sh-rtems-gcc






gcc − GNU project C and C++ compiler

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.

When you invoke GCC, it normally does preprocessing,
compilation, assembly and linking.  The ‘‘overall options’’
allow you to stop this process at an intermediate stage.
For example, the −c option says not to run the linker.  Then
the output consists of object files output by the assembler.

     Other options are passed on to one stage of processing.
Some options control the preprocessor and others the
compiler itself.  Yet other options control the assembler
and linker; most of these are not documented here, since you
rarely need to use any of them.

     Most of the command line options that you can use with
GCC are useful for C programs; when an option is only useful
with another language (usually C++), the explanation says so
explicitly.  If the description for a particular option does
not mention a source language, you can use that option with
all supported languages.

     The gcc program accepts options and file names as
operands.  Many options have multi‐letter names; therefore
multiple single‐letter options may not be grouped: −dr is
very different from −d −r.

     You can mix options and other arguments.  For the most
part, the order you use doesn’t matter.  Order does matter
when you use several options of the same kind; for example,
if you specify −L more than once, the directories are
searched in the order specified.

     Many options have long names starting with −f or with
−W−−−for example, −fforce‐mem, −fstrength‐reduce, −Wformat
and so on.  Most of these have both positive and negative
forms; the negative form of −ffoo would be −fno‐foo.  This
manual documents only one of these two forms, whichever one
is not the default.













                             ‐2‐




     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
    language −v  −###  −−help  −−target‐help  −−version

C Language Options
    −ansi  −std=standard  −aux‐info filename −fno‐asm  −fno‐
    builtin −fno‐builtin‐function −fhosted  −ffreestanding
    −trigraphs  −no‐integrated‐cpp  −traditional
    −traditional‐cpp −fallow‐single‐precision  −fcond‐
    mismatch −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‐
    location=[onceevery‐line]

Warning Options
    −fsyntax‐only  −pedantic  −pedantic‐errors −w  −W  −Wall
    −Waggregate‐return −Wcast‐align  −Wcast‐qual  −Wchar‐
    subscripts  −Wcomment −Wconversion  −Wno‐deprecated‐
    declarations −Wdisabled‐optimization  −Wdiv‐by‐zero
    −Werror −Wfloat‐equal  −Wformat  −Wformat=2 −Wformat‐
    nonliteral  −Wformat‐security −Wimplicit  −Wimplicit‐int
    −Wimplicit‐function‐declaration −Werror‐implicit‐
    function‐declaration −Wimport  −Winline −Wlarger‐than‐
    len  −Wlong‐long −Wmain  −Wmissing‐braces −Wmissing‐
    format‐attribute  −Wmissing‐noreturn −Wmultichar  −Wno‐









                             ‐3‐


    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‐verbose=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
    −finline‐functions  −finline‐limit=n  −fkeep‐inline‐
    functions −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‐precision‐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











                             ‐4‐


Preprocessor Options
    −$  −Aquestion=answer  −A‐question[=answer] −C  −dD  −dI
    −dM  −dN −Dmacro[=defn]  −E  −H −idirafter dir −include
    file  −imacros file −iprefix file  −iwithprefix dir
    −iwithprefixbefore dir  −isystem dir −M  −MM  −MF  −MG
    −MP  −MQ  −MT  −nostdinc  −P  −remap −trigraphs  −undef
    −Umacro  −Wp,option

Assembler Option
    −Wa,option

Linker Options
     object‐file‐name  −llibrary −nostartfiles
    −nodefaultlibs  −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









                             ‐5‐


    −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=sdata‐type
    −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‐serialize‐
    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









                             ‐6‐


    −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   −mminimal‐
    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  −mrelocatable‐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=cpu‐
    type −membedded‐data  −muninit‐const‐in‐rodata
    −membedded‐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‐
    outgoing‐args  −m128bit‐long‐double −m96bit‐long‐double
    −mregparm=num  −momit‐leaf‐frame‐pointer −mno‐red‐zone
    −mcmodel=code‐model −m32 −m64

    HPPA Options










                             ‐7‐


    −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‐precision=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









                             ‐8‐


    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 −mparallel‐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










                             ‐9‐


    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‐literals −mno‐text‐
    section‐literals −mtarget‐align −mno‐target‐align
    −mlongcalls −mno‐longcalls

Code Generation Options
    −fcall‐saved‐reg  −fcall‐used‐reg −ffixed‐reg
    −fexceptions −fnon‐call‐exceptions  −funwind‐tables
    −fasynchronous‐unwind‐tables −finhibit‐size‐directive
    −finstrument‐functions −fno‐common  −fno‐ident  −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‐









                            ‐10‐


    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
    preprocessed.

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











                            ‐11‐


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 subprogram
    renaming declaration).  Such files are also called
    specs.

file.adb
    Ada source code file containing a library unit body (a
    subprogram or package body).  Such files are also called
    bodies.

file.s
    Assembler code.

file.S
    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:














                            ‐12‐


            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
    subsequent 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
    preprocessed source code, which is sent to the standard
    output.

    Input files which don’t require preprocessing are









                            ‐13‐


    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
    executable file in a.out, the object file for
    source.suffix 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
    version number of the compiler driver program and of the
    preprocessor and the compiler proper.

−###
    Like −v except the commands are not executed and all
    command arguments are quoted.  This is useful for shell
    scripts to capture the driver‐generated command lines.

−pipe
    Use pipes rather than temporary files for communication
    between the various stages of compilation.  This fails
    to work on some systems where the assembler is unable to
    read from a pipe; but the GNU assembler has no trouble.

‐‐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 command line
    options which have no documentation associated with them
    will also be displayed.

‐‐target‐help
    Print (on the standard output) a description of target
    specific command line options for each tool.

‐‐version
    Display the version number and copyrights of the invoked
    GCC.











                            ‐14‐


     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 programs
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 automatically
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
programs in any language; or command‐line options meaningful
for C and related languages; or options that are meaningful
only for C++ programs.

     Options Controlling C Dialect

     The following options control the dialect of C (or
languages derived from C, such as C++ 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
    incompatible 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 compiler, it
    disables recognition of C++ style // comments as well as
    the "inline" keyword.

    The alternate keywords "__asm__", "__extension__",
    "__inline__" and "__typeof__" continue to work despite
    −ansi.  You would not want to use them in an ISO C
    program, of course, but it is useful to put them in
    header files that might be included in compilations done
    with −ansi.  Alternate predefined macros such as
    "__unix__" and "__vax__" are also available, with or
    without −ansi.

    The −ansi option does not cause non‐ISO programs to be









                            ‐15‐


    rejected gratuitously.  For that, −pedantic is required
    in addition to −ansi.

    The macro "__STRICT_ANSI__" is predefined when the −ansi
    option is used.  Some header files may notice this macro
    and refrain from declaring certain functions or defining
    certain macros that the ISO standard doesn’t call for;
    this is to avoid interfering with any programs that
    might use these names for other things.

    Functions which would normally be built in but do not
    have semantics defined by ISO C (such as "alloca" and
    "ffs") are not built‐in functions with −ansi is used.

−std=
    Determine the language standard.  This option is
    currently only supported when compiling C.  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/c99status.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









                            ‐16‐


        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 translation
    unit, including those in header files.  This option is
    silently ignored in any language other than C.

    Besides declarations, the file indicates, in comments,
    the origin of each declaration (source file and line),
    whether the declaration was implicit, prototyped or
    unprototyped (I, N for new or O for old, respectively,
    in the first character after the line number and the
    colon), and whether it came from a declaration or a
    definition (C or F, respectively, in the following
    character).  In the case of function definitions, a K&R‐
    style list of arguments followed by their declarations
    is also provided, inside comments, after the
    declaration.

−fno‐asm
    Do not recognize "asm", "inline" or "typeof" as a
    keyword, so that code can use these words as
    identifiers.  You can use the keywords "__asm__",
    "__inline__" and "__typeof__" instead.  −ansi implies
    −fno‐asm.

    In C++, this switch only affects the "typeof" keyword,
    since "asm" and "inline" are standard keywords.  You may
    want to use the −fno‐gnu‐keywords flag instead, which
    has the same effect.  In C99 mode (−std=c99 or
    −std=gnu99), this switch only affects the "asm" and
    "typeof" keywords, since "inline" is a standard keyword
    in ISO C99.

−fno‐builtin

−fno‐builtin‐function (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









                            ‐17‐


    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 efficient code.

    With the −fno‐builtin‐function option, not available
    when compiling C++, only the built‐in function function
    is disabled.  function must not begin with __builtin_.
    If a function is named this is not built‐in in this
    version of GCC, this option is ignored.  There is no
    corresponding −fbuiltin‐function option; if you wish to
    enable built‐in functions selectively when using −fno‐
    builtin or −ffreestanding, you may define macros such
    as:

            #define abs(n)          __builtin_abs ((n))
            #define strcpy(d, s)    __builtin_strcpy ((d), (s))


−fhosted
    Assert that compilation takes place in a hosted
    environment.  This implies −fbuiltin.  A hosted
    environment is one in which the entire standard library
    is available, and in which "main" has a return type of
    "int".  Examples are nearly everything except a kernel.
    This is equivalent to −fno‐freestanding.

−ffreestanding
    Assert that compilation takes place in a freestanding
    environment.  This implies −fno‐builtin.  A freestanding
    environment is one in which the standard library may not
    exist, and program startup may not necessarily be at
    "main".  The most obvious example is an OS kernel.  This
    is equivalent to −fno‐hosted.

−trigraphs
    Support ISO C trigraphs.  The −ansi option (and −std
    options for strict ISO C conformance) implies
    −trigraphs.

−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.










                            ‐18‐


    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
    compilers.  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 sequences \x and \a evaluate as
        the literal characters x and a respectively.
        Without −traditional, \x is a prefix for the
        hexadecimal representation of a character, and \a
        produces a bell.

        This option is deprecated and may be removed.

        You may wish to use −fno‐builtin as well as
        −traditional 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









                            ‐19‐


        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
    preprocessors.  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.
    Likewise, the option −fno‐signed‐char is equivalent to
    −funsigned‐char.

−fsigned‐bitfields

−funsigned‐bitfields

−fno‐signed‐bitfields

−fno‐unsigned‐bitfields
    These options control whether a bit‐field is signed or









                            ‐20‐


    unsigned, when the declaration does not use either
    "signed" or "unsigned".  By default, such a bit‐field is
    signed, because this is consistent: the basic integer
    types such as "int" are signed types.

    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 −traditional.

    Traditional K&R C promotes all floating point operations
    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 language
your program is in.  For example, you might compile a file
"firstClass.C" like this:

             g++ ‐g ‐frepo ‐O ‐c firstClass.C

In this example, only −frepo is an option meant only for C++
programs; you can use the other options with any language
supported by GCC.

     Here is a list of options that are only for compiling
C++ programs:

−fno‐access‐control
    Turn off all access checking.  This switch is mainly
    useful for working around bugs in the access control
    code.










                            ‐21‐


−fcheck‐new
    Check that the pointer returned by "operator new" is
    non‐null before attempting to modify the storage
    allocated.  The current Working Paper requires that
    "operator 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 throw(), G++ will check the return value.
    See also new (nothrow).

−fconserve‐space
    Put uninitialized or runtime‐initialized global
    variables into the common segment, as C does.  This
    saves space in the executable at the cost of not
    diagnosing duplicate definitions.  If you compile with
    this flag and your program mysteriously crashes after
    "main()" has completed, you may have an object that is
    being destroyed twice because two definitions were
    merged.

    This option is no longer useful on most targets, now
    that support has been added for putting variables into
    BSS without making them common.

−fno‐const‐strings
    Give string constants type "char *" instead of type
    "const char *".  By default, G++ uses type "const char
    *" as required by the standard.  Even if you use −fno‐
    const‐strings, you cannot actually modify the value of a
    string constant, unless you also use −fwritable‐strings.

    This option might be removed in a future release of G++.
    For maximum portability, you should structure your code
    so that it works with string constants that have type
    "const char *".

−fdollars‐in‐identifiers
    Accept $ in identifiers.  You can also explicitly
    prohibit use of $ with the option −fno‐dollars‐in‐
    identifiers.  (GNU C allows $ by default on most target
    systems, 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.











                            ‐22‐


−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
    definition.

    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 old‐style
    code that would otherwise be invalid, or have different
    behavior.

−fno‐gnu‐keywords
    Do not recognize "typeof" as a keyword, so that code can
    use this word as an identifier.  You can use the keyword
    "__typeof__" instead.  −ansi implies −fno‐gnu‐keywords.

−fno‐implicit‐templates
    Never emit code for non‐inline templates which are
    instantiated implicitly (i.e. by use); only emit code
    for explicit instantiations.

−fno‐implicit‐inline‐templates
    Don’t emit code for implicit instantiations of inline
    templates, either.  The default is to handle inlines
    differently so that compiles with and without
    optimization will need the same set of explicit
    instantiations.









                            ‐23‐


−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 exception
    handling uses the same information, but it will generate
    it as needed.

−fstats
    Emit statistics about front‐end processing at the end of
    the compilation.  This information is generally only
    useful to the G++ development team.











                            ‐24‐


−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
    destructors, but will only work if your C library
    supports "__cxa_atexit".

−fvtable‐gc
    Emit special relocations for vtables and virtual
    function 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
    directories specific to C++, but do still search the
    other standard directories.  (This option is used when
    building the C++ library.)

     In addition, these optimization, warning, and code
generation options have meanings only for C++ programs:

−fno‐default‐inline
    Do not assume inline for functions defined inside a
    class scope.
      Note that these functions will have linkage like
    inline functions; they just won’t be inlined by default.

−Wabi (C++ only)
    Warn when G++ generates code that is probably not
    compatible with the vendor‐neutral C++ ABI.  Although an
    effort has been made to warn about all such cases, there









                            ‐25‐


    are probably some cases that are not warned about, even
    though G++ is generating incompatible code.  There may
    also be cases where warnings are emitted even though the
    code that is generated will be compatible.

    You should rewrite your code to avoid these warnings if
    you are concerned about the fact that code generated by
    G++ may not be binary compatible with code generated by
    other compilers.

    The known 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
    constructors or destructors in a class are private and
    the class has no friends or public static member
    functions.

−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









                            ‐26‐


    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 initializers
    for i and j will be rearranged to match the declaration
    order of the members.

     The following −W... options are not affected by −Wall.

−Weffc++ (C++ only)
    Warn about violations of the following style guidelines
    from Scott Meyers’ Effective C++ book:

    •   Item 11:  Define a copy constructor and an
        assignment operator for classes with dynamically
        allocated 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









                            ‐27‐


    explicit template specification support in G++, if the
    name of the friend is an unqualified‐id (i.e., friend
    foo(int)), the C++ language specification demands that
    the friend declare or define an ordinary, nontemplate
    function.  (Section 14.5.3).  Before G++ implemented
    explicit specification, unqualified‐ids could be
    interpreted as a particular specialization of a
    templatized function.  Because this non‐conforming
    behavior is no longer the default behavior for G++,
    −Wnon‐template‐friend allows the compiler to check
    existing code for potential trouble spots, and is on by
    default.  This new compiler behavior can be turned off
    with −Wno‐non‐template‐friend which keeps the conformant
    compiler code but disables the helpful warning.

−Wold‐style‐cast (C++ only)
    Warn if an old‐style (C‐style) cast to a non‐void type
    is used within a C++ program.  The new‐style casts
    (static_cast, reinterpret_cast, and const_cast) are less
    vulnerable to unintended effects, and much easier to
    grep for.

−Woverloaded‐virtual (C++ only)
    Warn when a function declaration hides virtual functions
    from a base class.  For example, in:

            struct A {
              virtual void f();
            };

            struct B: public A {
              void f(int);
            };

    the "A" class version of "f" is hidden in "B", and code
    like 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
    conversion to an unsigned type of the same size.
    Previous versions of G++ would try to preserve
    unsignedness, but the standard mandates the current
    behavior.










                            ‐28‐


−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 compile
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 "NXConstantString".

−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.









                            ‐29‐


−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
    location information; that is, in case the message is
    too long to fit on a single physical line and has to be
    wrapped, the source location won’t be emitted (as
    prefix) again, over and over, in subsequent continuation
    lines.  This is the default behavior.

−fdiagnostics‐show‐location=every‐line
    Only meaningful in line‐wrapping mode.  Instructs the
    diagnostic messages reporter to emit the same source
    location information (as prefix) for physical lines that
    result from the process of breaking a message which is
    too long to fit on a single line.

     Options to Request or Suppress Warnings

     Warnings are diagnostic messages that report
constructions which are not inherently erroneous but which
are risky or suggest there may have been an error.

     You can request many specific warnings with options
beginning −W, for example −Wimplicit to request warnings on
implicit declarations.  Each of these specific warning
options also has a negative form beginning −Wno‐ to turn off
warnings; for example, −Wno‐implicit.  This manual lists









                            ‐30‐


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 anything
    beyond that.

−pedantic
    Issue all the warnings demanded by strict ISO C and ISO
    C++; reject all programs that use forbidden extensions,
    and some other programs that do not follow ISO C and ISO
    C++.  For ISO C, follows the version of the ISO C
    standard specified by any −std option used.

    Valid ISO C and ISO C++ programs should compile properly
    with or without this option (though a rare few will
    require −ansi or a −std option specifying the required
    version of ISO C).  However, without this option,
    certain GNU extensions and traditional C and C++
    features are supported as well.  With this option, they
    are rejected.

    −pedantic does not cause warning messages for use of the
    alternate keywords whose names begin and end with __.
    Pedantic warnings are also disabled in the expression
    that follows "__extension__".  However, only system
    header files should use these escape routes; application
    programs should avoid them.

    Some users try to use −pedantic to check programs for
    strict ISO C conformance.  They soon find that it does
    not do quite what they want: it finds some non‐ISO
    practices, but not all−−−only those for which ISO C
    requires a diagnostic, and some others for which
    diagnostics have been added.

    A feature to report any failure to conform to ISO C
    might be useful in some instances, but would require
    considerable additional work and would be quite
    different from −pedantic.  We don’t have plans to
    support such a feature in the near future.

    Where the standard specified with −std represents a GNU
    extended dialect of C, such as gnu89 or gnu99, there is
    a corresponding base standard, the version of ISO C on
    which the GNU extended dialect is based.  Warnings from
    −pedantic are given where they are required by the base
    standard.  (It would not make sense for such warnings to
    be given only for features not in the specified GNU C
    dialect, since by definition the GNU dialects of C









                            ‐31‐


    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 extension, 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
    extensions.  Other library implementations may not
    support all these features; GCC does not support warning
    about features that go beyond a particular library’s
    limitations.  However, if −pedantic is used with
    −Wformat, warnings will be given about format features
    not in the selected standard version (but not for
    "strfmon" formats, since those are not in any version of
    the C standard).

    −Wformat is included in −Wall.  For more control over
    some aspects of format checking, the options −Wno‐
    format‐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.









                            ‐32‐


−Wno‐format‐extra‐args
    If −Wformat is specified, do not warn about excess
    arguments to a "printf" or "scanf" format function.  The
    C standard specifies that such arguments are ignored.

    Where the unused arguments lie between used arguments
    that are specified with $ operand number specifications,
    normally warnings are still given, since the
    implementation could not know what type to pass to
    "va_arg" to skip the unused arguments.  However, in the
    case of "scanf" formats, this option will suppress the
    warning if the unused arguments are all pointers, since
    the Single Unix Specification says that such unused
    arguments are allowed.

−Wformat‐nonliteral
    If −Wformat is specified, also warn if the format string
    is not a string literal and so cannot be checked, unless
    the format function takes its format arguments as a
    "va_list".

−Wformat‐security
    If −Wformat is specified, also warn about uses of format
    functions that represent possible security problems.  At
    present, this warns about calls to "printf" and "scanf"
    functions where the format string is not a string
    literal and there are no format arguments, as in "printf
    (foo);".  This may be a security hole if the format
    string came from untrusted input and contains %n.  (This
    is currently a subset of what −Wformat‐nonliteral warns
    about, but in future warnings may be added to −Wformat‐
    security that are not included in −Wformat‐nonliteral.)

−Wformat=2
    Enable −Wformat plus format checks not included in
    −Wformat.  Currently equivalent to −Wformat −Wformat‐
    nonliteral −Wformat‐security.

−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‐
    declaration.

−Wmain
    Warn if the type of main is suspicious.  main should be
    a function with external linkage, returning int, taking









                            ‐33‐


    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
    confusion to which "if" statement an "else" branch
    belongs.  Here is an example of such a case:

            {
              if (a)
                if (b)
                  foo ();
              else
                bar ();
            }

    In C, every "else" branch belongs to the innermost
    possible "if" statement, which in this example is "if
    (b)".  This is often not what the programmer expected,
    as illustrated in the above example by indentation the
    programmer chose.  When there is the potential for this
    confusion, GCC will issue a warning when this flag is
    specified.  To eliminate the warning, add explicit
    braces around the innermost "if" statement so there is
    no way the "else" could belong to the enclosing "if".
    The resulting code would look like this:

            {
              if (a)
                {
                  if (b)
                    foo ();
                  else
                    bar ();
                }
            }












                            ‐34‐


−Wsequence‐point
    Warn about code that may have undefined semantics
    because of violations of sequence point rules in the C
    standard.

    The C standard defines the order in which expressions in
    a C program are evaluated in terms of sequence points,
    which represent a partial ordering between the execution
    of parts of the program: those executed before the
    sequence point, and those executed after it.  These
    occur after the evaluation of a full expression (one
    which is not part of a larger expression), after the
    evaluation of the first operand of a "&&", "⎪⎪", "? :"
    or "," (comma) operator, before a function is called
    (but after the evaluation of its arguments and the
    expression denoting the called function), and in certain
    other places.  Other than as expressed by the sequence
    point rules, the order of evaluation of subexpressions
    of an expression is not specified.  All these rules
    describe only a partial order rather than a total order,
    since, for example, if two functions are called within
    one expression with no sequence point between them, the
    order in which the functions are called is not
    specified.  However, the standards committee have ruled
    that function calls do not overlap.

    It is not specified when between sequence points
    modifications to the values of objects take effect.
    Programs whose behavior depends on this have undefined
    behavior; the C standard specifies that ‘‘Between the
    previous and next sequence point an object shall have
    its stored value modified at most once by the evaluation
    of an expression.  Furthermore, the prior value shall be
    read only to determine the value to be stored.’’.  If a
    program breaks these rules, the results on any
    particular implementation are entirely unpredictable.

    Examples of code with undefined behavior are "a = a++;",
    "a[n] = b[n++]" and "a[i++] = i;".  Some more
    complicated cases are not diagnosed by this option, and
    it may give an occasional false positive result, but in
    general it has been found fairly effective at detecting
    this sort of problem in programs.

    The present implementation of this option only works for
    C programs.  A future implementation may also work for
    C++ programs.

    The C standard is worded confusingly, therefore there is
    some debate over the precise meaning of the sequence
    point rules in subtle cases.  Links to discussions of
    the problem, including proposed formal definitions, may
    be found on our readings page, at
    <http://gcc.gnu.org/readings.html>.









                            ‐35‐


−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 produces
    a diagnostic message, even when −Wno‐return‐type is
    specified.  The only exceptions are main and functions
    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.










                            ‐36‐


    In order to get a warning about an unused function
    parameter, you must either specify −W −Wunused or
    separately specify −Wunused‐parameter.

−Wuninitialized
    Warn if an automatic variable is used without first
    being initialized or if a variable may be clobbered by a
    "setjmp" call.

    These warnings are possible only in optimizing
    compilation, because they require data flow information
    that is computed only when optimizing.  If you don’t
    specify −O, you simply won’t get these warnings.

    These warnings occur only for variables that are
    candidates 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:















                            ‐37‐


            {
              int save_y;
              if (change_y) save_y = y, y = new_y;
              ...
              if (change_y) y = save_y;
            }

    This has no bug because "save_y" is used only if it is
    set.

    This option also warns when a non‐volatile automatic
    variable might be changed by a call to "longjmp".  These
    warnings as well are possible only in optimizing
    compilation.

    The compiler sees only the calls to "setjmp".  It cannot
    know where "longjmp" will be called; in fact, a signal
    handler could call it at any point in the code.  As a
    result, you may get a warning even when there is in fact
    no problem because "longjmp" cannot in fact be called at
    the place which would cause a problem.

    Some spurious warnings can be avoided if you declare all
    the functions you use that never return as "noreturn".

−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.









                            ‐38‐


    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 values,
    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 normally
    suppressed, on the assumption that they usually do not
    indicate real problems and would only make the compiler
    output harder to read.  Using this command line option
    tells GCC to emit warnings from system headers as if
    they occurred in user code.  However, note that using
    −Wall in conjunction with this option will not warn
    about unknown pragmas in system headers−−−for that,
    −Wunknown‐pragmas must also be used.

     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
        equivalent 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









                            ‐39‐


        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 following
        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 floating‐point 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 computation introduces,
    and allow for it when performing comparisons (and when
    producing output, but that’s a different problem).  In
    particular, instead of testing for equality, you would
    check to see whether the two values have ranges that
    overlap; and this is done with the relational operators,
    so equality comparisons are probably mistaken.










                            ‐40‐


−Wtraditional (C only)
    Warn about certain constructs that behave differently in
    traditional and ISO C.  Also warn about ISO C constructs
    that have no traditional C equivalent, and/or
    problematic constructs which should be avoided.

    •   Macro parameters that appear within string literals
        in the macro body.  In traditional C macro
        replacement takes place within string literals, but
        does not in ISO C.

    •   In traditional C, some preprocessor directives did
        not exist.  Traditional preprocessors would only
        consider a line to be a directive if the # appeared
        in column 1 on the line.  Therefore −Wtraditional
        warns about directives that traditional C
        understands but would ignore because the # does not
        appear as the first character on the line.  It also
        suggests you hide directives like #pragma not
        understood by traditional C by indenting them.  Some
        traditional implementations would not recognize
        #elif, so it suggests avoiding it altogether.

    •   A function‐like macro that appears without
        arguments.

    •   The unary plus operator.

    •   The U integer constant suffix, or the F or L
        floating point constant suffixes.  (Traditional C
        does support the L suffix on integer constants.)
        Note, these suffixes appear in macros defined in the
        system headers of most modern systems, e.g. the
        _MIN/_MAX macros in "<limits.h>".  Use of these
        macros in user code might normally lead to spurious
        warnings, however gcc’s integrated preprocessor has
        enough context to avoid warning in these cases.

    •   A function declared external in one block and then
        used after the end of the block.

    •   A "switch" statement has an operand of type "long".

    •   A non‐"static" function declaration follows a
        "static" one.  This construct is not accepted by
        some traditional C compilers.

    •   The ISO type of an integer constant has a different
        width or signedness from its traditional type.  This
        warning is only issued if the base of the constant
        is ten.  I.e. hexadecimal or octal values, which
        typically represent bit patterns, are not warned
        about.










                            ‐41‐


    •   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‐matching
    type.  For example, warn if "int malloc()" is cast to
    "anything *".

−Wcast‐qual
    Warn whenever a pointer is cast so as to remove a type
    qualifier from the target type.  For example, warn if a
    "const char *" is cast to an ordinary "char *".

−Wcast‐align
    Warn whenever a pointer is cast such that the required
    alignment of the target is increased.  For example, warn
    if a "char *" is cast to an "int *" on machines where









                            ‐42‐


    integers can only be accessed at two‐ or four‐byte
    boundaries.

−Wwrite‐strings
    When compiling C, give string constants the type "const
    char[length]" so that copying the address of one into a
    non‐"const" "char *" pointer will get a warning; when
    compiling C++, warn about the deprecated conversion from
    string constants to "char *".  These warnings will help
    you find at compile time code that can try to write into
    a string constant, but only if you have been very
    careful about using "const" in declarations and
    prototypes.  Otherwise, it will just be a nuisance; this
    is why we did not make −Wall request these warnings.

−Wconversion
    Warn if a prototype causes a type conversion that is
    different from what would happen to the same argument in
    the absence of a prototype.  This includes conversions
    of fixed point to floating and vice versa, and
    conversions changing the width or signedness of a fixed
    point argument except when the same as the default
    promotion.

    Also, warn if a negative integer constant expression is
    implicitly converted to an unsigned type.  For example,
    warn about the assignment "x = −1" if "x" is unsigned.
    But do not warn about explicit casts like "(unsigned)
    −1".

−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 previous
    prototype declaration.  This warning is issued even if
    the definition itself provides a prototype.  The aim is
    to detect global functions that fail to be declared in
    header files.









                            ‐43‐


−Wmissing‐declarations
    Warn if a global function is defined without a previous
    declaration.  Do so even if the definition itself
    provides a prototype.  Use this option to detect global
    functions that are not declared in header files.

−Wmissing‐noreturn
    Warn about functions which might be candidates for
    attribute "noreturn".  Note these are only possible
    candidates, not absolute ones.  Care should be taken to
    manually verify functions actually do not ever return
    before adding the "noreturn" attribute, otherwise subtle
    code generation bugs could be introduced.  You will not
    get a warning for "main" in hosted C environments.

−Wmissing‐format‐attribute
    If −Wformat is enabled, also warn about functions which
    might be candidates for "format" attributes.  Note these
    are only possible candidates, not absolute ones.  GCC
    will guess that "format" attributes might be appropriate
    for any function that calls a function like "vprintf" or
    "vscanf", but this might not always be the case, and
    some functions for which "format" attributes are
    appropriate may not be detected.  This option has no
    effect unless −Wformat is enabled (possibly by −Wall).

−Wno‐deprecated‐declarations
    Do not warn about uses of functions, variables, and
    types marked as deprecated by using the "deprecated"
    attribute.  (@pxref{Function Attributes},
    @pxref{Variable 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 mis‐aligned
    for little benefit.  For instance, in this code, the
    variable "f.x" in "struct bar" will be misaligned even
    though "struct bar" does not itself have the packed
    attribute:

            struct foo {
              int x;
              char a, b, c, d;
            } __attribute__((packed));
            struct bar {
              char z;
              struct foo f;
            };


−Wpadded
    Warn if padding is included in a structure, either to
    align an element of the structure or to align the whole









                            ‐44‐


    structure.  Sometimes when this happens it is possible
    to rearrange the fields of the structure to reduce the
    padding and so make the structure smaller.

−Wredundant‐decls
    Warn if anything is declared more than once in the same
    scope, even in cases where multiple declaration is valid
    and changes nothing.

−Wnested‐externs (C only)
    Warn if an "extern" declaration is encountered within a
    function.

−Wunreachable‐code
    Warn if the compiler detects that code will never be
    executed.

    This option is intended to warn when the compiler
    detects that at least a whole line of source code will
    never be executed, because some condition is never
    satisfied or because it is after a procedure that never
    returns.

    It is possible for this option to produce a warning even
    though there are circumstances under which part of the
    affected line can be executed, so care should be taken
    when removing apparently‐unreachable code.

    For instance, when a function is inlined, a warning may
    mean that the line is unreachable in only one inlined
    copy of the function.

    This option is not made part of −Wall because in a
    debugging version of a program there is often
    substantial code which checks correct functioning of the
    program and is, hopefully, unreachable because the
    program does work.  Another common use of unreachable
    code is to provide behavior which is selectable at
    compile‐time.

−Winline
    Warn if a function can not be inlined and it was
    declared as inline.

−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









                            ‐45‐


    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
debugging either your program or GCC:

−g  Produce debugging information in the operating system’s
    native format (stabs, COFF, XCOFF, or DWARF).  GDB can
    work with this debugging information.

    On most systems that use stabs format, −g enables use of
    extra debugging information that only GDB can use; this
    extra information makes debugging work better in GDB but
    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 variables
    you declared may not exist at all; flow of control may
    briefly move where you did not expect it; some
    statements may not be executed because they compute
    constant results or their values were already at hand;
    some statements may execute in different places because
    they were moved out of loops.

    Nevertheless it proves possible to debug optimized
    output.  This makes it reasonable to use the optimizer
    for programs that might have bugs.

    The following options are useful when GCC is generated
    with the capability for more than one debugging format.

−ggdb
    Produce debugging information for use by GDB.  This
    means to use the most expressive format available (DWARF
    2, stabs, or the native format if neither of those are
    supported), including GDB extensions if at all possible.

−gstabs
    Produce debugging information in stabs format (if that
    is supported), without GDB extensions.  This is the
    format used by DBX on most BSD systems.  On MIPS, Alpha









                            ‐46‐


    and System V Release 4 systems this option produces
    stabs debugging output which is not understood by DBX or
    SDB.  On System V Release 4 systems this option requires
    the GNU assembler.

−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 format
    (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 format
    (if that is supported), using GNU extensions understood
    only by the GNU debugger (GDB).  The use of these
    extensions is likely to make other debuggers crash or
    refuse to read the program.

−gdwarf‐2
    Produce debugging information in DWARF version 2 format
    (if that is supported).  This is the format used by DBX
    on IRIX 6.

−gvms
    Produce debugging information in VMS debug format (if
    that is supported).  This is the format used by DEBUG on
    VMS systems.

−glevel










                            ‐47‐


−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 making
    backtraces in parts of the program that you don’t plan
    to debug.  This includes descriptions of functions and
    external variables, but no information about local
    variables and no line numbers.

    Level 3 includes extra information, such as all the
    macro definitions present in the program.  Some
    debuggers support macro expansion when you use −g3.

    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
    suitable for the analysis program "prof".  You must use
    this option when compiling the source files you want
    data about, and you must also use it when linking.

−pg Generate extra code to write profile information
    suitable for the analysis program "gprof".  You must use
    this option when compiling the source files you want
    data about, and you must also use it when linking.

−a  Generate extra code to write profile information for
    basic blocks, which will record the number of times each
    basic block is executed, the basic block start address,
    and the function name containing the basic block.  If −g
    is used, the line number and filename of the start of
    the basic block will also be recorded.  If not
    overridden by the machine description, the default
    action is to append to the text file bb.out.

    This data could be analyzed by a program like "tcov".
    Note, however, that the format of the data is not what
    "tcov" expects.  Eventually GNU "gprof" should be
    extended to process this data.

−Q  Makes the compiler print out each function name as it is
    compiled, and print some statistics about each pass when









                            ‐48‐


    it finishes.

−ftime‐report
    Makes the compiler print some statistics about the time
    consumed by each pass when it finishes.

−fmem‐report
    Makes the compiler print some statistics about permanent
    memory allocation when it finishes.

−fprofile‐arcs
    Instrument arcs during compilation to generate coverage
    data or for profile‐directed block ordering.  During
    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 program
    with −fprofile‐arcs plus optimization and code
    generation options, generate the arc profile information
    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 spanning
    tree for the graph.  Only arcs that are not on the
    spanning tree have to be instrumented: the compiler adds
    code to count the number of times that these arcs are
    executed.  When an arc is the only exit or only entrance
    to a block, the instrumentation code can be added to the
    block; otherwise, a new basic block must be created to
    hold the instrumentation code.

−ftest‐coverage
    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.









                            ‐49‐


        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
        "sourcename.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 debugging 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
        preprocessing, 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.










                            ‐50‐


    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
        reorganisation pass, to file.30.mach.

    n   Dump after register renumbering, to file.25.rnreg.

    N   Dump after the register move pass, to
        file.18.regmove.

    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.











                            ‐51‐


    z   Dump after the peephole pass, to file.24.peephole2.

    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
        indicating 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
        annotation.

    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
        compiling it.  Usually used with r.

    y   Dump debugging information during parsing, to
        standard error.

−fdump‐unnumbered
    When doing debugging dumps (see −d option above),
    suppress instruction numbers and line number note
    output.  This makes it more feasible to use diff on
    debugging dumps for compiler invocations with different
    options, in particular with and without −g.

−fdump‐translation‐unit (C and C++ only)

−fdump‐translation‐unit‐options (C and C++ only)
    Dump a representation of the tree structure for the
    entire translation unit to a file.  The file name is
    made by appending .tu to the source file name.  If the
    options form is used, options controls the details of
    the dump as described for the −fdump‐tree options.

−fdump‐class‐hierarchy (C++ only)

−fdump‐class‐hierarchy‐options (C++ only)
    Dump a representation of each class’s hierarchy and
    virtual function table layout to a file.  The file name
    is made by appending .class to the source file name.  If
    the options form is used, options controls the details
    of the dump as described for the −fdump‐tree options.

−fdump‐tree‐switch (C++ only)











                            ‐52‐


−fdump‐tree‐switchoptions (C++ only)
    Control the dumping at various stages of processing the
    intermediate language tree to a file.  The file name is
    generated by appending a switch specific suffix to the
    source file name.  If the options form is used, options
    is a list of separated options that control the
    details of the dump. Not all options are applicable to
    all dumps, those which are not meaningful will be
    ignored. The following options are available

    address
        Print the address of each node.  Usually this is not
        meaningful as it changes according to the
        environment and source file. Its primary use is for
        tying up a dump file with a debug environment.

    slim
        Inhibit dumping of members of a scope or body of a
        function merely because that scope has been reached.
        Only dump such items when they are directly
        reachable by some other path.

    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‐verbose also
    includes dependence info.










                            ‐53‐


−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 preprocessed
    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{‐, without spaces
    between multiple switches.  This is supposed to ease
    shell‐processing.

−print‐prog‐name=program
    Like −print‐file‐name, but searches for a program such









                            ‐54‐


    as cpp.

−print‐libgcc‐file‐name
    Same as −print‐file‐name=libgcc.a.

    This is useful when you use −nostdlib or −nodefaultlibs
    but you do want to link with libgcc.a.  You can do

            gcc ‐nostdlib <files>... ‘gcc ‐print‐libgcc‐file‐name‘


−print‐search‐dirs
    Print the name of the configured installation directory
    and a list of program and library directories gcc will
    search−−−and don’t do anything else.

    This is useful when gcc prints the error message
    installation problem, cannot exec cpp0: No such file or
    directory.  To resolve this you either need to put cpp0
    and the other compiler components where gcc expects to
    find them, or you can set the environment variable
    GCC_EXEC_PREFIX to the directory where you installed
    them.  Don’t forget the trailing ’/’.

−dumpmachine
    Print the compiler’s target machine (for example,
    i686−pc‐linux‐gnu)−−−and don’t do anything else.

−dumpversion
    Print the compiler version (for example, 3.0)−−−and
    don’t do anything else.

−dumpspecs
    Print the compiler’s built‐in specs−−−and don’t do
    anything else.  (This is used when GCC itself is being
    built.)

     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.









                            ‐55‐


    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
    tradeoff.  The compiler does not perform loop unrolling
    or function inlining when you specify −O2.  As compared
    to −O, this option increases both compilation time and
    the performance of the generated code.

    −O2 turns on all optional optimizations except for loop
    unrolling, function inlining, and register renaming.  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‐
    functions 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 memory.

    This option prevents undesirable excess precision on
    machines such as the 68000 where the floating registers
    (of the 68881) keep more precision than a "double" is
    supposed to have.  Similarly for the x86 architecture.
    For most programs, the excess precision does only good,
    but a few programs rely on the precise definition of
    IEEE floating point.  Use −ffloat‐store for such









                            ‐56‐


    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 functions
    defined inside class scope are compiled inline by
    default; i.e., you don’t need to add inline in front of
    the member function name.

−fno‐defer‐pop
    Always pop the arguments to each function call as soon
    as that function returns.  For machines which must pop
    arguments after a function call, the compiler normally
    lets arguments accumulate on the stack for several
    function calls and pops them all at once.

−fforce‐mem
    Force memory operands to be copied into registers before
    doing arithmetic on them.  This produces better code by
    making all memory references potential common
    subexpressions.  When they are not common
    subexpressions, instruction combination should eliminate
    the separate register‐load.  The −O2 option turns on
    this option.

−fforce‐addr
    Force memory address constants to be copied into
    registers before doing arithmetic on them.  This may
    produce better code just as −fforce‐mem may.

−fomit‐frame‐pointer
    Don’t keep the frame pointer in a register for functions
    that don’t need one.  This avoids the instructions to
    save, set up and restore frame pointers; it also makes
    an extra register available in many functions.  It also
    makes debugging impossible on some machines.

    On some machines, such as the VAX, this flag has no
    effect, because the standard calling sequence
    automatically handles the frame pointer and nothing is
    saved by pretending it doesn’t exist.  The machine‐
    description macro "FRAME_POINTER_REQUIRED" controls
    whether a target machine supports this flag.

−foptimize‐sibling‐calls
    Optimize sibling and tail recursive calls.

−ftrapv
    This option generates traps for signed overflow on
    addition, subtraction, multiplication operations.











                            ‐57‐


−fno‐inline
    Don’t pay attention to the "inline" keyword.  Normally
    this option is used to keep the compiler from expanding
    any functions inline.  Note that if you are not
    optimizing, no functions can be expanded inline.

−finline‐functions
    Integrate all simple functions into their callers.  The
    compiler heuristically decides which functions are
    simple enough to be worth integrating in this way.

    If all calls to a given function are integrated, and the
    function is declared "static", then the function is
    normally not output as assembler code in its own right.

−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 particular
    context, an abstract measurement of function’s size.  In
    no way, it represents a count of assembly instructions
    and as such its exact meaning might change from one
    release to an another.

−fkeep‐inline‐functions
    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 optimization
    isn’t turned on, even if the variables aren’t
    referenced.

    GCC enables this option by default.  If you want to
    force the compiler to check if the variable was
    referenced, regardless of whether or not optimization is
    turned on, use the −fno‐keep‐static‐consts option.










                            ‐58‐


−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‐
    constants 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 support
    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









                            ‐59‐


    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 similar
    optimizations.

    This option should never be turned on by any −O option
    since it can result in incorrect output for programs
    which depend on an exact implementation of IEEE or ISO
    rules/specifications for math functions.

    The default is −fno‐unsafe‐math‐optimizations.

−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 ‘‘non‐stop’’ IEEE
    arithmetic, for example.

    This option should never be turned on by any −O option
    since it can result in incorrect output for programs
    which depend on an exact implementation of IEEE or ISO
    rules/specifications for math functions.

    The default is −ftrapping‐math.

−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 supported 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
differently.











                            ‐60‐


     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 subsumed
    by the first is found.  If so, the first branch is
    redirected to either the destination of the second
    branch or a point immediately following it, depending on
    whether the condition is known to be true or false.

−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 elmination
    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









                            ‐61‐


    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
    optimization for programs which depend on that behavior.

−fexpensive‐optimizations
    Perform a number of minor optimizations that are
    relatively expensive.

−foptimize‐register‐move

−fregmove
    Attempt to reassign register numbers in move
    instructions and as operands of other simple
    instructions in order to maximize the amount of register
    tying.  This is especially helpful on machines with two‐
    operand instructions.  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.











                            ‐62‐


−fschedule‐insns2
    Similar to −fschedule‐insns, but requests an additional
    pass of instruction scheduling after register allocation
    has been done.  This is especially useful on machines
    with a relatively small number of registers and where
    memory load instructions take more than one cycle.

−fno‐sched‐interblock
    Don’t schedule instructions across basic blocks.  This
    is normally enabled by default when scheduling before
    register allocation, i.e.  with −fschedule‐insns or at
    −O2 or higher.

−fno‐sched‐spec
    Don’t allow speculative motion of non‐load instructions.
    This is normally enabled by default when scheduling
    before register allocation, i.e.  with −fschedule‐insns
    or at −O2 or higher.

−fsched‐spec‐load
    Allow speculative motion of some load instructions.
    This only makes sense when scheduling before register
    allocation, i.e. with −fschedule‐insns or at −O2 or
    higher.

−fsched‐spec‐load‐dangerous
    Allow speculative motion of more load instructions.
    This only makes sense when scheduling before register
    allocation, i.e. with −fschedule‐insns or at −O2 or
    higher.

−ffunction‐sections

−fdata‐sections
    Place each function or data item into its own section in
    the output file if the target supports arbitrary
    sections.  The name of the function or the name of the
    data item determines the section’s name in the output
    file.

    Use these options on systems where the linker can
    perform optimizations to improve locality of reference
    in the instruction space.  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
    benefits from doing so.  When you specify these options,
    the assembler and linker will create larger object and
    executable files and will also be slower.  You will not
    be able to use "gprof" on all systems if you specify
    this option and you may have problems with debugging if









                            ‐63‐


    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
    determined at compile time or upon entry to the loop.
    −funroll‐loops implies both −fstrength‐reduce and
    −frerun‐cse‐after‐loop.  This option makes code larger,
    and may or may not make it run faster.

−funroll‐all‐loops
    Unroll all loops, even if their number of iterations is
    uncertain when the loop is entered.  This usually makes
    programs run more slowly.  −funroll‐all‐loops implies
    the same options as −funroll‐loops,

−fprefetch‐loop‐arrays
    If supported by the target machine, generate
    instructions 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 various
    approaches to improving loop optimizations.









                            ‐64‐


    Please let us (<gcc@gcc.gnu.org> and <fortran@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‐peephole2
    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‐
    probabilities, to improve optimizations based on the
    number of times each branch was taken.  When the program
    compiled with −fprofile‐arcs exits it saves arc
    execution counts to a file called sourcename.da for each
    source file  The information in this data file is very
    dependent on the structure of the generated code, so you
    must use the same source code and the same optimization
    options for both compilations.

    With −fbranch‐probabilities, GCC puts a REG_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 different
    runs of the compiler to produce code that has different
    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









                            ‐65‐


    rules applicable to the language being compiled.  For C
    (and C++), this activates optimizations based on the
    type of expressions.  In particular, an object of one
    type is assumed never to reside at the same address as
    an object of a different type, unless the types are
    almost the same.  For example, an "unsigned int" can
    alias an "int", but not a "void*" or a "double".  A
    character type may alias any other type.

    Pay special attention to code like this:

            union a_union {
              int i;
              double d;
            };

            int f() {
              a_union t;
              t.d = 3.0;
              return t.i;
            }

    The practice of reading from a different union member
    than the one most recently written to (called ‘‘type‐
    punning’’) is common.  Even with −fstrict‐aliasing,
    type‐punning is allowed, provided the memory is accessed
    through the union type.  So, the code above will work as
    expected.  However, this code might not:

            int f() {
              a_union t;
              int* ip;
              t.d = 3.0;
              ip = &t.i;
              return *ip;
            }

    Every language that wishes to perform language‐specific
    alias analysis should define a function that computes,
    given an "tree" node, an alias set for the node.  Nodes
    in different alias sets are not allowed to alias.  For
    an example, see the C front‐end function
    "c_get_alias_set".

−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.










                            ‐66‐


    −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‐functions.
    In this case, no dummy operations need be executed.

    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









                            ‐67‐


    form.  Requires −fssa.  Like −fssa, this is an
    experimental 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
    constant 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
    processors 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 number
        of instructions to consider when searching for a
        block with valid live register information.
        Increasing this arbitrarily chosen value means more
        aggressive optimization, increasing the compile









                            ‐68‐


        time.  This parameter should be removed when the
        delay slot code is rewritten to maintain the
        control‐flow graph.

    max‐gcse‐memory
        The approximate maximum amount of memory that will
        be allocated in order to perform the global common
        subexpression elimination optimization.  If more
        memory than specified is required, the optimization
        will not be done.

    max‐gcse‐passes
        The maximum number of passes of GCSE to run.

    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
preprocessing.  Some of these options make sense only
together with −E because they cause the preprocessor output
to be unsuitable for actual compilation.

     You can use −Wp,option to bypass the compiler driver
and pass option directly through to the preprocessor.  If
option contains commas, it is split into multiple options at
the commas.  However, many options are modified, translated
or interpreted by the compiler driver before being passed to
the preprocessor, and −Wp forcibly bypasses this phase.  The
preprocessor’s direct interface is undocumented and subject
to change, so whenever possible you should avoid using −Wp
and let the driver handle the options instead.

−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 definition,
    but if you are invoking the preprocessor from a shell or
    shell‐like program you may need to use the shell’s









                            ‐69‐


    quoting syntax to protect characters such as spaces that
    have a meaning in the shell syntax.

    If you wish to define a function‐like macro on the
    command line, write its argument list with surrounding
    parentheses before the equals sign (if any).
    Parentheses are meaningful to most shells, so you will
    need to quote the option.  With sh and csh,
    −D’name(args...)=definitionworks.

    −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 common
    predefined macros remain defined.

−I dir
    Add the directory dir to the list of directories to be
    searched for header files.  Directories named by −I are
    searched before the standard system include directories.

    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
    −Wtrigraphs.  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.)










                            ‐70‐


−Wtrigraphs
    Warn if any trigraphs are encountered.  This option used
    to take effect only if −trigraphs was also specified,
    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 constructs
    that have no traditional C equivalent, and problematic
    constructs which should be avoided.

−Wimport
    Warn the first time #import is used.

−Wundef
    Warn whenever an identifier which is not a macro is
    encountered in an #if directive, outside of defined.
    Such identifiers are replaced with zero.

−Werror
    Make all warnings into hard errors.  Source code which
    triggers warnings will be rejected.

−Wsystem‐headers
    Issue warnings for code in system headers.  These are
    normally unhelpful in finding bugs in your own code,
    therefore suppressed.  If you are responsible for the
    system library, you may want to see them.

−w  Suppress all warnings, including those which GNU CPP
    issues by default.

−pedantic
    Issue all the mandatory diagnostics listed in the C
    standard.  Some of them are left out by default, since
    they trigger frequently on harmless code.

−pedantic‐errors
    Issue all the mandatory diagnostics, and make all
    mandatory diagnostics into errors.  This includes
    mandatory diagnostics that GCC issues without −pedantic
    but treats as warnings.

−M  Instead of outputting the result of preprocessing,
    output a rule suitable for make describing the
    dependencies of the main source file.  The preprocessor
    outputs one make rule containing the object file name
    for that source file, a colon, and the names of all the
    included files, including those coming from −include or
    −imacros command line options.

    Unless specified explicitly (with −MT or −MQ), the









                            ‐71‐


    object file name consists of the basename of the source
    file with any suffix replaced with object file suffix.
    If there are many included files then the rule is split
    into several lines using \−newline.  The rule has no
    commands.

    This option does not suppress the preprocessor’s debug
    output, such as −dM.  To avoid mixing such debug output
    with the dependency rules you should explicitly specify
    the dependency output file with −MF, or use an
    environment variable like DEPENDENCIES_OUTPUT.  Debug
    output will still be sent to the regular output stream
    as normal.

    Passing −M to the driver implies −E.

−MM Like −M but do not mention header files that are found
    in system header directories, nor header files that are
    included, directly or indirectly, from such a header.

    This implies that the choice of angle brackets or double
    quotes in an #include directive does not in itself
    determine whether that header will appear in −MM
    dependency output.  This is a slight change in semantics
    from GCC versions 3.0 and earlier.

−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 suppresses
    preprocessed output, as a missing header file is
    ordinarily an error.

    This feature is used in automatic updating of makefiles.

−MP This option instructs CPP to add a phony target for each
    dependency other than the main file, causing each to
    depend on nothing.  These dummy rules work around errors
    make gives if you remove header files without updating
    the Makefile to match.

    This is typical output:

            test.o: test.c test.h

            test.h:









                            ‐72‐


−MT target
    Change the target of the rule emitted by dependency
    generation.  By default CPP takes the name of the main
    input file, including any path, deletes any file suffix
    such as .c, and appends the platform’s usual object
    suffix.  The result is the target.

    An −MT option will set the target to be exactly the
    string you specify.  If you want multiple targets, you
    can specify them as a single argument to −MT, or use
    multiple −MT options.

    For example, −MT ’$(objpfx)foo.o’ might give

            $(objpfx)foo.o: foo.c


−MQ target
    Same as −MT, but it quotes any characters which are
    special to Make.  −MQ ’$(objpfx)foo.o’ gives

            $$(objpfx)foo.o: foo.c

    The default target is automatically quoted, as if it
    were given with −MQ.

−MD −MD is equivalent to −M −MF file, except that −E is not
    implied.  The driver determines file based on whether an
    −o option is given.  If it is, the driver uses its
    argument but with a suffix of .d, otherwise it take the
    basename of the input file and applies a .d suffix.

    If −MD is used in conjunction with −E, any −o switch is
    understood to specify the dependency output file (but
    @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
    compilation 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









                            ‐73‐


    conformance or extensions; it merely selects which base
    syntax to expect.  If you give none of these options,
    cpp will deduce the language from the extension of the
    source file: .c, .cc, .m, 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 customary
        shorthand for this version of the standard.

        The −ansi option is equivalent to −std=c89.

    ""iso9899:199409""
        The 1990 C standard, as amended in 1994.

    ""iso9899:1999""

    ""c99""

    ""iso9899:199x""

    ""c9x""
        The revised ISO C standard, published in December
        1999.  Before publication, this was known as C9X.

    ""gnu89""
        The 1990 C standard plus GNU extensions.  This is
        the default.

    ""gnu99""

    ""gnu9x""
        The 1999 C standard plus GNU extensions.

−I‐ Split the include path.  Any directories specified with
    −I options before −I‐ are searched only for headers









                            ‐74‐


    requested with "#include "file""; they are not searched
    for "#include <file>".  If additional directories are
    specified with −I options after the −I‐, those
    directories are searched for all #include directives.

    In addition, −I‐ inhibits the use of the directory of
    the current file directory as the first search directory
    for "#include "file"".

−nostdinc
    Do not search the standard system directories for header
    files.  Only the directories you have specified with −I
    options (and the directory of the current file, if
    appropriate) are searched.

−nostdinc++
    Do not search for header files in the C++−specific
    standard directories, but do still search the other
    standard directories.  (This option is used when
    building the C++ library.)

−include file
    Process file as if "#include "file"" appeared as the
    first line of the primary source file.  However, the
    first directory searched for file is the preprocessor’s
    working directory instead of the directory containing
    the main source file.  If not found there, it is
    searched for in the remainder of the "#include "...""
    search chain as normal.

    If multiple −include options are given, the files are
    included in the order they appear on the command line.

−imacros file
    Exactly like −include, except that any output produced
    by scanning file is thrown away.  Macros it defines
    remain defined.  This allows you to acquire all the
    macros from a header without also processing its
    declarations.

    All files specified by −imacros are processed before all
    files specified by −include.

−idirafter dir
    Search dir for header files, but do it after all
    directories specified with −I and the standard system
    directories have been exhausted.  dir is treated as a
    system include directory.

−iprefix prefix
    Specify prefix as the prefix for subsequent −iwithprefix
    options.  If the prefix represents a directory, you
    should include the final /.










                            ‐75‐


−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
    directories.  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 comments, so
    that you can pass a file preprocessed with −C to the
    compiler without problems.  In this mode the integrated
    preprocessor is little more than a tokenizer for the
    front ends.

    −fpreprocessed is implicit if the input file has one of
    the extensions .i, .ii or .mi.  These are the extensions
    that GCC uses for preprocessed files created by −save‐
    temps.

−ftabstop=width
    Set the distance between tab stops.  This helps the
    preprocessor report correct column numbers in warnings
    or errors, even if tabs appear on the line.  If the
    value is less than 1 or greater than 100, the option is
    ignored.  The default is 8.

−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.











                            ‐76‐


−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 during
        the execution of the preprocessor, including
        predefined macros.  This gives you a way of finding
        out what is predefined in your version of the
        preprocessor.  Assuming you have no file foo.h, the
        command

                touch foo.h; cpp ‐dM foo.h

        will show all the predefined macros.

    D   Like M except in two respects: it does not include
        the predefined macros, and it outputs both the
        #define directives and the result of preprocessing.
        Both kinds of output go to the standard output file.

    N   Like D, but emit only the macro names, not their
        expansions.

    I   Output #include directives in addition to the result
        of preprocessing.

−P  Inhibit generation of linemarkers in the output from the
    preprocessor.  This might be useful when running the
    preprocessor on something that is not C code, and will
    be sent to a program which might be confused by the
    linemarkers.

−C  Do not discard comments.  All comments are passed
    through to the output file, except for comments in
    processed directives, which are deleted along with the
    directive.

    You should be prepared for side effects when using −C;
    it causes the preprocessor to treat comments as tokens









                            ‐77‐


    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 for \, so ’??/n’ is a character constant for a
    newline.  By default, GCC ignores trigraphs, but in
    standard‐conforming modes it converts them.  See the
    −std and −ansi options.

    The nine trigraphs and their replacements are

            Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??’  ??!  ??‐
            Replacement:      [    ]    {    }    #    \    ^    ⎪    ~


−remap
    Enable special code to work around file systems which
    only permit very short file names, such as MS‐DOS.

−$  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









                            ‐78‐


    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 contents.)  If linking is
    done, these object files are used as input to the
    linker.

−c

−S

−E  If any of these options is used, then the linker is not
    run, and object file names should not be used as
    arguments.

−llibrary

−l library
    Search the library named library when linking.  (The
    second alternative with the library as a separate
    argument is only for POSIX compliance and is not
    recommended.)

    It makes a difference where in the command you write
    this option; the linker searches and processes libraries
    and object files in the order they are specified.  Thus,
    foo.o −lz bar.o searches library z after file foo.o but
    before bar.o.  If bar.o refers to functions in z, those
    functions may not be loaded.









                            ‐79‐


    The linker searches a standard list of directories for
    the library, which is actually a file named
    liblibrary.a.  The linker then uses this file as if it
    had been specified precisely by name.

    The directories searched include several standard system
    directories plus any that you specify with −L.

    Normally the files found this way are library
    files−−−archive files whose members are object files.
    The linker handles an archive file by scanning through
    it for members which define symbols that have so far
    been referenced but not defined.  But if the file that
    is found is an ordinary object file, it is linked in the
    usual fashion.  The only difference between using an −l
    option and specifying a file name is that −l surrounds
    library with lib and .a and searches several
    directories.

−lobjc
    You need this special case of the −l option in order to
    link an Objective‐C program.

−nostartfiles
    Do not use the standard system startup files when
    linking.  The standard system libraries are used
    normally, unless −nostdlib or −nodefaultlibs is used.

−nodefaultlibs
    Do not use the standard system libraries when linking.
    Only the libraries you specify will be passed to the
    linker.  The standard startup files are used normally,
    unless −nostartfiles is used.  The compiler may generate
    calls to memcmp, memset, 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.

−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









                            ‐80‐


    particular machines, or special needs for some
    languages.

    In most cases, you need libgcc.a even when you want to
    avoid other standard libraries.  In other words, when
    you specify −nostdlib or −nodefaultlibs you should
    usually specify −lgcc as well.  This ensures that you
    have no unresolved references to internal GCC library
    subroutines.  (For example, __main, used to ensure C++
    constructors will be called.)

−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 suboptions) 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
    application wishes to throw and catch exceptions across
    different shared libraries.  In that case, each of the
    libraries as well as the application itself should use
    the shared libgcc.

    Therefore, the G++ and GCJ drivers automatically add
    −shared‐libgcc whenever you build a shared library or a
    main executable, because C++ and Java programs typically
    use exceptions, so this is the right thing to do.

    If, instead, you use the GCC driver to create shared
    libraries, you may find that they will not always be
    linked with the shared libgcc.  If GCC finds, at its
    configuration time, that you have a GNU linker that does
    not support option ‐‐eh‐frame‐hdr, it will link the









                            ‐81‐


    shared version of libgcc into shared libraries by
    default.  Otherwise, it will take advantage of the
    linker and optimize away the linking with the shared
    version of libgcc, linking with the static version of
    libgcc by default.  This allows exceptions to propagate
    through such shared libraries, without incurring
    relocation costs at library load time.

    However, if a library or main executable is supposed to
    throw or catch exceptions, you must link it using the
    G++ or GCJ driver, as appropriate for the languages used
    in the program, or using the option −shared‐libgcc, such
    that it is linked with the shared libgcc.

−symbolic
    Bind references to global symbols when building a shared
    object.  Warn about any unresolved references (unless
    overridden by the link editor option −Xlinker −z
    −Xlinker defs).  Only a few systems support this option.

−Xlinker option
    Pass option as an option to the linker.  You can use
    this to supply system‐specific linker options which GCC
    does not know how to recognize.

    If you want to pass an option that takes an argument,
    you must use −Xlinker twice, once for the option and
    once for the argument.  For example, to pass −assert
    definitions, you must write −Xlinker −assert −Xlinker
    definitions.  It does not work to write −Xlinker
    "−assert definitions", because this passes the entire
    string as a single argument, which is not what the
    linker expects.

−Wl,option
    Pass option as an option to the linker.  If option
    contains commas, it is split into multiple options at
    the commas.

−u symbol
    Pretend the symbol symbol is undefined, to force linking
    of library modules to define it.  You can use −u
    multiple times with different symbols to force loading
    of additional library modules.

     Options for Directory Search

     These options specify directories to search for header
files, for libraries and for parts of the compiler:

−Idir
    Add the directory dir to the head of the list of
    directories to be searched for header files.  This can
    be used to override a system header file, substituting









                            ‐82‐


    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 compiler
    itself.

    The compiler driver program runs one or more of the
    subprograms cpp, cc1, as and ld.  It tries prefix as a
    prefix for each program it tries to run, both with and
    without machine/version/.









                            ‐83‐


    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 prefix.

    The run‐time support file libgcc.a can also be searched
    for using the −B prefix, if needed.  If it is not found
    there, the two standard prefixes above are tried, and
    that is all.  The file is left out of the link if it is
    not found by those means.

    Another way to specify a prefix much like the −B prefix
    is to use the environment variable GCC_EXEC_PREFIX.

    As a special kludge, if the path provided by −B is
    [dir/]stageN/, where N is a number in the range 0 to 9,
    then it will be replaced by [dir/]include.  This is to
    help with boot‐strapping the compiler.

−specs=file
    Process file after the compiler reads in the standard
    specs file, in order to override the defaults that the
    gcc driver program uses when determining what switches
    to pass to cc1, cc1plus, as, ld, etc.  More than one
    −specs=file can be specified on the command line, and
    they are processed in order, from left to right.

     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.










                            ‐84‐


     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
    specified as the machine type when configuring GCC as a
    cross‐compiler.  For example, if a cross‐compiler was
    configured with configure i386v, meaning to compile for
    an 80386 running System V, then you would specify −b
    i386v to run that cross compiler.

    When you do not specify −b, it normally means to compile
    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
alternate 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, assembler
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









                            ‐85‐


from GCC 3.0.

     The only way that the driver program depends on the
target machine is in the parsing and handling of special
machine‐specific 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
versions, 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
different target machines, such as VAX vs. 68000 vs. 80386.

     In addition, each of these target machine types can
have its own special options, starting with −m, to choose
among various hardware models or configurations−−−for
example, 68010 vs 68020, floating coprocessor or none.  A
single installed version of the compiler can compile for any
model or configuration, according to the options specified.

     Some configurations of the compiler also support
additional special options, usually for compatibility with
other compilers on the same platform.

     These options are defined by the macro
"TARGET_SWITCHES" in the machine description.  The default
for the options is also defined by that macro, which enables
you to change the defaults.

     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 configured; the
defaults for the most common choices are given below.

−m68000










                            ‐86‐


−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 instructions
    that have to be emulated by software on the 68040.  Use
    this option if your 68040 does not have code to emulate
    those instructions.

−m68060
    Generate output for a 68060.  This is the default when
    the compiler is configured for 68060−based systems.

    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.









                            ‐87‐


    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 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.  The embedded targets
    m68k‐*−aout and m68k‐*−coff do provide software 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









                            ‐88‐


    normally used on Unix, so you cannot use it if you need
    to call libraries compiled with the Unix compiler.

    Also, you must provide function prototypes for all
    functions that take variable numbers of arguments
    (including "printf"); otherwise incorrect code will be
    generated for calls to those functions.

    In addition, seriously incorrect code will result if you
    call a function with too many arguments.  (Normally,
    extra arguments are harmlessly ignored.)

    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
    boundaries produces code that runs somewhat faster on
    processors with 32−bit busses at the expense of more
    memory.

    Warning: if you use the −malign‐int switch, GCC will
    align structures containing the above types  differently
    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.










                            ‐89‐


−m6811

−m68hc11
    Generate output for a 68HC11.  This is the default when
    the compiler is configured for 68HC11−based systems.

−m6812

−m68hc12
    Generate output for a 68HC12.  This is the default when
    the compiler is configured for 68HC12−based systems.

−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 handle
    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.










                            ‐90‐


    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 instructions.
    This is the default.

−mno‐fpu

−msoft‐float
    Generate output containing library calls for floating
    point.  Warning: the requisite libraries are not
    available for all SPARC targets.  Normally the
    facilities of the machine’s usual C compiler are used,
    but this cannot be done directly in cross‐compilation.
    You must make your own arrangements to provide suitable
    library functions for cross‐compilation.  The embedded
    targets sparc‐*−aout and sparclite‐*−* do provide
    software floating point support.

    −msoft‐float changes the calling convention in the
    output file; therefore, it is only useful if you compile
    all of a program with this option.  In particular, you
    need to compile libgcc.a, the library that comes with
    GCC, with −msoft‐float in order for this to work.

−mhard‐quad‐float
    Generate output containing quad‐word (long double)
    floating point instructions.

−msoft‐quad‐float
    Generate output containing library calls for quad‐word
    (long double) floating point instructions.  The
    functions called are those specified in the SPARC ABI.
    This is the default.

    As of this writing, there are no sparc implementations
    that have hardware support for the quad‐word floating
    point instructions.  They all invoke a trap handler for
    one of these instructions, and then the trap handler
    emulates the effect of the instruction.  Because of the
    trap handler overhead, this is much slower than calling
    the ABI library routines.  Thus the −msoft‐quad‐float
    option is the default.

−mno‐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









                            ‐91‐


    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









                            ‐92‐


    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.
    Supported values for cpu_type are v7, cypress, v8,
    supersparc, 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
    implementation.  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.









                            ‐93‐


    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,
    sparclite86x, 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 program
    must be linked in the low 32 bits of the address space.
    Pointers are 64 bits.  Programs can be statically 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









                            ‐94‐


    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.  Pointers
    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 references.
    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 Convex
    machine.  The preprocessor symbol "__convex__c1__" is
    defined.

−mc2
    Generate output for C2.  Uses instructions not available
    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 preprocessor
    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 preprocessor
    symbol "__convex_c34__" is defined.










                            ‐95‐


−mc38
    Generate output for C38xx.  Uses instructions not
    available on C1.  Scheduling and other optimizations are
    chosen for max performance on C38.  The preprocessor
    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‐processor
    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 supported
    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 support









                            ‐96‐


    byte and halfword write operations.  −mnbw implies
    −mndw.

−msmall
    Use a small memory model that assumes that all function
    addresses are either within a single 256 KB segment 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"
    instructions 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 handle
    the separation of a mtsrim insn and a storem instruction









                            ‐97‐


    (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
    arguments 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 generated 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 calling
    standards for the APCS 26−bit option.  This option
    replaces the −m2 and −m3 options of previous releases of
    the compiler.










                            ‐98‐


−mapcs‐32
    Generate code for a processor running with a 32−bit
    program counter, and conforming to the function calling
    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
    information can be used to locate the start if functions
    inside an executable piece of code.  The default is
    −msched‐prolog.

−mhard‐float
    Generate output containing floating point instructions.
    This is the default.

−msoft‐float
    Generate output containing library calls for floating
    point.  Warning: the requisite libraries are not
    available for all ARM targets.  Normally the facilities
    of the machine’s usual C compiler are used, but this
    cannot be done directly in cross‐compilation.  You must
    make your own arrangements to provide suitable library
    functions for cross‐compilation.

    −msoft‐float changes the calling convention in the
    output file; therefore, it is only useful if you compile
    all of a program with this option.  In particular, you
    need to compile libgcc.a, the library that comes with
    GCC, with −msoft‐float in order for this to work.

−mlittle‐endian
    Generate code for a processor running in little‐endian
    mode.  This is the default for all standard
    configurations.

−mbig‐endian
    Generate code for a processor running in big‐endian
    mode; the default is to compile code for a little‐endian
    processor.









                            ‐99‐


−mwords‐little‐endian
    This option only applies when generating code for big‐
    endian processors.  Generate code for a little‐endian
    word order but a big‐endian byte order.  That is, a byte
    order of the form 32107654.  Note: this option should
    only be used if you require compatibility with code for
    big‐endian ARM processors generated by versions of the
    compiler prior to 2.8.

−malignment‐traps
    Generate code that will not trap if the MMU has
    alignment 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
    misaligned 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
    architecture 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‐alignment‐
    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.











                            ‐100‐


−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 linking 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
    instructions it can emit when generating assembly code.
    Permissible names are: arm2, arm250, arm3, arm6, arm60,
    arm600, arm610, arm620, arm7, arm7m, arm7d, arm7dm,
    arm7di, arm7dmi, arm70, arm700, arm700i, arm710,
    arm710c, arm7100, arm7500, arm7500fe, arm7tdmi, 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 instructions can be
    used, it specifies that GCC should tune the performance
    of the code as if the target were of the type specified
    in this option, but still choosing the instructions that
    it will generate based on the cpu specified by a −mcpu=
    option.  For some ARM implementations better performance
    can be obtained by using this option.

−march=name
    This specifies the name of the target ARM architecture.
    GCC uses this name to determine what kind of
    instructions it can emit when generating assembly code.
    This option can be used in conjunction with or instead
    of the −mcpu= option.  Permissible names are: armv2,
    armv2a, armv3, armv3m, armv4, armv4t, armv5, armv5t,
    armv5te.

−mfpe=number

−mfp=number
    This specifies the version of the floating point









                            ‐101‐


    emulation available on the target.  Permissible values
    are 2 and 3.  −mfp= is a synonym for −mfpe=, for
    compatibility 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 register.
    This switch is needed if the target function will lie
    outside of the 64 megabyte addressing range of the
    offset based version of subroutine call instruction.

    Even if this switch is enabled, not all function calls
    will be turned into long calls.  The heuristic is that
    static functions, functions which have the short‐call
    attribute, functions that are inside the scope of a
    #pragma no_long_calls directive and functions whose
    definitions have already been compiled within the
    current compilation unit, will not be turned into long
    calls.  The exception to this rule is that weak function
    definitions, functions with the long‐call attribute or
    the section attribute, and functions that are within the
    scope of a #pragma long_calls directive, will always be
    turned into long calls.

    This feature is not enabled by default.  Specifying
    −mno‐long‐calls will restore the default behavior, as
    will placing the function 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.









                            ‐102‐


−msingle‐pic‐base
    Treat the register used for PIC addressing as read‐only,
    rather than loading it in the prologue for each
    function.  The run‐time system is responsible for
    initializing this register with an appropriate value
    before execution begins.

−mpic‐register=reg
    Specify the register to be used for PIC addressing.  The
    default is R10 unless stack‐checking is enabled, when R9
    is used.

−mpoke‐function‐name
    Write the name of each function into the text section,
    directly preceding the function prologue.  The generated
    code is similar to this:

                 t0
                     .ascii "arm_poke_function_name", 0
                     .align
                 t1
                     .word 0xff000000 + (t1 ‐ t0)
                 arm_poke_function_name
                     mov     ip, sp
                     stmfd   sp!, {fp, ip, lr, pc}
                     sub     fp, ip, #4

    When performing a stack backtrace, code can inspect the
    value of "pc" stored at "fp + 0".  If the trace function
    then looks at location "pc − 12" and the top 8 bits are
    set, then we know that there is a function name embedded
    immediately preceding this location and has length
    "((pc[−3]) & 0xff000000)".

−mthumb
    Generate code for the 16−bit Thumb instruction set.  The
    default is to use the 32−bit ARM instruction set.

−mtpcs‐frame
    Generate a stack frame that is compliant with the Thumb
    Procedure Call Standard for all non‐leaf functions.  (A
    leaf function is one that does not call any other
    functions.)  The default is −mno‐tpcs‐frame.

−mtpcs‐leaf‐frame
    Generate a stack frame that is compliant with the Thumb
    Procedure Call Standard for all leaf functions.  (A leaf
    function is one that does not call any other functions.)
    The default is −mno‐apcs‐leaf‐frame.

−mcallee‐super‐interworking
    Gives all externally visible functions in the file being
    compiled an ARM instruction set header which switches to
    Thumb mode before executing the rest of the function.









                            ‐103‐


    This allows these functions to be called from non‐
    interworking code.

−mcaller‐super‐interworking
    Allows calls via function pointers (including virtual
    functions) to execute correctly regardless of whether
    the target code has been compiled for interworking or
    not.  There is a small overhead in the cost of executing
    a function pointer if this option is enabled.

     MN10200 Options

     These −m options are defined for Matsushita MN10200
architectures:

−mrelax
    Indicate to the linker that it should perform a
    relaxation optimization pass to shorten branches, calls
    and absolute memory addresses.  This option only has an
    effect when used on the command line for the final link
    step.

    This option makes symbolic debugging impossible.

     MN10300 Options

     These −m options are defined for Matsushita MN10300
architectures:

−mmult‐bug
    Generate code to avoid bugs in the multiply instructions
    for the MN10300 processors.  This is the default.

−mno‐mult‐bug
    Do not generate code to avoid bugs in the multiply
    instructions for the MN10300 processors.

−mam33
    Generate code 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
    relaxation optimization pass to shorten branches, calls
    and absolute memory addresses.  This option only has an
    effect when used on the command line for the final link









                            ‐104‐


    step.

    This option makes symbolic debugging impossible.

     M32R/D Options

     These −m options are defined for Mitsubishi M32R/D
architectures:

−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"
    instructions to load their addresses), and assume all
    subroutines 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"
    instructions 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.











                            ‐105‐


−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
architectures:

−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
    version, 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 specified
    in the 88open Object Compatibility Standard, ‘‘OCS’’.
    This extra information allows debugging of code that has









                            ‐106‐


    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 configurations
    use −mocs‐frame‐position; other 88k configurations 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
    specification, but uses more memory.

−mshort‐data‐num
    Generate smaller data references by making them relative
    to "r0", which allows loading a value using a single
    instruction (rather than the usual two).  You control
    which data references are affected by specifying 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
    particular, a load instruction may execute before a









                            ‐107‐


    preceding store instruction.  Such reordering violates
    sequential 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 references
    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,
    detection 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 number 503
    when one is detected.  Use of −mno‐check‐zero‐division









                            ‐108‐


    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 instruction
    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
    execution 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.









                            ‐109‐


     IBM RS/6000 and PowerPC Options

     These −m options are defined for the IBM RS/6000 and
PowerPC:

−mpower

−mno‐power

−mpower2

−mno‐power2

−mpowerpc

−mno‐powerpc

−mpowerpc‐gpopt

−mno‐powerpc‐gpopt

−mpowerpc‐gfxopt

−mno‐powerpc‐gfxopt

−mpowerpc64

−mno‐powerpc64
    GCC supports two related instruction set architectures
    for the RS/6000 and PowerPC.  The POWER instruction set
    are those instructions supported by the rios chip set
    used in the original RS/6000 systems and the PowerPC
    instruction set is the architecture of the Motorola
    MPC5xx, MPC6xx, MPC8xx microprocessors, and the IBM 4xx
    microprocessors.

    Neither architecture is a subset of the other.  However
    there is a large common subset of instructions supported
    by both.  An MQ register is included in processors
    supporting the POWER architecture.

    You use these options to specify which instructions are
    available on the processor you are using.  The default
    value of these options is determined when configuring
    GCC.  Specifying the −mcpu=cpu_type overrides the
    specification of these options.  We recommend you use
    the −mcpu=cpu_type option rather than the options listed
    above.

    The −mpower option allows GCC to generate instructions
    that are found only in the POWER architecture and to use
    the MQ register.  Specifying −mpower2 implies −power and
    also allows GCC to generate instructions that are
    present in the POWER2 architecture but not the original









                            ‐110‐


    POWER architecture.

    The −mpowerpc option allows GCC to generate instructions
    that are found only in the 32−bit subset of the PowerPC
    architecture.  Specifying −mpowerpc‐gpopt implies
    −mpowerpc and also allows GCC to use the optional
    PowerPC architecture instructions in the General Purpose
    group, including floating‐point square root.  Specifying
    −mpowerpc‐gfxopt implies −mpowerpc and also allows GCC
    to use the optional PowerPC architecture instructions in
    the Graphics group, including floating‐point select.

    The −mpowerpc64 option allows GCC to generate the
    additional 64−bit instructions that are found in the
    full PowerPC64 architecture and to treat GPRs as 64−bit,
    doubleword quantities.  GCC defaults to −mno‐powerpc64.

    If you specify both −mno‐power and −mno‐powerpc, GCC
    will use only the instructions in the common subset of
    both architectures plus some special AIX common‐mode
    calls, and will not use the MQ register.  Specifying
    both −mpower and −mpowerpc permits GCC to use any
    instruction from either architecture and to allow use of
    the MQ register; specify this for the Motorola MPC601.

−mnew‐mnemonics

−mold‐mnemonics
    Select which mnemonics to use in the generated assembler
    code.  With −mnew‐mnemonics, GCC uses the assembler
    mnemonics defined for the PowerPC architecture.  With
    −mold‐mnemonics it uses the assembler mnemonics defined
    for the POWER architecture.  Instructions defined in
    only one architecture have only one mnemonic; GCC uses
    that mnemonic irrespective of which of these options is
    specified.

    GCC defaults to the mnemonics appropriate for the
    architecture in use.  Specifying −mcpu=cpu_type
    sometimes overrides the value of these option.  Unless
    you are building a cross‐compiler, you should normally
    not specify either −mnew‐mnemonics or −mold‐mnemonics,
    but should instead accept the default.

−mcpu=cpu_type
    Set architecture type, register usage, choice of
    mnemonics, and instruction scheduling parameters for
    machine type cpu_type.  Supported values for cpu_type
    are 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.









                            ‐111‐


    Code generated under this option will run on any POWER
    or PowerPC processor.  GCC will use only the
    instructions in the common subset of both architectures,
    and will not use the MQ register.  GCC assumes a generic
    processor model for scheduling purposes.

    −mcpu=power, −mcpu=power2, −mcpu=powerpc, and
    −mcpu=powerpc64 specify generic POWER, POWER2, pure
    32−bit PowerPC (i.e., not MPC601), and 64−bit PowerPC
    architecture machine types, with an appropriate, generic
    processor model assumed for scheduling purposes.

    The other options specify a specific processor.  Code
    generated under those options will run best on that
    processor, and may not run at all on others.

    The −mcpu options automatically enable or disable 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










                            ‐112‐


    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 specified,
    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 available in the TOC.

    If you receive a linker error message that saying you
    have overflowed the available TOC space, you can reduce
    the amount of TOC space used with the −mno‐fp‐in‐toc and
    −mno‐sum‐in‐toc options.  −mno‐fp‐in‐toc prevents GCC
    from putting floating‐point constants in the TOC and
    −mno‐sum‐in‐toc forces GCC to generate code to calculate
    the sum of an address and a constant at run‐time instead
    of putting that sum into the TOC.  You may specify one
    or both of these options.  Each causes GCC to produce









                            ‐113‐


    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 standard
    location (/usr/lpp/ppe.poe/), or the specs file must be
    overridden with the −specs= option to specify the
    appropriate directory location.  The Parallel
    Environment does not support threads, so the −mpe option
    and the −pthread option are incompatible.

−msoft‐float











                            ‐114‐


−mhard‐float
    Generate code that does not use (uses) the floating‐
    point register set.  Software floating point emulation
    is provided if you use the −msoft‐float option, and pass
    the option to GCC when linking.

−mmultiple

−mno‐multiple
    Generate code that uses (does not use) the load multiple
    word instructions and the store multiple word
    instructions.  These instructions are generated by
    default on POWER systems, and not generated on PowerPC
    systems.  Do not use −mmultiple on little endian PowerPC
    systems, since those instructions do not work when the
    processor is in little endian mode.  The exceptions are
    PPC740 and PPC750 which permit 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.











                            ‐115‐


−mno‐bit‐align

−mbit‐align
    On System V.4 and embedded PowerPC systems do not (do)
    force structures and unions that contain bit‐fields to
    be aligned to the base type of the bit‐field.

    For example, by default a structure containing nothing
    but 8 "unsigned" bit‐fields of length 1 would be aligned
    to a 4 byte boundary and have a size of 4 bytes.  By
    using −mno‐bit‐align, the structure would be aligned to
    a 1 byte boundary and be one byte in size.

−mno‐strict‐align

−mstrict‐align
    On System V.4 and embedded PowerPC systems do not (do)
    assume that unaligned memory references will be handled
    by the system.

−mrelocatable

−mno‐relocatable
    On embedded PowerPC systems generate code that allows
    (does not allow) the program to be relocated to a
    different address at runtime.  If you use −mrelocatable
    on any module, all objects linked together must be
    compiled with −mrelocatable or −mrelocatable‐lib.

−mrelocatable‐lib

−mno‐relocatable‐lib
    On embedded PowerPC systems generate code that allows
    (does not allow) the program to be relocated to a
    different address at runtime.  Modules compiled with
    −mrelocatable‐lib can be linked with either modules
    compiled without −mrelocatable and −mrelocatable‐lib or
    with modules compiled with the −mrelocatable options.

−mno‐toc

−mtoc
    On System V.4 and embedded PowerPC systems do not (do)
    assume that register 2 contains a pointer to a global
    area pointing to the addresses used in the program.

−mlittle

−mlittle‐endian
    On System V.4 and embedded PowerPC systems compile code
    for the processor in little endian mode.  The −mlittle‐
    endian option is the same as −mlittle.











                            ‐116‐


−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 powerpc‐*−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 configured 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.









                            ‐117‐


−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









                            ‐118‐


    is aligned to an 8 byte boundary, a function "__eabi" is
    called to from "main" to set up the eabi environment,
    and the −msdata option can use both "r2" and "r13" to
    point to two separate small data areas.  Selecting −mno‐
    eabi means that the stack is aligned to a 16 byte
    boundary, do not call an initialization function from
    "main", and the −msdata option will only use "r13" to
    point to a single small data area.  The −meabi option is
    on by default if you configured GCC using one of the
    powerpc*−*−eabi* options.

−msdata=eabi
    On System V.4 and embedded PowerPC systems, put small
    initialized "const" global and static data in the
    .sdata2 section, which is pointed to by register "r2".
    Put small initialized non‐"const" global and static data
    in the .sdata section, which is pointed to by register
    "r13".  Put small uninitialized global and static data
    in the .sbss section, which is adjacent to the .sdata
    section.  The −msdata=eabi option is incompatible with
    the −mrelocatable option.  The −msdata=eabi option also
    sets the −memb option.

−msdata=sysv
    On System V.4 and embedded PowerPC systems, put small
    global and static data in the .sdata section, which is
    pointed to by register "r13".  Put small uninitialized
    global and static data in the .sbss section, which is
    adjacent to the .sdata section.  The −msdata=sysv option
    is incompatible with the −mrelocatable option.

−msdata=default

−msdata
    On System V.4 and embedded PowerPC systems, if −meabi is
    used, compile code the same as −msdata=eabi, otherwise
    compile code the same as −msdata=sysv.

−msdata‐data
    On System V.4 and embedded PowerPC systems, put small
    global and static data in the .sdata section.  Put small
    uninitialized global and static data in the .sbss
    section.  Do not use register "r13" to address small
    data however.  This is the default behavior unless other
    −msdata options are used.

−msdata=none

−mno‐sdata
    On embedded PowerPC systems, put all initialized global
    and static data in the .data section, and all
    uninitialized data in the .bss section.











                            ‐119‐


−G num
    On embedded PowerPC systems, put global and static items
    less than or equal to num bytes into the small data or
    bss sections instead of the normal data or bss section.
    By default, num is 8.  The −G num switch is also passed
    to the linker.  All modules should be compiled with the
    same −G num value.

−mregnames

−mno‐regnames
    On System V.4 and embedded PowerPC systems do (do not)
    emit register names in the assembly language output
    using symbolic forms.

−pthread
    Adds support for multithreading with the pthreads
    library.  This option sets flags for both the
    preprocessor 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, including
    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 calling
    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.











                            ‐120‐


−mhc‐struct‐return
    Return structures of more than one word in memory,
    rather than in a register.  This provides compatibility
    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
    registers, when convenient.  This is the default.  For
    compatibility 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
computers:

−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.  Additionally,
    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.  Additionally,
    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.











                            ‐121‐


−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
    (conditional 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
    undesirable 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
    pointers.

−mlong32
    Force long, int, and pointer types to be 32 bits wide.

    If none of −mlong32, −mlong64, or −mint64 are set, the









                            ‐122‐


    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‐as
    Generate code for the MIPS assembler, and invoke mips‐
    tfile to add normal debug information.  This is the
    default for all platforms except for the OSF/1 reference
    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









                            ‐123‐


    hardware names (ie, a0 instead of $4).  The only known
    assembler that supports this option is the Algorithmics
    assembler.

−mgpopt

−mno‐gpopt
    The −mgpopt switch says to write all of the data
    declarations 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
    program (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 program,
    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
    temporary file names passed to the assembler embedded in
    the object file, which means the objects will not
    compare 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.











                            ‐124‐


−mhard‐float
    Generate output containing floating point instructions.
    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 functions
    that are not through pointers.

−mhalf‐pic

−mno‐half‐pic
    Put pointers to extern references into the data section
    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 preferred 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.









                            ‐125‐


−msingle‐float

−mdouble‐float
    The −msingle‐float switch tells gcc to assume that the
    floating point coprocessor only supports single
    precision operations, as on the r4650 chip.  The
    −mdouble‐float switch permits gcc to use double
    precision operations.  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 instructions 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









                            ‐126‐


    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 common
    "_flush_func()", that is, the address of the memory
    range for which the cache is being flushed, the size of
    the memory range, and the number 3 (to flush both
    caches).  The default depends on the target gcc was
    configured for, but commonly is either _flush_func or
    __cpu_flush.

     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
family of computers:

−mcpu=cpu‐type
    Tune to cpu‐type everything applicable about the
    generated code, except for the ABI and the set of
    available instructions.  The choices for cpu‐type are
    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










                            ‐127‐


−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 supports
        only single precision arithmetics, thus the double
        and extended precision arithmetics is still done
        using 387.  Later version, present only in Pentium4
        and the future AMD x86−64 chips supports double
        precision arithmetics too.

        For 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 experimental,
        because gcc register allocator does not model









                            ‐128‐


        separate functional units well resulting in instable
        performance.

−masm=dialect
    Output asm instructions using selected dialect.
    Supported choices are intel or att (the default one).

−mieee‐fp

−mno‐ieee‐fp
    Control whether or not the compiler uses IEEE floating
    point comparisons.  These handle correctly the case
    where the result of a comparison is unordered.

−msoft‐float
    Generate output containing library calls for floating
    point.  Warning: the requisite libraries are not part of
    GCC.  Normally the facilities of the machine’s usual C
    compiler are used, but this can’t be done directly in
    cross‐compilation.  You must make your own arrangements
    to provide suitable library functions for cross‐
    compilation.

    On machines where a function returns floating point
    results in the 80387 register stack, some floating point
    opcodes may be emitted even if −msoft‐float is used.

−mno‐fp‐ret‐in‐387
    Do not use the FPU registers for return values of
    functions.

    The usual calling convention has functions return values
    of types "float" and "double" in an FPU register, even
    if there is no FPU.  The idea is that the operating
    system should emulate an FPU.

    The option −mno‐fp‐ret‐in‐387 causes such values to be
    returned in ordinary CPU registers instead.

−mno‐fancy‐math‐387
    Some 387 emulators do not support the "sin", "cos" and
    "sqrt" instructions for the 387.  Specify this option to
    avoid generating those instructions.  This option is the
    default on FreeBSD, OpenBSD and NetBSD.  This option is
    overridden when −march indicates that the target cpu
    will always have an FPU and so the instruction will not
    need emulation.  As of revision 2.6.1, these
    instructions are not generated unless you also use the
    −funsafe‐math‐optimizations switch.

−malign‐double

−mno‐align‐double
    Control whether GCC aligns "double", "long double", and









                            ‐129‐


    "long long" variables on a two word boundary or a one
    word boundary.  Aligning "double" variables on a two
    word boundary will produce code that runs somewhat
    faster on a Pentium at the expense of more memory.

    Warning: if you use the −malign‐double switch,
    structures containing the above types will be aligned
    differently than the published application binary
    interface specifications for the 386.

−m128bit‐long‐double
    Control the size of "long double" type. i386 application
    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 variables
    into the "bss" or "data" segments.  −msvr3−shlib places
    them into "bss".  These options are meaningful only on
    System V Release 3.

−mrtd
    Use a different function‐calling convention, in which
    functions that take a fixed number of arguments return
    with the "ret" num instruction, which pops their
    arguments while returning.  This saves one instruction
    in the caller since there is no need to pop the
    arguments there.

    You can specify that an individual function is called
    with this calling sequence with the function attribute
    stdcall.  You can also override the −mrtd option by
    using the function attribute cdecl.

    Warning: this calling convention is incompatible with
    the one normally used on Unix, so you cannot use it if
    you need to call libraries compiled with the Unix
    compiler.










                            ‐130‐


    Also, you must provide function prototypes for all
    functions that take variable numbers of arguments
    (including "printf"); otherwise incorrect code will be
    generated for calls to those functions.

    In addition, seriously incorrect code will result if you
    call a function with too many arguments.  (Normally,
    extra arguments are harmlessly ignored.)

−mregparm=num
    Control how many registers are used to pass integer
    arguments.  By default, no registers are used to pass
    arguments, and at most 3 registers can be used.  You can
    control this behavior for a specific function by using
    the function attribute regparm.

    Warning: if you use this switch, and num is nonzero,
    then you must build all modules with the same value,
    including any libraries.  This includes the system
    libraries and startup modules.

−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 correct alignment (4
    bytes for x86, and 8 bytes for x86−64).

    On Pentium and PentiumPro, "double" and "long double"
    values should be aligned to an 8 byte boundary (see
    −malign‐double) or suffer significant run time
    performance penalties.  On Pentium III, the Streaming
    SIMD Extension (SSE) data type "__m128" suffers similar
    penalties if it is not 16 byte aligned.

    To ensure proper alignment of this values on the stack,
    the stack boundary must be as aligned as that required
    by any value stored on the stack.  Further, every
    function must be generated such that it keeps the stack
    aligned.  Thus calling a function compiled with a higher
    preferred stack boundary from a function compiled with a
    lower preferred stack boundary will most likely misalign
    the stack.  It is recommended that libraries that use
    callbacks always use the default setting.

    This extra alignment does consume extra stack space, and
    generally increases code size.  Code that is sensitive
    to stack space usage, such as embedded systems and
    operating system kernels, may want to reduce the
    preferred alignment to −mpreferred‐stack‐boundary=2.

−mmmx










                            ‐131‐


−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 performance because
    of improved scheduling and reduced dependencies.

−maccumulate‐outgoing‐args
    If enabled, the maximum amount of space required for
    outgoing arguments will be computed in the function
    prologue.  This is faster on most modern CPUs because of
    reduced dependencies, improved scheduling and reduced
    stack usage when preferred stack boundary is not equal
    to 2.  The drawback is a notable increase in code size.
    This switch implies −mno‐push‐args.

−mthreads
    Support thread‐safe exception handling on Mingw32.  Code
    that relies on thread‐safe exception handling must
    compile and link all code with the −mthreads option.
    When compiling, −mthreads defines −D_MT; when linking,
    it links in a special thread helper library −lmingwthrd
    which cleans up per thread exception handling data.

−mno‐align‐stringops
    Do not align destination of inlined string operations.
    This switch reduces code size and improves performance
    in case the destination is already aligned, but gcc
    don’t know about it.

−minline‐all‐stringops
    By default GCC inlines string operations only when









                            ‐132‐


    destination is known to be aligned at least to 4 byte
    boundary.  This enables more inlining, increase code
    size, but may improve performance of code that depends
    on fast memcpy, strlen and memset for short lengths.

−momit‐leaf‐frame‐pointer
    Don’t keep the frame pointer in a register for leaf
    functions.  This avoids the instructions to save, set up
    and restore frame pointers and makes an extra register
    available in leaf functions.  The option −fomit‐frame‐
    pointer removes the frame pointer for all functions
    which might make debugging harder.

     These −m switches are supported in addition to the
above on AMD x86−64 processors in 64−bit environments.

−m32

−m64
    Generate code for a 32−bit or 64−bit environment.  The
    32−bit environment sets int, long and pointer to 32 bits
    and generates code that runs on any i386 system.  The
    64−bit environment sets int to 32 bits and long and
    pointer to 64 bits and generates code for AMD’s x86−64
    architecture.

−mno‐red‐zone
    Do not use a so called red zone for x86−64 code.  The
    red zone is mandated by the x86−64 ABI, it is a 128−byte
    area beyond the location of the stack pointer that will
    not be modified by signal or interrupt handlers and
    therefore can be used for temporary data without
    adjusting the stack pointer.  The flag −mno‐red‐zone
    disables this red zone.

−mcmodel=small
    Generate code for the small code model: the program and
    its symbols must be linked in the lower 2 GB of the
    address space.  Pointers are 64 bits.  Programs can be
    statically or dynamically linked.  This is the default
    code model.

−mcmodel=kernel
    Generate code for the kernel code model.  The kernel
    runs in the negative 2 GB of the address space.  This
    model has to be used for Linux kernel code.

−mcmodel=medium
    Generate code for the medium model: The program is
    linked in the lower 2 GB of the address space but
    symbols can be located anywhere in the address space.
    Programs can be statically or dynamically linked, but
    building of shared libraries are not supported with the
    medium model.









                            ‐133‐


−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
computers:

−march=architecture‐type
    Generate code for the specified architecture.  The
    choices for architecture‐type are 1.0 for PA 1.0, 1.1
    for PA 1.1, and 2.0 for PA 2.0 processors.  Refer to
    /usr/lib/sched.models on an HP‐UX system to determine
    the proper architecture option for your machine.  Code
    compiled for lower numbered architectures 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 (current 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
    perform 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









                            ‐134‐


    MIG generated code under MACH.

−mno‐space‐regs
    Generate code that assumes the target has no space
    registers.  This allows GCC to generate faster indirect
    calls and use unscaled index address modes.

    Such code is suitable for level 0 PA systems and
    kernels.

−mfast‐indirect‐calls
    Generate code that assumes calls never cross space
    boundaries.  This allows GCC to emit code which performs
    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
    understands.

−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 facilities
    of the machine’s usual C compiler are used, but this
    cannot be done directly in cross‐compilation.  You must
    make your own arrangements to provide suitable library
    functions for cross‐compilation.  The embedded target
    hppa1.1−*−pro does provide software floating point









                            ‐135‐


    support.

    −msoft‐float changes the calling convention in the
    output file; therefore, it is only useful if you compile
    all of a program with this option.  In particular, you
    need to compile libgcc.a, the library that comes with
    GCC, with −msoft‐float in order for this to work.

     Intel 960 Options

     These −m options are defined for the Intel 960
implementations:

−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









                            ‐136‐


    i960.  Complex addressing modes may not be worthwhile on
    the K‐series, but they definitely are on the C‐series.
    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 implemented
    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
implementations:

−mno‐soft‐float

−msoft‐float
    Use (do not use) the hardware floating‐point
    instructions for floating‐point operations.  When
    −msoft‐float is specified, functions in libgcc.a will be









                            ‐137‐


    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 floating‐
    point operations.   If you are compiling for an Alpha
    without floating‐point operations, you must ensure that
    the library is built so as not to call them.

    Note that Alpha implementations without floating‐point
    operations are required to have floating‐point
    registers.

−mfp‐reg

−mno‐fp‐regs
    Generate code that uses (does not use) the floating‐
    point register set.  −mno‐fp‐regs implies −msoft‐float.
    If the floating‐point register set is not used, floating
    point operands are passed in integer registers as if
    they were integers and floating‐point results are passed
    in "$0" instead of "$f0".  This is a non‐standard
    calling sequence, so any function with a floating‐point
    argument or return value called by code compiled with
    −mno‐fp‐regs must also be compiled with that option.

    A typical use of this option is building a kernel that
    does not use, and hence need not save and restore, any
    floating‐point registers.

−mieee
    The Alpha architecture implements floating‐point
    hardware optimized for maximum performance.  It is
    mostly compliant with the IEEE floating point standard.
    However, for full compliance, software assistance is
    required.  This option generates code fully IEEE
    compliant code except that the inexact‐flag is not
    maintained (see below).  If this option is turned on,
    the preprocessor macro "_IEEE_FP" is defined during
    compilation.  The resulting code is less efficient but
    is able to correctly support denormalized numbers and
    exceptional IEEE values such as not‐a‐number and
    plus/minus infinity.  Other Alpha compilers call this
    option −ieee_with_no_inexact.

−mieee‐with‐inexact
    This is like −mieee except the generated code also
    maintains the IEEE inexact‐flag.  Turning on this option
    causes the generated code to implement fully‐compliant
    IEEE math.  In addition to "_IEEE_FP", "_IEEE_FP_EXACT"
    is defined as a preprocessor macro.  On some Alpha
    implementations the resulting code may execute
    significantly slower than the code generated by default.
    Since there is very little code that depends on the
    inexact‐flag, you should normally not specify this









                            ‐138‐


    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 architecture
        manual for details).

    sui Like su, but inexact traps are enabled as well.

−mfp‐rounding‐mode=rounding‐mode
    Selects the IEEE rounding mode.  Other Alpha compilers
    call this option −fprm rounding‐mode.  The rounding‐mode
    can be one of:

    n   Normal IEEE rounding mode.  Floating point numbers
        are rounded towards the nearest machine number or
        towards the even machine number in case of a tie.

    m   Round towards minus infinity.

    c   Chopped rounding mode.  Floating point numbers are
        rounded towards zero.

    d   Dynamic rounding mode.  A field in the floating
        point control register (fpcr, see Alpha architecture
        reference manual) controls the rounding mode in
        effect.  The C library initializes this register for
        rounding towards plus infinity.  Thus, unless your
        program modifies the fpcr, d corresponds to round
        towards plus infinity.

−mtrap‐precision=trap‐precision
    In the Alpha architecture, floating point traps are
    imprecise.  This means without software assistance it is
    impossible to recover from a floating trap and program
    execution normally needs to be terminated.  GCC can
    generate code that can assist operating system trap
    handlers in determining the exact location that caused a
    floating point trap.  Depending on the requirements of
    an application, different levels of precisions can be
    selected:









                            ‐139‐


    p   Program precision.  This option is the default and
        means a trap handler can only identify which program
        caused a floating point exception.

    f   Function precision.  The trap handler can determine
        the function that caused a floating point exception.

    i   Instruction precision.  The trap handler can
        determine the exact instruction that caused a
        floating point exception.

        Other Alpha compilers provide the equivalent options
        called −scope_safe and −resumption_safe.

−mieee‐conformant
    This option marks the generated code as IEEE conformant.
    You must not use this option unless you also specify
    −mtrap‐precision=i and either −mfp‐trap‐mode=su or −mfp‐
    trap‐mode=sui.  Its only effect is to emit the line
    .eflag 48 in the function prologue of the generated
    assembly file.  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 integer
    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










                            ‐140‐


−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 instructions.
    This option is mostly useful for debugging, as GCC
    detects the capabilities of the assembler when it is
    built and sets the default accordingly.

−msmall‐data

−mlarge‐data
    When −mexplicit‐relocs is in effect, static data is
    accessed via gp‐relative relocations.  When −msmall‐data
    is used, objects 8 bytes long or smaller are placed in a
    small data area (the ".sdata" and ".sbss" sections) and
    are accessed via 16−bit relocations off of the "$gp"
    register.  This limits the size of the small data area
    to 64KB, but allows the variables to be directly
    accessed via a single instruction.

    The default is −mlarge‐data.  With this option the data
    area is limited to just below 2GB.  Programs that
    require more than 2GB of data must use "malloc" or
    "mmap" to allocate the data in the heap instead of in
    the program’s data segment.

    When generating code for shared libraries, −fpic implies
    −msmall‐data and −fPIC implies −mlarge‐data.










                            ‐141‐


−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
    processor 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 extension.

    pca56

    21164pc

    21164PC
        Schedules as an EV5 and supports the BWX and MAX
        extensions.

    ev6

    21264
        Schedules as an EV6 and supports the BWX, FIX, and
        MAX extensions.

    ev67

    21264a
        Schedules as an EV6 and supports the BWX, CIX, FIX,
        and MAX extensions.

−mtune=cpu_type
    Set only the instruction scheduling parameters for









                            ‐142‐


    machine type cpu_type.  The instruction set is not
    changed.

−mmemory‐latency=time
    Sets the latency the scheduler should assume for typical
    memory references as seen by the application.  This
    number is highly dependent on the memory access patterns
    used by the application and the size of the external
    cache on the machine.

    Valid options for time are

    number
        A decimal number representing clock cycles.

    L1

    L2

    L3

    main
        The compiler contains estimates of the number of
        clock cycles for ‘‘typical’’ EV4 & EV5 hardware for
        the Level 1, 2 & 3 caches (also called Dcache,
        Scache, and Bcache), as well as to main memory.
        Note that L3 is only valid for EV5.

     DEC Alpha/VMS Options

     These −m options are defined for the DEC Alpha/VMS
implementations:

−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
implementations:

−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
implementations:









                            ‐143‐


−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 boundaries.
    −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.











                            ‐144‐


−mdalign
    Align doubles at 64−bit boundaries.  Note that this
    changes the calling conventions, and thus some functions
    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
    multiple 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 function
    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 trampoline.
    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









                            ‐145‐


systems:

−G  Create a shared object.  It is recommended that
    −symbolic or −shared be used instead.

−Qy Identify the versions of each tool used by the compiler,
    in a ".ident" assembler directive in the output.

−Qn Refrain from adding ".ident" directives to the output
    file (this is the default).

−YP,dirs
    Search the directories dirs, and no others, for
    libraries specified with −l.

−Ym,dir
    Look in the directory dir to find the M4 preprocessor.
    The assembler uses this option.

     TMS320C3x/C4x Options

     These −m options are defined for TMS320C3x/C4x
implementations:

−mcpu=cpu_type
    Set the instruction set, register set, and instruction
    scheduling parameters for machine type cpu_type.
    Supported 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) register
    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










                            ‐146‐


−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 violated
    the small memory model by modifying the DP register, say
    within an object library.

−mmpyi

−mno‐mpyi
    For the C3x use the 24−bit MPYI instruction for integer
    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









                            ‐147‐


    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 guaranteed
    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 disabled 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
    instructions test if the iteration count is negative to
    terminate 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 compatibility
    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.










                            ‐148‐


−mparallel‐mpy

−mno‐parallel‐mpy
    Allow the generation of MPY⎪⎪ADD and MPY⎪⎪SUB parallel
    instructions, provided −mparallel‐insns is also
    specified.  These instructions have tight register
    constraints 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
    function.  The external functions are slower, but use
    less code space if more than one function saves the same
    number of registers.  The −mprolog‐function option is on
    by default if you optimize.

−mspace
    Try to make the code as small as possible.  At present,
    this just turns on the −mep and −mprolog‐function
    options.

−mtda=n
    Put static or global variables whose size is n bytes or
    less into the tiny data area that register "ep" points
    to.  The tiny data area can hold up to 256 bytes in
    total (128 bytes for byte references).

−msda=n
    Put static or global variables whose size is n bytes or
    less into the small data area that register "gp" points









                            ‐149‐


    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‐section,
    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 configured; the
defaults for the most common choices are given below.










                            ‐150‐


−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 configuration.

−mmulti‐add
    Try and generate multiply‐add floating point
    instructions "polyF" and "dotF".  This option is only
    available 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.










                            ‐151‐


−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"
    instruction.

    This calling convention is incompatible with the one
    normally used on Unix, so you cannot use it if you need
    to call libraries compiled with the Unix compiler.

    Also, you must provide function prototypes for all
    functions that take variable numbers of arguments
    (including "printf"); otherwise incorrect code will be
    generated for calls to those functions.

    In addition, seriously incorrect code will result if you
    call a function with too many arguments.  (Normally,
    extra arguments are harmlessly ignored.)

    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 compiler.

−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









                            ‐152‐


    512MB.  This may be useful for operating systems or ROM
    code.

−mnohimem
    Assume code will be loaded in the first 512MB of virtual
    address space.  This is the default for all platforms.

     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 programs
    (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 symbol
    or numeric value, __stack is the default.

−mno‐interrupts
    Generated code is not compatible with hardware
    interrupts.  Code size will be smaller.

−mcall‐prologues
    Functions prologues/epilogues expanded as call to
    appropriate subroutines.  Code size will be smaller.











                            ‐153‐


−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











                            ‐154‐


−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
    register.  The result is not position independent code,
    and violates the IA‐64 ABI.










                            ‐155‐


−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 register ranges
    can be specified separated by a comma.









                            ‐156‐


     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
    automatically 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
    instructions 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
    instructions 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











                            ‐157‐


−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
    instructions.  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.










                            ‐158‐


−mtune=architecture‐type
    Tune to architecture‐type everything applicable about
    the generated code, except for the ABI and the set of
    available instructions.  The choices for architecture‐
    type are the same as for −march=architecture‐type.

−mmax‐stack‐frame=n
    Warn when the stack frame of a function exceeds n bytes.

−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 addressing
    modes other than post‐increment.

−mstack‐align

−mno‐stack‐align

−mdata‐align

−mno‐data‐align

−mconst‐align

−mno‐const‐align
    These options (no‐options) 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.











                            ‐159‐


−m32−bit

−m16−bit

−m8−bit
    Similar to the stack‐ data‐ and const‐align options
    above, these options arrange for stack‐frame, writable
    data and constants to all be 32−bit, 16−bit or 8−bit
    aligned.  The default is 32−bit alignment.

−mno‐prologue‐epilogue

−mprologue‐epilogue
    With −mno‐prologue‐epilogue, the normal function
    prologue and epilogue 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 functions
    from the PLT part of the GOT rather than (traditional on
    other architectures) calls to the PLT.  The default is
    −mgotplt.

−maout
    Legacy no‐op option only recognized with the cris‐axis‐
    aout target.

−melf
    Legacy no‐op option only recognized with the cris‐axis‐
    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.

−mlinux
    Legacy no‐op option only recognized with the cris‐axis‐
    linux‐gnu target.

−sim
    This option, recognized for the cris‐axis‐aout and cris‐
    axis‐elf arranges to link with input‐output functions
    from a simulator library.  Code, initialized data and
    zero‐initialized data are allocated consecutively.










                            ‐160‐


−sim2
    Like −sim, but pass linker options to locate initialized
    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 instructions
    by default, rather than sign‐extending ones.

−mknuthdiv

−mno‐knuthdiv
    Make the result of a division yielding a remainder have
    the same sign as the divisor.  With the default, −mno‐
    knuthdiv, the sign of the remainder follows the sign of
    the dividend.  Both methods are arithmetically valid,
    the latter being almost exclusively used.

−mtoplevel‐symbols

−mno‐toplevel‐symbols
    Prepend (do not prepend) a : to all global symbols, so
    the assembly code can be used with the "PREFIX" assembly
    directive.











                            ‐161‐


−melf
    Generate an executable in the ELF format, rather than
    the default mmo format used by the mmix simulator.

−mbranch‐predict

−mno‐branch‐predict
    Use (do not use) the probable‐branch instructions, when
    static branch prediction indicates a probable branch.

−mbase‐addresses

−mno‐base‐addresses
    Generate (do not generate) code that uses base
    addresses.  Using a base address automatically generates
    a request (handled by the assembler and the linker) for
    a constant to be set up in a global register.  The
    register is used for one or more base address requests
    within the range 0 to 255 from the value held in the
    register.  The generally leads to short and fast code,
    but the number of different data items that can be
    addressed is limited.  This means that a program that
    uses lots of static data may require −mno‐base‐
    addresses.

     PDP‐11 Options

     These options are defined for the PDP‐11:

−mfpu
    Use hardware FPP floating point.  This is the default.
    (FIS floating point on the PDP‐11/40 is not supported.)

−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.











                            ‐162‐


−mbcopy‐builtin
    Use inline "movstrhi" patterns for copying memory.  This
    is the default.

−mbcopy
    Do not use inline "movstrhi" patterns for copying
    memory.

−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.











                            ‐163‐


−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
different configurations.  The compiler’s default options
can be set to match a particular Xtensa configuration by
copying a configuration file into the GCC sources when
building 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
    density 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/accumulate
    operations to a combination of core instructions 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 multiply









                            ‐164‐


    or MAC16 instructions if they are available or generate
    library calls to perform the multiply operations 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
    maximum 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 typically
    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 instructions
    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 "double" operations
    are always emulated with calls to library functions.









                            ‐165‐


−mfused‐madd

−mno‐fused‐madd
    Enable or disable use of fused multiply/add and
    multiply/subtract instructions in the floating‐point
    option.  This has no effect if the floating‐point option
    is not also enabled.  Disabling fused multiply/add and
    multiply/subtract instructions forces the compiler to
    use separate instructions for the multiply and
    add/subtract operations.  This may be desirable in some
    cases where strict IEEE 754−compliant results are
    required: the fused multiply add/subtract instructions
    do not round the intermediate result, thereby producing
    results with more bits of precision than specified by
    the IEEE standard.  Disabling fused multiply
    add/subtract instructions also ensures that the program
    output is not sensitive to the compiler’s ability to
    combine multiply and add/subtract operations.

−mserialize‐volatile

−mno‐serialize‐volatile
    When this option is enabled, GCC inserts "MEMW"
    instructions before "volatile" memory references to
    guarantee sequential consistency.  The default is
    −mserialize‐volatile.  Use −mno‐serialize‐volatile to
    omit the "MEMW" instructions.

−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 literals and improve
    code size.  With −mtext‐section‐literals, the literals
    are interspersed in the text section in order to keep
    them as close as possible to their references.  This may
    be necessary for large assembly files.

−mtarget‐align

−mno‐target‐align
    When this option is enabled, GCC instructs the assembler
    to automatically align instructions to reduce branch
    penalties at the expense of some code density.  The
    assembler attempts to widen density instructions to
    align branch targets and the instructions following call
    instructions.  If there are not enough preceding safe
    density instructions to align a target, no widening will
    be performed.  The default is −mtarget‐align.  These
    options do not affect the treatment of auto‐aligned









                            ‐166‐


    instructions like "LOOP", which the assembler will
    always align, either by widening density instructions or
    by inserting no‐op instructions.

−mlongcalls

−mno‐longcalls
    When this option is enabled, GCC instructs the assembler
    to translate direct calls to indirect calls unless it
    can determine that the target of a direct call is in the
    range allowed by the call instruction.  This translation
    typically occurs for calls to functions in other source
    files.  Specifically, the assembler translates a direct
    "CALL" instruction into an "L32R" followed by a "CALLX"
    instruction.  The default is −mno‐longcalls.  This
    option should be used in programs where the call target
    can potentially be out of range.  This option is
    implemented in the assembler, not the compiler, so the
    assembly code generated by GCC 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 information for all
    functions, which can produce significant data size
    overhead, although it does not affect execution.  If you
    do not specify this option, GCC will enable it by
    default for languages like C++ which normally require
    exception handling, and disable it for languages like C
    that do not normally require it.  However, you may need
    to enable this option when compiling C code that needs
    to interoperate properly with exception handlers written
    in C++.  You may also wish to disable this option if you
    are compiling older C++ programs that don’t use
    exception handling.

−fnon‐call‐exceptions
    Generate code that allows trapping instructions to throw
    exceptions.  Note that this requires platform‐specific









                            ‐167‐


    runtime support that does not exist everywhere.
    Moreover, it only allows trapping instructions to throw
    exceptions, i.e. memory references or floating point
    instructions.  It does not allow exceptions to be thrown
    from arbitrary signal handlers such as "SIGALRM".

−funwind‐tables
    Similar to −fexceptions, except that it will just
    generate any needed static data, but will not affect the
    generated code in any other way.  You will normally not
    enable this option; instead, a language processor that
    needs this handling would enable it on your behalf.

−fasynchronous‐unwind‐tables
    Generate unwind table in dwarf2 format, if supported by
    target machine.  The table is exact at each instruction
    boundary, so it can be used for stack unwinding from
    asynchronous events (such as debugger or garbage
    collector).

−fpcc‐struct‐return
    Return ‘‘short’’ "struct" and "union" values in memory
    like longer ones, rather than in registers.  This
    convention is less efficient, but it has the advantage
    of allowing intercallability between GCC‐compiled files
    and files compiled with other compilers, particularly
    the Portable C Compiler (pcc).

    The precise convention for returning structures in
    memory depends on the target configuration macros.

    Short structures and unions are those whose size and
    alignment match that of some integer type.

    Warning: code compiled with the −fpcc‐struct‐return
    switch is not binary compatible with code compiled with
    the −freg‐struct‐return switch.  Use it to conform to a
    non‐default application binary interface.

−freg‐struct‐return
    Return "struct" and "union" values in registers when
    possible.  This is more efficient for small structures
    than −fpcc‐struct‐return.

    If you specify neither −fpcc‐struct‐return nor −freg‐
    struct‐return, GCC defaults to whichever convention is
    standard for the target.  If there is no standard
    convention, GCC defaults to −fpcc‐struct‐return, except
    on targets where GCC is the principal compiler.  In
    those cases, we can choose the standard, and we chose
    the more efficient register return alternative.

    Warning: code compiled with the −freg‐struct‐return
    switch is not binary compatible with code compiled with









                            ‐168‐


    the −fpcc‐struct‐return switch.  Use it to conform to a
    non‐default application binary interface.

−fshort‐enums
    Allocate to an "enum" type only as many bytes as it
    needs for the declared range of possible values.
    Specifically, the "enum" type will be equivalent to the
    smallest integer type which has enough room.

    Warning: the −fshort‐enums switch causes GCC to generate
    code that is not binary compatible with code generated
    without that switch.  Use it to conform to a non‐default
    application binary interface.

−fshort‐double
    Use the same size for "double" as for "float".

    Warning: the −fshort‐double switch causes GCC to
    generate code that is not binary compatible with code
    generated without that switch.  Use it to conform to a
    non‐default application binary interface.

−fshort‐wchar
    Override the underlying type for wchar_t to be short
    unsigned int instead of the default for the target.
    This option is useful for building programs to run under
    WINE.

    Warning: the −fshort‐wchar switch causes GCC to generate
    code that is not binary compatible with code generated
    without that switch.  Use it to conform to a non‐default
    application binary interface.

−fshared‐data
    Requests that the data and non‐"const" variables of this
    compilation be shared data rather than private data.
    The distinction makes sense only on certain operating
    systems, where shared data is shared between processes
    running the same program, while private data exists in
    one copy per process.

−fno‐common
    In C, allocate even uninitialized global variables in
    the data section of the object file, rather than
    generating them as common blocks.  This has the effect
    that if the same variable is declared (without "extern")
    in two different compilations, you will get an error
    when you link them.  The only reason this might be
    useful is if you wish to verify that the program will
    work on other systems which always work this way.

−fno‐ident
    Ignore the #ident directive.










                            ‐169‐


−fno‐gnu‐linker
    Do not output global initializations (such as C++
    constructors and destructors) in the form used by the
    GNU linker (on systems where the GNU linker is the
    standard 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.  (collect2
    is included in the GCC distribution.)  For systems 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 anything
    else that would cause trouble if the function is split
    in the middle, and the two halves are placed at
    locations far apart in memory.  This option is used when
    compiling crtstuff.c; you should not need to use it for
    anything else.

−fverbose‐asm
    Put extra commentary information in the generated
    assembly code to make it more readable.  This option is
    generally only of use to those who actually need to read
    the generated assembly code (perhaps while debugging the
    compiler itself).

    −fno‐verbose‐asm, the default, causes the extra
    information to be omitted and is useful when comparing
    two assembler files.

−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 maximum size, you
    get an error message from the linker indicating that









                            ‐170‐


    −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.

−fPIC
    If supported for the target machine, emit position‐
    independent code, suitable for dynamic linking and
    avoiding any limit on the size of the global offset
    table.  This option makes a difference on the m68k,
    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;
    generated code should never refer to it (except perhaps
    as a stack pointer, frame pointer or in some other fixed
    role).

    reg must be the name of a register.  The register names
    accepted are machine‐specific and are defined in the
    "REGISTER_NAMES" macro in the machine description macro
    file.

    This flag does not have a negative form, because it
    specifies a three‐way choice.

−fcall‐used‐reg
    Treat the register named reg as an allocable register
    that is clobbered by function calls.  It may be
    allocated for temporaries or variables that do not live
    across a call.  Functions compiled this way will not
    save and restore the register reg.

    It is an error to used this flag with the frame pointer
    or stack pointer.  Use of this flag for other registers
    that have fixed pervasive roles in the machine’s
    execution model will produce disastrous results.

    This flag does not have a negative form, because it
    specifies a three‐way choice.

−fcall‐saved‐reg
    Treat the register named reg as an allocable register
    saved by functions.  It may be allocated even for
    temporaries or variables that live across a call.









                            ‐171‐


    Functions compiled this way will save and restore the
    register reg if they use it.

    It is an error to used this flag with the frame pointer
    or stack pointer.  Use of this flag for other registers
    that have fixed pervasive roles in the machine’s
    execution model will produce disastrous results.

    A different sort of disaster will result from the use of
    this flag for a register in which function values may be
    returned.

    This flag does not have a negative form, because it
    specifies a three‐way choice.

−fpack‐struct
    Pack all structure members together without holes.

    Warning: the −fpack‐struct switch causes GCC to generate
    code that is not binary compatible with code generated
    without that switch.  Additionally, it makes the code
    suboptimial.  Use it to conform to a non‐default
    application binary interface.

−finstrument‐functions
    Generate instrumentation calls for entry and exit to
    functions.  Just after function entry and just before
    function exit, the following profiling functions will be
    called with the address of the current function and its
    call site.  (On some platforms,
    "__builtin_return_address" does not work beyond the
    current function, so the call site information may not
    be available to the profiling functions otherwise.)

            void __cyg_profile_func_enter (void *this_fn,
                                           void *call_site);
            void __cyg_profile_func_exit  (void *this_fn,
                                           void *call_site);

    The first argument is the address of the start of the
    current function, which may be looked up exactly in the
    symbol table.

    This instrumentation is also done for functions expanded
    inline in other functions.  The profiling calls will
    indicate where, conceptually, the inline function is
    entered and exited.  This means that addressable
    versions of such functions must be available.  If all
    your uses of a function are expanded inline, this may
    mean an additional expansion of code size.  If you use
    extern inline in your C code, an addressable version of
    such functions must be provided.  (This is normally the
    case anyways, but if you get lucky and the optimizer
    always expands the functions inline, you might have









                            ‐172‐


    gotten away without providing static copies.)

    A function may be given the attribute
    "no_instrument_function", in which case this
    instrumentation will not be done.  This can be used, for
    example, for the profiling functions listed above, high‐
    priority interrupt routines, and any functions from
    which the profiling functions cannot safely be called
    (perhaps signal handlers, if the profiling routines
    generate output or allocate memory).

−fstack‐check
    Generate code to verify that you do not go beyond the
    boundary of the stack.  You should specify this flag if
    you are running in an environment with multiple threads,
    but only rarely need to specify it in a single‐threaded
    environment since stack overflow is automatically
    detected on nearly all systems if there is only one
    stack.

    Note that this switch does not actually cause checking
    to be done; the operating system must do that.  The
    switch causes generation of code to ensure that the
    operating system sees the stack being extended.

−fstack‐limit‐register=reg

−fstack‐limit‐symbol=sym

−fno‐stack‐limit
    Generate code to ensure that the stack does not grow
    beyond a certain value, either the value of a register
    or the address of a symbol.  If the stack would grow
    beyond the value, a signal is raised.  For most targets,
    the signal is raised before the stack overruns the
    boundary, so it is possible to catch the signal without
    taking special precautions.

    For instance, if the stack starts at absolute address
    0x80000000 and grows downwards, you can use the flags
    −fstack‐limit‐symbol=__stack_limit and
    −Wl,−−defsym,__stack_limit=0x7ffe0000 to enforce a stack
    limit of 128KB.  Note that this may only work with the
    GNU linker.

−fargument‐alias

−fargument‐noalias

−fargument‐noalias‐global
    Specify the possible relationships among parameters and
    between parameters and global data.

    −fargument‐alias specifies that arguments (parameters)









                            ‐173‐


    may alias each other and may alias global
    storage.−fargument‐noalias specifies that arguments do
    not alias each other, but may alias global
    storage.−fargument‐noalias‐global specifies that
    arguments do not alias each other and do not alias
    global storage.

    Each language will automatically use whatever option is
    required by the language standard.  You should not need
    to use these options yourself.

−fleading‐underscore
    This option and its counterpart, −fno‐leading‐
    underscore, forcibly change the way C symbols are
    represented in the object file.  One use is to help link
    with legacy assembly code.

    Warning: the −fleading‐underscore switch causes GCC to
    generate code that is not binary compatible with code
    generated without that switch.  Use it to conform to a
    non‐default application binary interface.  Not all
    targets provide complete support for this switch.

This section describes several environment variables that
affect how GCC operates.  Some of them work by specifying
directories or prefixes to use when searching for various
kinds of files.  Some are used to specify other aspects of
the compilation environment.

     Note that you can also specify places to search using
options such as −B, −I and −L.  These take precedence over
places specified using environment variables, which in turn
take precedence over those specified by the configuration of
GCC.

LANG

LC_CTYPE

LC_MESSAGES

LC_ALL
    These environment variables control the way that GCC
    uses localization information that allow GCC to work
    with different national conventions.  GCC inspects the
    locale categories LC_CTYPE and LC_MESSAGES if it has
    been configured to do so.  These locale categories can
    be set to any value supported by your installation.  A
    typical value is en_UK for English in the United
    Kingdom.

    The LC_CTYPE environment variable specifies character
    classification.  GCC uses it to determine the character
    boundaries in a string; this is needed for some









                            ‐174‐


    multibyte encodings that contain quote and escape
    characters that would otherwise be interpreted as a
    string end or escape.

    The LC_MESSAGES environment variable specifies the
    language to use in diagnostic messages.

    If the LC_ALL environment variable is set, it overrides
    the value of LC_CTYPE and LC_MESSAGES; otherwise,
    LC_CTYPE and LC_MESSAGES default to the value of the
    LANG environment variable.  If none of these variables
    are set, GCC defaults to traditional C English behavior.

TMPDIR
    If TMPDIR is set, it specifies the directory to use for
    temporary files.  GCC uses temporary files to hold the
    output of one stage of compilation which is to be used
    as input to the next stage: for example, the output of
    the preprocessor, which is the input to the compiler
    proper.

GCC_EXEC_PREFIX
    If GCC_EXEC_PREFIX is set, it specifies a prefix to use
    in the names of the subprograms executed by the
    compiler.  No slash is added when this prefix is
    combined with the name of a subprogram, but you can
    specify a prefix that ends with a slash if you wish.

    If GCC_EXEC_PREFIX is not set, GCC will attempt to
    figure out an appropriate prefix to use based on the
    pathname it was invoked with.

    If GCC cannot find the subprogram using the specified
    prefix, it tries looking in the usual places for the
    subprogram.

    The default value of GCC_EXEC_PREFIX is prefix/lib/gcc‐
    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 normally
    begins with /usr/local/lib/gcc‐lib (more precisely, with
    the value of GCC_INCLUDE_DIR), GCC tries replacing that
    beginning with the specified prefix to produce an
    alternate directory name.  Thus, with −Bfoo/, GCC will
    search foo/bar where it would normally search









                            ‐175‐


    /usr/local/lib/bar.  These alternate directories are
    searched first; the standard directories come next.

COMPILER_PATH
    The value of COMPILER_PATH is a colon‐separated list of
    directories, much like PATH.  GCC tries the directories
    thus specified when searching for subprograms, if it
    can’t find the subprograms using GCC_EXEC_PREFIX.

LIBRARY_PATH
    The value of LIBRARY_PATH is a colon‐separated list of
    directories, much like PATH.  When configured as a
    native compiler, GCC tries the directories thus
    specified when searching for special linker files, if it
    can’t find them using GCC_EXEC_PREFIX.  Linking using
    GCC also uses these directories when searching for
    ordinary libraries for the −l option (but directories
    specified with −L come first).

LANG
    This variable is used to pass locale information to the
    compiler.  One way in which this information is used is
    to determine the character set to be used when character
    literals, string literals and comments are parsed in C
    and C++.  When the compiler is configured to allow
    multibyte characters, the following values for LANG are
    recognized:

    C‐JIS
        Recognize JIS characters.

    C‐SJIS
        Recognize SJIS characters.

    C‐EUCJP
        Recognize EUCJP characters.

        If LANG is not defined, or if it has some other
        value, then the compiler will use mblen and mbtowc
        as defined by the default locale to recognize and
        translate multibyte characters.

     Some additional environments variables affect the
behavior of the preprocessor.

CPATH

C_INCLUDE_PATH

CPLUS_INCLUDE_PATH

OBJC_INCLUDE_PATH
    Each variable’s value is a list of directories separated
    by a special character, much like PATH, in which to look









                            ‐176‐


    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 equivalent
    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.

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



1.  On some systems, gcc −shared needs to build
    supplementary stub code for constructors to work.  On
    multi‐libbed systems, gcc −shared must select the
    correct support libraries to link against.  Failing to
    supply the correct flags may lead to subtle defects.
    Supplying them in cases where they are not necessary is









                            ‐177‐


    innocuous.

gpl(7), gfdl(7), fsf‐funding(7), cpp(1), gcov(1), g77(1),
as(1), ld(1), gdb(1), adb(1), dbx(1), sdb(1) and the Info
entries for gcc, cpp, g77, as, ld, binutils and gdb.

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

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

     Permission is granted to copy, distribute and/or modify
this document under the terms of the GNU Free Documentation
License, Version 1.1 or any later version published by the
Free Software Foundation; with the Invariant Sections being
‘‘GNU General Public License’’ and ‘‘Funding Free
Software’’, the Front‐Cover texts being (a) (see below), and
with the Back‐Cover Texts being (b) (see below).  A copy of
the license is included in the gfdl(7) man page.

     (a) The FSF’s Front‐Cover Text is:

          A GNU Manual

(b) The FSF’s Back‐Cover Text is:

          You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development.