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.

DESCRIPTION

When you invoke GCC, it normally does preprocessing, com
pilation, 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 com
piler 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 exam
ple, 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 -pipe -pass-exit-codes -x lan_ guage -v -### --help --target-help --version
C Language Options: -ansi -std=standard -aux-info filename -fno-asm -fno-builtin -fno-builtin-function -fhosted -ffree standing -trigraphs -no-integrated-cpp -traditional -traditional-cpp -fallow-single-precision -fcond-mis match -fsigned-bitfields -fsigned-char -funsigned-bitfields -funsigned-char -fwritable-strings
C++ Language Options: -fno-access-control -fcheck-new -fconserve-space -fno-const-strings -fdollars-in-identifiers -fno-elide-constructors -fno-enforce-eh-specs -fexternal-templates -falt-external-templates -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 -fuse-cxa-atexit -fvtable-gc -fno-weak -nostdinc++ -fno-default-inline -Wabi -Wctor-dtor-privacy -Wnon-virtual-dtor -Wreorder -Weffc++ -Wno-deprecated -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo -Wsynth
Objective-C Language Options: -fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -gen-decls -Wno-protocol -Wselector
Language Independent Options: -fmessage-length=n -fdiagnostics-show-loca tion=[once|every-line]
Warning Options: -fsyntax-only -pedantic -pedantic-errors -w -W -Wall -Waggregate-return -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wno-depre cated-declarations -Wdisabled-optimization -Wdiv-by-zero -Werror -Wfloat-equal -Wformat -Wformat=2 -Wformat-nonliteral -Wformat-security -Wimplicit -Wimplicit-int -Wimplicit-function-declaration -Wer ror-implicit-function-declaration -Wimport -Winline -Wlarger-than-len -Wlong-long -Wmain -Wmissing-braces -Wmissing-format-attribute -Wmissing-noreturn -Wmultichar -Wno-format-extra-args -Wno-format-y2k -Wno-import -Wpacked -Wpadded -Wparentheses -Wpointer-arith -Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wswitch -Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized -Wunknown-pragmas -Wunreachable-code -Wunused -Wunused-function -Wunused-label -Wunused-parameter -Wunused-value -Wunused-variable -Wwrite-strings
C-only Warning Options: -Wbad-function-cast -Wmissing-declarations -Wmissing-prototypes -Wnested-externs -Wstrict-prototypes -Wtraditional
Debugging Options: -dletters -dumpspecs -dumpmachine -dumpversion -fdump-unnumbered -fdump-translation-unit[-n] -fdump-class-hierarchy[-n] -fdump-tree-original[-n] -fdump-tree-optimized[-n] -fdump-tree-inlined[-n] -fmem-report -fpretend-float -fprofile-arcs -fsched-ver bose=n -ftest-coverage -ftime-report -g -glevel -gcoff -gdwarf -gdwarf-1 -gdwarf-1+ -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 -fbounds-check -fbranch-probabilities -fcaller-saves -fcprop-registers -fcse-follow-jumps -fcse-skip-blocks -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 -fin line-functions -finline-limit=n -fkeep-inline-func tions -fkeep-static-consts -fmerge-constants -fmerge-all-constants -fmove-all-movables -fno-branch-count-reg -fno-default-inline -fno-defer-pop -fno-function-cse -fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2 -funsafe-math-optimizations -fno-trapping-math -fomit-frame-pointer -foptimize-register-move -foptimize-sibling-calls -fprefetch-loop-arrays -freduce-all-givs -fregmove -frename-registers -frerun-cse-after-loop -frerun-loop-opt -fschedule-insns -fschedule-insns2 -fno-sched-interblock -fno-sched-spec -fsched-spec-load -fsched-spec-load-dangerous -fsingle-preci sion-constant -fssa -fssa-ccp -fssa-dce -fstrength-reduce -fstrict-aliasing -fthread-jumps -ftrapv -funroll-all-loops -funroll-loops --param name=value -O -O0 -O1 -O2 -O3 -Os
Preprocessor Options: -$ -Aquestion=answer -A-question[=answer] -C -dD -dI -dM -dN -Dmacro[=defn] -E -H -idirafter dir -include file -imacros file -iprefix file -iwithpre fix dir -iwithprefixbefore dir -isystem dir -M -MM -MF -MG -MP -MQ -MT -nostdinc -P -remap -tri graphs -undef -Umacro -Wp,option
Assembler Option: -Wa,option
Linker Options: object-file-name -llibrary -nostartfiles -node
faultlibs -nostdlib -s -static -static-libgcc -shared -shared-libgcc -symbolic -Wl,option -Xlinker option -u symbol
Directory Options: -Bprefix -Idir -I- -Ldir -specs=file
Target Options: -b machine -V version
Machine Dependent Options: M680x0 Options; -m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020 -mfpa -mnobitfield -mrtd -mshort -msoft-float -mpcrel -malign-int -mstrict-align; M68hc1x Options; -m6811 -m6812 -m68hc11 -m68hc12 -mauto-incdec -mshort -msoft-reg-count=count; VAX Options; -mg -mgnu -munix; SPARC Options; -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -m32 -m64 -mapp-regs -mbroken-saverestore -mcypress -mfaster-structs -mflat -mfpu -mhard-float -mhard-quad-float -mimpure-text -mlive-g0 -mno-app-regs -mno-faster-structs -mno-flat -mno-fpu -mno-impure-text -mno-stack-bias -mno-unaligned-doubles -msoft-float -msoft-quad-float -msparclite -mstack-bias -msupersparc -munaligned-doubles -mv8; Convex Options; -mc1 -mc2 -mc32 -mc34 -mc38 -margcount -mnoargcount -mlong32 -mlong64 -mvolatile-cache -mvolatile-nocache; AMD29K Options; -m29000 -m29050 -mbw -mnbw -mdw -mndw -mlarge -mnormal -msmall -mkernel-registers -mno-reuse-arg-regs -mno-stack-check -mno-storem-bug -mreuse-arg-regs -msoft-float -mstack-check -mstorem-bug -muser-registers; ARM Options; -mapcs-frame -mno-apcs-frame -mapcs-26 -mapcs-32 -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 -malignment-traps -mno-alignment-traps -msoft-float -mhard-float -mfpe -mthumb-interwork -mno-thumb-interwork -mcpu=name -march=name -mfpe=name -mstructure-size-boundary=n -mbsd -mxopen -mno-symrename -mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport -mpoke-function-name -mthumb -marm -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking -mcallee-super-interworking; MN10200 Options; -mrelax; MN10300 Options; -mmult-bug -mno-mult-bug -mam33 -mno-am33 -mno-crt0 -mrelax; M32R/D Options; -m32rx -m32r -mcode-model=model-type -msdata=sdatatype -G num; M88K Options; -m88000 -m88100 -m88110 -mbig-pic -mcheck-zero-division -mhandle-large-shift -midentify-revision -mno-check-zero-division -mno-ocs-debug-info -mno-ocs-frame-position -mno-optimize-arg-area -mno-seri alize-volatile -mno-underscores -mocs-debug-info -mocs-frame-position -moptimize-arg-area -mserialize-volatile -mshort-data-num -msvr3 -msvr4 -mtrap-large-shift -muse-div-instruction -mversion-03.00 -mwarn-passed-structs; RS/6000 and PowerPC Options; -mcpu=cpu-type -mtune=cpu-type -mpower -mno-power -mpower2 -mno-power2 -mpowerpc -mpowerpc64 -mno-powerpc -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt -mnew-mnemonics -mold-mnemonics -mfull-toc -mmini mal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-call -mno-xl-call -mpe -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 -mrelo catable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mcall-aix -mcall-sysv -mcall-netbsd -maix-struct-return -msvr4-struct-return -mabi=altivec -mabi=no-altivec -mprototype -mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata -msdata=opt -mvxworks -G num -pthread; RT Options; -mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs -mfull-fp-blocks -mhc-struct-return -min-line-mul -mminimum-fp-blocks -mnohc-struct-return; MIPS Options; -mabicalls -march=cpu-type -mtune=cpu=type -mcpu=cputype -membedded-data -muninit-const-in-rodata -mem bedded-pic -mfp32 -mfp64 -mfused-madd -mno-fused-madd -mgas -mgp32 -mgp64 -mgpopt -mhalf-pic -mhard-float -mint64 -mips1 -mips2 -mips3 -mips4 -mlong64 -mlong32 -mlong-calls -mmemcpy -mmips-as -mmips-tfile -mno-abicalls -mno-embedded-data -mno-uninit-const-in-rodata -mno-embedded-pic -mno-gpopt -mno-long-calls -mno-memcpy -mno-mips-tfile -mno-rnames -mno-stats -mrnames -msoft-float -m4650 -msingle-float -mmad -mstats -EL -EB -G num -nocpp -mabi=32 -mabi=n32 -mabi=64 -mabi=eabi -mfix7000 -mno-crt0 -mflush-func=func -mno-flush-func; i386 and x86-64 Options; -mcpu=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 -m3dnow -mthreads -mno-align-stringops -minline-all-stringops -mpush-args -maccumulate-out going-args -m128bit-long-double -m96bit-long-double -mregparm=num -momit-leaf-frame-pointer -mno-red-zone -mcmodel=code-model -m32 -m64; HPPA Options; -march=architecture-type -mbig-switch -mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas -mjump-in-delay -mlong-load-store -mno-big-switch -mno-disable-fpregs -mno-disable-indexing -mno-fast-indirect-calls -mno-gas -mno-jump-in-delay -mno-long-load-store -mno-portable-runtime -mno-soft-float -mno-space-regs -msoft-float -mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu-type -mspace-regs; Intel 960 Options; -mcpu-type -masm-compat -mclean-linkage -mcode-align -mcomplex-addr -mleaf-procedures -mic-compat -mic2.0-compat -mic3.0-compat -mintel-asm -mno-clean-linkage -mno-code-align -mno-complex-addr -mno-leaf-procedures -mno-old-align -mno-strict-align -mno-tail-call -mnumerics -mold-align -msoft-float -mstrict-align -mtail-call; DEC Alpha Options; -mno-fp-regs -msoft-float -malpha-as -mgas -mieee -mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-preci sion=mode -mbuild-constants -mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix -mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data -mlarge-data -mmemory-latency=time; DEC Alpha/VMS Options; -mvms-return-codes; Clipper Options; -mc300 -mc400; H8/300 Options; -mrelax -mh -ms -mint32 -malign-300; SH Options; -m1 -m2 -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4 -m5-64media -m5-64media-nofpu -m5-32media -m5-32media-nofpu -m5-compact -m5-compact-nofpu -mb -ml -mdalign -mrelax -mbigtable -mfmovd -mhitachi -mnomacsave -mieee -misize -mpadstruct -mspace -mprefergot -musermode; System V Options; -Qy -Qn -YP,paths -Ym,dir; ARC Options; -EB -EL -mmangle-cpu -mcpu=cpu -mtext=text-section -mdata=data-section -mrodata=readonly-data-section; TMS320C3x/C4x Options; -mcpu=cpu -mbig -msmall -mregparm -mmemparm -mfast-fix -mmpyi -mbk -mti -mdp-isr-reload -mrpts=count -mrptb -mdb -mloop-unsigned -mparal lel-insns -mparallel-mpy -mpreserve-float; V850 Options; -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n -mzda=n -mv850 -mbig-switch; NS32K Options; -m32032 -m32332 -m32532 -m32081 -m32381 -mmult-add -mnomult-add -msoft-float -mrtd -mnortd -mregparam -mnoregparam -msb -mnosb -mbitfield -mnobitfield -mhimem -mnohimem; AVR Options; -mmcu=mcu -msize -minit-stack=n -mno-interrupts -mcall-prologues -mno-tablejump -mtiny-stack; MCore Options; -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates -mno-relax-immediates -mwide-bitfields -mno-wide-bitfields -m4byte-functions -mno-4byte-functions -mcallgraph-data -mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340 -mstack-increment; MMIX Options; -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict -mbase-addresses -mno-base-addresses; IA-64 Options; -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop -mb-step -mregister-names -mno-sdata -mconstant-gp -mauto-pic -minline-divide-min-latency -minline-divide-max-throughput -mno-dwarf2-asm -mfixed-range=register-range; D30V Options; -mextmem -mextmemory -monchip -mno-asm-optimize -masm-optimize -mbranch-cost=n -mcond-exec=n; S/390 and zSeries Options; -mhard-float -msoft-float -mbackchain -mno-backchain -msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug; CRIS Options; -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -melinux-stacksize=n -metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt -melf -maout -melinux -mlinux -sim -sim2; PDP-11 Options; -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mbcopy -mbcopy-builtin -mint32 -mno-int16 -mint16 -mno-int32 -mfloat32 -mno-float64 -mfloat64 -mno-float32 -mabshi -mno-abshi -mbranch-expensive -mbranch-cheap -msplit -mno-split -munix-asm -mdec-asm; Xstormy16 Options; -msim; Xtensa Options; -mbig-endian -mlittle-endian -mdensity -mno-density -mmac16 -mno-mac16 -mmul16 -mno-mul16 -mmul32 -mno-mul32 -mnsa -mno-nsa -mminmax -mno-minmax -msext -mno-sext -mbooleans -mno-booleans -mhard-float -msoft-float -mfused-madd -mno-fused-madd -mserialize-volatile -mno-serialize-volatile -mtext-section-liter als -mno-text-section-literals -mtarget-align -mno-target-align -mlongcalls -mno-longcalls
Code Generation Options: -fcall-saved-reg -fcall-used-reg -ffixed-reg -fexcep tions -fnon-call-exceptions -funwind-tables -fasyn chronous-unwind-tables -finhibit-size-directive -fin strument-functions -fno-common -fno-ident -fno-gnu-linker -fpcc-struct-return -fpic -fPIC -freg-struct-return -fshared-data -fshort-enums -fshort-double -fshort-wchar -fvolatile -fvolatile-global -fvolatile-static -fverbose-asm -fpack-struct -fstack-check -fstack-limit-register=reg -fstack-limit-symbol=sym -fargument-alias -fargument-noalias -fargument-noalias-global -fleading-underscore
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. The first three stages apply to an individual source file, and end by producing an object file; 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 library libobjc.a to make an Objective-C program work.
file.mi: Objective-C source code which should not be prepro
cessed.
file.h: C header file (not to be compiled or linked).
file.cc
file.cp
file.cxx
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.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.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 sub
program renaming declaration). Such files are also
called specs.
file.adb: Ada source code file containing a library unit body (a
subprogram or package body). Such files are also
called bodies.
file.s: Assembler code.
file.S: 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 cpp-output
c++ c++-cpp-output
objective-c objc-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
java
-x none: Turn off any specification of a language, so that sub
sequent files are handled according to their file name
suffixes (as they are if -x has not been used at all).
-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 prepro
cessed source code, which is sent to the standard out
put.; 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.; Since only one output file can be specified, it does
not make sense to use -o when compiling more than one
input file, unless you are producing an executable
file as output.; If -o is not specified, the default is to put an exe
cutable file in a.out, the object file for source.suf_ fix in source.o, its assembler file in source.s, 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 ver
sion 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 communica
tion 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.
--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 -W option is also specified then com
mand line options which have no documentation associ
ated 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, .c++, .cp, or .cxx; 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, C++ programs often require class libraries as
well as a compiler that understands the C++ language---and
under some circumstances, you might want to compile pro
grams from standard input, or otherwise without a suffix
that flags them as C++ programs. g++ is a program that calls GCC with the default language set to C++, and auto
matically specifies linking against the C++ library. 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 pro grams 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 lan guages derived from C, such as C++ and Objective-C) that the compiler accepts:
-ansi: In C mode, support all ISO C89 programs. In C++ mode,
remove GNU extensions that conflict with ISO C++.; This turns off certain features of GCC that are incom
patible with ISO C89 (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 com
piler, it disables recognition of C++ style // com
ments 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 func
tions 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 cur
rently only supported when compiling C. A value for
this option must be provided; possible values are; c89
iso9899:1990
ISO C89 (same as -ansi).; iso9899:199409
ISO C89 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-3.1/c99sta tus.html> for more information. The names c9x and iso9899:199x are deprecated.; gnu89
Default, ISO C89 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.; 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 C89 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 trans
lation 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 declara
tions is also provided, inside comments, after the
declaration.
-fno-asm: Do not recognize "asm", "inline" or "typeof" as a key
word, 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 key
word in ISO C99.
-fno-builtin -fno-builtin-function (C and Objective-C only): 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 C++, -fno-builtin is always in effect. The -fbuiltin option has no effect. Therefore, in C++, the only way to get the optimization benefits of
built-in functions is to call the function using the
__builtin_ prefix. The GNU C++ Standard Library uses built-in functions to implement many functions (like
"std::strchr"), so that you automatically get effi
cient code.; With the -fno-builtin-function option, not available when compiling C++, only the built-in function func_
tion is disabled. function must not begin with __builtin_. If a function is named this is not builtin 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 envi
ronment. This implies -fbuiltin. A hosted environ ment 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 ker
nel. This is equivalent to -fno-freestanding.
-ffreestanding: Assert that compilation takes place in a freestanding
environment. This implies -fno-builtin. A freestand ing 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 ker
nel. This is equivalent to -fno-hosted.
-trigraphs: Support ISO C trigraphs. The -ansi option (and -std options for strict ISO C conformance) implies -tri graphs.
-no-integrated-cpp: Invoke the external cpp during compilation. The
default is to use the integrated cpp (internal cpp).
This option also allows a user-supplied cpp via the -B option. This flag is applicable in both C and C++
modes.; We do not guarantee to retain this option in future,
and we may change its semantics.
-traditional: Attempt to support some aspects of traditional C com
pilers. Specifically:; � All "extern" declarations take effect globally
even if they are written inside of a function
definition. This includes implicit declarations
of functions.; � The newer keywords "typeof", "inline", "signed",
"const" and "volatile" are not recognized. (You
can still use the alternative keywords such as
"__typeof__", "__inline__", and so on.); � Comparisons between pointers and integers are
always allowed.; � Integer types "unsigned short" and "unsigned char"
promote to "unsigned int".; � Out-of-range floating point literals are not an
error.; � Certain constructs which ISO regards as a single
invalid preprocessing number, such as 0xe-0xd, are treated as expressions instead.; � String ``constants'' are not necessarily constant;
they are stored in writable space, and identical
looking constants are allocated separately. (This
is the same as the effect of -fwritable-strings.); � All automatic variables not declared "register"
are preserved by "longjmp". Ordinarily, GNU C
follows ISO C: automatic variables not declared
"volatile" may be clobbered.; � The character escape sequenxeanevaluate
as the literal characters x and a respectively.
Without -traditionaxlis a prefix for the hex adecimal representation of a character, and
produces a bell.; This option is deprecated and may be removed.; You may wish to use -fno-builtin as well as -tradi tional if your program uses names that are normally GNU C built-in functions for other purposes of its
own.; You cannot use -traditional if you include any header files that rely on ISO C features. Some vendors are
starting to ship systems with ISO C header files and
you cannot use -traditional on such systems to compile files that include any system headers.; The -traditional option also enables -traditional-cpp.
-traditional-cpp: Attempt to support some aspects of traditional C pre
processors. 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++.
-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. Like wise, 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 inte
ger types such as "int" are signed types.; However, when -traditional is used, bit-fields are all unsigned no matter what.
-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. The option -traditional also has this effect.; Writing into string constants is a very bad idea;
``constants'' should be constant.
-fallow-single-precision: Do not promote single precision math operations to
double precision, even when compiling with -tradi tional.; Traditional K&R C promotes all floating point opera
tions to double precision, regardless of the sizes of
the operands. On the architecture for which you are
compiling, single precision may be faster than double
precision. If you must use -traditional, but want to use single precision operations when the operands are
single precision, use this option. This option has
no effect when compiling with ISO or GNU C conventions
(the default).
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 lan guage 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 lan
guage supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-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 allo
cated. The current Working Paper requires that "oper
ator new" never return a null pointer, so this check
is normally unnecessary.; An alternative to using this option is to specify that
your "operator new" does not throw any exceptions; if
you declare it tthhrrooww(()), G++ will check the return value. See also new (nothrow).
-fconserve-space: Put uninitialized or runtime-initialized global vari
ables 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 *".
-fdollars-in-identifiers: Accept $ in identifiers. You can also explicitly pro
hibit use of $ with the option -fno-dollars-in-identi fiers. (GNU C allows $ by default on most target sys tems, but there are a few exceptions.) Traditional C
allowed the character $ to form part of identifiers.
However, ISO C and C++ forbid $ in identifiers.
-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.
-fexternal-templates: Cause #pragma interface and implementation to apply to template instantiation; template instances are emitted
or not according to the location of the template defi
nition.; This option is deprecated.
-falt-external-templates: Similar to -fexternal-templates, but template instances are emitted or not according to the place
where they are first instantiated.; This option is deprecated.
-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 oldstyle 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 opti
mization will need the same set of explicit instantia
tions.
-fno-implement-inlines: To save space, do not emit out-of-line copies of
inline functions controlled by #pragma implementation. This will cause linker 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 messages about nonconformant code from
errors to warnings. By default, G++ effectively sets
-pedantic-errors without -pedantic; this option reverses that. This behavior and this option are
superseded by -pedantic, which works as it does for GNU C.
-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 excep
tion 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.
-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 destruc
tors, but will only work if your C library supports
"__cxa_atexit".
-fvtable-gc: Emit special relocations for vtables and virtual func
tion references so that the linker can identify unused
virtual functions and zero out vtable slots that refer
to them. This is most useful with -ffunction-sections and -Wl,--gc-sections, in order to also discard the functions themselves.; This optimization requires GNU as and GNU ld. Not all
systems support this option. -Wl,--gc-sections is ignored without -static.
-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 direc
tories 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 genera tion 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 com
patible 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 gener
ated by G++ may not be binary compatible with code
generated by other compilers.; The known incompatibilites at this point include:; � 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.
� 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.
-Wctor-dtor-privacy (C++ only): Warn when a class seems unusable, because all the con
structors or destructors in a class are private and
the class has no friends or public static member func
tions.
-Wnon-virtual-dtor (C++ only): Warn when a class declares a non-virtual destructor
that should probably be virtual, because it looks like
the class will be used polymorphically.
-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) { }

};
Here the compiler will warn that the member initializ ers for i and j will be rearranged to match the decla ration order of the members.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only): Warn about violations of the following style guide
lines from Scott Meyers' Effective C++ book:; � Item 11: Define a copy constructor and an assign
ment operator for classes with dynamically allo
cated memory.; � Item 12: Prefer initialization to assignment in
constructors.; � Item 14: Make destructors virtual in base
classes.; � Item 15: Have "operator=" return a reference to
"*this".; � Item 23: Don't try to return a reference when you
must return an object.; and about violations of the following style guidelines
from Scott Meyers' More Effective C++ book:; � Item 6: Distinguish between prefix and postfix
forms of increment and decrement operators.; � Item 7: Never overload "&&", "||", or ",".; If you use this option, you should be aware that the
standard library headers do not obey all of these
guidelines; you can use grep -v to filter out those warnings.
-Wno-deprecated (C++ only): Do not warn about usage of deprecated features.
-Wno-non-template-friend (C++ only): Disable warnings when non-templatized friend functions
are declared within a template. With 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 tem
platized function. Because this non-conforming behav
ior 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 confor mant 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 grep for.
-Woverloaded-virtual (C++ only): Warn when a function declaration hides virtual func
tions 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 this:: 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 enumeral type to a signed type over a con
version to an unsigned type of the same size. Previ
ous versions of G++ would try to preserve unsigned
ness, but the standard mandates the current behavior.
-Wsynth (C++ only): Warn when G++'s synthesis behavior does not match that
of cfront. For instance:

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 Dialect
This section describes the command-line options that are only meaningful for Objective-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 com pile a file "some_class.m" like this:: gcc -g -fgnu-runtime -O -c some_class.m
In this example, only -fgnu-runtime is an option meant only for Objective-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 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 "NXCon
stantString".
-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.
-gen-decls: Dump interface declarations for all classes seen in
the source file to a file named sourcename.decl.
-Wno-protocol: Do not warn if methods required by a protocol are not
implemented in the class adopting it.
-Wselector: Warn if a selector has multiple methods of different
types defined.
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 loca
tion 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 pre
fix) 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 begin
ning -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 @ref{C++ Dialect Options} and @ref{Objective-C Dialect Options}.
-fsyntax-only: Check the code for syntax errors, but don't do any
thing 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 prop
erly 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 diag nostics 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 dif
ferent 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 defini
tion 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.
-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.
-Wcomment: Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a Backslash-Newline appears in a // comment.
-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 exten
sion, not in the C standard) families.; The formats are checked against the format features
supported by GNU libc version 2.2. These include all
ISO C89 and C99 features, as well as features from the
Single Unix Specification and some BSD and GNU exten
sions. Other library implementations may not support
all these features; GCC does not support warning about
features that go beyond a particular library's limita
tions. 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).; -Wformat is included in -Wall. For more control over some aspects of format checking, the options -Wno-for mat-y2k, -Wno-format-extra-args, -Wformat-nonliteral, -Wformat-security and -Wformat=2 are available, but are not included in -Wall.
-Wno-format-y2k: If -Wformat is specified, do not 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 specifica
tions, 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.
-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 for mat functions that represent possible security prob
lems. 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 con
tains %n. (This is currently a subset of what -Wfor mat-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.
-Wimplicit-int: Warn when a declaration does not specify a type.
-Wimplicit-function-declaration -Werror-implicit-function-declaration: Give a warning (or error) whenever a function is used
before being declared.
-Wimplicit: Same as -Wimplicit-int and -Wimplicit-function-decla ration.
-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.
-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 } };
-Wparentheses: Warn if parentheses are omitted in certain contexts,
such as when there is an assignment in a context where
a truth value is expected, or when operators are
nested whose precedence people often get confused
about.; Also warn about constructions where there may be con
fusion 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 enclos ing "if". The resulting code would look like this:: {
if (a)
{
if (b)
foo ();

else

bar ();

}

}
-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 expres
sion), 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 func
tion), 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 modi
fications to the values of objects take effect. Pro
grams 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 evalua
tion 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 dis
cussions of the problem, including proposed formal
definitions, may be found on our readings page, at
<http://gcc.gnu.org/readings.html>.
-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++, a function without return type always pro
duces a diagnostic message, even when -Wno-return-type is specified. The only exceptions are main and func tions defined in system headers.
-Wswitch: Warn whenever a "switch" statement has an index of
enumeral 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.
-Wtrigraphs: Warn if any trigraphs are encountered that might
change the meaning of the program (trigraphs within
comments are not warned about).
-Wunused-function: Warn whenever a static function is declared but not
defined or a non-inline static function is unused.
-Wunused-label: Warn whenever a label is declared but not used.; 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; To suppress this warning use the unused attribute.
-Wunused-value: Warn whenever a statement computes a result that is
explicitly not used.; To suppress this warning cast the expression to void.
-Wunused: All all the above -Wunused options combined.; In order to get a warning about an unused function
parameter, you must either specify -W -Wunused or sep arately 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 informa
tion that is computed only when optimizing. If you
don't specify -O, you simply won't get these warnings.; These warnings occur only for variables that are can
didates for register allocation. Therefore, they do
not occur for a variable that is declared "volatile",
or whose address is taken, or whose size is other than
1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in
registers.; 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 can not 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 "nore turn".
-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:
-Wunknown-pragmas: Warn when a #pragma directive is encountered which is
not 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.
-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.
-Wdiv-by-zero: Warn about compile-time integer division by zero.
This is default. To inhibit the warning messages, use
-Wno-div-by-zero. Floating point division by zero is not warned about, as it can be a legitimate way of
obtaining infinities and NaNs.
-Wmultichar: Warn if a multicharacter constant ('FOOF') is used. This is default. To inhibit the warning messages, use
-Wno-multichar. Usually they indicate a typo in the user's code, as they have implementation-defined val
ues, and should not be used in portable code.
-Wsystem-headers: Print warning messages for constructs found in system
header files. Warnings from system headers are nor
mally 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 head
ers---for that, -Wunknown-pragmas must also be used.
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.
-W Print extra warning messages for these events:: � 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;

}
� 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.
� An unsigned value is compared against zero with <: or <=.
� A comparison like x<=y<=z appears; this is equiva: lent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical
notation.
� Storage-class specifiers like "static" are not the: first things in a declaration. According to the C
Standard, this usage is obsolescent.
� 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. (But don't warn about the GNU
extension of "volatile void" return types. That
extension will be warned about if -pedantic is specified.)
� If -Wall or -Wunused is also specified, warn about: unused arguments.
� 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.)
� An aggregate has a partly bracketed initializer.: For example, the following code would evoke such a
warning, because braces are missing around the
initializer for "x.h":

struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
� An aggregate has an initializer which does not: initialize all members. For example, the follow
ing code would cause such a warning, because "x.h"
would be implicitly initialized to zero:

struct s { int f, g, h; };
struct s x = { 3, 4 };
-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 floatingpoint values as approximations to infinitely precise
real numbers. If you are doing this, then you need to
compute (by analysing the code, or in some other way)
the maximum or likely maximum error that the computa
tion introduces, and allow for it when performing com
parisons (and when producing output, but that's a dif
ferent 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.
-Wtraditional (C only): Warn about certain constructs that behave differently
in traditional and ISO C. Also warn about ISO C con
structs that have no traditional C equivalent, and/or
problematic constructs which should be avoided.; � Macro parameters that appear within string liter
als in the macro body. In traditional C macro
replacement takes place within string literals,
but does not in ISO C.; � 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 tradi tional C understands but would ignore because the
# does not appear as the first character on the
line. It also suggests you hide directives like
#pragma not understood by traditional C by indent ing them. Some traditional implementations would
not recognize #elif, so it suggests avoiding it altogether.; � A function-like macro that appears without argu
ments.; � The unary plus operator.; � The U integer constant suffix, or the F or L
floating point constant suffixes. (Traditional C
does support the L suffix on integer constants.)
Note, these suffixes appear in macros defined in
the system headers of most modern systems, e.g.
the _MIN/_MAX macros in "<limits.h>". Use of these macros in user code might normally lead to
spurious warnings, however gcc's integrated pre
processor has enough context to avoid warning in
these cases.; � A function declared external in one block and then
used after the end of the block.; � A "switch" statement has an operand of type
"long".; � A non-"static" function declaration follows a
"static" one. This construct is not accepted by
some traditional C compilers.; � The ISO type of an integer constant has a differ
ent 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 val
ues, which typically represent bit patterns, are
not warned about.; � Usage of ISO string concatenation is detected.; � Initialization of automatic aggregates.; � Identifier conflicts with labels. Traditional C
lacks a separate namespace for labels.; � Initialization of unions. If the initializer is
zero, the warning is omitted. This is done under
the assumption that the zero initializer in user
code appears conditioned on e.g. "__STDC__" to
avoid missing initializer warnings and relies on
default initialization to zero in the traditional
C case.; � Conversions by prototypes between fixed/floating
point values and vice versa. The absence of these
prototypes when compiling with traditional C would
cause serious problems. This is a subset of the
possible conversion warnings, for the full set use
-Wconversion.
-Wundef: Warn if an undefined identifier is evaluated in an #if directive.
-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-match
ing 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 fourbyte 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 dec
larations 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 conver
sions 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".
-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 -W; to get the other warnings of -W without this warning, use -W -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.)
-Wmissing-prototypes (C only): Warn if a global function is defined without a previ
ous 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: Warn if a global function is defined without a previ
ous 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-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, other
wise 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 (pos sibly by -Wall).
-Wno-deprecated-declarations: Do not warn about uses of functions, variables, and
types marked as deprecated by using the "deprecated"
attribute. (@pxref{Function Attributes}, @pxref{Vari
able Attributes}, @pxref{Type Attributes}.)
-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 misaligned for little benefit. For instance, in this
code, the variable "f.x" in "struct bar" will be mis
aligned 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 substan
tial code which checks correct functioning of the pro
gram and is, hopefully, unreachable because the pro
gram 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.
-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.
-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.
-Werror: Make all warnings into errors.
Options for Debugging Your Program or GCC
GCC has various special options that are used for debug ging either your program or GCC:
-g Produce debugging information in the operating sys: tem's native format (stabs, COFF, XCOFF, or DWARF).
GDB can work with this debugging information.; On most systems that use stabs format, -g enables use
of extra debugging information that only GDB can use;
this extra information makes debugging work better in
GDB but 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+, -gstabs, -gxcoff+, -gxcoff, -gdwarf-1+, -gdwarf-1, or -gvms (see below).; Unlike most other C compilers, GCC allows you to use
-g with -O. The shortcuts taken by optimized code may occasionally produce surprising results: some vari
ables you declared may not exist at all; flow of con
trol may briefly move where you did not expect it;
some statements may not be executed because they com
pute 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 for
mat.
-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 pro
duces stabs debugging output which is not understood
by DBX or SDB. On System V Release 4 systems this
option requires the GNU assembler.
-gstabs+: Produce debugging information in stabs format (if that
is supported), using GNU extensions understood only by
the GNU debugger (GDB). The use of these extensions
is likely to make other debuggers crash or refuse to
read the program.
-gcoff: Produce debugging information in COFF format (if that
is supported). This is the format used by SDB on most
System V systems prior to System V Release 4.
-gxcoff: Produce debugging information in XCOFF format (if that
is supported). This is the format used by the DBX
debugger on IBM RS/6000 systems.
-gxcoff+: Produce debugging information in XCOFF format (if that
is supported), using GNU extensions understood only by
the GNU debugger (GDB). The use of these extensions
is likely to make other debuggers crash or refuse to
read the program, and may cause assemblers other than
the GNU assembler (GAS) to fail with an error.
-gdwarf: Produce debugging information in DWARF version 1 for
mat (if that is supported). This is the format used
by SDB on most System V Release 4 systems.
-gdwarf+: Produce debugging information in DWARF version 1 for
mat (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 for
mat (if that is supported). This is the format used
by DBX on IRIX 6.
-gvms: Produce debugging information in VMS debug format (if
that is supported). This is the format used by DEBUG
on VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gcofflevel
-gxcofflevel
-gvmslevel: Request debugging information and also use level to
specify how much information. The default level is 2.; Level 1 produces minimal information, enough for mak
ing backtraces in parts of the program that you don't
plan to debug. This includes descriptions of func
tions 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 debug
gers support macro expansion when you use -g3.; Note that in order to avoid confusion between DWARF1
debug level 2, and DWARF2, neither -gdwarf nor -gdwarf-2 accept a concatenated debug level. Instead use an additional -glevel option to change the debug level for DWARF1 or DWARF2.
-p Generate extra code to write profile information suit: able 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 suit: able 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 perma
nent memory allocation when it finishes.
-fprofile-arcs: Instrument arcs during compilation to generate cover
age data or for profile-directed block ordering. Dur
ing execution the program records how many times each
branch is executed and how many times it is taken.
When the compiled program exits it saves this data to
a file called sourcename.da for each source file.; For profile-directed block ordering, compile the pro
gram with -fprofile-arcs plus optimization and code generation options, generate the arc profile informa
tion by running the program on a selected workload,
and then compile the program again with the same
optimization and code generation options plus
-fbranch-probabilities.; The other use of -fprofile-arcs is for use with "gcov", when it is used with the -ftest-coverage option.; With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a span
ning tree for the graph. Only arcs that are not on
the spanning tree have to be instrumented: the com
piler adds code to count the number of times that
these arcs are executed. When an arc is the only exit
or only entrance to a block, the instrumentation code
can be added to the block; otherwise, a new basic
block must be created to hold the instrumentation
code.
-ftest-coverage: Create data files for the gcov code-coverage utility. The data file names begin with the name of your source
file:; sourcename.bb
A mapping from basic blocks to line numbers, which
"gcov" uses to associate basic block execution
counts with line numbers.; sourcename.bbg
A list of all arcs in the program flow graph.
This allows "gcov" to reconstruct the program flow
graph, so that it can compute all basic block and
arc execution counts from the information in the
"sourcename.da" file.; Use -ftest-coverage with -fprofile-arcs; the latter option adds instrumentation to the program, which then
writes execution counts to another data file:; sourcename.da
Runtime arc execution counts, used in conjunction
with the arc information in the file "source
name.bbg".; Coverage data will map better to the source files if
-ftest-coverage is used without optimization.
-dletters: Says to make debugging dumps during compilation at
times specified by letters. This is used for debug
ging the compiler. The file names for most of the
dumps are made by appending a pass number and a word
to the source file name (e.g. foo.c.00.rtl or foo.c.01.sibling). Here are the possible letters for use in letters, and their meanings:; A Annotate the assembler output with miscellaneous
debugging information.; b Dump after computing branch probabilities, to
file.14.bp.; B Dump after block reordering, to file.29.bbro.; c Dump after instruction combination, to the file
file.16.combine.; C Dump after the first if conversion, to the file
file.17.ce.; d Dump after delayed branch scheduling, to
file.31.dbr.; D Dump all macro definitions, at the end of prepro
cessing, in addition to normal output.; e Dump after SSA optimizations, to file.04.ssa and
file.07.ussa.; E Dump after the second if conversion, to
file.26.ce2.; f Dump after life analysis, to file.15.life.; F Dump after purging "ADDRESSOF" codes, to
file.09.addressof.; g Dump after global register allocation, to
file.21.greg.; h Dump after finalization of EH handling code, to
file.02.eh.; k Dump after reg-to-stack conversion, to
file.28.stack.; o Dump after post-reload optimizations, to
file.22.postreload.; G Dump after GCSE, to file.10.gcse.; i Dump after sibling call optimizations, to
file.01.sibling.; j Dump after the first jump optimization, to
file.03.jump.; k Dump after conversion from registers to stack, to
file.32.stack.; l Dump after local register allocation, to
file.20.lreg.; L Dump after loop optimization, to file.11.loop.; M Dump after performing the machine dependent reor
ganisation pass, to file.30.mach.; n Dump after register renumbering, to file.25.rnreg.; N Dump after the register move pass, to file.18.reg_
move.; r Dump after RTL generation, to file.00.rtl.; R Dump after the second scheduling pass, to
file.27.sched2.; s Dump after CSE (including the jump optimization
that sometimes follows CSE), to file.08.cse.; S Dump after the first scheduling pass, to
file.19.sched.; t Dump after the second CSE pass (including the jump
optimization that sometimes follows CSE), to
file.12.cse2.; w Dump after the second flow pass, to file.23.flow2.; X Dump after SSA dead code elimination, to
file.06.ssadce.; z Dump after the peephole pass, to file.24.peep_
hole2.; a Produce all the dumps listed above.; m Print statistics on memory usage, at the end of
the run, to standard error.; p Annotate the assembler output with a comment indi
cating which pattern and alternative was used.
The length of each instruction is also printed.; P Dump the RTL in the assembler output as a comment
before each instruction. Also turns on -dp anno tation.; v For each of the other indicated dump files (except
for file.00.rtl), dump a representation of the control flow graph suitable for viewing with VCG
to file.pass.vcg.; x Just generate RTL for a function instead of com
piling it. Usually used with r.; y Dump debugging information during parsing, to
standard error.
-fdump-unnumbered: When doing debugging dumps (see -d option above), sup
press 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-tree-switch (C++ only)
-fdump-tree-switch-options (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 suf
fix 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 meaning
ful will be ignored. The following options are avail
able; 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.; all Turn on all options.; 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.
-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-ver bose also includes dependence info.
-fpretend-float: When running a cross-compiler, pretend that the target
machine uses the same floating point format as the
host machine. This causes incorrect output of the
actual floating constants, but the actual instruction
sequence will probably be the same as GCC would make
when running on the target machine.
-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 prepro cessed foo.i output file even though the compiler now
normally uses an integrated preprocessor.
-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.
-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{-, with out spaces between multiple switches. This is sup
posed 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 -nodefault libs 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 direc
tory 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.)
Options That Control Optimization
These options control various sorts of optimizations:
-O
-O1 Optimize. Optimizing compilation takes somewhat more: time, and a lot more memory for a large function.; Without -O, 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.; With -O, the compiler tries to reduce code size and
execution time, without performing any optimizations
that take a great deal of compilation time.
-O2 Optimize even more. GCC performs nearly all supported: optimizations that do not involve a space-speed trade
off. 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 optional optimizations except for loop unrolling, function inlining, and register renam
ing. It also turns on the -fforce-mem option on all machines and frame pointer elimination on machines
where doing so does not interfere with debugging.; Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations: specified by -O2 and also turns on the -finline-func tions and -frename-registers options.
-O0 Do not optimize.
-Os Optimize for size. -Os enables all -O2 optimizations: that do not typically increase code size. It also
performs further optimizations designed to reduce code
size.; 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
which is not the default. You can figure out the other
form by either removing no- or adding it.
-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 mem
ory.; This option prevents undesirable excess precision on
machines such as the 68000 where the floating regis
ters (of the 68881) keep more precision than a "dou
ble" 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.
-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 func
tions 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.
-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 subexpres
sions, instruction combination should eliminate the
separate register-load. The -O2 option turns on this option.
-fforce-addr: Force memory address constants to be copied into reg
isters before doing arithmetic on them. This may pro
duce better code just as -fforce-mem may.
-fomit-frame-pointer: Don't keep the frame pointer in a register for func
tions that don't need one. This avoids the instruc
tions to save, set up and restore frame pointers; it
also makes an extra register available in many func
tions. 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 automat
ically handles the frame pointer and nothing is saved
by pretending it doesn't exist. The machinedescription macro "FRAME_POINTER_REQUIRED" controls
whether a target machine supports this flag.
-foptimize-sibling-calls: Optimize sibling and tail recursive calls.
-ftrapv: This option generates traps for signed overflow on
addition, subtraction, multiplication operations.
-fno-inline: Don't pay attention to the "inline" keyword. Normally
this option is used to keep the compiler from expand
ing 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.
-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 (ie 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++.; Note: pseudo instruction represents, in this particu
lar 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: Even if all calls to a given function are integrated,
and the function is declared "static", nevertheless
output a separate run-time callable version of the
function. This switch does not affect "extern inline"
functions.
-fkeep-static-consts: Emit variables declared "static const" when optimiza
tion 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 refer
enced, 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) accross compilation
units.; This option is default for optimized compilation if
assembler and linker support it. Use -fno-merge-con stants to inhibit this behavior.
-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.
-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 sup
port such instructions, which include x86, PowerPC,
IA-64 and S/390.
-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.
-ffast-math: Sets -fno-math-errno, -funsafe-math-optimizations, and -fno-trapping-math.; 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.; 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 -fmath-errno.
-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 sim
ilar 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.
-fno-trapping-math: Compile code assuming that floating-point operations
cannot generate user-visible traps. Setting this
option may allow faster code if one relies on ``nonstop'' 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.
-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 currenly only sup
ported by the Java and Fortran 77 front-ends, where
this option defaults to true and false respectively.
The following options control specific optimizations. The
-O2 option turns on all of these optimizations except -funroll-loops and -funroll-all-loops. On most machines, the -O option turns on the -fthread-jumps and -fdelayed-branch options, but specific machines may handle it dif ferently.
You can use the following flags in the rare cases when ``fine-tuning'' of optimizations to be performed is desired.
Not all of the optimizations performed by GCC have -f options to control them.
-fstrength-reduce: Perform the optimizations of loop strength reduction
and elimination of iteration variables.
-fthread-jumps: Perform optimizations where we check to see if a jump
branches to a location where another comparison sub
sumed 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.
-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.
-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".
-frerun-cse-after-loop: Re-run common subexpression elimination after loop
optimizations has been performed.
-frerun-loop-opt: Run the loop optimizer twice.
-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 elmina
tion pass by adding -fno-gcse to the command line.
-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.
-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.
-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 opti mization for programs which depend on that behavior.
-fexpensive-optimizations: Perform a number of minor optimizations that are rela
tively expensive.
-foptimize-register-move -fregmove: Attempt to reassign register numbers in move instruc
tions 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. GCC enables this optimization by
default with -O2 or higher.; Note -fregmove and -foptimize-register-move are the same optimization.
-fdelayed-branch: If supported for the target machine, attempt to
reorder instructions to exploit instruction slots
available after delayed branch instructions.
-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.
-fschedule-insns2: Similar to -fschedule-insns, but requests an addi tional pass of instruction scheduling after register
allocation has been done. This is especially useful
on machines with a relatively small number of regis
ters and where memory load instructions take more than
one cycle.
-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 instruc
tions. 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.
-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 per
form optimizations to improve locality of reference in
the instruction space. HPPA processors running HP-UX
and Sparc processors running Solaris 2 have linkers
with such optimizations. Other systems using the ELF
object format as well as AIX may have these
optimizations in the future.; Only use these options when there are significant ben
efits 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 spec
ify this option and you may have problems with debug
ging if you specify both this option and -g.
-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.; For all machines, optimization level 2 and higher
enables this flag by default.
-funroll-loops: Unroll loops whose number of iterations can be deter
mined 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,
-fprefetch-loop-arrays: If supported by the target machine, generate instruc
tions to prefetch memory to improve the performance of
loops that access large arrays.
-fmove-all-movables: Forces all invariant computations in loops to be moved
outside the loop.
-freduce-all-givs: Forces all general-induction variables in loops to be
strength-reduced.; Note: When compiling programs written in Fortran,
-fmove-all-movables and -freduce-all-givs are enabled by default when you use the optimizer.; These options may generate better or worse code;
results are highly dependent on the structure of loops
within the source code.; These two options are intended to be removed someday,
once they have helped determine the efficacy of vari
ous approaches to improving loop optimizations.; Please let us (<gcc@gcc.gnu.org> and <for tran@gnu.org>) know how use of these options affects the performance of your production code. We're very
interested in code that runs slower when these options are enabled.
-fno-peephole -fno-peephole2: Disable any machine-specific peephole optimizations.
The difference between -fno-peephole and -fno-peep hole2 is in how they are implemented in the compiler;
some targets use one, some use the other, a few use
both.
-fbranch-probabilities: After running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-proba bilities, to improve optimizations based on the number of times each branch was taken. When the program com
piled with -fprofile-arcs exits it saves arc execution counts to a file called sourcename.da for each source file The information in this data file is very depen
dent on the structure of the generated code, so you
must use the same source code and the same optimiza
tion options for both compilations.; With -fbranch-probabilities, GCC puts a REG_EXEC_COUNT note on the first instruction of each basic block, and
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.
-fno-guess-branch-probability: Do not guess branch probabilities using a randomized
model.; Sometimes gcc will opt to use a randomized model to
guess branch probabilities, when none are available
from either profiling feedback (-fprofile-arcs) or __builtin_expect. This means that different runs of the compiler on the same program may produce different
object code.; In a hard real-time system, people don't want differ
ent runs of the compiler to produce code that has dif
ferent behavior; minimizing non-determinism is of
paramount import. This switch allows users to reduce
non-determinism, possibly at the expense of inferior
optimization.
-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 ``typepunning'') 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-spe cific 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 func tion "c_get_alias_set".
-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, use a machine-dependent
default.
-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.; 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, use a machine-dependent default
which is very likely to be 1, meaning no alignment.
-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.; If n is not specified, use a machine-dependent
default.
-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-func tions. In this case, no dummy operations need be exe cuted.; If n is not specified, use a machine-dependent
default.
-fssa: Perform optimizations in static single assignment
form. Each function's flow graph is translated into
SSA form, optimizations are performed, and the flow
graph is translated back from SSA form. Users should
not specify this option, since it is not yet ready for
production use.
-fssa-ccp: Perform Sparse Conditional Constant Propagation in SSA
form. Requires -fssa. Like -fssa, this is an experi mental feature.
-fssa-dce: Perform aggressive dead-code elimination in SSA form.
Requires -fssa. Like -fssa, this is an experimental feature.
-fsingle-precision-constant: Treat floating point constant as single precision con
stant instead of implicitly converting it to double
precision constant.
-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 pro
cessors with lots of registers. It can, however, make
debugging impossible, since variables will no longer
stay in a ``home register''.
-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.
--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.; In each case, the value is an integer. The allowable
choices for name are given in the following table:; 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 num
ber 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 com
pile 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 optimiza
tion will not be done.; max-gcse-passes
The maximum number of passes of GCSE to run.; 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
If an function contains more than this many
instructions, it will not be inlined. This option
is precisely equivalent to -finline-limit.
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 prepro cessing. 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, trans
lated or interpreted by the compiler driver before being
passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocu
mented and subject to change, so whenever possible you
should avoid using -Wp and let the driver handle the options instead.
-D name: Predefine name as a macro, with definition "1".
-D name=definition: Predefine name as a macro, with definition definition. There are no restrictions on the contents of defini_
tion, but 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). Paren
theses 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 macros. The com
mon 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 direc
tories.; It is dangerous to specify a standard system include
directory in an -I option. This defeats the special
treatment of system headers . It can also defeat the
repairs to buggy system headers which GCC makes when
it is installed.
-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 and -Wtri graphs. 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: Warn if any trigraphs are encountered. This option
used to take effect only if -trigraphs was also speci fied, but now works independently. Warnings are not
given for trigraphs within comments, as they do not
affect the meaning of the program.
-Wtraditional: Warn about certain constructs that behave differently
in traditional and ISO C. Also warn about ISO C con
structs 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.
-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 -pedan tic but treats as warnings.
-M Instead of outputting the result of preprocessing,: output a rule suitable for make describing the depen dencies 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 usi-newline. The
rule has no commands.; This option does not suppress the preprocessor's debug
output, such as -dM. To avoid mixing such debug out put 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.
-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 dou
ble 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 seman
tics from GCC versions 3.0 and earlier.
-MF file: @anchor{-MF} 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.
-MG When used with -M or -MM, -MG says to treat missing: header files as generated files and assume they live
in the same directory as the source file. It sup
presses preprocessed output, as a missing header file
is ordinarily an error.; This feature is used in automatic updating of make
files.
-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 suf
fix 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 @pxref{-MF}), 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 compi
lation process.
-MMD: Like -MD except mention only user header files, not system -header files.
-x c
-x c++
-x objective-c -x assembler-with-cpp: Specify the source language: C, C++, Objective-C, or
assembly. This has nothing to do with standards con
formance 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, 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 only knows about the standards for C;
other language standards will be added in the future.; standard may be one of:; ""iso9899:1990""
""c89""
The ISO C standard from 1990. c89 is the custom ary 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.
-I- Split the include path. Any directories specified with -I options before -I- are searched only for head ers requested with "#include "file""; they are not
searched for "#include <file>". If additional direc
tories are specified with -I options after the -I-, those directories are searched for all #include direc tives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search direc
tory for "#include "file"".
-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 preproces
sor's working directory instead of the directory con taining 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 dec
larations.
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 -iwithpre fix 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.
Use of these options is discouraged.
-isystem dir
Search dir for header files, after all directories
specified by -I but before the standard system direc
tories. Mark it as a system directory, so that it
gets the same special treatment as is applied to the
standard system directories.
-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 com
ments, 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 tok
enizer 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 cre
ated 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.
-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.
-A- Cancel all predefined assertions and all assertions preceding it on the command line. Also, undefine all
predefined macros and all macros preceding it on the
command line. (This is a historical wart and may
change in the future.)
-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 dur ing the execution of the preprocessor, including
predefined macros. This gives you a way of find
ing 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 preprocess ing. Both kinds of output go to the standard out
put 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 #.
-gcc
Define the macros __GNUC__, __GNUC_MINOR__ and
__GNUC_PATCHLEVEL__. These are defined automatically
when you use gcc -E; you can turn them off in that case with -no-gcc.
-traditional Try to imitate the behavior of old-fashioned C, as
opposed to ISO C.
-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 foso '??/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.
-$ Forbid the use of $ in identifiers. The C standard allows implementations to define extra characters that can appear in identifiers. By default GNU CPP permits $, a common extension.
-h
--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.
-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.
Options for Linking
These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.
object-file-name A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file con tents.) 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 rec
ommended.)
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 spec
ified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to func tions 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 libli_ brary.a. The linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard sys tem 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 program.
-nostartfiles Do not use the standard system startup files when
linking. The standard system libraries are used nor
mally, 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 gener ate calls to memcmp, memset, and memcpy for System V
(and ISO C) environments or to bcopy and bzero for BSD
environments. These entries are usually resolved by
entries in libc. These entry points should be sup
plied 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, and
memcpy for System V (and ISO C) environments or to
bcopy and bzero for BSD environments. 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 lan
guages.
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.)
-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.
-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 subop tions) when you specify this option.[1]
-shared-libgcc -static-libgcc On systems that provide libgcc as a shared library,
these options force the use of either the shared or
static version respectively. If no shared version of
libgcc was built when the compiler was configured,
these options have no effect.
There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the appli cation 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 typi
cally 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 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 lan
guages 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 link
ing 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 inadvertantly changed. If you
really need to change the search order for system
directories, use the -nostdinc and/or -isystem options.
-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.
-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 com
piler 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-lib/. 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 pre fix.
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 pre
fix is to use the environment variable GCC_EXEC_PRE FIX.
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.
Specifying Target Machine and Compiler Version
By default, GCC compiles code for the same type of machine that you are using. However, it can also be installed as a cross-compiler, to compile for some other type of machine. In fact, several different configurations of GCC, for different target machines, can be installed side by side. Then you specify which one to use with the -b option.
In addition, older and newer versions of GCC can be installed side by side. One of them (probably the newest) will be the default, but you may sometimes wish to use another.
-b machine
The argument machine specifies the target machine for compilation. This is useful when you have installed
GCC as a cross-compiler.
The value to use for machine is the same as was speci fied 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.
When you do not specify -b, it normally means to com pile for the same type of machine that you are using.
-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 default version, when you do not specify -V, is the last version of GCC that you installed.
The -b and -V options actually work by controlling part of the file name used for the executable files and libraries used for compilation. A given version of GCC, for a given target machine, is normally kept in the directory /usr/local/lib/gcc-lib/machine/version.
Thus, sites can customize the effect of -b or -V either by changing the names of these directories or adding alter
nate names (or symbolic links). If in directory
/usr/local/lib/gcc-lib/ the file 80386 is a link to the file i386v, then -b 80386 becomes an alias for -b i386v.
In one respect, the -b or -V do not completely change to a different compiler: the top-level driver program gcc that you originally invoked continues to run and invoke the
other executables (preprocessor, compiler per se, assem
bler and linker) that do the real work. However, since no
real work is done in the driver program, it usually does
not matter that the driver program in use is not the one
for the specified target. It is common for the interface
to the other executables to change incompatibly between
compiler versions, so unless the version specified is very
close to that of the driver (for example, -V 3.0 with a driver program from GCC version 3.0.1), use of -V may not
work; for example, using -V 2.95.2 will not work with a driver program from GCC 3.0.
The only way that the driver program depends on the target machine is in the parsing and handling of special machinespecific options. However, this is controlled by a file which is found, along with the other executables, in the directory for the specified version and target machine. As a result, a single installed driver program adapts to any specified target machine, and sufficiently similar compiler versions.
The driver program executable does control one significant thing, however: the default version and target machine. Therefore, you can install different instances of the driver program, compiled for different targets or ver sions, under different names.
For example, if the driver for version 2.0 is installed as
ogcc and that for version 2.1 is installed as gcc, then the command gcc will use version 2.1 by default, while ogcc will use 2.0 by default. However, you can choose either version with either command with the -V option.
Hardware Models and Configurations
Earlier we discussed the standard option -b which chooses among different installed compilers for completely differ ent 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 speci fied.
Some configurations of the compiler also support addi tional 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.
M680x0 Options
These are the -m options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was config ured; the defaults for the most common choices are given below.
-m68000
-mc68000
Generate output for a 68000. This is the default when
the compiler is configured for 68000-based systems.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68020
-mc68020
Generate output for a 68020. This is the default when
the compiler is configured for 68020-based systems.
-m68881
Generate output containing 68881 instructions for
floating point. This is the default for most 68020
systems unless --nfp was specified when the compiler was configured.
-m68030
Generate output for a 68030. This is the default when
the compiler is configured for 68030-based systems.
-m68040
Generate output for a 68040. This is the default when
the compiler is configured for 68040-based systems.
This option inhibits the use of 68881/68882 instruc tions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when
the compiler is configured for 68060-based systems.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when
the compiler is configured for CPU32-based systems.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.
-m5200
Generate output for a 520X ``coldfire'' family cpu.
This is the default when the compiler is configured
for 520X-based systems.
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5202.
-m68020-40 Generate output for a 68040, without using any of the
new instructions. This results in code which can run
relatively efficiently on either a 68020/68881 or a
68030 or a 68040. The generated code does use the
68881 instructions that are emulated on the 68040.
-m68020-60 Generate output for a 68060, without using any of the
new instructions. This results in code which can run
relatively efficiently on either a 68020/68881 or a
68030 or a 68040. The generated code does use the
68881 instructions that are emulated on the 68060.
-mfpa
Generate output containing Sun FPA instructions for
floating point.
-msoft-float Generate output containing library calls for floating
point. Warning: the requisite libraries are not available for all m68k targets. Normally the facili
ties 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. The embedded
targets m68k-*-aout and m68k-*-coff do provide soft ware floating point support.
-mshort
Consider type "int" to be 16 bits wide, like "short
int".
-mnobitfield Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
-mbitfield Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a configuration designed for a 68020.
-mrtd
Use a different function-calling convention, in which
functions that take a fixed number of arguments return
with the "rtd" instruction, which pops their arguments
while returning. This saves one instruction in the
caller since there is no need to pop the arguments
there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix com piler.
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. (Nor mally, extra arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
-malign-int -mno-align-int Control whether GCC aligns "int", "long", "long long",
"float", "double", and "long double" variables on a
32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit bound aries produces code that runs somewhat faster on pro
cessors with 32-bit busses at the expense of more mem
ory.
Warning: if you use the -malign-int switch, GCC will align structures containing the above types differ
ently than most published application binary interface
specifications for the m68k.
-mpcrel
Use the pc-relative addressing mode of the 68000
directly, instead of using a global offset table. At
present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align -mstrict-align Do not (do) assume that unaligned memory references
will be handled by the system.
M68hc1x Options
These are the -m options defined for the 68hc11 and 68hc12 microcontrollers. The default values for these options depends on which style of microcontroller was selected when the compiler was configured; the defaults for the most common choices are given below.
-m6811
-m68hc11
Generate output for a 68HC11. This is the default
when the compiler is configured for 68HC11-based sys
tems.
-m6812
-m68hc12
Generate output for a 68HC12. This is the default
when the compiler is configured for 68HC12-based sys
tems.
-mauto-incdec Enable the use of 68HC12 pre and post auto-increment
and auto-decrement addressing modes.
-mshort
Consider type "int" to be 16 bits wide, like "short
int".
-msoft-reg-count=count Specify the number of pseudo-soft registers which are
used for the code generation. The maximum number is
32. Using more pseudo-soft register may or may not
result in better code depending on the program. The
default is 4 for 68HC11 and 2 for 68HC12.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and
so on) that the Unix assembler for the VAX cannot han
dle across long ranges.
-mgnu
Do output those jump instructions, on the assumption
that you will assemble with the GNU assembler.
-mg Output code for g-format floating point numbers
instead of d-format.
SPARC Options
These -m switches are supported on the SPARC:
-mno-app-regs -mapp-regs Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI
reserves for applications. This is the default.
To be fully SVR4 ABI compliant at the cost of some
performance loss, specify -mno-app-regs. You should compile libraries and system software with this
option.
-mfpu
-mhard-float Generate output containing floating point instruc
tions. This is the default.
-mno-fpu
-msoft-float Generate output containing library calls for floating
point. Warning: the requisite libraries are not available for all SPARC targets. Normally the facili
ties 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 suit
able library functions for cross-compilation. The
embedded targets sparc-*-aout and sparclite-*-* do provide software floating point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you com
pile all of a program with this option. In particu
lar, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.
-mhard-quad-float Generate output containing quad-word (long double)
floating point instructions.
-msoft-quad-float Generate output containing library calls for quad-word
(long double) floating point instructions. The func
tions called are those specified in the SPARC ABI.
This is the default.
As of this writing, there are no sparc implementations
that have hardware support for the quad-word floating
point instructions. They all invoke a trap handler
for one of these instructions, and then the trap han
dler emulates the effect of the instruction. Because
of the trap handler overhead, this is much slower than
calling the ABI library routines. Thus the -msoft-quad-float option is the default.
-mno-flat
-mflat
With -mflat, the compiler does not generate save/restore instructions and will use a ``flat'' or
single register window calling convention. This model
uses %i7 as the frame pointer and is compatible with
the normal register window model. Code from either
may be intermixed. The local registers and the input
registers (0--5) are still treated as ``call saved''
registers and will be saved on the stack as necessary.
With -mno-flat (the default), the compiler emits save/restore instructions (except for leaf functions)
and is the normal mode of operation.
-mno-unaligned-doubles -munaligned-doubles Assume that doubles have 8 byte alignment. This is
the default.
With -munaligned-doubles, GCC assumes that doubles have 8 byte alignment only if they are contained in
another type, or if they have an absolute address.
Otherwise, it assumes they have 4 byte alignment.
Specifying this option avoids some rare compatibility
problems with code generated by other compilers. It
is not the default because it results in a performance
loss, especially for floating point code.
-mno-faster-structs -mfaster-structs With -mfaster-structs, the compiler assumes that structures should have 8 byte alignment. This enables
the use of pairs of "ldd" and "std" instructions for
copies in structure assignment, in place of twice as
many "ld" and "st" pairs. However, the use of this
changed alignment directly violates the Sparc ABI.
Thus, it's intended only for use on targets where the
developer acknowledges that their resulting code will
not be directly in line with the rules of the ABI.
-mv8
-msparclite These two options select variations on the SPARC
architecture.
By default (unless specifically configured for the Fujitsu SPARClite), GCC generates code for the v7 variant of the SPARC architecture.
-mv8 will give you SPARC v8 code. The only difference from v7 code is that the compiler emits the integer
multiply and integer divide instructions which exist
in SPARC v8 but not in SPARC v7.
-msparclite will give you SPARClite code. This adds the integer multiply, integer divide step and scan
("ffs") instructions which exist in SPARClite but not
in SPARC v7.
These options are deprecated and will be deleted in a
future GCC release. They have been replaced with
-mcpu=xxx.
-mcypress
-msupersparc These two options select the processor for which the
code is optimized.
With -mcypress (the default), the compiler optimizes code for the Cypress CY7C602 chip, as used in the
SparcStation/SparcServer 3xx series. This is also
appropriate for the older SparcStation 1, 2, IPX etc.
With -msupersparc the compiler optimizes code for the SuperSparc cpu, as used in the SparcStation 10, 1000
and 2000 series. This flag also enables use of the
full SPARC v8 instruction set.
These options are deprecated and will be deleted in a
future GCC release. They have been replaced with
-mcpu=xxx.
-mcpu=cpu_type
Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type. Sup ported values for cpu_type are v7, cypress, v8, super sparc, sparclite, hypersparc, sparclite86x, f930, f934, sparclet, tsc701, v9, and ultrasparc.
Default instruction scheduling parameters are used for
values that select an architecture and not an imple
mentation. These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported implementations.: v7: cypress
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc
-mtune=cpu_type Set the instruction scheduling parameters for machine
type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type would.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular cpu implementation. Those
are cypress, supersparc, hypersparc, f930, f934, spar clite86x, tsc701, and ultrasparc.
These -m switches are supported in addition to the above on the SPARCLET processor.
-mlittle-endian Generate code for a processor running in little-endian
mode.
-mlive-g0
Treat register "%g0" as a normal register. GCC will
continue to clobber it as necessary but will not
assume it always reads as 0.
-mbroken-saverestore Generate code that does not use non-trivial forms of
the "save" and "restore" instructions. Early versions
of the SPARCLET processor do not correctly handle
"save" and "restore" instructions used with arguments.
They correctly handle them used without arguments. A
"save" instruction used without arguments increments
the current window pointer but does not allocate a new
stack frame. It is assumed that the window overflow
trap handler will properly handle this case as will
interrupt handlers.
These -m switches are supported in addition to the above on SPARC V9 processors in 64-bit environments.
-mlittle-endian Generate code for a processor running in little-endian
mode.
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The
32-bit environment sets int, long and pointer to 32
bits. The 64-bit environment sets int to 32 bits and
long and pointer to 64 bits.
-mcmodel=medlow Generate code for the Medium/Low code model: the pro
gram must be linked in the low 32 bits of the address
space. Pointers are 64 bits. Programs can be stati
cally or dynamically linked.
-mcmodel=medmid Generate code for the Medium/Middle code model: the
program must be linked in the low 44 bits of the
address space, the text segment must be less than 2G
bytes, and data segment must be within 2G of the text
segment. Pointers are 64 bits.
-mcmodel=medany Generate code for the Medium/Anywhere code model: the
program may be linked anywhere in the address space,
the text segment must be less than 2G bytes, and data
segment must be within 2G of the text segment. Point
ers are 64 bits.
-mcmodel=embmedany Generate code for the Medium/Anywhere code model for
embedded systems: assume a 32-bit text and a 32-bit
data segment, both starting anywhere (determined at
link time). Register %g4 points to the base of the
data segment. Pointers are still 64 bits. Programs
are statically linked, PIC is not supported.
-mstack-bias -mno-stack-bias With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047
which must be added back when making stack frame ref
erences. Otherwise, assume no such offset is present.
Convex Options
These -m options are defined for Convex:
-mc1
Generate output for C1. The code will run on any Con
vex machine. The preprocessor symbol "__convex__c1__"
is defined.
-mc2
Generate output for C2. Uses instructions not avail
able on C1. Scheduling and other optimizations are
chosen for max performance on C2. The preprocessor
symbol "__convex_c2__" is defined.
-mc32
Generate output for C32xx. Uses instructions not
available on C1. Scheduling and other optimizations
are chosen for max performance on C32. The preproces
sor symbol "__convex_c32__" is defined.
-mc34
Generate output for C34xx. Uses instructions not
available on C1. Scheduling and other optimizations
are chosen for max performance on C34. The preproces
sor symbol "__convex_c34__" is defined.
-mc38
Generate output for C38xx. Uses instructions not
available on C1. Scheduling and other optimizations
are chosen for max performance on C38. The preproces
sor symbol "__convex_c38__" is defined.
-margcount Generate code which puts an argument count in the word
preceding each argument list. This is compatible with
regular CC, and a few programs may need the argument
count word. GDB and other source-level debuggers do
not need it; this info is in the symbol table.
-mnoargcount Omit the argument count word. This is the default.
-mvolatile-cache Allow volatile references to be cached. This is the
default.
-mvolatile-nocache Volatile references bypass the data cache, going all
the way to memory. This is only needed for multi-pro
cessor code that does not use standard synchronization
instructions. Making non-volatile references to
volatile locations will not necessarily work.
-mlong32
Type long is 32 bits, the same as type int. This is
the default.
-mlong64
Type long is 64 bits, the same as type long long.
This option is useless, because no library support
exists for it.
AMD29K Options
These -m options are defined for the AMD Am29000:
-mdw
Generate code that assumes the "DW" bit is set, i.e.,
that byte and halfword operations are directly sup
ported by the hardware. This is the default.
-mndw
Generate code that assumes the "DW" bit is not set.
-mbw
Generate code that assumes the system supports byte
and halfword write operations. This is the default.
-mnbw
Generate code that assumes the systems does not sup
port byte and halfword write operations. -mnbw implies -mndw.
-msmall
Use a small memory model that assumes that all func
tion addresses are either within a single 256 KB seg
ment or at an absolute address of less than 256k.
This allows the "call" instruction to be used instead
of a "const", "consth", "calli" sequence.
-mnormal
Use the normal memory model: Generate "call" instruc
tions only when calling functions in the same file and
"calli" instructions otherwise. This works if each
file occupies less than 256 KB but allows the entire
executable to be larger than 256 KB. This is the
default.
-mlarge
Always use "calli" instructions. Specify this option
if you expect a single file to compile into more than
256 KB of code.
-m29050
Generate code for the Am29050.
-m29000
Generate code for the Am29000. This is the default.
-mkernel-registers Generate references to registers "gr64-gr95" instead
of to registers "gr96-gr127". This option can be used
when compiling kernel code that wants a set of global
registers disjoint from that used by user-mode code.
Note that when this option is used, register names in -f flags must use the normal, user-mode, names.
-muser-registers Use the normal set of global registers, "gr96-gr127".
This is the default.
-mstack-check -mno-stack-check Insert (or do not insert) a call to "__msp_check"
after each stack adjustment. This is often used for
kernel code.
-mstorem-bug
-mno-storem-bug -mstorem-bug handles 29k processors which cannot han dle the separation of a mtsrim insn and a storem
instruction (most 29000 chips to date, but not the
29050).
-mno-reuse-arg-regs -mreuse-arg-regs -mno-reuse-arg-regs tells the compiler to only use incoming argument registers for copying out arguments.
This helps detect calling a function with fewer argu
ments than it was declared with.
-mno-impure-text -mimpure-text -mimpure-text, used in addition to -shared, tells the compiler to not pass -assert pure-text to the linker when linking a shared object.
-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.
-mno-multm Do not generate multm or multmu instructions. This is
useful for some embedded systems which do not have
trap handlers for these instructions.
ARM Options
These -m options are defined for Advanced RISC Machines (ARM) architectures:
-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 gen
erated for leaf functions. The default is -mno-apcs-frame.
-mapcs
This is a synonym for -mapcs-frame.
-mapcs-26
Generate code for a processor running with a 26-bit
program counter, and conforming to the function call
ing standards for the APCS 26-bit option. This option
replaces the -m2 and -m3 options of previous releases of the compiler.
-mapcs-32
Generate code for a processor running with a 32-bit
program counter, and conforming to the function call
ing standards for the APCS 32-bit option. This option
replaces the -m6 option of previous releases of the compiler.
-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 infor
mation 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 instruc
tions. 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 facili
ties 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 suit
able library functions for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you com
pile all of a program with this option. In particu
lar, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.
-mlittle-endian Generate code for a processor running in little-endian
mode. This is the default for all standard configura
tions.
-mbig-endian Generate code for a processor running in big-endian
mode; the default is to compile code for a littleendian processor.
-mwords-little-endianThis option only applies when generating code for bigendian 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 ver
sions of the compiler prior to 2.8.
-malignment-traps Generate code that will not trap if the MMU has align
ment traps enabled. On ARM architectures prior to
ARMv4, there were no instructions to access half-word
objects stored in memory. However, when reading from
memory a feature of the ARM architecture allows a word
load to be used, even if the address is unaligned, and
the processor core will rotate the data as it is being
loaded. This option tells the compiler that such mis
aligned accesses will cause a MMU trap and that it
should instead synthesise the access as a series of
byte accesses. The compiler can still use word
accesses to load half-word data if it knows that the
address is aligned to a word boundary.
This option is ignored when compiling for ARM archi tecture 4 or later, since these processors have instructions to directly access half-word objects in memory.
-mno-alignment-traps Generate code that assumes that the MMU will not trap
unaligned accesses. This produces better code when
the target instruction set does not have half-word
memory operations (i.e. implementations prior to
ARMv4).
Note that you cannot use this option to access unaligned word objects, since the processor will only fetch one 32-bit aligned object from memory.
The default setting for most targets is -mno-align ment-traps, since this produces better code when there are no half-word memory instructions available.
-mshort-load-bytes -mno-short-load-words These are deprecated aliases for -malignment-traps.
-mno-short-load-bytes -mshort-load-words This are deprecated aliases for -mno-alignment-traps.
-mbsd
This option only applies to RISC iX. Emulate the
native BSD-mode compiler. This is the default if
-ansi is not specified.
-mxopen
This option only applies to RISC iX. Emulate the
native X/Open-mode compiler.
-mno-symrename This option only applies to RISC iX. Do not run the
assembler post-processor, symrename, after code has been assembled. Normally it is necessary to modify
some of the standard symbols in preparation for link
ing with the RISC iX C library; this option suppresses
this pass. The post-processor is never run when the
compiler is built for cross-compilation.
-mcpu=name
This specifies the name of the target ARM processor.
GCC uses this name to determine what kind of instruc
tions it can emit when generating assembly code. Per
missible 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, arm8, strongarm, strongarm110, strongarm1100, arm8, arm810, arm9, arm9e, arm920, arm920t, arm940t, arm9tdmi, arm10tdmi, arm1020t, xscale.
-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 instruc
tions 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 imple mentations better performance can be obtained by using
this option.
-march=name
This specifies the name of the target ARM architec
ture. 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.
-mfpe=number
-mfp=number
This specifies the version of the floating point emu
lation available on the target. Permissible values
are 2 and 3. -mfp= is a synonym for -mfpe=, for com patibility with older versions of GCC.
-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 and 32. The default
value varies for different toolchains. For the COFF
targeted toolchain the default value is 8. Specifying
the larger number can produce faster, more efficient
code, but can also increase the size of the program.
The two values are potentially incompatible. Code
compiled with one value cannot necessarily expect to
work with code or libraries compiled with the other
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 regis
ter. 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 inside the scope of a
#pragma no_long_calls directive and functions whose definitions have already been compiled within the cur
rent compilation unit, will not be turned into long
calls. The exception to this rule is that weak func
tion definitions, functions with the long-call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls direc tive, 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 calls within the scope of a
#pragma long_calls_off directive. Note 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-baseTreat the register used for PIC addressing as readonly, rather than loading it in the prologue for each
function. The run-time system is responsible for ini
tializing 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.
-mpoke-function-name Write the name of each function into the text section,
directly preceding the function prologue. The gener
ated 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 func
tions. (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 execut
ing a function pointer if this option is enabled.
MN10200 Options
These -m options are defined for Matsushita MN10200 archi tectures:
-mrelax
Indicate to the linker that it should perform a relax
ation optimization pass to shorten branches, calls and
absolute memory addresses. This option only has an
effect when used on the command line for the final
link step.
This option makes symbolic debugging impossible.
MN10300 Options
These -m options are defined for Matsushita MN10300 archi tectures:
-mmult-bug Generate code to avoid bugs in the multiply instruc
tions for the MN10300 processors. This is the
default.
-mno-mult-bug Do not generate code to avoid bugs in the multiply
instructions for the MN10300 processors.
-mam33
Generate code which uses features specific to the AM33
processor.
-mno-am33
Do not generate code which uses features specific to
the AM33 processor. This is the default.
-mno-crt0
Do not link in the C run-time initialization object
file.
-mrelax
Indicate to the linker that it should perform a relax
ation optimization pass to shorten branches, calls and
absolute memory addresses. This option only has an
effect when used on the command line for the final
link step.
This option makes symbolic debugging impossible.
M32R/D Options
These -m options are defined for Mitsubishi M32R/D archi tectures:
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mcode-model=small Assume all objects live in the lower 16MB of memory
(so that their addresses can be loaded with the "ld24"
instruction), and assume all subroutines are reachable
with the "bl" instruction. This is the default.
The addressability of a particular object can be set with the "model" attribute.
-mcode-model=medium Assume objects may be anywhere in the 32-bit address
space (the compiler will generate "seth/add3" instruc
tions to load their addresses), and assume all subrou
tines are reachable with the "bl" instruction.
-mcode-model=large Assume objects may be anywhere in the 32-bit address
space (the compiler will generate "seth/add3" instruc
tions to load their addresses), and assume subroutines
may not be reachable with the "bl" instruction (the
compiler will generate the much slower "seth/add3/jl"
instruction sequence).
-msdata=none Disable use of the small data area. Variables will be
put into one of .data, bss, or .rodata (unless the "section" attribute has been specified). This is the
default.
The small data area consists of sections .sdata and .sbss. Objects may be explicitly put in the small data area with the "section" attribute using one of
these sections.
-msdata=sdata Put small global and static data in the small data
area, but do not generate special code to reference
them.
-msdata=use Put small global and static data in the small data
area, and generate special instructions to reference
them.
-G num
Put global and static objects less than or equal to
num bytes into the small data or bss sections instead
of the normal data or bss sections. The default value
of num is 8. The -msdata option must be set to one of sdata or use for this option to have any effect.
All modules should be compiled with the same -G num value. Compiling with different values of num may or
may not work; if it doesn't the linker will give an
error message---incorrect code will not be generated.
M88K Options
These -m options are defined for Motorola 88k architec tures:
-m88000
Generate code that works well on both the m88100 and
the m88110.
-m88100
Generate code that works best for the m88100, but that
also runs on the m88110.
-m88110
Generate code that works best for the m88110, and may
not run on the m88100.
-mbig-pic
Obsolete option to be removed from the next revision.
Use -fPIC.
-midentify-revision Include an "ident" directive in the assembler output
recording the source file name, compiler name and ver
sion, timestamp, and compilation flags used.
-mno-underscores In assembler output, emit symbol names without adding
an underscore character at the beginning of each name.
The default is to use an underscore as prefix on each
name.
-mocs-debug-info -mno-ocs-debug-info Include (or omit) additional debugging information
(about registers used in each stack frame) as speci
fied in the 88open Object Compatibility Standard,
``OCS''. This extra information allows debugging of
code that has had the frame pointer eliminated. The
default for DG/UX, SVr4, and Delta 88 SVr3.2 is to
include this information; other 88k configurations
omit this information by default.
-mocs-frame-position When emitting COFF debugging information for automatic
variables and parameters stored on the stack, use the
offset from the canonical frame address, which is the
stack pointer (register 31) on entry to the function.
The DG/UX, SVr4, Delta88 SVr3.2, and BCS configura
tions use -mocs-frame-position; other 88k configura tions have the default -mno-ocs-frame-position.
-mno-ocs-frame-position When emitting COFF debugging information for automatic
variables and parameters stored on the stack, use the
offset from the frame pointer register (register 30).
When this option is in effect, the frame pointer is
not eliminated when debugging information is selected
by the -g switch.
-moptimize-arg-area Save space by reorganizing the stack frame. This
option generates code that does not agree with the
88open specifications, but uses less memory.
-mno-optimize-arg-area Do not reorganize the stack frame to save space. This
is the default. The generated conforms to the speci
fication, but uses more memory.
-mshort-data-num Generate smaller data references by making them rela
tive to "r0", which allows loading a value using a
single instruction (rather than the usual two). You
control which data references are affected by specify
ing num with this option. For example, if you specify
-mshort-data-512, then the data references affected are those involving displacements of less than 512
bytes. -mshort-data-num is not effective for num greater than 64k.
-mserialize-volatile -mno-serialize-volatile Do, or don't, generate code to guarantee sequential
consistency of volatile memory references. By
default, consistency is guaranteed.
The order of memory references made by the MC88110 processor does not always match the order of the instructions requesting those references. In particu lar, a load instruction may execute before a preceding store instruction. Such reordering violates sequen tial consistency of volatile memory references, when there are multiple processors. When consistency must be guaranteed, GCC generates special instructions, as needed, to force execution in the proper order.
The MC88100 processor does not reorder memory refer
ences and so always provides sequential consistency.
However, by default, GCC generates the special
instructions to guarantee consistency even when you
use -m88100, so that the code may be run on an MC88110 processor. If you intend to run your code only on the
MC88100 processor, you may use -mno-serialize-volatile.
The extra code generated to guarantee consistency may
affect the performance of your application. If you
know that you can safely forgo this guarantee, you may
use -mno-serialize-volatile.
-msvr4
-msvr3
Turn on (-msvr4) or off (-msvr3) compiler extensions related to System V release 4 (SVr4). This controls
the following:
1. Which variant of the assembler syntax to emit.
2. -msvr4 makes the C preprocessor recognize #pragma weak that is used on System V release 4.
3. -msvr4 makes GCC issue additional declaration directives used in SVr4.
-msvr4 is the default for the m88k-motorola-sysv4 and m88k-dg-dgux m88k configurations. -msvr3 is the default for all other m88k configurations.
-mversion-03.00 This option is obsolete, and is ignored.
-mno-check-zero-division
-mcheck-zero-division Do, or don't, generate code to guarantee that integer
division by zero will be detected. By default, detec
tion is guaranteed.
Some models of the MC88100 processor fail to trap upon
integer division by zero under certain conditions. By
default, when compiling code that might be run on such
a processor, GCC generates code that explicitly checks
for zero-valued divisors and traps with exception num
ber 503 when one is detected. Use of -mno-check-zero-division suppresses such checking for code generated to run on an MC88100 processor.
GCC assumes that the MC88110 processor correctly
detects all instances of integer division by zero.
When -m88110 is specified, no explicit checks for zero-valued divisors are generated, and both -mcheck-zero-division and -mno-check-zero-division are ignored.
-muse-div-instruction Use the div instruction for signed integer division on
the MC88100 processor. By default, the div instruc
tion is not used.
On the MC88100 processor the signed integer division instruction div) traps to the operating system on a negative operand. The operating system transparently completes the operation, but at a large cost in execu tion time. By default, when compiling code that might be run on an MC88100 processor, GCC emulates signed integer division using the unsigned integer division instruction divu), thereby avoiding the large penalty of a trap to the operating system. Such emulation has its own, smaller, execution cost in both time and space. To the extent that your code's important signed integer division operations are performed on two nonnegative operands, it may be desirable to use the div instruction directly.
On the MC88110 processor the div instruction (also
known as the divs instruction) processes negative
operands without trapping to the operating system.
When -m88110 is specified, -muse-div-instruction is ignored, and the div instruction is used for signed
integer division.
Note that the result of dividing "INT_MIN" by -1 is
undefined. In particular, the behavior of such a
division with and without -muse-div-instruction may differ.
-mtrap-large-shift -mhandle-large-shift Include code to detect bit-shifts of more than 31
bits; respectively, trap such shifts or emit code to
handle them properly. By default GCC makes no special
provision for large bit shifts.
-mwarn-passed-structs Warn when a function passes a struct as an argument or
result. Structure-passing conventions have changed
during the evolution of the C language, and are often
the source of portability problems. By default, GCC
issues no such warning.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and Pow erPC:
-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 Pow_
erPC 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. How ever there is a large common subset of instructions supported by both. An MQ register is included in pro cessors 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 con
figuring 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 instruc tions 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 Gen
eral Purpose group, including floating-point square
root. Specifying -mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use the optional PowerPC archi
tecture 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 assem
bler code. With -mnew-mnemonics, GCC uses the assem bler 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 some times 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 rios, rios1, rsc, rios2, rs64a, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, power, power2, powerpc, 403, 505, 801, 821, 823, and 860 and common.
-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 instruc
tions 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 pur
poses.
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 other -m options as follows:
common
-mno-power, -mno-powerc
power
power2
rios1
rios2
rsc -mpower, -mno-powerpc, -mno-new-mnemonics
powerpc
rs64a
602
603
603e
604
620
630
740
7400
7450
750
505 -mno-power, -mpowerpc, -mnew-mnemonics
601 -mpower, -mpowerpc, -mnew-mnemonics
403
821
860 -mno-power, -mpowerpc, -mnew-mnemonics, -msoft- float
-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 speci fied, the code generated will use the architecture,
registers, and mnemonics set by -mcpu, but the scheduling parameters set by -mtune.
-maltivec
-mno-altivec These switches enable or disable the use of built-in
functions that allow access to the AltiVec instruction
set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.
-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 avail
able 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-call
-mno-xl-call On AIX, pass floating-point arguments to prototyped
functions beyond the register save area (RSA) on the
stack in addition to argument FPRs. 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 stan
dard 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.
-msoft-float -mhard-floatGenerate code that does not use (uses) the floatingpoint register set. Software floating point emulation
is provided if you use the -msoft-float option, and pass the option to GCC when linking.
-mmultiple -mno-multiple Generate code that uses (does not use) the load multi
ple 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 Pow erPC 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 han
dled by the system.
-mrelocatable -mno-relocatable On embedded PowerPC systems generate code that allows
(does not allow) the program to be relocated to a dif
ferent 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 dif
ferent 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.
-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 pow
erpc-*-eabiaix.
-mcall-sysv-eabi Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi Specify both -mcall-sysv and -mno-eabi options.
-mcall-aix On System V.4 and embedded PowerPC systems compile
code using calling conventions that are similar to
those used on AIX. This is the default if you config
ured GCC using powerpc-*-eabiaix.
-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.
-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 incom patible 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, other wise 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 com
piled 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.
-pthread
Adds support for multithreading with the pthreads
library. This option sets flags for both the prepro
cessor and linker.
IBM RT Options
These -m options are defined for the IBM RT PC:
-min-line-mul Use an in-line code sequence for integer multiplies.
This is the default.
-mcall-lib-mul Call "lmul$$" for integer multiples.
-mfull-fp-blocks Generate full-size floating point data blocks, includ
ing the minimum amount of scratch space recommended by
IBM. This is the default.
-mminimum-fp-blocks Do not include extra scratch space in floating point
data blocks. This results in smaller code, but slower
execution, since scratch space must be allocated
dynamically.
-mfp-arg-in-fpregs Use a calling sequence incompatible with the IBM call
ing convention in which floating point arguments are
passed in floating point registers. Note that
"varargs.h" and "stdarg.h" will not work with floating
point operands if this option is specified.
-mfp-arg-in-gregs Use the normal calling convention for floating point
arguments. This is the default.
-mhc-struct-return Return structures of more than one word in memory,
rather than in a register. This provides compatibil
ity with the MetaWare HighC (hc) compiler. Use the
option -fpcc-struct-return for compatibility with the Portable C Compiler (pcc).
-mnohc-struct-return Return some structures of more than one word in regis
ters, when convenient. This is the default. For com
patibility with the IBM-supplied compilers, use the
option -fpcc-struct-return or the option -mhc-struct-return.
MIPS Options
These -m options are defined for the MIPS family of com puters:
-march=cpu-type Assume the defaults for the machine type cpu-type when generating instructions. The choices for cpu-type are r2000, r3000, r3900, r4000, r4100, r4300, r4400, r4600, r4650, r5000, r6000, r8000, and orion. Addi tionally, the r2000, r3000, r4000, r5000, and r6000 can be abbreviated as r2k (or r2K), r3k, etc.
-mtune=cpu-type Assume the defaults for the machine type cpu-type when scheduling instructions. The choices for cpu-type are r2000, r3000, r3900, r4000, r4100, r4300, r4400, r4600, r4650, r5000, r6000, r8000, and orion. Addi tionally, the r2000, r3000, r4000, r5000, and r6000 can be abbreviated as r2k (or r2K), r3k, etc. While picking a specific cpu-type will schedule things
appropriately for that particular chip, the compiler
will not generate any code that does not meet level 1
of the MIPS ISA (instruction set architecture) without
a -mipsX or -mabi switch being used.
-mcpu=cpu-type
This is identical to specifying both -march and -mtune.
-mips1
Issue instructions from level 1 of the MIPS ISA. This
is the default. r3000 is the default cpu-type at this ISA level.
-mips2
Issue instructions from level 2 of the MIPS ISA
(branch likely, square root instructions). r6000 is the default cpu-type at this ISA level.
-mips3
Issue instructions from level 3 of the MIPS ISA
(64-bit instructions). r4000 is the default cpu-type at this ISA level.
-mips4
Issue instructions from level 4 of the MIPS ISA (con
ditional move, prefetch, enhanced FPU instructions).
r8000 is the default cpu-type at this ISA level.
-mfp32
Assume that 32 32-bit floating point registers are
available. This is the default.
-mfp64
Assume that 32 64-bit floating point registers are
available. This is the default when the -mips3 option is used.
-mfused-madd -mno-fused-madd Generate code that uses (does not use) the floating
point multiply and accumulate instructions, when they
are available. These instructions are generated by
default if they are available, but this may be unde
sirable if the extra precision causes problems or on
certain chips in the mode where denormals are rounded
to zero where denormals generated by multiply and
accumulate instructions cause exceptions anyway.
-mgp32
Assume that 32 32-bit general purpose registers are
available. This is the default.
-mgp64
Assume that 32 64-bit general purpose registers are
available. This is the default when the -mips3 option is used.
-mint64
Force int and long types to be 64 bits wide. See
-mlong32 for an explanation of the default, and the width of pointers.
-mlong64
Force long types to be 64 bits wide. See -mlong32 for an explanation of the default, and the width of point
ers.
-mlong32
Force long, int, and pointer types to be 32 bits wide.
If none of -mlong32, -mlong64, or -mint64 are set, the size of ints, longs, and pointers depends on the ABI
and ISA chosen. For -mabi=32, and -mabi=n32, ints and longs are 32 bits wide. For -mabi=64, ints are 32 bits, and longs are 64 bits wide. For -mabi=eabi and either -mips1 or -mips2, ints and longs are 32 bits wide. For -mabi=eabi and higher ISAs, ints are 32 bits, and longs are 64 bits wide. The width of
pointer types is the smaller of the width of longs or
the width of general purpose registers (which in turn
depends on the ISA).
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi Generate code for the indicated ABI. The default
instruction level is -mips1 for 32, -mips3 for n32, and -mips4 otherwise. Conversely, with -mips1 or -mips2, the default ABI is 32; otherwise, the default ABI is 64.
-mmips-asGenerate code for the MIPS assembler, and invoke mipstfile to add normal debug information. This is the
default for all platforms except for the OSF/1 refer
ence platform, using the OSF/rose object format. If
the either of the -gstabs or -gstabs+ switches are used, the mips-tfile program will encapsulate the stabs within MIPS ECOFF.
-mgas
Generate code for the GNU assembler. This is the
default on the OSF/1 reference platform, using the
OSF/rose object format. Also, this is the default if
the configure option --with-gnu-as is used.
-msplit-addresses -mno-split-addresses Generate code to load the high and low parts of
address constants separately. This allows GCC to
optimize away redundant loads of the high order bits
of addresses. This optimization requires GNU as and
GNU ld. This optimization is enabled by default for
some embedded targets where GNU as and GNU ld are
standard.
-mrnames
-mno-rnames The -mrnames switch says to output code using the MIPS software names for the registers, instead of the hard
ware names (ie, a0 instead of $4). The only known
assembler that supports this option is the Algorith
mics assembler.
-mgpopt
-mno-gpopt The -mgpopt switch says to write all of the data dec larations before the instructions in the text section,
this allows the MIPS assembler to generate one word
memory references instead of using two words for short
global or static data items. This is on by default if
optimization is selected.
-mstats
-mno-stats For each non-inline function processed, the -mstats switch causes the compiler to emit one line to the
standard error file to print statistics about the pro
gram (number of registers saved, stack size, etc.).
-mmemcpy
-mno-memcpy The -mmemcpy switch makes all block moves call the appropriate string function (memcpy or bcopy) instead of possibly generating inline code.
-mmips-tfile -mno-mips-tfile The -mno-mips-tfile switch causes the compiler not postprocess the object file with the mips-tfile pro gram, after the MIPS assembler has generated it to add
debug support. If mips-tfile is not run, then no local variables will be available to the debugger. In
addition, stage2 and stage3 objects will have the tem porary file names passed to the assembler embedded in
the object file, which means the objects will not com
pare the same. The -mno-mips-tfile switch should only be used when there are bugs in the mips-tfile program that prevents compilation.
-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.
-mhard-float Generate output containing floating point instruc
tions. This is the default if you use the unmodified
sources.
-mabicalls -mno-abicalls Emit (or do not emit) the pseudo operations .abicalls, .cpload, and .cprestore that some System V.4 ports use for position independent code.
-mlong-calls -mno-long-calls Do all calls with the JALR instruction, which requires loading up a function's address into a register before
the call. You need to use this switch, if you call
outside of the current 512 megabyte segment to func
tions that are not through pointers.
-mhalf-pic -mno-half-pic Put pointers to extern references into the data sec
tion and load them up, rather than put the references
in the text section.
-membedded-pic -mno-embedded-pic Generate PIC code suitable for some embedded systems.
All calls are made using PC relative address, and all
data is addressed using the $gp register. No more
than 65536 bytes of global data may be used. This
requires GNU as and GNU ld which do most of the work.
This currently only works on targets which use ECOFF;
it does not work with ELF.
-membedded-data -mno-embedded-data Allocate variables to the read-only data section first
if possible, then next in the small data section if
possible, otherwise in data. This gives slightly
slower code than the default, but reduces the amount
of RAM required when executing, and thus may be pre
ferred for some embedded systems.
-muninit-const-in-rodata -mno-uninit-const-in-rodata When used together with -membedded-data, it will always store uninitialized const variables in the
read-only data section.
-msingle-float -mdouble-float The -msingle-float switch tells gcc to assume that the floating point coprocessor only supports single preci
sion operations, as on the r4650 chip. The -mdouble-float switch permits gcc to use double precision oper ations. This is the default.
-mmad
-mno-mad
Permit use of the mad, madu and mul instructions, as on the r4650 chip.
-m4650
Turns on -msingle-float, -mmad, and, at least for now, -mcpu=r4650.
-mips16
-mno-mips16 Enable 16-bit instructions.
-mentry
Use the entry and exit pseudo ops. This option can
only be used with -mips16.
-EL Compile code for the processor in little endian mode. The requisite libraries are assumed to exist.
-EB Compile code for the processor in big endian mode. The requisite libraries are assumed to exist.
-G num
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. This allows the
assembler to emit one word memory reference instruc
tions based on the global pointer (gp or $28), instead
of the normal two words used. By default, num is 8
when the MIPS assembler is used, and 0 when the GNU
assembler is used. The -G num switch is also passed to the assembler and linker. All modules should be
compiled with the same -G num value.
-nocpp
Tell the MIPS assembler to not run its preprocessor
over user assembler files (with a .s suffix) when
assembling them.
-mfix7000
Pass an option to gas which will cause nops to be
inserted if the read of the destination register of an
mfhi or mflo instruction occurs in the following two
instructions.
-no-crt0
Do not include the default crt0.
-mflush-func=func -mno-flush-func Specifies the function to call to flush the I and D
caches, or to not call any such function. If called,
the function must take the same arguments as the com
mon "_flush_func()", that is, the address of the mem
ory range for which the cache is being flushed, the
size of the memory range, and the number 3 (to flush
both caches). The default depends on the target gcc
was configured for, but commonly is either _flush_func or __cpu_flush.
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.
Intel 386 and AMD x86-64 Options
These -m options are defined for the i386 and x86-64 fam ily of computers:
-mcpu=cpu-type
Tune to cpu-type everything applicable about the gen erated code, except for the ABI and the set of avail
able instructions. The choices for cpu-type are i386, i486, i586, i686, pentium, pentium-mmx, pentiumpro, pentium2, pentium3, pentium4, k6, k6-2, k6-3, athlon, athlon-tbird, athlon-4, athlon-xp and athlon-mp.
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. i586 is equivalent to pentium and i686 is equivalent to pentiumpro. k6 and athlon are the AMD chips as opposed to the Intel ones.
-march=cpu-type Generate instructions for the machine type cpu-type. The choices for cpu-type are the same as for -mcpu. Moreover, specifying -march=cpu-type implies -mcpu=cpu-type.
-m386
-m486
-mpentium
-mpentiumpro These options are synonyms for -mcpu=i386, -mcpu=i486, -mcpu=pentium, and -mcpu=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 precesion 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 sup
ports 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 i387 you need to use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For x86-64 compiler, these
extensions are enabled by default.
The resulting code should be considerably faster in 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 x86-64 compiler.
sse,387
Attempt to utilize both instruction sets at once.
This effectivly 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 experi
mental, because gcc register allocator does not
model separate functional units well resulting in
instable performance.
-masm=dialect
Output asm instructions using selected dialect. Sup
ported choices are intel or att (the default one).
-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 val ues of types "float" and "double" in an FPU register, even if there is no FPU. The idea is that the operat ing 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 instruc
tion 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, struc tures containing the above types will be aligned dif
ferently than the published application binary inter
face specifications for the 386 and will not be binary
compatible with structures in code compiled without
that switch.
-m128bit-long-double Control the size of "long double" type. i386 applica
tion binary interface specify the size to be 12 bytes,
while modern architectures (Pentium and newer) prefer
"long double" aligned to 8 or 16 byte boundary. This
is impossible to reach with 12 byte long doubles in
the array accesses.
Warning: if you use the -m128bit-long-double switch, the structures and arrays containing "long double"
will change their size as well as function calling
convention for function taking "long double" will be
modified.
-m96bit-long-double Set the size of "long double" to 96 bits as required
by the i386 application binary interface. This is the
default.
-msvr3-shlib -mno-svr3-shlib Control whether GCC places uninitialized local vari
ables 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 argu
ments 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 com
piler.
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. (Nor mally, 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.
-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), in which case the default is the minimum cor rect 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 perfor mance 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 sen
sitive 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
-m3dnow
-mno-3dnow These switches enable or disable the use of built-in
functions that allow direct access to the MMX, SSE 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 per
formance 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 han dling 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
don'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 reg
ister available in leaf functions. The option -fomit-frame-pointer removes the frame pointer for all func tions which might make debugging harder.
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 han
dlers 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 sym
bols 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.
HPPA Options
These -m options are defined for the HPPA family of com puters:
-march=architecture-type Generate code for the specified architecture. The
choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. Code
compiled for lower numbered architectures will run on
higher numbered architectures, but not the other way
around.
PA 2.0 support currently requires gas snapshot 19990413 or later. The next release of binutils (cur rent is 2.9.1) will probably contain PA 2.0 support.
-mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-mbig-switch Generate code suitable for big switch tables. Use
this option only if the assembler/linker complain
about out of range branches within a switch table.
-mjump-in-delay Fill delay slots of function calls with unconditional
jump instructions by modifying the return pointer for
the function call to be the target of the conditional
jump.
-mdisable-fpregs Prevent floating point registers from being used in
any manner. This is necessary for compiling kernels
which perform lazy context switching of floating point
registers. If you use this option and attempt to per
form floating point operations, the compiler will
abort.
-mdisable-indexing Prevent the compiler from using indexing address
modes. This avoids some rather obscure problems when
compiling MIG generated code under MACH.
-mno-space-regs Generate code that assumes the target has no space
registers. This allows GCC to generate faster indi
rect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and ker nels.
-mfast-indirect-calls Generate code that assumes calls never cross space
boundaries. This allows GCC to emit code which per
forms faster indirect calls.
This option will not work in the presence of shared libraries or nested functions.
-mlong-load-store Generate 3-instruction load and store sequences as
sometimes required by the HP-UX 10 linker. This is
equivalent to the +k option to the HP compilers.
-mportable-runtime Use the portable calling conventions proposed by HP
for ELF systems.
-mgas
Enable the use of assembler directives only GAS under
stands.
-mschedule=cpu-type Schedule code according to the constraints for the
machine type cpu-type. The choices for cpu-type are 700 7100, 7100LC, 7200, and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine.
-mlinker-opt Enable the optimization pass in the HPUX linker. Note
this makes symbolic debugging impossible. It also
triggers a bug in the HPUX 8 and HPUX 9 linkers in
which they give bogus error messages when linking some
programs.
-msoft-float Generate output containing library calls for floating
point. Warning: the requisite libraries are not available for all HPPA targets. Normally the facili
ties 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 suit
able library functions for cross-compilation. The
embedded target hppa1.1-*-pro does provide software floating point support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you com
pile all of a program with this option. In particu
lar, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.
Intel 960 Options
These -m options are defined for the Intel 960 implementa tions:
-mcpu-type
Assume the defaults for the machine type cpu-type for some of the other options, including instruction
scheduling, floating point support, and addressing
modes. The choices for cpu-type are ka, kb, mc, ca, cf, sa, and sb. The default is kb.
-mnumerics -msoft-float The -mnumerics option indicates that the processor does support floating-point instructions. The -msoft-float option indicates that floating-point support should not be assumed.
-mleaf-procedures -mno-leaf-procedures Do (or do not) attempt to alter leaf procedures to be
callable with the "bal" instruction as well as "call".
This will result in more efficient code for explicit
calls when the "bal" instruction can be substituted by
the assembler or linker, but less efficient code in
other cases, such as calls via function pointers, or
using a linker that doesn't support this optimization.
-mtail-call -mno-tail-call Do (or do not) make additional attempts (beyond those
of the machine-independent portions of the compiler)
to optimize tail-recursive calls into branches. You
may not want to do this because the detection of cases
where this is not valid is not totally complete. The
default is -mno-tail-call.
-mcomplex-addr -mno-complex-addr Assume (or do not assume) that the use of a complex
addressing mode is a win on this implementation of the
i960. Complex addressing modes may not be worthwhile
on the K-series, but they definitely are on the Cseries. The default is currently -mcomplex-addr for all processors except the CB and CC.
-mcode-align -mno-code-align Align code to 8-byte boundaries for faster fetching
(or don't bother). Currently turned on by default for
C-series implementations only.
-mic-compat -mic2.0-compat -mic3.0-compat Enable compatibility with iC960 v2.0 or v3.0.
-masm-compat -mintel-asm Enable compatibility with the iC960 assembler.
-mstrict-align -mno-strict-align Do not permit (do permit) unaligned accesses.
-mold-align Enable structure-alignment compatibility with Intel's
gcc release version 1.3 (based on gcc 1.37). This
option implies -mstrict-align.
-mlong-double-64 Implement type long double as 64-bit floating point numbers. Without the option long double is imple mented by 80-bit floating point numbers. The only
reason we have it because there is no 128-bit long double support in fp-bit.c yet. So it is only useful for people using soft-float targets. Otherwise, we
should recommend against use of it.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementa tions:
-mno-soft-float -msoft-float Use (do not use) the hardware floating-point instruc
tions for floating-point operations. When -msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations. Unless they are
replaced by routines that emulate the floating-point
operations, or compiled in such a way as to call such
emulations routines, these routines will issue float
ing-point operations. If you are compiling for an
Alpha without floating-point operations, you must
ensure that the library is built so as not to call
them.
Note that Alpha implementations without floating-point operations are required to have floating-point regis ters.
-mfp-reg
-mno-fp-regsGenerate code that uses (does not use) the floatingpoint register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating point operands are passed in integer
registers as if they were integers and floating-point
results are passed in "$0" instead of "$f0". This is
a non-standard calling sequence, so any function with
a floating-point argument or return value called by
code compiled with -mno-fp-regs must also be compiled with that option.
A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.
-mieee
The Alpha architecture implements floating-point hard
ware optimized for maximum performance. It is mostly
compliant with the IEEE floating point standard. How
ever, for full compliance, software assistance is
required. This option generates code fully IEEE com
pliant code except that the inexact-flag is not main tained (see below). If this option is turned on, the
preprocessor macro "_IEEE_FP" is defined during compi
lation. The resulting code is less efficient but is
able to correctly support denormalized numbers and
exceptional IEEE values such as not-a-number and
plus/minus infinity. Other Alpha compilers call this
option -ieee_with_no_inexact.
-mieee-with-inexact This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option causes the generated code to implement fullycompliant IEEE math. In addition to "_IEEE_FP",
"_IEEE_FP_EXACT" is defined as a preprocessor macro.
On some Alpha implementations the resulting code may
execute significantly slower than the code generated
by default. Since there is very little code that
depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this
option -ieee_with_inexact.
-mfp-trap-mode=trap-mode This option controls what floating-point related traps
are enabled. Other Alpha compilers call this option
-fptm trap-mode. The trap mode can be set to one of four values:
n This is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps are enabled as well.
su Like su, but the instructions are marked to be safe for software completion (see Alpha architec ture manual for details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode Selects the IEEE rounding mode. Other Alpha compilers
call this option -fprm rounding-mode. The roundingmode can be one of:
n Normal IEEE rounding mode. Floating point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating point numbers are rounded towards zero.
d Dynamic rounding mode. A field in the floating point control register (fpcr, see Alpha architec ture reference manual) controls the rounding mode in effect. The C library initializes this regis ter for rounding towards plus infinity. Thus, unless your program modifies the fpcr, d corre sponds to round towards plus infinity.
-mtrap-precision=trap-precision In the Alpha architecture, floating point traps are
imprecise. This means without software assistance it
is impossible to recover from a floating trap and pro
gram execution normally needs to be terminated. GCC
can generate code that can assist operating system
trap handlers in determining the exact location that
caused a floating point trap. Depending on the
requirements of an application, different levels of
precisions can be selected:
p Program precision. This option is the default and means a trap handler can only identify which pro gram caused a floating point exception.
f Function precision. The trap handler can deter mine the function that caused a floating point exception.
i Instruction precision. The trap handler can determine the exact instruction that caused a floating point exception.
Other Alpha compilers provide the equivalent options
called -scope_safe and -resumption_safe.
-mieee-conformant This option marks the generated code as IEEE confor
mant. You must not use this option unless you also
specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file. Under DEC Unix, this has
the effect that IEEE-conformant math library routines
will be linked in.
-mbuild-constants Normally GCC examines a 32- or 64-bit integer constant
to see if it can construct it from smaller constants
in two or three instructions. If it cannot, it will
output the constant as a literal and generate code to
load it from the data segment at runtime.
Use this option to require GCC to construct all inte ger constants using code, even if it takes more instructions (the maximum is six).
You would typically use this option to build a shared library dynamic loader. Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment.
-malpha-as -mgas
Select whether to generate code to be assembled by the
vendor-supplied assembler (-malpha-as) or by the GNU assembler -mgas.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the
optional BWX, CIX, FIX and MAX instruction sets. The
default is to use the instruction sets supported by
the CPU type specified via -mcpu= option or that of the CPU on which GCC was built if none was specified.
-mfloat-vax -mfloat-ieee Generate code that uses (does not use) VAX F and G
floating point arithmetic instead of IEEE single and
double precision.
-mexplicit-relocs -mno-explicit-relocs Older Alpha assemblers provided no way to generate
symbol relocations except via assembler macros. Use
of these macros does not allow optimial instruction
scheduling. GNU binutils as of version 2.12 supports
a new syntax that allows the compiler to explicitly
mark which relocations should apply to which instruc
tions. This option is mostly useful for debugging, as
GCC detects the capabilities of the assembler when it
is built and sets the default accordingly.
-msmall-data -mlarge-data When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the ".sdata" and ".sbss" sections) and are accessed via 16-bit relocations off
of the "$gp" register. This limits the size of the
small data area to 64KB, but allows the variables to
be directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that
require more than 2GB of data must use "malloc" or
"mmap" to allocate the data in the heap instead of in
the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-mcpu=cpu_type
Set the instruction set and instruction scheduling
parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip
number. GCC supports scheduling parameters for the
EV4, EV5 and EV6 family of processors and will choose
the default values for the instruction set from the
processor you specify. If you do not specify a pro
cessor type, GCC will default to the processor on
which the compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set
extensions.
ev5
21164
Schedules as an EV5 and has no instruction set
extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX exten
sion.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX
extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and
MAX extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX,
FIX, and MAX extensions.
-mtune=cpu_type Set only the instruction scheduling parameters for
machine type cpu_type. The instruction set is not changed.
-mmemory-latency=time Sets the latency the scheduler should assume for typi
cal memory references as seen by the application.
This number is highly dependent on the memory access
patterns used by the application and the size of the
external cache on the machine.
Valid options for time are
number A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of
clock cycles for ``typical'' EV4 & EV5 hardware
for the Level 1, 2 & 3 caches (also called Dcache,
Scache, and Bcache), as well as to main memory.
Note that L3 is only valid for EV5.
DEC Alpha/VMS Options
These -m options are defined for the DEC Alpha/VMS imple mentations:
-mvms-return-codes Return VMS condition codes from main. The default is
to return POSIX style condition (e.g. error) codes.
Clipper Options
These -m options are defined for the Clipper implementa tions:
-mc300
Produce code for a C300 Clipper processor. This is
the default.
-mc400
Produce code for a C400 Clipper processor, i.e. use
floating point registers f8--f15.
H8/300 Options
These -m options are defined for the H8/300 implementa tions:
-mrelax
Shorten some address references at link time, when
possible; uses the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8/S.
-ms2600
Generate code for the H8/S2600. This switch must be
used with -ms.
-mint32
Make "int" data 32 bits by default.
-malign-300 On the H8/300H and H8/S, use the same alignment rules
as for the H8/300. The default for the H8/300H and
H8/S is to align longs and floats on 4 byte bound
aries. -malign-300 causes them to be aligned on 2 byte boundaries. This option has no effect on the
H8/300.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point
unit.
-m4-single-only Generate code for the SH4 with a floating-point unit
that only supports single-precision arithmetic.
-m4-single Generate code for the SH4 assuming the floating-point
unit is in single-precision mode by default.
-m4 Generate code for the SH4.
-mb Compile code for the processor in big endian mode.
-ml Compile code for the processor in little endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this
changes the calling conventions, and thus some func
tions from the standard C library will not work unless
you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when
possible; uses the linker option -relax.
-mbigtable Use 32-bit offsets in "switch" tables. The default is
to use 16-bit offsets.
-mfmovd
Enable the use of the instruction "fmovd".
-mhitachi
Comply with the calling conventions defined by
Hitachi.
-mnomacsave Mark the "MAC" register as call-clobbered, even if
-mhitachi is given.
-mieee
Increase IEEE-compliance of floating-point code.
-misize
Dump instruction size and location in the assembly
code.
-mpadstruct This option is deprecated. It pads structures to mul
tiple of 4 bytes, which is incompatible with the SH
ABI.
-mspace
Optimize for space instead of speed. Implied by -Os.
-mprefergot When generating position-independent code, emit func
tion calls using the Global Offset Table instead of
the Procedure Linkage Table.
-musermode Generate a library function call to invalidate
instruction cache entries, after fixing up a trampo
line. This library function call doesn't assume it
can write to the whole memory address space. This is
the default when the target is "sh-*-linux*".
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on those systems:
-G Create a shared object. It is recommended that -sym bolic or -shared be used instead.
-Qy Identify the versions of each tool used by the com piler, in a ".ident" assembler directive in the out
put.
-Qn Refrain from adding ".ident" directives to the output file (this is the default).
-YP,dirs
Search the directories dirs, and no others, for
libraries specified with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor.
The assembler uses this option.
TMS320C3x/C4x Options
These -m options are defined for TMS320C3x/C4x implementa tions:
-mcpu=cpu_type
Set the instruction set, register set, and instruction
scheduling parameters for machine type cpu_type. Sup ported values for cpu_type are c30, c31, c32, c40, and c44. The default is c40 to generate code for the TMS320C40.
-mbig-memory -mbig
-msmall-memory -msmall
Generates code for the big or small memory model. The
small memory model assumed that all data fits into one
64K word page. At run-time the data page (DP) regis
ter must be set to point to the 64K page containing
the .bss and .data program sections. The big memory
model is the default and requires reloading of the DP
register for every direct memory access.
-mbk
-mno-bk
Allow (disallow) allocation of general integer
operands into the block count register BK.
-mdb
-mno-db
Enable (disable) generation of code using decrement
and branch, DBcond(D), instructions. This is enabled
by default for the C4x. To be on the safe side, this
is disabled for the C3x, since the maximum iteration
count on the C3x is 2^{23 + 1} (but who iterates loops
more than 2^{23} times on the C3x?). Note that GCC
will try to reverse a loop so that it can utilise the
decrement and branch instruction, but will give up if
there is more than one memory reference in the loop.
Thus a loop where the loop counter is decremented can
generate slightly more efficient code, in cases where
the RPTB instruction cannot be utilised.
-mdp-isr-reload -mparanoid Force the DP register to be saved on entry to an
interrupt service routine (ISR), reloaded to point to
the data section, and restored on exit from the ISR.
This should not be required unless someone has vio
lated the small memory model by modifying the DP reg
ister, say within an object library.
-mmpyi
-mno-mpyi
For the C3x use the 24-bit MPYI instruction for inte
ger multiplies instead of a library call to guarantee
32-bit results. Note that if one of the operands is a
constant, then the multiplication will be performed
using shifts and adds. If the -mmpyi option is not specified for the C3x, then squaring operations are
performed inline instead of a library call.
-mfast-fix -mno-fast-fix The C3x/C4x FIX instruction to convert a floating
point value to an integer value chooses the nearest
integer less than or equal to the floating point value
rather than to the nearest integer. Thus if the
floating point number is negative, the result will be
incorrectly truncated an additional code is necessary
to detect and correct this case. This option can be
used to disable generation of the additional code
required to correct the result.
-mrptb
-mno-rptb
Enable (disable) generation of repeat block sequences
using the RPTB instruction for zero overhead looping.
The RPTB construct is only used for innermost loops
that do not call functions or jump across the loop
boundaries. There is no advantage having nested RPTB
loops due to the overhead required to save and restore
the RC, RS, and RE registers. This is enabled by
default with -O2.
-mrpts=count
-mno-rpts
Enable (disable) the use of the single instruction
repeat instruction RPTS. If a repeat block contains a
single instruction, and the loop count can be guaran
teed to be less than the value count, GCC will emit a
RPTS instruction instead of a RPTB. If no value is
specified, then a RPTS will be emitted even if the
loop count cannot be determined at compile time. Note
that the repeated instruction following RPTS does not
have to be reloaded from memory each iteration, thus
freeing up the CPU buses for operands. However, since
interrupts are blocked by this instruction, it is dis
abled by default.
-mloop-unsigned -mno-loop-unsigned The maximum iteration count when using RPTS and RPTB
(and DB on the C40) is 2^{31 + 1} since these instruc
tions test if the iteration count is negative to ter
minate the loop. If the iteration count is unsigned
there is a possibility than the 2^{31 + 1} maximum
iteration count may be exceeded. This switch allows
an unsigned iteration count.
-mti
Try to emit an assembler syntax that the TI assembler
(asm30) is happy with. This also enforces compatibil
ity with the API employed by the TI C3x C compiler.
For example, long doubles are passed as structures
rather than in floating point registers.
-mregparm
-mmemparm
Generate code that uses registers (stack) for passing
arguments to functions. By default, arguments are
passed in registers where possible rather than by
pushing arguments on to the stack.
-mparallel-insns -mno-parallel-insns Allow the generation of parallel instructions. This
is enabled by default with -O2.
-mparallel-mpy -mno-parallel-mpy Allow the generation of MPY||ADD and MPY||SUB parallel
instructions, provided -mparallel-insns is also speci fied. These instructions have tight register con
straints which can pessimize the code generation of
large functions.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls -mno-long-calls Treat all calls as being far away (near). If calls
are assumed to be far away, the compiler will always
load the functions address up into a register, and
call indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use
the same index pointer 4 or more times to copy pointer
into the "ep" register, and use the shorter "sld" and
"sst" instructions. The -mep option is on by default if you optimize.
-mno-prolog-function
-mprolog-function Do not use (do use) external functions to save and
restore registers at the prolog and epilog of a func
tion. The external functions are slower, but use less
code space if more than one function saves the same
number of registers. The -mprolog-function option is on by default if you optimize.
-mspace
Try to make the code as small as possible. At pre
sent, this just turns on the -mep and -mprolog-func tion options.
-mtda=n
Put static or global variables whose size is n bytes
or less into the tiny data area that register "ep"
points to. The tiny data area can hold up to 256
bytes in total (128 bytes for byte references).
-msda=n
Put static or global variables whose size is n bytes
or less into the small data area that register "gp"
points to. The small data area can hold up to 64
kilobytes.
-mzda=n
Put static or global variables whose size is n bytes
or less into the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mbig-switch Generate code suitable for big switch tables. Use
this option only if the assembler/linker complain
about out of range branches within a switch table.
ARC Options
These options are defined for ARC implementations:
-EL Compile code for little endian mode. This is the default.
-EB Compile code for big endian mode.
-mmangle-cpu Prepend the name of the cpu to all public symbol
names. In multiple-processor systems, there are many
ARC variants with different instruction and register
set characteristics. This flag prevents code compiled
for one cpu to be linked with code compiled for
another. No facility exists for handling variants
that are ``almost identical''. This is an all or
nothing option.
-mcpu=cpu
Compile code for ARC variant cpu. Which variants are
supported depend on the configuration. All variants
support -mcpu=base, this is the default.
-mtext=text-section -mdata=data-section -mrodata=readonly-data-section Put functions, data, and readonly data in text-sec_ tion, data-section, and readonly-data-section respectively by default. This can be overridden with
the "section" attribute.
NS32K Options
These are the -m options defined for the 32000 series. The default values for these options depends on which style of 32000 was selected when the compiler was config ured; the defaults for the most common choices are given below.
-m32032
-m32032
Generate output for a 32032. This is the default when
the compiler is configured for 32032 and 32016 based
systems.
-m32332
-m32332
Generate output for a 32332. This is the default when
the compiler is configured for 32332-based systems.
-m32532
-m32532
Generate output for a 32532. This is the default when
the compiler is configured for 32532-based systems.
-m32081
Generate output containing 32081 instructions for
floating point. This is the default for all systems.
-m32381
Generate output containing 32381 instructions for
floating point. This also implies -m32081. The 32381 is only compatible with the 32332 and 32532 cpus.
This is the default for the pc532-netbsd configura
tion.
-mmulti-add Try and generate multiply-add floating point instruc
tions "polyF" and "dotF". This option is only avail
able if the -m32381 option is in effect. Using these instructions requires changes to register allocation
which generally has a negative impact on performance.
This option should only be enabled when compiling code
particularly likely to make heavy use of multiply-add
instructions.
-mnomulti-add Do not try and generate multiply-add floating point
instructions "polyF" and "dotF". This is the default
on all platforms.
-msoft-float Generate output containing library calls for floating
point. Warning: the requisite libraries may not be available.
-mnobitfield Do not use the bit-field instructions. On some
machines it is faster to use shifting and masking
operations. This is the default for the pc532.
-mbitfield Do use the bit-field instructions. This is the
default for all platforms except the pc532.
-mrtd
Use a different function-calling convention, in which
functions that take a fixed number of arguments return
pop their arguments on return with the "ret" instruc
tion.
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 com piler.
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. (Nor mally, extra arguments are harmlessly ignored.)
This option takes its name from the 680x0 "rtd" instruction.
-mregparam Use a different function-calling convention where the
first two arguments are passed in registers.
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 com piler.
-mnoregparam Do not pass any arguments in registers. This is the
default for all targets.
-msb
It is OK to use the sb as an index register which is
always loaded with zero. This is the default for the
pc532-netbsd target.
-mnosb
The sb register is not available for use or has not
been initialized to zero by the run time system. This
is the default for all targets except the
pc532-netbsd. It is also implied whenever -mhimem or -fpic is set.
-mhimem
Many ns32000 series addressing modes use displacements
of up to 512MB. If an address is above 512MB then
displacements from zero can not be used. This option
causes code to be generated which can be loaded above
512MB. This may be useful for operating systems or
ROM code.
-mnohimem
Assume code will be loaded in the first 512MB of vir
tual address space. This is the default for all plat
forms.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify ATMEL AVR instruction set or MCU type.
Instruction set avr1 is for the minimal AVR core, not supported by the C compiler, only for assembler pro grams (MCU types: at90s1200, attiny10, attiny11, attiny12, attiny15, attiny28).
Instruction set avr2 (default) is for the classic AVR core with up to 8K program memory space (MCU types: at90s2313, at90s2323, attiny22, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515, at90c8534, at90s8535).
Instruction set avr3 is for the classic AVR core with up to 128K program memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
Instruction set avr4 is for the enhanced AVR core with up to 8K program memory space (MCU types: atmega8, atmega83, atmega85).
Instruction set avr5 is for the enhanced AVR core with up to 128K program memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323, atmega64, atmega128, at43usb355, at94k).
-msize
Output instruction sizes to the asm file.
-minit-stack=N Specify the initial stack address, which may be a sym
bol or numeric value, __stack is the default.
-mno-interrupts Generated code is not compatible with hardware inter
rupts. Code size will be smaller.
-mcall-prologues Functions prologues/epilogues expanded as call to
appropriate subroutines. Code size will be smaller.
-mno-tablejump Do not generate tablejump insns which sometimes
increase code size.
-mtiny-stack Change only the low 8 bits of the stack pointer.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
-mhardlit
-mhardlit
-mno-hardlit Inline constants into the code stream if it can be
done in two instructions or less.
-mdiv
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate -mrelax-immediate -mno-relax-immediate Allow arbitrary sized immediates in bit operations.
-mwide-bitfields -mwide-bitfields -mno-wide-bitfields Always treat bit-fields as int-sized.
-m4byte-functions -m4byte-functions -mno-4byte-functions Force all functions to be aligned to a four byte
boundary.
-mcallgraph-data -mcallgraph-data -mno-callgraph-data Emit callgraph information.
-mslow-bytes -mslow-bytes -mno-slow-bytes Prefer word access when reading byte quantities.
-mlittle-endian -mlittle-endian -mbig-endian Generate code for a little endian target.
-m210
-m210
-m340
Generate code for the 210 processor.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian Generate code for a big endian target. This is the
default for HPUX.
-mlittle-endian Generate code for a little endian target. This is the
default for AIX5 and Linux.
-mgnu-as
-mno-gnu-as Generate (or don't) code for the GNU assembler. This
is the default.
-mgnu-ld
-mno-gnu-ld Generate (or don't) code for the GNU linker. This is
the default.
-mno-pic
Generate code that does not use a global pointer reg
ister. The result is not position independent code,
and violates the IA-64 ABI.
-mvolatile-asm-stop -mno-volatile-asm-stop Generate (or don't) a stop bit immediately before and
after volatile asm statements.
-mb-step
Generate code that works around Itanium B step errata.
-mregister-names -mno-register-names Generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler
output more readable.
-mno-sdata -msdata
Disable (or enable) optimizations that use the small
data section. This may be useful for working around
optimizer bugs.
-mconstant-gp Generate code that uses a single constant global
pointer value. This is useful when compiling kernel
code.
-mauto-pic Generate code that is self-relocatable. This implies
-mconstant-gp. This is useful when compiling firmware code.
-minline-divide-min-latency Generate code for inline divides using the minimum
latency algorithm.
-minline-divide-max-throughput Generate code for inline divides using the maximum
throughput algorithm.
-mno-dwarf2-asm -mdwarf2-asm Don't (or do) generate assembler code for the DWARF2
line number debugging info. This may be useful when
not using the GNU assembler.
-mfixed-range=register-range Generate code treating the given register range as
fixed registers. A fixed register is one that the
register allocator can not use. This is useful when
compiling kernel code. A register range is specified
as two registers separated by a dash. Multiple regis
ter ranges can be specified separated by a comma.
D30V Options
These -m options are defined for D30V implementations:
-mextmem
Link the .text, .data, .bss, .strings, .rodata, .rodata1, .data1 sections into external memory, which starts at location "0x80000000".
-mextmemory Same as the -mextmem switch.
-monchip
Link the .text section into onchip text memory, which starts at location "0x0". Also link .data, .bss, .strings, .rodata, .rodata1, .data1 sections into onchip data memory, which starts at location
"0x20000000".
-mno-asm-optimize -masm-optimize Disable (enable) passing -O to the assembler when
optimizing. The assembler uses the -O option to auto
matically parallelize adjacent short instructions
where possible.
-mbranch-cost=n Increase the internal costs of branches to n. Higher
costs means that the compiler will issue more instruc
tions to avoid doing a branch. The default is 2.
-mcond-exec=n Specify the maximum number of conditionally executed
instructions that replace a branch. The default is 4.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float -msoft-float Use (do not use) the hardware floating-point instruc
tions and registers for floating-point operations.
When -msoft-float is specified, functions in libgcc.a will be used to perform floating-point operations.
When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the
default.
-mbackchain -mno-backchain Generate (or do not generate) code which maintains an
explicit backchain within the stack frame that points
to the caller's frame. This is currently needed to
allow debugging. The default is to generate the
backchain.
-msmall-exec -mno-small-exec Generate (or do not generate) code using the "bras"
instruction to do subroutine calls. This only works
reliably if the total executable size does not exceed
64k. The default is to use the "basr" instruction
instead, which does not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the Linux for S/390 ABI. When -m64 is specified, generate code compliant to the Linux for zSeries ABI. This
allows GCC in particular to generate 64-bit instruc
tions. For the s390 targets, the default is -m31, while the s390x targets default to -m64.
-mmvcle
-mno-mvcle Generate (or do not generate) code using the "mvcle"
instruction to perform block moves. When -mno-mvcle is specifed, use a "mvc" loop instead. This is the
default.
-mdebug
-mno-debug Print (or do not print) additional debug information
when compiling. The default is to not print debug
information.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type -mcpu=architecture-type Generate code for the specified architecture. The
choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX.
Default is v0 except for cris-axis-linux-gnu, where
the default is v10.
-mtune=architecture-type Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of
available instructions. The choices for architecturetype are the same as for -march=architecture-type.
-mmax-stack-frame=n Warn when the stack frame of a function exceeds n
bytes.
-melinux-stacksize=n Only available with the cris-axis-aout target. Arranges for indications in the program to the kernel
loader that the stack of the program should be set to
n bytes.
-metrax4
-metrax100 The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
-mpdebug
Enable CRIS-specific verbose debug-related information
in the assembly code. This option also has the effect
to turn off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.
-mcc-init
Do not use condition-code results from previous
instruction; always emit compare and test instructions
before use of condition codes.
-mno-side-effects Do not emit instructions with side-effects in address
ing modes other than post-increment.
-mstack-align -mno-stack-align -mdata-align -mno-data-align
-mconst-align -mno-const-align These options (no-options) arranges (eliminate
arrangements) for the stack-frame, individual data and
constants to be aligned for the maximum single data
access size for the chosen CPU model. The default is
to arrange for 32-bit alignment. ABI details such as
structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options
above, these options arrange for stack-frame, writable
data and constants to all be 32-bit, 16-bit or 8-bit
aligned. The default is 32-bit alignment.
-mno-prologue-epilogue -mprologue-epilogue With -mno-prologue-epilogue, the normal function pro logue and epilogue that sets up the stack-frame are
omitted and no return instructions or return sequences
are generated in the code. Use this option only
together with visual inspection of the compiled code:
no warnings or errors are generated when call-saved
registers must be saved, or storage for local variable
needs to be allocated.
-mno-gotplt -mgotplt
With -fpic and -fPIC, don't generate (do generate) instruction sequences that load addresses for func
tions from the PLT part of the GOT rather than (tradi
tional on other architectures) calls to the PLT. The
default is -mgotplt.
-maoutLegacy no-op option only recognized with the crisaxis-aout target.
-melfLegacy no-op option only recognized with the crisaxis-elf and cris-axis-linux-gnu targets.
-melinux
Only recognized with the cris-axis-aout target, where
it selects a GNU/linux-like multilib, include files
and instruction set for -march=v8.
-mlinuxLegacy no-op option only recognized with the crisaxis-linux-gnu target.
-sim
This option, recognized for the cris-axis-aout and
cris-axis-elf arranges to link with input-output func
tions from a simulator library. Code, initialized
data and zero-initialized data are allocated consecu
tively.
-sim2
Like -sim, but pass linker options to locate initial ized data at 0x40000000 and zero-initialized data at
0x80000000.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs -mno-libfuncs Specify that intrinsic library functions are being
compiled, passing all values in registers, no matter
the size.
-mepsilon
-mno-epsilon Generate floating-point comparison instructions that
compare with respect to the "rE" epsilon register.
-mabi=mmixware -mabi=gnu
Generate code that passes function parameters and
return values that (in the called function) are seen
as registers "$0" and up, as opposed to the GNU ABI
which uses global registers "$231" and up.
-mzero-extend -mno-zero-extend When reading data from memory in sizes shorter than 64
bits, use (do not use) zero-extending load instruc
tions by default, rather than sign-extending ones.
-mknuthdiv -mno-knuthdiv Make the result of a division yielding a remainder
have the same sign as the divisor. With the default,
-mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically
valid, the latter being almost exclusively used.
-mtoplevel-symbols -mno-toplevel-symbols Prepend (do not prepend) a : to all global symbols, so
the assembly code can be used with the "PREFIX" assem
bly directive.
-melf
Generate an executable in the ELF format, rather than
the default mmo format used by the mmix simulator.
-mbranch-predict -mno-branch-predict Use (do not use) the probable-branch instructions,
when static branch prediction indicates a probable
branch.
-mbase-addresses -mno-base-addresses Generate (do not generate) code that uses base
addresses. Using a base address automatically gener ates a request (handled by the assembler and the
linker) for a constant to be set up in a global regis
ter. The register is used for one or more base
address requests within the range 0 to 255 from the
value held in the register. The generally leads to
short and fast code, but the number of different data
items that can be addressed is limited. This means
that a program that uses lots of static data may
require -mno-base-addresses.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default.
(FIS floating point on the PDP-11/40 is not sup
ported.)
-msoft-float Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix
assembler syntax).
-mno-ac0
Return floating-point results in memory. This is the
default.
-m40
Generate code for a PDP-11/40.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10.
-mbcopy-builtin Use inline "movstrhi" patterns for copying memory.
This is the default.
-mbcopy
Do not use inline "movstrhi" patterns for copying mem
ory.
-mint16
-mno-int32 Use 16-bit "int". This is the default.
-mint32
-mno-int16 Use 32-bit "int".
-mfloat64
-mno-float32 Use 64-bit "float". This is the default.
-mfloat32
-mno-float64 Use 32-bit "float".
-mabshi
Use "abshi2" pattern. This is the default.
-mno-abshi Do not use "abshi2" pattern.
-mbranch-expensive Pretend that branches are expensive. This is for
experimenting with code generation only.
-mbranch-cheap Do not pretend that branches are expensive. This is
the default.
-msplit
Generate code for a system with split I&D.
-mno-split Generate code for a system without split I&D. This is
the default.
-munix-asm Use Unix assembler syntax. This is the default when
configured for pdp11-*-bsd.
-mdec-asm
Use DEC assembler syntax. This is the default when
configured for any PDP-11 target other than
pdp11-*-bsd.
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for
the simulator.
Xtensa Options
The Xtensa architecture is designed to support many dif ferent configurations. The compiler's default options can be set to match a particular Xtensa configuration by copy ing a configuration file into the GCC sources when build ing GCC. The options below may be used to override the default options.
-mbig-endian -mlittle-endian Specify big-endian or little-endian byte ordering for
the target Xtensa processor.
-mdensity
-mno-density Enable or disable use of the optional Xtensa code den
sity instructions.
-mmac16
-mno-mac16 Enable or disable use of the Xtensa MAC16 option.
When enabled, GCC will generate MAC16 instructions
from standard C code, with the limitation that it will
use neither the MR register file nor any instruction
that operates on the MR registers. When this option
is disabled, GCC will translate 16-bit multiply/accu
mulate operations to a combination of core instruc
tions and library calls, depending on whether any
other multiplier options are enabled.
-mmul16
-mno-mul16 Enable or disable use of the 16-bit integer multiplier
option. When enabled, the compiler will generate
16-bit multiply instructions for multiplications of 16
bits or smaller in standard C code. When this option
is disabled, the compiler will either use 32-bit mul
tiply or MAC16 instructions if they are available or
generate library calls to perform the multiply opera
tions using shifts and adds.
-mmul32
-mno-mul32 Enable or disable use of the 32-bit integer multiplier
option. When enabled, the compiler will generate
32-bit multiply instructions for multiplications of 32
bits or smaller in standard C code. When this option
is disabled, the compiler will generate library calls
to perform the multiply operations using either shifts
and adds or 16-bit multiply instructions if they are
available.
-mnsa
-mno-nsa
Enable or disable use of the optional normalization
shift amount ("NSA") instructions to implement the
built-in "ffs" function.
-mminmax
-mno-minmax Enable or disable use of the optional minimum and max
imum value instructions.
-msext
-mno-sext
Enable or disable use of the optional sign extend
("SEXT") instruction.
-mbooleans -mno-booleans Enable or disable support for the boolean register
file used by Xtensa coprocessors. This is not typi
cally useful by itself but may be required for other
options that make use of the boolean registers (e.g.,
the floating-point option).
-mhard-float -msoft-float Enable or disable use of the floating-point option.
When enabled, GCC generates floating-point instruc
tions for 32-bit "float" operations. When this option
is disabled, GCC generates library calls to emulate
32-bit floating-point operations using integer
instructions. Regardless of this option, 64-bit "dou
ble" operations are always emulated with calls to
library functions.
-mfused-madd -mno-fused-madd Enable or disable use of fused multiply/add and multi
ply/subtract instructions in the floating-point
option. This has no effect if the floating-point
option is not also enabled. Disabling fused multi
ply/add and multiply/subtract instructions forces the
compiler to use separate instructions for the multiply
and add/subtract operations. This may be desirable in
some cases where strict IEEE 754-compliant results are
required: the fused multiply add/subtract instructions
do not round the intermediate result, thereby produc
ing results with more bits of precision than specified
by the IEEE standard. Disabling fused multiply
add/subtract instructions also ensures that the pro
gram output is not sensitive to the compiler's ability
to combine multiply and add/subtract operations.
-mserialize-volatile -mno-serialize-volatile When this option is enabled, GCC inserts "MEMW"
instructions before "volatile" memory references to
guarantee sequential consistency. The default is
-mserialize-volatile. Use -mno-serialize-volatile to omit the "MEMW" instructions.
-mtext-section-literals -mno-text-section-literals Control the treatment of literal pools. The default
is -mno-text-section-literals, which places literals in a separate section in the output file. This allows
the literal pool to be placed in a data RAM/ROM, and
it also allows the linker to combine literal pools
from separate object files to remove redundant liter
als and improve code size. With -mtext-section-liter als, the literals are interspersed in the text section in order to keep them as close as possible to their
references. This may be necessary for large assembly
files.
-mtarget-align -mno-target-align When this option is enabled, GCC instructs the assem
bler to automatically align instructions to reduce
branch penalties at the expense of some code density.
The assembler attempts to widen density instructions
to align branch targets and the instructions following
call instructions. If there are not enough preceding
safe density instructions to align a target, no widen
ing will be performed. The default is -mtarget-align. These options do not affect the treatment of autoaligned instructions like "LOOP", which the assembler
will always align, either by widening density instruc
tions or by inserting no-op instructions.
-mlongcalls -mno-longcalls When this option is enabled, GCC instructs the assem
bler to translate direct calls to indirect calls
unless it can determine that the target of a direct
call is in the range allowed by the call instruction.
This translation typically occurs for calls to func
tions in other source files. Specifically, the assem
bler translates a direct "CALL" instruction into an
"L32R" followed by a "CALLX" instruction. The default
is -mno-longcalls. This option should be used in pro grams where the call target can potentially be out of
range. This option is implemented in the assembler,
not the compiler, so the assembly code generated by
GCC will still show direct call instructions---look at
the disassembled object code to see the actual
instructions. Note that the assembler will use an
indirect call for every cross-file call, not just
those that really will be out of range.
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.
-fexceptions Enable exception handling. Generates extra code
needed to propagate exceptions. For some targets,
this implies GCC will generate frame unwind informa
tion for all functions, which can produce significant
data size overhead, although it does not affect execu
tion. If you do not specify this option, GCC will
enable it by default for languages like C++ which nor
mally 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 com
piling 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 platformspecific runtime support that does not exist every
where. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or float
ing 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 gen erate 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 con
vention 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 con form 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 con
vention, 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 con form 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 gener ate code that is not binary compatible with code gen
erated 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 gen erate 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 gener ate code that is not binary compatible with code gen
erated 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 gen
erating 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 pro
gram will work on other systems which always work this
way.
-fno-ident Ignore the #ident directive.
-fno-gnu-linker Do not output global initializations (such as C++ con
structors and destructors) in the form used by the GNU
linker (on systems where the GNU linker is the stan
dard method of handling them). Use this option when
you want to use a non-GNU linker, which also requires
using the collect2 program to make sure the system linker includes constructors and destructors. (col lect2 is included in the GCC distribution.) For sys tems which must use collect2, the compiler driver gcc is configured to do this automatically.
-finhibit-size-directive Don't output a ".size" assembler directive, or any
thing 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 debug
ging the compiler itself).
-fno-verbose-asm, the default, causes the extra infor mation to be omitted and is useful when comparing two
assembler files.
-fvolatile Consider all memory references through pointers to be
volatile.
-fvolatile-global Consider all memory references to extern and global
data items to be volatile. GCC does not consider
static data items to be volatile because of this
switch.
-fvolatile-static Consider all memory references to static data to be
volatile.
-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 maxi
mum 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 16k on the m88k, 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.
-fPICIf supported for the target machine, emit positionindependent 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,
m88k, and the Sparc.
Position-independent code requires special support, and therefore works only on certain machines.
-ffixed-reg
Treat the register named reg as a fixed register; gen
erated 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 allo
cated 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 tem
poraries or variables that live across a call. Func
tions compiled this way will save and restore the reg
ister 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 Pack all structure members together without holes.
Warning: the -fpack-struct switch causes GCC to gener ate code that is not binary compatible with code
generated without that switch. Additionally, it makes
the code suboptimial. Use it to conform to a nondefault 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 avail
able. 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 pro
vided. (This is normally the case anyways, but if you
get lucky and the optimizer always expands the func
tions inline, you might have gotten away without pro
viding static copies.)
A function may be given the attribute "no_instru ment_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 sin
gle-threaded environment since stack overflow is auto
matically 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 tar
gets, 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,--def sym,__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 stor
age.-fargument-noalias specifies that arguments do not alias each other, but may alias global storage.-fargu ment-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-under score, forcibly change the way C symbols are repre sented 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.

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 configu
ration 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_UK for English in the United King dom.; The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the charac
ter 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 over rides the value of LC_CTYPE and LC_MESSAGES; other wise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these vari ables 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 out
put 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 com
bined with the name of a subprogram, but you can spec
ify 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 pre_ fix/lib/gcc-lib/ 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 nor
mally begins with /usr/local/lib/gcc-lib (more pre cisely, 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 nor mally search /usr/local/lib/bar. These alternate directories are searched first; the standard directo
ries 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 subpro
grams, 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 speci
fied 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.
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 sepa
rated 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 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. The 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.
DEPENDENCIES_OUTPUT @anchor{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 equiva
lent to combining the options -MM and -MF, with an optional -MT switch too.
SUNPRO_DEPENDENCIES This variable is the same as the environment variable
DEPENDENCIES_OUTPUT, 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

For instructions on reporting bugs, see
<http://gcc.gnu.org/bugs.html>. Use of the gccbug script to report bugs is recommended.

FOOTNOTES

1. On some systems, gcc -shared needs to build supplemen: tary stub code for constructors to work. On multilibbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply
the correct flags may lead to subtle defects. Supply
ing them in cases where they are not necessary is
innocuous.

AUTHOR

See the Info entry for gcc, or <http://gcc.gnu.org/online docs/gcc/Contributors.html>, for contributors to GCC.

COPYRIGHT

Copyright (c) 1988, 1989, 1992, 1993, 1994, 1995, 1996,
1997, 1998, 1999, 2000, 2001, 2002, 2003 Free Software
Foundation, Inc.

Permission is granted to copy, distribute and/or modify
this document under the terms of the GNU Free Documenta
tion License, Version 1.1 or any later version published
by the Free Software Foundation; with the Invariant Sec
tions 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.

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