GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-pedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] infile...
Only the most useful options are listed here; see below for the
remainder. g++ accepts mostly the same options as gcc.
In Apple's version of GCC, both cc and gcc are actually symbolic links
to a compiler named like gcc-version. Similarly, c++ and g++ are links
to a compiler named like g++-version.
Note that Apple's GCC includes a number of extensions to standard GCC
(flagged below with ``APPLE ONLY''), and that not all generic GCC
options are available or supported on Darwin / Mac OS X. In
particular, Apple does not currently support the compilation of
Fortran, Ada, or Java, although there are third parties who have made
these work.
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 stage 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 gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dr is very different from -d -r.
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.
Many options have long names starting with -f or with -W---for example,
-fforce-mem, -fstrength-reduce, -Wformat and so on. Most of these have
both positive and negative forms; the negative form of -ffoo would be
-fno-foo. This manual documents only one of these two forms, whichever
one is not the default.
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 -combine -pipe -pass-exit-codes -ObjC (APPLE
ONLY) -ObjC++ (APPLE ONLY) -arch arch (APPLE ONLY) -Xarch_arch
option (APPLE ONLY) -fsave-repository=file -x language -v -###
--help --target-help --version
C Language Options
-ansi -std=standard -aux-info filename -faltivec (APPLE ONLY)
-fasm-blocks (APPLE ONLY) -fno-asm -fno-builtin
-fno-builtin-function -fhosted -ffreestanding -fms-extensions
-trigraphs -no-integrated-cpp -traditional -traditional-cpp
-fallow-single-precision -fcond-mismatch -fconstant-cfstrings
(APPLE ONLY) -fnon-lvalue-assign (APPLE ONLY) -fno-nested-functions
-fpch-preprocess (APPLE ONLY) -fsigned-bitfields -fsigned-char
-fpascal-strings (APPLE ONLY) -Wno-#warnings (APPLE ONLY)
-Wextra-tokens (APPLE ONLY) -Wnewline-eof (APPLE ONLY)
-Wno-altivec-long-deprecated (APPLE ONLY) -funsigned-bitfields
-funsigned-char -fwritable-strings
C++ Language Options
-fabi-version=n -fno-access-control -fcheck-new -fconserve-space
-fno-const-strings -fno-elide-constructors -fno-enforce-eh-specs
-ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates -fno-implicit-inline-templates
-fno-implement-inlines -fms-extensions -fno-nonansi-builtins
-fno-operator-names -fno-optional-diags -fpermissive -frepo
-fno-rtti -fstats -ftemplate-depth-n -fno-threadsafe-statics
-fuse-cxa-atexit -fno-weak -nostdinc++ -fno-default-inline
-fvisibility-inlines-hidden -fvisibility-ms-compat -Wabi
-Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++
-Wno-deprecated -Wstrict-null-sentinel -Wno-non-template-friend
-Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions
-Wsign-promo
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-call-cxx-cdtors (APPLE ONLY)
-fobjc-sjlj-exceptions -fobjc-gc -freplace-objc-classes -fzero-link
-gen-decls -Wno-protocol -Wselector -Wstrict-selector-match
-Wundeclared-selector
Language Independent Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
Warning Options
-fsyntax-only -pedantic -pedantic-errors -w -Wextra -Wall
-Waggregate-return -Wcast-align -Wcast-qual -Wchar-subscripts
-Wcomment -Wconversion -Wno-deprecated-declarations
-Wdisabled-optimization -Wno-div-by-zero -Wno-endif-labels
-Werror -Werror-implicit-function-declaration -Wfatal-errors
-Wfloat-equal -Wformat -Wformat=2 -Wno-format-extra-args
-Wformat-nonliteral -Wformat-security -Wformat-y2k -Wimplicit
-Wimplicit-function-declaration -Wimplicit-int -Wimport
-Wno-import -Winit-self -Winline -Wno-int-to-pointer-cast
-Wno-invalid-offsetof -Winvalid-pch -Wlarger-than-len -Wlong-long
-Wmain -Wmissing-braces -Wmissing-field-initializers
-Wmissing-format-attribute -Wmissing-include-dirs
-Wmissing-noreturn -Wmost (APPLE ONLY) -Wno-multichar -Wnonnull
-Wpacked -Wpadded -Wparentheses -Wpointer-arith
-Wno-pointer-to-int-cast -Wredundant-decls -Wreturn-type
-Wsequence-point -Wshadow -Wstack-protector -Wsign-compare
-Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch -Wswitch-default
-Wswitch-enum -Wsystem-headers -Wtrigraphs -Wundef
-Wuninitialized -Wunknown-pragmas -Wunreachable-code -Wunused
-Wunused-function -Wunused-label -Wunused-parameter
-Wunused-value -Wunused-variable -Wwrite-strings
-Wvariadic-macros
C-only Warning Options
-Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes
-Wnested-externs -Wold-style-definition -Wstrict-prototypes
-Wtraditional -Wdeclaration-after-statement -Wno-discard-qual
-Wno-pointer-sign
Debugging Options
-dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered
-fdump-translation-unit[-n] -fdump-class-hierarchy[-n]
-fdump-ipa-all -fdump-ipa-cgraph -fdump-tree-all
-fdump-tree-original[-n] -fdump-tree-optimized[-n]
-fdump-tree-inlined[-n] -fdump-tree-cfg -fdump-tree-vcg
-fdump-tree-alias -fdump-tree-ch -fdump-tree-ssa[-n]
-fdump-tree-pre[-n] -fdump-tree-ccp[-n] -fdump-tree-dce[-n]
-fdump-tree-gimple[-raw] -fdump-tree-mudflap[-n] -fdump-tree-scev
[-n] -fdump-tree-ddall [-n] -fdump-tree-elck [-n]
-fdump-tree-dom[-n] -fdump-tree-dse[-n] -fdump-tree-phiopt[-n]
-fdump-tree-forwprop[-n] -fdump-tree-copyrename[-n] -fdump-tree-nrv
-fdump-tree-vect -fdump-tree-sra[-n] -fdump-tree-fre[-n]
-fdump-tree-loop[-n] -fdump-tree-vect[-n]
-ftree-vectorizer-verbose=n -flimit-debug-info
-feliminate-dwarf2-dups -feliminate-unused-debug-types
-feliminate-unused-debug-symbols -fmem-report -fopt-diary
-fprofile-arcs -ftree-based-profiling -frandom-seed=string
-fsched-verbose=n -ftest-coverage -ftime-report -fvar-tracking -g
-glevel -gcoff -gdwarf-2 -ggdb -gstabs -gstabs+ -gvms -gxcoff
-gxcoff+ -p -pg -print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-prog-name=program
-print-search-dirs -Q -save-temps -time
Optimization Options
-falign-functions=n -falign-jumps=n -falign-labels=n
-falign-loops=n -falign-loops-max-skip=n -falign-jumps-max-skip=n
-fbounds-check -fmudflap -fmudflapth -fmudflapir
-fbranch-probabilities -fprofile-values -fvpt
-fbranch-target-load-optimize -fbranch-target-load-optimize2
-fbtr-bb-exclusive -fcaller-saves -fcprop-registers
-fcreate-profile -fcse-follow-jumps -fcse-skip-blocks
-fcx-limited-range -fdata-sections -fdelayed-branch
-fdelete-null-pointer-checks -fexpensive-optimizations -ffast-math
-ffloat-store -fforce-addr -fforce-mem -ffunction-sections -fgcse
-fgcse-lm -fgcse-sm -fgcse-las -fgcse-after-reload
-floop-optimize -fcrossjumping -fif-conversion -fif-conversion2
-finline-functions -finline-limit=n -fkeep-inline-functions
-fkeep-static-consts -fmerge-constants -fmerge-all-constants
-fmodulo-sched -fno-branch-count-reg -fno-default-inline
-fno-defer-pop -floop-optimize2 -fmove-loop-invariants
-fno-function-cse -fno-guess-branch-probability -fno-inline
-fno-math-errno -fno-peephole -fno-peephole2
-funsafe-math-optimizations -ffinite-math-only -fno-trapping-math
-fno-zero-initialized-in-bss -mstackrealign -fomit-frame-pointer
-foptimize-register-move -foptimize-sibling-calls
-fprefetch-loop-arrays -fprofile-generate -fprofile-use -fregmove
-frename-registers -freorder-blocks -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop -frerun-loop-opt
-frounding-math -fschedule-insns -fschedule-insns2
-fno-sched-interblock -fno-sched-spec -fsched-spec-load
-fsched-spec-load-dangerous -fsched-stalled-insns=n
-sched-stalled-insns-dep=n -fsched2-use-superblocks
-fsched2-use-traces -freschedule-modulo-scheduled-loops
-fsignaling-nans -fsingle-precision-constant
-fspeculative-prefetching -fstack-protector -fstack-protector-all
-fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps
-funroll-all-loops -funroll-loops -fpeel-loops
-fsplit-ivs-in-unroller -funswitch-loops
-fvariable-expansion-in-unroller -ftree-pre -ftree-ccp -ftree-dce
-ftree-loop-optimize -ftree-loop-linear -ftree-loop-im
-ftree-loop-ivcanon -fivopts -ftree-dominator-opts -ftree-dse
-ftree-copyrename -ftree-ch -ftree-sra -ftree-ter -ftree-lrs
-ftree-fre -ftree-vectorize -fuse-profile -fweb -fscalar-evolutions
-fall-data-deps --param name=value -O -O0 -O1 -O2 -O3 -Os -Oz
(APPLE ONLY) -fast (APPLE ONLY)
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN
-Dmacro[=defn] -E -H -idirafter dir -include file -imacros file
-iprefix file -iwithprefix dir -iwithprefixbefore dir -isystem
dir -iwithsysroot (APPLE ONLY) dir -M -MM -MF -MG -MP -MQ -MT
-nostdinc -P -fworking-directory -remap -trigraphs -undef
-Umacro -Wp,option -Xpreprocessor option
Assembler Option
-Wa,option -Xassembler option
Linker Options
object-file-name -llibrary -nostartfiles -nodefaultlibs
-nostdlib -pie -s -static -static-libgcc -shared -shared-libgcc
-symbolic -Wl,option -Xlinker option -u symbol
Directory Options
-Bprefix -Idir -iquotedir -Ldir -specs=file -I-
Target Options
-V version -b machine
Machine Dependent Options
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name
-mapcs-stack-check -mno-apcs-stack-check -mapcs-float
-mno-apcs-float -mapcs-reentrant -mno-apcs-reentrant
-msched-prolog -mno-sched-prolog -mlittle-endian -mbig-endian
-mwords-little-endian -mfloat-abi=name -msoft-float -mhard-float
-mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name
-march=name -mfpu=name -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
-mcirrus-fix-invalid-insns -mno-cirrus-fix-invalid-insns
-mpoke-function-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking
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 -miphoneos-version-min=version
-mkernel -mone-byte-bool -Xarch_arch
i386 and x86-64 Options -mtune=cpu-type -march=cpu-type
-mfpmath=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387
-msoft-float -msvr3-shlib -mno-wide-multiply -mrtd
-malign-double -mpreferred-stack-boundary=num -mmmx -msse -msse2
-msse3 -mssse3 -m3dnow -mthreads -mno-align-stringops
-minline-all-stringops -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mregparm=num
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs
-mcmodel=code-model -m32 -m64
PowerPC Options See RS/6000 and PowerPC Options.
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type -mpower
-mno-power -mpower2 -mno-power2 -mpowerpc -mpowerpc64
-mno-powerpc -maltivec -mno-altivec -mpim-altivec -mno-pim-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mnew-mnemonics -mold-mnemonics -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 -mstring
-mno-string -mupdate -mno-update -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
-mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-sysv -mcall-netbsd -maix-struct-return
-msvr4-struct-return -mabi=altivec -mabi=no-altivec -mabi=spe
-mabi=no-spe -misel=yes -misel=no -mspe=yes -mspe=no
-mfloat-gprs=yes -mfloat-gprs=no -mfloat-gprs=single
-mfloat-gprs=double -mprototype -mno-prototype -msim -mmvme
-mads -myellowknife -memb -msdata -msdata=opt -mvxworks
-mwindiss -G num -pthread
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -funwind-tables -fasynchronous-unwind-tables
-finhibit-size-directive -finstrument-functions -fno-common
-fno-ident -fpcc-struct-return -fpic -fPIC -fpie -fPIE
-freg-struct-return -fshared-data -fshort-enums -fshort-double
-fshort-wchar -fverbose-asm -fpack-struct[=n] -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym
-fargument-alias -fargument-noalias -fargument-noalias-global
-fleading-underscore -ftls-model=model -ftrapv -fwrapv
-fbounds-check -fvisibility
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 which must be preprocessed.
file.i
C source code which should not be preprocessed.
file.ii
C++ source code which 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 which 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 which should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into
a precompiled header.
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code which 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 which must be preprocessed. (APPLE ONLY)
file.mii
Objective-C++ source code which should not be preprocessed. (APPLE
ONLY)
file.hh
file.H
C++ header file to be turned into a precompiled header.
file.f
file.for
file.FOR
Fortran source code which should not be preprocessed.
file.F
file.fpp
file.FPP
Fortran source code which must be preprocessed (with the
traditional preprocessor).
file.r
Fortran source code which must be preprocessed with a RATFOR
preprocessor (not included with GCC).
file.f90
file.f95
Fortran 90/95 source code which should not be preprocessed.
file.ads
Ada source code file which 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. Apple's version of GCC runs the preprocessor on
these files as well as those ending in .S.
file.S
Assembler code which 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 c-cpp-output
c++ c++-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
f77 f77-cpp-input ratfor
f95
java
treelang
-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).
-ObjC
-ObjC++
These are similar in effect to -x objective-c and -x objective-c++,
but affect only the choice of compiler for files already identified
as source files. (APPLE ONLY)
-arch arch
Compile for the specified target architecture arch. The allowable
values are i386, ppc and ppc64. Multiple options work, and direct
the compiler to produce ``universal'' binaries including object
code for each architecture specified with -arch. This option only
works if assembler and libraries are available for each
architecture specified. (APPLE ONLY)
-Xarch_arch option
Apply option to the command line for architecture arch. This is
useful for specifying an option that should only apply to one
architecture when building a ``universal'' binary. (APPLE ONLY)
-fsave-repository=file
Save debug info in separate object file. This is available only
while building PCH in -gfull mode.
-pass-exit-codes
Normally the gcc program will exit 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 will instead return with
numerically highest error produced by any phase that returned an
error indication.
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 which don't require preprocessing are ignored.
-o file
Place output in file file. This applies regardless 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.
-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 all command
arguments are quoted. This is useful for shell scripts to capture
the driver-generated command lines.
-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.
-combine
If you are compiling multiple source files, this option tells the
driver to pass all the source files to the compiler at once (for
those languages for which the compiler can handle this). This will
allow intermodule analysis (IMA) to be performed by the compiler.
Currently the only language for which this is supported is C. If
you pass source files for multiple languages to the driver, using
this option, the driver will invoke the compiler(s) that support
IMA once each, passing each compiler all the source files
appropriate for it. For those languages that do not support IMA
this option will be ignored, and the compiler will be invoked once
for each source file in that language. If you use this option in
conjunction with -save-temps, the compiler will generate multiple
pre-processed files (one for each source file), but only one
(combined) .o or .s file.
--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 will also be passed on to the various processes invoked by
gcc, so that they can display the command line options they accept.
If the -Wextra option is also specified then command line options
which have no documentation associated with them will also be
displayed.
--target-help
Print (on the standard output) a description of target specific
command line options for each tool.
--version
Display the version number and copyrights of the invoked GCC.
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 or .H; 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 treats .c, .h and .i files as C++ source files
instead of C source files unless -x is used, and automatically
specifies linking against the C++ library. This 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, support all ISO C90 programs. In C++ mode, remove GNU
extensions that conflict with ISO C++.
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, -pedantic 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 which would normally be built in but do not have
semantics defined by ISO C (such as "alloca" and "ffs") are not
built-in functions with -ansi is used.
-std=
Determine the language standard. This option is currently only
supported when compiling C or C++. A value for this option must be
provided; possible values are
c89
iso9899:1990
ISO C90 (same as -ansi).
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. Note that this standard is not yet fully supported;
see <http://gcc.gnu.org/gcc-4.0/c99status.html> for more
information. The names c9x and iso9899:199x are deprecated.
gnu89
Default, ISO C90 plus GNU extensions (including some C99
features).
gnu99
gnu9x
ISO C99 plus GNU extensions. When ISO C99 is fully implemented
in GCC, this will become the default. The name gnu9x is
deprecated.
c++98
The 1998 ISO C++ standard plus amendments.
gnu++98
The same as -std=c++98 plus GNU extensions. This is the
default for C++ code.
Even when this option is not specified, you can still use some of
the features of newer standards in so far as they do not conflict
with previous C standards. For example, you may use "__restrict__"
even when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same
effects as -ansi, except that features that were not in ISO C90 but
are in the specified version (for example, // comments and the
"inline" keyword in ISO C99) are not disabled.
-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.
-faltivec
This flag is provided for compatibility with Metrowerks CodeWarrior
and MrC compilers as well as previous Apple versions of GCC. It
causes the -mpim-altivec option to be turned on.
-fasm-blocks
Enable the use of blocks and entire functions of assembly code
within a C or C++ file. The syntax follows that used in
CodeWarrior. (APPLE ONLY)
-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. -ansi implies
-fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm"
and "inline" are standard keywords. You may want to use the
-fno-gnu-keywords flag instead, which has the same effect. 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 that 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 this 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))
-fhosted
Assert that compilation takes place in 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.
-ffreestanding
Assert that compilation takes place in 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.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
Some cases of unnamed fields in structures and unions are only
accepted with this option.
-trigraphs
Support ISO C trigraphs. The -ansi option (and -std options for
strict ISO C conformance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling.
This option allows a user supplied "cc1", "cc1plus", or "cc1obj"
via the -B option. The user supplied compilation step can then add
in an additional preprocessing step after normal preprocessing but
before compiling. The default is to use the integrated cpp
(internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and
"cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-
standard C compiler. They are now only supported with the -E
switch. The preprocessor continues to support a pre-standard mode.
See the GNU CPP manual for details.
-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++.
-fno-nested-functions
Disable nested functions. This option is not supported for C++ or
Objective-C++. On Darwin, nested functions are disabled by
default.
-fpch-preprocess
Enable PCH processing even when -E or -save-temps is used.
-fnon-lvalue-assign
C and C++ forbid the use of casts and conditional expressions as
lvalues, e.g.:
float *p, q, r;
((int *)p)++;
(cond ? q : r) = 3.0;
As a transitional measure, the Apple version of GCC 4.0 allows
casts and conditional expressions to be used as lvalues in certain
situations. This is accomplished via the -fnon-lvalue-assign
switch, which is on by default. Whenever an lvalue cast or an
lvalue conditional expression is encountered, the compiler will
issue a deprecation warning and then rewrite the expression as
follows:
(type)expr ---becomes---> *(type *)&expr
cond ? expr1 : expr2 ---becomes---> *(cond ? &expr1 : &expr2)
To disallow lvalue casts and lvalue conditional expressions
altogether, specify -fno-non-lvalue-assign; lvalue casts and lvalue
conditional expressions will be disallowed in future versions of
Apple's GCC.
-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.
-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.
-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.
-fconstant-cfstrings
Enable the automatic creation of a CoreFoundation-type constant
string whenever a special builtin
"__builtin__CFStringMakeConstantString" is called on a literal
string. (APPLE ONLY)
-fpascal-strings
Allow Pascal-style string literals to be constructed. (APPLE ONLY)
-fwritable-strings
Store string constants in the writable data segment and don't
uniquize them. This is for compatibility with old programs which
assume they can write into string constants.
Writing into string constants is a very bad idea; ``constants''
should be constant.
This option is deprecated.
Options Controlling C++ Dialect
This section describes the command-line options that are only
meaningful for C++ programs; but 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 -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs;
you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. Version 2 is the version of the C++
ABI that first appeared in G++ 3.4. Version 1 is the version of
the C++ ABI that first appeared in G++ 3.2. Version 0 will always
be the version that conforms most closely to the C++ ABI
specification. Therefore, the ABI obtained using version 0 will
change as ABI bugs are fixed.
The default is version 2.
-fno-access-control
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
-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" will only return 0 if it is declared throw(), in
which case the compiler will always check 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).
-fconserve-space
Put uninitialized or runtime-initialized global variables into the
common segment, as C does. This saves space in the executable at
the cost of not diagnosing duplicate definitions. If you compile
with this flag and your program mysteriously crashes after "main()"
has completed, you may have an object that is being destroyed twice
because two definitions were merged.
This option is no longer useful on most targets, now that support
has been added for putting variables into BSS without making them
common.
-fno-const-strings
Give string constants type "char *" instead of type "const char *".
By default, G++ uses type "const char *" as required by the
standard. Even if you use -fno-const-strings, you cannot actually
modify the value of a string constant, unless you also use
-fwritable-strings.
This option might be removed in a future release of G++. For
maximum portability, you should structure your code so that it
works with string constants that have type "const char *".
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a
temporary which 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.
-fno-enforce-eh-specs
Don't check for violation of exception specifications at runtime.
This option violates the C++ standard, but may be useful for
reducing code size in production builds, much like defining NDEBUG.
The compiler will still optimize based on the exception
specifications.
-ffor-scope
-fno-for-scope
If -ffor-scope is specified, the scope of variables declared in a
for-init-statement is limited to the for loop itself, as specified
by the C++ standard. If -fno-for-scope is specified, the scope of
variables declared in a for-init-statement extends to the end of
the enclosing scope, as was the case in old versions of G++, and
other (traditional) implementations of C++.
The default if neither flag is given to follow the standard, but to
allow and give a warning for old-style code that would otherwise be
invalid, or have different behavior.
-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. -ansi implies -fno-gnu-keywords.
-fno-implicit-templates
Never emit code for non-inline templates which are instantiated
implicitly (i.e. by use); only emit code for explicit
instantiations.
-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 will need the same set of
explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
errors if these functions are not inlined everywhere they are
called.
-fms-extensions
Disable pedantic warnings about constructs used in MFC, such as
implicit int and getting a pointer to member function via non-
standard syntax.
-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.
-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 will allow some nonconforming
code to compile.
-frepo
Enable automatic template instantiation at link time. This option
also implies -fno-implicit-templates.
-fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ runtime 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 it will generate
it as needed.
-fstats
Emit statistics about front-end processing at the end of the
compilation. This information is generally only useful to the G++
development team.
-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.
-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 will only work if your C library supports
"__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This will
cause "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 methods where the addresses of the two functions
were 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 behaviour of this switch is not quite the same as marking the
methods as hidden directly. Normally if there is a class with
default visibility which has a hidden method, the effect of this is
that the method must be defined in only one shared object. This
switch does not have this restriction.
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.
-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 which are defined in
more than one different shared object: those declarations are
permitted if they would have been permitted when this option was
not used.
This option is discouraged, rather, it is preferable for types to
be explicitly exported as desired on a per-class basis.
Unfortunately because Visual Studio can't compare two different
hidden types as unequal for the purposes of type_info and exception
handling, users are able to write code that relies upon this
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 will be different, so changing one will
not change the other; and that pointers to function members defined
in different shared objects will 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++ will use weak symbols if they are
available. This option exists only for testing, and should not be
used by end-users; it will result in inferior code and has no
benefits. This option may be removed in a future release of G++.
-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.)
In addition, these optimization, warning, and code generation options
have meanings only for C++ programs:
-fno-default-inline
Do not assume inline for functions defined inside a class scope.
Note that these functions will have linkage like inline
functions; they just won't be inlined by default.
-Wabi (C++ only)
Warn when G++ generates code that is probably not compatible with
the vendor-neutral C++ ABI. 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 will be 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.
The known incompatibilities at this point include:
o Incorrect handling of tail-padding for bit-fields. G++ may
attempt to pack data into the same byte as a base class. For
example:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place "B::f2" into the same byte
as"A::f1"; other compilers will not. You can avoid this
problem by explicitly padding "A" so that its size is a
multiple of the byte size on your platform; that will cause G++
and other compilers to layout "B" identically.
o Incorrect handling of tail-padding for virtual bases. G++ does
not use tail padding when laying out virtual bases. For
example:
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place "B" into the tail-padding for
"A"; other compilers will. You can avoid this problem by
explicitly padding "A" so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++
and other compilers to layout "C" identically.
o Incorrect handling of bit-fields with declared widths greater
than that of their underlying types, when the bit-fields appear
in a union. For example:
union U { int i : 4096; };
Assuming that an "int" does not have 4096 bits, G++ will make
the union too small by the number of bits in an "int".
o Empty classes can be placed at incorrect offsets. For example:
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the "A" base class of "C" at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes
that the "A" data member of "B" is already at offset zero.
o Names of template functions whose types involve "typename" or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (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.
-Wnon-virtual-dtor (C++ only)
Warn when a class appears to be polymorphic, thereby requiring a
virtual destructor, yet it declares a non-virtual one. This
warning is enabled by -Wall.
-Wreorder (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 will rearrange 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.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)
Warn about violations of the following style guidelines from Scott
Meyers' Effective C++ book:
o Item 11: Define a copy constructor and an assignment operator
for classes with dynamically allocated memory.
o Item 12: Prefer initialization to assignment in constructors.
o Item 14: Make destructors virtual in base classes.
o Item 15: Have "operator=" return a reference to *this.
o Item 23: Don't try to return a reference when you must return
an object.
Also warn about violations of the following style guidelines from
Scott Meyers' More Effective C++ book:
o Item 6: Distinguish between prefix and postfix forms of
increment and decrement operators.
o Item 7: Never overload "&&", "||", or ",".
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-deprecated (C++ only)
Do not warn about usage of deprecated features.
-Wstrict-null-sentinel (C++ only)
Warn also 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 not a
null pointer, it is guaranteed to of the same size as a pointer.
But this use is not portable across different compilers.
-Wno-non-template-friend (C++ only)
Disable warnings when non-templatized friend functions are declared
within a template. Since the advent of explicit template
specification support in G++, if the name of the friend is an
unqualified-id (i.e., friend foo(int)), the C++ language
specification demands that the friend declare or define an
ordinary, nontemplate function. (Section 14.5.3). Before G++
implemented explicit specification, unqualified-ids could be
interpreted as a particular specialization of a templatized
function. Because this non-conforming behavior is no longer the
default behavior for G++, -Wnon-template-friend allows the compiler
to check existing code for potential trouble spots and is on by
default. This new compiler behavior can be turned off with
-Wno-non-template-friend which keeps the conformant compiler code
but disables the helpful warning.
-Wold-style-cast (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 (static_cast,
reinterpret_cast, and const_cast) are less vulnerable to unintended
effects and much easier to search for.
-Woverloaded-virtual (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();
will fail to compile.
-Wno-pmf-conversions (C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (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++ would try to
preserve unsignedness, but the standard mandates the current
behavior.
struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const
A&);, while cfront will use the user-defined operator =.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++
languages themselves. See
This section describes the command-line options that are only
meaningful for Objective-C and Objective-C++ programs, but 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, will
override 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 (e.g., "[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. Currently, this option is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-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 that
will run 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 that will run all such default destructors,
in reverse order.
The "- (id) .cxx_construct" and/or "- (void) .cxx_destruct" methods
thusly generated will 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 will be invoked by the runtime
immediately after a new object instance is allocated; the "- (void)
.cxx_destruct" methods will be 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-sjlj-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++ and Java. This
option is available in conjunction with the NeXT setjmp base
exceptions on Mac OS X 10.3 and later. This option is on by
default with the NeXT runtime.
@try {
...
@throw expr;
...
}
@catch (AnObjCClass *exc) {
...
@throw expr;
...
@throw;
...
}
@catch (AnotherClass *exc) {
...
}
@catch (id allOthers) {
...
}
@finally {
...
@throw expr;
...
}
The @throw statement may appear anywhere in an Objective-C or
Objective-C++ program; when used inside of a @catch block, the
@throw may appear without an argument (as shown above), in which
case the object caught by the @catch will be rethrown.
Note that only (pointers to) Objective-C objects may be thrown and
caught using this scheme. When an object is thrown, it will be
caught by the nearest @catch clause capable of handling objects of
that type, analogously to how "catch" blocks work in C++ and Java.
A "@catch(id ...)" clause (as shown above) may also be provided to
catch any and all Objective-C exceptions not caught by previous
@catch clauses (if any).
The @finally clause, if present, will be executed upon exit from
the immediately preceding "@try ... @catch" section. This will
happen regardless of whether any exceptions are thrown, caught or
rethrown inside the "@try ... @catch" section, analogously to the
behavior of the "finally" clause in Java.
There are several caveats to using the new exception mechanism:
o Although currently designed to be binary compatible with
"NS_HANDLER"-style idioms provided by the "NSException" class,
the new exceptions can only be used on Mac OS X 10.3 (Panther)
and later systems, due to additional functionality needed in
the (NeXT) Objective-C runtime.
o As mentioned above, the new exceptions do not support handling
types other than Objective-C objects. Furthermore, when used
from Objective-C++, the Objective-C exception model does not
interoperate with C++ exceptions at this time. This means you
cannot @throw an exception from Objective-C and "catch" it in
C++, or vice versa (i.e., "throw ... @catch").
The -fobjc-sjlj-exceptions switch also enables the use of
synchronization blocks for thread-safe execution:
@synchronized (ObjCClass *guard) {
...
}
Upon entering the @synchronized block, a thread of execution shall
first check whether a lock has been placed on the corresponding
"guard" object by another thread. If it has, the current thread
shall wait until the other thread relinquishes its lock. Once
"guard" becomes available, the current thread will place its own
lock on it, execute the code contained in the @synchronized block,
and finally relinquish the lock (thereby making "guard" available
to other threads).
Unlike Java, Objective-C does not allow for entire methods to be
marked @synchronized. Note that throwing exceptions out of
@synchronized blocks is allowed, and will cause the guarding object
to be unlocked properly.
-fobjc-gc
Enable garbage collection (GC) for Objective-C objects. The
resulting binary requires additional runtime support which is not
present in any released version of Mac OS X.
When the -fobjc-gc switch is specified, the compiler will replace
assignments to instance variables (ivars) and to certain kinds of
pointers to Objective-C object instances with calls to interceptor
functions provided by the runtime garbage collector. Two type
qualifiers, "__strong" and "__weak", also become available. The
"__strong" qualifier may be used to indicate that assignments to
variables of this type should generate a GC interceptor call, e.g.:
__strong void *p; // assignments to 'p' will have interceptor calls
int *q; // assignments to 'q' ordinarly will not
...
(__strong int *)q = 0; // this assignment will call an interceptor
Conversely, the "__weak" type qualifier may be used to suppress
interceptor call generation:
__weak id q; // assignments to 'q' will not have interceptor calls
id p; // assignments to 'p' will have interceptor calls
...
(__weak id)p = 0; // suppress interceptor call for this assignment
-fobjc-gc-only
Use this option to indicate that the Objective-C program supports
garbage collection (GC) only - that is, it does not contain
retain/release logic. This flag implies -fobjc-gc as well. With
this flag, framework is marked as not honoring retain/release.
-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.
-gen-decls
Dump interface declarations for all classes seen in the source file
to a file named sourcename.decl.
-Wno-protocol
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.
-Wselector
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
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
will omit such warnings if any differences found are confined to
types which share the same size and alignment.
-Wundeclared-selector
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, ...). The options
described below can be used to control the diagnostic messages
formatting algorithm, e.g. how many characters per line, how often
source location information should be reported. Right now, only the
C++ front end can honor these options. However it is expected, in the
near future, that the remaining front ends would be able to digest them
correctly.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. The default is 72 characters for g++ and 0 for the
rest of the front ends supported by GCC. If n is zero, then no
line-wrapping will be done; each error message will appear on a
single line.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit once source location information; 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.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there may have
been an error.
You can request many specific warnings with options beginning -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.
The following options control the amount and kinds of warnings produced
by GCC; for further, language-specific options also refer to C++
Dialect Options and Objective-C and Objective-C++ Dialect Options.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond
that.
-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 will 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.
-pedantic does not cause warning messages for use of the alternate
keywords whose names begin and end with __. 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 -pedantic 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 -pedantic. 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 gnu89 or gnu99, there is a corresponding base
standard, the version of ISO C on which the GNU extended dialect is
based. Warnings from -pedantic are given where they are required
by the base standard. (It would 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
Like -pedantic, except that errors are produced rather than
warnings.
-w Inhibit all warning messages.
-Wno-import
Inhibit warning messages about the use of #import.
-Wno-#warnings
Inhibit warning messages issued by #warning.
-Wextra-tokens
Warn about extra tokens at the end of prepreprocessor directives.
(APPLE ONLY)
-Wnewline-eof
Warn about files missing a newline at the end of the file. (APPLE
ONLY)
-Wno-altivec-long-deprecated
Do not warn about the use of the deprecated 'long' keyword in
AltiVec data types. (APPLE ONLY)
-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.
-Wcomment
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.
-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.
-Wformat
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
-pedantic is used with -Wformat, warnings will be 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).
Since -Wformat also checks for null format arguments for several
functions, -Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some aspects
of format checking, the options -Wformat-y2k,
-Wno-format-extra-args, -Wno-format-zero-length,
-Wformat-nonliteral, -Wformat-security, and -Wformat=2 are
available, but are not included in -Wall.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats which
may yield only a two-digit year.
-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 will suppress the warning if
the unused arguments are all pointers, since the Single Unix
Specification says that such unused arguments are allowed.
-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=2
Enable -Wformat plus format checks not included in -Wformat.
Currently equivalent to -Wformat -Wformat-nonliteral
-Wformat-security -Wformat-y2k.
-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.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables which are initialized with
themselves. Note this option can only be used with the
-Wuninitialized option, which in turn only works with -O1 and
above.
For example, GCC will warn about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
-Wimplicit-int
Warn when a declaration does not specify a type. This warning is
enabled by -Wall.
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
Give a warning (or error) whenever a function is used before being
declared. The form -Wno-error-implicit-function-declaration is not
supported. This warning is enabled by -Wall (as a warning, not an
error).
-Wimplicit
Same as -Wimplicit-int and -Wimplicit-function-declaration. This
warning is enabled by -Wall.
-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 -Wall.
-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 and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-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. Only the warning for an assignment used
as a truth value is supported when compiling C++; the other
warnings are only supported when compiling C.
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 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, 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 will issue 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" could
belong to the enclosing "if". The resulting code would look like
this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
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 standard.
The C standard defines the order in which expressions in a 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 standard specifies 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 present implementation of this option only works for C
programs. A future implementation may also work for C++ programs.
The C 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 <http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall.
-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".
For C, also warn if the return type of a function has a type
qualifier such as "const". Such a type qualifier has no effect,
since the value returned by a function is not an lvalue. ISO C
prohibits qualified "void" return types on function definitions, so
such return types always receive a warning even without this
option.
For C++, a function without return type always produces a
diagnostic message, even when -Wno-return-type is specified. The
only exceptions are main and functions defined in system headers.
This warning is enabled by -Wall.
-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. 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.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning
of the program (trigraphs within comments are not warned about).
This warning 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-parameter
Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the unused attribute.
-Wunused-variable
Warn whenever a local variable or non-constant static variable is
unused aside from its declaration This warning is enabled by -Wall.
To suppress this warning use the unused attribute.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not
used. This warning is enabled by -Wall.
To suppress this warning cast the expression to void.
-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 automatic variable is used without first being
initialized or if a variable may be clobbered by a "setjmp" call.
These warnings are possible only in optimizing compilation, because
they require data flow information that is computed only when
optimizing. If you don't specify -O, you simply won't get these
warnings.
If you want to warn about code which uses the uninitialized value
of the variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized or clobbered
elements of structure, union or array variables as well as for
variables which are uninitialized or clobbered 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 will 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.
These warnings are made optional because GCC is not smart enough to
see all the reasons why the code might be correct despite 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. Here is another common
case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because "save_y" is used only if it is set.
This option also warns when a non-volatile automatic variable might
be changed by a call to "longjmp". These warnings as well are
possible only in optimizing compilation.
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 which 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.
-Wunknown-pragmas
understood by GCC. If this command line option is used, warnings
will even be issued for unknown pragmas in system header files.
This is not the case if the warnings were only enabled by the -Wall
command line option.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It
warns about code which 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.
-Wstrict-aliasing=2
This option is only active when -fstrict-aliasing is active. It
warns about all code which might break the strict aliasing rules
that the compiler is using for optimization. This warning catches
all cases, but it will also give a warning for some ambiguous cases
that are safe.
-Wall
All of the above -W options combined. 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.
-Wmost
This is equivalent to -Wall -Wno-parentheses. (APPLE ONLY)
The following -W... options 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.
-Wextra
(This option used to be called -W. The older name is still
supported, but the newer name is more descriptive.) Print extra
warning messages for these events:
o A function can return either with or without a value. (Falling
off the end of the function body is considered returning
without a value.) For example, this function would evoke such
a warning:
foo (a)
{
if (a > 0)
return a;
}
o An expression-statement or the left-hand side of a comma
expression contains no side effects. To suppress the warning,
cast the unused expression to void. For example, an expression
such as x[i,j] will cause a warning, but x[(void)i,j] will not.
o An unsigned value is compared against zero with < or >=.
o Storage-class specifiers like "static" are not the first things
in a declaration. According to the C Standard, this usage is
obsolescent.
o If -Wall or -Wunused is also specified, warn about unused
arguments.
o A comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to
unsigned. (But don't warn if -Wno-sign-compare is also
specified.)
o An aggregate has an initializer which does not initialize all
members. This warning can be independently controlled by
-Wmissing-field-initializers.
o A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
o An empty body occurs in an if or else statement.
o A pointer is compared against integer zero with <, <=, >, or
>=.
o A variable might be changed by longjmp or vfork.
o Any of several floating-point events that often indicate
errors, such as overflow, underflow, loss of precision, etc.
o *<(C++ only)>
An enumerator and a non-enumerator both appear in a conditional
expression.
o *<(C++ only)>
A non-static reference or non-static const member appears in a
class without constructors.
o *<(C++ only)>
Ambiguous virtual bases.
o *<(C++ only)>
Subscripting an array which has been declared register.
o *<(C++ only)>
Taking the address of a variable which has been declared
register.
o *<(C++ only)>
A base class is not initialized in a derived class' copy
constructor.
-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 will not warn about unknown pragmas
in system headers---for that, -Wunknown-pragmas must also be used.
-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 would 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.
-Wfour-char-constants
Warn about four char constants, e.g. OSType 'APPL'. This warning
is disabled by default.
-Wtraditional (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 which
should be avoided.
o Macro parameters that appear within string literals in the
macro body. In traditional C macro replacement takes place
within string literals, but does not in ISO C.
o In traditional C, some preprocessor directives did not exist.
Traditional preprocessors would only consider 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 would ignore because the # does not appear as
the first character on the line. It also suggests you hide
indenting them. Some traditional implementations would not
recognize #elif, so it suggests avoiding it altogether.
o A function-like macro that appears without arguments.
o The unary plus operator.
o 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.
o A function declared external in one block and then used after
the end of the block.
o A "switch" statement has an operand of type "long".
o A non-"static" function declaration follows a "static" one.
This construct is not accepted by some traditional C compilers.
o 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.
o Usage of ISO string concatenation is detected.
o Initialization of automatic aggregates.
o Identifier conflicts with labels. Traditional C lacks a
separate namespace for labels.
o 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.
o Conversions by prototypes between fixed/floating point values
and vice versa. The absence of these prototypes when compiling
with traditional C would cause serious problems. This is a
subset of the possible conversion warnings, for the full set
use -Wconversion.
o Use of ISO C style function definitions. This warning
intentionally is not issued for prototype declarations or
variadic functions because these ISO C features will 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.
-Wdeclaration-after-statement (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 and was
not supported by GCC versions before GCC 3.0.
-Wno-discard-qual
This flag allows user to suppress warning that is issued when
qualification is discarded in situations like, initialization,
assignment and argument passing.
-Wundef
Warn if an undefined identifier is evaluated in an #if directive.
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
-Wshadow
Warn whenever a local variable shadows another local variable,
parameter or global variable or whenever a built-in function is
shadowed.
-Wlarger-than-len
Warn whenever an object of larger than len bytes is defined.
-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.
-Wbad-function-cast (C only)
Warn whenever a function call is cast to a non-matching type. For
example, warn if "int malloc()" is cast to "anything *".
-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 *".
-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.
-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 will get a warning; when compiling C++, warn about
the deprecated conversion from string constants to "char *". These
warnings will 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 will just be a nuisance; this is why we did not make
-Wall request these warnings.
-Wconversion
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.
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the
assignment "x = -1" if "x" is unsigned. But do not warn about
explicit casts like "(unsigned) -1".
-Wshorten-64-to-32
Warn if a value is implicitly converted from a 64 bit type to a 32
bit type.
-Wsign-compare
Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. This warning is also enabled by -Wextra; to get the
other warnings of -Wextra without this warning, use -Wextra
-Wno-sign-compare.
-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.)
-Wstrict-prototypes (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 which specifies the
argument types.)
-Wold-style-definition (C only)
Warn if an old-style function definition is used. A warning is
given even if there is a previous prototype.
-Wmissing-prototypes (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. The aim is to detect global functions that
fail to be declared in header files.
-Wmissing-declarations (C only)
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.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For
example, the following code would cause 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 would not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
This warning is included in -Wextra. To get other -Wextra warnings
without this one, use -Wextra -Wno-missing-field-initializers.
-Wmissing-noreturn
Warn about functions which might be candidates for attribute
"noreturn". Note these are only possible candidates, not absolute
ones. Care should be taken to manually verify functions actually
do not ever return before adding the "noreturn" attribute,
otherwise subtle code generation bugs could be introduced. You
will not get a warning for "main" in hosted C environments.
-Wmissing-format-attribute
If -Wformat is enabled, also warn about functions which might be
candidates for "format" attributes. Note these are only possible
candidates, not absolute ones. GCC will guess 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. This option has no effect unless
-Wformat is enabled (possibly by -Wall).
-Wno-multichar
Do not warn if a multicharacter constant ('FOO') 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. This flag does not control warning for a constant with four
characters, use -Wfour-char-constants instead.
-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 which have not been normalized; this option
controls that warning.
There are four levels of warning that GCC supports. The default is
-Wnormalized=nfc, which warns about any identifier which is not in
the ISO 10646 ``C'' normalized form, NFC. NFC is the recommended
form for most uses.
Unfortunately, there are some characters which ISO C and ISO C++
allow in identifiers that when turned into NFC aren't allowable as
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. You would only want to do this if you were
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'', will display just like a regular "n" which
has been placed in a superscript. ISO 10646 defines the NFKC
normalisation scheme to convert all these into a standard form as
well, and GCC will warn 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
is unable to be fixed to display these characters distinctly.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as
deprecated by using the "deprecated" attribute.
-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" will be
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;
};
-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.
-Wnested-externs (C only)
Warn if an "extern" declaration is encountered within a function.
-Wunreachable-code
Warn if the compiler detects that code will never be executed.
This option is intended to warn when the compiler detects that at
least a whole line of source code will never be executed, because
some condition is never satisfied or because it is after a
procedure that never returns.
It is possible for this option to produce a warning even though
there are circumstances under which part of the affected line can
be executed, so care should be taken when removing apparently-
unreachable code.
For instance, when a function is inlined, a warning may mean that
the line is unreachable in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging
version of a program there is often substantial code which checks
correct functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable
code is to provide behavior which is selectable at compile-time.
-Winline
Warn if a function can not be inlined and it was declared as
inline. Even with this option, the compiler will 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.
-Wno-invalid-offsetof (C++ only)
Suppress warnings from applying the offsetof macro to a non-POD
type. According to the 1998 ISO C++ standard, applying offsetof to
a non-POD type is undefined. In existing C++ implementations,
however, offsetof typically gives meaningful results even when
applied to certain kinds of non-POD types. (Such as a simple struct
that fails to be a POD type only by virtue of having a
constructor.) 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.
-Wno-int-to-pointer-cast (C only)
Suppress warnings from casts to pointer type of an integer of a
different size.
-Wno-pointer-to-int-cast (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 can't
be used.
-Wlong-long
Warn if long long type is used. This is default. To inhibit the
warning messages, use -Wno-long-long. Flags -Wlong-long and
-Wno-long-long are taken into account only when -pedantic flag is
used.
-Wvariadic-macros
Warn if variadic macros are used in pedantic ISO C90 mode, or the
GNU alternate syntax when in pedantic ISO C99 mode. This is
default. To inhibit the warning messages, use
-Wno-variadic-macros.
-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 were unable to
handle the code effectively. Often, the problem is that your code
is too big or too complex; GCC will refuse to optimize programs
when the optimization itself is likely to take inordinate amounts
of time.
-Wno-pointer-sign
Don't warn for pointer argument passing or assignment with
different signedness. Only useful in the negative form since this
warning is enabled by default. This option is only supported for C
and Objective-C.
-Wstack-protector
This option is only active when -fstack-protector is active. It
warns about functions that will not be protected against stack
smashing.
-Werror
Make all warnings into errors.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debugging either your
program or GCC:
-g Produce debugging information in the operating system's native
format (stabs, COFF, XCOFF, or DWARF 2). 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 will probably make other
debuggers crash or refuse to read the program. If you want to
control for certain whether to generate the extra information, use
-gstabs+ or -gstabs (see below).
GCC allows you to use -g with -O. The shortcuts taken by optimized
code may occasionally produce surprising results: 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 were already
at hand; some statements may execute in different places because
they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might
have bugs.
The following options are useful when GCC is generated with the
capability for more than one debugging format.
-ggdb
Produce debugging information for use by GDB. This means to use
the most expressive format available (DWARF 2, stabs, or the native
format if neither of those are supported), including GDB extensions
if at all possible.
-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 which is not understood
by DBX or SDB. On System V Release 4 systems this option requires
the GNU assembler.
-flimit-debug-info
Limit debug information produced to reduce size of debug binary.
-feliminate-unused-debug-symbols
Produce debugging information in stabs format (if that is
supported), for only symbols that are actually used.
-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.
-gdwarf-2
Produce debugging information in DWARF version 2 format (if that is
supported). This is the format used by DBX on IRIX 6. With this
option, GCC uses features of DWARF version 3 when they are useful;
version 3 is upward compatible with version 2, but may still cause
problems for older debuggers.
(Other debug formats, such as -gcoff, are not supported in Darwin
or Mac OS X.)
-glevel
-ggdblevel
-gstabslevel
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, but no
information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support macro
expansion when you use -g3.
-gdwarf-2 does not accept a concatenated debug level, because GCC
used to support an option -gdwarf that meant to generate debug
information in version 1 of the DWARF format (which is very
different from version 2), and it would have been too confusing.
That debug format is long obsolete, but the option cannot be
changed now. Instead use an additional -glevel option to change
the debug level for DWARF2.
-feliminate-dwarf2-dups
Compress DWARF2 debugging information by eliminating duplicated
information about each symbol. This option only makes sense when
generating DWARF2 debugging information with -gdwarf-2.
-p Generate extra code to write profile information suitable for the
analysis program prof. You must use this option when compiling the
source files you want data about, and you must also use it when
linking.
-pg Generate extra code to write profile information suitable for the
analysis program gprof. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
-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.
-fmem-report
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
-fopt-diary
Enable optimization diary entries using DWARF encoding. This option
does nothing unless gdwarf-2 is specified.
-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. 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).
@bullet
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.
@cvmmfu
Link your object files with -lgcov or -fprofile-arcs (the
latter implies the former).
@dwnngv
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. Also "fork" calls are detected and
correctly handled (double counting will not happen).
@exoohw
For profile-directed optimizations, compile the source files
again with the same optimization and code generation options
plus -fbranch-probabilities.
@fyppix
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.
-ftree-based-profiling
This option is used in addition to -fprofile-arcs or
-fbranch-probabilities to control whether those optimizations are
performed on a tree-based or rtl-based internal representation. If
you use this option when compiling with -fprofile-arcs, you must
also use it when compiling later with -fbranch-probabilities.
Currently the tree-based optimization is in an early stage of
development, and this option is recommended only for those people
working on improving it.
-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 will match the source files more
closely, if you do not optimize.
-dletters
-fdump-rtl-pass
Says to make debugging dumps during compilation at times specified
by letters. This is used for debugging the RTL-based passes of
the compiler. The file names for most of the dumps are made by
appending a pass number and a word to the dumpname. dumpname 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.
Most debug dumps can be enabled either passing a letter to the -d
option, or with a long -fdump-rtl switch; here are the possible
letters for use in letters and pass, and their meanings:
-dA Annotate the assembler output with miscellaneous debugging
information.
-db
-fdump-rtl-bp
Dump after computing branch probabilities, to file.09.bp.
-dB
-fdump-rtl-bbro
Dump after block reordering, to file.30.bbro.
-dc
-fdump-rtl-combine
Dump after instruction combination, to the file
file.17.combine.
-dC
-fdump-rtl-ce1
-fdump-rtl-ce2
-dC and -fdump-rtl-ce1 enable dumping after the first if
conversion, to the file file.11.ce1. -dC and -fdump-rtl-ce2
enable dumping after the second if conversion, to the file
file.18.ce2.
-dd
-fdump-rtl-btl
-fdump-rtl-dbr
-dd and -fdump-rtl-btl enable dumping after branch target load
optimization, to file.31.btl. -dd and -fdump-rtl-dbr enable
dumping after delayed branch scheduling, to file.36.dbr.
-dD Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
-dE
-fdump-rtl-ce3
Dump after the third if conversion, to file.28.ce3.
-df
-fdump-rtl-cfg
-fdump-rtl-life
-df and -fdump-rtl-cfg enable dumping after control and data
flow analysis, to file.08.cfg. -df and -fdump-rtl-cfg enable
dumping dump after life analysis, to file.16.life.
-dg
-fdump-rtl-greg
Dump after global register allocation, to file.23.greg.
-dG
-fdump-rtl-gcse
-fdump-rtl-bypass
-dG and -fdump-rtl-gcse enable dumping after GCSE, to
file.05.gcse. -dG and -fdump-rtl-bypass enable dumping after
jump bypassing and control flow optimizations, to
file.07.bypass.
-dh
-fdump-rtl-eh
Dump after finalization of EH handling code, to file.02.eh.
-di
-fdump-rtl-sibling
Dump after sibling call optimizations, to file.01.sibling.
-dj
-fdump-rtl-jump
Dump after the first jump optimization, to file.03.jump.
-dk
-fdump-rtl-stack
Dump after conversion from registers to stack, to
file.33.stack.
-dl
-fdump-rtl-lreg
Dump after local register allocation, to file.22.lreg.
-dL
-fdump-rtl-loop
-fdump-rtl-loop2
-dL and -fdump-rtl-loop enable dumping after the first loop
optimization pass, to file.06.loop. -dL and -fdump-rtl-loop2
enable dumping after the second pass, to file.13.loop2.
-dm
-fdump-rtl-sms
Dump after modulo scheduling, to file.20.sms.
-dM
-fdump-rtl-mach
Dump after performing the machine dependent reorganization
pass, to file.35.mach.
-dn
-fdump-rtl-rnreg
Dump after register renumbering, to file.29.rnreg.
-dN
-fdump-rtl-regmove
Dump after the register move pass, to file.19.regmove.
-do
-fdump-rtl-postreload
Dump after post-reload optimizations, to file.24.postreload.
-dr
-fdump-rtl-expand
Dump after RTL generation, to file.00.expand.
-dR
-fdump-rtl-sched2
Dump after the second scheduling pass, to file.32.sched2.
-ds
-fdump-rtl-cse
Dump after CSE (including the jump optimization that sometimes
follows CSE), to file.04.cse.
-dS
-fdump-rtl-sched
Dump after the first scheduling pass, to file.21.sched.
-dt
-fdump-rtl-cse2
Dump after the second CSE pass (including the jump optimization
that sometimes follows CSE), to file.15.cse2.
-dT
-fdump-rtl-tracer
Dump after running tracer, to file.12.tracer.
-dV
-fdump-rtl-vpt
-fdump-rtl-vartrack
-dV and -fdump-rtl-vpt enable dumping after the value profile
transformations, to file.10.vpt. -dV and -fdump-rtl-vartrack
enable dumping after variable tracking, to file.34.vartrack.
-dw
-fdump-rtl-flow2
Dump after the second flow pass, to file.26.flow2.
-dz
-fdump-rtl-peephole2
Dump after the peephole pass, to file.27.peephole2.
-dZ
-fdump-rtl-web
Dump after live range splitting, to file.14.web.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dH Produce a core dump whenever an error occurs.
-dm Print statistics on memory usage, at the end of the run, to
standard error.
-dp Annotate the assembler output with a comment indicating which
pattern and alternative was used. The length of each
instruction is also printed.
-dP Dump the RTL in the assembler output as a comment before each
instruction. Also turns on -dp annotation.
-dv For each of the other indicated dump files (either with -d or
-fdump-rtl-pass), dump a representation of the control flow
graph suitable for viewing with VCG to file.pass.vcg.
-dx Just generate RTL for a function instead of compiling it.
Usually used with r (-fdump-rtl-expand).
-dy Dump debugging information during parsing, to standard error.
-fdump-unnumbered
When doing debugging dumps (see -d option above), suppress
instruction numbers and line number note 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-translation-unit (C and C++ only)
-fdump-translation-unit-options (C and C++ only)
Dump a representation of the tree structure for the entire
translation unit to a file. The file name is made by appending .tu
to the source file name. If the -options form is used, options
controls the details of the dump as described for the -fdump-tree
options.
-fdump-class-hierarchy (C++ only)
-fdump-class-hierarchy-options (C++ only)
Dump a representation of each class's hierarchy and virtual
function table layout to a file. The file name is made by
appending .class to the source file name. If the -options form is
used, options controls the details of the dump as described for the
-fdump-tree options.
-fdump-ipa-switch
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. The following
dumps are possible:
all Enables all inter-procedural analysis dumps; currently the only
produced dump is the cgraph dump.
cgraph
Dumps information about call-graph optimization, unused
function removal, and inlining decisions.
-fdump-tree-switch (C and C++ only)
-fdump-tree-switch-options (C and C++ only)
Control the dumping at various stages of processing the
intermediate language tree to a file. The file name is generated
by appending a switch specific suffix to the source file name. If
the -options form is used, options is a list of - separated options
that control the details of the dump. Not all options are
applicable to all dumps, those which are not meaningful will be
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.
slim
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.
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).
stats
Enable dumping various statistics about the pass (not honored
by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
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.
all Turn on all options, except raw, slim and lineno.
The following tree dumps are possible:
original
Dump before any tree based optimization, to file.original.
optimized
Dump after all tree based optimization, to file.optimized.
inlined
Dump after function inlining, to file.inlined.
gimple
Dump each function before and after the gimplification pass to
a file. The file name is made by appending .gimple to the
source file name.
cfg Dump the control flow graph of each function to a file. The
file name is made by appending .cfg to the source file name.
vcg Dump the control flow graph of each function to a file in VCG
format. The file name is made by appending .vcg to the source
file name. Note that if the file contains more than one
function, the generated file cannot be used directly by VCG.
You will need to cut and paste each function's graph into its
own separate file first.
ch Dump each function after copying loop headers. The file name
is made by appending .ch to the source file name.
ssa Dump SSA related information to a file. The file name is made
by appending .ssa to the source file name.
alias
Dump aliasing information for each function. The file name is
made by appending .alias to the source file name.
ccp Dump each function after CCP. The file name is made by
appending .ccp to the source file name.
pre Dump trees after partial redundancy elimination. The file name
is made by appending .pre to the source file name.
fre Dump trees after full redundancy elimination. The file name is
made by appending .fre to the source file name.
dce Dump each function after dead code elimination. The file name
is made by appending .dce to the source file name.
mudflap
Dump each function after adding mudflap instrumentation. The
file name is made by appending .mudflap to the source file
name.
scev
Dump the information gathered by the scalar evolution analyzer.
The file name is made by appending .scev to the source file
name.
ddall
Dump all the data dependence relations. The file name is made
by appending .ddall to the source file name.
elck
Dump each function after performing checks elimination based on
scalar evolution informations. The file name is made by
appending .elck to the source file name.
sra Dump each function after performing scalar replacement of
aggregates. The file name is made by appending .sra to the
source file name.
dom Dump each function after applying dominator tree optimizations.
The file name is made by appending .dom to the source file
name.
dse Dump each function after applying dead store elimination. The
file name is made by appending .dse to the source file name.
phiopt
Dump each function after optimizing PHI nodes into straightline
code. The file name is made by appending .phiopt to the source
file name.
forwprop
Dump each function after forward propagating single use
variables. The file name is made by appending .forwprop to the
source file name.
copyrename
Dump each function after applying the copy rename optimization.
The file name is made by appending .copyrename to the source
file name.
nrv Dump each function after applying the named return value
optimization on generic trees. The file name is made by
appending .nrv to the source file name.
loop
Dump each function after applying tree-level loop
optimizations. The file name is made by appending .loop to the
source file name.
vect
Dump each function after applying vectorization of loops. The
file name is made by appending .vect to the source file name.
all Enable all the available tree dumps with the flags provided in
this option.
-ftree-vectorizer-verbose=n
This option controls the amount of debugging output the vectorizer
prints. This information is written to standard error, unless
-fdump-tree-all or -fdump-tree-vect is specified, in which case it
is output to the usual dump listing file, .vect.
-frandom-seed=string
This option provides a seed that GCC uses when it would otherwise
use random numbers. It is used to generate 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 should be different for every file you compile.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls
the amount of debugging output the scheduler prints. This
information is written to standard error, unless -dS or -dR is
specified, in which case it is output to the usual dump listing
file, .sched or .sched2 respectively. However for n greater than
nine, the output is always printed to standard error.
For n greater than zero, -fsched-verbose outputs the same
information as -dRS. 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.
-save-temps
Store the usual ``temporary'' intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling foo.c with -c -save-temps would produce
files foo.i and foo.s, as well as foo.o. This creates a
preprocessed foo.i output file even though the compiler now
normally uses an integrated preprocessor.
When used in combination with the -x command line option,
-save-temps is sensible enough to avoid over writing 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.
-time
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). 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.
-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, -Oz (APPLE ONLY), ...), debugging information (-g) and the
debug info format supports it.
-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
@samp{-, without spaces between multiple switches. This is
supposed to ease shell-processing.
-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 will search---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 /.
-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)---and don't do
anything else.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else.
(This is used when GCC itself is being built.)
-feliminate-unused-debug-types
Normally, when producing DWARF2 output, GCC will emit debugging
information for all types declared in a compilation unit,
regardless of whether or not they are actually used in that
compilation unit. Sometimes this is useful, such as 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. With this option,
GCC will avoid producing debug symbol output for types that are
nowhere used in the source file being compiled.
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 would 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. Optimization levels -O2 and above, in particular, enable
unit-at-a-time mode, which allows the compiler to consider information
gained from later functions in the file when compiling a function.
Compiling multiple files at once to a single output file in unit-at-a-
time 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.
-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: -fdefer-pop
-fdelayed-branch -fguess-branch-probability -fcprop-registers
-floop-optimize -fif-conversion -fif-conversion2 -ftree-ccp
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-ter -ftree-lrs
-ftree-sra -ftree-copyrename -ftree-fre -ftree-ch -fmerge-constants
-O also turns on -fomit-frame-pointer on machines where doing so
does not interfere with debugging.
-O doesn't turn on -ftree-sra for the Ada compiler. This option
must be explicitly specified on the command line to be enabled for
the Ada compiler.
-O2 Optimize even more. GCC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. The
compiler does not perform loop unrolling or function inlining when
you specify -O2. 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 -O. It also turns
on the following optimization flags: -fthread-jumps -fcrossjumping
-foptimize-sibling-calls -fcse-follow-jumps -fcse-skip-blocks
-fgcse -fgcse-lm -fexpensive-optimizations -fstrength-reduce
-frerun-cse-after-loop -frerun-loop-opt -fcaller-saves -fforce-mem
-fpeephole2 -fschedule-insns -fschedule-insns2 -fsched-interblock
-fsched-spec -fregmove -fstrict-aliasing
-fdelete-null-pointer-checks -freorder-blocks -freorder-functions
-funit-at-a-time -falign-functions -falign-jumps -falign-loops
-falign-labels -ftree-pre
Please note the warning under -fgcse about invoking -O2 on programs
that use computed gotos.
In Apple's version of GCC, -fstrict-aliasing, -freorder-blocks, and
-fsched-interblock are disabled by default when optimizing.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2
and also turns on the -finline-functions, -funswitch-loops and
-fgcse-after-reload options.
-O0 Do not optimize. This is the default.
-fast
Optimize for maximum performance. -fast changes the overall
optimization strategy of GCC in order to produce the fastest
possible running code for PPC7450 and G5 architectures. By default,
-fast optimizes for G5. Programs optimized for G5 will not run on
PPC7450. To optimize for PPC7450, add -mcpu=7450 on command line.
-fast currently enables the following optimization flags (for G5
and PPC7450). These flags may change in the future. You cannot
override any of these options if you use -fast except by setting
-mcpu=7450 (or -fPIC, see below).
-O3 -falign-loops-max-skip=15 -falign-jumps-max-skip=15
-falign-loops=16 -falign-jumps=16 -falign-functions=16
-malign-natural (except when -fastf is specified) -ffast-math
-fstrict-aliasing -funroll-loops -ftree-loop-linear
-ftree-loop-memset -mcpu=G5 -mpowerpc-gpopt -mtune=G5 (unless
-mtune=G4 is specified). -fsched-interblock -fgcse-sm -mpowerpc64
To build shared libraries with -fast, specify -fPIC on the command
line as -fast turns on -mdynamic-no-pic otherwise.
Important notes: -ffast-math results in code that is not
necessarily IEEE-compliant. -fstrict-aliasing is highly likely to
break non-standard-compliant programs. -malign-natural only works
properly if the entire program is compiled with it, and none of the
standard headers/libraries contain any code that changes alignment
when this option is used.
On Intel target, -fast currently enables the following optimization
flags:
-O3 -fomit-frame-pointer -fstrict-aliasing
-momit-leaf-frame-pointer -fno-tree-pre -falign-loops
All choices of flags enabled by -fast are subject to change without
notice.
-Os Optimize for size, but not at the expense of speed. -Os enables
all -O2 optimizations that do not typically increase code size.
However, instructions are chosen for best performance, regardless
of size. To optimize solely for size on Darwin, use -Oz (APPLE
ONLY).
The following options are set for -O2, but are disabled under -Os:
-falign-functions -falign-jumps -falign-loops -falign-labels
-freorder-blocks -freorder-blocks-and-partition
-fprefetch-loop-arrays
When optimizing with -Os or -Oz (APPLE ONLY) on Darwin, any
function up to 30 ``estimated insns'' in size will be considered
for inlining. When compiling C and Objective-C sourcefiles with
-Os or -Oz on Darwin, functions explictly marked with the "inline"
keyword up to 450 ``estimated insns'' in size will be considered
for inlining. When compiling for Apple POWERPC targets, -Os and
-Oz (APPLE ONLY) disable use of the string instructions even though
they would usually be smaller, because the kernel can't emulate
them correctly in some rare cases. This behavior is not portable
to any other gcc environment, and will not affect most programs at
all. If you really want the string instructions, use -mstring.
-Oz (APPLE ONLY) Optimize for size, regardless of performance. -Oz
enables the same optimization flags that -Os uses, but -Oz also
enables other optimizations intended solely to reduce code size.
In particular, instructions that encode into fewer bytes are
preferred over longer instructions that execute in fewer cycles.
-Oz on Darwin is very similar to -Os in FSF distributions of GCC.
-Oz employs the same inlining limits and avoids string instructions
just like -Os.
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
would be -fno-foo. In the table below, only one of the forms is
listed---the one you typically will 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-default-inline
Do not make member functions inline by default merely because they
are defined inside the class scope (C++ only). Otherwise, when you
specify -O, member functions defined inside class scope are
compiled inline by default; i.e., you don't need to add inline in
front of the member function name.
-fno-defer-pop
Always pop the arguments to each function call as soon as that
function returns. For machines which must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at once.
Disabled at levels -O, -O2, -O3, -Os, -Oz (APPLE ONLY).
-fforce-mem
Force memory operands to be copied into registers before doing
arithmetic on them. This produces better code by making all memory
references potential common subexpressions. When they are not
common subexpressions, instruction combination should eliminate the
separate register-load.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-fforce-addr
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
-fforce-mem may.
-fomit-frame-pointer
Don't keep the frame pointer in a register for functions that don't
need one. This avoids the instructions to save, set up and restore
frame pointers; it also makes an extra register available in many
functions. It also makes debugging impossible on some machines.
On some machines, such as the VAX, this flag has no effect, because
the standard calling sequence automatically handles the frame
pointer and nothing is saved by pretending it doesn't exist. The
machine-description macro "FRAME_POINTER_REQUIRED" controls whether
a target machine supports this flag.
Enabled at levels -O, -O2, -O3, -Os, -Oz (APPLE ONLY).
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-fno-inline
Don't pay attention to the "inline" keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded
inline.
-finline-functions
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be 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 level -O3.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined.
This flag allows the control of this limit for functions that are
explicitly marked as inline (i.e., marked with the inline keyword
or defined within the class definition in c++). n is the size of
functions that can be inlined in number of pseudo instructions (not
counting parameter handling). The default value of n is 600.
Increasing this value can result in more inlined code at the cost
of compilation time and memory consumption. Decreasing usually
makes the compilation faster and less code will be inlined (which
presumably means slower programs). This option is particularly
useful for programs that use inlining heavily such as those based
on recursive templates with C++.
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:
@item max-inline-insns-single
is set to I<n>/2.
@item max-inline-insns-auto
is set to I<n>/2.
@item min-inline-insns
is set to 130 or I<n>/4, whichever is smaller.
@item max-inline-insns-rtl
is set to I<n>.
See below for a documentation of the individual parameters
controlling inlining.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way, it represents
a count of assembly instructions and as such its exact meaning
might change from one release to an another.
-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 C. In C++, emit any and all inline
functions into the object file.
-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 the variable was 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 -O, -O2, -O3, -Os, -Oz (APPLE ONLY).
-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 non-automatic
variable to have distinct location, so using this option will
result 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.
-fno-branch-count-reg
Do not use ``decrement and branch'' instructions on a count
register, but instead generate a sequence of instructions that
decrement a register, compare it against zero, 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.
The default is -fbranch-count-reg, enabled when -fstrength-reduce
is enabled.
-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.
-fbounds-check
For front-ends that support it, generate additional code to check
that indices used to access arrays are within the declared range.
This is currently only supported by the Java and Fortran front-
ends, where this option defaults to true and false respectively.
-fmudflap -fmudflapth -fmudflapir
For front-ends that support it (C and C++), instrument all risky
pointer/array dereferencing operations, some standard library
string/heap functions, and some other associated constructs with
range/validity tests. Modules so instrumented should be immune to
buffer overflows, invalid heap use, and some other classes of C/C++
programming errors. The instrumentation relies on a separate
runtime library (libmudflap), which will be linked into a program
if -fmudflap is given at link time. Run-time behavior of the
instrumented program is controlled by the MUDFLAP_OPTIONS
environment variable. See "env MUDFLAP_OPTIONS=-help a.out" for
its options.
Use -fmudflapth instead of -fmudflap to compile and to link if your
program is multi-threaded. Use -fmudflapir, in addition to
-fmudflap or -fmudflapth, if instrumentation should ignore pointer
reads. This produces less instrumentation (and therefore faster
execution) and still provides some protection against outright
memory corrupting writes, but allows erroneously read data to
propagate within a program.
-fstrength-reduce
Perform the optimizations of loop strength reduction and
elimination of iteration variables.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-fthread-jumps
Perform optimizations where we 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 -O2, -O3, -Os, -Oz (APPLE ONLY).
-fcse-follow-jumps
In common subexpression elimination, 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 will follow the jump when the condition tested is
false.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow
jumps which 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, -Oz (APPLE ONLY).
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations
has been performed.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-frerun-loop-opt
Run the loop optimizer twice.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-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 runtime performance if you disable
the global common subexpression elimination pass by adding
-fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination
will attempt to move loads which 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 gcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global
common subexpression elimination. This pass will attempt 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
cleanup redundant spilling.
-floop-optimize
Perform loop optimizations: move constant expressions out of loops,
simplify exit test conditions and optionally do strength-reduction
as well.
Enabled at levels -O, -O2, -O3, -Os, -Oz (APPLE ONLY).
-floop-optimize2
Perform loop optimizations using the new loop optimizer. The
optimizations (loop unrolling, peeling and unswitching, loop
invariant motion) are enabled by separate flags.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies
equivalent code and save code size. The resulting code may or may
not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-fif-conversion
Attempt to transform conditional jumps into branch-less
equivalents. This include 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 "if-conversion2".
Enabled at levels -O, -O2, -O3, -Os, -Oz (APPLE ONLY).
-fif-conversion2
Use conditional execution (where available) to transform
conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os, -Oz (APPLE ONLY).
-fdelete-null-pointer-checks
Use global dataflow analysis to identify and eliminate useless
checks for null pointers. The compiler assumes that dereferencing
a null pointer would have halted the program. If a pointer is
checked after it has already been dereferenced, it cannot be null.
In some environments, this assumption is not true, and programs can
safely dereference null pointers. Use
-fno-delete-null-pointer-checks to disable this optimization for
programs which depend on that behavior.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively
expensive.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-foptimize-register-move
-fregmove
Attempt to reassign register numbers in move instructions and as
operands of other simple instructions in order to maximize the
amount of register tying. This is especially helpful on machines
with two-operand instructions.
Note -fregmove and -foptimize-register-move are the same
optimization.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-fdelayed-branch
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed
branch instructions.
Enabled at levels -O, -O2, -O3, -Os, -Oz (APPLE ONLY).
-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, -Os, -Oz (APPLE ONLY).
-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, -Oz (APPLE ONLY).
-fno-sched-interblock
Don't schedule instructions across basic blocks. This is normally
enabled by default when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Don't allow speculative motion of non-load instructions. This is
normally enabled by default when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-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=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.
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) will be examined for a
dependency on a stalled insn that is candidate for premature
removal from the queue of stalled insns. Has an effect only during
the second scheduling pass, and only if -fsched-stalled-insns is
used and its value is not zero.
-fsched2-use-superblocks
When scheduling after register allocation, do use superblock
scheduling algorithm. Superblock scheduling allows motion across
basic block boundaries resulting on 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.
-fsched2-use-traces
Use -fsched2-use-superblocks algorithm when scheduling after
register allocation and additionally perform code duplication in
order to increase the size of superblocks using tracer pass. See
-ftracer for details on trace formation.
This mode should produce faster but significantly longer programs.
Also without -fbranch-probabilities the traces constructed may not
match the reality and hurt the performance. This only makes sense
when scheduling after register allocation, i.e. with
-fschedule-insns2 or at -O2 or higher.
-freschedule-modulo-scheduled-loops
The modulo scheduling comes before the traditional scheduling, if a
loop was modulo scheduled we may want to prevent the later
scheduling passes from changing its schedule, we use this option to
control that.
-fcaller-saves
Enable values to be allocated in registers that will be 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 than would otherwise be
produced.
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, -Oz (APPLE ONLY).
-ftree-pre
Perform Partial Redundancy Elimination (PRE) on trees. This flag
is enabled by default at -O2 and -O3.
-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 faster than PRE, though it exposes fewer redundancies.
This flag is enabled by default at -O and higher.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees.
This flag is enabled by default at -O and higher.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled
by default at -O and higher.
-ftree-dominator-opts
Perform dead code elimination (DCE) on trees. This flag is enabled
by default at -O 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 -O and higher.
It is not enabled for -Os or -Oz (APPLE ONLY), since it usually
increases code size.
-ftree-elim-checks
Perform elimination of checks based on scalar evolution
informations. This flag is disabled by default.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by
default at -O and higher.
-ftree-loop-linear
Perform linear loop transformations on tree. This flag can improve
cache performance and allow further loop optimizations to take
place. This flag is known to have bugs that cause incorrect code
to be generated in some rare cases. Note this flag is included in
-fast.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only
invariants that would be 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 the loop for
that determining number of iterations requires complicated
analysis. Later optimizations then may determine the number
easily. Useful especially in connection with unrolling.
-fivopts
Perform induction variable optimizations (strength reduction,
induction variable merging and induction variable elimination) on
trees.
-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 -O and
higher.
-ftree-copyrename
Perform copy renaming on trees. This pass attempts to rename
compiler temporaries to other variables at copy locations, usually
resulting in variable names which more closely resemble the
original variables. This flag is enabled by default at -O and
higher.
-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 -O and higher.
-ftree-lrs
Perform live range splitting during the SSA->normal phase.
Distinct live ranges of a variable are split into unique variables,
allowing for better optimization later. This is enabled by default
at -O and higher.
-ftree-vectorize
Perform loop vectorization on trees.
In Apple's version of GCC, -fstrict-aliasing is enabled by default
when loop vectorization is enabled. See -fstrict-aliasing document
for more information.
-ftracer
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function allowing
other optimizations to do better job.
-funroll-loops
Unroll loops whose number of iterations can be determined at
compile time or upon entry to the loop. -funroll-loops implies
both -fstrength-reduce and -frerun-cse-after-loop. This option
makes code larger, and may or may not make it run faster.
-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,
-fsplit-ivs-in-unroller
Enables expressing 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.
Combination of -fweb and CSE is often sufficient to obtain the same
effect. However in cases the loop body is more complicated than a
single basic block, this is not reliable. It also does not work at
all on some of the architectures due to restrictions in the CSE
pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler will create multiple copies of some
local variables when unrolling a loop which can result in superior
code.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that access
large arrays.
These options may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
-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, -Oz (APPLE ONLY).
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC will use 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 will be 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.
The default is -fguess-branch-probability at levels -O, -O2, -O3,
-Os, -Oz (APPLE ONLY).
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce
number of taken branches and improve code locality.
Enabled 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, for linkonce sections, for functions with a
user-defined section attribute and on any architecture that does
not support named sections.
-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.
Also profile feedback must be available in to make this option
effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-fstrict-aliasing
Allows 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() {
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 will
work as expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-specific alias
analysis should define a function that computes, given an "tree"
node, an alias set for the node. Nodes in different alias sets are
not allowed to alias. For an example, see the C front-end function
"c_get_alias_set".
Enabled at levels -O2, -O3, -Os, -Oz (APPLE ONLY).
-falign-functions
-falign-functions=n
Align the start of functions to the next power-of-two greater than
n, skipping up to n bytes. For instance, -falign-functions=32
aligns functions to the next 32-byte boundary, but
-falign-functions=24 would align to the next 32-byte boundary only
if this can be done by skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are equivalent and
mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two; in
that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels
-falign-labels=n
Align all branch targets to a power-of-two boundary, skipping up to
n bytes like -falign-functions. This option can easily make code
slower, because it must insert dummy operations for when the branch
target is reached in the usual flow of the code.
-fno-align-labels and -falign-labels=1 are equivalent and mean that
labels will not be 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.
Enabled at levels -O2, -O3.
-falign-loops-max-skip
-falign-loops-max-skip=n
Align loops to a power-of-two boundary, but do not skip more than n
bytes to do so.
-falign-loops
-falign-loops=n
Align loops to a power-of-two boundary, skipping up to n bytes like
-falign-functions. The hope is that the loop will be executed many
times, which will make up for any execution of the dummy
operations.
-fno-align-loops and -falign-loops=1 are equivalent and mean that
loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping, skipping up to n
bytes like -falign-functions. In this case, no dummy operations
need be executed.
-falign-jumps-max-skip
-falign-jumps-max-skip=n
Align branch targets to a power-of-two boundary, but do not skip
more than n bytes to do so.
-fno-align-jumps and -falign-jumps=1 are equivalent and mean that
loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-funit-at-a-time
Parse the whole compilation unit before starting to produce code.
This allows some extra optimizations to take place but consumes
more memory (in general). There are some compatibility issues with
unit-at-at-time mode:
o enabling unit-at-a-time mode may change the order in which
functions, variables, and top-level "asm" statements are
emitted, and will likely break code relying on some particular
ordering. The majority of such top-level "asm" statements,
though, can be replaced by "section" attributes.
o unit-at-a-time mode removes unreferenced static variables and
functions are removed. This may result in undefined references
when an "asm" statement refers directly to variables or
functions that are otherwise unused. In that case either the
variable/function shall be listed as an operand of the "asm"
statement operand or, in the case of top-level "asm" statements
the attribute "used" shall be used on the declaration.
o Static functions now can use non-standard passing conventions
that may break "asm" statements calling functions directly.
Again, attribute "used" will prevent this behavior.
As a temporary workaround, -fno-unit-at-a-time can be used, but
this scheme may not be supported by future releases of GCC.
Enabled at levels -O2, -O3.
-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 will no longer stay in a
``home register''.
Enabled by default with -funroll-loops.
-fno-cprop-registers
After register allocation and post-register allocation instruction
splitting, we perform a copy-propagation pass to try to reduce
scheduling dependencies and occasionally eliminate the copy.
Disabled at levels -O, -O2, -O3, -Os, -Oz (APPLE ONLY).
-fprofile-generate
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", "-fvpt".
-fprofile-use
Enable profile feedback directed optimizations, and optimizations
generally profitable only with profile feedback available.
The following options are enabled: "-fbranch-probabilities",
"-fvpt", "-funroll-loops", "-fpeel-loops", "-ftracer".
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.
-ffast-math
Sets -fno-math-errno, -funsafe-math-optimizations,
-fno-trapping-math, -ffinite-math-only, -fno-rounding-math,
-fno-signaling-nans and fcx-limited-range.
This option causes the preprocessor macro "__FAST_MATH__" to be
defined.
This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions.
-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.
(APPLE ONLY) The Darwin math libraries never set errno, so there is
no point in having the compiler generate code that assumes they
might. Therefore, the default is -fno-math-errno on Darwin.
-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 should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that
arguments and results are not NaNs or +-Infs.
This option should never be turned on by any -O option since it can
result in incorrect output for programs which depend on an exact
implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-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 implies -fno-signaling-nans. 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 which 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.
-fsingle-precision-constant
Treat floating point constant as single precision constant instead
of implicitly converting it to double precision constant.
-fcx-limited-range
-fno-cx-limited-range
When enabled, this option states that a range reduction step is not
needed when performing complex division. 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.
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 the 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.
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.c, instead of guessing which path a branch is mostly to take,
the REG_BR_PROB values are used to exactly determine which path is
taken more often.
-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 and adds REG_VALUE_PROFILE notes to
instructions for their later usage in optimizations.
Enabled with -fprofile-generate and -fprofile-use.
-fvpt
If combined with -fprofile-arcs, it instructs the compiler to add a
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 operation using
the knowledge about the value of the denominator.
-fspeculative-prefetching
If combined with -fprofile-arcs, it instructs the compiler to add a
code to gather information about addresses of memory references in
the program.
With -fbranch-probabilities, it reads back the data gathered and
issues prefetch instructions according to them. In addition to the
opportunities noticed by -fprefetch-loop-arrays, it also notices
more complicated memory access patterns---for example accesses to
the data stored in linked list whose elements are usually allocated
sequentially.
In order to prevent issuing double prefetches, usage of
-fspeculative-prefetching implies -fno-prefetch-loop-arrays.
Enabled with -fprofile-generate and -fprofile-use.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This
optimization will most benefit processors with lots of registers.
Depending on the debug information format adopted by the target,
however, it can make debugging impossible, since variables will no
longer stay in a ``home register''.
Not enabled by default at any level because it has known bugs.
-ftracer
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function allowing
other optimizations to do better job.
Enabled with -fprofile-use.
-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 and -fweb. It also turns on complete loop
peeling (i.e. complete removal of loops with small constant number
of iterations). This option makes code larger, and may or may not
make it run faster.
Enabled with -fprofile-use.
-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 the loops for that there is enough information that they do
not roll much (from profile feedback). It also turns on complete
loop peeling (i.e. complete removal of loops with small constant
number of iterations).
Enabled with -fprofile-use.
-fmove-loop-invariants
Enables the loop invariant motion pass in the new loop optimizer.
Enabled at level -O1
-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).
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that access
large arrays.
Disabled at levels -Os and -Oz (APPLE ONLY).
-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 and SPARC
processors running Solaris 2 have linkers with such optimizations.
AIX may have these optimizations in the future.
Only use these options when there are significant benefits from
doing so. When you specify these options, the assembler and linker
will create larger object and executable files and will also be
slower. You will not be able to use "gprof" on all systems if you
specify this option and you may have problems with debugging if you
specify both this option and -g.
-fbranch-target-load-optimize
Perform branch target register load optimization before prologue /
epilogue threading. The use of target registers can typically be
exposed only during reload, thus hoisting loads out of loops and
doing inter-block scheduling needs a separate optimization pass.
-fbranch-target-load-optimize2
Perform branch target register load optimization after prologue /
epilogue threading.
-fbtr-bb-exclusive
When performing branch target register load optimization, don't
reuse branch target registers in within any basic block.
-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 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.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
--param name=value
In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC will not inline
functions that contain more that 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 each case, the value is an integer. The allowable choices for
name are given in the following table:
sra-max-structure-size
The maximum structure size, in bytes, at which the scalar
replacement of aggregates (SRA) optimization will perform block
copies. The default value, 0, implies that GCC will select the
most appropriate size itself.
sra-field-structure-ratio
The threshold ratio (as a percentage) between instantiated
fields and the complete structure size. We say that if the
ratio of the number of bytes in instantiated fields to the
number of bytes in the complete structure exceeds this
parameter, then block copies are not used. The default is 75.
max-crossjump-edges
The maximum number of incoming edges to consider for
crossjumping. 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 compile
time increase with probably small improvement in executable
size.
min-crossjump-insns
The minimum number of instructions which must be matched at the
end of two blocks before crossjumping will be performed on
them. This value is ignored in the case where all instructions
in the block being crossjumped from are matched. The default
value is 5.
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. The default value is 8.
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 is searched, the time savings
from filling the delay slot will be minimal so stop searching.
Increasing values mean more aggressive optimization, making the
compile time increase with probably small improvement in
executable run 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
compile 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 that will be allocated
in order to perform the global common subexpression elimination
optimization. If more memory than specified is required, the
optimization will not be done.
max-gcse-passes
The maximum number of passes of GCSE to run. The default is 1.
max-pending-list-length
The maximum number of pending dependencies scheduling will
allow 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-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 will consider for inlining. This only affects
functions declared inline and methods implemented in a class
declaration (C++). The default value is 450.
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 will be investigated. To those functions, a
different (more restrictive) limit compared to functions
declared inline can be applied. The default value is 90.
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 backend. This parameter is
ignored when -funit-at-a-time is not used. The default value
is 2700.
large-function-growth
Specifies maximal growth of large function caused by inlining
in percents. This parameter is ignored when -funit-at-a-time
is not used. The default value is 100 which limits large
function growth to 2.0 times the original size.
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused
by inlining. This parameter is ignored when -funit-at-a-time
is not used. The default value is 50 which limits unit growth
to 1.5 times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies maximum number of instructions out-of-line copy of
self recursive inline function can grow into by performing
recursive inlining.
For functions declared inline --param max-inline-insns-
recursive is taken into acount. For function not declared
inline, recursive inlining happens only when -finline-functions
(included in -O3) is enabled and --param max-inline-insns-
recursive-auto is used. The default value is 450.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies maximum recursion depth used by the recursive
inlining.
For functions declared inline --param max-inline-recursive-
depth is taken into acount. For function not declared inline,
recursive inlining happens only when -finline-functions
(included in -O3) is enabled and --param max-inline-recursive-
depth-auto is used. The default value is 450.
inline-call-cost
Specify cost of call instruction relative to simple arithmetics
operations (having cost of 1). Increasing this cost disqualify
inlinining of non-leaf functions and at same time increase size
of leaf function that is believed to reduce function size by
being inlined. In effect it increase amount of inlining for
code having large abstraction penalty (many functions that just
pass the argumetns to other functions) and decrease inlining
for code with low abstraction penalty. Default value is 16.
max-unrolled-insns
The maximum number of instructions that a loop should have if
that loop is unrolled, and if the loop is unrolled, it
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 should have if that loop is
unrolled, and if the loop is unrolled, it 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 should have if
that loop is peeled, and if the loop is peeled, it determines
how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
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-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.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables below
that all candidates are considered for each use in induction
variable optimizations. Only the most relevant candidates are
considered if there are more candidates, 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 number of candidates in the set is smaller than this value,
we always try to remove unnecessary ivs from the set during its
optimization when a new iv is added to the set.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions
analyzer. Large expressions slow the analyzer.
max-iterations-to-track
The maximum number of iterations of a loop the brute force
algorithm for analysis of # of iterations of the loop tries to
evaluate.
hot-bb-count-fraction
Select fraction of the maximal count of repetitions of basic
block in program given basic block needs to have to be
considered hot.
hot-bb-frequency-fraction
Select fraction of the maximal frequency of executions of basic
block in function given basic block needs to have to be
considered hot
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 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 rather hokey argument, as most of the
duplicates will be 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-ratio
tracer-min-branch-ratio-feedback
Stop forward growth if the best edge do have probability lower
than this threshold.
Similarly to tracer-dynamic-coverage two values are present,
one for compilation for profile feedback and one for
compilation without. The value for compilation with profile
feedback needs to be more conservative (higher) in order to
make tracer effective.
max-cse-path-length
Maximum number of basic blocks on path that cse considers. The
default is 10.
global-var-threshold
Counts the number of function calls (n) and the number of call-
clobbered variables (v). If nxv is larger than this limit, a
single artificial variable will be created to represent all the
call-clobbered variables at function call sites. This
artificial variable will then be made to alias every call-
clobbered variable. (done as "int * size_t" on the host
machine; beware overflow).
max-aliased-vops
Maximum number of virtual operands allowed to represent aliases
before triggering the alias grouping heuristic. Alias grouping
reduces compile times and memory consumption needed for
aliasing at the expense of precision loss in alias information.
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
which 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 compile time increase with
probably slightly better performance. The default value is
100.
max-cselib-memory-location
The maximum number of memory locations cselib should take into
acount. Increasing values mean more aggressive optimization,
making the compile time increase with probably slightly better
performance. The default value is 500.
reorder-blocks-duplicate
reorder-blocks-duplicate-feedback
Used by basic block reordering pass to decide whether to use
unconditional branch or duplicate the code on its destination.
Code is duplicated when its estimated size is smaller than this
value multiplied by the estimated size of unconditional jump in
the hot spots of the program.
The reorder-block-duplicate-feedback is used only when profile
feedback is available and may be set to higher values than
reorder-block-duplicate since information about the hot spots
is more accurate.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for
interblock scheduling. The default value is 10.
max-sched-region-insns
The maximum number of insns in a region to be considered for
interblock scheduling. The default value is 100.
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. The default is 10000.
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's.
The default value is 256.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that will receive
stack smashing protection when -fstack-protection is used.
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.
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 which GCC does not know
how to 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.
-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 will be 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 will need to 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.
-undef
Do not predefine any system-specific or GCC-specific macros. The
standard predefined macros remain defined.
-I dir
Add the directory dir to the list of directories to be searched for
header files. Directories named by -I are searched before the
standard system include directories. If the directory dir is a
standard system include directory, the option is ignored to ensure
that the default search order for system directories and the
special treatment of system headers are not defeated .
-o file
Write output to file. This is the same as specifying file as the
second non-option argument to cpp. gcc has a different
interpretation of a second non-option argument, so you must use -o
to specify the output file.
-Wall
Turns on all optional warnings which are desirable for normal code.
At present this is -Wcomment, -Wtrigraphs, -Wmultichar and a
warning about integer promotion causing a change of sign in "#if"
expressions. Note that many of the preprocessor's warnings are on
by default and have no options to control them.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment,
or whenever a backslash-newline appears in a // comment. (Both
forms have the same effect.)
-Wtrigraphs
@anchor{Wtrigraphs} Most trigraphs in comments cannot affect the
meaning of the program. However, a trigraph that would form an
escaped newline (??/ at the end of a line) can, by changing where
the comment begins or ends. Therefore, only trigraphs that would
form escaped newlines produce warnings inside a comment.
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.
-Wtraditional
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 problematic constructs which
should be avoided.
-Wimport
Warn the first time #import is used.
-Wundef
Warn whenever an identifier which is not a macro is encountered in
an #if directive, outside of defined. Such identifiers are
replaced with zero.
-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 will also warn 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 CPP will report 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
-Wendif-labels
Warn whenever an #else or an #endif are followed by text. This
usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments, but often are not
in older programs. This warning is on by default.
-Werror
Make all warnings into hard errors. Source code which triggers
warnings will be rejected.
-Wsystem-headers
Issue warnings for code in system headers. These are normally
unhelpful in finding bugs in your own code, therefore suppressed.
If you are responsible for the system library, you may want to see
them.
-w Suppress all warnings, including those which GNU CPP issues by
default.
-pedantic
Issue all the mandatory diagnostics listed in the C standard. Some
of them are left out by default, since they trigger frequently on
harmless code.
-pedantic-errors
Issue all the mandatory diagnostics, and make all mandatory
diagnostics into errors. This includes mandatory diagnostics that
GCC issues without -pedantic but treats as warnings.
-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 basename of the source file with any suffix
replaced with object file suffix. 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 will still be 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 will appear in -MM dependency output. This is a slight
change in semantics from GCC versions 3.0 and earlier.
@anchor{dashMF}
-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 have sent preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the
default dependency output file.
-dependency-file
Like -MF. (APPLE ONLY)
-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.
-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, including any
path, deletes any file suffix such as .c, and appends the
platform's usual object suffix. The result is the target.
An -MT option will set 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 take the basename of the input file 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.
-fpch-deps
When using precompiled headers, this flag will cause the
dependency-output flags to also list the files from the precompiled
header's dependencies. If not specified only the precompiled
header would be 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.
"<filename>"" in the output to mark the place where the precompiled
header was found, and its filename. When -fpreprocessed is in use,
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.
safe 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.
-x c
-x c++
-x objective-c
-x objective-c++
-x assembler-with-cpp
Specify the source language: C, C++, Objective-C, Objective-C++, or
assembly. This has nothing to do with standards conformance or
extensions; it merely selects which base syntax to expect. If you
give none of these options, cpp will deduce the language from the
extension of the source file: .c, .cc, .m, .mm, or .S. Some other
common extensions for C++ and assembly are also recognized. If cpp
does not recognize the extension, it will treat the file as C; this
is the most generic mode.
Note: Previous versions of cpp accepted a -lang option which
selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the -l
option.
-std=standard
-ansi
Specify the standard to which the code should conform. Currently
CPP knows about C and C++ standards; others may be added in the
future.
standard may be one of:
"iso9899:1990"
"c89"
The ISO C standard from 1990. c89 is the customary shorthand
for this version of the standard.
The -ansi option is equivalent to -std=c89.
"iso9899:199409"
The 1990 C standard, as amended in 1994.
"iso9899:1999"
"c99"
"iso9899:199x"
"c9x"
The revised ISO C standard, published in December 1999. Before
publication, this was known as C9X.
"gnu89"
The 1990 C standard plus GNU extensions. This is the default.
"gnu99"
"gnu9x"
The 1999 C standard plus GNU extensions.
"c++98"
The 1998 ISO C++ standard plus amendments.
"gnu++98"
The same as -std=c++98 plus GNU extensions. This is the
default for C++ code.
-I- Split the include path. 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"".
This option has been deprecated.
-nostdinc
Do not search the standard system directories for header files.
Only the directories you have specified with -I 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.)
-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.
-idirafter dir
Search dir for header files, but do it after all directories
specified with -I and the standard system directories have been
exhausted. dir is treated as a system include directory.
-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.
-isystem dir
Search dir for header files, after all directories specified by -I
but before the standard system directories. Mark it as a system
directory, so that it gets the same special treatment as is applied
to the standard system directories.
-iquote dir
Search dir only for header files requested with "#include "file"";
they are not searched for "#include <file>", before all directories
specified by -I and before the standard system directories.
-fdollars-in-identifiers
@anchor{fdollars-in-identifiers} Accept $ in identifiers.
-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.
-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.
-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 UTF-32 or UTF-16, whichever
corresponds to the width of "wchar_t". 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.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that
will let the compiler know the current working directory at the
time of preprocessing. When this option is enabled, the
preprocessor will emit, after the initial linemarker, a second
linemarker with the current working directory followed by two
slashes. GCC will use 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.
-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.
-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.
-dCHARS
CHARS is a sequence of one or more of the following characters, and
must not be preceded by a space. Other characters are interpreted
by the compiler proper, or reserved for future versions of GCC, and
so are silently ignored. If you specify characters whose behavior
conflicts, the result is undefined.
M 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
will show all the predefined macros.
D Like M 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.
N Like D, but emit only the macro names, not their expansions.
I Output #include directives in addition to the result of
preprocessing.
-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.
-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.
-traditional-cpp
Try to imitate the behavior of old-fashioned C preprocessors, as
opposed to ISO C preprocessors.
-trigraphs
Process trigraph sequences. 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. By default, GCC ignores
trigraphs, but in standard-conforming modes it converts them. See
the -std and -ansi options.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
-remap
Enable special code to work around file systems which only permit
very short file names, such as MS-DOS.
--help
--target-help
Print text describing all the command line options instead of
preprocessing anything.
-v Verbose mode. Print out GNU CPP's version number at the beginning
of execution, and report the final form of the include path.
-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 ...! .
-version
--version
Print out GNU CPP's version number. With one dash, proceed to
preprocess as normal. With two dashes, exit immediately.
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 which GCC does not know
how to 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.
In addition to the options listed below, Apple's GCC also accepts and
passes nearly all of the options defined by the linker ld and by the
library tool libtool. Common options include -framework, -dynamic,
-bundle, -flat_namespace, and so forth. See the ld and libtool man
pages for further details.
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.
-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.)
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.
The linker searches a standard list of directories for the library,
which is actually a file named liblibrary.a. The linker then uses
this file as if it had been specified precisely by name.
The directories searched include several standard system
directories plus any that you specify with -L.
Normally the files found this way are library files---archive files
whose members are object files. The linker handles an archive file
by scanning through it for members which define symbols that have
so far been referenced but not defined. But if the file that is
found is an ordinary object file, it is linked in the usual
fashion. The only difference between using an -l option and
specifying a file name is that -l surrounds library with lib and .a
and searches several directories.
-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 or
-nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the
libraries you specify will be passed to the linker. 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.
-nostdlib
Do not use the standard system startup files or libraries when
linking. No startup files and only the libraries you specify will
be passed to the linker. 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 that
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. (For example, __main, used to ensure C++
constructors will be called.)
-pie
Produce a position independent executable on targets which support
it. For predictable results, you must also specify the same set of
options that were used to generate code (-fpie, -fPIE, or model
suboptions) when you specify this option.
-s Remove all symbol table and relocation information from the
executable.
-static
On systems that support dynamic linking, this prevents linking with
the shared libraries. On other systems, this option has no effect.
This option will not work on Mac OS X unless all libraries
(including libgcc.a) have also been compiled with -static. Since
neither a static version of libSystem.dylib nor crt0.o are
provided, this option is not useful to most people.
-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
that were used to generate code (-fpic, -fPIC, or model suboptions)
when you specify this option.[1]
This option is not supported on Mac OS X.
-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++ and GCJ drivers automatically add -shared-libgcc
whenever you build a shared library or a main executable, because
C++ and Java 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 will not always be 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 will link the shared version of libgcc into
shared libraries by default. Otherwise, it will take advantage of
the linker and optimize 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++ or GCJ driver, as
appropriate for the languages used in the program, or using the
option -shared-libgcc, such that it is linked with the shared
libgcc.
-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.
-Xlinker option
Pass option as an option to the linker. You can use this to supply
system-specific linker options which GCC does not know how to
recognize.
If you want to pass an option that takes an 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.
-Wl,option
Pass option as an option to the linker. If option contains commas,
it is split into multiple options at the commas.
-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.
Options for Directory Search
These options specify directories to search for header files, for
libraries and for parts of the compiler:
-Idir
Add the directory dir to the head of the list of directories to be
searched for header files. This can be used to override a system
header file, substituting your own version, since these directories
are searched before the 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). If
you use more than one -I option, the directories are scanned in
left-to-right order; the standard system directories come after.
If a standard system include directory, or a directory specified
with -isystem, is also specified with -I, the -I option will be
ignored. The directory will still be 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.
The option -iwithsysroot (APPLE ONLY), if specified with an
absolute path, will prepend the system root directory (if
applicable) to the path and add it to the beginning of the system
search paths. If specified with a relative path, -iwithsysroot
will behave identically to -isystem.
-iquotedir
Add the directory dir to the head of the list of directories to be
searched for header files only for the case of #include "file";
they are not searched for #include <file>, otherwise just like -I.
-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 each subprogram to be run, the compiler driver first tries the
-B prefix, if any. If that name is not found, or if -B was not
specified, the driver tries two standard prefixes, which are
/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 will check to see if the path provided by the -B
refers to a directory, and if necessary it will add 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
includes 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 run-time 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 will be replaced
by [dir/]include. This is to help with boot-strapping the
compiler.
-specs=file
Process file after the compiler reads in the standard specs file,
in order to override the defaults that 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.
-I- This option has been deprecated. Please use -iquote instead for -I
directories before the -I- and remove the -I-. Any directories you
specify with -I options before the -I- option are searched only for
the case of #include "file"; they are not searched for #include
<file>.
If additional directories are specified with -I options after the
-I-, these directories are searched for all #include directives.
(Ordinarily all -I directories are used this way.)
In addition, the -I- option inhibits the use of the current
directory (where the current input file came from) as the first
search directory for #include "file". There is no way to override
this effect of -I-. With -I. you can specify searching the
directory which was current when the compiler was invoked. That is
not exactly the same as what the preprocessor does by default, but
it is often satisfactory.
-I- does not inhibit the use of the standard system directories for
header files. Thus, -I- and -nostdinc are independent.
Specifying Target Machine and Compiler Version
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
version other than the one that was installed last. Sometimes this is
inconvenient, so GCC provides options that will switch to another
cross-compiler or version.
-b machine
The argument machine specifies the target machine for compilation.
The value to use for machine is the same as was specified as the
machine type when configuring GCC as a cross-compiler. For
example, if a cross-compiler was configured with configure i386v,
meaning to compile for an 80386 running System V, then you would
specify -b i386v to run that cross compiler.
-V version
The argument version specifies which version of GCC to run. This
is useful when multiple versions are installed. For example,
version might be 2.0, meaning to run GCC version 2.0.
The -V and -b options work by running the <machine>-gcc-<version>
executable, so there's no real reason to use them if you can just run
that directly.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses among
different installed compilers for completely different target machines,
such as VAX vs. 68000 vs. 80386.
In addition, each of these target machine types can have its own
special options, starting with -m, to choose among various hardware
models or configurations---for example, 68010 vs 68020, floating
coprocessor or none. A single installed version of the compiler can
compile for any model or configuration, according to the options
specified.
Some configurations of the compiler also support additional special
options, usually for compatibility with other compilers on the same
platform.
These options are defined by the macro "TARGET_SWITCHES" in the machine
description. The default for the options is also defined by that
macro, which enables you to change the defaults.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM)
architectures:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-
gnu, atpcs, aapcs 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 will cause the stack frames
not to be generated for leaf functions. The default is
-mno-apcs-frame.
-mapcs
This is a synonym for -mapcs-frame.
-mthumb-interwork
Generate code which supports calling between the ARM and Thumb
instruction sets. Without this option 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.
-mno-sched-prolog
Prevent the reordering of instructions in the function prolog, or
the merging of those instruction with the instructions in the
function's body. This means that all functions will 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 if functions inside an executable
piece of code. The default is -msched-prolog.
-mhard-float
Generate output containing floating point instructions. This is
the default.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all ARM
targets. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation.
You must make your own arrangements to provide suitable library
functions for cross-compilation.
-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for this to
work.
-mfloat-abi=name
Specifies which ABI to use for floating point values. Permissible
values are: soft, softfp and hard.
soft and hard are equivalent to -msoft-float and -mhard-float
respectively. softfp allows the generation of floating point
instructions, but still uses the soft-float calling conventions.
-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.
-mwords-little-endian
This option only applies when generating code for big-endian
processors. Generate code for a little-endian word order but a
big-endian byte order. That is, a byte order of the form 32107654.
Note: this option should only be used if you require compatibility
with code for big-endian ARM processors generated by versions of
the compiler prior to 2.8.
-mcpu=name
This specifies the name of the target ARM processor. GCC uses this
name to determine what kind of instructions it can emit when
generating assembly code. Permissible names are: arm2, arm250,
arm3, arm6, arm60, arm600, arm610, arm620, arm7, arm7m, arm7d,
arm7dm, arm7di, arm7dmi, arm70, arm700, arm700i, arm710, arm710c,
arm7100, arm7500, arm7500fe, arm7tdmi, arm7tdmi-s, arm8, strongarm,
strongarm110, strongarm1100, 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,
arm1176jz-s, arm1176jzf-s, xscale, iwmmxt, ep9312.
-mtune=name
This option is very similar to the -mcpu= option, except that
instead of specifying the actual target processor type, and hence
restricting which instructions can be used, it specifies that GCC
should tune the performance of the code as if the target were of
the type specified in this option, but still choosing the
instructions that it will generate based on the cpu specified by a
-mcpu= option. For some ARM implementations better performance can
be obtained by using this option.
-march=name
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:
armv2, armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t,
armv5te, armv6, armv6j, iwmmxt, ep9312.
-mfpu=name
-mfpe=number
-mfp=number
This specifies what floating point hardware (or hardware emulation)
is available on the target. Permissible names are: fpa, fpe2,
fpe3, maverick, vfp. -mfp and -mfpe are synonyms for
-mfpu=fpenumber, for compatibility with older versions of GCC.
If -msoft-float is specified this specifies the format of floating
point values.
-mstructure-size-boundary=n
The size of all structures and unions will be 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 the 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.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn"
function. It will be 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 will lie 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 will be
turned into long calls. The heuristic is that static functions,
functions which have the short-call attribute, functions that are
whose definitions have already been compiled within the current
compilation unit, will not be turned into long calls. The
exception to this rule is that weak function definitions, functions
with the long-call attribute or the section attribute, and
directive, will always be turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls
will restore the default behavior, as will placing the function
these switches have no effect on how the compiler generates code to
handle function calls via function pointers.
-mnop-fun-dllimport
Disable support for the "dllimport" attribute.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather
than loading it in the prologue for each function. The run-time
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. The default is
R10 unless stack-checking is enabled, when R9 is used.
-mcirrus-fix-invalid-insns
Insert NOPs into the instruction stream to in order to work around
problems with invalid Maverick instruction combinations. This
option is only valid if the -mcpu=ep9312 option has been used to
enable generation of instructions for the Cirrus Maverick floating
point co-processor. This option is not enabled by default, since
the problem is only present in older Maverick implementations. The
default can be re-enabled by use of the
-mno-cirrus-fix-invalid-insns switch.
-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
Generate code for the 16-bit Thumb instruction set. The default is
to use the 32-bit ARM instruction set.
-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.
-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.
Darwin Options
These options are defined for all architectures running the Darwin
operating system.
FSF GCC on Darwin does not create ``universal'' object files; it will
create an object file for the single architecture that it was built to
target. Apple's GCC on Darwin does create ``universal'' files if
multiple -arch options are used; it does so by running the compiler or
linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is
determined by the flags that specify the ISA that GCC is targetting,
like -mcpu or -march. The -force_cpusubtype_ALL option can be used to
override this.
The Darwin tools vary in their behavior when presented with an ISA
mismatch. The assembler, as, will only permit instructions to be used
that are valid for the subtype of the file it is generating, so you
cannot put 64-bit instructions in an ppc750 object file. The linker
for shared libraries, /usr/bin/libtool, will fail and print an error if
asked to create a shared library with a less restrictive subtype than
its input files (for instance, trying to put a ppc970 object file in a
ppc7400 library). The linker for executables, ld, will quietly give
the executable the most restrictive subtype of any of its input files.
-Fdir
Add the framework directory dir to the head of the list of
directories to be searched for header files. These directories are
interleaved with those specified by -I options and are scanned in a
left-to-right order.
A framework directory is a directory with frameworks in it. A
framework is a directory with a "Headers" and/or "PrivateHeaders"
directory contained directly in it that ends in ".framework". The
name of a framework is the name of this directory excluding the
".framework". Headers associated with the framework are found in
one of those two directories, with "Headers" being searched first.
A subframework is a framework directory that is in a framework's
"Frameworks" directory. Includes of subframework headers can only
appear in a header of a framework that contains the subframework,
or in a sibling subframework header. Two subframeworks are
siblings if they occur in the same framework. A subframework
should not have the same name as a framework, a warning will be
issued if this is violated. Currently a subframework cannot have
subframeworks, in the future, the mechanism may be extended to
support this. The standard frameworks can be found in
"/System/Library/Frameworks" and "/Library/Frameworks". An example
include looks like "#include <Framework/header.h>", where Framework
denotes the name of the framework and header.h is found in the
"PrivateHeaders" or "Headers" directory.
-iframeworkdir
Like -F except the directory is a treated as a system directory.
The main effect is to not warn about constructs contained within
header files found via dir.
-gused
Emit debugging information for symbols that are used. For STABS
debugging format, this enables -feliminate-unused-debug-symbols.
This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is
version. Typical values of version include 10.1, 10.2, and 10.3.9.
This value can also be set with the MACOSX_DEPLOYMENT_TARGET
environment variable. If both the command-line option is specified
and the environment variable is set, the command-line option will
take precedence.
If the compiler was built to use the system's headers by default,
then the default for this option is the system version on which the
compiler is running, otherwise the default is to make choices which
are compatible with as many systems and code bases as possible.
This value is not set by default for ARM targets.
-miphoneos-version-min=version
The earliest version of iPhone OS that this executable will run on
is version.
This value can also be set with the IPHONEOS_DEPLOYMENT_TARGET
environment variable. If both the command-line option is specified
and the environment variable is set, the command-line option will
take precedence.
On ARM targets, if not specified by the command-line option or
environment variable, this value defaults to 2.0.
-mkernel
Enable kernel development mode. The -mkernel option sets -static,
-fno-common, -fno-cxa-atexit, -fno-exceptions,
-fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti
where applicable. This mode also sets -mno-altivec, -msoft-float,
-fno-builtin and -mlong-branch for PowerPC targets.
-mone-byte-bool
Override the defaults for bool so that sizeof(bool)==1. By default
sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when
compiling for Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code
that is not binary compatible with code generated without that
switch. Using this switch may require recompiling all other
modules in a program, including system libraries. Use this switch
to conform to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turn around development. Needed to
enable gdb to dynamically load ".o" files into already running
programs. -findirect-data and -ffix-and-continue are provided for
backwards compatibility.
-fapple-kext
-findirect-virtual-calls
-fterminated-vtables
Alter vtables, destructors, and other implementation details to
more closely resemble the GCC 2.95 ABI. This is to make kernel
extensions loadable by Darwin kernels, and is required to build any
Darwin kernel extension. In addition, virtual calls are not made
directly, instead, code is generated to always go through the
virtual table, as virtual tables can be patched by the kernel
module loader. Vtables are altered by adding a zero word at the
end of every vtable. -fno-exceptions and -static must also be used
with this flag. -findirect-virtual-calls and -fterminated-vtables
are accepted for backwards compatibility but will be removed in the
future. On Intel x86-based Apple platforms, the kernel and its
extensions run with a four-byte aligned stack
(-mpreferred-stack-boundary=2); function prologues inside kernel
extentions won't keep the usual 16-byte alignment required
everywhere else in OS X. (APPLE ONLY)
-all_load
Loads all members of static archive libraries. See man ld(1) for
more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong
architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker
will bind all undefined references when the file is loaded or
launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more
information.
-bundle_loader executable
This option specifies the executable that will be loading the build
output file being linked. See man ld(1) for more information.
-dynamiclib
When passed this option, GCC will produce a dynamic library instead
of an executable when linking, using the Darwin libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of
one controlled by the -mcpu or -march option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker
man page describes them in detail.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 family of
computers:
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code,
except for the ABI and the set of available instructions. The
choices for cpu-type are:
generic
Produce code optimized for the most common IA32/AMD64/EM64T
processors. If you know the CPU on which your code will run,
then you should use the corresponding -mtune option instead of
-mtune=generic. But, if you do not know exactly what CPU users
of your application will have, then you should use this option.
As new processors are deployed in the marketplace, the behavior
of this option will change. Therefore, if you upgrade to a
newer version of GCC, the code generated option will change to
reflect the processors that were most common when that version
of GCC was released.
There is no -march=generic option because -march indicates the
instruction set the compiler can use, and there is no generic
instruction set applicable to all processors. In contrast,
-mtune indicates the processor (or, in this case, collection of
processors) for which the code is optimized.
i386
Original Intel's i386 CPU.
i486
Intel's i486 CPU. (No scheduling is implemented for this
chip.)
i586, pentium
Intel Pentium CPU with no MMX support.
pentium-mmx
Intel PentiumMMX CPU based on Pentium core with MMX instruction
set support.
pentiumpro
Intel PentiumPro CPU.
i686
Same as "generic", but when used as "march" option, PentiumPro
instruction set will be used, so the code will run on all i686
familly chips.
pentium2
Intel Pentium2 CPU based on PentiumPro core with MMX
instruction set support.
pentium3, pentium3m
Intel Pentium3 CPU based on PentiumPro core with MMX and SSE
instruction set support.
pentium-m
Low power version of Intel Pentium3 CPU with MMX, SSE and SSE2
instruction set support. Used by Centrino notebooks.
pentium4, pentium4m
Intel Pentium4 CPU with MMX, SSE and SSE2 instruction set
support.
prescott
Improved version of Intel Pentium4 CPU with MMX, SSE, SSE2 and
SSE3 instruction set support.
nocona
Improved version of Intel Pentium4 CPU with 64-bit extensions,
MMX, SSE, SSE2 and SSE3 instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2, k6-3
Improved versions of AMD K6 CPU with MMX and 3dNOW! instruction
set support.
athlon, athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and SSE
prefetch instructions support.
athlon-4, athlon-xp, athlon-mp
Improved AMD Athlon CPU with MMX, 3dNOW!, enhanced 3dNOW! and
full SSE instruction set support.
k8, opteron, athlon64, athlon-fx
AMD K8 core based CPUs with x86-64 instruction set support.
(This supersets MMX, SSE, SSE2, 3dNOW!, enhanced 3dNOW! and
64-bit instruction set extensions.)
winchip-c6
IDT Winchip C6 CPU, dealt in same way as i486 with additional
MMX instruction set support.
winchip2
IDT Winchip2 CPU, dealt in same way as i486 with additional MMX
and 3dNOW! instruction set support.
c3 Via C3 CPU with MMX and 3dNOW! instruction set support. (No
scheduling is implemented for this chip.)
c3-2
Via C3-2 CPU with MMX and SSE instruction set support. (No
scheduling is implemented for this chip.)
While picking a specific cpu-type will schedule things
appropriately for that particular chip, the compiler will not
generate any code that does not run on the i386 without the
-march=cpu-type option being used.
-march=cpu-type
Generate instructions for the machine type cpu-type. The choices
for cpu-type are the same as for -mtune. Moreover, specifying
-march=cpu-type implies -mtune=cpu-type.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-m386
-m486
-mpentium
-mpentiumpro
These options are synonyms for -mtune=i386, -mtune=i486,
-mtune=pentium, and -mtune=pentiumpro respectively. These synonyms
are deprecated.
-mfpmath=unit
Generate floating point arithmetics for selected unit unit. The
choices for unit are:
387 Use the standard 387 floating point coprocessor present
majority of chips and emulated otherwise. Code compiled with
this option will run almost everywhere. The temporary results
are computed in 80bit precision instead of precision specified
by the type resulting in slightly different results compared to
most of other chips. See -ffloat-store for more detailed
description.
This is the default choice for i386 compiler.
sse Use scalar floating point instructions present in the SSE
instruction set. This instruction set is supported by Pentium3
and newer chips, in the AMD line by Athlon-4, Athlon-xp and
Athlon-mp chips. The earlier version of SSE instruction set
supports only single precision arithmetics, thus the double and
extended precision arithmetics is still done using 387. Later
version, present only in Pentium4 and the future AMD x86-64
chips supports double precision arithmetics too.
For the i386 compiler, you need to use -march=cpu-type, -msse
or -msse2 switches to enable SSE extensions and make this
option effective. For the x86-64 compiler, these extensions
are enabled by default.
The resulting code should be considerably faster in the
majority of cases and avoid the numerical instability problems
of 387 code, but may break some existing code that expects
temporaries to be 80bit.
This is the default choice for the x86-64 compiler.
sse,387
Attempt to utilize both instruction sets at once. This
effectively double the amount of available registers and on
chips with separate execution units for 387 and SSE the
execution resources too. Use this option with care, as it is
still experimental, because the GCC register allocator does not
model separate functional units well resulting in instable
performance.
-masm=dialect
Output asm instructions using selected dialect. Supported choices
are intel or att (the default one). Darwin does not support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating point
comparisons. These handle correctly the case where the result of a
comparison is unordered.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC. Normally the
facilities of the machine's usual C compiler are used, but this
can't be done directly in cross-compilation. You must make your
own arrangements to provide suitable library functions for cross-
compilation.
On machines where a function returns floating point results in the
80387 register stack, some floating point opcodes may be emitted
even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types
"float" and "double" in an FPU register, even if there is no FPU.
The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in
ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt"
instructions for the 387. Specify this option to avoid generating
those instructions. This option is the default on FreeBSD, OpenBSD
and NetBSD. This option is overridden when -march indicates that
the target cpu will always have an FPU and so the instruction will
not need emulation. As of revision 2.6.1, these instructions are
not generated unless you also use the -funsafe-math-optimizations
switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long"
variables on a two word boundary or a one word boundary. Aligning
"double" variables on a two word boundary will produce code that
runs somewhat faster on a Pentium at the expense of more memory.
Warning: if you use the -malign-double switch, structures
containing the above types will be aligned differently than the
published application binary interface specifications for the 386
and will not be binary compatible with structures in code compiled
without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The i386
application binary interface specifies the size to be 96 bits, so
-m96bit-long-double is the default in 32 bit mode.
Modern architectures (Pentium and newer) would prefer "long double"
to be aligned to an 8 or 16 byte boundary. In arrays or structures
conforming to the ABI, this would not be possible. So specifying a
-m128bit-long-double will align "long double" to a 16 byte boundary
by padding the "long double" with an additional 32 bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice
as its ABI specifies that "long double" is to be aligned on 16 byte
boundary.
Notice that neither of these options enable any extra precision
over the x87 standard of 80 bits for a "long double".
Warning: if you override the default value for your target ABI, the
structures and arrays containing "long double" variables will
change their size as well as function calling convention for
function taking "long double" will be modified. Hence they will
not be binary compatible with arrays or structures in code compiled
without that switch.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized local variables into the
"bss" or "data" segments. -msvr3-shlib places them into "bss".
These options are meaningful only on System V Release 3.
-mrtd
Use a different function-calling convention, in which functions
that take a fixed number of arguments return with the "ret" num
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
You can specify that an individual function is called with this
calling sequence with the function attribute stdcall. You can also
override the -mrtd option by using the function attribute cdecl.
Warning: this calling convention is incompatible with the one
normally used on Unix, so you cannot use it if you need to call
libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including "printf"); otherwise
incorrect code will be generated for calls to those functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By
default, no registers are used to pass arguments, and at most 3
registers can be used. You can control this behavior for a
specific function by using the function attribute regparm.
Warning: if you use this switch, and num is nonzero, then you must
build all modules with the same value, including any libraries.
This includes the system libraries and startup modules.
-mstackrealign
Realign the stack at entry. On the Intel x86, the -mstackrealign
option will generate an alternate prologue/epilogue that realigns
the runtime stack. This supports mixing legacy codes that keep a
4-byte aligned stack with modern codes that keep a 16-byte stack
for SSE compatibility. The alternate prologue and epilogue are
slower and bigger than the regular ones, and they require one
dedicated register for the entire function. This also lowers the
number of registers available if used in conjunction with the
"regparm" attribute. Nested functions encountered while
-mstackrealign is on will generate warnings, and they will not
realign the stack when called.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num
byte boundary. If -mpreferred-stack-boundary is not specified, the
default is 4 (16 bytes or 128 bits), except when optimizing for
code size (-Os or -Oz (APPLE ONLY)), in which case the default is
the minimum correct alignment (4 bytes for x86, and 8 bytes for
x86-64).
On Pentium and PentiumPro, "double" and "long double" values should
be aligned to an 8 byte boundary (see -malign-double) or suffer
significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type "__m128" suffers similar
penalties if it is not 16 byte aligned.
To ensure proper alignment of this values on the stack, the stack
boundary must be as aligned as that required by any value stored on
the stack. Further, every function must be generated such that it
keeps the stack aligned. Thus calling a function compiled with a
higher preferred stack boundary from a function compiled with a
lower preferred stack boundary will most likely misalign the stack.
It is recommended that libraries that use callbacks always use the
default setting.
This extra alignment does consume extra stack space, and generally
increases code size. Code that is sensitive to stack space usage,
such as embedded systems and operating system kernels, may want to
reduce the preferred alignment to -mpreferred-stack-boundary=2.
-mmmx
-mno-mmx
-msse
-mno-sse
-msse2
-mno-sse2
-msse3
-mno-sse3
-mssse3
-mno-ssse3
-m3dnow
-mno-3dnow
These switches enable or disable the use of built-in functions that
allow direct access to the MMX, SSE, SSE2, SSE3 and 3Dnow
extensions of the instruction set.
To have SSE/SSE2 instructions generated automatically from
floating-point code, see -mfpmath=sse.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is
shorter and usually equally fast as method using SUB/MOV operations
and is enabled by default. In some cases disabling it may improve
performance because of improved scheduling and reduced
dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing
arguments will be computed in the function prologue. This is
faster on most modern CPUs because of reduced dependencies,
improved scheduling and reduced stack usage when preferred stack
boundary is not equal to 2. The drawback is a notable increase in
code size. This switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on Mingw32. Code that
relies on thread-safe exception handling must compile and link all
code with the -mthreads option. When compiling, -mthreads defines
-D_MT; when linking, it links in a special thread helper library
-lmingwthrd which cleans up per thread exception handling data.
-mno-align-stringops
Do not align destination of inlined string operations. This switch
reduces code size and improves performance in case the destination
is already aligned, but GCC doesn't know about it.
-minline-all-stringops
By default GCC inlines string operations only when destination is
known to be aligned at least to 4 byte boundary. This enables more
inlining, increase code size, but may improve performance of code
that depends on fast memcpy, strlen and memset for short lengths.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions.
This avoids the instructions to save, set up and restore frame
pointers and makes an extra register available in leaf functions.
The option -fomit-frame-pointer removes the frame pointer for all
functions which might make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from
the TLS segment register (%gs for 32-bit, %fs for 64-bit), or
whether the thread base pointer must be added. Whether or not this
is legal depends on the operating system, and whether it maps the
segment to cover the entire TLS area.
For systems that use GNU libc, the default is on.
These -m switches are supported in addition to the above on AMD x86-64
processors in 64-bit environments.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit
environment sets int, long and pointer to 32 bits and generates
code that runs on any i386 system. The 64-bit environment sets int
to 32 bits and long and pointer to 64 bits and generates code for
AMD's x86-64 architecture.
-mno-red-zone
Do not use a so called red zone for x86-64 code. The red zone is
mandated by the x86-64 ABI, it is a 128-byte area beyond the
location of the stack pointer that will not be modified by signal
or interrupt handlers and therefore can be used for temporary data
without adjusting the stack pointer. The flag -mno-red-zone
disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols
must be linked in the lower 2 GB of the address space. Pointers
are 64 bits. Programs can be statically or dynamically linked.
This is the default code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the
negative 2 GB of the address space. This model has to be used for
Linux kernel code.
-mcmodel=medium
Generate code for the medium model: The program is linked in the
lower 2 GB of the address space but symbols can be located anywhere
in the address space. Programs can be statically or dynamically
linked, but building of shared libraries are not supported with the
medium model.
-mcmodel=large
Generate code for the large model: This model makes no assumptions
about addresses and sizes of sections. Currently GCC does not
implement this model.
PowerPC Options
These are listed under
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpower
-mno-power
-mpower2
-mno-power2
-mpowerpc
-mno-powerpc
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
GCC supports two related instruction set architectures for the
RS/6000 and PowerPC. The POWER instruction set are those
instructions supported by the rios chip set used in the original
RS/6000 systems and the PowerPC instruction set is the architecture
of the Motorola MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM
4xx microprocessors.
Neither architecture is a subset of the other. However there is a
large common subset of instructions supported by both. An MQ
register is included in processors supporting the POWER
architecture.
You use these options to specify which instructions are available
on the processor you are using. The default value of these options
is determined when configuring GCC. Specifying the -mcpu=cpu_type
overrides the specification of these options. We recommend you use
the -mcpu=cpu_type option rather than the options listed above.
The -mpower option allows GCC to generate instructions that are
found only in the POWER architecture and to use the MQ register.
Specifying -mpower2 implies -power and also allows GCC to generate
instructions that are present in the POWER2 architecture but not
the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are
found only in the 32-bit subset of the PowerPC architecture.
Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to
use the optional PowerPC architecture instructions in the General
Purpose group, including floating-point square root. Specifying
-mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use the
optional PowerPC architecture instructions in the Graphics group,
including floating-point select.
The -mpowerpc64 option allows GCC to generate the additional 64-bit
instructions that are found in the full PowerPC64 architecture and
to treat GPRs as 64-bit, doubleword quantities. GCC defaults to
-mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC will use only
the instructions in the common subset of both architectures plus
some special AIX common-mode calls, and will not use the MQ
register. Specifying both -mpower and -mpowerpc permits GCC to use
any instruction from either architecture and to allow use of the MQ
register; specify this for the Motorola MPC601.
-mnew-mnemonics
-mold-mnemonics
Select which mnemonics to use in the generated assembler code.
With -mnew-mnemonics, GCC uses the assembler mnemonics defined for
the PowerPC architecture. With -mold-mnemonics it uses the
assembler mnemonics defined for the POWER architecture.
Instructions defined in only one architecture have only one
mnemonic; GCC uses that mnemonic irrespective of which of these
options is specified.
GCC defaults to the mnemonics appropriate for the architecture in
use. Specifying -mcpu=cpu_type sometimes overrides the value of
these option. Unless you are building a cross-compiler, you should
normally not specify either -mnew-mnemonics or -mold-mnemonics, but
should instead accept the default.
-mcpu=cpu_type
Set architecture type, register usage, choice of mnemonics, and
instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp,
505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450,
750, 801, 821, 823, 860, 970, 8540, common, ec603e, G3, G4, G5,
power, power2, power3, power4, power5, powerpc, powerpc64, rios,
rios1, rios2, rsc, and rs64a.
-mcpu=common selects a completely generic processor. Code
generated under this option will run on any POWER or PowerPC
processor. GCC will use only the instructions in the common subset
of both architectures, and will not use the MQ register. GCC
assumes a generic processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64
specify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not
MPC601), and 64-bit PowerPC architecture machine types, with an
appropriate, generic processor model assumed for scheduling
purposes.
The other options specify a specific processor. Code generated
under those options will run best on that processor, and may not
run at all on others.
The -mcpu options automatically enable or disable the following
options: -maltivec, -mhard-float, -mmfcrf, -mmultiple,
-mnew-mnemonics, -mpower, -mpower2, -mpowerpc64, -mpowerpc-gpopt,
-mpowerpc-gfxopt, -mstring. The particular options set for any
particular CPU will vary between compiler versions, depending on
what setting seems to produce optimal code for that CPU; it doesn't
necessarily reflect the actual hardware's capabilities. If you
wish to set an individual option to a particular value, you may
specify it after the -mcpu option, like -mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or
disabled by the -mcpu option at present, since AIX does not have
full support for these options. You may still enable or disable
them individually if you're sure it'll work in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type
cpu_type, but do not set the architecture type, register usage, or
choice of mnemonics, as -mcpu=cpu_type would. The same values for
cpu_type are used for -mtune as for -mcpu. If both are specified,
the code generated will use the architecture, registers, and
mnemonics set by -mcpu, but the scheduling parameters set by
-mtune.
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and
also enable the use of built-in functions that allow more direct
access to the AltiVec instruction set. You may also need to set
-mabi=altivec to adjust the current ABI with AltiVec ABI
enhancements.
-mpim-altivec
-mno-pim-altivec
Enable (or disable) built-in compiler support for the syntactic
extensions as well as operations and predicates defined in the
Motorola AltiVec Technology Programming Interface Manual (PIM).
This includes the recognition of "vector" and "pixel" as (context-
dependent) keywords, the definition of built-in functions such as
"vec_add", and the use of parenthesized comma expression as AltiVec
literals. Note that unlike the option -maltivec, the extension
does not require the inclusion of any special header files; if
"<altivec.h>" is included, a warning will be issued and the
contents of the header will be ignored. The preprocessor shall
provide an "__APPLE_ALTIVEC__" manifest constant when -mpim-altivec
is specified. (APPLE ONLY)
In addition, the -mpim-altivec option disables the inlining of
functions containing AltiVec instructions into functions that do
not make use of the vector unit. Certain other optimizations, such
as inline vectorization of "memset" and "memcpy" calls, are also
disabled. These adjustments make it possible to compile programs
whose use of AltiVec instructions is preceded by a run-time check
for the presence of AltiVec functionality, and that can therefore
be made to run on G3 processors. Note that all of these
optimizations may be re-enabled by supplying the -maltivec option,
or an -mcpu option specifying a processor that supports AltiVec
instructions.
-mabi=spe
Extend the current ABI with SPE ABI extensions. This does not
change the default ABI, instead it adds the SPE ABI extensions to
the current ABI.
-mabi=no-spe
Disable Booke SPE ABI extensions for the current ABI.
-misel=yes/no
-misel
This switch enables or disables the generation of ISEL
instructions.
-mspe=yes/no
-mspe
This switch enables or disables the generation of SPE simd
instructions.
-mfloat-gprs=yes/single/double/no
-mfloat-gprs
This switch enables or disables the generation of floating point
operations on the general purpose registers for architectures that
support it.
The argument yes or single enables the use of single-precision
floating point operations.
The argument double enables the use of single and double-precision
floating point operations.
The argument no disables floating point operations on the general
purpose registers.
This option is currently only available on the MPC854x.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4
targets (including GNU/Linux). The 32-bit environment sets int,
long and pointer to 32 bits and generates code that runs on any
PowerPC variant. The 64-bit environment sets int to 32 bits and
long and pointer to 64 bits, and generates code for PowerPC64, as
for -mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created
for every executable file. The -mfull-toc option is selected by
default. In that case, GCC will allocate at least one TOC entry
for each unique non-automatic variable reference in your program.
GCC will also place floating-point constants in the TOC. However,
only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have
overflowed the available TOC space, you can reduce the amount of
TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options.
-mno-fp-in-toc prevents GCC from putting floating-point constants
in the TOC and -mno-sum-in-toc forces GCC to generate code to
calculate the sum of an address and a constant at run-time instead
of putting that sum into the TOC. You may specify one or both of
these options. Each causes GCC to produce very slightly slower and
larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both
of these options, specify -mminimal-toc instead. This option
causes GCC to make only one TOC entry for every file. When you
specify this option, GCC will produce code that is slower and
larger but which uses extremely little TOC space. You may wish to
use this option only on files that contain less frequently executed
code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers,
64-bit "long" type, and the infrastructure needed to support them.
Specifying -maix64 implies -mpowerpc64 and -mpowerpc, while -maix32
disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults
to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XLC semantics when
using AIX-compatible ABI. Pass floating-point arguments to
prototyped functions beyond the register save area (RSA) on the
stack in addition to argument FPRs. Do not assume that most
significant double in 128 bit long double value is properly rounded
when comparing values.
The AIX calling convention was extended but not initially
documented to handle an obscure K&R C case of calling a function
that takes the address of its arguments with fewer arguments than
declared. AIX XL compilers access floating point arguments which
do not fit in the RSA from the stack when a subroutine is compiled
without optimization. Because always storing floating-point
arguments on the stack is inefficient and rarely needed, this
option is not enabled by default and only is necessary when calling
subroutines compiled by AIX XL compilers without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an
application written to use message passing with special startup
code to enable the application to run. The system must have PE
installed in the standard location (/usr/lpp/ppe.poe/), or the
specs file must be overridden with the -specs= option to specify
the appropriate directory location. The Parallel Environment does
not support threads, so the -mpe option and the -pthread option are
incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
-malign-natural overrides the ABI-defined alignment of larger
types, such as floating-point doubles, on their natural size-based
boundary. The option -malign-power instructs GCC to follow the
ABI-specified alignment rules. GCC defaults to the standard
alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and
-malign-power is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register
set. Software floating point emulation is provided if you use the
-msoft-float option, and pass the option to GCC when linking.
(APPLE ONLY) While the -msoft-float option is supported, the
libraries that do the floating point emulation are not shipped on
Apple PowerPCs, with the effect that the emulation does not work.
However, the option may be useful for a different reason. Normally
the compiler can use floating point registers in contexts where you
might not expect it, for example, to copy data from one memory
location to another. The -msoft-float option will prevent it from
doing this.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word
instructions and the store multiple word instructions. These
instructions are generated by default on POWER systems, and not
generated on PowerPC systems. Do not use -mmultiple on little
endian PowerPC systems, since those instructions do not work when
the processor is in little endian mode. The exceptions are PPC740
and PPC750 which permit the instructions usage in little endian
mode.
-mstring
-mno-string
Generate code that uses (does not use) the load string instructions
and the store string word instructions to save multiple registers
and do small block moves. These instructions are generated by
default on POWER systems, and not generated on PowerPC systems. Do
not use -mstring on little endian PowerPC systems, since those
instructions do not work when the processor is in little endian
mode. The exceptions are PPC740 and PPC750 which permit the
instructions usage in little endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store
instructions that update the base register to the address of the
calculated memory location. These instructions are generated by
default. If you use -mno-update, there is a small window between
the time that the stack pointer is updated and the address of the
previous frame is stored, which means code that walks the stack
frame across interrupts or signals may get corrupted data.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating point multiply
and accumulate instructions. These instructions are generated by
default if hardware floating is used.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force
structures and unions that contain bit-fields to be aligned to the
base type of the bit-field.
For example, by default a structure containing nothing but 8
"unsigned" bit-fields of length 1 would be aligned to a 4 byte
boundary and have a size of 4 bytes. By using -mno-bit-align, the
structure would be aligned to a 1 byte boundary and be one byte in
size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that
unaligned memory references will be handled by the system.
-mrelocatable
-mno-relocatable
On embedded PowerPC systems generate code that allows (does not
allow) the program to be relocated to a different address at
runtime. If you use -mrelocatable on any module, all objects
linked together must be compiled with -mrelocatable or
-mrelocatable-lib.
-mrelocatable-lib
-mno-relocatable-lib
On embedded PowerPC systems generate code that allows (does not
allow) the program to be relocated to a different address at
runtime. Modules compiled with -mrelocatable-lib can be linked
with either modules compiled without -mrelocatable and
-mrelocatable-lib or with modules compiled with the -mrelocatable
options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that
register 2 contains a pointer to a global area pointing to the
addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in little endian mode. The -mlittle-endian option is the
same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the
processor in big endian mode. The -mbig-endian option is the same
as -mbig.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not
relocatable, but that its external references are relocatable. The
resulting code is suitable for applications, but not shared
libraries.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot
restricted instructions during the second scheduling pass. The
argument priority takes the value 0/1/2 to assign
no/highest/second-highest priority to dispatch slot restricted
instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the
target during instruction scheduling. The argument dependence_type
takes one of the following values: no: no dependence is costly,
all: all dependences are costly, true_store_to_load: a true
dependence from store to load is costly, store_to_load: any
dependence from store to load is costly, number: any dependence
which latency >= number is costly.
-minsert-sched-nops=scheme
This option controls which nop insertion scheme will be used during
the second scheduling pass. The argument scheme takes one of the
following values: no: Don't insert nops. pad: Pad with nops any
dispatch group which has vacant issue slots, according to the
scheduler's grouping. regroup_exact: Insert nops to force costly
dependent insns into separate groups. Insert exactly as many nops
as needed to force an insn to a new group, according to the
estimated processor grouping. number: Insert nops to force costly
dependent insns into separate groups. Insert number nops to force
an insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using
calling conventions that adheres to the March 1995 draft of the
System V Application Binary Interface, PowerPC processor
supplement. This is the default unless you configured GCC using
powerpc-*-eabiaix.
-mcall-sysv-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-solaris
On System V.4 and embedded PowerPC systems compile code for the
Solaris operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the
Linux-based GNU system.
-mcall-gnu
On System V.4 and embedded PowerPC systems compile code for the
Hurd-based GNU system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the
NetBSD operating system.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified
by the SVR4 ABI).
-mabi=altivec
Extend the current ABI with AltiVec ABI extensions. This does not
change the default ABI, instead it adds the AltiVec ABI extensions
to the current ABI.
-mabi=no-altivec
Disable AltiVec ABI extensions for the current ABI.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to
variable argument functions are properly prototyped. Otherwise,
the compiler must insert an instruction before every non prototyped
call to set or clear bit 6 of the condition code register (CR) to
indicate whether floating point values were passed in the floating
point registers in case the function takes a variable arguments.
With -mprototype, only calls to prototyped variable argument
functions will set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is
called sim-crt0.o and that the standard C libraries are libsim.a
and libc.a. This is the default for powerpc-*-eabisim.
configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libmvme.a and
libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is
called crt0.o and the standard C libraries are libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are
compiling for a VxWorks system.
-mwindiss
Specify that you are compiling for the WindISS simulation
environment.
-memb
On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags
header to indicate that eabi extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to
the Embedded Applications Binary Interface (eabi) which is a set of
modifications to the System V.4 specifications. Selecting -meabi
means that the stack is aligned to an 8 byte boundary, a function
"__eabi" is called to from "main" to set up the eabi environment,
and the -msdata option can use both "r2" and "r13" to point to two
separate small data areas. Selecting -mno-eabi means that the
stack is aligned to a 16 byte boundary, do not call an
initialization function from "main", and the -msdata option will
only use "r13" to point to a single small data area. The -meabi
option is on by default if you configured GCC using one of the
powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized
"const" global and static data in the .sdata2 section, which is
pointed to by register "r2". Put small initialized non-"const"
global and static data in the .sdata section, which is pointed to
by register "r13". Put small uninitialized global and static data
in the .sbss section, which is adjacent to the .sdata section. The
-msdata=eabi option is incompatible with the -mrelocatable option.
The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and
static data in the .sdata section, which is pointed to by register
"r13". Put small uninitialized global and static data in the .sbss
section, which is adjacent to the .sdata section. The -msdata=sysv
option is incompatible with the -mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used,
compile code the same as -msdata=eabi, otherwise compile code the
same as -msdata=sysv.
-msdata-data
On System V.4 and embedded PowerPC systems, put small global and
static data in the .sdata section. Put small uninitialized global
and static data in the .sbss section. Do not use register "r13" to
address small data however. This is the default behavior unless
other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static
data in the .data section, and all uninitialized data in the .bss
section.
-G num
On embedded PowerPC systems, put global and static items less than
or equal to num bytes into the small data or bss sections instead
of the normal data or bss section. By default, num is 8. The -G
num switch is also passed to the linker. All modules should be
compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit
register names in the assembly language output using symbolic
forms.
-mlongcall
-mno-longcall
-mlong-branch
-mno-long-branch
Default to making all function calls indirectly, using a register,
so that functions which reside further than 32 megabytes
(33,554,432 bytes) from the current location can be called. This
setting can be overridden by the "shortcall" function attribute, or
Some linkers are capable of detecting out-of-range calls and
generating glue code on the fly. On these systems, long calls are
unnecessary and generate slower code. As of this writing, the AIX
linker can do this, as can the GNU linker for PowerPC/64. It is
planned to add this feature to the GNU linker for 32-bit PowerPC
systems as well.
callee, L42'', plus a ``branch island'' (glue code). The two
target addresses represent the callee and the ``branch island''.
The Darwin/PPC linker will prefer the first address and generate a
``bl callee'' if the PPC ``bl'' instruction will reach the callee
directly; otherwise, the linker will generate ``bl L42'' to call
the ``branch island''. The ``branch island'' is appended to the
body of the calling function; it computes the full 32-bit address
of the callee and jumps to it.
On Mach-O (Darwin) systems, -mlongcall directs the compiler emit to
the glue for every direct call, and the Darwin linker decides
whether to use or discard it. -mlong-branch is a synonym for
-mlongcall.
In the future, we may cause GCC to ignore all longcall
specifications when the linker is known to generate glue.
-pthread
Adds support for multithreading with the pthreads library. This
option sets flags for both the preprocessor and linker.
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 would be -fno-foo. In the table below, only one of the forms
is listed---the one which is not the default. You can figure out the
other form by either removing no- or adding it.
-fbounds-check
For front-ends that support it, generate additional code to check
that indices used to access arrays are within the declared range.
This is currently only supported by the Java and Fortran 77 front-
ends, where this option defaults to true and false respectively.
-ftrapv
This option generates traps for signed overflow on addition,
subtraction, multiplication operations.
-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 other. This option is enabled by
default for the Java front-end, as required by the Java language
specification.
-fexceptions
Enable exception handling. Generates extra code needed to
propagate exceptions. For some targets, this implies GCC will
generate 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 will enable it
by default for languages like C++ which normally require exception
handling, and disable 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".
-funwind-tables
Similar to -fexceptions, except that it will just generate any
needed static data, but will not affect the generated code in any
other way. You will normally not enable this option; instead, a
language processor that needs this handling would enable it on your
behalf.
-fasynchronous-unwind-tables
Generate unwind table in dwarf2 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).
-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
will be equivalent to the smallest integer type which 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-double
Use the same size for "double" as for "float".
Warning: the -fshort-double 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.
-fshared-data
Requests that the data and non-"const" variables of this
compilation be shared data rather than private data. The
distinction makes sense only on certain operating systems, where
shared data is shared between processes running the same program,
while private data exists in one copy per process.
-fno-common
In C, allocate even uninitialized global variables in the data
section of the object file, rather than generating them as common
blocks. This has the effect that if the same variable is declared
(without "extern") in two different compilations, you will get an
error when you link them. The only reason this might be useful is
if you wish to verify that the program will work on other systems
which always work this way.
-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.
-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 and 32k on the m68k and RS/6000. The 386 has no such
limit.)
Position-independent code requires special support, and therefore
works only on certain machines. For the 386, GCC supports PIC for
System V but not for the Sun 386i. Code generated for the IBM
RS/6000 is always position-independent.
-fpic is not supported on Mac OS X.
-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
the m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore
works only on certain machines.
-fPIC is the default on Darwin and Mac OS X.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but generated
position independent code can be only linked into executables.
Usually these options are used when -pie GCC option will be used
during linking.
-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 will not save and restore the register reg.
It is an error to used 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 will produce
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 will save and
restore the register reg if they use it.
It is an error to used 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 will produce
disastrous results.
A different sort of disaster will result 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 will be 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.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions.
Just after function entry and just before function exit, the
following profiling functions will be 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 will 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 anyways,
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 will not be 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).
-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 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 must do that. The switch causes generation of
code to ensure that the operating system sees the stack being
extended.
-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 the stack would grow beyond the value, a signal is
raised. 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.
-fargument-alias
-fargument-noalias
-fargument-noalias-global
Specify the possible relationships among parameters and between
parameters and global data.
-fargument-alias specifies that arguments (parameters) may alias
each other and may alias global storage.-fargument-noalias
specifies that arguments do not alias each other, but may alias
global storage.-fargument-noalias-global specifies that arguments
do not alias each other and do not alias global storage.
Each language will automatically use whatever option is required by
the language standard. You should not need to use these options
yourself.
-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".
The default without -fpic is "initial-exec"; with -fpic the default
is "global-dynamic".
-fvisibility=default|internal|hidden|protected
Set the default ELF image symbol visibility to the specified
option---all symbols will be 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 ie;
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 will be "hidden". The
default if -fvisibility isn't specified is "default", i.e., make
every symbol public---this causes the same behavior as previous
versions of GCC.
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
<http://people.redhat.com/~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 DLL's 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)
These can be nested up to sixteen times. 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 ie; 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.
before including any such headers.
An overview of these techniques, their benefits and how to use them
is at <http://www.nedprod.com/programs/gccvisibility.html>.
ENVIRONMENT
This section describes several environment variables that affect how
GCC operates. Some of them work by specifying directories or prefixes
to use when searching for various kinds of files. Some are used to
specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as
-B, -I and -L. These take precedence over places specified using
environment variables, which in turn take precedence over those
specified by the configuration of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses
localization information that allow GCC to work with different
national conventions. GCC inspects the locale categories LC_CTYPE
and LC_MESSAGES if it has been configured to do so. These locale
categories can be set to any value supported by your installation.
A typical value is en_GB.UTF-8 for English in the United Kingdom
encoded in UTF-8.
The LC_CTYPE environment variable specifies character
classification. GCC uses it to determine the character boundaries
in a string; this is needed for some multibyte encodings that
contain quote and escape characters that would otherwise be
interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use
in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES
default to the value of the LANG environment variable. If none of
these variables are set, GCC defaults to traditional C English
behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary
files. GCC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for
example, the output of the preprocessor, which is the input to the
compiler proper.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler. No slash is
added when this prefix is combined with the name of a subprogram,
but you can specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an
appropriate prefix to use based on the pathname it was invoked
with.
If GCC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where
prefix is the value of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are
used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with /usr/local/lib/gcc
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with -Bfoo/, GCC will search
foo/bar where it would normally search /usr/local/lib/bar. These
alternate directories are searched first; the standard directories
come next.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of
directories, much like PATH. GCC tries the directories thus
specified when searching for subprograms, if it can't find the
subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories,
much like PATH. When configured as a native compiler, GCC tries
the directories thus specified when searching for special linker
files, if it can't find them using GCC_EXEC_PREFIX. Linking using
GCC also uses these directories when searching for ordinary
libraries for the -l option (but directories specified with -L come
first).
LANG
This variable is used to pass locale information to the compiler.
One way in which this information is used is to determine the
character set to be used when character literals, string literals
and comments are parsed in C and C++. When the compiler is
configured to allow multibyte characters, the following values for
LANG are recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the
compiler will use mblen and mbtowc as defined by the default locale
to recognize and translate multibyte characters.
MACOSX_DEPLOYMENT_TARGET
IPHONEOS_DEPLOYMENT_TARGET
These variables are used to set the target OS version, as described
for command-line options -mmacosx-version-min and
-miphoneos-version-min. Only one OS version can be specified per
architecture, with MACOSX_DEPLOYMENT_TARGET taking precedence on
non-ARM targets and IPHONEOS_DEPLOYMENT_TARGET taking precedence on
ARM targets.
If either command-line option -mmacosx-version-min or
-miphoneos-version-min is specified, both of these environment
variables are ignored.
Some additional environments variables affect the behavior of the
preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a
special character, much like PATH, in which to look for header
files. The special character, "PATH_SEPARATOR", is target-
dependent and determined at GCC build time. For Microsoft Windows-
based targets it is a semicolon, and for almost all other targets
it is a colon.
CPATH specifies a list of directories to be searched as if
specified with -I, but after any paths given with -I options on the
command line. This environment variable is used regardless of
which language is being preprocessed.
The remaining environment variables apply only when preprocessing
the particular language indicated. Each specifies a list of
directories to be searched as if specified with -isystem, but after
any paths given with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to
search its current working directory. Empty elements can appear at
the beginning or end of a path. For instance, if the value of
CPATH is ":/special/include", that has the same effect as
-I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files
processed by the compiler. System header files are ignored in the
dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which
case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
file target, in which case the rules are written to file file using
target as the target name.
In other words, this environment variable is equivalent to
combining the options -MM and -MF, with an optional -MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above),
except that system header files are not ignored, so it implies -M
rather than -MM. However, the dependence on the main input file is
omitted.
BUGS
To report bugs to Apple, see <http://developer.apple.com/bugreporter>.
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code
for constructors to work. On multi-libbed systems, gcc -shared
must select the correct support libraries to link against. Failing
to supply the correct flags may lead to subtle defects. Supplying
them in cases where they are not necessary is innocuous.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1),
adb(1), dbx(1), sdb(1) and the Info entries for gcc, cpp, as, ld,
binutils and gdb.
AUTHOR
See the Info entry for gcc, or
<http://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors
to GCC.
COPYRIGHT
Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,
1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``GNU General Public License'' and ``Funding
Free Software'', the Front-Cover texts being (a) (see below), and with
the Back-Cover Texts being (b) (see below). A copy of the license is
included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
POD ERRORS
Hey! The above document had some coding errors, which are explained
below:
Around line 3008:
Expected '=item *'
Around line 3013:
Expected '=item *'
Around line 3019:
Expected '=item *'
Around line 3024:
Expected '=item *'
Around line 3029:
Expected '=item *'
Around line 3034:
Expected '=item *'
Around line 6799:
You can't have =items (as at line 6812) unless the first thing
after the =over is an =item
gcc-4.0.1 2008-05-03 GCC(1)
Mac OS X 10.7 - Generated Thu Nov 3 11:58:06 CDT 2011