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 -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
-fallow-parameterless-variadic-functions -fno-asm -fno-builtin
-fno-builtin-function -fcond-mismatch -ffreestanding -fgimple
-fgnu-tm -fgnu89-inline -fhosted -flax-vector-conversions
-fms-extensions -foffload=arg -foffload-options=arg -fopenacc
-fopenacc-dim=geom -fopenmp -fopenmp-simd
-fpermitted-flt-eval-methods=standard -fplan9-extensions
-fsigned-bitfields -funsigned-bitfields -fsigned-char
-funsigned-char -fsso-struct=endianness
C++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n
-fargs-in-order=n -fchar8_t -fcheck-new -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-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fmodule-header[=kind] -fmodule-only -fmodules-ts
-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 -fpermissive
-fno-pretty-templates -fno-rtti -fsized-deallocation
-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 -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-exceptions
-Wextra-semi -Wno-inaccessible-base -Wno-inherited-variadic-ctor
-Wno-init-list-lifetime -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 -Wsign-promo -Wsized-deallocation
-Wsuggest-final-methods -Wsuggest-final-types -Wsuggest-override
-Wno-terminate -Wuseless-cast -Wno-vexing-parse
-Wvirtual-inheritance -Wno-virtual-move-assign -Wvolatile
-Wzero-as-null-pointer-constant
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
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|json] -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-labels
-fno-diagnostics-show-line-numbers -fno-diagnostics-show-cwe
-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]
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w
-Wextra -Wall -Wabi=n -Waddress -Wno-address-of-packed-member
-Waggregate-return -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 -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-c2x-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 -Wcast-align
-Wcast-align=strict -Wcast-function-type -Wcast-qual
-Wchar-subscripts -Wclobbered -Wcomment -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 -Werror
-Werror=* -Wexpansion-to-defined -Wfatal-errors -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 -Wno-if-not-aligned -Wno-ignored-attributes
-Wignored-qualifiers -Wno-incompatible-pointer-types -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 -Wjump-misses-init
-Wlarger-than=byte-size -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
-Wno-missing-profile -Wno-multichar -Wmultistatement-macros
-Wnonnull -Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc]
-Wnull-dereference -Wno-odr -Wopenacc-parallelism -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-prio-ctor-dtor -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
-Wsuggest-attribute=[pure|const|noreturn|format|malloc] -Wswitch
-Wno-switch-bool -Wswitch-default -Wswitch-enum
-Wno-switch-outside-range -Wno-switch-unreachable -Wsync-nand
-Wsystem-headers -Wtautological-compare -Wtrampolines -Wtrigraphs
-Wtrivial-auto-var-init -Wtsan -Wtype-limits -Wundef -Wuninitialized
-Wunknown-pragmas -Wunsuffixed-float-constants -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 -Wno-varargs -Wvariadic-macros
-Wvector-operation-performance -Wvla -Wvla-larger-than=byte-size
-Wno-vla-larger-than -Wvolatile-register-var -Wwrite-strings
-Wzero-length-bounds
Static Analyzer Options
-fanalyzer -fanalyzer-call-summaries -fanalyzer-checker=name
-fno-analyzer-feasibility -fanalyzer-fine-grained
-fno-analyzer-state-merge -fno-analyzer-state-purge
-fanalyzer-transitivity -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-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-file-leak -Wno-analyzer-free-of-non-heap
-Wno-analyzer-malloc-leak -Wno-analyzer-mismatching-deallocation
-Wno-analyzer-null-argument -Wno-analyzer-null-dereference
-Wno-analyzer-possible-null-argument
-Wno-analyzer-possible-null-dereference
-Wno-analyzer-shift-count-negative -Wno-analyzer-shift-count-overflow
-Wno-analyzer-stale-setjmp-buffer
-Wno-analyzer-tainted-allocation-size
-Wno-analyzer-tainted-array-index -Wno-analyzer-tainted-divisor
-Wno-analyzer-tainted-offset -Wno-analyzer-tainted-size
-Wanalyzer-too-complex
-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-write-to-const
-Wno-analyzer-write-to-string-literal
C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-parameter-type
-Wmissing-prototypes -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 -ggdb
-grecord-gcc-switches -gno-record-gcc-switches -gstabs -gstabs+
-gstrict-dwarf -gno-strict-dwarf -gas-loc-support
-gno-as-loc-support -gas-locview-support -gno-as-locview-support
-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 -gxcoff -gxcoff+
-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]]]] -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
-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-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-stack-alignment
-fipa-icf -fira-algorithm=algorithm -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 -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-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-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
-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
-fasan-shadow-offset=number -fsanitize-sections=s1,s2,...
-fsanitize-undefined-trap-on-error -fbounds-check
-fcf-protection=[full|branch|return|none|check] -fharden-compares
-fharden-conditional-branches -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
-fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug
-finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
-fprofile-prefix-map=old=new
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 -flarge-source-files
-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 -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 -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
-foff-stack-trampolines -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 -ftrapv -fwrapv
-fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
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] -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 -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
-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
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 -march=gpu -mtune=gpu -mstack-size=bytes
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 -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
AVR Options -mmcu=mcu -mabsdata -maccumulate-args
-mbranch-cost=cost -mcall-prologues -mgas-isr-prologues -mint8
-mdouble=bits -mlong-double=bits -mn_flash=size -mno-interrupts
-mmain-is-OS_task -mrelax -mrmw -mstrict-X -mtiny-stack
-mfract-convert-truncate -mshort-calls -nodevicelib -nodevicespecs
-Waddr-space-convert -Wmisspelled-isr
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 -mbig-endian -mlittle-endian -march=cpu -msim
-msdata=sdata-type
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
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops
-mdata-model=model
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
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
-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
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
-mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
eBPF Options -mbig-endian -mlittle-endian -mkernel=version
-mframe-limit=bytes -mxbpf -mco-re -mno-co-re -mjmpext -mjmp32
-malu32 -mcpu=version
FR30 Options -msmall-model -mno-lsim
FT32 Options -msim -mlra -mnodiv -mft32b -mcompress -mnopm
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 -mglibc -muclibc -mmusl -mbionic -mandroid
-tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr -mint32
-malign-300
HPPA Options -march=architecture-type -mcaller-copies
-mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas
-mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay
-mlinker-opt -mlong-calls -mlong-load-store -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 -msio -mwsio -munix=unix-std -nolibdld -static
-threads
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 -mbarrel-shift-enabled -mdivide-enabled
-mmultiply-enabled -msign-extend-enabled -muser-enabled
LoongArch Options -march=cpu-type -mtune=cpu-type -mabi=base-abi-
type -mfpu=fpu-type -msoft-float -msingle-float -mdouble-float
-mbranch-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 -mmax-inline-memcpy-size=n -mcmodel=code-model
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
M32C Options -mcpu=cpu -msim -memregs=number
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 -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
MeP Options -mabsdiff -mall-opts -maverage -mbased=n -mbitops
-mc=n -mclip -mconfig=name -mcop -mcop32 -mcop64 -mivc2 -mdc
-mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax -mmult
-mno-opts -mrepeat -ms -msatur -msdram -msim -msimnovec -mtf
-mtiny=n
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 -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
-mcheck-zero-division -mno-check-zero-division -mdivide-traps
-mdivide-breaks -mload-store-pairs -mno-load-store-pairs
-munaligned-access -mno-unaligned-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 -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 -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 -meb -mel -mmul.x -mno-crt0
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 -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
Nios II Options -G num -mgpopt=option -mgpopt -mno-gpopt
-mgprel-sec=regexp -mr0rel-sec=regexp -mel -meb -mno-bypass-cache
-mbypass-cache -mno-cache-volatile -mcache-volatile -mno-fast-sw-div
-mfast-sw-div -mhw-mul -mno-hw-mul -mhw-mulx -mno-hw-mulx
-mno-hw-div -mhw-div -mcustom-insn=N -mno-custom-insn
-mcustom-fpu-cfg=name -mhal -msmallc -msys-crt0=name
-msys-lib=name -march=arch -mbmx -mno-bmx -mcdx -mno-cdx
Nvidia PTX Options -m64 -mmainkernel -moptimize
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 -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10
-mint32 -mno-int16 -mint16 -mno-int32 -msplit -munix-asm
-mdec-asm -mgnu-asm -mlra
picoChip Options -mae=ae_type -mvliw-lookahead=N -msymbol-as-address
-mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
PRU Options -mmcu=mcu -minrt -mno-relax -mloop -mabi=variant
RISC-V Options -mbranch-cost=N-instruction -mplt -mno-plt -mabi=ABI-
string -mfdiv -mno-fdiv -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 -mexplicit-relocs
-mno-explicit-relocs -mrelax -mno-relax -mriscv-attribute
-mmo-riscv-attribute -malign-data=type -mbig-endian -mlittle-endian
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
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 -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
-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
-mpower8-vector -mno-power8-vector -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 -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 -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
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u
-mscore7 -mscore7d
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 -mclear-hwcap -mno-clear-hwcap -mimpure-text
-mno-impure-text -pthreads
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 -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 -mlra -mno-lra
System V Options -Qy -Qn -YP,paths -Ym,dir
TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian
-mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
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 -mg -mgnu -munix -mlra
Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float
-msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode
VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64
-mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy
-Xbind-now
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
-mmove-max=bits -mstore-max=bits -mmmx -msse -msse2 -msse3
-mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f
-mavx512pf -mavx512er -mavx512cd -mavx512vl -mavx512bw -mavx512dq
-mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mfsgsbase
-mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mptwrite
-mprefetchwt1 -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 -mforce-indirect-call -mavx512vbmi2
-mavx512bf16 -menqcmd -mvpclmulqdq -mavx512bitalg -mmovdiri
-mmovdir64b -mavx512vpopcntdq -mavx5124fmaps -mavx512vnni
-mavx5124vnniw -mprfchw -mrdpid -mrdseed -msgx
-mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8
-mamx-bf16 -muintr -mhreset -mavxvnni -mavx512fp16 -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 -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
x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread -municode -mwin32 -mwindows
-fno-set-stack-executable
Xstormy16 Options -msim
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
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
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
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.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
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
d
f77 f77-cpp-input f95 f95-cpp-input
go
-x none
Turn off any specification of a language, so that subsequent files
are handled according to their file name suffixes (as they are 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. See
also -fmacro-prefix-map, -fdebug-prefix-map and -fprofile-prefix-map.
-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; and preprocessed C++ files use the
suffix .ii. 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.
This turns off certain features of GCC that are incompatible with ISO
C90 (when compiling C code), or of standard C++ (when compiling C++
code), such as the "asm" and "typeof" keywords, and predefined macros
such as "unix" and "vax" that identify the type of system you are
using. It also enables the undesirable and rarely used ISO trigraph
feature. For the C compiler, it disables recognition of C++ style //
comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and
"__typeof__" continue to work despite -ansi. You would not want to
use them in an ISO C program, of course, but it is useful to put them
in header files that might be included in compilations done with
-ansi. Alternate predefined macros such as "__unix__" and "__vax__"
are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -Wpedantic is required in addition to -ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is
used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the ISO
standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions that are normally built in but do not have semantics
defined by ISO C (such as "alloca" and "ffs") are not built-in
functions when -ansi is used.
-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.
c2x 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. This is the default for C code.
gnu2x
The next version of the ISO C standard, still under development,
plus GNU extensions. The support for this version is
experimental and incomplete.
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++2b
c++23
The next revision of the ISO C++ standard, planned for 2023.
Support is highly experimental, and will almost certainly change
in incompatible ways in future releases.
gnu++2b
gnu++23
GNU dialect of -std=c++2b. 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.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very
useful as it is not possible to read the arguments. This is only
supported for C as this construct is allowed by C++.
-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.
-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.
-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=-lgfortran -foffload-options=-lm
-foffload-options="-lgfortran -lm" -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906 -foffload-options=-lm
-fopenacc
"!$acc" in Fortran. When -fopenacc is specified, the compiler
generates accelerated code according to the OpenACC Application
Programming Interface v2.6 <https://www.openacc.org>. This option
implies -pthread, and thus is only supported on targets that have
support for -pthread.
-fopenacc-dim=geom
Specify default compute dimensions for parallel offload regions that
do not explicitly specify. The geom value is a triple of
':'-separated sizes, in order 'gang', 'worker' and, 'vector'. A size
can be omitted, to use a target-specific default value.
-fopenmp
"!$omp" in Fortran. When -fopenmp is specified, the compiler
generates parallel code according to the OpenMP Application Program
Interface v4.5 <https://www.openmp.org>. This option implies
-pthread, and thus is only supported on targets that have support for
-pthread. -fopenmp implies -fopenmp-simd.
-fopenmp-simd
C/C++ and "!$omp" in Fortran. Other OpenMP directives are ignored.
-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.
-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.
-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++ 11, changes the mangling of
"__alignof__" to be distinct from that of "alignof", and dependent
operator names.
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 11 (GCC 7
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.
-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.
char ca[] = u8"xx"; // error: char-array initialized from wide
// string
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
-fconcepts-ts
Below -std=c++20, -fconcepts enables support for the C++ Extensions
for Concepts Technical Specification, ISO 19217 (2015).
With -std=c++20 and above, Concepts are part of the language
standard, so -fconcepts defaults to on. But the standard
specification of Concepts differs significantly from the TS, so some
constructs that were allowed in the TS but didn't make it into the
standard can still be enabled by -fconcepts-ts.
-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).
-fcoroutines
Enable support for the C++ coroutines extension (experimental).
-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.
-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-ts
-fno-modules-ts
Enable support for C++20 modules. The -fno-modules-ts 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.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to
warnings. Thus, using -fpermissive allows some nonconforming code to
compile.
-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.
-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.
-fstrong-eval-order
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. Enabled by default with -std=c++17.
-fstrong-eval-order=some enables just the ordering of member access
and shift expressions, and is the default without -std=c++17.
-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 with
-std=c++23 regardless of -Wno-deprecated.
-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) { }
};
S(allow_ctad_t) -> S<void>; // guide with incomplete parameter type will never be considered
-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.
-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.
-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.
-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-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.
-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)
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);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
fails to compile.
-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-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.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal 0 is used as null pointer constant. This can be
useful to facilitate the conversion to "nullptr" in C++11.
-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-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 after in-class function definitions.
-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.
-Wuse-after-free
-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 reallocatio 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 legacy code gradually
transition 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++ and Objective-C++ only)
Warn when an expression is casted to its own type.
-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).
The -fconstant-cfstrings option, if also present, overrides the
-fconstant-string-class setting and cause "@"..."" literals to be
laid out as constant CoreFoundation strings.
-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 and Mac OS X. 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 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 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
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.
"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.
-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-cwe
Diagnostic messages can optionally have an associated
@url{https://cwe.mitre.org/index.html, CWE} 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-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=separate-events.
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-format=FORMAT
Select a different format for printing diagnostics. FORMAT is text
or json. The default is text.
The json format consists of a top-level JSON array containing JSON
objects representing the diagnostics.
The JSON is emitted as one line, without formatting; the examples
below have been formatted for clarity.
Diagnostics can have child diagnostics. For example, this error and
note:
misleading-indentation.c:15:3: warning: this 'if' clause does not
guard... [-Wmisleading-indentation]
15 | if (flag)
| ^~
misleading-indentation.c:17:5: note: ...this statement, but the latter
is misleadingly indented as if it were guarded by the 'if'
17 | y = 2;
| ^
might be printed in JSON form (after formatting) like this:
[
{
"kind": "warning",
"locations": [
{
"caret": {
"display-column": 3,
"byte-column": 3,
"column": 3,
"file": "misleading-indentation.c",
"line": 15
},
"finish": {
"display-column": 4,
"byte-column": 4,
"column": 4,
"file": "misleading-indentation.c",
"line": 15
}
}
],
"message": "this \u2018if\u2019 clause does not guard...",
"option": "-Wmisleading-indentation",
"option_url": "https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmisleading-indentation",
"children": [
{
"kind": "note",
"locations": [
{
"caret": {
"display-column": 5,
"byte-column": 5,
"column": 5,
"file": "misleading-indentation.c",
"line": 17
}
}
],
"escape-source": false,
"message": "...this statement, but the latter is ..."
}
]
"escape-source": false,
"column-origin": 1,
}
]
where the "note" is a child of the "warning".
A diagnostic has a "kind". If this is "warning", then there is an
"option" key describing the command-line option controlling the
warning.
A diagnostic can contain zero or more locations. Each location has
an optional "label" string and up to three positions within it: a
"caret" position and optional "start" and "finish" positions. A
position is described by a "file" name, a "line" number, and three
numbers indicating a column position:
* "display-column" counts display columns, accounting for tabs and
multibyte characters.
* "byte-column" counts raw bytes.
* "column" is equal to one of the previous two, as dictated by the
-fdiagnostics-column-unit option.
All three columns are relative to the origin specified by
-fdiagnostics-column-origin, which is typically equal to 1 but may be
set, for instance, to 0 for compatibility with other utilities that
number columns from 0. The column origin is recorded in the JSON
output in the "column-origin" tag. In the remaining examples below,
the extra column number outputs have been omitted for brevity.
For example, this error:
bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
'struct s'} and 'T' {aka 'struct t'})
64 | return callee_4a () + callee_4b ();
| ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
| | |
| | T {aka struct t}
| S {aka struct s}
has three locations. Its primary location is at the "+" token at
column 23. It has two secondary locations, describing the left and
right-hand sides of the expression, which have labels. It might be
printed in JSON form as:
{
"children": [],
"kind": "error",
"locations": [
{
"caret": {
"column": 23, "file": "bad-binary-ops.c", "line": 64
}
},
{
"caret": {
"column": 10, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 21, "file": "bad-binary-ops.c", "line": 64
},
"label": "S {aka struct s}"
},
{
"caret": {
"column": 25, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 36, "file": "bad-binary-ops.c", "line": 64
},
"label": "T {aka struct t}"
}
],
"escape-source": false,
"message": "invalid operands to binary + ..."
}
If a diagnostic contains fix-it hints, it has a "fixits" array,
consisting of half-open intervals, similar to the output of
-fdiagnostics-parseable-fixits. For example, this diagnostic with a
replacement fix-it hint:
demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
mean 'color'?
8 | return ptr->colour;
| ^~~~~~
| color
might be printed in JSON form as:
{
"children": [],
"fixits": [
{
"next": {
"column": 21,
"file": "demo.c",
"line": 8
},
"start": {
"column": 15,
"file": "demo.c",
"line": 8
},
"string": "color"
}
],
"kind": "error",
"locations": [
{
"caret": {
"column": 15,
"file": "demo.c",
"line": 8
},
"finish": {
"column": 20,
"file": "demo.c",
"line": 8
}
}
],
"escape-source": false,
"message": "\u2018struct s\u2019 has no member named ..."
}
where the fix-it hint suggests replacing the text from "start" up to
but not including "next" with "string"'s value. Deletions are
expressed via an empty value for "string", insertions by having
"start" equal "next".
If the diagnostic has a path of control-flow events associated with
it, it has a "path" array of objects representing the events. Each
event object has a "description" string, a "location" object, along
with a "function" string and a "depth" number for representing
interprocedural paths. The "function" represents the current
function at that event, and the "depth" represents the stack depth
relative to some baseline: the higher, the more frames are within the
stack.
For example, the intraprocedural example shown for
-fdiagnostics-path-format= might have this JSON for its path:
"path": [
{
"depth": 0,
"description": "when 'PyList_New' fails, returning NULL",
"function": "test",
"location": {
"column": 10,
"file": "test.c",
"line": 25
}
},
{
"depth": 0,
"description": "when 'i < count'",
"function": "test",
"location": {
"column": 3,
"file": "test.c",
"line": 27
}
},
{
"depth": 0,
"description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",
"function": "test",
"location": {
"column": 5,
"file": "test.c",
"line": 29
}
}
]
Diagnostics have a boolean attribute "escape-source", hinting whether
non-ASCII bytes should be escaped when printing the pertinent lines
of source code ("true" for diagnostics involving source encoding
issues).
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
Make all warnings into errors.
-Werror=
Make 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 -Wimplicit to request warnings on implicit declarations.
Each of these specific warning options also has a negative form beginning
-Wno- to turn off warnings; for example, -Wno-implicit. 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-value. 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 emits a diagnostic 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 and some instances of other warnings may not be issued
at all unless optimization is enabled. While optimization in general
improves the efficacy of control and data flow sensitive warnings, in
some cases it may also cause false positives.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject
all programs that use forbidden extensions, and some other programs
that do not follow ISO C and ISO C++. For ISO C, follows the version
of the ISO 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 required version of ISO C). However, without this
option, certain GNU extensions and traditional C and C++ features are
supported as well. With this option, they are rejected.
-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 users try to use -Wpedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they
want: it finds some non-ISO practices, but not all---only those for
which ISO C requires a diagnostic, and some others for which
diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful
in some instances, but would require considerable additional work and
would be quite different from -Wpedantic. We don't have plans to
support such a feature in the near future.
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, since there are errors enabled by
this option and not enabled by the latter and vice versa.
-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 -Warray-bounds=1 (only with -O2) -Warray-compare
-Warray-parameter=2 (C and Objective-C only) -Wbool-compare
-Wbool-operation -Wc++11-compat -Wc++14-compat -Wcatch-value (C++
and Objective-C++ only) -Wchar-subscripts -Wcomment
-Wdangling-pointer=2 -Wduplicate-decl-specifier (C and Objective-C
only) -Wenum-compare (in C/ObjC; this is on by default in C++)
-Wformat -Wformat-overflow -Wformat-truncation -Wint-in-bool-context
-Wimplicit (C and Objective-C only) -Wimplicit-int (C and Objective-C
only) -Wimplicit-function-declaration (C and Objective-C only)
-Winit-self (only for C++) -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 (only
for C/C++) -Wmissing-attributes -Wmissing-braces (only for C/ObjC)
-Wmultistatement-macros -Wnarrowing (only for C++) -Wnonnull
-Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpessimizing-move
(only for C++) -Wpointer-sign -Wrange-loop-construct (only for C++)
-Wreorder -Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare
(only in C++) -Wsizeof-array-div -Wsizeof-pointer-div
-Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1
-Wswitch -Wtautological-compare -Wtrigraphs -Wuninitialized
-Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value
-Wunused-variable -Wuse-after-free=3 -Wvla-parameter (C and
Objective-C only) -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.)
-Wclobbered -Wcast-function-type -Wdeprecated-copy (C++ only)
-Wempty-body -Wenum-conversion (C only) -Wignored-qualifiers
-Wimplicit-fallthrough=3 -Wmissing-field-initializers
-Wmissing-parameter-type (C only) -Wold-style-declaration (C only)
-Woverride-init -Wsign-compare (C only) -Wstring-compare
-Wredundant-move (only for C++) -Wtype-limits -Wuninitialized
-Wshift-negative-value (in C++11 to C++17 and in C99 and newer)
-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.
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.
This option also enables warnings about psABI-related changes. The
known psABI changes at this point include:
* For SysV/x86-64, unions with "long double" members are passed in
memory as specified in psABI. Prior to GCC 4.4, this was not the
case. For example:
union U {
long double ld;
int i;
};
"union U" is now always passed in memory.
-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-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.
-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.
-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 in C99 and later dialects
of C, and also by -Wall.
-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 in C99 and later
dialects of C, and also by -Wall. The warning is made into 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.
-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 provides 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.
-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 opions
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 from allocations 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.
-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-type
Warn whenever a function is defined with a return type that defaults
to "int". Also warn about any "return" statement with no return
value in a function whose return type is not "void" (falling off the
end of the function body is considered returning without a value).
For C only, 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.
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 outside the enumeration range also provoke
warnings when this option is used (even if there is a "default"
label). 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 outside the enumeration range also
provoke warnings when this option is used. 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.
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. -Wunused-const-variable=1 is enabled by
-Wunused-variable 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.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable=1
This is the warning level that is enabled by -Wunused-variable
for C. It warns only 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=2
This warning level also warns for unused constant static
variables in headers (excluding system headers). This is the
warning level of -Wunused-const-variable and must be explicitly
requested since in C++ this isn't an error and in C it might be
harder 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.
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.
-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-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 calls to string manipulation functions such as "memcpy" and
"strcpy" that are determined to 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 functions that operate on character
arrays but not for raw memory functions like "memcpy" which always
make use of Object Size type-0. 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. 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
}
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.
-Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]
Warn for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
-Wmissing-noreturn
-Wsuggest-attribute=malloc
Warn about functions that might be candidates for attributes
"pure", "const" or "noreturn" or "malloc". 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-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.
-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.
-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 out of bounds accesses to
trailing struct members of one-element array types and 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.
-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 arguments 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 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. At level 1 the warning diagnoses inconsistencies
involving array parameters declared using the "T[static N]" form.
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 with the keyword "static" specifies that the
array argument must have at least four elements.
void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)
void g (void)
{
int *p = (int *)malloc (4);
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.
-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.
-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.
-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. See
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.
-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.
-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.
-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.
-Wtsan
Warn about unsupported features in ThreadSanitizer.
ThreadSanitizer does not support "std::atomic_thread_fence" and can
report false positives.
This warning is enabled by default.
-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-c2x-compat (C and Objective-C only)
Warn about features not present in ISO C11, but present in ISO C2X.
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__".
-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.
-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.
-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.
-Wclobbered
Warn for variables that might be changed by "longjmp" or "vfork".
This warning is also enabled by -Wextra.
-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. For example
int f (int c1, int c2, x)
{
char *p = strchr ((char[]){ c1, c2 }, c3);
return p ? *p : 'x'; // warning: dangling pointer to a compound literal
}
In the following function the store of the address of the local
variable "x" in the escaped pointer *p also triggers the warning.
void g (int **p)
{
int x = 7;
*p = &x; // warning: storing the address of a local variable in *p
}
-Wdangling-pointer=2
At level 2, in addition to unconditional uses the warning also
diagnoses conditional uses of dangling pointers.
For example, because the array a in the following function is out
of scope when the pointer s that was set to point is used, the
warning triggers at this level.
void f (char *s)
{
if (!s)
{
char a[12] = "tmpname";
s = a;
}
strcat (s, ".tmp"); // warning: dangling pointer to a may be used
...
}
-Wdangling-pointer=2 is included in -Wall.
-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.
-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.
-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 (!func) // 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 "inptr_t" or "uinptr_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
Warn if in a loop with constant number of iterations the compiler
detects undefined behavior in some statement during one or more of
the iterations.
-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.
Additionally, using -Wno-attributes=, it is possible to suppress
warnings about unknown scoped attributes (in C++11 and C2X). 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__".
-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 C2X mode, because it is
equivalent to (void) in that case, but is considered an old-style
definition for older standards.
-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.
-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-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 };
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.
-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.
-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.
-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.
-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.
-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]);
void f (int, int[]); // warning: argument 2 previously declared as a VLA
void g (int n)
{
if (n > 4)
return;
int a[n];
f (sizeof a, a); // warning: access to a by f may be out of bounds
...
}
-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.
-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.
-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-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-shift-count-negative
-Wanalyzer-shift-count-overflow -Wanalyzer-stale-setjmp-buffer
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value -Wanalyzer-write-to-const
-Wanalyzer-write-to-string-literal
This option is only available if GCC was configured with analyzer
support enabled.
-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-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.
-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".
-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).
-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.
-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).
-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".
-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".
-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.
-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.
-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.
-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.
-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 both -fanalyzer and -fanalyzer-checker=taint to
enable 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 @url{https://cwe.mitre.org/data/definitions/789.html, CWE-789:
Memory Allocation with Excessive Size Value}.
-Wno-analyzer-tainted-array-index
This warning requires both -fanalyzer and -fanalyzer-checker=taint to
enable 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 @url{https://cwe.mitre.org/data/definitions/129.html, CWE-129:
Improper Validation of Array Index}.
-Wno-analyzer-tainted-divisor
This warning requires both -fanalyzer and -fanalyzer-checker=taint to
enable 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.
-Wno-analyzer-tainted-offset
This warning requires both -fanalyzer and -fanalyzer-checker=taint to
enable 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 @url{https://cwe.mitre.org/data/definitions/823.html, CWE-823:
Use of Out-of-range Pointer Offset}.
-Wno-analyzer-tainted-size
This warning requires both -fanalyzer and -fanalyzer-checker=taint to
enable 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.
-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.
-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".
-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-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.
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, and --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.
Some checkers are disabled by default (even with -fanalyzer), such as
the "taint" checker that implements -Wanalyzer-tainted-array-index,
and this option is required to enable them.
Note: currently, -fanalyzer-checker=taint disables the following
warnings from -fanalyzer:
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-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.
-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.
-fanalyzer-transitivity
This option enables transitivity of constraints within 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. This option is overridden by
-fdump-analyzer-stderr.
-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-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 -gstabs+,
-gstabs, -gxcoff+, -gxcoff, or -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/Mac OS X, 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.
-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.
-gstabs
Produce debugging information in stabs format (if that is supported),
without GDB extensions. This is the format used by DBX on most BSD
systems. On MIPS, Alpha and System V Release 4 systems this option
produces stabs debugging output that is not understood by DBX. On
System V Release 4 systems this option requires the GNU assembler.
-gstabs+
Produce debugging information in stabs format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB). The
use of these extensions is likely to make other debuggers crash or
refuse to read the program.
-gxcoff
Produce debugging information in XCOFF format (if that is supported).
This is the format used by the DBX debugger on IBM RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is supported),
using GNU extensions understood only by the GNU debugger (GDB). The
use of these extensions is likely to make other debuggers crash or
refuse to read the program, and may cause assemblers other than the
GNU assembler (GAS) to fail with an error.
-gvms
Produce debugging information in Alpha/VMS debug format (if that is
supported). This is the format used by DEBUG on Alpha/VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gxcofflevel
-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.
-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), zlib (use zlib compression in
ELF gABI format), or zlib-gnu (use zlib compression in traditional
GNU 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 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 -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-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.
-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.
-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 currently
not implemented and treated equal to -ffp-contract=off.
The default is -ffp-contract=fast.
-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-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.
-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.
-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 Nios II ELF,
it defaults to off. On AVR, CR16, 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 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.
Enabled at levels -O2, -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-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.
-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-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. This flag is enabled by default at -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-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.
-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. 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.
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 only allows vectorization if the vector code
would entirely replace the scalar code that is being vectorized. For
example, if each iteration of a vectorized loop would only be able to
handle exactly four iterations of the scalar loop, the very-cheap
model would only allow vectorization if the scalar iteration count is
known to be a multiple of four.
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. 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.
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.
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.
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.
-fno-allocation-dce
Do not remove unused C++ allocations in dead code elimination.
-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.
-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.
This option should not be used in combination with -flto. Instead
relying on a linker plugin should provide safer and more precise
information.
-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. 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-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.
-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 agressively 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 trianing may not cover all hardware configurations
program is run on.) With "-fprofile-partial-training" profile
feedback will be 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.
-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 prevents undesirable excess precision on machines such as
the 68000 where the floating registers (of the 68881) keep more
precision than a "double" is supposed to have. Similarly for the x86
architecture. For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE
floating point. Use -ffloat-store for such programs, after modifying
them to store all pertinent intermediate computations into variables.
-fexcess-precision=style
This option allows further 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.
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, if -fexcess-precision=standard
is specified then excess precision follows the rules specified in ISO
C99; 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 if a strict
conformance option such as -std=c99 is used. -ffast-math enables
-fexcess-precision=fast by default regardless of whether a strict
conformance option is used.
-fexcess-precision=standard is not implemented for languages other
than 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.
-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 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 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 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 should never be turned on by any -O option 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.
-frounding-math
Disable transformations and optimizations that assume default
floating-point rounding behavior. This is round-to-zero for all
floating point to integer conversions, and round-to-nearest for all
other arithmetic truncations. 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 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 C2X, does not allow these
functions to do so.
The default is -ffp-int-builtin-inexact, allowing the exception to be
raised, unless C2X 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.
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.
-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 minimal, maximal and default value of a parameter,
one can use --help=param -Q options.
In each case, the value is an integer. The following choices of name
are recognized for all targets:
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.
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 bound applied to calls which are considered relevant with
-finline-small-functions.
max-inline-insns-size
This is bound applied to calls which are optimized for size.
Small growth may be desirable to anticipate optimization
oppurtunities 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 function 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-level
The maximum number of branches unswitched in a single loop.
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-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.
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-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.
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.
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-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-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.
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-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.
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.
max-fsm-thread-length
Maximum number of basic blocks on a 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.
max-vrp-switch-assertions
The maximum number of assertions to add along the default edge of
a switch statement during VRP.
evrp-sparse-threshold
Maximum number of basic blocks before EVRP uses a sparse cache.
evrp-mode
Specifies the mode Early VRP should operate in.
vrp1-mode
Specifies the mode VRP pass 1 should operate in.
vrp2-mode
Specifies the mode VRP pass 2 should operate in.
ranger-debug
Specifies the type of debug output to be issued for ranges.
evrp-switch-limit
Specifies the maximum number of switch cases before EVRP ignores
a switch.
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.
avoid-fma-max-bits
Maximum number of bits for which we avoid creating FMAs.
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 when comparing to the number of (scaled) blocks.
uninit-control-dep-attempts
Maximum number of nested calls to search for control dependencies
during uninitialized variable analysis.
fsm-scale-path-blocks
Scale factor to apply to the number of blocks in a threading path
when comparing to the number of (scaled) statements.
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.
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.
ranger-logical-depth
Maximum depth of logical expression evaluation ranger will look
through when evaluating outgoing edge ranges.
relation-block-limit
Maximum number of relations the oracle will register in a basic
block.
min-pagesize
Minimum page size for warning 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
Specify mode of OpenACC privatization diagnostics for
-fopt-info-omp-note and applicable -fdump-tree-*-details. With
--param=openacc-privatization=quiet, don't diagnose. This is the
current default. With --param=openacc-privatization=noisy, do
diagnose.
The following choices of name are available on AArch64 targets:
aarch64-sve-compare-costs
When vectorizing for SVE, consider using "unpacked" vectors for
smaller elements and use the cost model to pick the cheapest
approach. Also use the cost model to choose between SVE and
Advanced SIMD vectorization.
Using unpacked vectors 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 propotional to
this param when division approximation is enabled. The default
value is 2.
aarch64-autovec-preference
Force an ISA selection strategy for auto-vectorization. Accepts
values from 0 to 4, inclusive.
0 Use the default heuristics.
1 Use only Advanced SIMD for auto-vectorization.
2 Use only SVE for auto-vectorization.
3 Use both Advanced SIMD and SVE. Prefer Advanced SIMD when
the costs are deemed equal.
4 Use both Advanced SIMD and SVE. Prefer SVE when the costs
are deemed equal.
The default value is 0.
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 i386 and x86_64
targets:
x86-stlf-window-ninsns
Instructions number above which STFL stall penalty can be
compensated.
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.
--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.
-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 free-standing
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 *__gcov_info_start[];
extern const struct gcov_info *__gcov_info_end[];
static void
filename (const char *f, void *arg)
{
puts (f);
}
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 *
allocate (unsigned length, void *arg)
{
return malloc (length);
}
static void
dump_gcov_info (void)
{
const struct gcov_info **info = __gcov_info_start;
const struct gcov_info **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()
{
dump_gcov_info();
return 0;
}
-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.
-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.
-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 target
-fsanitize=hwaddress is currently supported on is AArch64.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See
<https://github.com/google/kasan> 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.
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 and 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.
-fsanitize=bounds-strict
This option enables strict instrumentation of array bounds. Most
out of bounds accesses are detected, including flexible array
members and 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_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.
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-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option instructs the compiler
to report undefined behavior using "__builtin_trap" rather than a
"libubsan" library routine. 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.
-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]
Enable code instrumentation of control-flow transfers 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 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 "none" turns off instrumentation.
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 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.
-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.
-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-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.
The maximum value of N and M is 65535.
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.
-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.
-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 in two respects: it does not include the
predefined macros, and 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.
-flarge-source-files
Adjust GCC to expect large source files, at the expense of slower
compilation and higher memory usage.
Specifically, GCC normally tracks both column numbers and line
numbers within source files and it normally prints both of these
numbers in diagnostics. However, once it has processed a certain
number of source lines, it stops tracking column numbers and only
tracks line numbers. This means that diagnostics for later lines do
not include column numbers. It also means that options like
-Wmisleading-indentation cease to work at that point, although the
compiler prints a note if this happens. Passing -flarge-source-files
significantly increases the number of source lines that GCC can
process before it stops tracking columns.
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.)
-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.)
-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.
-foff-stack-trampolines
Certain platforms (such as the Apple M1) do not permit an executable
stack. Generate calls to "__builtin_nested_func_ptr_created" and
"__builtin_nested_func_ptr_deleted" in order to allocate and
deallocate trampoline space on the executable heap. Please note that
these functions are implemented in libgcc, and will not be compiled
in unless you provide --enable-off-stack-trampolines when building
gcc. PLEASE NOTE: The 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.
-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.
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.
-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.
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-sibling
Dump after sibling call optimizations.
-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-unshare
Dump after all rtl has been unshared.
-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.
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-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 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 about the time consumed by
each pass when it finishes.
-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.
-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.
-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.
-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.
-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.
-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
armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod
armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres
armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
armv8.7-a : Armv8.7-A : armv8.6-a, +ls64
armv8.8-a : Armv8.8-a : armv8.7-a, +mops
armv9-a : Armv9-A : armv8.5-a, +sve, +sve2
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,
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, neoverse-512tvb,
neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1, neoverse-v2,
qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81,
octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93,
octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, thunderx,
thunderxt88, thunderxt88p1, thunderxt81, tsv110, 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-x1,
cortex-x2, cortex-a510, cortex-a710, ampere1, ampere1a, 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 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
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.
-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.
-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.
-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 does not change code generation from
GCC, but is passed 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.
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. This option is enabled by default for
-march=armv8.7-a.
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.
pauth
Enable the Pointer Authentication Extension.
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
fiji
Compile for GCN3 Fiji devices (gfx803).
gfx900
Compile for GCN5 Vega 10 devices (gfx900).
gfx906
Compile for GCN5 Vega 20 devices (gfx906).
-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
Compile binaries suitable for devices with the XNACK feature enabled.
Some devices always require XNACK and some allow the user to
configure XNACK. The compiled code must match the device mode. The
default is -mno-xnack. At present this option is a placeholder for
support that is not yet implemented.
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.
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
Explain what alignment considerations lead to the decision to make an
instruction short or long.
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
Enable bbit peephole2.
-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
Enable Local Register Allocation. This is still experimental for
ARC, so by default the compiler uses standard reload (i.e. -mno-lra).
-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
Enable Rcq constraint handling. Most short code generation depends
on this. This is the default.
-mRcw
Enable Rcw constraint handling. Most ccfsm condexec mostly depends
on this. This is the default.
-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.
-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, armv9-a, 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.
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.
-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-m55, cortex-x1, 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,
xgene1.
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.
+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 and cortex-m35p. 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.
-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
valid models are soft, which generates calls to "__aeabi_read_tp",
cp15, which fetches the thread pointer from "cp15" directly
(supported in the arm6k architecture), and auto, which 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" or "cortex-m55". 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.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
The default for this option is avr2.
GCC supports the following AVR devices and ISAs:
"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", "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", "ata5795", "ata5790", "ata5790n", "ata5791",
"ata6613c", "ata6614q", "ata5782", "ata5831", "ata8210",
"ata8510", "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".
"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", "attiny417", "attiny804", "attiny806", "attiny807",
"attiny814", "attiny816", "attiny817", "attiny1604",
"attiny1606", "attiny1607", "attiny1614", "attiny1616",
"attiny1617", "attiny3214", "attiny3216", "attiny3217",
"atmega808", "atmega809", "atmega1608", "atmega1609",
"atmega3208", "atmega3209", "atmega4808", "atmega4809".
"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".
"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"
"TINY" Tiny core devices with 512 B up to 4 KiB of program
memory. mcu = "attiny4", "attiny5", "attiny9", "attiny10",
"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.
-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.
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.
-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.
-mn-flash=num
Assume that the flash memory has a size of num times 64 KiB.
-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.
-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 internally for multilib selection. It is not an
optimization option, and you don't need to set it by hand.
-msp8
Treat the stack pointer register as an 8-bit register, i.e. assume
the high byte of the stack pointer is zero. In general, you don't
need to set this option by hand.
This option is used internally by the compiler to select and build
multilibs for architectures "avr2" and "avr25". These architectures
mix devices with and without "SPH". For any setting other than
-mmcu=avr2 or -mmcu=avr25 the compiler driver adds or removes this
option from the compiler proper's command line, because the compiler
then knows if the device or architecture has an 8-bit stack pointer
and thus no "SPH" register or not.
-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
-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 "lib<mcu>.a".
-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.
"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
("http://nongnu.org/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_XMEGA__"
The device / architecture belongs to the XMEGA family of devices.
"__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_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-
register moves.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__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_*".
"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-Libc. See
the --with-avrlibc configure option.
"__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").
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, 