obscure K&R C case of calling a function that takes the
address of its arguments with fewer arguments than declared. IBM XL
compilers access floating-point arguments that do not fit in the RSA
from the stack when a subroutine is compiled without optimization.
Because always storing floating-point arguments on the stack is
inefficient and rarely needed, this option is not enabled by default
and only is necessary when calling subroutines compiled by IBM XL
compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an
application written to use message passing with special startup code
to enable the application to run. The system must have PE installed
in the standard location (/usr/lpp/ppe.poe/), or the specs file must
be overridden with the -specs= option to specify the appropriate
directory location. The Parallel Environment does not support
threads, so the -mpe option and the -pthread option are incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
-malign-natural overrides the ABI-defined alignment of larger types,
such as floating-point doubles, on their natural size-based boundary.
The option -malign-power instructs GCC to follow the ABI-specified
alignment rules. GCC defaults to the standard alignment defined in
the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power
is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register
set. Software floating-point emulation is provided if you use the
-msoft-float option, and pass the option to GCC when linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and not
generated on PowerPC systems. Do not use -mmultiple on little-endian
PowerPC systems, since those instructions do not work when the
processor is in little-endian mode. The exceptions are PPC740 and
PPC750 which permit these instructions in little-endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions
that update the base register to the address of the calculated memory
location. These instructions are generated by default. If you use
-mno-update, there is a small window between the time that the stack
pointer is updated and the address of the previous frame is stored,
which means code that walks the stack frame across interrupts or
signals may get corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed load
or store instructions. These instructions can incur a performance
penalty on Power6 processors in certain situations, such as when
stepping through large arrays that cross a 16M boundary. This option
is enabled by default when targeting Power6 and disabled otherwise.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply
and accumulate instructions. These instructions are generated by
default if hardware floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-independent
-ffp-contract=fast option, and -mno-fused-madd is mapped to
-ffp-contract=off.
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and
multiply-accumulate instructions on the IBM 405, 440, 464 and 476
processors. These instructions are generated by default when
targeting those processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb
instruction on the IBM 405, 440, 464 and 476 processors. This
instruction is generated by default when targeting those processors.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force
structures and unions that contain bit-fields to be aligned to the
base type of the bit-field.
For example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 is aligned to a 4-byte boundary and
has a size of 4 bytes. By using -mno-bit-align, the structure is
aligned to a 1-byte boundary and is 1 byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that
unaligned memory references are handled by the system.
-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static executable to be
relocated to a different address at run time. A simple embedded
PowerPC system loader should relocate the entire contents of ".got2"
and 4-byte locations listed in the ".fixup" section, a table of
32-bit addresses generated by this option. For this to work, all
objects linked together must be compiled with -mrelocatable or
-mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte
boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section to
allow static executables to be relocated at run time, but
-mrelocatable-lib does not use the smaller stack alignment of
-mrelocatable. Objects compiled with -mrelocatable-lib may be linked
with objects compiled with any combination of the -mrelocatable
options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that
register 2 contains a pointer to a global area pointing to the
addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in little-endian mode. The -mlittle-endian option is the
same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in big-endian mode. The -mbig-endian option is the same as
-mbig.
-mdynamic-no-pic
On Darwin / macOS systems, compile code so that it is not
relocatable, but that its external references are relocatable. The
resulting code is suitable for applications, but not shared
libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than
loading it in the prologue for each function. The runtime system is
responsible for initializing this register with an appropriate value
before execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot
restricted instructions during the second scheduling pass. The
argument priority takes the value 0, 1, or 2 to assign no, highest,
or second-highest (respectively) priority to dispatch-slot restricted
instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the
target during instruction scheduling. The argument dependence_type
takes one of the following values:
no No dependence is costly.
all All dependences are costly.
true_store_to_load
A true dependence from store to load is costly.
store_to_load
Any dependence from store to load is costly.
number
Any dependence for which the latency is greater than or equal to
number is costly.
-minsert-sched-nops=scheme
This option controls which NOP insertion scheme is used during the
second scheduling pass. The argument scheme takes one of the
following values:
no Don't insert NOPs.
pad Pad with NOPs any dispatch group that has vacant issue slots,
according to the scheduler's grouping.
regroup_exact
Insert NOPs to force costly dependent insns into separate groups.
Insert exactly as many NOPs as needed to force an insn to a new
group, according to the estimated processor grouping.
number
Insert NOPs to force costly dependent insns into separate groups.
Insert number NOPs to force an insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling
conventions that adhere to the March 1995 draft of the System V
Application Binary Interface, PowerPC processor supplement. This is
the default unless you configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX
operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the
FreeBSD operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the
NetBSD operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the
OpenBSD operating system.
-mtraceback=traceback_type
Select the type of traceback table. Valid values for traceback_type
are full, part, and no.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by
the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove such
extension. Valid values are: altivec, no-altivec, ibmlongdouble,
ieeelongdouble, elfv1, elfv2, and for AIX: vec-extabi, vec-default.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double.
This is not likely to work if your system defaults to using IEEE
extended-precision long double. If you change the long double type
from IEEE extended-precision, the compiler issues a warning unless
you use the -Wno-psabi option. Requires -mlong-double-128 to be
enabled.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double.
This is not likely to work if your system defaults to using IBM
extended-precision long double. If you change the long double type
from IBM extended-precision, the compiler issues a warning unless you
use the -Wno-psabi option. Requires -mlong-double-128 to be enabled.
-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is the default ABI
for big-endian PowerPC 64-bit Linux. Overriding the default ABI
requires special system support and is likely to fail in spectacular
ways.
-mabi=elfv2
Change the current ABI to use the ELFv2 ABI. This is the default ABI
for little-endian PowerPC 64-bit Linux. Overriding the default ABI
requires special system support and is likely to fail in spectacular
ways.
-mgnu-attribute
-mno-gnu-attribute
Emit .gnu_attribute assembly directives to set tag/value pairs in a
.gnu.attributes section that specify ABI variations in function
parameters or return values.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to
variable argument functions are properly prototyped. Otherwise, the
compiler must insert an instruction before every non-prototyped call
to set or clear bit 6 of the condition code register ("CR") to
indicate whether floating-point values are passed in the floating-
point registers in case the function takes variable arguments. With
-mprototype, only calls to prototyped variable argument functions set
or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called
sim-crt0.o and that the standard C libraries are libsim.a and libc.a.
This is the default for powerpc-*-eabisim configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called
crt0.o and the standard C libraries are libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called
crt0.o and the standard C libraries are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called
crt0.o and the standard C libraries are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are
compiling for a VxWorks system.
-memb
On embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags
header to indicate that eabi extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the
Embedded Applications Binary Interface (EABI), which is a set of
modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8-byte boundary, a function
"__eabi" is called from "main" to set up the EABI environment, and
the -msdata option can use both "r2" and "r13" to point to two
separate small data areas. Selecting -mno-eabi means that the stack
is aligned to a 16-byte boundary, no EABI initialization function is
called from "main", and the -msdata option only uses "r13" to point
to a single small data area. The -meabi option is on by default if
you configured GCC using one of the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized
"const" global and static data in the ".sdata2" section, which is
pointed to by register "r2". Put small initialized non-"const"
global and static data in the ".sdata" section, which is pointed to
by register "r13". Put small uninitialized global and static data in
the ".sbss" section, which is adjacent to the ".sdata" section. The
-msdata=eabi option is incompatible with the -mrelocatable option.
The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and
static data in the ".sdata" section, which is pointed to by register
"r13". Put small uninitialized global and static data in the ".sbss"
section, which is adjacent to the ".sdata" section. The -msdata=sysv
option is incompatible with the -mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used,
compile code the same as -msdata=eabi, otherwise compile code the
same as -msdata=sysv.
-msdata=data
On System V.4 and embedded PowerPC systems, put small global data in
the ".sdata" section. Put small uninitialized global data in the
".sbss" section. Do not use register "r13" to address small data
however. This is the default behavior unless other -msdata options
are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static
data in the ".data" section, and all uninitialized data in the ".bss"
section.
-mreadonly-in-sdata
Put read-only objects in the ".sdata" section as well. This is the
default.
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or structure
copies) less than or equal to num bytes. The minimum value for num
is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The
default value is target-specific.
-mblock-compare-inline-limit=num
Generate non-looping inline code for all block compares (such as
calls to "memcmp" or structure compares) less than or equal to num
bytes. If num is 0, all inline expansion (non-loop and loop) of block
compare is disabled. The default value is target-specific.
-mblock-compare-inline-loop-limit=num
Generate an inline expansion using loop code for all block compares
that are less than or equal to num bytes, but greater than the limit
for non-loop inline block compare expansion. If the block length is
not constant, at most num bytes will be compared before "memcmp" is
called to compare the remainder of the block. The default value is
target-specific.
-mstring-compare-inline-limit=num
Compare at most num string bytes with inline code. If the difference
or end of string is not found at the end of the inline compare a call
to "strcmp" or "strncmp" will take care of the rest of the
comparison. The default is 64 bytes.
-G num
On embedded PowerPC systems, put global and static items less than or
equal to num bytes into the small data or BSS sections instead of the
normal data or BSS section. By default, num is 8. The -G num switch
is also passed to the linker. All modules should be compiled with
the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register
names in the assembly language output using symbolic forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer and
more expensive calling sequence is required. This is required for
calls farther than 32 megabytes (33,554,432 bytes) from the current
location. A short call is generated if the compiler knows the call
cannot be that far away. This setting can be overridden by the
Some linkers are capable of detecting out-of-range calls and
generating glue code on the fly. On these systems, long calls are
unnecessary and generate slower code. As of this writing, the AIX
linker can do this, as can the GNU linker for PowerPC/64. It is
planned to add this feature to the GNU linker for 32-bit PowerPC
systems as well.
On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers,
GCC can generate long calls using an inline PLT call sequence (see
-mpltseq). PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian)
do not support inline PLT calls.
L42", plus a branch island (glue code). The two target addresses
represent the callee and the branch island. The Darwin/PPC linker
prefers the first address and generates a "bl callee" if the PPC "bl"
instruction reaches the callee directly; otherwise, the linker
generates "bl L42" to call the branch island. The branch island is
appended to the body of the calling function; it computes the full
32-bit address of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to
the glue for every direct call, and the Darwin linker decides whether
to use or discard it.
In the future, GCC may ignore all longcall specifications when the
linker is known to generate glue.
-mpltseq
-mno-pltseq
Implement (do not implement) -fno-plt and long calls using an inline
PLT call sequence that supports lazy linking and long calls to
functions in dlopen'd shared libraries. Inline PLT calls are only
supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer
GNU linkers, and are enabled by default if the support is detected
when configuring GCC, and, in the case of 32-bit PowerPC, if GCC is
configured with --enable-secureplt. -mpltseq code and -mbss-plt
32-bit PowerPC relocatable objects may not be linked together.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation
specifying the function argument. The relocation allows the linker
to reliably associate function call with argument setup instructions
for TLS optimization, which in turn allows GCC to better schedule the
sequence.
-mrecip
-mno-recip
This option enables use of the reciprocal estimate and reciprocal
square root estimate instructions with additional Newton-Raphson
steps to increase precision instead of doing a divide or square root
and divide for floating-point arguments. You should use the
-ffast-math option when using -mrecip (or at least
-funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math
and -fno-trapping-math). Note that while the throughput of the
sequence is generally higher than the throughput of the non-
reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994)
for reciprocal square roots.
-mrecip=opt
This option controls which reciprocal estimate instructions may be
used. opt is a comma-separated list of options, which may be
preceded by a "!" to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the reciprocal approximation instructions for both single
and double precision.
divf
Enable the single-precision reciprocal approximation
instructions.
divd
Enable the double-precision reciprocal approximation
instructions.
rsqrt
Enable the reciprocal square root approximation instructions for
both single and double precision.
rsqrtf
Enable the single-precision reciprocal square root approximation
instructions.
rsqrtd
Enable the double-precision reciprocal square root approximation
instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal
estimate instructions, except for the "FRSQRTE", "XSRSQRTEDP", and
"XVRSQRTEDP" instructions which handle the double-precision
reciprocal square root calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions
provide higher-precision estimates than is mandated by the PowerPC
ABI. Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8
automatically selects -mrecip-precision. The double-precision square
root estimate instructions are not generated by default on low-
precision machines, since they do not provide an estimate that
converges after three steps.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an
external library. The only type supported at present is mass, which
specifies to use IBM's Mathematical Acceleration Subsystem (MASS)
libraries for vectorizing intrinsics using external libraries. GCC
currently emits calls to "acosd2", "acosf4", "acoshd2", "acoshf4",
"asind2", "asinf4", "asinhd2", "asinhf4", "atan2d2", "atan2f4",
"atand2", "atanf4", "atanhd2", "atanhf4", "cbrtd2", "cbrtf4",
"cosd2", "cosf4", "coshd2", "coshf4", "erfcd2", "erfcf4", "erfd2",
"erff4", "exp2d2", "exp2f4", "expd2", "expf4", "expm1d2", "expm1f4",
"hypotd2", "hypotf4", "lgammad2", "lgammaf4", "log10d2", "log10f4",
"log1pd2", "log1pf4", "log2d2", "log2f4", "logd2", "logf4", "powd2",
"powf4", "sind2", "sinf4", "sinhd2", "sinhf4", "sqrtd2", "sqrtf4",
"tand2", "tanf4", "tanhd2", and "tanhf4" when generating code for
power7. Both -ftree-vectorize and -funsafe-math-optimizations must
also be enabled. The MASS libraries must be specified at link time.
-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when the
-funsafe-math-optimizations option is used to optimize rounding of
floating-point values to 64-bit integer and back to floating point.
The "friz" instruction does not return the same value if the
floating-point number is too large to fit in an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain register
("r11") when calling through a pointer on AIX and 64-bit Linux
systems where a function pointer points to a 3-word descriptor giving
the function address, TOC value to be loaded in register "r2", and
static chain value to be loaded in register "r11". The
-mpointers-to-nested-functions is on by default. You cannot call
through pointers to nested functions or pointers to functions
compiled in other languages that use the static chain if you use
-mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the reserved
stack location in the function prologue if the function calls through
a pointer on AIX and 64-bit Linux systems. If the TOC value is not
saved in the prologue, it is saved just before the call through the
pointer. The -mno-save-toc-indirect option is the default.
-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure parameters with a
maximum alignment of 64 bits, for compatibility with older versions
of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align a
structure parameter on a 128-bit boundary when that structure
contained a member requiring 128-bit alignment. This is corrected in
more recent versions of GCC. This option may be used to generate
code that is compatible with functions compiled with older versions
of GCC.
The -mno-compat-align-parm option is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported
locations are global for global canary or tls for per-thread canary
in the TLS block (the default with GNU libc version 2.4 or later).
With the latter choice the options -mstack-protector-guard-reg=reg
and -mstack-protector-guard-offset=offset furthermore specify which
register to use as base register for reading the canary, and from
what offset from that base register. The default for those is as
specified in the relevant ABI.
-mpcrel
-mno-pcrel
Generate (do not generate) pc-relative addressing. The -mpcrel
option requires that the medium code model (-mcmodel=medium) and
prefixed addressing (-mprefixed) options are enabled.
-mprefixed
-mno-prefixed
Generate (do not generate) addressing modes using prefixed load and
store instructions. The -mprefixed option requires that the option
-mcpu=power10 (or later) is enabled.
-mmma
-mno-mma
Generate (do not generate) the MMA instructions. The -mma option
requires that the option -mcpu=power10 (or later) is enabled.
-mrop-protect
-mno-rop-protect
Generate (do not generate) ROP protection instructions when the
target processor supports them. Currently this option disables the
shrink-wrap optimization (-fshrink-wrap).
-mprivileged
-mno-privileged
Generate (do not generate) code that will run in privileged state.
-mblock-ops-unaligned-vsx
-mno-block-ops-unaligned-vsx
Generate (do not generate) unaligned vsx loads and stores for inline
expansion of "memcpy" and "memmove".
--param rs6000-vect-unroll-limit=
The vectorizer will check with target information to determine
whether it would be beneficial to unroll the main vectorized loop and
by how much. This parameter sets the upper bound of how much the
vectorizer will unroll the main loop. The default value is four.
RX Options
These command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits
(-m32bit-doubles) in size. The default is -m32bit-doubles. Note RX
floating-point hardware only works on 32-bit values, which is why the
default is -m32bit-doubles.
-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-point
hardware. The default is enabled for the RX600 series and disabled
for the RX200 series.
Floating-point instructions are only generated for 32-bit floating-
point values, however, so the FPU hardware is not used for doubles if
the -m64bit-doubles option is used.
Note If the -fpu option is enabled then -funsafe-math-optimizations
is also enabled automatically. This is because the RX FPU
instructions are themselves unsafe.
-mcpu=name
Selects the type of RX CPU to be targeted. Currently three types are
supported, the generic RX600 and RX200 series hardware and the
specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610 does
not support the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point unit and so
-nofpu is enabled by default when this type is selected.
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The default is
-mlittle-endian-data, i.e. to store data in the little-endian format.
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static variables
which can be placed into the small data area. Using the small data
area can lead to smaller and faster code, but the size of area is
limited and it is up to the programmer to ensure that the area does
not overflow. Also when the small data area is used one of the RX's
registers (usually "r13") is reserved for use pointing to this area,
so it is no longer available for use by the compiler. This could
result in slower and/or larger code if variables are pushed onto the
stack instead of being held in this register.
Note, common variables (variables that have not been initialized) and
constants are not placed into the small data area as they are
assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note, this
feature is not enabled by default with higher optimization levels
(-O2 etc) because of the potentially detrimental effects of reserving
a register. It is up to the programmer to experiment and discover
whether this feature is of benefit to their program. See the
description of the -mpid option for a description of how the actual
register to hold the small data area pointer is chosen.
-msim
-mno-sim
Use the simulator runtime. The default is to use the libgloss board-
specific runtime.
-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is compatible with
Renesas's AS100 assembler. This syntax can also be handled by the
GAS assembler, but it has some restrictions so it is not generated by
default.
-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that can be used
as an operand in a RX instruction. Although the RX instruction set
does allow constants of up to 4 bytes in length to be used in
instructions, a longer value equates to a longer instruction. Thus
in some circumstances it can be beneficial to restrict the size of
constants that are used in instructions. Constants that are too big
are instead placed into a constant pool and referenced via register
indirection.
The value N can be between 0 and 4. A value of 0 (the default) or 4
means that constants of any size are allowed.
-mrelax
Enable linker relaxation. Linker relaxation is a process whereby the
linker attempts to reduce the size of a program by finding shorter
versions of various instructions. Disabled by default.
-mint-register=N
Specify the number of registers to reserve for fast interrupt handler
functions. The value N can be between 0 and 4. A value of 1 means
that register "r13" is reserved for the exclusive use of fast
interrupt handlers. A value of 2 reserves "r13" and "r12". A value
of 3 reserves "r13", "r12" and "r11", and a value of 4 reserves "r13"
through "r10". A value of 0, the default, does not reserve any
registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the
accumulator register. This is only necessary if normal code might
use the accumulator register, for example because it performs 64-bit
multiplications. The default is to ignore the accumulator as this
makes the interrupt handlers faster.
-mpid
-mno-pid
Enables the generation of position independent data. When enabled
any access to constant data is done via an offset from a base address
held in a register. This allows the location of constant data to be
determined at run time without requiring the executable to be
relocated, which is a benefit to embedded applications with tight
memory constraints. Data that can be modified is not affected by
this option.
Note, using this feature reserves a register, usually "r13", for the
constant data base address. This can result in slower and/or larger
code, especially in complicated functions.
The actual register chosen to hold the constant data base address
depends upon whether the -msmall-data-limit and/or the -mint-register
command-line options are enabled. Starting with register "r13" and
proceeding downwards, registers are allocated first to satisfy the
requirements of -mint-register, then -mpid and finally
-msmall-data-limit. Thus it is possible for the small data area
register to be "r8" if both -mint-register=4 and -mpid are specified
on the command line.
By default this feature is not enabled. The default can be restored
via the -mno-pid command-line option.
-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
Prevents GCC from issuing a warning message if it finds more than one
fast interrupt handler when it is compiling a file. The default is
to issue a warning for each extra fast interrupt handler found, as
the RX only supports one such interrupt.
-mallow-string-insns
-mno-allow-string-insns
Enables or disables the use of the string manipulation instructions
"SMOVF", "SCMPU", "SMOVB", "SMOVU", "SUNTIL" "SWHILE" and also the
"RMPA" instruction. These instructions may prefetch data, which is
not safe to do if accessing an I/O register. (See section 12.2.7 of
the RX62N Group User's Manual for more information).
The default is to allow these instructions, but it is not possible
for GCC to reliably detect all circumstances where a string
instruction might be used to access an I/O register, so their use
cannot be disabled automatically. Instead it is reliant upon the
programmer to use the -mno-allow-string-insns option if their program
accesses I/O space.
When the instructions are enabled GCC defines the C preprocessor
symbol "__RX_ALLOW_STRING_INSNS__", otherwise it defines the symbol
"__RX_DISALLOW_STRING_INSNS__".
-mjsr
-mno-jsr
Use only (or not only) "JSR" instructions to access functions. This
option can be used when code size exceeds the range of "BSR"
instructions. Note that -mno-jsr does not mean to not use "JSR" but
instead means that any type of branch may be used.
Note: The generic GCC command-line option -ffixed-reg has special
significance to the RX port when used with the "interrupt" function
attribute. This attribute indicates a function intended to process fast
interrupts. GCC ensures that it only uses the registers "r10", "r11",
"r12" and/or "r13" and only provided that the normal use of the
corresponding registers have been restricted via the -ffixed-reg or
-mint-register command-line options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and
registers for floating-point operations. When -msoft-float is
specified, functions in libgcc.a are used to perform floating-point
operations. When -mhard-float is specified, the compiler generates
IEEE floating-point instructions. This is the default.
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions for
decimal-floating-point operations. When -mno-hard-dfp is specified,
functions in libgcc.a are used to perform decimal-floating-point
operations. When -mhard-dfp is specified, the compiler generates
decimal-floating-point hardware instructions. This is the default
for -march=z9-ec or higher.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of 64
bits makes the "long double" type equivalent to the "double" type.
This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller's frame as backchain
pointer into the callee's stack frame. A backchain may be needed to
allow debugging using tools that do not understand DWARF call frame
information. When -mno-packed-stack is in effect, the backchain
pointer is stored at the bottom of the stack frame; when
-mpacked-stack is in effect, the backchain is placed into the topmost
word of the 96/160 byte register save area.
In general, code compiled with -mbackchain is call-compatible with
code compiled with -mno-backchain; however, use of the backchain for
debugging purposes usually requires that the whole binary is built
with -mbackchain. Note that the combination of -mbackchain,
-mpacked-stack and -mhard-float is not supported. In order to build
a linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack is
specified, the compiler uses the all fields of the 96/160 byte
register save area only for their default purpose; unused fields
still take up stack space. When -mpacked-stack is specified,
register save slots are densely packed at the top of the register
save area; unused space is reused for other purposes, allowing for
more efficient use of the available stack space. However, when
-mbackchain is also in effect, the topmost word of the save area is
always used to store the backchain, and the return address register
is always saved two words below the backchain.
As long as the stack frame backchain is not used, code generated with
-mpacked-stack is call-compatible with code generated with
-mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for
S/390 or zSeries generated code that uses the stack frame backchain
at run time, not just for debugging purposes. Such code is not call-
compatible with code compiled with -mpacked-stack. Also, note that
the combination of -mbackchain, -mpacked-stack and -mhard-float is
not supported. In order to build a linux kernel use -msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to do
subroutine calls. This only works reliably if the total executable
size does not exceed 64k. The default is to use the "basr"
instruction instead, which does not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux for
S/390 ABI. When -m64 is specified, generate code compliant to the
GNU/Linux for zSeries ABI. This allows GCC in particular to generate
64-bit instructions. For the s390 targets, the default is -m31,
while the s390x targets default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the instructions
available on z/Architecture. When -mesa is specified, generate code
using the instructions available on ESA/390. Note that -mesa is not
possible with -m64. When generating code compliant to the GNU/Linux
for S/390 ABI, the default is -mesa. When generating code compliant
to the GNU/Linux for zSeries ABI, the default is -mzarch.
-mhtm
-mno-htm
The -mhtm option enables a set of builtins making use of instructions
available with the transactional execution facility introduced with
the IBM zEnterprise EC12 machine generation S/390 System z Built-in
Functions. -mhtm is enabled by default when using -march=zEC12.
-mvx
-mno-vx
When -mvx is specified, generate code using the instructions
available with the vector extension facility introduced with the IBM
z13 machine generation. This option changes the ABI for some vector
type values with regard to alignment and calling conventions. In
case vector type values are being used in an ABI-relevant context a
GAS .gnu_attribute command will be added to mark the resulting binary
with the ABI used. -mvx is enabled by default when using -march=z13.
-mzvector
-mno-zvector
The -mzvector option enables vector language extensions and builtins
using instructions available with the vector extension facility
introduced with the IBM z13 machine generation. This option adds
support for vector to be used as a keyword to define vector type
variables and arguments. vector is only available when GNU
extensions are enabled. It will not be expanded when requesting
strict standard compliance e.g. with -std=c99. In addition to the
GCC low-level builtins -mzvector enables a set of builtins added for
compatibility with AltiVec-style implementations like Power and Cell.
In order to make use of these builtins the header file vecintrin.h
needs to be included. -mzvector is disabled by default.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to
perform block moves. When -mno-mvcle is specified, use a "mvc" loop
instead. This is the default unless optimizing for size.
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling.
The default is to not print debug information.
-march=cpu-type
Generate code that runs on cpu-type, which is the name of a system
representing a certain processor type. Possible values for cpu-type
are z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8,
z196/arch9, zEC12, z13/arch11, z14/arch12, z15/arch13, z16/arch14,
z17/arch15, and native.
The default is -march=z900.
Specifying native as cpu type can be used to select the best
architecture option for the host processor. -march=native has no
effect if GCC does not recognize the processor.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions. The list
of cpu-type values is the same as for -march. The default is the
value used for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to
trace routines in the operating system. This option is off by
default, even when compiling for the TPF OS.
-mtpf-trace-skip
-mno-tpf-trace-skip
Generate code that changes (does not change) the default branch
targets enabled by -mtpf-trace to point to specialized trace routines
providing the ability of selectively skipping function trace entries
for the TPF OS. This option is off by default, even when compiling
for the TPF OS and specifying -mtpf-trace.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply
and accumulate instructions. These instructions are generated by
default if hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame size.
Because this is a compile-time check it doesn't need to be a real
problem when the program runs. It is intended to identify functions
that most probably cause a stack overflow. It is useful to be used
in an environment with limited stack size e.g. the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls "alloca" or uses dynamically-
sized arrays. This is generally a bad idea with a limited stack
size.
-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the S/390 back end emits additional
instructions in the function prologue that trigger a trap if the
stack size is stack-guard bytes above the stack-size (remember that
the stack on S/390 grows downward). If the stack-guard option is
omitted the smallest power of 2 larger than the frame size of the
compiled function is chosen. These options are intended to be used
to help debugging stack overflow problems. The additionally emitted
code causes only little overhead and hence can also be used in
production-like systems without greater performance degradation. The
given values have to be exact powers of 2 and stack-size has to be
greater than stack-guard without exceeding 64k. In order to be
efficient the extra code makes the assumption that the stack starts
at an address aligned to the value given by stack-size. The stack-
guard option can only be used in conjunction with stack-size.
-mhotpatch=pre-halfwords,post-halfwords
If the hotpatch option is enabled, a "hot-patching" function prologue
is generated for all functions in the compilation unit. The funtion
label is prepended with the given number of two-byte NOP instructions
(pre-halfwords, maximum 1000000). After the label, 2 * post-
halfwords bytes are appended, using the largest NOP like instructions
the architecture allows (maximum 1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the
"hotpatch" attribute.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a
way that the floating-point unit is not used.
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-
precision floating-point operations are used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit is in
single-precision mode by default.
-m2a
Generate code for the SH2a-FPU assuming the floating-point unit is in
double-precision mode by default.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only
supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in
single-precision mode by default.
-m4 Generate code for the SH4.
-m4-100
Generate code for SH4-100.
-m4-100-nofpu
Generate code for SH4-100 in such a way that the floating-point unit
is not used.
-m4-100-single
Generate code for SH4-100 assuming the floating-point unit is in
single-precision mode by default.
-m4-100-single-only
Generate code for SH4-100 in such a way that no double-precision
floating-point operations are used.
-m4-200
Generate code for SH4-200.
-m4-200-nofpu
Generate code for SH4-200 without in such a way that the floating-
point unit is not used.
-m4-200-single
Generate code for SH4-200 assuming the floating-point unit is in
single-precision mode by default.
-m4-200-single-only
Generate code for SH4-200 in such a way that no double-precision
floating-point operations are used.
-m4-300
Generate code for SH4-300.
-m4-300-nofpu
Generate code for SH4-300 without in such a way that the floating-
point unit is not used.
-m4-300-single
Generate code for SH4-300 in such a way that no double-precision
floating-point operations are used.
-m4-300-single-only
Generate code for SH4-300 in such a way that no double-precision
floating-point operations are used.
-m4-340
Generate code for SH4-340 (no MMU, no FPU).
-m4-500
Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the
assembler.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the
floating-point unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision
floating-point operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in
single-precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the
assembler. GCC doesn't generate any DSP instructions at the moment.
-mb Compile code for the processor in big-endian mode.
-ml Compile code for the processor in little-endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the
calling conventions, and thus some functions from the standard C
library do not work unless you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when possible; uses the
linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use 16-bit
offsets.
-mbitops
Enable the use of bit manipulation instructions on SH2A.
-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign for
alignment constraints.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the
Renesas conventions were available. This option is the default for
all targets of the SH toolchain.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mrenesas is
given.
-mieee
-mno-ieee
Control the IEEE compliance of floating-point comparisons, which
affects the handling of cases where the result of a comparison is
unordered. By default -mieee is implicitly enabled. If
-ffinite-math-only is enabled -mno-ieee is implicitly set, which
results in faster floating-point greater-equal and less-equal
comparisons. The implicit settings can be overridden by specifying
either -mieee or -mno-ieee.
-minline-ic_invalidate
Inline code to invalidate instruction cache entries after setting up
nested function trampolines. This option has no effect if -musermode
is in effect and the selected code generation option (e.g. -m4) does
not allow the use of the "icbi" instruction. If the selected code
generation option does not allow the use of the "icbi" instruction,
and -musermode is not in effect, the inlined code manipulates the
instruction cache address array directly with an associative write.
This not only requires privileged mode at run time, but it also fails
if the cache line had been mapped via the TLB and has become
unmapped.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4
bytes, which is incompatible with the SH ABI.
-matomic-model=model
Sets the model of atomic operations and additional parameters as a
comma separated list. For details on the atomic built-in functions
see __atomic Builtins. The following models and parameters are
supported:
none
Disable compiler generated atomic sequences and emit library
calls for atomic operations. This is the default if the target
is not "sh*-*-linux*".
soft-gusa
Generate GNU/Linux compatible gUSA software atomic sequences for
the atomic built-in functions. The generated atomic sequences
require additional support from the interrupt/exception handling
code of the system and are only suitable for SH3* and SH4*
single-core systems. This option is enabled by default when the
target is "sh*-*-linux*" and SH3* or SH4*. When the target is
SH4A, this option also partially utilizes the hardware atomic
instructions "movli.l" and "movco.l" to create more efficient
code, unless strict is specified.
soft-tcb
Generate software atomic sequences that use a variable in the
thread control block. This is a variation of the gUSA sequences
which can also be used on SH1* and SH2* targets. The generated
atomic sequences require additional support from the
interrupt/exception handling code of the system and are only
suitable for single-core systems. When using this model, the
gbr-offset= parameter has to be specified as well.
soft-imask
Generate software atomic sequences that temporarily disable
interrupts by setting "SR.IMASK = 1111". This model works only
when the program runs in privileged mode and is only suitable for
single-core systems. Additional support from the
interrupt/exception handling code of the system is not required.
This model is enabled by default when the target is
"sh*-*-linux*" and SH1* or SH2*.
hard-llcs
Generate hardware atomic sequences using the "movli.l" and
"movco.l" instructions only. This is only available on SH4A and
is suitable for multi-core systems. Since the hardware
instructions support only 32 bit atomic variables access to 8 or
16 bit variables is emulated with 32 bit accesses. Code compiled
with this option is also compatible with other software atomic
model interrupt/exception handling systems if executed on an SH4A
system. Additional support from the interrupt/exception handling
code of the system is not required for this model.
gbr-offset=
This parameter specifies the offset in bytes of the variable in
the thread control block structure that should be used by the
generated atomic sequences when the soft-tcb model has been
selected. For other models this parameter is ignored. The
specified value must be an integer multiple of four and in the
range 0-1020.
strict
This parameter prevents mixed usage of multiple atomic models,
even if they are compatible, and makes the compiler generate
atomic sequences of the specified model only.
-mtas
Generate the "tas.b" opcode for "__atomic_test_and_set". Notice that
depending on the particular hardware and software configuration this
can degrade overall performance due to the operand cache line flushes
that are implied by the "tas.b" instruction. On multi-core SH4A
processors the "tas.b" instruction must be used with caution since it
can result in data corruption for certain cache configurations.
-mprefergot
When generating position-independent code, emit function calls using
the Global Offset Table instead of the Procedure Linkage Table.
-musermode
-mno-usermode
Don't allow (allow) the compiler generating privileged mode code.
Specifying -musermode also implies -mno-inline-ic_invalidate if the
inlined code would not work in user mode. -musermode is the default
when the target is "sh*-*-linux*". If the target is SH1* or SH2*
-musermode has no effect, since there is no user mode.
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to be used for integer division operations.
strategy can be one of:
call-div1
Calls a library function that uses the single-step division
instruction "div1" to perform the operation. Division by zero
calculates an unspecified result and does not trap. This is the
default except for SH4, SH2A and SHcompact.
call-fp
Calls a library function that performs the operation in double
precision floating point. Division by zero causes a floating-
point exception. This is the default for SHcompact with FPU.
Specifying this for targets that do not have a double precision
FPU defaults to "call-div1".
call-table
Calls a library function that uses a lookup table for small
divisors and the "div1" instruction with case distinction for
larger divisors. Division by zero calculates an unspecified
result and does not trap. This is the default for SH4.
Specifying this for targets that do not have dynamic shift
instructions defaults to "call-div1".
When a division strategy has not been specified the default strategy
is selected based on the current target. For SH2A the default
strategy is to use the "divs" and "divu" instructions instead of
library function calls.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue
rather than around each call. Generally beneficial for performance
and size. Also needed for unwinding to avoid changing the stack
frame around conditional code.
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed division
to name. This only affects the name used in the call division
strategies, and the compiler still expects the same sets of
input/output/clobbered registers as if this option were not present.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers.
A fixed register is one that the register allocator cannot use. This
is useful when compiling kernel code. A register range is specified
as two registers separated by a dash. Multiple register ranges can
be specified separated by a comma.
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher numbers
make the compiler try to generate more branch-free code if possible.
If not specified the value is selected depending on the processor
type that is being compiled for.
-mzdcbranch
-mno-zdcbranch
Assume (do not assume) that zero displacement conditional branch
instructions "bt" and "bf" are fast. If -mzdcbranch is specified,
the compiler prefers zero displacement branch code sequences. This
is enabled by default when generating code for SH4 and SH4A. It can
be explicitly disabled by specifying -mno-zdcbranch.
-mcbranch-force-delay-slot
Force the usage of delay slots for conditional branches, which stuffs
the delay slot with a "nop" if a suitable instruction cannot be
found. By default this option is disabled. It can be enabled to
work around hardware bugs as found in the original SH7055.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply
and accumulate instructions. These instructions are generated by
default if hardware floating point is used. The machine-dependent
-mfused-madd option is now mapped to the machine-independent
-ffp-contract=fast option, and -mno-fused-madd is mapped to
-ffp-contract=off.
-mfsca
-mno-fsca
Allow or disallow the compiler to emit the "fsca" instruction for
sine and cosine approximations. The option -mfsca must be used in
combination with -funsafe-math-optimizations. It is enabled by
default when generating code for SH4A. Using -mno-fsca disables sine
and cosine approximations even if -funsafe-math-optimizations is in
effect.
-mfsrra
-mno-fsrra
Allow or disallow the compiler to emit the "fsrra" instruction for
reciprocal square root approximations. The option -mfsrra must be
used in combination with -funsafe-math-optimizations and
-ffinite-math-only. It is enabled by default when generating code
for SH4A. Using -mno-fsrra disables reciprocal square root
approximations even if -funsafe-math-optimizations and
-ffinite-math-only are in effect.
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional move
instruction patterns. This can result in faster code on the SH4
processor.
-mfdpic
Generate code using the FDPIC ABI.
Solaris 2 Options
These -m options are supported on Solaris 2:
-mclear-hwcap
-mclear-hwcap tells the compiler to remove the hardware capabilities
generated by the Solaris assembler. This is only necessary when
object files use ISA extensions not supported by the current machine,
but check at runtime whether or not to use them.
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to not
pass -z text to the linker when linking a shared object. Using this
option, you can link position-dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against allocatable
but non-writable sections" linker error message. However, the
necessary relocations trigger copy-on-write, and the shared object is
not actually shared across processes. Instead of using
-mimpure-text, you should compile all source code with -fpic or
-fPIC.
These switches are supported in addition to the above on Solaris 2:
-pthreads
This is a synonym for -pthread.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global registers 2
through 4, which the SPARC SVR4 ABI reserves for applications. Like
the global register 1, each global register 2 through 4 is then
treated as an allocable register that is clobbered by function calls.
This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance loss,
specify -mno-app-regs. You should compile libraries and system
software with this option.
-mflat
-mno-flat
With -mflat, the compiler does not generate save/restore instructions
and uses a "flat" or single register window model. This model is
compatible with the regular register window model. The local
registers and the input registers (0--5) are still treated as "call-
saved" registers and are saved on the stack as needed.
With -mno-flat (the default), the compiler generates save/restore
instructions (except for leaf functions). This is the normal
operating mode.
-mfpu
-mhard-float
Generate output containing floating-point instructions. This is the
default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all SPARC
targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation. You
must make your own arrangements to provide suitable library functions
for cross-compilation. The embedded targets sparc-*-aout and
sparclite-*-* do provide software floating-point support.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-point
instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double)
floating-point instructions. The functions called are those
specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have
hardware support for the quad-word floating-point instructions. They
all invoke a trap handler for one of these instructions, and then the
trap handler emulates the effect of the instruction. Because of the
trap handler overhead, this is much slower than calling the ABI
library routines. Thus the -msoft-quad-float option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte
alignment only if they are contained in another type, or if they have
an absolute address. Otherwise, it assumes they have 4-byte
alignment. Specifying this option avoids some rare compatibility
problems with code generated by other compilers. It is not the
default because it results in a performance loss, especially for
floating-point code.
-muser-mode
-mno-user-mode
Do not generate code that can only run in supervisor mode. This is
relevant only for the "casa" instruction emitted for the LEON3
processor. This is the default.
-mfaster-structs
-mno-faster-structs
With -mfaster-structs, the compiler assumes that structures should
have 8-byte alignment. This enables the use of pairs of "ldd" and
"std" instructions for copies in structure assignment, in place of
twice as many "ld" and "st" pairs. However, the use of this changed
alignment directly violates the SPARC ABI. Thus, it's intended only
for use on targets where the developer acknowledges that their
resulting code is not directly in line with the rules of the ABI.
-mstd-struct-return
-mno-std-struct-return
With -mstd-struct-return, the compiler generates checking code in
functions returning structures or unions to detect size mismatches
between the two sides of function calls, as per the 32-bit ABI.
The default is -mno-std-struct-return. This option has no effect in
64-bit mode.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. Supported values for cpu_type
are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7,
leon5, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9,
ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4,
niagara7 and m8.
Native Solaris and GNU/Linux toolchains also support the value
native, which selects the best architecture option for the host
processor. -mcpu=native has no effect if GCC does not recognize the
processor.
Default instruction scheduling parameters are used for values that
select an architecture and not an implementation. These are v7, v8,
sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported
implementations.
v7 cypress, leon3v7
v8 supersparc, hypersparc, leon, leon3, leon5
sparclite
f930, f934, sparclite86x
sparclet
tsc701
v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4,
niagara7, m8
By default (unless configured otherwise), GCC generates code for the
V7 variant of the SPARC architecture. With -mcpu=cypress, the
compiler additionally optimizes it for the Cypress CY7C602 chip, as
used in the SPARCStation/SPARCServer 3x series. This is also
appropriate for the older SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC
architecture. The only difference from V7 code is that the compiler
emits the integer multiply and integer divide instructions which
exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the
compiler additionally optimizes it for the SuperSPARC chip, as used
in the SPARCStation 10, 1000 and 2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of
the SPARC architecture. This adds the integer multiply, integer
divide step and scan ("ffs") instructions which exist in SPARClite
but not in SPARC-V7. With -mcpu=f930, the compiler additionally
optimizes it for the Fujitsu MB86930 chip, which is the original
SPARClite, with no FPU. With -mcpu=f934, the compiler additionally
optimizes it for the Fujitsu MB86934 chip, which is the more recent
SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of
the SPARC architecture. This adds the integer multiply,
multiply/accumulate, integer divide step and scan ("ffs")
instructions which exist in SPARClet but not in SPARC-V7. With
-mcpu=tsc701, the compiler additionally optimizes it for the TEMIC
SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC
architecture. This adds 64-bit integer and floating-point move
instructions, 3 additional floating-point condition code registers
and conditional move instructions. With -mcpu=ultrasparc, the
compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi
chips. With -mcpu=ultrasparc3, the compiler additionally optimizes
it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With
-mcpu=niagara, the compiler additionally optimizes it for Sun
UltraSPARC T1 chips. With -mcpu=niagara2, the compiler additionally
optimizes it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the
compiler additionally optimizes it for Sun UltraSPARC T3 chips. With
-mcpu=niagara4, the compiler additionally optimizes it for Sun
UltraSPARC T4 chips. With -mcpu=niagara7, the compiler additionally
optimizes it for Oracle SPARC M7 chips. With -mcpu=m8, the compiler
additionally optimizes it for Oracle M8 chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type,
but do not set the instruction set or register set that the option
-mcpu=cpu_type does.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type,
but the only useful values are those that select a particular CPU
implementation. Those are cypress, supersparc, hypersparc, leon,
leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc,
ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8.
With native Solaris and GNU/Linux toolchains, native can also be
used.
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The
difference from the V8 ABI is that the global and out registers are
considered 64 bits wide. This is enabled by default on Solaris in
32-bit mode for all SPARC-V9 processors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the UltraSPARC
Visual Instruction Set extensions. The default is -mno-vis.
-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of version 2.0
of the UltraSPARC Visual Instruction Set extensions. The default is
-mvis2 when targeting a cpu that supports such instructions, such as
UltraSPARC-III and later. Setting -mvis2 also sets -mvis.
-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of version 3.0
of the UltraSPARC Visual Instruction Set extensions. The default is
-mvis3 when targeting a cpu that supports such instructions, such as
niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis.
-mvis3b
-mno-vis3b
With -mvis3b, GCC generates code that takes advantage of version 3.0
of the UltraSPARC Visual Instruction Set extensions, plus the
additional VIS instructions introduced in the Oracle SPARC
Architecture 2011. The default is -mvis3b when targeting a cpu that
supports such instructions, such as niagara-7 and later. Setting
-mvis3b also sets -mvis3, -mvis2 and -mvis.
-mvis4
-mno-vis4
With -mvis4, GCC generates code that takes advantage of version 4.0
of the UltraSPARC Visual Instruction Set extensions. The default is
-mvis4 when targeting a cpu that supports such instructions, such as
niagara-7 and later. Setting -mvis4 also sets -mvis3b, -mvis3,
-mvis2 and -mvis.
-mvis4b
-mno-vis4b
With -mvis4b, GCC generates code that takes advantage of version 4.0
of the UltraSPARC Visual Instruction Set extensions, plus the
additional VIS instructions introduced in the Oracle SPARC
Architecture 2017. The default is -mvis4b when targeting a cpu that
supports such instructions, such as m8 and later. Setting -mvis4b
also sets -mvis4, -mvis3b, -mvis3, -mvis2 and -mvis.
-mcbcond
-mno-cbcond
With -mcbcond, GCC generates code that takes advantage of the
UltraSPARC Compare-and-Branch-on-Condition instructions. The default
is -mcbcond when targeting a CPU that supports such instructions,
such as Niagara-4 and later.
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the
UltraSPARC Fused Multiply-Add Floating-point instructions. The
default is -mfmaf when targeting a CPU that supports such
instructions, such as Niagara-3 and later.
-mfsmuld
-mno-fsmuld
With -mfsmuld, GCC generates code that takes advantage of the
Floating-point Multiply Single to Double (FsMULd) instruction. The
default is -mfsmuld when targeting a CPU supporting the architecture
versions V8 or V9 with FPU except -mcpu=leon.
-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of the
UltraSPARC Population Count instruction. The default is -mpopc when
targeting a CPU that supports such an instruction, such as Niagara-2
and later.
-msubxc
-mno-subxc
With -msubxc, GCC generates code that takes advantage of the
UltraSPARC Subtract-Extended-with-Carry instruction. The default is
-msubxc when targeting a CPU that supports such an instruction, such
as Niagara-7 and later.
-mfix-at697f
Enable the documented workaround for the single erratum of the Atmel
AT697F processor (which corresponds to erratum #13 of the AT697E
processor).
-mfix-ut699
Enable the documented workarounds for the floating-point errata and
the data cache nullify errata of the UT699 processor.
-mfix-ut700
Enable the documented workaround for the back-to-back store errata of
the UT699E/UT700 processor.
-mfix-gr712rc
Enable the documented workaround for the back-to-back store errata of
the GR712RC processor.
These -m options are supported in addition to the above on SPARC-V9
processors in 64-bit environments:
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long and pointer to 32 bits. The 64-bit
environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=which
Set the code model to one of
medlow
The Medium/Low code model: 64-bit addresses, programs must be
linked in the low 32 bits of memory. Programs can be statically
or dynamically linked.
medmid
The Medium/Middle code model: 64-bit addresses, programs must be
linked in the low 44 bits of memory, the text and data segments
must be less than 2GB in size and the data segment must be
located within 2GB of the text segment.
medany
The Medium/Anywhere code model: 64-bit addresses, programs may be
linked anywhere in memory, the text and data segments must be
less than 2GB in size and the data segment must be located within
2GB of the text segment.
embmedany
The Medium/Anywhere code model for embedded systems: 64-bit
addresses, the text and data segments must be less than 2GB in
size, both starting anywhere in memory (determined at link time).
The global register %g4 points to the base of the data segment.
Programs are statically linked and PIC is not supported.
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
default
The default memory model for the processor and operating system.
rmo Relaxed Memory Order
pso Partial Store Order
tso Total Store Order
sc Sequential Consistency
These memory models are formally defined in Appendix D of the
SPARC-V9 architecture manual, as set in the processor's "PSTATE.MM"
field.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame
pointer if present, are offset by -2047 which must be added back when
making stack frame references. This is the default in 64-bit mode.
Otherwise, assume no such offset is present.
Options for System V
These additional options are available on System V Release 4 for
compatibility with other compilers on those systems:
-G Create a shared object. It is recommended that -symbolic or -shared
be used instead.
-Qy Identify the versions of each tool used by the compiler, in a
".ident" assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output file (this is
the default).
-YP,dirs
Search the directories dirs, and no others, for libraries specified
with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The assembler
uses this option.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be
far away, the compiler always loads the function's address into a
register, and calls indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index
pointer 4 or more times to copy pointer into the "ep" register, and
use the shorter "sld" and "sst" instructions. The -mep option is on
by default if you optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers
at the prologue and epilogue of a function. The external functions
are slower, but use less code space if more than one function saves
the same number of registers. The -mprolog-function option is on by
default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just
turns on the -mep and -mprolog-function options.
-mtda=n
Put static or global variables whose size is n bytes or less into the
tiny data area that register "ep" points to. The tiny data area can
hold up to 256 bytes in total (128 bytes for byte references).
-msda=n
Put static or global variables whose size is n bytes or less into the
small data area that register "gp" points to. The small data area
can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into the
first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mv850e3v5
Specify that the target processor is the V850E3V5. The preprocessor
constant "__v850e3v5__" is defined if this option is used.
-mv850e2v4
Specify that the target processor is the V850E3V5. This is an alias
for the -mv850e3v5 option.
-mv850e2v3
Specify that the target processor is the V850E2V3. The preprocessor
constant "__v850e2v3__" is defined if this option is used.
-mv850e2
Specify that the target processor is the V850E2. The preprocessor
constant "__v850e2__" is defined if this option is used.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor
constants "__v850e1__" and "__v850e__" are defined if this option is
used.
-mv850es
Specify that the target processor is the V850ES. This is an alias
for the -mv850e1 option.
-mv850e
Specify that the target processor is the V850E. The preprocessor
constant "__v850e__" is defined if this option is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor
-mv850e2v3 nor -mv850e3v5 are defined then a default target processor
is chosen and the relevant __v850*__ preprocessor constant is
defined.
The preprocessor constants "__v850" and "__v851__" are always
defined, regardless of which processor variant is the target.
-mdisable-callt
-mno-disable-callt
This option suppresses generation of the "CALLT" instruction for the
v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850
architecture.
This option is enabled by default when the RH850 ABI is in use (see
-mrh850-abi), and disabled by default when the GCC ABI is in use. If
"CALLT" instructions are being generated then the C preprocessor
symbol "__V850_CALLT__" is defined.
-mrelax
-mno-relax
Pass on (or do not pass on) the -mrelax command-line option to the
assembler.
-mlong-jumps
-mno-long-jumps
Disable (or re-enable) the generation of PC-relative jump
instructions.
-msoft-float
-mhard-float
Disable (or re-enable) the generation of hardware floating point
instructions. This option is only significant when the target
architecture is V850E2V3 or higher. If hardware floating point
instructions are being generated then the C preprocessor symbol
"__FPU_OK__" is defined, otherwise the symbol "__NO_FPU__" is
defined.
-mloop
Enables the use of the e3v5 LOOP instruction. The use of this
instruction is not enabled by default when the e3v5 architecture is
selected because its use is still experimental.
-mrh850-abi
-mghs
Enables support for the RH850 version of the V850 ABI. This is the
default. With this version of the ABI the following rules apply:
* Integer sized structures and unions are returned via a memory
pointer rather than a register.
* Large structures and unions (more than 8 bytes in size) are
passed by value.
* Functions are aligned to 16-bit boundaries.
* The -m8byte-align command-line option is supported.
* The -mdisable-callt command-line option is enabled by default.
The -mno-disable-callt command-line option is not supported.
When this version of the ABI is enabled the C preprocessor symbol
"__V850_RH850_ABI__" is defined.
-mgcc-abi
Enables support for the old GCC version of the V850 ABI. With this
version of the ABI the following rules apply:
* Integer sized structures and unions are returned in register
"r10".
* Large structures and unions (more than 8 bytes in size) are
passed by reference.
* Functions are aligned to 32-bit boundaries, unless optimizing for
size.
* The -m8byte-align command-line option is not supported.
* The -mdisable-callt command-line option is supported but not
enabled by default.
When this version of the ABI is enabled the C preprocessor symbol
"__V850_GCC_ABI__" is defined.
-m8byte-align
-mno-8byte-align
Enables support for "double" and "long long" types to be aligned on
8-byte boundaries. The default is to restrict the alignment of all
objects to at most 4-bytes. When -m8byte-align is in effect the C
preprocessor symbol "__V850_8BYTE_ALIGN__" is defined.
-mbig-switch
Generate code suitable for big switch tables. Use this option only
if the assembler/linker complain about out of range branches within a
switch table.
-mapp-regs
This option causes r2 and r5 to be used in the code generated by the
compiler. This setting is the default.
-mno-app-regs
This option causes r2 and r5 to be treated as fixed registers.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on) that the
Unix assembler for the VAX cannot handle across long ranges.
-mgnu
Do output those jump instructions, on the assumption that the GNU
assembler is being used.
-md
-md-float
Use the D_floating data format for double-precision floating-point
numbers instead of G_floating.
-mg
-mg-float
Use the G_floating data format for double-precision floating-point
numbers instead of D_floating.
-mlra
-mno-lra
Enable Local Register Allocation. This is still experimental for the
VAX, so by default the compiler uses standard reload.
Visium Options
-mdebug
A program which performs file I/O and is destined to run on an MCM
target should be linked with this option. It causes the libraries
libc.a and libdebug.a to be linked. The program should be run on the
target under the control of the GDB remote debugging stub.
-msim
A program which performs file I/O and is destined to run on the
simulator should be linked with option. This causes libraries libc.a
and libsim.a to be linked.
-mfpu
-mhard-float
Generate code containing floating-point instructions. This is the
default.
-mno-fpu
-msoft-float
Generate code containing library calls for floating-point.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling
parameters for machine type cpu_type. Supported values for cpu_type
are mcm, gr5 and gr6.
mcm is a synonym of gr5 present for backward compatibility.
By default (unless configured otherwise), GCC generates code for the
GR5 variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium
architecture. The only difference from GR5 code is that the compiler
will generate block move instructions.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type,
but do not set the instruction set or register set that the option
-mcpu=cpu_type would.
-msv-mode
Generate code for the supervisor mode, where there are no
restrictions on the access to general registers. This is the
default.
-muser-mode
Generate code for the user mode, where the access to some general
registers is forbidden: on the GR5, registers r24 to r31 cannot be
accessed in this mode; on the GR6, only registers r29 to r31 are
affected.
VMS Options
These -m options are defined for the VMS implementations:
-mvms-return-codes
Return VMS condition codes from "main". The default is to return
POSIX-style condition (e.g. error) codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main
routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
-mpointer-size=size
Set the default size of pointers. Possible options for size are 32 or
short for 32 bit pointers, 64 or long for 64 bit pointers, and no for
supporting only 32 bit pointers. The later option disables "pragma
pointer_size".
VxWorks Options
The options in this section are defined for all VxWorks targets. Options
specific to the target hardware are listed with the other options for
that target.
-mrtp
GCC can generate code for both VxWorks kernels and real time
processes (RTPs). This option switches from the former to the
latter. It also defines the preprocessor macro "__RTP__".
-msmp
Select SMP runtimes for linking. Not available on architectures
other than PowerPC, nor on VxWorks version 7 or later, in which the
selection is part of the VxWorks build configuration and the library
paths are the same for either choice.
-non-static
Link an RTP executable against shared libraries rather than static
libraries. The options -static and -shared can also be used for
RTPs; -static is the default.
-Bstatic
-Bdynamic
These options are passed down to the linker. They are defined for
compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent to
-Wl,-z,now and is defined for compatibility with Diab.
-Xbind-now
Disable lazy binding of function calls. This option is the default
and is defined for compatibility with Diab.
x86 Options
These -m options are defined for the x86 family of computers.
-march=cpu-type
Generate instructions for the machine type cpu-type. In contrast to
-mtune=cpu-type, which merely tunes the generated code for the
specified cpu-type, -march=cpu-type allows GCC to generate code that
may not run at all on processors other than the one indicated.
Specifying -march=cpu-type implies -mtune=cpu-type, except where
noted otherwise.
The choices for cpu-type are:
native
This selects the CPU to generate code for at compilation time by
determining the processor type of the compiling machine. Using
-march=native enables all instruction subsets supported by the
local machine (hence the result might not run on different
machines). Using -mtune=native produces code optimized for the
local machine under the constraints of the selected instruction
set.
x86-64
A generic CPU with 64-bit extensions, MMX, SSE, SSE2, and FXSR
instruction set support.
x86-64-v2
x86-64-v3
x86-64-v4
These choices for cpu-type select the corresponding micro-
architecture level from the x86-64 psABI. On ABIs other than the
x86-64 psABI they select the same CPU features as the x86-64
psABI documents for the particular micro-architecture level.
Since these cpu-type values do not have a corresponding -mtune
setting, using -march with these values enables generic tuning.
Specific tuning can be enabled using the -mtune=other-cpu-type
option with an appropriate other-cpu-type value.
i386
Original Intel i386 CPU.
i486
Intel i486 CPU. (No scheduling is implemented for this chip.)
i586
pentium
Intel Pentium CPU with no MMX support.
lakemont
Intel Lakemont MCU, based on Intel Pentium CPU.
pentium-mmx
Intel Pentium MMX CPU, based on Pentium core with MMX instruction
set support.
pentiumpro
Intel Pentium Pro CPU with no MMX support.
i686
When used with -march, the Pentium Pro instruction set is used,
so the code runs on all i686 family chips. When used with
-mtune, it has the same meaning as generic.
pentium2
Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR
instruction set support.
pentium3
pentium3m
Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR
and SSE instruction set support.
pentium-m
Intel Pentium M; low-power version of Intel Pentium III CPU with
MMX, SSE, SSE2 and FXSR instruction set support. Used by
Centrino notebooks.
pentium4
pentium4m
Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set
support.
prescott
Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3
and FXSR instruction set support.
nocona
Improved version of Intel Pentium 4 CPU with 64-bit extensions,
MMX, SSE, SSE2, SSE3 and FXSR instruction set support.
core2
Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3,
SSSE3, CX16, SAHF and FXSR instruction set support.
nehalem
corei7
Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction
set support.
westmere
Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL
instruction set support.
sandybridge
corei7-avx
Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE
and PCLMUL instruction set support.
ivybridge
core-avx-i
Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
haswell
core-avx2
Intel Haswell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE,
PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE
and HLE instruction set support.
broadwell
Intel Broadwell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE,
PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA,
MOVBE, HLE, RDSEED, ADCX and PREFETCHW instruction set support.
skylake
Intel Skylake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE,
PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA,
MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC,
XSAVES and SGX instruction set support.
skylake-avx512
Intel Skylake Server CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ
and AVX512CD instruction set support.
cascadelake
Intel Cascade Lake CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD and AVX512VNNI instruction set support.
cannonlake
Intel Cannon Lake Server CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD, PKU, AVX512VBMI, AVX512IFMA and SHA instruction set
support.
cooperlake
Intel Cooper Lake CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD, AVX512VNNI and AVX512BF16 instruction set support.
icelake-client
Intel Ice Lake Client CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI,
VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID and
AVX512VPOPCNTDQ instruction set support.
icelake-server
Intel Ice Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI,
VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB instruction set
support.
tigerlake
Intel Tiger Lake CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI,
VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and
KEYLOCKER instruction set support.
rocketlake
Intel Rocket Lake CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD
PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES,
AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ
instruction set support.
alderlake
raptorlake
meteorlake
gracemont
Intel Alder Lake/Raptor Lake/Meteor Lake/Gracemont CPU with
64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC,
XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB,
MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C,
FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET,
KL, WIDEKL and AVX-VNNI instruction set support.
arrowlake
Intel Arrow Lake CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW,
PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE,
PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG,
ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES,
VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR,
AVXIFMA, AVXVNNIINT8, AVXNECONVERT and CMPCCXADD instruction set
support.
arrowlake-s
lunarlake
Intel Arrow Lake S/Lunar Lake CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES,
PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT,
FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI,
MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT,
PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL,
AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD,
AVXVNNIINT16, SHA512, SM3 and SM4 instruction set support.
pantherlake
Intel Panther Lake CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW,
PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE,
PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG,
ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES,
VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA,
AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3
and SM4 instruction set support.
sapphirerapids
emeraldrapids
Intel Sapphire Rapids/Emerald Rapids CPU with 64-bit extensions,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF,
FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2,
LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI,
VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B,
ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR,
AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16 and
AVX512BF16 instruction set support.
graniterapids
Intel Granite Rapids CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI,
VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B,
ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR,
AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16, AVX512BF16,
AMX-FP16 and PREFETCHI instruction set support.
graniterapids-d
Intel Granite Rapids D CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI,
VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B,
ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR,
AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16,
AMX-FP16, PREFETCHI and AMX-COMPLEX instruction set support.
diamondrapids
Intel Diamond Rapids CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX,
XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT,
FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT,
XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI,
VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B,
ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR,
AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16,
AMX-FP16, PREFETCHI, AMX-COMPLEX, AVX10.1-512, AVX-IFMA, AVX-NE-
CONVERT, AVX-VNNI-INT16, AVX-VNNI-INT8, CMPccXADD, SHA512, SM3,
SM4, AVX10.2-512, APX_F, AMX-AVX512, AMX-FP8, AMX-TF32, MOVRS and
AMX-MOVRS instruction set support.
bonnell
atom
Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2,
SSE3 and SSSE3 instruction set support.
silvermont
slm Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR,
PCLMUL, PREFETCHW and RDRND instruction set support.
goldmont
Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL,
PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES,
XSAVEOPT, CLFLUSHOPT and FSGSBASE instruction set support.
goldmont-plus
Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR,
PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC,
XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID and SGX
instruction set support.
tremont
Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL,
PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES,
XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID, SGX, CLWB, GFNI-
SSE, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set
support.
sierraforest
Intel Sierra Forest CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW,
PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE,
PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B,
CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT,
PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL,
AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD
and UINTR instruction set support.
grandridge
Intel Grand Ridge CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW,
PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE,
PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B,
CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT,
PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL,
AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD
and UINTR instruction set support.
clearwaterforest
Intel Clearwater Forest CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW,
PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE,
PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B,
CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT,
PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI,
ENQCMD, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD,
AVXVNNIINT16, SHA512, SM3, SM4, USER_MSR and PREFETCHI
instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2
k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction
set support.
athlon
athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch
instructions support.
athlon-4
athlon-xp
athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and
full SSE instruction set support.
k8
opteron
athlon64
athlon-fx
Processors based on the AMD K8 core with x86-64 instruction set
support, including the AMD Opteron, Athlon 64, and Athlon 64 FX
processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced
3DNow! and 64-bit instruction set extensions.)
k8-sse3
opteron-sse3
athlon64-sse3
Improved versions of AMD K8 cores with SSE3 instruction set
support.
amdfam10
barcelona
CPUs based on AMD Family 10h cores with x86-64 instruction set
support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!,
enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
bdver1
CPUs based on AMD Family 15h cores with x86-64 instruction set
support. (This supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16,
MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and
64-bit instruction set extensions.)
bdver2
AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP,
LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3,
SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
bdver3
AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE,
AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A,
SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set
extensions.)
bdver4
AMD Family 15h core based CPUs with x86-64 instruction set
support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4,
FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX,
SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit
instruction set extensions.)
znver1
AMD Family 17h core based CPUs with x86-64 instruction set
support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX,
AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16,
MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM,
XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set
extensions.)
znver2
AMD Family 17h core based CPUs with x86-64 instruction set
support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE,
AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16,
MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM,
XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit
instruction set extensions.)
znver3
AMD Family 19h core based CPUs with x86-64 instruction set
support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE,
AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16,
MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM,
XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU,
VPCLMULQDQ, VAES, and 64-bit instruction set extensions.)
znver4
AMD Family 19h core based CPUs with x86-64 instruction set
support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE,
AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16,
MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM,
XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU,
VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD,
AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2,
AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI and 64-bit
instruction set extensions.)
znver5
AMD Family 1ah core based CPUs with x86-64 instruction set
support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE,
AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16,
MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM,
XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU,
VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD,
AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2,
AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI, AVXVNNI,
MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, PREFETCHI and 64-bit
instruction set extensions.)
btver1
CPUs based on AMD Family 14h cores with x86-64 instruction set
support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A,
CX16, ABM and 64-bit instruction set extensions.)
btver2
CPUs based on AMD Family 16h cores with x86-64 instruction set
support. This includes MOVBE, F16C, BMI, AVX, PCLMUL, AES,
SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and
64-bit instruction set extensions.
winchip-c6
IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX
instruction set support.
winchip2
IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX
and 3DNow! instruction set support.
c3 VIA C3 CPU with MMX and 3DNow! instruction set support. (No
scheduling is implemented for this chip.)
c3-2
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set
support. (No scheduling is implemented for this chip.)
c7 VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set
support. (No scheduling is implemented for this chip.)
samuel-2
VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set
support. (No scheduling is implemented for this chip.)
nehemiah
VIA Eden Nehemiah CPU with MMX and SSE instruction set support.
(No scheduling is implemented for this chip.)
esther
VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set
support. (No scheduling is implemented for this chip.)
eden-x2
VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction
set support. (No scheduling is implemented for this chip.)
eden-x4
VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1,
SSE4.2, AVX and AVX2 instruction set support. (No scheduling is
implemented for this chip.)
nano
Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3
instruction set support. (No scheduling is implemented for this
chip.)
nano-1000
VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3
instruction set support. (No scheduling is implemented for this
chip.)
nano-2000
VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3
instruction set support. (No scheduling is implemented for this
chip.)
nano-3000
VIA Nano 3xx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and
SSE4.1 instruction set support. (No scheduling is implemented
for this chip.)
nano-x2
VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3
and SSE4.1 instruction set support. (No scheduling is
implemented for this chip.)
nano-x4
VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3
and SSE4.1 instruction set support. (No scheduling is
implemented for this chip.)
lujiazui
ZHAOXIN lujiazui CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL, RDRND, XSAVE,
XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, FXSR, RDSEED
instruction set support. While the CPUs do support AVX and F16C,
these aren't enabled by "-march=lujiazui" for performance
reasons.
yongfeng
ZHAOXIN yongfeng CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE,
XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED,
AVX2, FMA, SHA, LZCNT instruction set support.
shijidadao
ZHAOXIN shijidadao CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE,
XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED,
AVX2, FMA, SHA, LZCNT instruction set support.
geode
AMD Geode embedded processor with MMX and 3DNow! instruction set
support.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions. While
picking a specific cpu-type schedules things appropriately for that
particular chip, the compiler does not generate any code that cannot
run on the default machine type unless you use a -march=cpu-type
option. For example, if GCC is configured for i686-pc-linux-gnu then
-mtune=pentium4 generates code that is tuned for Pentium 4 but still
runs on i686 machines.
The choices for cpu-type are the same as for -march. In addition,
-mtune supports 2 extra choices for cpu-type:
generic
Produce code optimized for the most common IA32/AMD64/EM64T
processors. If you know the CPU on which your code will run,
then you should use the corresponding -mtune or -march option
instead of -mtune=generic. But, if you do not know exactly what
CPU users of your application will have, then you should use this
option.
As new processors are deployed in the marketplace, the behavior
of this option will change. Therefore, if you upgrade to a newer
version of GCC, code generation controlled by this option will
change to reflect the processors that are most common at the time
that version of GCC is released.
There is no -march=generic option because -march indicates the
instruction set the compiler can use, and there is no generic
instruction set applicable to all processors. In contrast,
-mtune indicates the processor (or, in this case, collection of
processors) for which the code is optimized.
intel
Produce code optimized for the most current Intel processors,
which are Haswell and Silvermont for this version of GCC. If you
know the CPU on which your code will run, then you should use the
corresponding -mtune or -march option instead of -mtune=intel.
But, if you want your application performs better on both Haswell
and Silvermont, then you should use this option.
As new Intel processors are deployed in the marketplace, the
behavior of this option will change. Therefore, if you upgrade
to a newer version of GCC, code generation controlled by this
option will change to reflect the most current Intel processors
at the time that version of GCC is released.
There is no -march=intel option because -march indicates the
instruction set the compiler can use, and there is no common
instruction set applicable to all processors. In contrast,
-mtune indicates the processor (or, in this case, collection of
processors) for which the code is optimized.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-mfpmath=unit
Generate floating-point arithmetic for selected unit unit. The
choices for unit are:
387 Use the standard 387 floating-point coprocessor present on the
majority of chips and emulated otherwise. Code compiled with
this option runs almost everywhere. The temporary results are
computed in 80-bit precision instead of the precision specified
by the type, resulting in slightly different results compared to
most of other chips. See -ffloat-store for more detailed
description.
This is the default choice for non-Darwin x86-32 targets.
sse Use scalar floating-point instructions present in the SSE
instruction set. This instruction set is supported by Pentium
III and newer chips, and in the AMD line by Athlon-4, Athlon XP
and Athlon MP chips. The earlier version of the SSE instruction
set supports only single-precision arithmetic, thus the double
and extended-precision arithmetic are still done using 387. A
later version, present only in Pentium 4 and AMD x86-64 chips,
supports double-precision arithmetic too.
For the x86-32 compiler, you must use -march=cpu-type, -msse or
-msse2 switches to enable SSE extensions and make this option
effective. For the x86-64 compiler, these extensions are enabled
by default.
The resulting code should be considerably faster in the majority
of cases and avoid the numerical instability problems of 387
code, but may break some existing code that expects temporaries
to be 80 bits.
This is the default choice for the x86-64 compiler, Darwin x86-32
targets, and the default choice for x86-32 targets with the SSE2
instruction set when -ffast-math is enabled.
sse,387
sse+387
both
Attempt to utilize both instruction sets at once. This
effectively doubles the amount of available registers, and on
chips with separate execution units for 387 and SSE the execution
resources too. Use this option with care, as it is still
experimental, because the GCC register allocator does not model
separate functional units well, resulting in unstable
performance.
-masm=dialect
Output assembly instructions using selected dialect. Also affects
which dialect is used for basic "asm" and extended "asm". Supported
choices (in dialect order) are att or intel. The default is att.
Darwin does not support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point
comparisons. These correctly handle the case where the result of a
comparison is unordered.
-m80387
-mhard-float
Generate output containing 80387 instructions for floating point.
-mno-80387
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC. Normally the
facilities of the machine's usual C compiler are used, but this
cannot be done directly in cross-compilation. You must make your own
arrangements to provide suitable library functions for cross-
compilation.
On machines where a function returns floating-point results in the
80387 register stack, some floating-point opcodes may be emitted even
if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types
"float" and "double" in an FPU register, even if there is no FPU.
The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in
ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt"
instructions for the 387. Specify this option to avoid generating
those instructions. This option is overridden when -march indicates
that the target CPU always has an FPU and so the instruction does not
need emulation. These instructions are not generated unless you also
use the -funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long"
variables on a two-word boundary or a one-word boundary. Aligning
"double" variables on a two-word boundary produces code that runs
somewhat faster on a Pentium at the expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures containing
the above types are aligned differently than the published
application binary interface specifications for the x86-32 and are
not binary compatible with structures in code compiled without that
switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The x86-32
application binary interface specifies the size to be 96 bits, so
-m96bit-long-double is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer "long double" to be
aligned to an 8- or 16-byte boundary. In arrays or structures
conforming to the ABI, this is not possible. So specifying
-m128bit-long-double aligns "long double" to a 16-byte boundary by
padding the "long double" with an additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as
its ABI specifies that "long double" is aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over
the x87 standard of 80 bits for a "long double".
Warning: if you override the default value for your target ABI, this
changes the size of structures and arrays containing "long double"
variables, as well as modifying the function calling convention for
functions taking "long double". Hence they are not binary-compatible
with code compiled without that switch.
-mlong-double-64
-mlong-double-80
-mlong-double-128
These switches control the size of "long double" type. A size of 64
bits makes the "long double" type equivalent to the "double" type.
This is the default for 32-bit Bionic C library. A size of 128 bits
makes the "long double" type equivalent to the "__float128" type.
This is the default for 64-bit Bionic C library.
Warning: if you override the default value for your target ABI, this
changes the size of structures and arrays containing "long double"
variables, as well as modifying the function calling convention for
functions taking "long double". Hence they are not binary-compatible
with code compiled without that switch.
-malign-data=type
Control how GCC aligns variables. Supported values for type are
compat uses increased alignment value compatible uses GCC 4.8 and
earlier, abi uses alignment value as specified by the psABI, and
cacheline uses increased alignment value to match the cache line
size. compat is the default.
-mlarge-data-threshold=threshold
When -mcmodel=medium or -mcmodel=large is specified, data objects
larger than threshold are placed in large data sections. The default
is 65535.
-mrtd
Use a different function-calling convention, in which functions that
take a fixed number of arguments return with the "ret num"
instruction, which pops their arguments while returning. This saves
one instruction in the caller since there is no need to pop the
arguments there.
You can specify that an individual function is called with this
calling sequence with the function attribute "stdcall". You can also
override the -mrtd option by using the function attribute "cdecl".
Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including "printf"); otherwise
incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function
with too many arguments. (Normally, extra arguments are harmlessly
ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By
default, no registers are used to pass arguments, and at most 3
registers can be used. You can control this behavior for a specific
function by using the function attribute "regparm".
Warning: if you use this switch, and num is nonzero, then you must
build all modules with the same value, including any libraries. This
includes the system libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and double arguments
and return values. You can control this behavior for a specific
function by using the function attribute "sseregparm".
Warning: if you use this switch then you must build all modules with
the same value, including any libraries. This includes the system
libraries and startup modules.
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers. This is
the default on VxWorks to match the ABI of the Sun Studio compilers
until version 12. Only use this option if you need to remain
compatible with existing code produced by those previous compiler
versions or older versions of GCC.
-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32
is specified, the significands of results of floating-point
operations are rounded to 24 bits (single precision); -mpc64 rounds
the significands of results of floating-point operations to 53 bits
(double precision) and -mpc80 rounds the significands of results of
floating-point operations to 64 bits (extended double precision),
which is the default. When this option is used, floating-point
operations in higher precisions are not available to the programmer
without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the
default 80 bits can speed some programs by 2% or more. Note that
some mathematical libraries assume that extended-precision (80-bit)
floating-point operations are enabled by default; routines in such
libraries could suffer significant loss of accuracy, typically
through so-called "catastrophic cancellation", when this option is
used to set the precision to less than extended precision.
-mdaz-ftz
The flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the
MXCSR register are used to control floating-point calculations.SSE
and AVX instructions including scalar and vector instructions could
benefit from enabling the FTZ and DAZ flags when -mdaz-ftz is
specified. Don't set FTZ/DAZ flags when -mno-daz-ftz or -shared is
specified, -mdaz-ftz will set FTZ/DAZ flags even with -shared.
-mstackrealign
Realign the stack at entry. On the x86, the -mstackrealign option
generates an alternate prologue and epilogue that realigns the run-
time stack if necessary. This supports mixing legacy codes that keep
4-byte stack alignment with modern codes that keep 16-byte stack
alignment for SSE compatibility. See also the attribute
"force_align_arg_pointer", applicable to individual functions.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte
boundary. If -mpreferred-stack-boundary is not specified, the
default is 4 (16 bytes or 128 bits).
Warning: When generating code for the x86-64 architecture with SSE
extensions disabled, -mpreferred-stack-boundary=3 can be used to keep
the stack boundary aligned to 8 byte boundary. Since x86-64 ABI
require 16 byte stack alignment, this is ABI incompatible and
intended to be used in controlled environment where stack space is
important limitation. This option leads to wrong code when functions
compiled with 16 byte stack alignment (such as functions from a
standard library) are called with misaligned stack. In this case,
SSE instructions may lead to misaligned memory access traps. In
addition, variable arguments are handled incorrectly for 16 byte
aligned objects (including x87 long double and __int128), leading to
wrong results. You must build all modules with
-mpreferred-stack-boundary=3, including any libraries. This includes
the system libraries and startup modules.
-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num byte
boundary. If -mincoming-stack-boundary is not specified, the one
specified by -mpreferred-stack-boundary is used.
On Pentium and Pentium Pro, "double" and "long double" values should
be aligned to an 8-byte boundary (see -malign-double) or suffer
significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type "__m128" may not work
properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack
boundary must be as aligned as that required by any value stored on
the stack. Further, every function must be generated such that it
keeps the stack aligned. Thus calling a function compiled with a
higher preferred stack boundary from a function compiled with a lower
preferred stack boundary most likely misaligns the stack. It is
recommended that libraries that use callbacks always use the default
setting.
This extra alignment does consume extra stack space, and generally
increases code size. Code that is sensitive to stack space usage,
such as embedded systems and operating system kernels, may want to
reduce the preferred alignment to -mpreferred-stack-boundary=2.
-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mptwrite
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mrdseed
-msgx
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-madx
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mhle
-mtbm
-mmwaitx
-mclzero
-mpku
-mavx512vbmi2
-mavx512bf16
-mavx512fp16
-mgfni
-mvaes
-mwaitpkg
-mvpclmulqdq
-mavx512bitalg
-mmovdiri
-mmovdir64b
-menqcmd
-muintr
-mtsxldtrk
-mavx512vpopcntdq
-mavx512vp2intersect
-mavx512vnni
-mavxvnni
-mcldemote
-mserialize
-mamx-tile
-mamx-int8
-mamx-bf16
-mhreset
-mkl
-mwidekl
-mavxifma
-mavxvnniint8
-mavxneconvert
-mcmpccxadd
-mamx-fp16
-mprefetchi
-mraoint
-mamx-complex
-mavxvnniint16
-msm3
-msha512
-msm4
-mapxf
-musermsr
-mavx10.1
-mavx10.1-256
-mavx10.1-512
-mavx10.2
-mamx-avx512
-mamx-tf32
-mamx-transpose
-mamx-fp8
-mmovrs
-mamx-movrs
These switches enable the use of instructions in the MMX, SSE,
AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA,
AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA,
PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, RDSEED, SGX, XOP, LWP,
3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR,
XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU,
AVX512VBMI2, GFNI, VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI,
MOVDIR64B, AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX512VNNI,
SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL,
AVXVNNI, AVX512-FP16, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD,
AMX-FP16, PREFETCHI, RAOINT, AMX-COMPLEX, AVXVNNIINT16, SM3, SHA512,
SM4, APX_F, USER_MSR, AVX10.1, AVX10.2, AMX-AVX512, AMX-TF32, AMX-
TRANSPOSE, AMX-FP8, MOVRS, AMX-MOVRS or CLDEMOTE extended instruction
sets. Each has a corresponding -mno- option to disable use of these
instructions.
These extensions are also available as built-in functions: see x86
Built-in Functions, for details of the functions enabled and disabled
by these switches.
Note that -msse4 enables both SSE4.1 and SSE4.2 support, while
-mno-sse4 turns off those features; neither form of the option
affects SSE4A support, controlled separately by -msse4a.
To generate SSE/SSE2 instructions automatically from floating-point
code (as opposed to 387 instructions), see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it
generates new AVX instructions or AVX equivalence for all SSEx
instructions when needed.
These options enable GCC to use these extended instructions in
generated code, even without -mfpmath=sse. Applications that perform
run-time CPU detection must compile separate files for each supported
architecture, using the appropriate flags. In particular, the file
containing the CPU detection code should be compiled without these
options.
-mdump-tune-features
This option instructs GCC to dump the names of the x86 performance
tuning features and default settings. The names can be used in
-mtune-ctrl=feature-list.
-mtune-ctrl=feature-list
This option is used to do fine grain control of x86 code generation
features. feature-list is a comma separated list of feature names.
See also -mdump-tune-features. When specified, the feature is turned
on if it is not preceded with ^, otherwise, it is turned off.
-mtune-ctrl=feature-list is intended to be used by GCC developers.
Using it may lead to code paths not covered by testing and can
potentially result in compiler ICEs or runtime errors.
-mno-default
This option instructs GCC to turn off all tunable features. See also
-mtune-ctrl=feature-list and -mdump-tune-features.
-mcld
This option instructs GCC to emit a "cld" instruction in the prologue
of functions that use string instructions. String instructions
depend on the DF flag to select between autoincrement or
autodecrement mode. While the ABI specifies the DF flag to be
cleared on function entry, some operating systems violate this
specification by not clearing the DF flag in their exception
dispatchers. The exception handler can be invoked with the DF flag
set, which leads to wrong direction mode when string instructions are
used. This option can be enabled by default on 32-bit x86 targets by
configuring GCC with the --enable-cld configure option. Generation
of "cld" instructions can be suppressed with the -mno-cld compiler
option in this case.
-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction before a
transfer of control flow out of the function to minimize the AVX to
SSE transition penalty as well as remove unnecessary "zeroupper"
intrinsics.
-mprefer-avx128
This option instructs GCC to use 128-bit AVX instructions instead of
256-bit AVX instructions in the auto-vectorizer.
-mprefer-vector-width=opt
This option instructs GCC to use opt-bit vector width in instructions
instead of default on the selected platform.
-mpartial-vector-fp-math
This option enables GCC to generate floating-point operations that
might affect the set of floating-point status flags on partial
vectors, where vector elements reside in the low part of the 128-bit
SSE register. Unless -fno-trapping-math is specified, the compiler
guarantees correct behavior by sanitizing all input operands to have
zeroes in the unused upper part of the vector register. Note that by
using built-in functions or inline assembly with partial vector
arguments, NaNs, denormal or invalid values can leak into the upper
part of the vector, causing possible performance issues when
-fno-trapping-math is in effect. These issues can be mitigated by
manually sanitizing the upper part of the partial vector argument
register or by using -mdaz-ftz to set denormals-are-zero (DAZ) flag
in the MXCSR register.
This option is enabled by default.
-mmove-max=bits
This option instructs GCC to set the maximum number of bits can be
moved from memory to memory efficiently to bits. The valid bits are
128, 256 and 512.
-mstore-max=bits
This option instructs GCC to set the maximum number of bits can be
stored to memory efficiently to bits. The valid bits are 128, 256
and 512.
none
No extra limitations applied to GCC other than defined by the
selected platform.
128 Prefer 128-bit vector width for instructions.
256 Prefer 256-bit vector width for instructions.
512 Prefer 512-bit vector width for instructions.
-mnoreturn-no-callee-saved-registers
This option optimizes functions with "noreturn" attribute or
"_Noreturn" specifier by not saving in the function prologue callee-
saved registers which are used in the function (except for the "BP"
register). This option can interfere with debugging of the caller of
the "noreturn" function or any function further up in the call stack,
so it is not enabled by default.
-mcx16
This option enables GCC to generate "CMPXCHG16B" instructions in
64-bit code to implement compare-and-exchange operations on 16-byte
aligned 128-bit objects. This is useful for atomic updates of data
structures exceeding one machine word in size. The compiler uses
this instruction to implement __sync Builtins. However, for __atomic
Builtins operating on 128-bit integers, a library call is always
used.
-msahf
This option enables generation of "SAHF" instructions in 64-bit code.
Early Intel Pentium 4 CPUs with Intel 64 support, prior to the
introduction of Pentium 4 G1 step in December 2005, lacked the "LAHF"
and "SAHF" instructions which are supported by AMD64. These are load
and store instructions, respectively, for certain status flags. In
64-bit mode, the "SAHF" instruction is used to optimize "fmod",
"drem", and "remainder" built-in functions; see Other Builtins for
details.
-mmovbe
This option enables use of the "movbe" instruction to optimize byte
swapping of four and eight byte entities.
-mshstk
The -mshstk option enables shadow stack built-in functions from x86
Control-flow Enforcement Technology (CET).
-mcrc32
This option enables built-in functions "__builtin_ia32_crc32qi",
"__builtin_ia32_crc32hi", "__builtin_ia32_crc32si" and
"__builtin_ia32_crc32di" to generate the "crc32" machine instruction.
-mmwait
This option enables built-in functions "__builtin_ia32_monitor", and
"__builtin_ia32_mwait" to generate the "monitor" and "mwait" machine
instructions.
-mrecip
This option enables use of "RCPSS" and "RSQRTSS" instructions (and
their vectorized variants "RCPPS" and "RSQRTPS") with an additional
Newton-Raphson step to increase precision instead of "DIVSS" and
"SQRTSS" (and their vectorized variants) for single-precision
floating-point arguments. These instructions are generated only when
-funsafe-math-optimizations is enabled together with
-ffinite-math-only and -fno-trapping-math. Note that while the
throughput of the sequence is higher than the throughput of the non-
reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of "RSQRTSS" (or
"RSQRTPS") already with -ffast-math (or the above option
combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional Newton-
Raphson step for vectorized single-float division and vectorized
"sqrtf(x)" already with -ffast-math (or the above option
combination), and doesn't need -mrecip.
-mrecip=opt
This option controls which reciprocal estimate instructions may be
used. opt is a comma-separated list of options, which may be
preceded by a ! to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the approximation for scalar division.
vec-div
Enable the approximation for vectorized division.
sqrt
Enable the approximation for scalar square root.
vec-sqrt
Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal
approximations, except for square root.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an
external library. Supported values for type are svml for the Intel
short vector math library, aocl for the math library (LibM) from AMD
Optimizing CPU Libraries (AOCL) and acml for the end-of-life AMD core
math library (to which AOCL-LibM is the successor). To use this
option, both -ftree-vectorize and -funsafe-math-optimizations have to
be enabled, and an SVML or ACML ABI-compatible library must be
specified at link time.
GCC currently emits calls to "vmldExp2", "vmldLn2", "vmldLog102",
"vmldPow2", "vmldTanh2", "vmldTan2", "vmldAtan2", "vmldAtanh2",
"vmldCbrt2", "vmldSinh2", "vmldSin2", "vmldAsinh2", "vmldAsin2",
"vmldCosh2", "vmldCos2", "vmldAcosh2", "vmldAcos2", "vmlsExp4",
"vmlsLn4", "vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4",
"vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4",
"vmlsAsinh4", "vmlsAsin4", "vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and
"vmlsAcos4" for corresponding function type when -mveclibabi=svml is
used, "amd_vrs4_acosf", "amd_vrs16_acosf", "amd_vrd8_asin",
"amd_vrs4_asinf", "amd_vrs8_asinf", "amd_vrs16_asinf",
"amd_vrd2_atan", "amd_vrd8_atan", "amd_vrs4_atanf", "amd_vrs8_atanf",
"amd_vrs16_atanf", "amd_vrd2_cos", "amd_vrd4_cos", "amd_vrd8_cos",
"amd_vrs4_cosf", "amd_vrs8_cosf", "amd_vrs16_cosf", "amd_vrs4_coshf",
"amd_vrs8_coshf", "amd_vrd2_erf", "amd_vrd4_erf", "amd_vrd8_erf",
"amd_vrs4_erff", "amd_vrs8_erff", "amd_vrs16_erff", "amd_vrd2_exp",
"amd_vrd4_exp", "amd_vrd8_exp", "amd_vrs4_expf", "amd_vrs8_expf",
"amd_vrs16_expf", "amd_vrd2_exp10", "amd_vrs4_exp10f",
"amd_vrd2_exp2", "amd_vrd4_exp2", "amd_vrd8_exp2", "amd_vrs4_exp2f",
"amd_vrs8_exp2f", "amd_vrs16_exp2f", "amd_vrs4_expm1f",
"amd_vrd2_log", "amd_vrd4_log", "amd_vrd8_log", "amd_vrs4_logf",
"amd_vrs8_logf", "amd_vrs16_logf", "amd_vrd2_log10",
"amd_vrs4_log10f", "amd_vrs8_log10f", "amd_vrs16_log10f",
"amd_vrd2_log1p", "amd_vrs4_log1pf", "amd_vrd2_log2",
"amd_vrd4_log2", "amd_vrd8_log2", "amd_vrs4_log2f", "amd_vrs8_log2f",
"amd_vrs16_log2f", "amd_vrd2_pow", "amd_vrd4_pow", "amd_vrd8_pow",
"amd_vrs4_powf", "amd_vrs8_powf", "amd_vrs16_powf", "amd_vrd2_sin",
"amd_vrd4_sin", "amd_vrd8_sin", "amd_vrs4_sinf", "amd_vrs8_sinf",
"amd_vrs16_sinf", "amd_vrd2_tan", "amd_vrd4_tan", "amd_vrd8_tan",
"amd_vrs16_tanf", "amd_vrs4_tanhf", "amd_vrs8_tanhf",
"amd_vrs16_tanhf" for the corresponding function type when
-mveclibabi=aocl is used, and "__vrd2_sin", "__vrd2_cos",
"__vrd2_exp", "__vrd2_log", "__vrd2_log2", "__vrd2_log10",
"__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf", "__vrs4_logf",
"__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf" for the
corresponding function type when -mveclibabi=acml is used.
-mabi=name
Generate code for the specified calling convention. Permissible
values are sysv for the ABI used on GNU/Linux and other systems, and
ms for the Microsoft ABI. The default is to use the Microsoft ABI
when targeting Microsoft Windows and the SysV ABI on all other
systems. You can control this behavior for specific functions by
using the function attributes "ms_abi" and "sysv_abi".
-mforce-indirect-call
Force all calls to functions to be indirect. This is useful when
using Intel Processor Trace where it generates more precise timing
information for function calls.
-mmanual-endbr
Insert ENDBR instruction at function entry only via the "cf_check"
function attribute. This is useful when used with the option
-fcf-protection=branch to control ENDBR insertion at the function
entry.
-mcet-switch
By default, CET instrumentation is turned off on switch statements
that use a jump table and indirect branch track is disabled. Since
jump tables are stored in read-only memory, this does not result in a
direct loss of hardening. But if the jump table index is attacker-
controlled, the indirect jump may not be constrained by CET. This
option turns on CET instrumentation to enable indirect branch track
for switch statements with jump tables which leads to the jump
targets reachable via any indirect jumps.
-mcall-ms2sysv-xlogues
Due to differences in 64-bit ABIs, any Microsoft ABI function that
calls a System V ABI function must consider RSI, RDI and XMM6-15 as
clobbered. By default, the code for saving and restoring these
registers is emitted inline, resulting in fairly lengthy prologues
and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and
epilogues that use stubs in the static portion of libgcc to perform
these saves and restores, thus reducing function size at the cost of
a few extra instructions.
-mtls-dialect=type
Generate code to access thread-local storage using the gnu or gnu2
conventions. gnu is the conservative default; gnu2 is more
efficient, but it may add compile- and run-time requirements that
cannot be satisfied on all systems.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is
shorter and usually equally fast as method using SUB/MOV operations
and is enabled by default. In some cases disabling it may improve
performance because of improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing
arguments is computed in the function prologue. This is faster on
most modern CPUs because of reduced dependencies, improved scheduling
and reduced stack usage when the preferred stack boundary is not
equal to 2. The drawback is a notable increase in code size. This
switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on MinGW. Programs that rely
on thread-safe exception handling must compile and link all code with
the -mthreads option. When compiling, -mthreads defines -D_MT; when
linking, it links in a special thread helper library -lmingwthrd
which cleans up per-thread exception-handling data.
-mms-bitfields
-mno-ms-bitfields
Enable/disable bit-field layout compatible with the native Microsoft
Windows compiler.
If "packed" is used on a structure, or if bit-fields are used, it may
be that the Microsoft ABI lays out the structure differently than the
way GCC normally does. Particularly when moving packed data between
functions compiled with GCC and the native Microsoft compiler (either
via function call or as data in a file), it may be necessary to
access either format.
This option is enabled by default for Microsoft Windows targets.
This behavior can also be controlled locally by use of variable or
type attributes. For more information, see x86 Variable Attributes
and x86 Type Attributes.
The Microsoft structure layout algorithm is fairly simple with the
exception of the bit-field packing. The padding and alignment of
members of structures and whether a bit-field can straddle a storage-
unit boundary are determine by these rules:
1. Structure members are stored sequentially in the order in which
they are
declared: the first member has the lowest memory address and the
last member the highest.
2. Every data object has an alignment requirement. The alignment
requirement
for all data except structures, unions, and arrays is either the
size of the object or the current packing size (specified with
either the "aligned" attribute or the "pack" pragma), whichever
is less. For structures, unions, and arrays, the alignment
requirement is the largest alignment requirement of its members.
Every object is allocated an offset so that:
offset % alignment_requirement == 0
3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte
allocation
unit if the integral types are the same size and if the next bit-
field fits into the current allocation unit without crossing the
boundary imposed by the common alignment requirements of the bit-
fields.
MSVC interprets zero-length bit-fields in the following ways:
1. If a zero-length bit-field is inserted between two bit-fields that
are normally coalesced, the bit-fields are not coalesced.
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;
The size of "t1" is 8 bytes with the zero-length bit-field. If
the zero-length bit-field were removed, "t1"'s size would be 4
bytes.
2. If a zero-length bit-field is inserted after a bit-field, "foo",
and the
alignment of the zero-length bit-field is greater than the member
that follows it, "bar", "bar" is aligned as the type of the zero-
length bit-field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;
For "t2", "bar" is placed at offset 2, rather than offset 1.
Accordingly, the size of "t2" is 4. For "t3", the zero-length
bit-field does not affect the alignment of "bar" or, as a result,
the size of the structure.
Taking this into account, it is important to note the following:
1. If a zero-length bit-field follows a normal bit-field, the
type of the
zero-length bit-field may affect the alignment of the
structure as whole. For example, "t2" has a size of 4 bytes,
since the zero-length bit-field follows a normal bit-field,
and is of type short.
2. Even if a zero-length bit-field is not followed by a normal
bit-field, it may
still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;
Here, "t4" takes up 4 bytes.
3. Zero-length bit-fields following non-bit-field members are
ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;
Here, "t5" takes up 2 bytes.
-mno-align-stringops
Do not align the destination of inlined string operations. This
switch reduces code size and improves performance in case the
destination is already aligned, but GCC doesn't know about it.
-minline-all-stringops
By default GCC inlines string operations only when the destination is
known to be aligned to least a 4-byte boundary. This enables more
inlining and increases code size, but may improve performance of code
that depends on fast "memcpy" and "memset" for short lengths. The
option enables inline expansion of "strlen" for all pointer
alignments.
-minline-stringops-dynamically
For string operations of unknown size, use run-time checks with
inline code for small blocks and a library call for large blocks.
-mstringop-strategy=alg
Override the internal decision heuristic for the particular algorithm
to use for inlining string operations. The allowed values for alg
are:
rep_byte
rep_4byte
rep_8byte
Expand using i386 "rep" prefix of the specified size.
byte_loop
loop
unrolled_loop
Expand into an inline loop.
libcall
Always use a library call.
-mmemcpy-strategy=strategy
Override the internal decision heuristic to decide if
"__builtin_memcpy" should be inlined and what inline algorithm to use
when the expected size of the copy operation is known. strategy is a
comma-separated list of alg:max_size:dest_align triplets. alg is
specified in -mstringop-strategy, max_size specifies the max byte
size with which inline algorithm alg is allowed. For the last
triplet, the max_size must be "-1". The max_size of the triplets in
the list must be specified in increasing order. The minimal byte
size for alg is 0 for the first triplet and "max_size + 1" of the
preceding range.
-mmemset-strategy=strategy
The option is similar to -mmemcpy-strategy= except that it is to
control "__builtin_memset" expansion.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This
avoids the instructions to save, set up, and restore frame pointers
and makes an extra register available in leaf functions. The option
-fomit-leaf-frame-pointer removes the frame pointer for leaf
functions, which might make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the
TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the
thread base pointer must be added. Whether or not this is valid
depends on the operating system, and whether it maps the segment to
cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions with VEX
prefix. The option -mavx turns this on by default.
-mfentry
-mno-fentry
If profiling is active (-pg), put the profiling counter call before
the prologue. Note: On x86 architectures the attribute
"ms_hook_prologue" isn't possible at the moment for -mfentry and -pg.
-mrecord-mcount
-mno-record-mcount
If profiling is active (-pg), generate a __mcount_loc section that
contains pointers to each profiling call. This is useful for
automatically patching and out calls.
-mnop-mcount
-mno-nop-mcount
If profiling is active (-pg), generate the calls to the profiling
functions as NOPs. This is useful when they should be patched in
later dynamically. This is likely only useful together with
-mrecord-mcount.
-minstrument-return=type
Instrument function exit in -pg -mfentry instrumented functions with
call to specified function. This only instruments true returns ending
with ret, but not sibling calls ending with jump. Valid types are
none to not instrument, call to generate a call to __return__, or
nop5 to generate a 5 byte nop.
-mrecord-return
-mno-record-return
Generate a __return_loc section pointing to all return
instrumentation code.
-mfentry-name=name
Set name of __fentry__ symbol called at function entry for -pg
-mfentry functions.
-mfentry-section=name
Set name of section to record -mrecord-mcount calls (default
__mcount_loc).
-mskip-rax-setup
-mno-skip-rax-setup
When generating code for the x86-64 architecture with SSE extensions
disabled, -mskip-rax-setup can be used to skip setting up RAX
register when there are no variable arguments passed in vector
registers.
Warning: Since RAX register is used to avoid unnecessarily saving
vector registers on stack when passing variable arguments, the
impacts of this option are callees may waste some stack space,
misbehave or jump to a random location. GCC 4.4 or newer don't have
those issues, regardless the RAX register value.
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer divide is
much faster than 32-bit/64-bit integer divide. This option generates
a run-time check. If both dividend and divisor are within range of 0
to 255, 8-bit unsigned integer divide is used instead of
32-bit/64-bit integer divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol
Generate stack protection code using canary at guard. Supported
locations are global for global canary or tls for per-thread canary
in the TLS block (the default). This option has effect only when
-fstack-protector or -fstack-protector-all is specified.
With the latter choice the options -mstack-protector-guard-reg=reg
and -mstack-protector-guard-offset=offset furthermore specify which
segment register (%fs or %gs) to use as base register for reading the
canary, and from what offset from that base register. The default
for those is as specified in the relevant ABI.
-mstack-protector-guard-symbol=symbol overrides the offset with a
symbol reference to a canary in the TLS block.
-mgeneral-regs-only
Generate code that uses only the general-purpose registers. This
prevents the compiler from using floating-point, vector, mask and
bound registers.
-mrelax-cmpxchg-loop
When emitting a compare-and-swap loop for __sync Builtins and
__atomic Builtins lacking a native instruction, optimize for the
highly contended case by issuing an atomic load before the "CMPXCHG"
instruction, and using the "PAUSE" instruction to save CPU power when
restarting the loop.
-mindirect-branch=choice
Convert indirect call and jump with choice. The default is keep,
which keeps indirect call and jump unmodified. thunk converts
indirect call and jump to call and return thunk. thunk-inline
converts indirect call and jump to inlined call and return thunk.
thunk-extern converts indirect call and jump to external call and
return thunk provided in a separate object file. You can control
this behavior for a specific function by using the function attribute
"indirect_branch".
Note that -mcmodel=large is incompatible with -mindirect-branch=thunk
and -mindirect-branch=thunk-extern since the thunk function may not
be reachable in the large code model.
Note that -mindirect-branch=thunk-extern is compatible with
-fcf-protection=branch since the external thunk can be made to enable
control-flow check.
-mfunction-return=choice
Convert function return with choice. The default is keep, which
keeps function return unmodified. thunk converts function return to
call and return thunk. thunk-inline converts function return to
inlined call and return thunk. thunk-extern converts function return
to external call and return thunk provided in a separate object file.
You can control this behavior for a specific function by using the
function attribute "function_return".
Note that -mindirect-return=thunk-extern is compatible with
-fcf-protection=branch since the external thunk can be made to enable
control-flow check.
Note that -mcmodel=large is incompatible with -mfunction-return=thunk
and -mfunction-return=thunk-extern since the thunk function may not
be reachable in the large code model.
-mindirect-branch-register
Force indirect call and jump via register.
-mharden-sls=choice
Generate code to mitigate against straight line speculation (SLS)
with choice. The default is none which disables all SLS hardening.
return enables SLS hardening for function returns. indirect-jmp
enables SLS hardening for indirect jumps. all enables all SLS
hardening.
-mindirect-branch-cs-prefix
Add CS prefix to call and jmp to indirect thunk with branch target in
r8-r15 registers so that the call and jmp instruction length is 6
bytes to allow them to be replaced with lfence; call *%r8-r15 or
lfence; jmp *%r8-r15 at run-time.
-mapx-inline-asm-use-gpr32
For inline asm support with APX, by default the EGPR feature was
disabled to prevent potential illegal instruction with EGPR occurs.
To invoke egpr usage in inline asm, use new compiler option
-mapx-inline-asm-use-gpr32 and user should ensure the instruction
supports EGPR.
-mevex512
-mno-evex512
Enables/disables 512-bit vector. It will be default on if AVX512F is
enabled.
These -m switches are supported in addition to the above on x86-64
processors in 64-bit environments.
-m32
-m64
-mx32
-m16
-miamcu
Generate code for a 16-bit, 32-bit or 64-bit environment. The -m32
option sets "int", "long", and pointer types to 32 bits, and
generates code that runs in 32-bit mode.
The -m64 option sets "int" to 32 bits and "long" and pointer types to
64 bits, and generates code for the x86-64 architecture. For Darwin
only the -m64 option also turns off the -fno-pic and -mdynamic-no-pic
options.
The -mx32 option sets "int", "long", and pointer types to 32 bits,
and generates code for the x86-64 architecture.
The -m16 option is the same as -m32, except for that it outputs the
".code16gcc" assembly directive at the beginning of the assembly
output so that the binary can run in 16-bit mode.
The -miamcu option generates code which conforms to Intel MCU psABI.
It requires the -m32 option to be turned on.
-mno-red-zone
Do not use a so-called "red zone" for x86-64 code. The red zone is
mandated by the x86-64 ABI; it is a 128-byte area beyond the location
of the stack pointer that is not modified by signal or interrupt
handlers and therefore can be used for temporary data without
adjusting the stack pointer. The flag -mno-red-zone disables this
red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols
must be linked in the lower 2 GB of the address space. Pointers are
64 bits. Programs can be statically or dynamically linked. This is
the default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the
negative 2 GB of the address space. This model has to be used for
Linux kernel code.
-mcmodel=medium
Generate code for the medium model: the program is linked in the
lower 2 GB of the address space. Small symbols are also placed
there. Symbols with sizes larger than -mlarge-data-threshold are put
into large data or BSS sections and can be located above 2GB.
Programs can be statically or dynamically linked.
-mcmodel=large
Generate code for the large model. This model makes no assumptions
about addresses and sizes of sections.
-maddress-mode=long
Generate code for long address mode. This is only supported for
64-bit and x32 environments. It is the default address mode for
64-bit environments.
-maddress-mode=short
Generate code for short address mode. This is only supported for
32-bit and x32 environments. It is the default address mode for
32-bit and x32 environments.
-mneeded
-mno-needed
Emit GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target to
indicate the micro-architecture ISA level required to execute the
binary.
-mno-direct-extern-access
Without -fpic nor -fPIC, always use the GOT pointer to access
external symbols. With -fpic or -fPIC, treat access to protected
symbols as local symbols. The default is -mdirect-extern-access.
Warning: shared libraries compiled with -mno-direct-extern-access and
executable compiled with -mdirect-extern-access may not be binary
compatible if protected symbols are used in shared libraries and
executable.
-munroll-only-small-loops
Controls conservative small loop unrolling. It is default enabled by
O2, and unrolls loop with less than 4 insns by 1 time. Explicit
-f[no-]unroll-[all-]loops would disable this flag to avoid any
unintended unrolling behavior that user does not want.
-mlam=choice
LAM(linear-address masking) allows special bits in the pointer to be
used for metadata. The default is none. With u48, pointer bits in
positions 62:48 can be used for metadata; With u57, pointer bits in
positions 62:57 can be used for metadata.
x86 Windows Options
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading constant
values. The "CONST16" instruction is currently not a standard option
from Tensilica. When enabled, "CONST16" instructions are always used
in place of the standard "L32R" instructions. The use of "CONST16"
is enabled by default only if the "L32R" instruction is not
available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract
instructions in the floating-point option. This has no effect if the
floating-point option is not also enabled. Disabling fused
multiply/add and multiply/subtract instructions forces the compiler
to use separate instructions for the multiply and add/subtract
operations. This may be desirable in some cases where strict IEEE
754-compliant results are required: the fused multiply add/subtract
instructions do not round the intermediate result, thereby producing
results with more bits of precision than specified by the IEEE
standard. Disabling fused multiply add/subtract instructions also
ensures that the program output is not sensitive to the compiler's
ability to combine multiply and add/subtract operations.
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions before
"volatile" memory references to guarantee sequential consistency.
The default is -mserialize-volatile. Use -mno-serialize-volatile to
omit the "MEMW" instructions.
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code must be
position-independent code (PIC), this option disables PIC for
compiling kernel code.
-mtext-section-literals
-mno-text-section-literals
These options control the treatment of literal pools. The default is
-mno-text-section-literals, which places literals in a separate
section in the output file. This allows the literal pool to be
placed in a data RAM/ROM, and it also allows the linker to combine
literal pools from separate object files to remove redundant literals
and improve code size. With -mtext-section-literals, the literals
are interspersed in the text section in order to keep them as close
as possible to their references. This may be necessary for large
assembly files. Literals for each function are placed right before
that function.
-mauto-litpools
-mno-auto-litpools
These options control the treatment of literal pools. The default is
-mno-auto-litpools, which places literals in a separate section in
the output file unless -mtext-section-literals is used. With
-mauto-litpools the literals are interspersed in the text section by
the assembler. Compiler does not produce explicit ".literal"
directives and loads literals into registers with "MOVI" instructions
instead of "L32R" to let the assembler do relaxation and place
literals as necessary. This option allows assembler to create
several literal pools per function and assemble very big functions,
which may not be possible with -mtext-section-literals.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to
automatically align instructions to reduce branch penalties at the
expense of some code density. The assembler attempts to widen
density instructions to align branch targets and the instructions
following call instructions. If there are not enough preceding safe
density instructions to align a target, no widening is performed.
The default is -mtarget-align. These options do not affect the
treatment of auto-aligned instructions like "LOOP", which the
assembler always aligns, either by widening density instructions or
by inserting NOP instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate
direct calls to indirect calls unless it can determine that the
target of a direct call is in the range allowed by the call
instruction. This translation typically occurs for calls to
functions in other source files. Specifically, the assembler
translates a direct "CALL" instruction into an "L32R" followed by a
"CALLX" instruction. The default is -mno-longcalls. This option
should be used in programs where the call target can potentially be
out of range. This option is implemented in the assembler, not the
compiler, so the assembly code generated by GCC still shows direct
call instructions---look at the disassembled object code to see the
actual instructions. Note that the assembler uses an indirect call
for every cross-file call, not just those that really are out of
range.
-mabi=name
Generate code for the specified ABI. Permissible values are: call0,
windowed. Default ABI is chosen by the Xtensa core configuration.
-mabi=call0
When this option is enabled function parameters are passed in
registers "a2" through "a7", registers "a12" through "a15" are
caller-saved, and register "a15" may be used as a frame pointer.
When this version of the ABI is enabled the C preprocessor symbol
"__XTENSA_CALL0_ABI__" is defined.
-mabi=windowed
When this option is enabled function parameters are passed in
registers "a10" through "a15", and called function rotates register
window by 8 registers on entry so that its arguments are found in
registers "a2" through "a7". Register "a7" may be used as a frame
pointer. Register window is rotated 8 registers back upon return.
When this version of the ABI is enabled the C preprocessor symbol
"__XTENSA_WINDOWED_ABI__" is defined.
-mextra-l32r-costs=n
Specify an extra cost of instruction RAM/ROM access for "L32R"
instructions, in clock cycles. This affects, when optimizing for
speed, whether loading a constant from literal pool using "L32R" or
synthesizing the constant from a small one with a couple of
arithmetic instructions. The default value is 0.
-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned on
a natural object boundary as described in the architecture
specification. The default is -mno-strict-align for cores that
support both unaligned loads and stores in hardware and
-mstrict-align for all other cores.
zSeries Options
These are listed under
ENVIRONMENT
This section describes several environment variables that affect how GCC
operates. Some of them work by specifying directories or prefixes to use
when searching for various kinds of files. Some are used to specify
other aspects of the compilation environment.
Note that you can also specify places to search using options such as -B,
-I and -L. These take precedence over places specified using environment
variables, which in turn take precedence over those specified by the
configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses
localization information which allows GCC to work with different
national conventions. GCC inspects the locale categories LC_CTYPE
and LC_MESSAGES if it has been configured to do so. These locale
categories can be set to any value supported by your installation. A
typical value is en_GB.UTF-8 for English in the United Kingdom
encoded in UTF-8.
The LC_CTYPE environment variable specifies character classification.
GCC uses it to determine the character boundaries in a string; this
is needed for some multibyte encodings that contain quote and escape
characters that are otherwise interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in
diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of
LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default
to the value of the LANG environment variable. If none of these
variables are set, GCC defaults to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary
files. GCC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for
example, the output of the preprocessor, which is the input to the
compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing
-fcompare-debug to the compiler driver. See the documentation of
this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names
of the subprograms executed by the compiler. No slash is added when
this prefix is combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an
appropriate prefix to use based on the pathname it is invoked with.
If GCC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix
is the prefix to the installed compiler. In many cases prefix is the
value of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are
used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with /usr/local/lib/gcc (more
precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing
that beginning with the specified prefix to produce an alternate
directory name. Thus, with -Bfoo/, GCC searches foo/bar just before
it searches the standard directory /usr/local/lib/bar. If a standard
directory begins with the configured prefix then the value of prefix
is replaced by GCC_EXEC_PREFIX when looking for header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories,
much like PATH. GCC tries the directories thus specified when
searching for subprograms, if it cannot find the subprograms using
GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories,
much like PATH. When configured as a native compiler, GCC tries the
directories thus specified when searching for special linker files,
if it cannot find them using GCC_EXEC_PREFIX. Linking using GCC also
uses these directories when searching for ordinary libraries for the
-l option (but directories specified with -L come first).
LANG
This variable is used to pass locale information to the compiler.
One way in which this information is used is to determine the
character set to be used when character literals, string literals and
comments are parsed in C and C++. When the compiler is configured to
allow multibyte characters, the following values for LANG are
recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the
compiler uses "mblen" and "mbtowc" as defined by the default locale
to recognize and translate multibyte characters.
GCC_EXTRA_DIAGNOSTIC_OUTPUT
If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values,
then additional text will be emitted to stderr when fix-it hints are
emitted. -fdiagnostics-parseable-fixits and
-fno-diagnostics-parseable-fixits take precedence over this
environment variable.
fixits-v1
Emit parseable fix-it hints, equivalent to
-fdiagnostics-parseable-fixits. In particular, columns are
expressed as a count of bytes, starting at byte 1 for the initial
column.
fixits-v2
As "fixits-v1", but columns are expressed as display columns, as
per -fdiagnostics-column-unit=display.
Some additional environment variables affect the behavior of the
preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special
character, much like PATH, in which to look for header files. The
special character, "PATH_SEPARATOR", is target-dependent and
determined at GCC build time. For Microsoft Windows-based targets it
is a semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if specified
with -I, but after any paths given with -I options on the command
line. This environment variable is used regardless of which language
is being preprocessed.
The remaining environment variables apply only when preprocessing the
particular language indicated. Each specifies a list of directories
to be searched as if specified with -isystem, but after any paths
given with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to
search its current working directory. Empty elements can appear at
the beginning or end of a path. For instance, if the value of CPATH
is ":/special/include", that has the same effect as
-I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files processed
by the compiler. System header files are ignored in the dependency
output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which
case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form file
target, in which case the rules are written to file file using target
as the target name.
In other words, this environment variable is equivalent to combining
the options -MM and -MF, with an optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except
that system header files are not ignored, so it implies -M rather
than -MM. However, the dependence on the main input file is omitted.
SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp to be
used in replacement of the current date and time in the "__DATE__"
and "__TIME__" macros, so that the embedded timestamps become
reproducible.
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as
the number of seconds (excluding leap seconds) since 01 Jan 1970
00:00:00 represented in ASCII; identical to the output of "date +%s"
on GNU/Linux and other systems that support the %s extension in the
"date" command.
The value should be a known timestamp such as the last modification
time of the source or package and it should be set by the build
process.
BUGS
For instructions on reporting bugs, see
<https://trac.macports.org/newticket>.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code
for constructors to work. On multi-libbed systems, gcc -shared must
select the correct support libraries to link against. Failing to
supply the correct flags may lead to subtle defects. Supplying them
in cases where they are not necessary is innocuous. -shared
suppresses the addition of startup code to alter the floating-point
environment as done with -ffast-math, -Ofast or
-funsafe-math-optimizations on some targets.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1)
and the Info entries for gcc, cpp, as, ld, binutils and gdb.
AUTHOR
See the Info entry for gcc, or
<https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors
to GCC.
COPYRIGHT
Copyright (c) 1988-2025 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or any
later version published by the Free Software Foundation; with the
Invariant Sections being "GNU General Public License" and "Funding Free
Software", the Front-Cover texts being (a) (see below), and with the
Back-Cover Texts being (b) (see below). A copy of the license is
included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-15.3.0 2026-06-12 GCC(1)
gcc15 15.3.0 - Generated Sat Oct 10 07:19:20 CDT 2026