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* gcc: (gcc).                  The GNU Compiler Collection.
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   This file documents the use of the GNU compilers.

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File: gcc.info,  Node: i386 and x86-64 Options,  Next: HPPA Options,  Prev: MIPS Options,  Up: Submodel Options

3.17.15 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'
`-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 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 `long
     long' variables on a two word boundary or a one word boundary.
     Aligning `double' variables on a two word boundary will produce
     code that runs somewhat faster on a `Pentium' at the expense of
     more memory.

     *Warning:* if you use the `-malign-double' switch, structures
     containing the above types will be aligned differently than the
     published application binary interface specifications for the 386
     and will not be binary compatible with structures in code compiled
     without that switch.

`-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'.  *Note Function Attributes::.

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

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

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

`-mregparm=NUM'
     Control how many registers are used to pass integer arguments.  By
     default, no registers are used to pass arguments, and at most 3
     registers can be used.  You can control this behavior for a
     specific function by using the function attribute `regparm'.
     *Note Function Attributes::.

     *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'
`-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.

     *Note X86 Built-in Functions::, for details of the functions
     enabled and disabled by these switches.

     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 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.

`-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.


File: gcc.info,  Node: HPPA Options,  Next: Intel 960 Options,  Prev: i386 and x86-64 Options,  Up: Submodel Options

3.17.16 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 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
     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.


File: gcc.info,  Node: Intel 960 Options,  Next: DEC Alpha Options,  Prev: HPPA Options,  Up: Submodel Options

3.17.17 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 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.



File: gcc.info,  Node: DEC Alpha Options,  Next: DEC Alpha/VMS Options,  Prev: Intel 960 Options,  Up: Submodel Options

3.17.18 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 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 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:

    `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'
`-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'.

`-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 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.



File: gcc.info,  Node: DEC Alpha/VMS Options,  Next: Clipper Options,  Prev: DEC Alpha Options,  Up: Submodel Options

3.17.19 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.


File: gcc.info,  Node: Clipper Options,  Next: H8/300 Options,  Prev: DEC Alpha/VMS Options,  Up: Submodel Options

3.17.20 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.


File: gcc.info,  Node: H8/300 Options,  Next: SH Options,  Prev: Clipper Options,  Up: Submodel Options

3.17.21 H8/300 Options
----------------------

These `-m' options are defined for the H8/300 implementations:

`-mrelax'
     Shorten some address references at link time, when possible; uses
     the linker option `-relax'.  *Note `ld' and the H8/300:
     (ld.info)H8/300, for a fuller description.

`-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.


File: gcc.info,  Node: SH Options,  Next: System V Options,  Prev: H8/300 Options,  Up: Submodel Options

3.17.22 SH Options
------------------

These `-m' options are defined for the SH implementations:

`-m1'
     Generate code for the SH1.

`-m2'
     Generate code for the SH2.

`-m3'
     Generate code for the SH3.

`-m3e'
     Generate code for the SH3e.

`-m4-nofpu'
     Generate code for the SH4 without a floating-point unit.

`-m4-single-only'
     Generate code for the SH4 with a floating-point unit that only
     supports single-precision arithmetic.

`-m4-single'
     Generate code for the SH4 assuming the floating-point unit is in
     single-precision mode by default.

`-m4'
     Generate code for the SH4.

`-mb'
     Compile code for the processor in big endian mode.

`-ml'
     Compile code for the processor in little endian mode.

`-mdalign'
     Align doubles at 64-bit boundaries.  Note that this changes the
     calling conventions, and thus some 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*'.


File: gcc.info,  Node: System V Options,  Next: TMS320C3x/C4x Options,  Prev: SH Options,  Up: Submodel Options

3.17.23 Options for System V
----------------------------

These additional options are available on System V Release 4 for
compatibility with other compilers on those systems:

`-G'
     Create a shared object.  It is recommended that `-symbolic' or
     `-shared' be used instead.

`-Qy'
     Identify the versions of each tool used by the compiler, in a
     `.ident' assembler directive in the output.

`-Qn'
     Refrain from adding `.ident' directives to the output file (this is
     the default).

`-YP,DIRS'
     Search the directories DIRS, and no others, for libraries
     specified with `-l'.

`-Ym,DIR'
     Look in the directory DIR to find the M4 preprocessor.  The
     assembler uses this option.


File: gcc.info,  Node: TMS320C3x/C4x Options,  Next: V850 Options,  Prev: System V Options,  Up: Submodel Options

3.17.24 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'
`-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 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'.

`-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.



File: gcc.info,  Node: V850 Options,  Next: ARC Options,  Prev: TMS320C3x/C4x Options,  Up: Submodel Options

3.17.25 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 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.


File: gcc.info,  Node: ARC Options,  Next: NS32K Options,  Prev: V850 Options,  Up: Submodel Options

3.17.26 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.  *Note
     Variable Attributes::.



File: gcc.info,  Node: NS32K Options,  Next: AVR Options,  Prev: ARC Options,  Up: Submodel Options

3.17.27 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.

`-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.

`-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 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.



File: gcc.info,  Node: AVR Options,  Next: MCore Options,  Prev: NS32K Options,  Up: Submodel Options

3.17.28 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.

`-mno-tablejump'
     Do not generate tablejump insns which sometimes increase code size.

`-mtiny-stack'
     Change only the low 8 bits of the stack pointer.


File: gcc.info,  Node: MCore Options,  Next: IA-64 Options,  Prev: AVR Options,  Up: Submodel Options

3.17.29 MCore Options
---------------------

These are the `-m' options defined for the Motorola M*Core processors.

`-mhardlit'
`-mhardlit'
`-mno-hardlit'
     Inline constants into the code stream if it can be done in two
     instructions or less.

`-mdiv'
`-mdiv'
`-mno-div'
     Use the divide instruction.  (Enabled by default).

`-mrelax-immediate'
`-mrelax-immediate'
`-mno-relax-immediate'
     Allow arbitrary sized immediates in bit operations.

`-mwide-bitfields'
`-mwide-bitfields'
`-mno-wide-bitfields'
     Always treat bit-fields as int-sized.

`-m4byte-functions'
`-m4byte-functions'
`-mno-4byte-functions'
     Force all functions to be aligned to a four byte boundary.

`-mcallgraph-data'
`-mcallgraph-data'
`-mno-callgraph-data'
     Emit callgraph information.

`-mslow-bytes'
`-mslow-bytes'
`-mno-slow-bytes'
     Prefer word access when reading byte quantities.

`-mlittle-endian'
`-mlittle-endian'
`-mbig-endian'
     Generate code for a little endian target.

`-m210'
`-m210'
`-m340'
     Generate code for the 210 processor.


File: gcc.info,  Node: IA-64 Options,  Next: D30V Options,  Prev: MCore Options,  Up: Submodel Options

3.17.30 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.

`-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.


File: gcc.info,  Node: D30V Options,  Next: S/390 and zSeries Options,  Prev: IA-64 Options,  Up: Submodel Options

3.17.31 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.


File: gcc.info,  Node: S/390 and zSeries Options,  Next: CRIS Options,  Prev: D30V Options,  Up: Submodel Options

3.17.32 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'
`-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.



File: gcc.info,  Node: CRIS Options,  Next: MMIX Options,  Prev: S/390 and zSeries Options,  Up: Submodel Options

3.17.33 CRIS Options
--------------------

These options are defined specifically for the CRIS ports.

`-march=ARCHITECTURE-TYPE'
`-mcpu=ARCHITECTURE-TYPE'
     Generate code for the specified architecture.  The choices for
     ARCHITECTURE-TYPE are `v3', `v8' and `v10' for respectively
     ETRAX 4, ETRAX 100, and ETRAX 100 LX.  Default is `v0' except for
     cris-axis-linux-gnu, where the default is `v10'.

`-mtune=ARCHITECTURE-TYPE'
     Tune to ARCHITECTURE-TYPE everything applicable about the generated
     code, except for the ABI and the set of available instructions.
     The choices for 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.

`-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.

`-sim2'
     Like `-sim', but pass linker options to locate initialized data at
     0x40000000 and zero-initialized data at 0x80000000.


File: gcc.info,  Node: MMIX Options,  Next: PDP-11 Options,  Prev: CRIS Options,  Up: Submodel Options

3.17.34 MMIX Options
--------------------

These options are defined for the MMIX:

`-mlibfuncs'
`-mno-libfuncs'
     Specify that intrinsic library functions are being compiled,
     passing all values in registers, no matter the size.

`-mepsilon'
`-mno-epsilon'
     Generate floating-point comparison instructions that compare with
     respect to the `rE' epsilon register.

`-mabi=mmixware'
`-mabi=gnu'
     Generate code that passes function parameters and return values
     that (in the called function) are seen as registers `$0' and up,
     as opposed to the GNU ABI which uses global registers `$231' and
     up.

`-mzero-extend'
`-mno-zero-extend'
     When reading data from memory in sizes shorter than 64 bits, use
     (do not use) zero-extending load instructions by default, rather
     than sign-extending ones.

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

`-mtoplevel-symbols'
`-mno-toplevel-symbols'
     Prepend (do not prepend) a `:' to all global symbols, so the
     assembly code can be used with the `PREFIX' assembly directive.

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

`-mbranch-predict'
`-mno-branch-predict'
     Use (do not use) the probable-branch instructions, when static
     branch prediction indicates a probable branch.

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


File: gcc.info,  Node: PDP-11 Options,  Next: Xstormy16 Options,  Prev: MMIX Options,  Up: Submodel Options

3.17.35 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.

`-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'.

`-mdec-asm'
     Use DEC assembler syntax.  This is the default when configured for
     any PDP-11 target other than `pdp11-*-bsd'.


File: gcc.info,  Node: Xstormy16 Options,  Next: Xtensa Options,  Prev: PDP-11 Options,  Up: Submodel Options

3.17.36 Xstormy16 Options
-------------------------

These options are defined for Xstormy16:

`-msim'
     Choose startup files and linker script suitable for the simulator.


File: gcc.info,  Node: Xtensa Options,  Prev: Xstormy16 Options,  Up: Submodel Options

3.17.37 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 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.

`-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 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.


File: gcc.info,  Node: Code Gen Options,  Next: Environment Variables,  Prev: Submodel Options,  Up: Invoking GCC

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

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

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

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

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

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

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

`-freg-struct-return'
     Return `struct' and `union' values in registers when possible.
     This is more efficient for small structures than
     `-fpcc-struct-return'.

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

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

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

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

`-fshort-double'
     Use the same size for `double' as for `float'.

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

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

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

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

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

`-fno-ident'
     Ignore the `#ident' directive.

`-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 `-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.  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 gotten away without providing
     static copies.)

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

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

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

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

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

`-fargument-alias'
`-fargument-noalias'
`-fargument-noalias-global'
     Specify the possible relationships among parameters and between
     parameters and global data.

     `-fargument-alias' specifies that arguments (parameters) may alias
     each other and may alias global storage.
     `-fargument-noalias' specifies that arguments do not alias each
     other, but may alias global storage.
     `-fargument-noalias-global' specifies that arguments do not alias
     each other and do not alias global storage.

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

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

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


File: gcc.info,  Node: Environment Variables,  Next: Running Protoize,  Prev: Code Gen Options,  Up: Invoking GCC

3.19 Environment Variables Affecting GCC
========================================

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' (*note Directory Options::).  These take precedence
over places specified using environment variables, which in turn take
precedence over those specified by the configuration of GCC.  *Note
Controlling the Compilation Driver `gcc': (gccint)Driver.

`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 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 `/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 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'
     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' (*note Preprocessor
     Options::), with an optional `-MT' switch too.

`SUNPRO_DEPENDENCIES'
     This variable is the same as the environment variable
     `DEPENDENCIES_OUTPUT' (*note 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.  *Note Preprocessor Options::.


File: gcc.info,  Node: Running Protoize,  Prev: Environment Variables,  Up: Invoking GCC

3.20 Running Protoize
=====================

The program `protoize' is an optional part of GCC.  You can use it to
add prototypes to a program, thus converting the program to ISO C in
one respect.  The companion program `unprotoize' does the reverse: it
removes argument types from any prototypes that are found.

   When you run these programs, you must specify a set of source files
as command line arguments.  The conversion programs start out by
compiling these files to see what functions they define.  The
information gathered about a file FOO is saved in a file named `FOO.X'.

   After scanning comes actual conversion.  The specified files are all
eligible to be converted; any files they include (whether sources or
just headers) are eligible as well.

   But not all the eligible files are converted.  By default,
`protoize' and `unprotoize' convert only source and header files in the
current directory.  You can specify additional directories whose files
should be converted with the `-d DIRECTORY' option.  You can also
specify particular files to exclude with the `-x FILE' option.  A file
is converted if it is eligible, its directory name matches one of the
specified directory names, and its name within the directory has not
been excluded.

   Basic conversion with `protoize' consists of rewriting most function
definitions and function declarations to specify the types of the
arguments.  The only ones not rewritten are those for varargs functions.

   `protoize' optionally inserts prototype declarations at the
beginning of the source file, to make them available for any calls that
precede the function's definition.  Or it can insert prototype
declarations with block scope in the blocks where undeclared functions
are called.

   Basic conversion with `unprotoize' consists of rewriting most
function declarations to remove any argument types, and rewriting
function definitions to the old-style pre-ISO form.

   Both conversion programs print a warning for any function
declaration or definition that they can't convert.  You can suppress
these warnings with `-q'.

   The output from `protoize' or `unprotoize' replaces the original
source file.  The original file is renamed to a name ending with
`.save' (for DOS, the saved filename ends in `.sav' without the
original `.c' suffix).  If the `.save' (`.sav' for DOS) file already
exists, then the source file is simply discarded.

   `protoize' and `unprotoize' both depend on GCC itself to scan the
program and collect information about the functions it uses.  So
neither of these programs will work until GCC is installed.

   Here is a table of the options you can use with `protoize' and
`unprotoize'.  Each option works with both programs unless otherwise
stated.

`-B DIRECTORY'
     Look for the file `SYSCALLS.c.X' in DIRECTORY, instead of the
     usual directory (normally `/usr/local/lib').  This file contains
     prototype information about standard system functions.  This option
     applies only to `protoize'.

`-c COMPILATION-OPTIONS'
     Use  COMPILATION-OPTIONS as the options when running `gcc' to
     produce the `.X' files.  The special option `-aux-info' is always
     passed in addition, to tell `gcc' to write a `.X' file.

     Note that the compilation options must be given as a single
     argument to `protoize' or `unprotoize'.  If you want to specify
     several `gcc' options, you must quote the entire set of
     compilation options to make them a single word in the shell.

     There are certain `gcc' arguments that you cannot use, because they
     would produce the wrong kind of output.  These include `-g', `-O',
     `-c', `-S', and `-o' If you include these in the
     COMPILATION-OPTIONS, they are ignored.

`-C'
     Rename files to end in `.C' (`.cc' for DOS-based file systems)
     instead of `.c'.  This is convenient if you are converting a C
     program to C++.  This option applies only to `protoize'.

`-g'
     Add explicit global declarations.  This means inserting explicit
     declarations at the beginning of each source file for each function
     that is called in the file and was not declared.  These
     declarations precede the first function definition that contains a
     call to an undeclared function.  This option applies only to
     `protoize'.

`-i STRING'
     Indent old-style parameter declarations with the string STRING.
     This option applies only to `protoize'.

     `unprotoize' converts prototyped function definitions to old-style
     function definitions, where the arguments are declared between the
     argument list and the initial `{'.  By default, `unprotoize' uses
     five spaces as the indentation.  If you want to indent with just
     one space instead, use `-i " "'.

`-k'
     Keep the `.X' files.  Normally, they are deleted after conversion
     is finished.

`-l'
     Add explicit local declarations.  `protoize' with `-l' inserts a
     prototype declaration for each function in each block which calls
     the function without any declaration.  This option applies only to
     `protoize'.

`-n'
     Make no real changes.  This mode just prints information about the
     conversions that would have been done without `-n'.

`-N'
     Make no `.save' files.  The original files are simply deleted.
     Use this option with caution.

`-p PROGRAM'
     Use the program PROGRAM as the compiler.  Normally, the name `gcc'
     is used.

`-q'
     Work quietly.  Most warnings are suppressed.

`-v'
     Print the version number, just like `-v' for `gcc'.

   If you need special compiler options to compile one of your program's
source files, then you should generate that file's `.X' file specially,
by running `gcc' on that source file with the appropriate options and
the option `-aux-info'.  Then run `protoize' on the entire set of
files.  `protoize' will use the existing `.X' file because it is newer
than the source file.  For example:

     gcc -Dfoo=bar file1.c -aux-info file1.X
     protoize *.c

You need to include the special files along with the rest in the
`protoize' command, even though their `.X' files already exist, because
otherwise they won't get converted.

   *Note Protoize Caveats::, for more information on how to use
`protoize' successfully.


File: gcc.info,  Node: C Implementation,  Next: C Extensions,  Prev: Invoking GCC,  Up: Top

4 C Implementation-defined behavior
***********************************

A conforming implementation of ISO C is required to document its choice
of behavior in each of the areas that are designated "implementation
defined."  The following lists all such areas, along with the section
number from the ISO/IEC 9899:1999 standard.

* Menu:

* Translation implementation::
* Environment implementation::
* Identifiers implementation::
* Characters implementation::
* Integers implementation::
* Floating point implementation::
* Arrays and pointers implementation::
* Hints implementation::
* Structures unions enumerations and bit-fields implementation::
* Qualifiers implementation::
* Preprocessing directives implementation::
* Library functions implementation::
* Architecture implementation::
* Locale-specific behavior implementation::


File: gcc.info,  Node: Translation implementation,  Next: Environment implementation,  Up: C Implementation

4.1 Translation
===============

   * `How a diagnostic is identified (3.10, 5.1.1.3).'

   * `Whether each nonempty sequence of white-space characters other
     than new-line is retained or replaced by one space character in
     translation phase 3 (5.1.1.2).'


File: gcc.info,  Node: Environment implementation,  Next: Identifiers implementation,  Prev: Translation implementation,  Up: C Implementation

4.2 Environment
===============

The behavior of these points are dependent on the implementation of the
C library, and are not defined by GCC itself.


File: gcc.info,  Node: Identifiers implementation,  Next: Characters implementation,  Prev: Environment implementation,  Up: C Implementation

4.3 Identifiers
===============

   * `Which additional multibyte characters may appear in identifiers
     and their correspondence to universal character names (6.4.2).'

   * `The number of significant initial characters in an identifier
     (5.2.4.1, 6.4.2).'


File: gcc.info,  Node: Characters implementation,  Next: Integers implementation,  Prev: Identifiers implementation,  Up: C Implementation

4.4 Characters
==============

   * `The number of bits in a byte (3.6).'

   * `The values of the members of the execution character set (5.2.1).'

   * `The unique value of the member of the execution character set
     produced for each of the standard alphabetic escape sequences
     (5.2.2).'

   * `The value of a `char' object into which has been stored any
     character other than a member of the basic execution character set
     (6.2.5).'

   * `Which of `signed char' or `unsigned char' has the same range,
     representation, and behavior as "plain" `char' (6.2.5, 6.3.1.1).'

   * `The mapping of members of the source character set (in character
     constants and string literals) to members of the execution
     character set (6.4.4.4, 5.1.1.2).'

   * `The value of an integer character constant containing more than
     one character or containing a character or escape sequence that
     does not map to a single-byte execution character (6.4.4.4).'

   * `The value of a wide character constant containing more than one
     multibyte character, or containing a multibyte character or escape
     sequence not represented in the extended execution character set
     (6.4.4.4).'

   * `The current locale used to convert a wide character constant
     consisting of a single multibyte character that maps to a member
     of the extended execution character set into a corresponding wide
     character code (6.4.4.4).'

   * `The current locale used to convert a wide string literal into
     corresponding wide character codes (6.4.5).'

   * `The value of a string literal containing a multibyte character or
     escape sequence not represented in the execution character set
     (6.4.5).'


File: gcc.info,  Node: Integers implementation,  Next: Floating point implementation,  Prev: Characters implementation,  Up: C Implementation

4.5 Integers
============

   * `Any extended integer types that exist in the implementation
     (6.2.5).'

   * `Whether signed integer types are represented using sign and
     magnitude, two's complement, or one's complement, and whether the
     extraordinary value is a trap representation or an ordinary value
     (6.2.6.2).'

   * `The rank of any extended integer type relative to another extended
     integer type with the same precision (6.3.1.1).'

   * `The result of, or the signal raised by, converting an integer to a
     signed integer type when the value cannot be represented in an
     object of that type (6.3.1.3).'

   * `The results of some bitwise operations on signed integers (6.5).'


File: gcc.info,  Node: Floating point implementation,  Next: Arrays and pointers implementation,  Prev: Integers implementation,  Up: C Implementation

4.6 Floating point
==================

   * `The accuracy of the floating-point operations and of the library
     functions in `<math.h>' and `<complex.h>' that return
     floating-point results (5.2.4.2.2).'

   * `The rounding behaviors characterized by non-standard values of
     `FLT_ROUNDS'  (5.2.4.2.2).'

   * `The evaluation methods characterized by non-standard negative
     values of `FLT_EVAL_METHOD' (5.2.4.2.2).'

   * `The direction of rounding when an integer is converted to a
     floating-point number that cannot exactly represent the original
     value (6.3.1.4).'

   * `The direction of rounding when a floating-point number is
     converted to a narrower floating-point number (6.3.1.5).'

   * `How the nearest representable value or the larger or smaller
     representable value immediately adjacent to the nearest
     representable value is chosen for certain floating constants
     (6.4.4.2).'

   * `Whether and how floating expressions are contracted when not
     disallowed by the `FP_CONTRACT' pragma (6.5).'

   * `The default state for the `FENV_ACCESS' pragma (7.6.1).'

   * `Additional floating-point exceptions, rounding modes,
     environments, and classifications, and their macro names (7.6,
     7.12).'

   * `The default state for the `FP_CONTRACT' pragma (7.12.2).'

   * `Whether the "inexact" floating-point exception can be raised when
     the rounded result actually does equal the mathematical result in
     an IEC 60559 conformant implementation (F.9).'

   * `Whether the "underflow" (and "inexact") floating-point exception
     can be raised when a result is tiny but not inexact in an IEC
     60559 conformant implementation (F.9).'



File: gcc.info,  Node: Arrays and pointers implementation,  Next: Hints implementation,  Prev: Floating point implementation,  Up: C Implementation

4.7 Arrays and pointers
=======================

   * `The result of converting a pointer to an integer or vice versa
     (6.3.2.3).'

     A cast from pointer to integer discards most-significant bits if
     the pointer representation is larger than the integer type,
     sign-extends(1) if the pointer representation is smaller than the
     integer type, otherwise the bits are unchanged.

     A cast from integer to pointer discards most-significant bits if
     the pointer representation is smaller than the integer type,
     extends according to the signedness of the integer type if the
     pointer representation is larger than the integer type, otherwise
     the bits are unchanged.

     When casting from pointer to integer and back again, the resulting
     pointer must reference the same object as the original pointer,
     otherwise the behavior is undefined.  That is, one may not use
     integer arithmetic to avoid the undefined behavior of pointer
     arithmetic as proscribed in 6.5.6/8.

   * `The size of the result of subtracting two pointers to elements of
     the same array (6.5.6).'


   ---------- Footnotes ----------

   (1) Future versions of GCC may zero-extend, or use a target-defined
`ptr_extend' pattern.  Do not rely on sign extension.


File: gcc.info,  Node: Hints implementation,  Next: Structures unions enumerations and bit-fields implementation,  Prev: Arrays and pointers implementation,  Up: C Implementation

4.8 Hints
=========

   * `The extent to which suggestions made by using the `register'
     storage-class specifier are effective (6.7.1).'

   * `The extent to which suggestions made by using the inline function
     specifier are effective (6.7.4).'



File: gcc.info,  Node: Structures unions enumerations and bit-fields implementation,  Next: Qualifiers implementation,  Prev: Hints implementation,  Up: C Implementation

4.9 Structures, unions, enumerations, and bit-fields
====================================================

   * `Whether a "plain" int bit-field is treated as a `signed int'
     bit-field or as an `unsigned int' bit-field (6.7.2, 6.7.2.1).'

   * `Allowable bit-field types other than `_Bool', `signed int', and
     `unsigned int' (6.7.2.1).'

   * `Whether a bit-field can straddle a storage-unit boundary
     (6.7.2.1).'

   * `The order of allocation of bit-fields within a unit (6.7.2.1).'

   * `The alignment of non-bit-field members of structures (6.7.2.1).'

   * `The integer type compatible with each enumerated type (6.7.2.2).'



File: gcc.info,  Node: Qualifiers implementation,  Next: Preprocessing directives implementation,  Prev: Structures unions enumerations and bit-fields implementation,  Up: C Implementation

4.10 Qualifiers
===============

   * `What constitutes an access to an object that has
     volatile-qualified type (6.7.3).'



File: gcc.info,  Node: Preprocessing directives implementation,  Next: Library functions implementation,  Prev: Qualifiers implementation,  Up: C Implementation

4.11 Preprocessing directives
=============================

   * `How sequences in both forms of header names are mapped to headers
     or external source file names (6.4.7).'

   * `Whether the value of a character constant in a constant expression
     that controls conditional inclusion matches the value of the same
     character constant in the execution character set (6.10.1).'

   * `Whether the value of a single-character character constant in a
     constant expression that controls conditional inclusion may have a
     negative value (6.10.1).'

   * `The places that are searched for an included `<>' delimited
     header, and how the places are specified or the header is
     identified (6.10.2).'

   * `How the named source file is searched for in an included `""'
     delimited header (6.10.2).'

   * `The method by which preprocessing tokens (possibly resulting from
     macro expansion) in a `#include' directive are combined into a
     header name (6.10.2).'

   * `The nesting limit for `#include' processing (6.10.2).'

   * `Whether the `#' operator inserts a `\' character before the `\'
     character that begins a universal character name in a character
     constant or string literal (6.10.3.2).'

   * `The behavior on each recognized non-`STDC #pragma' directive
     (6.10.6).'

   * `The definitions for `__DATE__' and `__TIME__' when respectively,
     the date and time of translation are not available (6.10.8).'



File: gcc.info,  Node: Library functions implementation,  Next: Architecture implementation,  Prev: Preprocessing directives implementation,  Up: C Implementation

4.12 Library functions
======================

The behavior of these points are dependent on the implementation of the
C library, and are not defined by GCC itself.


File: gcc.info,  Node: Architecture implementation,  Next: Locale-specific behavior implementation,  Prev: Library functions implementation,  Up: C Implementation

4.13 Architecture
=================

   * `The values or expressions assigned to the macros specified in the
     headers `<float.h>', `<limits.h>', and `<stdint.h>' (5.2.4.2,
     7.18.2, 7.18.3).'

   * `The number, order, and encoding of bytes in any object (when not
     explicitly specified in this International Standard) (6.2.6.1).'

   * `The value of the result of the sizeof operator (6.5.3.4).'



File: gcc.info,  Node: Locale-specific behavior implementation,  Prev: Architecture implementation,  Up: C Implementation

4.14 Locale-specific behavior
=============================

The behavior of these points are dependent on the implementation of the
C library, and are not defined by GCC itself.


File: gcc.info,  Node: C Extensions,  Next: C++ Extensions,  Prev: C Implementation,  Up: Top

5 Extensions to the C Language Family
*************************************

GNU C provides several language features not found in ISO standard C.
(The `-pedantic' option directs GCC to print a warning message if any
of these features is used.)  To test for the availability of these
features in conditional compilation, check for a predefined macro
`__GNUC__', which is always defined under GCC.

   These extensions are available in C and Objective-C.  Most of them
are also available in C++.  *Note Extensions to the C++ Language: C++
Extensions, for extensions that apply _only_ to C++.

   Some features that are in ISO C99 but not C89 or C++ are also, as
extensions, accepted by GCC in C89 mode and in C++.

* Menu:

* Statement Exprs::     Putting statements and declarations inside expressions.
* Local Labels::        Labels local to a statement-expression.
* Labels as Values::    Getting pointers to labels, and computed gotos.
* Nested Functions::    As in Algol and Pascal, lexical scoping of functions.
* Constructing Calls::	Dispatching a call to another function.
* Typeof::              `typeof': referring to the type of an expression.
* Lvalues::             Using `?:', `,' and casts in lvalues.
* Conditionals::        Omitting the middle operand of a `?:' expression.
* Long Long::		Double-word integers---`long long int'.
* Complex::             Data types for complex numbers.
* Hex Floats::          Hexadecimal floating-point constants.
* Zero Length::         Zero-length arrays.
* Variable Length::     Arrays whose length is computed at run time.
* Variadic Macros::	Macros with a variable number of arguments.
* Escaped Newlines::    Slightly looser rules for escaped newlines.
* Multi-line Strings::  String literals with embedded newlines.
* Subscripting::        Any array can be subscripted, even if not an lvalue.
* Pointer Arith::       Arithmetic on `void'-pointers and function pointers.
* Initializers::        Non-constant initializers.
* Compound Literals::   Compound literals give structures, unions
                         or arrays as values.
* Designated Inits::	Labeling elements of initializers.
* Cast to Union::       Casting to union type from any member of the union.
* Case Ranges::		`case 1 ... 9' and such.
* Mixed Declarations::	Mixing declarations and code.
* Function Attributes:: Declaring that functions have no side effects,
                         or that they can never return.
* Attribute Syntax::    Formal syntax for attributes.
* Function Prototypes:: Prototype declarations and old-style definitions.
* C++ Comments::        C++ comments are recognized.
* Dollar Signs::        Dollar sign is allowed in identifiers.
* Character Escapes::   `\e' stands for the character <ESC>.
* Variable Attributes::	Specifying attributes of variables.
* Type Attributes::	Specifying attributes of types.
* Alignment::           Inquiring about the alignment of a type or variable.
* Inline::              Defining inline functions (as fast as macros).
* Extended Asm::        Assembler instructions with C expressions as operands.
                         (With them you can define ``built-in'' functions.)
* Constraints::         Constraints for asm operands
* Asm Labels::          Specifying the assembler name to use for a C symbol.
* Explicit Reg Vars::   Defining variables residing in specified registers.
* Alternate Keywords::  `__const__', `__asm__', etc., for header files.
* Incomplete Enums::    `enum foo;', with details to follow.
* Function Names::	Printable strings which are the name of the current
			 function.
* Return Address::      Getting the return or frame address of a function.
* Vector Extensions::   Using vector instructions through built-in functions.
* Other Builtins::      Other built-in functions.
* Target Builtins::     Built-in functions specific to particular targets.
* Pragmas::             Pragmas accepted by GCC.
* Unnamed Fields::      Unnamed struct/union fields within structs/unions.


File: gcc.info,  Node: Statement Exprs,  Next: Local Labels,  Up: C Extensions

5.1 Statements and Declarations in Expressions
==============================================

A compound statement enclosed in parentheses may appear as an expression
in GNU C.  This allows you to use loops, switches, and local variables
within an expression.

   Recall that a compound statement is a sequence of statements
surrounded by braces; in this construct, parentheses go around the
braces.  For example:

     ({ int y = foo (); int z;
        if (y > 0) z = y;
        else z = - y;
        z; })

is a valid (though slightly more complex than necessary) expression for
the absolute value of `foo ()'.

   The last thing in the compound statement should be an expression
followed by a semicolon; the value of this subexpression serves as the
value of the entire construct.  (If you use some other kind of statement
last within the braces, the construct has type `void', and thus
effectively no value.)

   This feature is especially useful in making macro definitions "safe"
(so that they evaluate each operand exactly once).  For example, the
"maximum" function is commonly defined as a macro in standard C as
follows:

     #define max(a,b) ((a) > (b) ? (a) : (b))

But this definition computes either A or B twice, with bad results if
the operand has side effects.  In GNU C, if you know the type of the
operands (here let's assume `int'), you can define the macro safely as
follows:

     #define maxint(a,b) \
       ({int _a = (a), _b = (b); _a > _b ? _a : _b; })

   Embedded statements are not allowed in constant expressions, such as
the value of an enumeration constant, the width of a bit-field, or the
initial value of a static variable.

   If you don't know the type of the operand, you can still do this,
but you must use `typeof' (*note Typeof::).

   Statement expressions are not supported fully in G++, and their fate
there is unclear.  (It is possible that they will become fully supported
at some point, or that they will be deprecated, or that the bugs that
are present will continue to exist indefinitely.)  Presently, statement
expressions do not work well as default arguments.

   In addition, there are semantic issues with statement-expressions in
C++.  If you try to use statement-expressions instead of inline
functions in C++, you may be surprised at the way object destruction is
handled.  For example:

     #define foo(a)  ({int b = (a); b + 3; })

does not work the same way as:

     inline int foo(int a) { int b = a; return b + 3; }

In particular, if the expression passed into `foo' involves the
creation of temporaries, the destructors for those temporaries will be
run earlier in the case of the macro than in the case of the function.

   These considerations mean that it is probably a bad idea to use
statement-expressions of this form in header files that are designed to
work with C++.  (Note that some versions of the GNU C Library contained
header files using statement-expression that lead to precisely this
bug.)


File: gcc.info,  Node: Local Labels,  Next: Labels as Values,  Prev: Statement Exprs,  Up: C Extensions

5.2 Locally Declared Labels
===========================

Each statement expression is a scope in which "local labels" can be
declared.  A local label is simply an identifier; you can jump to it
with an ordinary `goto' statement, but only from within the statement
expression it belongs to.

   A local label declaration looks like this:

     __label__ LABEL;

or

     __label__ LABEL1, LABEL2, ...;

   Local label declarations must come at the beginning of the statement
expression, right after the `({', before any ordinary declarations.

   The label declaration defines the label _name_, but does not define
the label itself.  You must do this in the usual way, with `LABEL:',
within the statements of the statement expression.

   The local label feature is useful because statement expressions are
often used in macros.  If the macro contains nested loops, a `goto' can
be useful for breaking out of them.  However, an ordinary label whose
scope is the whole function cannot be used: if the macro can be
expanded several times in one function, the label will be multiply
defined in that function.  A local label avoids this problem.  For
example:

     #define SEARCH(array, target)                     \
     ({                                                \
       __label__ found;                                \
       typeof (target) _SEARCH_target = (target);      \
       typeof (*(array)) *_SEARCH_array = (array);     \
       int i, j;                                       \
       int value;                                      \
       for (i = 0; i < max; i++)                       \
         for (j = 0; j < max; j++)                     \
           if (_SEARCH_array[i][j] == _SEARCH_target)  \
             { value = i; goto found; }                \
       value = -1;                                     \
      found:                                           \
       value;                                          \
     })


File: gcc.info,  Node: Labels as Values,  Next: Nested Functions,  Prev: Local Labels,  Up: C Extensions

5.3 Labels as Values
====================

You can get the address of a label defined in the current function (or
a containing function) with the unary operator `&&'.  The value has
type `void *'.  This value is a constant and can be used wherever a
constant of that type is valid.  For example:

     void *ptr;
     ...
     ptr = &&foo;

   To use these values, you need to be able to jump to one.  This is
done with the computed goto statement(1), `goto *EXP;'.  For example,

     goto *ptr;

Any expression of type `void *' is allowed.

   One way of using these constants is in initializing a static array
that will serve as a jump table:

     static void *array[] = { &&foo, &&bar, &&hack };

   Then you can select a label with indexing, like this:

     goto *array[i];

Note that this does not check whether the subscript is in bounds--array
indexing in C never does that.

   Such an array of label values serves a purpose much like that of the
`switch' statement.  The `switch' statement is cleaner, so use that
rather than an array unless the problem does not fit a `switch'
statement very well.

   Another use of label values is in an interpreter for threaded code.
The labels within the interpreter function can be stored in the
threaded code for super-fast dispatching.

   You may not use this mechanism to jump to code in a different
function.  If you do that, totally unpredictable things will happen.
The best way to avoid this is to store the label address only in
automatic variables and never pass it as an argument.

   An alternate way to write the above example is

     static const int array[] = { &&foo - &&foo, &&bar - &&foo,
                                  &&hack - &&foo };
     goto *(&&foo + array[i]);

This is more friendly to code living in shared libraries, as it reduces
the number of dynamic relocations that are needed, and by consequence,
allows the data to be read-only.

   ---------- Footnotes ----------

   (1) The analogous feature in Fortran is called an assigned goto, but
that name seems inappropriate in C, where one can do more than simply
store label addresses in label variables.


File: gcc.info,  Node: Nested Functions,  Next: Constructing Calls,  Prev: Labels as Values,  Up: C Extensions

5.4 Nested Functions
====================

A "nested function" is a function defined inside another function.
(Nested functions are not supported for GNU C++.)  The nested function's
name is local to the block where it is defined.  For example, here we
define a nested function named `square', and call it twice:

     foo (double a, double b)
     {
       double square (double z) { return z * z; }

       return square (a) + square (b);
     }

   The nested function can access all the variables of the containing
function that are visible at the point of its definition.  This is
called "lexical scoping".  For example, here we show a nested function
which uses an inherited variable named `offset':

     bar (int *array, int offset, int size)
     {
       int access (int *array, int index)
         { return array[index + offset]; }
       int i;
       ...
       for (i = 0; i < size; i++)
         ... access (array, i) ...
     }

   Nested function definitions are permitted within functions in the
places where variable definitions are allowed; that is, in any block,
before the first statement in the block.

   It is possible to call the nested function from outside the scope of
its name by storing its address or passing the address to another
function:

     hack (int *array, int size)
     {
       void store (int index, int value)
         { array[index] = value; }

       intermediate (store, size);
     }

   Here, the function `intermediate' receives the address of `store' as
an argument.  If `intermediate' calls `store', the arguments given to
`store' are used to store into `array'.  But this technique works only
so long as the containing function (`hack', in this example) does not
exit.

   If you try to call the nested function through its address after the
containing function has exited, all hell will break loose.  If you try
to call it after a containing scope level has exited, and if it refers
to some of the variables that are no longer in scope, you may be lucky,
but it's not wise to take the risk.  If, however, the nested function
does not refer to anything that has gone out of scope, you should be
safe.

   GCC implements taking the address of a nested function using a
technique called "trampolines".  A paper describing them is available as

`http://people.debian.org/~aaronl/Usenix88-lexic.pdf'.

   A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (*note Local Labels::).  Such a jump returns instantly to the
containing function, exiting the nested function which did the `goto'
and any intermediate functions as well.  Here is an example:

     bar (int *array, int offset, int size)
     {
       __label__ failure;
       int access (int *array, int index)
         {
           if (index > size)
             goto failure;
           return array[index + offset];
         }
       int i;
       ...
       for (i = 0; i < size; i++)
         ... access (array, i) ...
       ...
       return 0;

      /* Control comes here from `access'
         if it detects an error.  */
      failure:
       return -1;
     }

   A nested function always has internal linkage.  Declaring one with
`extern' is erroneous.  If you need to declare the nested function
before its definition, use `auto' (which is otherwise meaningless for
function declarations).

     bar (int *array, int offset, int size)
     {
       __label__ failure;
       auto int access (int *, int);
       ...
       int access (int *array, int index)
         {
           if (index > size)
             goto failure;
           return array[index + offset];
         }
       ...
     }


File: gcc.info,  Node: Constructing Calls,  Next: Typeof,  Prev: Nested Functions,  Up: C Extensions

5.5 Constructing Function Calls
===============================

Using the built-in functions described below, you can record the
arguments a function received, and call another function with the same
arguments, without knowing the number or types of the arguments.

   You can also record the return value of that function call, and
later return that value, without knowing what data type the function
tried to return (as long as your caller expects that data type).

 -- Built-in Function: void * __builtin_apply_args ()
     This built-in function returns a pointer to data describing how to
     perform a call with the same arguments as were passed to the
     current function.

     The function saves the arg pointer register, structure value
     address, and all registers that might be used to pass arguments to
     a function into a block of memory allocated on the stack.  Then it
     returns the address of that block.

 -- Built-in Function: void * __builtin_apply (void (*FUNCTION)(), void
          *ARGUMENTS, size_t SIZE)
     This built-in function invokes FUNCTION with a copy of the
     parameters described by ARGUMENTS and SIZE.

     The value of ARGUMENTS should be the value returned by
     `__builtin_apply_args'.  The argument SIZE specifies the size of
     the stack argument data, in bytes.

     This function returns a pointer to data describing how to return
     whatever value was returned by FUNCTION.  The data is saved in a
     block of memory allocated on the stack.

     It is not always simple to compute the proper value for SIZE.  The
     value is used by `__builtin_apply' to compute the amount of data
     that should be pushed on the stack and copied from the incoming
     argument area.

 -- Built-in Function: void __builtin_return (void *RESULT)
     This built-in function returns the value described by RESULT from
     the containing function.  You should specify, for RESULT, a value
     returned by `__builtin_apply'.

   The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within
the expressions that are substituted for `a' and `b'.  Eventually we
hope to design a new form of declaration syntax that allows you to
declare variables whose scopes start only after their initializers;
this will be a more reliable way to prevent such conflicts.


File: gcc.info,  Node: Typeof,  Next: Lvalues,  Prev: Constructing Calls,  Up: C Extensions

5.6 Referring to a Type with `typeof'
=====================================

Another way to refer to the type of an expression is with `typeof'.
The syntax of using of this keyword looks like `sizeof', but the
construct acts semantically like a type name defined with `typedef'.

   There are two ways of writing the argument to `typeof': with an
expression or with a type.  Here is an example with an expression:

     typeof (x[0](1))

This assumes that `x' is an array of pointers to functions; the type
described is that of the values of the functions.

   Here is an example with a typename as the argument:

     typeof (int *)

Here the type described is that of pointers to `int'.

   If you are writing a header file that must work when included in ISO
C programs, write `__typeof__' instead of `typeof'.  *Note Alternate
Keywords::.

   A `typeof'-construct can be used anywhere a typedef name could be
used.  For example, you can use it in a declaration, in a cast, or
inside of `sizeof' or `typeof'.

   `typeof' is often useful in conjunction with the
statements-within-expressions feature.  Here is how the two together can
be used to define a safe "maximum" macro that operates on any
arithmetic type and evaluates each of its arguments exactly once:

     #define max(a,b) \
       ({ typeof (a) _a = (a); \
           typeof (b) _b = (b); \
         _a > _b ? _a : _b; })

Some more examples of the use of `typeof':

   * This declares `y' with the type of what `x' points to.

          typeof (*x) y;

   * This declares `y' as an array of such values.

          typeof (*x) y[4];

   * This declares `y' as an array of pointers to characters:

          typeof (typeof (char *)[4]) y;

     It is equivalent to the following traditional C declaration:

          char *y[4];

     To see the meaning of the declaration using `typeof', and why it
     might be a useful way to write, let's rewrite it with these macros:

          #define pointer(T)  typeof(T *)
          #define array(T, N) typeof(T [N])

     Now the declaration can be rewritten this way:

          array (pointer (char), 4) y;

     Thus, `array (pointer (char), 4)' is the type of arrays of 4
     pointers to `char'.

   _Compatibility Note:_ In addition to `typeof', GCC 2 supported a
more limited extension which permitted one to write

     typedef T = EXPR;

with the effect of declaring T to have the type of the expression EXPR.
This extension does not work with GCC 3 (versions between 3.0 and 3.2
will crash; 3.2.1 and later give an error).  Code which relies on it
should be rewritten to use `typeof':

     typedef typeof(EXPR) T;

This will work with all versions of GCC.


File: gcc.info,  Node: Lvalues,  Next: Conditionals,  Prev: Typeof,  Up: C Extensions

5.7 Generalized Lvalues
=======================

Compound expressions, conditional expressions and casts are allowed as
lvalues provided their operands are lvalues.  This means that you can
take their addresses or store values into them.

   Standard C++ allows compound expressions and conditional expressions
as lvalues, and permits casts to reference type, so use of this
extension is deprecated for C++ code.

   For example, a compound expression can be assigned, provided the last
expression in the sequence is an lvalue.  These two expressions are
equivalent:

     (a, b) += 5
     a, (b += 5)

   Similarly, the address of the compound expression can be taken.
These two expressions are equivalent:

     &(a, b)
     a, &b

   A conditional expression is a valid lvalue if its type is not void
and the true and false branches are both valid lvalues.  For example,
these two expressions are equivalent:

     (a ? b : c) = 5
     (a ? b = 5 : (c = 5))

   A cast is a valid lvalue if its operand is an lvalue.  A simple
assignment whose left-hand side is a cast works by converting the
right-hand side first to the specified type, then to the type of the
inner left-hand side expression.  After this is stored, the value is
converted back to the specified type to become the value of the
assignment.  Thus, if `a' has type `char *', the following two
expressions are equivalent:

     (int)a = 5
     (int)(a = (char *)(int)5)

   An assignment-with-arithmetic operation such as `+=' applied to a
cast performs the arithmetic using the type resulting from the cast,
and then continues as in the previous case.  Therefore, these two
expressions are equivalent:

     (int)a += 5
     (int)(a = (char *)(int) ((int)a + 5))

   You cannot take the address of an lvalue cast, because the use of its
address would not work out coherently.  Suppose that `&(int)f' were
permitted, where `f' has type `float'.  Then the following statement
would try to store an integer bit-pattern where a floating point number
belongs:

     *&(int)f = 1;

   This is quite different from what `(int)f = 1' would do--that would
convert 1 to floating point and store it.  Rather than cause this
inconsistency, we think it is better to prohibit use of `&' on a cast.

   If you really do want an `int *' pointer with the address of `f',
you can simply write `(int *)&f'.


File: gcc.info,  Node: Conditionals,  Next: Long Long,  Prev: Lvalues,  Up: C Extensions

5.8 Conditionals with Omitted Operands
======================================

The middle operand in a conditional expression may be omitted.  Then if
the first operand is nonzero, its value is the value of the conditional
expression.

   Therefore, the expression

     x ? : y

has the value of `x' if that is nonzero; otherwise, the value of `y'.

   This example is perfectly equivalent to

     x ? x : y

In this simple case, the ability to omit the middle operand is not
especially useful.  When it becomes useful is when the first operand
does, or may (if it is a macro argument), contain a side effect.  Then
repeating the operand in the middle would perform the side effect
twice.  Omitting the middle operand uses the value already computed
without the undesirable effects of recomputing it.


File: gcc.info,  Node: Long Long,  Next: Complex,  Prev: Conditionals,  Up: C Extensions

5.9 Double-Word Integers
========================

ISO C99 supports data types for integers that are at least 64 bits wide,
and as an extension GCC supports them in C89 mode and in C++.  Simply
write `long long int' for a signed integer, or `unsigned long long int'
for an unsigned integer.  To make an integer constant of type `long
long int', add the suffix `LL' to the integer.  To make an integer
constant of type `unsigned long long int', add the suffix `ULL' to the
integer.

   You can use these types in arithmetic like any other integer types.
Addition, subtraction, and bitwise boolean operations on these types
are open-coded on all types of machines.  Multiplication is open-coded
if the machine supports fullword-to-doubleword a widening multiply
instruction.  Division and shifts are open-coded only on machines that
provide special support.  The operations that are not open-coded use
special library routines that come with GCC.

   There may be pitfalls when you use `long long' types for function
arguments, unless you declare function prototypes.  If a function
expects type `int' for its argument, and you pass a value of type `long
long int', confusion will result because the caller and the subroutine
will disagree about the number of bytes for the argument.  Likewise, if
the function expects `long long int' and you pass `int'.  The best way
to avoid such problems is to use prototypes.


File: gcc.info,  Node: Complex,  Next: Hex Floats,  Prev: Long Long,  Up: C Extensions

5.10 Complex Numbers
====================

ISO C99 supports complex floating data types, and as an extension GCC
supports them in C89 mode and in C++, and supports complex integer data
types which are not part of ISO C99.  You can declare complex types
using the keyword `_Complex'.  As an extension, the older GNU keyword
`__complex__' is also supported.

   For example, `_Complex double x;' declares `x' as a variable whose
real part and imaginary part are both of type `double'.  `_Complex
short int y;' declares `y' to have real and imaginary parts of type
`short int'; this is not likely to be useful, but it shows that the set
of complex types is complete.

   To write a constant with a complex data type, use the suffix `i' or
`j' (either one; they are equivalent).  For example, `2.5fi' has type
`_Complex float' and `3i' has type `_Complex int'.  Such a constant
always has a pure imaginary value, but you can form any complex value
you like by adding one to a real constant.  This is a GNU extension; if
you have an ISO C99 conforming C library (such as GNU libc), and want
to construct complex constants of floating type, you should include
`<complex.h>' and use the macros `I' or `_Complex_I' instead.

   To extract the real part of a complex-valued expression EXP, write
`__real__ EXP'.  Likewise, use `__imag__' to extract the imaginary
part.  This is a GNU extension; for values of floating type, you should
use the ISO C99 functions `crealf', `creal', `creall', `cimagf',
`cimag' and `cimagl', declared in `<complex.h>' and also provided as
built-in functions by GCC.

   The operator `~' performs complex conjugation when used on a value
with a complex type.  This is a GNU extension; for values of floating
type, you should use the ISO C99 functions `conjf', `conj' and `conjl',
declared in `<complex.h>' and also provided as built-in functions by
GCC.

   GCC can allocate complex automatic variables in a noncontiguous
fashion; it's even possible for the real part to be in a register while
the imaginary part is on the stack (or vice-versa).  None of the
supported debugging info formats has a way to represent noncontiguous
allocation like this, so GCC describes a noncontiguous complex variable
as if it were two separate variables of noncomplex type.  If the
variable's actual name is `foo', the two fictitious variables are named
`foo$real' and `foo$imag'.  You can examine and set these two
fictitious variables with your debugger.

   A future version of GDB will know how to recognize such pairs and
treat them as a single variable with a complex type.


File: gcc.info,  Node: Hex Floats,  Next: Zero Length,  Prev: Complex,  Up: C Extensions

5.11 Hex Floats
===============

ISO C99 supports floating-point numbers written not only in the usual
decimal notation, such as `1.55e1', but also numbers such as `0x1.fp3'
written in hexadecimal format.  As a GNU extension, GCC supports this
in C89 mode (except in some cases when strictly conforming) and in C++.
In that format the `0x' hex introducer and the `p' or `P' exponent
field are mandatory.  The exponent is a decimal number that indicates
the power of 2 by which the significant part will be multiplied.  Thus
`0x1.f' is 1 15/16, `p3' multiplies it by 8, and the value of `0x1.fp3'
is the same as `1.55e1'.

   Unlike for floating-point numbers in the decimal notation the
exponent is always required in the hexadecimal notation.  Otherwise the
compiler would not be able to resolve the ambiguity of, e.g., `0x1.f'.
This could mean `1.0f' or `1.9375' since `f' is also the extension for
floating-point constants of type `float'.


File: gcc.info,  Node: Zero Length,  Next: Variable Length,  Prev: Hex Floats,  Up: C Extensions

5.12 Arrays of Length Zero
==========================

Zero-length arrays are allowed in GNU C.  They are very useful as the
last element of a structure which is really a header for a
variable-length object:

     struct line {
       int length;
       char contents[0];
     };

     struct line *thisline = (struct line *)
       malloc (sizeof (struct line) + this_length);
     thisline->length = this_length;

   In ISO C89, you would have to give `contents' a length of 1, which
means either you waste space or complicate the argument to `malloc'.

   In ISO C99, you would use a "flexible array member", which is
slightly different in syntax and semantics:

   * Flexible array members are written as `contents[]' without the `0'.

   * Flexible array members have incomplete type, and so the `sizeof'
     operator may not be applied.  As a quirk of the original
     implementation of zero-length arrays, `sizeof' evaluates to zero.

   * Flexible array members may only appear as the last member of a
     `struct' that is otherwise non-empty.

   GCC versions before 3.0 allowed zero-length arrays to be statically
initialized, as if they were flexible arrays.  In addition to those
cases that were useful, it also allowed initializations in situations
that would corrupt later data.  Non-empty initialization of zero-length
arrays is now treated like any case where there are more initializer
elements than the array holds, in that a suitable warning about "excess
elements in array" is given, and the excess elements (all of them, in
this case) are ignored.

   Instead GCC allows static initialization of flexible array members.
This is equivalent to defining a new structure containing the original
structure followed by an array of sufficient size to contain the data.
I.e. in the following, `f1' is constructed as if it were declared like
`f2'.

     struct f1 {
       int x; int y[];
     } f1 = { 1, { 2, 3, 4 } };

     struct f2 {
       struct f1 f1; int data[3];
     } f2 = { { 1 }, { 2, 3, 4 } };

The convenience of this extension is that `f1' has the desired type,
eliminating the need to consistently refer to `f2.f1'.

   This has symmetry with normal static arrays, in that an array of
unknown size is also written with `[]'.

   Of course, this extension only makes sense if the extra data comes at
the end of a top-level object, as otherwise we would be overwriting
data at subsequent offsets.  To avoid undue complication and confusion
with initialization of deeply nested arrays, we simply disallow any
non-empty initialization except when the structure is the top-level
object.  For example:

     struct foo { int x; int y[]; };
     struct bar { struct foo z; };

     struct foo a = { 1, { 2, 3, 4 } };        // Valid.
     struct bar b = { { 1, { 2, 3, 4 } } };    // Invalid.
     struct bar c = { { 1, { } } };            // Valid.
     struct foo d[1] = { { 1 { 2, 3, 4 } } };  // Invalid.


File: gcc.info,  Node: Variable Length,  Next: Variadic Macros,  Prev: Zero Length,  Up: C Extensions

5.13 Arrays of Variable Length
==============================

Variable-length automatic arrays are allowed in ISO C99, and as an
extension GCC accepts them in C89 mode and in C++.  (However, GCC's
implementation of variable-length arrays does not yet conform in detail
to the ISO C99 standard.)  These arrays are declared like any other
automatic arrays, but with a length that is not a constant expression.
The storage is allocated at the point of declaration and deallocated
when the brace-level is exited.  For example:

     FILE *
     concat_fopen (char *s1, char *s2, char *mode)
     {
       char str[strlen (s1) + strlen (s2) + 1];
       strcpy (str, s1);
       strcat (str, s2);
       return fopen (str, mode);
     }

   Jumping or breaking out of the scope of the array name deallocates
the storage.  Jumping into the scope is not allowed; you get an error
message for it.

   You can use the function `alloca' to get an effect much like
variable-length arrays.  The function `alloca' is available in many
other C implementations (but not in all).  On the other hand,
variable-length arrays are more elegant.

   There are other differences between these two methods.  Space
allocated with `alloca' exists until the containing _function_ returns.
The space for a variable-length array is deallocated as soon as the
array name's scope ends.  (If you use both variable-length arrays and
`alloca' in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with `alloca'.)

   You can also use variable-length arrays as arguments to functions:

     struct entry
     tester (int len, char data[len][len])
     {
       ...
     }

   The length of an array is computed once when the storage is allocated
and is remembered for the scope of the array in case you access it with
`sizeof'.

   If you want to pass the array first and the length afterward, you can
use a forward declaration in the parameter list--another GNU extension.

     struct entry
     tester (int len; char data[len][len], int len)
     {
       ...
     }

   The `int len' before the semicolon is a "parameter forward
declaration", and it serves the purpose of making the name `len' known
when the declaration of `data' is parsed.

   You can write any number of such parameter forward declarations in
the parameter list.  They can be separated by commas or semicolons, but
the last one must end with a semicolon, which is followed by the "real"
parameter declarations.  Each forward declaration must match a "real"
declaration in parameter name and data type.  ISO C99 does not support
parameter forward declarations.


File: gcc.info,  Node: Variadic Macros,  Next: Escaped Newlines,  Prev: Variable Length,  Up: C Extensions

5.14 Macros with a Variable Number of Arguments.
================================================

In the ISO C standard of 1999, a macro can be declared to accept a
variable number of arguments much as a function can.  The syntax for
defining the macro is similar to that of a function.  Here is an
example:

     #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)

   Here `...' is a "variable argument".  In the invocation of such a
macro, it represents the zero or more tokens until the closing
parenthesis that ends the invocation, including any commas.  This set of
tokens replaces the identifier `__VA_ARGS__' in the macro body wherever
it appears.  See the CPP manual for more information.

   GCC has long supported variadic macros, and used a different syntax
that allowed you to give a name to the variable arguments just like any
other argument.  Here is an example:

     #define debug(format, args...) fprintf (stderr, format, args)

   This is in all ways equivalent to the ISO C example above, but
arguably more readable and descriptive.

   GNU CPP has two further variadic macro extensions, and permits them
to be used with either of the above forms of macro definition.

   In standard C, you are not allowed to leave the variable argument out
entirely; but you are allowed to pass an empty argument.  For example,
this invocation is invalid in ISO C, because there is no comma after
the string:

     debug ("A message")

   GNU CPP permits you to completely omit the variable arguments in this
way.  In the above examples, the compiler would complain, though since
the expansion of the macro still has the extra comma after the format
string.

   To help solve this problem, CPP behaves specially for variable
arguments used with the token paste operator, `##'.  If instead you
write

     #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)

   and if the variable arguments are omitted or empty, the `##'
operator causes the preprocessor to remove the comma before it.  If you
do provide some variable arguments in your macro invocation, GNU CPP
does not complain about the paste operation and instead places the
variable arguments after the comma.  Just like any other pasted macro
argument, these arguments are not macro expanded.


File: gcc.info,  Node: Escaped Newlines,  Next: Multi-line Strings,  Prev: Variadic Macros,  Up: C Extensions

5.15 Slightly Looser Rules for Escaped Newlines
===============================================

Recently, the non-traditional preprocessor has relaxed its treatment of
escaped newlines.  Previously, the newline had to immediately follow a
backslash.  The current implementation allows whitespace in the form of
spaces, horizontal and vertical tabs, and form feeds between the
backslash and the subsequent newline.  The preprocessor issues a
warning, but treats it as a valid escaped newline and combines the two
lines to form a single logical line.  This works within comments and
tokens, including multi-line strings, as well as between tokens.
Comments are _not_ treated as whitespace for the purposes of this
relaxation, since they have not yet been replaced with spaces.


File: gcc.info,  Node: Multi-line Strings,  Next: Subscripting,  Prev: Escaped Newlines,  Up: C Extensions

5.16 String Literals with Embedded Newlines
===========================================

As an extension, GNU CPP permits string literals to cross multiple lines
without escaping the embedded newlines.  Each embedded newline is
replaced with a single `\n' character in the resulting string literal,
regardless of what form the newline took originally.

   CPP currently allows such strings in directives as well (other than
the `#include' family).  This is deprecated and will eventually be
removed.


File: gcc.info,  Node: Subscripting,  Next: Pointer Arith,  Prev: Multi-line Strings,  Up: C Extensions

5.17 Non-Lvalue Arrays May Have Subscripts
==========================================

In ISO C99, arrays that are not lvalues still decay to pointers, and
may be subscripted, although they may not be modified or used after the
next sequence point and the unary `&' operator may not be applied to
them.  As an extension, GCC allows such arrays to be subscripted in C89
mode, though otherwise they do not decay to pointers outside C99 mode.
For example, this is valid in GNU C though not valid in C89:

     struct foo {int a[4];};

     struct foo f();

     bar (int index)
     {
       return f().a[index];
     }


File: gcc.info,  Node: Pointer Arith,  Next: Initializers,  Prev: Subscripting,  Up: C Extensions

5.18 Arithmetic on `void'- and Function-Pointers
================================================

In GNU C, addition and subtraction operations are supported on pointers
to `void' and on pointers to functions.  This is done by treating the
size of a `void' or of a function as 1.

   A consequence of this is that `sizeof' is also allowed on `void' and
on function types, and returns 1.

   The option `-Wpointer-arith' requests a warning if these extensions
are used.


File: gcc.info,  Node: Initializers,  Next: Compound Literals,  Prev: Pointer Arith,  Up: C Extensions

5.19 Non-Constant Initializers
==============================

As in standard C++ and ISO C99, the elements of an aggregate
initializer for an automatic variable are not required to be constant
expressions in GNU C.  Here is an example of an initializer with
run-time varying elements:

     foo (float f, float g)
     {
       float beat_freqs[2] = { f-g, f+g };
       ...
     }


File: gcc.info,  Node: Compound Literals,  Next: Designated Inits,  Prev: Initializers,  Up: C Extensions

5.20 Compound Literals
======================

ISO C99 supports compound literals.  A compound literal looks like a
cast containing an initializer.  Its value is an object of the type
specified in the cast, containing the elements specified in the
initializer; it is an lvalue.  As an extension, GCC supports compound
literals in C89 mode and in C++.

   Usually, the specified type is a structure.  Assume that `struct
foo' and `structure' are declared as shown:

     struct foo {int a; char b[2];} structure;

Here is an example of constructing a `struct foo' with a compound
literal:

     structure = ((struct foo) {x + y, 'a', 0});

This is equivalent to writing the following:

     {
       struct foo temp = {x + y, 'a', 0};
       structure = temp;
     }

   You can also construct an array.  If all the elements of the
compound literal are (made up of) simple constant expressions, suitable
for use in initializers of objects of static storage duration, then the
compound literal can be coerced to a pointer to its first element and
used in such an initializer, as shown here:

     char **foo = (char *[]) { "x", "y", "z" };

   Compound literals for scalar types and union types are is also
allowed, but then the compound literal is equivalent to a cast.

   As a GNU extension, GCC allows initialization of objects with static
storage duration by compound literals (which is not possible in ISO
C99, because the initializer is not a constant).  It is handled as if
the object was initialized only with the bracket enclosed list if
compound literal's and object types match.  The initializer list of the
compound literal must be constant.  If the object being initialized has
array type of unknown size, the size is determined by compound literal
size.

     static struct foo x = (struct foo) {1, 'a', 'b'};
     static int y[] = (int []) {1, 2, 3};
     static int z[] = (int [3]) {1};

The above lines are equivalent to the following:
     static struct foo x = {1, 'a', 'b'};
     static int y[] = {1, 2, 3};
     static int z[] = {1, 0, 0};


File: gcc.info,  Node: Designated Inits,  Next: Cast to Union,  Prev: Compound Literals,  Up: C Extensions

5.21 Designated Initializers
============================

Standard C89 requires the elements of an initializer to appear in a
fixed order, the same as the order of the elements in the array or
structure being initialized.

   In ISO C99 you can give the elements in any order, specifying the
array indices or structure field names they apply to, and GNU C allows
this as an extension in C89 mode as well.  This extension is not
implemented in GNU C++.

   To specify an array index, write `[INDEX] =' before the element
value.  For example,

     int a[6] = { [4] = 29, [2] = 15 };

is equivalent to

     int a[6] = { 0, 0, 15, 0, 29, 0 };

The index values must be constant expressions, even if the array being
initialized is automatic.

   An alternative syntax for this which has been obsolete since GCC 2.5
but GCC still accepts is to write `[INDEX]' before the element value,
with no `='.

   To initialize a range of elements to the same value, write `[FIRST
... LAST] = VALUE'.  This is a GNU extension.  For example,

     int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

If the value in it has side-effects, the side-effects will happen only
once, not for each initialized field by the range initializer.

Note that the length of the array is the highest value specified plus
one.

   In a structure initializer, specify the name of a field to initialize
with `.FIELDNAME =' before the element value.  For example, given the
following structure,

     struct point { int x, y; };

the following initialization

     struct point p = { .y = yvalue, .x = xvalue };

is equivalent to

     struct point p = { xvalue, yvalue };

   Another syntax which has the same meaning, obsolete since GCC 2.5, is
`FIELDNAME:', as shown here:

     struct point p = { y: yvalue, x: xvalue };

   The `[INDEX]' or `.FIELDNAME' is known as a "designator".  You can
also use a designator (or the obsolete colon syntax) when initializing
a union, to specify which element of the union should be used.  For
example,

     union foo { int i; double d; };

     union foo f = { .d = 4 };

will convert 4 to a `double' to store it in the union using the second
element.  By contrast, casting 4 to type `union foo' would store it
into the union as the integer `i', since it is an integer.  (*Note Cast
to Union::.)

   You can combine this technique of naming elements with ordinary C
initialization of successive elements.  Each initializer element that
does not have a designator applies to the next consecutive element of
the array or structure.  For example,

     int a[6] = { [1] = v1, v2, [4] = v4 };

is equivalent to

     int a[6] = { 0, v1, v2, 0, v4, 0 };

   Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an `enum' type.  For
example:

     int whitespace[256]
       = { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
           ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };

   You can also write a series of `.FIELDNAME' and `[INDEX]'
designators before an `=' to specify a nested subobject to initialize;
the list is taken relative to the subobject corresponding to the
closest surrounding brace pair.  For example, with the `struct point'
declaration above:

     struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };

If the same field is initialized multiple times, it will have value from
the last initialization.  If any such overridden initialization has
side-effect, it is unspecified whether the side-effect happens or not.
Currently, gcc will discard them and issue a warning.


File: gcc.info,  Node: Case Ranges,  Next: Mixed Declarations,  Prev: Cast to Union,  Up: C Extensions

5.22 Case Ranges
================

You can specify a range of consecutive values in a single `case' label,
like this:

     case LOW ... HIGH:

This has the same effect as the proper number of individual `case'
labels, one for each integer value from LOW to HIGH, inclusive.

   This feature is especially useful for ranges of ASCII character
codes:

     case 'A' ... 'Z':

   *Be careful:* Write spaces around the `...', for otherwise it may be
parsed wrong when you use it with integer values.  For example, write
this:

     case 1 ... 5:

rather than this:

     case 1...5:


File: gcc.info,  Node: Cast to Union,  Next: Case Ranges,  Prev: Designated Inits,  Up: C Extensions

5.23 Cast to a Union Type
=========================

A cast to union type is similar to other casts, except that the type
specified is a union type.  You can specify the type either with `union
TAG' or with a typedef name.  A cast to union is actually a constructor
though, not a cast, and hence does not yield an lvalue like normal
casts.  (*Note Compound Literals::.)

   The types that may be cast to the union type are those of the members
of the union.  Thus, given the following union and variables:

     union foo { int i; double d; };
     int x;
     double y;

both `x' and `y' can be cast to type `union foo'.

   Using the cast as the right-hand side of an assignment to a variable
of union type is equivalent to storing in a member of the union:

     union foo u;
     ...
     u = (union foo) x  ==  u.i = x
     u = (union foo) y  ==  u.d = y

   You can also use the union cast as a function argument:

     void hack (union foo);
     ...
     hack ((union foo) x);


File: gcc.info,  Node: Mixed Declarations,  Next: Function Attributes,  Prev: Case Ranges,  Up: C Extensions

5.24 Mixed Declarations and Code
================================

ISO C99 and ISO C++ allow declarations and code to be freely mixed
within compound statements.  As an extension, GCC also allows this in
C89 mode.  For example, you could do:

     int i;
     ...
     i++;
     int j = i + 2;

   Each identifier is visible from where it is declared until the end of
the enclosing block.


File: gcc.info,  Node: Function Attributes,  Next: Attribute Syntax,  Prev: Mixed Declarations,  Up: C Extensions

5.25 Declaring Attributes of Functions
======================================

In GNU C, you declare certain things about functions called in your
program which help the compiler optimize function calls and check your
code more carefully.

   The keyword `__attribute__' allows you to specify special attributes
when making a declaration.  This keyword is followed by an attribute
specification inside double parentheses.  The following attributes are
currently defined for functions on all targets: `noreturn', `noinline',
`always_inline', `pure', `const', `format', `format_arg',
`no_instrument_function', `section', `constructor', `destructor',
`used', `unused', `deprecated', `weak', `malloc', and `alias'.  Several
other attributes are defined for functions on particular target
systems.  Other attributes, including `section' are supported for
variables declarations (*note Variable Attributes::) and for types
(*note Type Attributes::).

   You may also specify attributes with `__' preceding and following
each keyword.  This allows you to use them in header files without
being concerned about a possible macro of the same name.  For example,
you may use `__noreturn__' instead of `noreturn'.

   *Note Attribute Syntax::, for details of the exact syntax for using
attributes.

`noreturn'
     A few standard library functions, such as `abort' and `exit',
     cannot return.  GCC knows this automatically.  Some programs define
     their own functions that never return.  You can declare them
     `noreturn' to tell the compiler this fact.  For example,

          void fatal () __attribute__ ((noreturn));

          void
          fatal (...)
          {
            ... /* Print error message. */ ...
            exit (1);
          }

     The `noreturn' keyword tells the compiler to assume that `fatal'
     cannot return.  It can then optimize without regard to what would
     happen if `fatal' ever did return.  This makes slightly better
     code.  More importantly, it helps avoid spurious warnings of
     uninitialized variables.

     Do not assume that registers saved by the calling function are
     restored before calling the `noreturn' function.

     It does not make sense for a `noreturn' function to have a return
     type other than `void'.

     The attribute `noreturn' is not implemented in GCC versions
     earlier than 2.5.  An alternative way to declare that a function
     does not return, which works in the current version and in some
     older versions, is as follows:

          typedef void voidfn ();

          volatile voidfn fatal;

`noinline'
     This function attribute prevents a function from being considered
     for inlining.

`always_inline'
     Generally, functions are not inlined unless optimization is
     specified.  For functions declared inline, this attribute inlines
     the function even if no optimization level was specified.

`pure'
     Many functions have no effects except the return value and their
     return value depends only on the parameters and/or global
     variables.  Such a function can be subject to common subexpression
     elimination and loop optimization just as an arithmetic operator
     would be.  These functions should be declared with the attribute
     `pure'.  For example,

          int square (int) __attribute__ ((pure));

     says that the hypothetical function `square' is safe to call fewer
     times than the program says.

     Some of common examples of pure functions are `strlen' or `memcmp'.
     Interesting non-pure functions are functions with infinite loops
     or those depending on volatile memory or other system resource,
     that may change between two consecutive calls (such as `feof' in a
     multithreading environment).

     The attribute `pure' is not implemented in GCC versions earlier
     than 2.96.  

`const'
     Many functions do not examine any values except their arguments,
     and have no effects except the return value.  Basically this is
     just slightly more strict class than the `pure' attribute above,
     since function is not allowed to read global memory.

     Note that a function that has pointer arguments and examines the
     data pointed to must _not_ be declared `const'.  Likewise, a
     function that calls a non-`const' function usually must not be
     `const'.  It does not make sense for a `const' function to return
     `void'.

     The attribute `const' is not implemented in GCC versions earlier
     than 2.5.  An alternative way to declare that a function has no
     side effects, which works in the current version and in some older
     versions, is as follows:

          typedef int intfn ();

          extern const intfn square;

     This approach does not work in GNU C++ from 2.6.0 on, since the
     language specifies that the `const' must be attached to the return
     value.

`format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)'
     The `format' attribute specifies that a function takes `printf',
     `scanf', `strftime' or `strfmon' style arguments which should be
     type-checked against a format string.  For example, the
     declaration:

          extern int
          my_printf (void *my_object, const char *my_format, ...)
                __attribute__ ((format (printf, 2, 3)));

     causes the compiler to check the arguments in calls to `my_printf'
     for consistency with the `printf' style format string argument
     `my_format'.

     The parameter ARCHETYPE determines how the format string is
     interpreted, and should be `printf', `scanf', `strftime' or
     `strfmon'.  (You can also use `__printf__', `__scanf__',
     `__strftime__' or `__strfmon__'.)  The parameter STRING-INDEX
     specifies which argument is the format string argument (starting
     from 1), while FIRST-TO-CHECK is the number of the first argument
     to check against the format string.  For functions where the
     arguments are not available to be checked (such as `vprintf'),
     specify the third parameter as zero.  In this case the compiler
     only checks the format string for consistency.  For `strftime'
     formats, the third parameter is required to be zero.

     In the example above, the format string (`my_format') is the second
     argument of the function `my_print', and the arguments to check
     start with the third argument, so the correct parameters for the
     format attribute are 2 and 3.

     The `format' attribute allows you to identify your own functions
     which take format strings as arguments, so that GCC can check the
     calls to these functions for errors.  The compiler always (unless
     `-ffreestanding' is used) checks formats for the standard library
     functions `printf', `fprintf', `sprintf', `scanf', `fscanf',
     `sscanf', `strftime', `vprintf', `vfprintf' and `vsprintf'
     whenever such warnings are requested (using `-Wformat'), so there
     is no need to modify the header file `stdio.h'.  In C99 mode, the
     functions `snprintf', `vsnprintf', `vscanf', `vfscanf' and
     `vsscanf' are also checked.  Except in strictly conforming C
     standard modes, the X/Open function `strfmon' is also checked as
     are `printf_unlocked' and `fprintf_unlocked'.  *Note Options
     Controlling C Dialect: C Dialect Options.

`format_arg (STRING-INDEX)'
     The `format_arg' attribute specifies that a function takes a format
     string for a `printf', `scanf', `strftime' or `strfmon' style
     function and modifies it (for example, to translate it into
     another language), so the result can be passed to a `printf',
     `scanf', `strftime' or `strfmon' style function (with the
     remaining arguments to the format function the same as they would
     have been for the unmodified string).  For example, the
     declaration:

          extern char *
          my_dgettext (char *my_domain, const char *my_format)
                __attribute__ ((format_arg (2)));

     causes the compiler to check the arguments in calls to a `printf',
     `scanf', `strftime' or `strfmon' type function, whose format
     string argument is a call to the `my_dgettext' function, for
     consistency with the format string argument `my_format'.  If the
     `format_arg' attribute had not been specified, all the compiler
     could tell in such calls to format functions would be that the
     format string argument is not constant; this would generate a
     warning when `-Wformat-nonliteral' is used, but the calls could
     not be checked without the attribute.

     The parameter STRING-INDEX specifies which argument is the format
     string argument (starting from 1).

     The `format-arg' attribute allows you to identify your own
     functions which modify format strings, so that GCC can check the
     calls to `printf', `scanf', `strftime' or `strfmon' type function
     whose operands are a call to one of your own function.  The
     compiler always treats `gettext', `dgettext', and `dcgettext' in
     this manner except when strict ISO C support is requested by
     `-ansi' or an appropriate `-std' option, or `-ffreestanding' is
     used.  *Note Options Controlling C Dialect: C Dialect Options.

`no_instrument_function'
     If `-finstrument-functions' is given, profiling function calls will
     be generated at entry and exit of most user-compiled functions.
     Functions with this attribute will not be so instrumented.

`section ("SECTION-NAME")'
     Normally, the compiler places the code it generates in the `text'
     section.  Sometimes, however, you need additional sections, or you
     need certain particular functions to appear in special sections.
     The `section' attribute specifies that a function lives in a
     particular section.  For example, the declaration:

          extern void foobar (void) __attribute__ ((section ("bar")));

     puts the function `foobar' in the `bar' section.

     Some file formats do not support arbitrary sections so the
     `section' attribute is not available on all platforms.  If you
     need to map the entire contents of a module to a particular
     section, consider using the facilities of the linker instead.

`constructor'
`destructor'
     The `constructor' attribute causes the function to be called
     automatically before execution enters `main ()'.  Similarly, the
     `destructor' attribute causes the function to be called
     automatically after `main ()' has completed or `exit ()' has been
     called.  Functions with these attributes are useful for
     initializing data that will be used implicitly during the
     execution of the program.

     These attributes are not currently implemented for Objective-C.

`unused'
     This attribute, attached to a function, means that the function is
     meant to be possibly unused.  GCC will not produce a warning for
     this function.  GNU C++ does not currently support this attribute
     as definitions without parameters are valid in C++.

`used'
     This attribute, attached to a function, means that code must be
     emitted for the function even if it appears that the function is
     not referenced.  This is useful, for example, when the function is
     referenced only in inline assembly.

`deprecated'
     The `deprecated' attribute results in a warning if the function is
     used anywhere in the source file.  This is useful when identifying
     functions that are expected to be removed in a future version of a
     program.  The warning also includes the location of the declaration
     of the deprecated function, to enable users to easily find further
     information about why the function is deprecated, or what they
     should do instead.  Note that the warnings only occurs for uses:

          int old_fn () __attribute__ ((deprecated));
          int old_fn ();
          int (*fn_ptr)() = old_fn;

     results in a warning on line 3 but not line 2.

     The `deprecated' attribute can also be used for variables and
     types (*note Variable Attributes::, *note Type Attributes::.)

`weak'
     The `weak' attribute causes the declaration to be emitted as a weak
     symbol rather than a global.  This is primarily useful in defining
     library functions which can be overridden in user code, though it
     can also be used with non-function declarations.  Weak symbols are
     supported for ELF targets, and also for a.out targets when using
     the GNU assembler and linker.

`malloc'
     The `malloc' attribute is used to tell the compiler that a function
     may be treated as if it were the malloc function.  The compiler
     assumes that calls to malloc result in a pointers that cannot
     alias anything.  This will often improve optimization.

`alias ("TARGET")'
     The `alias' attribute causes the declaration to be emitted as an
     alias for another symbol, which must be specified.  For instance,

          void __f () { /* do something */; }
          void f () __attribute__ ((weak, alias ("__f")));

     declares `f' to be a weak alias for `__f'.  In C++, the mangled
     name for the target must be used.

     Not all target machines support this attribute.

`regparm (NUMBER)'
     On the Intel 386, the `regparm' attribute causes the compiler to
     pass up to NUMBER integer arguments in registers EAX, EDX, and ECX
     instead of on the stack.  Functions that take a variable number of
     arguments will continue to be passed all of their arguments on the
     stack.

`stdcall'
     On the Intel 386, the `stdcall' attribute causes the compiler to
     assume that the called function will pop off the stack space used
     to pass arguments, unless it takes a variable number of arguments.

     The PowerPC compiler for Windows NT currently ignores the `stdcall'
     attribute.

`cdecl'
     On the Intel 386, the `cdecl' attribute causes the compiler to
     assume that the calling function will pop off the stack space used
     to pass arguments.  This is useful to override the effects of the
     `-mrtd' switch.

     The PowerPC compiler for Windows NT currently ignores the `cdecl'
     attribute.

`longcall'
     On the RS/6000 and PowerPC, the `longcall' attribute causes the
     compiler to always call the function via a pointer, so that
     functions which reside further than 64 megabytes (67,108,864
     bytes) from the current location can be called.

`long_call/short_call'
     This attribute allows to specify how to call a particular function
     on ARM.  Both attributes override the `-mlong-calls' (*note ARM
     Options::) command line switch and `#pragma long_calls' settings.
     The `long_call' attribute causes the compiler to always call the
     function by first loading its address into a register and then
     using the contents of that register.   The `short_call' attribute
     always places the offset to the function from the call site into
     the `BL' instruction directly.

`dllimport'
     On the PowerPC running Windows NT, the `dllimport' attribute causes
     the compiler to call the function via a global pointer to the
     function pointer that is set up by the Windows NT dll library.
     The pointer name is formed by combining `__imp_' and the function
     name.

`dllexport'
     On the PowerPC running Windows NT, the `dllexport' attribute causes
     the compiler to provide a global pointer to the function pointer,
     so that it can be called with the `dllimport' attribute.  The
     pointer name is formed by combining `__imp_' and the function name.

`exception (EXCEPT-FUNC [, EXCEPT-ARG])'
     On the PowerPC running Windows NT, the `exception' attribute causes
     the compiler to modify the structured exception table entry it
     emits for the declared function.  The string or identifier
     EXCEPT-FUNC is placed in the third entry of the structured
     exception table.  It represents a function, which is called by the
     exception handling mechanism if an exception occurs.  If it was
     specified, the string or identifier EXCEPT-ARG is placed in the
     fourth entry of the structured exception table.

`function_vector'
     Use this attribute on the H8/300 and H8/300H to indicate that the
     specified function should be called through the function vector.
     Calling a function through the function vector will reduce code
     size, however; the function vector has a limited size (maximum 128
     entries on the H8/300 and 64 entries on the H8/300H) and shares
     space with the interrupt vector.

     You must use GAS and GLD from GNU binutils version 2.7 or later for
     this attribute to work correctly.

`interrupt'
     Use this attribute on the ARM, AVR, M32R/D and Xstormy16 ports to
     indicate that the specified function is an interrupt handler.  The
     compiler will generate function entry and exit sequences suitable
     for use in an interrupt handler when this attribute is present.

     Note, interrupt handlers for the H8/300, H8/300H and SH processors
     can be specified via the `interrupt_handler' attribute.

     Note, on the AVR interrupts will be enabled inside the function.

     Note, for the ARM you can specify the kind of interrupt to be
     handled by adding an optional parameter to the interrupt attribute
     like this:

          void f () __attribute__ ((interrupt ("IRQ")));

     Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT
     and UNDEF.

`interrupt_handler'
     Use this attribute on the H8/300, H8/300H and SH to indicate that
     the specified function is an interrupt handler.  The compiler will
     generate function entry and exit sequences suitable for use in an
     interrupt handler when this attribute is present.

`sp_switch'
     Use this attribute on the SH to indicate an `interrupt_handler'
     function should switch to an alternate stack.  It expects a string
     argument that names a global variable holding the address of the
     alternate stack.

          void *alt_stack;
          void f () __attribute__ ((interrupt_handler,
                                    sp_switch ("alt_stack")));

`trap_exit'
     Use this attribute on the SH for an `interrupt_handle' to return
     using `trapa' instead of `rte'.  This attribute expects an integer
     argument specifying the trap number to be used.

`eightbit_data'
     Use this attribute on the H8/300 and H8/300H to indicate that the
     specified variable should be placed into the eight bit data
     section.  The compiler will generate more efficient code for
     certain operations on data in the eight bit data area.  Note the
     eight bit data area is limited to 256 bytes of data.

     You must use GAS and GLD from GNU binutils version 2.7 or later for
     this attribute to work correctly.

`tiny_data'
     Use this attribute on the H8/300H to indicate that the specified
     variable should be placed into the tiny data section.  The
     compiler will generate more efficient code for loads and stores on
     data in the tiny data section.  Note the tiny data area is limited
     to slightly under 32kbytes of data.

`signal'
     Use this attribute on the AVR to indicate that the specified
     function is an signal handler.  The compiler will generate function
     entry and exit sequences suitable for use in an signal handler
     when this attribute is present.  Interrupts will be disabled
     inside function.

`naked'
     Use this attribute on the ARM or AVR ports to indicate that the
     specified function do not need prologue/epilogue sequences
     generated by the compiler.  It is up to the programmer to provide
     these sequences.

`model (MODEL-NAME)'
     Use this attribute on the M32R/D to set the addressability of an
     object, and the code generated for a function.  The identifier
     MODEL-NAME is one of `small', `medium', or `large', representing
     each of the code models.

     Small model objects live in the lower 16MB of memory (so that their
     addresses can be loaded with the `ld24' instruction), and are
     callable with the `bl' instruction.

     Medium model objects may live anywhere in the 32-bit address space
     (the compiler will generate `seth/add3' instructions to load their
     addresses), and are callable with the `bl' instruction.

     Large model objects may live anywhere in the 32-bit address space
     (the compiler will generate `seth/add3' instructions to load their
     addresses), and may not be reachable with the `bl' instruction
     (the compiler will generate the much slower `seth/add3/jl'
     instruction sequence).


   You can specify multiple attributes in a declaration by separating
them by commas within the double parentheses or by immediately
following an attribute declaration with another attribute declaration.

   Some people object to the `__attribute__' feature, suggesting that
ISO C's `#pragma' should be used instead.  At the time `__attribute__'
was designed, there were two reasons for not doing this.

  1. It is impossible to generate `#pragma' commands from a macro.

  2. There is no telling what the same `#pragma' might mean in another
     compiler.

   These two reasons applied to almost any application that might have
been proposed for `#pragma'.  It was basically a mistake to use
`#pragma' for _anything_.

   The ISO C99 standard includes `_Pragma', which now allows pragmas to
be generated from macros.  In addition, a `#pragma GCC' namespace is
now in use for GCC-specific pragmas.  However, it has been found
convenient to use `__attribute__' to achieve a natural attachment of
attributes to their corresponding declarations, whereas `#pragma GCC'
is of use for constructs that do not naturally form part of the
grammar.  *Note Miscellaneous Preprocessing Directives: (cpp)Other
Directives.


File: gcc.info,  Node: Attribute Syntax,  Next: Function Prototypes,  Prev: Function Attributes,  Up: C Extensions

5.26 Attribute Syntax
=====================

This section describes the syntax with which `__attribute__' may be
used, and the constructs to which attribute specifiers bind, for the C
language.  Some details may vary for C++ and Objective-C.  Because of
infelicities in the grammar for attributes, some forms described here
may not be successfully parsed in all cases.

   There are some problems with the semantics of attributes in C++.  For
example, there are no manglings for attributes, although they may affect
code generation, so problems may arise when attributed types are used in
conjunction with templates or overloading.  Similarly, `typeid' does
not distinguish between types with different attributes.  Support for
attributes in C++ may be restricted in future to attributes on
declarations only, but not on nested declarators.

   *Note Function Attributes::, for details of the semantics of
attributes applying to functions.  *Note Variable Attributes::, for
details of the semantics of attributes applying to variables.  *Note
Type Attributes::, for details of the semantics of attributes applying
to structure, union and enumerated types.

   An "attribute specifier" is of the form `__attribute__
((ATTRIBUTE-LIST))'.  An "attribute list" is a possibly empty
comma-separated sequence of "attributes", where each attribute is one
of the following:

   * Empty.  Empty attributes are ignored.

   * A word (which may be an identifier such as `unused', or a reserved
     word such as `const').

   * A word, followed by, in parentheses, parameters for the attribute.
     These parameters take one of the following forms:

        * An identifier.  For example, `mode' attributes use this form.

        * An identifier followed by a comma and a non-empty
          comma-separated list of expressions.  For example, `format'
          attributes use this form.

        * A possibly empty comma-separated list of expressions.  For
          example, `format_arg' attributes use this form with the list
          being a single integer constant expression, and `alias'
          attributes use this form with the list being a single string
          constant.

   An "attribute specifier list" is a sequence of one or more attribute
specifiers, not separated by any other tokens.

   An attribute specifier list may appear after the colon following a
label, other than a `case' or `default' label.  The only attribute it
makes sense to use after a label is `unused'.  This feature is intended
for code generated by programs which contains labels that may be unused
but which is compiled with `-Wall'.  It would not normally be
appropriate to use in it human-written code, though it could be useful
in cases where the code that jumps to the label is contained within an
`#ifdef' conditional.

   An attribute specifier list may appear as part of a `struct',
`union' or `enum' specifier.  It may go either immediately after the
`struct', `union' or `enum' keyword, or after the closing brace.  It is
ignored if the content of the structure, union or enumerated type is
not defined in the specifier in which the attribute specifier list is
used--that is, in usages such as `struct __attribute__((foo)) bar' with
no following opening brace.  Where attribute specifiers follow the
closing brace, they are considered to relate to the structure, union or
enumerated type defined, not to any enclosing declaration the type
specifier appears in, and the type defined is not complete until after
the attribute specifiers.

   Otherwise, an attribute specifier appears as part of a declaration,
counting declarations of unnamed parameters and type names, and relates
to that declaration (which may be nested in another declaration, for
example in the case of a parameter declaration), or to a particular
declarator within a declaration.  Where an attribute specifier is
applied to a parameter declared as a function or an array, it should
apply to the function or array rather than the pointer to which the
parameter is implicitly converted, but this is not yet correctly
implemented.

   Any list of specifiers and qualifiers at the start of a declaration
may contain attribute specifiers, whether or not such a list may in that
context contain storage class specifiers.  (Some attributes, however,
are essentially in the nature of storage class specifiers, and only make
sense where storage class specifiers may be used; for example,
`section'.)  There is one necessary limitation to this syntax: the
first old-style parameter declaration in a function definition cannot
begin with an attribute specifier, because such an attribute applies to
the function instead by syntax described below (which, however, is not
yet implemented in this case).  In some other cases, attribute
specifiers are permitted by this grammar but not yet supported by the
compiler.  All attribute specifiers in this place relate to the
declaration as a whole.  In the obsolescent usage where a type of `int'
is implied by the absence of type specifiers, such a list of specifiers
and qualifiers may be an attribute specifier list with no other
specifiers or qualifiers.

   An attribute specifier list may appear immediately before a
declarator (other than the first) in a comma-separated list of
declarators in a declaration of more than one identifier using a single
list of specifiers and qualifiers.  Such attribute specifiers apply
only to the identifier before whose declarator they appear.  For
example, in

     __attribute__((noreturn)) void d0 (void),
         __attribute__((format(printf, 1, 2))) d1 (const char *, ...),
          d2 (void)

the `noreturn' attribute applies to all the functions declared; the
`format' attribute only applies to `d1'.

   An attribute specifier list may appear immediately before the comma,
`=' or semicolon terminating the declaration of an identifier other
than a function definition.  At present, such attribute specifiers apply
to the declared object or function, but in future they may attach to the
outermost adjacent declarator.  In simple cases there is no difference,
but, for example, in

     void (****f)(void) __attribute__((noreturn));

at present the `noreturn' attribute applies to `f', which causes a
warning since `f' is not a function, but in future it may apply to the
function `****f'.  The precise semantics of what attributes in such
cases will apply to are not yet specified.  Where an assembler name for
an object or function is specified (*note Asm Labels::), at present the
attribute must follow the `asm' specification; in future, attributes
before the `asm' specification may apply to the adjacent declarator,
and those after it to the declared object or function.

   An attribute specifier list may, in future, be permitted to appear
after the declarator in a function definition (before any old-style
parameter declarations or the function body).

   Attribute specifiers may be mixed with type qualifiers appearing
inside the `[]' of a parameter array declarator, in the C99 construct by
which such qualifiers are applied to the pointer to which the array is
implicitly converted.  Such attribute specifiers apply to the pointer,
not to the array, but at present this is not implemented and they are
ignored.

   An attribute specifier list may appear at the start of a nested
declarator.  At present, there are some limitations in this usage: the
attributes correctly apply to the declarator, but for most individual
attributes the semantics this implies are not implemented.  When
attribute specifiers follow the `*' of a pointer declarator, they may
be mixed with any type qualifiers present.  The following describes the
formal semantics of this syntax.  It will make the most sense if you
are familiar with the formal specification of declarators in the ISO C
standard.

   Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration `T
D1', where `T' contains declaration specifiers that specify a type TYPE
(such as `int') and `D1' is a declarator that contains an identifier
IDENT.  The type specified for IDENT for derived declarators whose type
does not include an attribute specifier is as in the ISO C standard.

   If `D1' has the form `( ATTRIBUTE-SPECIFIER-LIST D )', and the
declaration `T D' specifies the type "DERIVED-DECLARATOR-TYPE-LIST
TYPE" for IDENT, then `T D1' specifies the type
"DERIVED-DECLARATOR-TYPE-LIST ATTRIBUTE-SPECIFIER-LIST TYPE" for IDENT.

   If `D1' has the form `* TYPE-QUALIFIER-AND-ATTRIBUTE-SPECIFIER-LIST
D', and the declaration `T D' specifies the type
"DERIVED-DECLARATOR-TYPE-LIST TYPE" for IDENT, then `T D1' specifies
the type "DERIVED-DECLARATOR-TYPE-LIST
TYPE-QUALIFIER-AND-ATTRIBUTE-SPECIFIER-LIST TYPE" for IDENT.

   For example,

     void (__attribute__((noreturn)) ****f) (void);

specifies the type "pointer to pointer to pointer to pointer to
non-returning function returning `void'".  As another example,

     char *__attribute__((aligned(8))) *f;

specifies the type "pointer to 8-byte-aligned pointer to `char'".  Note
again that this does not work with most attributes; for example, the
usage of `aligned' and `noreturn' attributes given above is not yet
supported.

   For compatibility with existing code written for compiler versions
that did not implement attributes on nested declarators, some laxity is
allowed in the placing of attributes.  If an attribute that only applies
to types is applied to a declaration, it will be treated as applying to
the type of that declaration.  If an attribute that only applies to
declarations is applied to the type of a declaration, it will be treated
as applying to that declaration; and, for compatibility with code
placing the attributes immediately before the identifier declared, such
an attribute applied to a function return type will be treated as
applying to the function type, and such an attribute applied to an array
element type will be treated as applying to the array type.  If an
attribute that only applies to function types is applied to a
pointer-to-function type, it will be treated as applying to the pointer
target type; if such an attribute is applied to a function return type
that is not a pointer-to-function type, it will be treated as applying
to the function type.


File: gcc.info,  Node: Function Prototypes,  Next: C++ Comments,  Prev: Attribute Syntax,  Up: C Extensions

5.27 Prototypes and Old-Style Function Definitions
==================================================

GNU C extends ISO C to allow a function prototype to override a later
old-style non-prototype definition.  Consider the following example:

     /* Use prototypes unless the compiler is old-fashioned.  */
     #ifdef __STDC__
     #define P(x) x
     #else
     #define P(x) ()
     #endif

     /* Prototype function declaration.  */
     int isroot P((uid_t));

     /* Old-style function definition.  */
     int
     isroot (x)   /* ??? lossage here ??? */
          uid_t x;
     {
       return x == 0;
     }

   Suppose the type `uid_t' happens to be `short'.  ISO C does not
allow this example, because subword arguments in old-style
non-prototype definitions are promoted.  Therefore in this example the
function definition's argument is really an `int', which does not match
the prototype argument type of `short'.

   This restriction of ISO C makes it hard to write code that is
portable to traditional C compilers, because the programmer does not
know whether the `uid_t' type is `short', `int', or `long'.  Therefore,
in cases like these GNU C allows a prototype to override a later
old-style definition.  More precisely, in GNU C, a function prototype
argument type overrides the argument type specified by a later
old-style definition if the former type is the same as the latter type
before promotion.  Thus in GNU C the above example is equivalent to the
following:

     int isroot (uid_t);

     int
     isroot (uid_t x)
     {
       return x == 0;
     }

GNU C++ does not support old-style function definitions, so this
extension is irrelevant.


File: gcc.info,  Node: C++ Comments,  Next: Dollar Signs,  Prev: Function Prototypes,  Up: C Extensions

5.28 C++ Style Comments
=======================

In GNU C, you may use C++ style comments, which start with `//' and
continue until the end of the line.  Many other C implementations allow
such comments, and they are likely to be in a future C standard.
However, C++ style comments are not recognized if you specify `-ansi',
a `-std' option specifying a version of ISO C before C99, or
`-traditional', since they are incompatible with traditional constructs
like `dividend//*comment*/divisor'.


File: gcc.info,  Node: Dollar Signs,  Next: Character Escapes,  Prev: C++ Comments,  Up: C Extensions

5.29 Dollar Signs in Identifier Names
=====================================

In GNU C, you may normally use dollar signs in identifier names.  This
is because many traditional C implementations allow such identifiers.
However, dollar signs in identifiers are not supported on a few target
machines, typically because the target assembler does not allow them.


File: gcc.info,  Node: Character Escapes,  Next: Variable Attributes,  Prev: Dollar Signs,  Up: C Extensions

5.30 The Character <ESC> in Constants
=====================================

You can use the sequence `\e' in a string or character constant to
stand for the ASCII character <ESC>.


File: gcc.info,  Node: Alignment,  Next: Inline,  Prev: Type Attributes,  Up: C Extensions

5.31 Inquiring on Alignment of Types or Variables
=================================================

The keyword `__alignof__' allows you to inquire about how an object is
aligned, or the minimum alignment usually required by a type.  Its
syntax is just like `sizeof'.

   For example, if the target machine requires a `double' value to be
aligned on an 8-byte boundary, then `__alignof__ (double)' is 8.  This
is true on many RISC machines.  On more traditional machine designs,
`__alignof__ (double)' is 4 or even 2.

   Some machines never actually require alignment; they allow reference
to any data type even at an odd addresses.  For these machines,
`__alignof__' reports the _recommended_ alignment of a type.

   If the operand of `__alignof__' is an lvalue rather than a type, its
value is the required alignment for its type, taking into account any
minimum alignment specified with GCC's `__attribute__' extension (*note
Variable Attributes::).  For example, after this declaration:

     struct foo { int x; char y; } foo1;

the value of `__alignof__ (foo1.y)' is 1, even though its actual
alignment is probably 2 or 4, the same as `__alignof__ (int)'.

   It is an error to ask for the alignment of an incomplete type.


File: gcc.info,  Node: Variable Attributes,  Next: Type Attributes,  Prev: Character Escapes,  Up: C Extensions

5.32 Specifying Attributes of Variables
=======================================

The keyword `__attribute__' allows you to specify special attributes of
variables or structure fields.  This keyword is followed by an
attribute specification inside double parentheses.  Ten attributes are
currently defined for variables: `aligned', `mode', `nocommon',
`packed', `section', `transparent_union', `unused', `deprecated',
`vector_size', and `weak'.  Some other attributes are defined for
variables on particular target systems.  Other attributes are available
for functions (*note Function Attributes::) and for types (*note Type
Attributes::).  Other front ends might define more attributes (*note
Extensions to the C++ Language: C++ Extensions.).

   You may also specify attributes with `__' preceding and following
each keyword.  This allows you to use them in header files without
being concerned about a possible macro of the same name.  For example,
you may use `__aligned__' instead of `aligned'.

   *Note Attribute Syntax::, for details of the exact syntax for using
attributes.

`aligned (ALIGNMENT)'
     This attribute specifies a minimum alignment for the variable or
     structure field, measured in bytes.  For example, the declaration:

          int x __attribute__ ((aligned (16))) = 0;

     causes the compiler to allocate the global variable `x' on a
     16-byte boundary.  On a 68040, this could be used in conjunction
     with an `asm' expression to access the `move16' instruction which
     requires 16-byte aligned operands.

     You can also specify the alignment of structure fields.  For
     example, to create a double-word aligned `int' pair, you could
     write:

          struct foo { int x[2] __attribute__ ((aligned (8))); };

     This is an alternative to creating a union with a `double' member
     that forces the union to be double-word aligned.

     As in the preceding examples, you can explicitly specify the
     alignment (in bytes) that you wish the compiler to use for a given
     variable or structure field.  Alternatively, you can leave out the
     alignment factor and just ask the compiler to align a variable or
     field to the maximum useful alignment for the target machine you
     are compiling for.  For example, you could write:

          short array[3] __attribute__ ((aligned));

     Whenever you leave out the alignment factor in an `aligned'
     attribute specification, the compiler automatically sets the
     alignment for the declared variable or field to the largest
     alignment which is ever used for any data type on the target
     machine you are compiling for.  Doing this can often make copy
     operations more efficient, because the compiler can use whatever
     instructions copy the biggest chunks of memory when performing
     copies to or from the variables or fields that you have aligned
     this way.

     The `aligned' attribute can only increase the alignment; but you
     can decrease it by specifying `packed' as well.  See below.

     Note that the effectiveness of `aligned' attributes may be limited
     by inherent limitations in your linker.  On many systems, the
     linker is only able to arrange for variables to be aligned up to a
     certain maximum alignment.  (For some linkers, the maximum
     supported alignment may be very very small.)  If your linker is
     only able to align variables up to a maximum of 8 byte alignment,
     then specifying `aligned(16)' in an `__attribute__' will still
     only provide you with 8 byte alignment.  See your linker
     documentation for further information.

`mode (MODE)'
     This attribute specifies the data type for the
     declaration--whichever type corresponds to the mode MODE.  This in
     effect lets you request an integer or floating point type
     according to its width.

     You may also specify a mode of `byte' or `__byte__' to indicate
     the mode corresponding to a one-byte integer, `word' or `__word__'
     for the mode of a one-word integer, and `pointer' or `__pointer__'
     for the mode used to represent pointers.

`nocommon'
     This attribute specifies requests GCC not to place a variable
     "common" but instead to allocate space for it directly.  If you
     specify the `-fno-common' flag, GCC will do this for all variables.

     Specifying the `nocommon' attribute for a variable provides an
     initialization of zeros.  A variable may only be initialized in one
     source file.

`packed'
     The `packed' attribute specifies that a variable or structure field
     should have the smallest possible alignment--one byte for a
     variable, and one bit for a field, unless you specify a larger
     value with the `aligned' attribute.

     Here is a structure in which the field `x' is packed, so that it
     immediately follows `a':

          struct foo
          {
            char a;
            int x[2] __attribute__ ((packed));
          };

`section ("SECTION-NAME")'
     Normally, the compiler places the objects it generates in sections
     like `data' and `bss'.  Sometimes, however, you need additional
     sections, or you need certain particular variables to appear in
     special sections, for example to map to special hardware.  The
     `section' attribute specifies that a variable (or function) lives
     in a particular section.  For example, this small program uses
     several specific section names:

          struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
          struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
          char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
          int init_data __attribute__ ((section ("INITDATA"))) = 0;

          main()
          {
            /* Initialize stack pointer */
            init_sp (stack + sizeof (stack));

            /* Initialize initialized data */
            memcpy (&init_data, &data, &edata - &data);

            /* Turn on the serial ports */
            init_duart (&a);
            init_duart (&b);
          }

     Use the `section' attribute with an _initialized_ definition of a
     _global_ variable, as shown in the example.  GCC issues a warning
     and otherwise ignores the `section' attribute in uninitialized
     variable declarations.

     You may only use the `section' attribute with a fully initialized
     global definition because of the way linkers work.  The linker
     requires each object be defined once, with the exception that
     uninitialized variables tentatively go in the `common' (or `bss')
     section and can be multiply "defined".  You can force a variable
     to be initialized with the `-fno-common' flag or the `nocommon'
     attribute.

     Some file formats do not support arbitrary sections so the
     `section' attribute is not available on all platforms.  If you
     need to map the entire contents of a module to a particular
     section, consider using the facilities of the linker instead.

`shared'
     On Windows NT, in addition to putting variable definitions in a
     named section, the section can also be shared among all running
     copies of an executable or DLL.  For example, this small program
     defines shared data by putting it in a named section `shared' and
     marking the section shareable:

          int foo __attribute__((section ("shared"), shared)) = 0;

          int
          main()
          {
            /* Read and write foo.  All running
               copies see the same value.  */
            return 0;
          }

     You may only use the `shared' attribute along with `section'
     attribute with a fully initialized global definition because of
     the way linkers work.  See `section' attribute for more
     information.

     The `shared' attribute is only available on Windows NT.

`transparent_union'
     This attribute, attached to a function parameter which is a union,
     means that the corresponding argument may have the type of any
     union member, but the argument is passed as if its type were that
     of the first union member.  For more details see *Note Type
     Attributes::.  You can also use this attribute on a `typedef' for
     a union data type; then it applies to all function parameters with
     that type.

`unused'
     This attribute, attached to a variable, means that the variable is
     meant to be possibly unused.  GCC will not produce a warning for
     this variable.

`deprecated'
     The `deprecated' attribute results in a warning if the variable is
     used anywhere in the source file.  This is useful when identifying
     variables that are expected to be removed in a future version of a
     program.  The warning also includes the location of the declaration
     of the deprecated variable, to enable users to easily find further
     information about why the variable is deprecated, or what they
     should do instead.  Note that the warnings only occurs for uses:

          extern int old_var __attribute__ ((deprecated));
          extern int old_var;
          int new_fn () { return old_var; }

     results in a warning on line 3 but not line 2.

     The `deprecated' attribute can also be used for functions and
     types (*note Function Attributes::, *note Type Attributes::.)

`vector_size (BYTES)'
     This attribute specifies the vector size for the variable,
     measured in bytes.  For example, the declaration:

          int foo __attribute__ ((vector_size (16)));

     causes the compiler to set the mode for `foo', to be 16 bytes,
     divided into `int' sized units.  Assuming a 32-bit int (a vector of
     4 units of 4 bytes), the corresponding mode of `foo' will be V4SI.

     This attribute is only applicable to integral and float scalars,
     although arrays, pointers, and function return values are allowed
     in conjunction with this construct.

     Aggregates with this attribute are invalid, even if they are of
     the same size as a corresponding scalar.  For example, the
     declaration:

          struct S { int a; };
          struct S  __attribute__ ((vector_size (16))) foo;

     is invalid even if the size of the structure is the same as the
     size of the `int'.

`weak'
     The `weak' attribute is described in *Note Function Attributes::.

`model (MODEL-NAME)'
     Use this attribute on the M32R/D to set the addressability of an
     object.  The identifier MODEL-NAME is one of `small', `medium', or
     `large', representing each of the code models.

     Small model objects live in the lower 16MB of memory (so that their
     addresses can be loaded with the `ld24' instruction).

     Medium and large model objects may live anywhere in the 32-bit
     address space (the compiler will generate `seth/add3' instructions
     to load their addresses).


   To specify multiple attributes, separate them by commas within the
double parentheses: for example, `__attribute__ ((aligned (16),
packed))'.


File: gcc.info,  Node: Type Attributes,  Next: Alignment,  Prev: Variable Attributes,  Up: C Extensions

5.33 Specifying Attributes of Types
===================================

The keyword `__attribute__' allows you to specify special attributes of
`struct' and `union' types when you define such types.  This keyword is
followed by an attribute specification inside double parentheses.  Five
attributes are currently defined for types: `aligned', `packed',
`transparent_union', `unused', and `deprecated'.  Other attributes are
defined for functions (*note Function Attributes::) and for variables
(*note Variable Attributes::).

   You may also specify any one of these attributes with `__' preceding
and following its keyword.  This allows you to use these attributes in
header files without being concerned about a possible macro of the same
name.  For example, you may use `__aligned__' instead of `aligned'.

   You may specify the `aligned' and `transparent_union' attributes
either in a `typedef' declaration or just past the closing curly brace
of a complete enum, struct or union type _definition_ and the `packed'
attribute only past the closing brace of a definition.

   You may also specify attributes between the enum, struct or union
tag and the name of the type rather than after the closing brace.

   *Note Attribute Syntax::, for details of the exact syntax for using
attributes.

`aligned (ALIGNMENT)'
     This attribute specifies a minimum alignment (in bytes) for
     variables of the specified type.  For example, the declarations:

          struct S { short f[3]; } __attribute__ ((aligned (8)));
          typedef int more_aligned_int __attribute__ ((aligned (8)));

     force the compiler to insure (as far as it can) that each variable
     whose type is `struct S' or `more_aligned_int' will be allocated
     and aligned _at least_ on a 8-byte boundary.  On a Sparc, having
     all variables of type `struct S' aligned to 8-byte boundaries
     allows the compiler to use the `ldd' and `std' (doubleword load and
     store) instructions when copying one variable of type `struct S' to
     another, thus improving run-time efficiency.

     Note that the alignment of any given `struct' or `union' type is
     required by the ISO C standard to be at least a perfect multiple of
     the lowest common multiple of the alignments of all of the members
     of the `struct' or `union' in question.  This means that you _can_
     effectively adjust the alignment of a `struct' or `union' type by
     attaching an `aligned' attribute to any one of the members of such
     a type, but the notation illustrated in the example above is a
     more obvious, intuitive, and readable way to request the compiler
     to adjust the alignment of an entire `struct' or `union' type.

     As in the preceding example, you can explicitly specify the
     alignment (in bytes) that you wish the compiler to use for a given
     `struct' or `union' type.  Alternatively, you can leave out the
     alignment factor and just ask the compiler to align a type to the
     maximum useful alignment for the target machine you are compiling
     for.  For example, you could write:

          struct S { short f[3]; } __attribute__ ((aligned));

     Whenever you leave out the alignment factor in an `aligned'
     attribute specification, the compiler automatically sets the
     alignment for the type to the largest alignment which is ever used
     for any data type on the target machine you are compiling for.
     Doing this can often make copy operations more efficient, because
     the compiler can use whatever instructions copy the biggest chunks
     of memory when performing copies to or from the variables which
     have types that you have aligned this way.

     In the example above, if the size of each `short' is 2 bytes, then
     the size of the entire `struct S' type is 6 bytes.  The smallest
     power of two which is greater than or equal to that is 8, so the
     compiler sets the alignment for the entire `struct S' type to 8
     bytes.

     Note that although you can ask the compiler to select a
     time-efficient alignment for a given type and then declare only
     individual stand-alone objects of that type, the compiler's
     ability to select a time-efficient alignment is primarily useful
     only when you plan to create arrays of variables having the
     relevant (efficiently aligned) type.  If you declare or use arrays
     of variables of an efficiently-aligned type, then it is likely
     that your program will also be doing pointer arithmetic (or
     subscripting, which amounts to the same thing) on pointers to the
     relevant type, and the code that the compiler generates for these
     pointer arithmetic operations will often be more efficient for
     efficiently-aligned types than for other types.

     The `aligned' attribute can only increase the alignment; but you
     can decrease it by specifying `packed' as well.  See below.

     Note that the effectiveness of `aligned' attributes may be limited
     by inherent limitations in your linker.  On many systems, the
     linker is only able to arrange for variables to be aligned up to a
     certain maximum alignment.  (For some linkers, the maximum
     supported alignment may be very very small.)  If your linker is
     only able to align variables up to a maximum of 8 byte alignment,
     then specifying `aligned(16)' in an `__attribute__' will still
     only provide you with 8 byte alignment.  See your linker
     documentation for further information.

`packed'
     This attribute, attached to an `enum', `struct', or `union' type
     definition, specified that the minimum required memory be used to
     represent the type.

     Specifying this attribute for `struct' and `union' types is
     equivalent to specifying the `packed' attribute on each of the
     structure or union members.  Specifying the `-fshort-enums' flag
     on the line is equivalent to specifying the `packed' attribute on
     all `enum' definitions.

     You may only specify this attribute after a closing curly brace on
     an `enum' definition, not in a `typedef' declaration, unless that
     declaration also contains the definition of the `enum'.

`transparent_union'
     This attribute, attached to a `union' type definition, indicates
     that any function parameter having that union type causes calls to
     that function to be treated in a special way.

     First, the argument corresponding to a transparent union type can
     be of any type in the union; no cast is required.  Also, if the
     union contains a pointer type, the corresponding argument can be a
     null pointer constant or a void pointer expression; and if the
     union contains a void pointer type, the corresponding argument can
     be any pointer expression.  If the union member type is a pointer,
     qualifiers like `const' on the referenced type must be respected,
     just as with normal pointer conversions.

     Second, the argument is passed to the function using the calling
     conventions of first member of the transparent union, not the
     calling conventions of the union itself.  All members of the union
     must have the same machine representation; this is necessary for
     this argument passing to work properly.

     Transparent unions are designed for library functions that have
     multiple interfaces for compatibility reasons.  For example,
     suppose the `wait' function must accept either a value of type
     `int *' to comply with Posix, or a value of type `union wait *' to
     comply with the 4.1BSD interface.  If `wait''s parameter were
     `void *', `wait' would accept both kinds of arguments, but it
     would also accept any other pointer type and this would make
     argument type checking less useful.  Instead, `<sys/wait.h>' might
     define the interface as follows:

          typedef union
            {
              int *__ip;
              union wait *__up;
            } wait_status_ptr_t __attribute__ ((__transparent_union__));

          pid_t wait (wait_status_ptr_t);

     This interface allows either `int *' or `union wait *' arguments
     to be passed, using the `int *' calling convention.  The program
     can call `wait' with arguments of either type:

          int w1 () { int w; return wait (&w); }
          int w2 () { union wait w; return wait (&w); }

     With this interface, `wait''s implementation might look like this:

          pid_t wait (wait_status_ptr_t p)
          {
            return waitpid (-1, p.__ip, 0);
          }

`unused'
     When attached to a type (including a `union' or a `struct'), this
     attribute means that variables of that type are meant to appear
     possibly unused.  GCC will not produce a warning for any variables
     of that type, even if the variable appears to do nothing.  This is
     often the case with lock or thread classes, which are usually
     defined and then not referenced, but contain constructors and
     destructors that have nontrivial bookkeeping functions.

`deprecated'
     The `deprecated' attribute results in a warning if the type is
     used anywhere in the source file.  This is useful when identifying
     types that are expected to be removed in a future version of a
     program.  If possible, the warning also includes the location of
     the declaration of the deprecated type, to enable users to easily
     find further information about why the type is deprecated, or what
     they should do instead.  Note that the warnings only occur for
     uses and then only if the type is being applied to an identifier
     that itself is not being declared as deprecated.

          typedef int T1 __attribute__ ((deprecated));
          T1 x;
          typedef T1 T2;
          T2 y;
          typedef T1 T3 __attribute__ ((deprecated));
          T3 z __attribute__ ((deprecated));

     results in a warning on line 2 and 3 but not lines 4, 5, or 6.  No
     warning is issued for line 4 because T2 is not explicitly
     deprecated.  Line 5 has no warning because T3 is explicitly
     deprecated.  Similarly for line 6.

     The `deprecated' attribute can also be used for functions and
     variables (*note Function Attributes::, *note Variable
     Attributes::.)


   To specify multiple attributes, separate them by commas within the
double parentheses: for example, `__attribute__ ((aligned (16),
packed))'.


File: gcc.info,  Node: Inline,  Next: Extended Asm,  Prev: Alignment,  Up: C Extensions

5.34 An Inline Function is As Fast As a Macro
=============================================

By declaring a function `inline', you can direct GCC to integrate that
function's code into the code for its callers.  This makes execution
faster by eliminating the function-call overhead; in addition, if any
of the actual argument values are constant, their known values may
permit simplifications at compile time so that not all of the inline
function's code needs to be included.  The effect on code size is less
predictable; object code may be larger or smaller with function
inlining, depending on the particular case.  Inlining of functions is an
optimization and it really "works" only in optimizing compilation.  If
you don't use `-O', no function is really inline.

   Inline functions are included in the ISO C99 standard, but there are
currently substantial differences between what GCC implements and what
the ISO C99 standard requires.

   To declare a function inline, use the `inline' keyword in its
declaration, like this:

     inline int
     inc (int *a)
     {
       (*a)++;
     }

   (If you are writing a header file to be included in ISO C programs,
write `__inline__' instead of `inline'.  *Note Alternate Keywords::.)
You can also make all "simple enough" functions inline with the option
`-finline-functions'.

   Note that certain usages in a function definition can make it
unsuitable for inline substitution.  Among these usages are: use of
varargs, use of alloca, use of variable sized data types (*note
Variable Length::), use of computed goto (*note Labels as Values::),
use of nonlocal goto, and nested functions (*note Nested Functions::).
Using `-Winline' will warn when a function marked `inline' could not be
substituted, and will give the reason for the failure.

   Note that in C and Objective-C, unlike C++, the `inline' keyword
does not affect the linkage of the function.

   GCC automatically inlines member functions defined within the class
body of C++ programs even if they are not explicitly declared `inline'.
(You can override this with `-fno-default-inline'; *note Options
Controlling C++ Dialect: C++ Dialect Options.)

   When a function is both inline and `static', if all calls to the
function are integrated into the caller, and the function's address is
never used, then the function's own assembler code is never referenced.
In this case, GCC does not actually output assembler code for the
function, unless you specify the option `-fkeep-inline-functions'.
Some calls cannot be integrated for various reasons (in particular,
calls that precede the function's definition cannot be integrated, and
neither can recursive calls within the definition).  If there is a
nonintegrated call, then the function is compiled to assembler code as
usual.  The function must also be compiled as usual if the program
refers to its address, because that can't be inlined.

   When an inline function is not `static', then the compiler must
assume that there may be calls from other source files; since a global
symbol can be defined only once in any program, the function must not
be defined in the other source files, so the calls therein cannot be
integrated.  Therefore, a non-`static' inline function is always
compiled on its own in the usual fashion.

   If you specify both `inline' and `extern' in the function
definition, then the definition is used only for inlining.  In no case
is the function compiled on its own, not even if you refer to its
address explicitly.  Such an address becomes an external reference, as
if you had only declared the function, and had not defined it.

   This combination of `inline' and `extern' has almost the effect of a
macro.  The way to use it is to put a function definition in a header
file with these keywords, and put another copy of the definition
(lacking `inline' and `extern') in a library file.  The definition in
the header file will cause most calls to the function to be inlined.
If any uses of the function remain, they will refer to the single copy
in the library.

   For future compatibility with when GCC implements ISO C99 semantics
for inline functions, it is best to use `static inline' only.  (The
existing semantics will remain available when `-std=gnu89' is
specified, but eventually the default will be `-std=gnu99' and that
will implement the C99 semantics, though it does not do so yet.)

   GCC does not inline any functions when not optimizing unless you
specify the `always_inline' attribute for the function, like this:

     /* Prototype.  */
     inline void foo (const char) __attribute__((always_inline));


File: gcc.info,  Node: Extended Asm,  Next: Constraints,  Prev: Inline,  Up: C Extensions

5.35 Assembler Instructions with C Expression Operands
======================================================

In an assembler instruction using `asm', you can specify the operands
of the instruction using C expressions.  This means you need not guess
which registers or memory locations will contain the data you want to
use.

   You must specify an assembler instruction template much like what
appears in a machine description, plus an operand constraint string for
each operand.

   For example, here is how to use the 68881's `fsinx' instruction:

     asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));

Here `angle' is the C expression for the input operand while `result'
is that of the output operand.  Each has `"f"' as its operand
constraint, saying that a floating point register is required.  The `='
in `=f' indicates that the operand is an output; all output operands'
constraints must use `='.  The constraints use the same language used
in the machine description (*note Constraints::).

   Each operand is described by an operand-constraint string followed by
the C expression in parentheses.  A colon separates the assembler
template from the first output operand and another separates the last
output operand from the first input, if any.  Commas separate the
operands within each group.  The total number of operands is currently
limited to 30; this limitation may be lifted in some future version of
GCC.

   If there are no output operands but there are input operands, you
must place two consecutive colons surrounding the place where the output
operands would go.

   As of GCC version 3.1, it is also possible to specify input and
output operands using symbolic names which can be referenced within the
assembler code.  These names are specified inside square brackets
preceding the constraint string, and can be referenced inside the
assembler code using `%[NAME]' instead of a percentage sign followed by
the operand number.  Using named operands the above example could look
like:

     asm ("fsinx %[angle],%[output]"
          : [output] "=f" (result)
          : [angle] "f" (angle));

Note that the symbolic operand names have no relation whatsoever to
other C identifiers.  You may use any name you like, even those of
existing C symbols, but must ensure that no two operands within the same
assembler construct use the same symbolic name.

   Output operand expressions must be lvalues; the compiler can check
this.  The input operands need not be lvalues.  The compiler cannot
check whether the operands have data types that are reasonable for the
instruction being executed.  It does not parse the assembler instruction
template and does not know what it means or even whether it is valid
assembler input.  The extended `asm' feature is most often used for
machine instructions the compiler itself does not know exist.  If the
output expression cannot be directly addressed (for example, it is a
bit-field), your constraint must allow a register.  In that case, GCC
will use the register as the output of the `asm', and then store that
register into the output.

   The ordinary output operands must be write-only; GCC will assume that
the values in these operands before the instruction are dead and need
not be generated.  Extended asm supports input-output or read-write
operands.  Use the constraint character `+' to indicate such an operand
and list it with the output operands.

   When the constraints for the read-write operand (or the operand in
which only some of the bits are to be changed) allows a register, you
may, as an alternative, logically split its function into two separate
operands, one input operand and one write-only output operand.  The
connection between them is expressed by constraints which say they need
to be in the same location when the instruction executes.  You can use
the same C expression for both operands, or different expressions.  For
example, here we write the (fictitious) `combine' instruction with
`bar' as its read-only source operand and `foo' as its read-write
destination:

     asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));

The constraint `"0"' for operand 1 says that it must occupy the same
location as operand 0.  A number in constraint is allowed only in an
input operand and it must refer to an output operand.

   Only a number in the constraint can guarantee that one operand will
be in the same place as another.  The mere fact that `foo' is the value
of both operands is not enough to guarantee that they will be in the
same place in the generated assembler code.  The following would not
work reliably:

     asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));

   Various optimizations or reloading could cause operands 0 and 1 to
be in different registers; GCC knows no reason not to do so.  For
example, the compiler might find a copy of the value of `foo' in one
register and use it for operand 1, but generate the output operand 0 in
a different register (copying it afterward to `foo''s own address).  Of
course, since the register for operand 1 is not even mentioned in the
assembler code, the result will not work, but GCC can't tell that.

   As of GCC version 3.1, one may write `[NAME]' instead of the operand
number for a matching constraint.  For example:

     asm ("cmoveq %1,%2,%[result]"
          : [result] "=r"(result)
          : "r" (test), "r"(new), "[result]"(old));

   Some instructions clobber specific hard registers.  To describe this,
write a third colon after the input operands, followed by the names of
the clobbered hard registers (given as strings).  Here is a realistic
example for the VAX:

     asm volatile ("movc3 %0,%1,%2"
                   : /* no outputs */
                   : "g" (from), "g" (to), "g" (count)
                   : "r0", "r1", "r2", "r3", "r4", "r5");

   You may not write a clobber description in a way that overlaps with
an input or output operand.  For example, you may not have an operand
describing a register class with one member if you mention that register
in the clobber list.  There is no way for you to specify that an input
operand is modified without also specifying it as an output operand.
Note that if all the output operands you specify are for this purpose
(and hence unused), you will then also need to specify `volatile' for
the `asm' construct, as described below, to prevent GCC from deleting
the `asm' statement as unused.

   If you refer to a particular hardware register from the assembler
code, you will probably have to list the register after the third colon
to tell the compiler the register's value is modified.  In some
assemblers, the register names begin with `%'; to produce one `%' in the
assembler code, you must write `%%' in the input.

   If your assembler instruction can alter the condition code register,
add `cc' to the list of clobbered registers.  GCC on some machines
represents the condition codes as a specific hardware register; `cc'
serves to name this register.  On other machines, the condition code is
handled differently, and specifying `cc' has no effect.  But it is
valid no matter what the machine.

   If your assembler instruction modifies memory in an unpredictable
fashion, add `memory' to the list of clobbered registers.  This will
cause GCC to not keep memory values cached in registers across the
assembler instruction.  You will also want to add the `volatile'
keyword if the memory affected is not listed in the inputs or outputs
of the `asm', as the `memory' clobber does not count as a side-effect
of the `asm'.

   You can put multiple assembler instructions together in a single
`asm' template, separated by the characters normally used in assembly
code for the system.  A combination that works in most places is a
newline to break the line, plus a tab character to move to the
instruction field (written as `\n\t').  Sometimes semicolons can be
used, if the assembler allows semicolons as a line-breaking character.
Note that some assembler dialects use semicolons to start a comment.
The input operands are guaranteed not to use any of the clobbered
registers, and neither will the output operands' addresses, so you can
read and write the clobbered registers as many times as you like.  Here
is an example of multiple instructions in a template; it assumes the
subroutine `_foo' accepts arguments in registers 9 and 10:

     asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
          : /* no outputs */
          : "g" (from), "g" (to)
          : "r9", "r10");

   Unless an output operand has the `&' constraint modifier, GCC may
allocate it in the same register as an unrelated input operand, on the
assumption the inputs are consumed before the outputs are produced.
This assumption may be false if the assembler code actually consists of
more than one instruction.  In such a case, use `&' for each output
operand that may not overlap an input.  *Note Modifiers::.

   If you want to test the condition code produced by an assembler
instruction, you must include a branch and a label in the `asm'
construct, as follows:

     asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
          : "g" (result)
          : "g" (input));

This assumes your assembler supports local labels, as the GNU assembler
and most Unix assemblers do.

   Speaking of labels, jumps from one `asm' to another are not
supported.  The compiler's optimizers do not know about these jumps, and
therefore they cannot take account of them when deciding how to
optimize.

   Usually the most convenient way to use these `asm' instructions is to
encapsulate them in macros that look like functions.  For example,

     #define sin(x)       \
     ({ double __value, __arg = (x);   \
        asm ("fsinx %1,%0": "=f" (__value): "f" (__arg));  \
        __value; })

Here the variable `__arg' is used to make sure that the instruction
operates on a proper `double' value, and to accept only those arguments
`x' which can convert automatically to a `double'.

   Another way to make sure the instruction operates on the correct data
type is to use a cast in the `asm'.  This is different from using a
variable `__arg' in that it converts more different types.  For
example, if the desired type were `int', casting the argument to `int'
would accept a pointer with no complaint, while assigning the argument
to an `int' variable named `__arg' would warn about using a pointer
unless the caller explicitly casts it.

   If an `asm' has output operands, GCC assumes for optimization
purposes the instruction has no side effects except to change the output
operands.  This does not mean instructions with a side effect cannot be
used, but you must be careful, because the compiler may eliminate them
if the output operands aren't used, or move them out of loops, or
replace two with one if they constitute a common subexpression.  Also,
if your instruction does have a side effect on a variable that otherwise
appears not to change, the old value of the variable may be reused later
if it happens to be found in a register.

   You can prevent an `asm' instruction from being deleted, moved
significantly, or combined, by writing the keyword `volatile' after the
`asm'.  For example:

     #define get_and_set_priority(new)              \
     ({ int __old;                                  \
        asm volatile ("get_and_set_priority %0, %1" \
                      : "=g" (__old) : "g" (new));  \
        __old; })

If you write an `asm' instruction with no outputs, GCC will know the
instruction has side-effects and will not delete the instruction or
move it outside of loops.

   The `volatile' keyword indicates that the instruction has important
side-effects.  GCC will not delete a volatile `asm' if it is reachable.
(The instruction can still be deleted if GCC can prove that
control-flow will never reach the location of the instruction.)  In
addition, GCC will not reschedule instructions across a volatile `asm'
instruction.  For example:

     *(volatile int *)addr = foo;
     asm volatile ("eieio" : : );

Assume `addr' contains the address of a memory mapped device register.
The PowerPC `eieio' instruction (Enforce In-order Execution of I/O)
tells the CPU to make sure that the store to that device register
happens before it issues any other I/O.

   Note that even a volatile `asm' instruction can be moved in ways
that appear insignificant to the compiler, such as across jump
instructions.  You can't expect a sequence of volatile `asm'
instructions to remain perfectly consecutive.  If you want consecutive
output, use a single `asm'.  Also, GCC will perform some optimizations
across a volatile `asm' instruction; GCC does not "forget everything"
when it encounters a volatile `asm' instruction the way some other
compilers do.

   An `asm' instruction without any operands or clobbers (an "old
style" `asm') will be treated identically to a volatile `asm'
instruction.

   It is a natural idea to look for a way to give access to the
condition code left by the assembler instruction.  However, when we
attempted to implement this, we found no way to make it work reliably.
The problem is that output operands might need reloading, which would
result in additional following "store" instructions.  On most machines,
these instructions would alter the condition code before there was time
to test it.  This problem doesn't arise for ordinary "test" and
"compare" instructions because they don't have any output operands.

   For reasons similar to those described above, it is not possible to
give an assembler instruction access to the condition code left by
previous instructions.

   If you are writing a header file that should be includable in ISO C
programs, write `__asm__' instead of `asm'.  *Note Alternate Keywords::.

5.35.1 i386 floating point asm operands
---------------------------------------

There are several rules on the usage of stack-like regs in asm_operands
insns.  These rules apply only to the operands that are stack-like regs:

  1. Given a set of input regs that die in an asm_operands, it is
     necessary to know which are implicitly popped by the asm, and
     which must be explicitly popped by gcc.

     An input reg that is implicitly popped by the asm must be
     explicitly clobbered, unless it is constrained to match an output
     operand.

  2. For any input reg that is implicitly popped by an asm, it is
     necessary to know how to adjust the stack to compensate for the
     pop.  If any non-popped input is closer to the top of the
     reg-stack than the implicitly popped reg, it would not be possible
     to know what the stack looked like--it's not clear how the rest of
     the stack "slides up".

     All implicitly popped input regs must be closer to the top of the
     reg-stack than any input that is not implicitly popped.

     It is possible that if an input dies in an insn, reload might use
     the input reg for an output reload.  Consider this example:

          asm ("foo" : "=t" (a) : "f" (b));

     This asm says that input B is not popped by the asm, and that the
     asm pushes a result onto the reg-stack, i.e., the stack is one
     deeper after the asm than it was before.  But, it is possible that
     reload will think that it can use the same reg for both the input
     and the output, if input B dies in this insn.

     If any input operand uses the `f' constraint, all output reg
     constraints must use the `&' earlyclobber.

     The asm above would be written as

          asm ("foo" : "=&t" (a) : "f" (b));

  3. Some operands need to be in particular places on the stack.  All
     output operands fall in this category--there is no other way to
     know which regs the outputs appear in unless the user indicates
     this in the constraints.

     Output operands must specifically indicate which reg an output
     appears in after an asm.  `=f' is not allowed: the operand
     constraints must select a class with a single reg.

  4. Output operands may not be "inserted" between existing stack regs.
     Since no 387 opcode uses a read/write operand, all output operands
     are dead before the asm_operands, and are pushed by the
     asm_operands.  It makes no sense to push anywhere but the top of
     the reg-stack.

     Output operands must start at the top of the reg-stack: output
     operands may not "skip" a reg.

  5. Some asm statements may need extra stack space for internal
     calculations.  This can be guaranteed by clobbering stack registers
     unrelated to the inputs and outputs.


   Here are a couple of reasonable asms to want to write.  This asm
takes one input, which is internally popped, and produces two outputs.

     asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));

   This asm takes two inputs, which are popped by the `fyl2xp1' opcode,
and replaces them with one output.  The user must code the `st(1)'
clobber for reg-stack.c to know that `fyl2xp1' pops both inputs.

     asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");


File: gcc.info,  Node: Constraints,  Next: Asm Labels,  Prev: Extended Asm,  Up: C Extensions

5.36 Constraints for `asm' Operands
===================================

Here are specific details on what constraint letters you can use with
`asm' operands.  Constraints can say whether an operand may be in a
register, and which kinds of register; whether the operand can be a
memory reference, and which kinds of address; whether the operand may
be an immediate constant, and which possible values it may have.
Constraints can also require two operands to match.

* Menu:

* Simple Constraints::  Basic use of constraints.
* Multi-Alternative::   When an insn has two alternative constraint-patterns.
* Modifiers::           More precise control over effects of constraints.
* Machine Constraints:: Special constraints for some particular machines.


File: gcc.info,  Node: Simple Constraints,  Next: Multi-Alternative,  Up: Constraints

5.36.1 Simple Constraints
-------------------------

The simplest kind of constraint is a string full of letters, each of
which describes one kind of operand that is permitted.  Here are the
letters that are allowed:

whitespace
     Whitespace characters are ignored and can be inserted at any
     position except the first.  This enables each alternative for
     different operands to be visually aligned in the machine
     description even if they have different number of constraints and
     modifiers.

`m'
     A memory operand is allowed, with any kind of address that the
     machine supports in general.

`o'
     A memory operand is allowed, but only if the address is
     "offsettable".  This means that adding a small integer (actually,
     the width in bytes of the operand, as determined by its machine
     mode) may be added to the address and the result is also a valid
     memory address.

     For example, an address which is constant is offsettable; so is an
     address that is the sum of a register and a constant (as long as a
     slightly larger constant is also within the range of
     address-offsets supported by the machine); but an autoincrement or
     autodecrement address is not offsettable.  More complicated
     indirect/indexed addresses may or may not be offsettable depending
     on the other addressing modes that the machine supports.

     Note that in an output operand which can be matched by another
     operand, the constraint letter `o' is valid only when accompanied
     by both `<' (if the target machine has predecrement addressing)
     and `>' (if the target machine has preincrement addressing).

`V'
     A memory operand that is not offsettable.  In other words,
     anything that would fit the `m' constraint but not the `o'
     constraint.

`<'
     A memory operand with autodecrement addressing (either
     predecrement or postdecrement) is allowed.

`>'
     A memory operand with autoincrement addressing (either
     preincrement or postincrement) is allowed.

`r'
     A register operand is allowed provided that it is in a general
     register.

`i'
     An immediate integer operand (one with constant value) is allowed.
     This includes symbolic constants whose values will be known only at
     assembly time.

`n'
     An immediate integer operand with a known numeric value is allowed.
     Many systems cannot support assembly-time constants for operands
     less than a word wide.  Constraints for these operands should use
     `n' rather than `i'.

`I', `J', `K', ... `P'
     Other letters in the range `I' through `P' may be defined in a
     machine-dependent fashion to permit immediate integer operands with
     explicit integer values in specified ranges.  For example, on the
     68000, `I' is defined to stand for the range of values 1 to 8.
     This is the range permitted as a shift count in the shift
     instructions.

`E'
     An immediate floating operand (expression code `const_double') is
     allowed, but only if the target floating point format is the same
     as that of the host machine (on which the compiler is running).

`F'
     An immediate floating operand (expression code `const_double') is
     allowed.

`G', `H'
     `G' and `H' may be defined in a machine-dependent fashion to
     permit immediate floating operands in particular ranges of values.

`s'
     An immediate integer operand whose value is not an explicit
     integer is allowed.

     This might appear strange; if an insn allows a constant operand
     with a value not known at compile time, it certainly must allow
     any known value.  So why use `s' instead of `i'?  Sometimes it
     allows better code to be generated.

     For example, on the 68000 in a fullword instruction it is possible
     to use an immediate operand; but if the immediate value is between
     -128 and 127, better code results from loading the value into a
     register and using the register.  This is because the load into
     the register can be done with a `moveq' instruction.  We arrange
     for this to happen by defining the letter `K' to mean "any integer
     outside the range -128 to 127", and then specifying `Ks' in the
     operand constraints.

`g'
     Any register, memory or immediate integer operand is allowed,
     except for registers that are not general registers.

`X'
     Any operand whatsoever is allowed.

`0', `1', `2', ... `9'
     An operand that matches the specified operand number is allowed.
     If a digit is used together with letters within the same
     alternative, the digit should come last.

     This number is allowed to be more than a single digit.  If multiple
     digits are encountered consecutavely, they are interpreted as a
     single decimal integer.  There is scant chance for ambiguity,
     since to-date it has never been desirable that `10' be interpreted
     as matching either operand 1 _or_ operand 0.  Should this be
     desired, one can use multiple alternatives instead.

     This is called a "matching constraint" and what it really means is
     that the assembler has only a single operand that fills two roles
     which `asm' distinguishes.  For example, an add instruction uses
     two input operands and an output operand, but on most CISC
     machines an add instruction really has only two operands, one of
     them an input-output operand:

          addl #35,r12

     Matching constraints are used in these circumstances.  More
     precisely, the two operands that match must include one input-only
     operand and one output-only operand.  Moreover, the digit must be a
     smaller number than the number of the operand that uses it in the
     constraint.

`p'
     An operand that is a valid memory address is allowed.  This is for
     "load address" and "push address" instructions.

     `p' in the constraint must be accompanied by `address_operand' as
     the predicate in the `match_operand'.  This predicate interprets
     the mode specified in the `match_operand' as the mode of the memory
     reference for which the address would be valid.

OTHER-LETTERS
     Other letters can be defined in machine-dependent fashion to stand
     for particular classes of registers or other arbitrary operand
     types.  `d', `a' and `f' are defined on the 68000/68020 to stand
     for data, address and floating point registers.



File: gcc.info,  Node: Multi-Alternative,  Next: Modifiers,  Prev: Simple Constraints,  Up: Constraints

5.36.2 Multiple Alternative Constraints
---------------------------------------

Sometimes a single instruction has multiple alternative sets of possible
operands.  For example, on the 68000, a logical-or instruction can
combine register or an immediate value into memory, or it can combine
any kind of operand into a register; but it cannot combine one memory
location into another.

   These constraints are represented as multiple alternatives.  An
alternative can be described by a series of letters for each operand.
The overall constraint for an operand is made from the letters for this
operand from the first alternative, a comma, the letters for this
operand from the second alternative, a comma, and so on until the last
alternative.

   If all the operands fit any one alternative, the instruction is
valid.  Otherwise, for each alternative, the compiler counts how many
instructions must be added to copy the operands so that that
alternative applies.  The alternative requiring the least copying is
chosen.  If two alternatives need the same amount of copying, the one
that comes first is chosen.  These choices can be altered with the `?'
and `!' characters:

`?'
     Disparage slightly the alternative that the `?' appears in, as a
     choice when no alternative applies exactly.  The compiler regards
     this alternative as one unit more costly for each `?' that appears
     in it.

`!'
     Disparage severely the alternative that the `!' appears in.  This
     alternative can still be used if it fits without reloading, but if
     reloading is needed, some other alternative will be used.


File: gcc.info,  Node: Modifiers,  Next: Machine Constraints,  Prev: Multi-Alternative,  Up: Constraints

5.36.3 Constraint Modifier Characters
-------------------------------------

Here are constraint modifier characters.

`='
     Means that this operand is write-only for this instruction: the
     previous value is discarded and replaced by output data.

`+'
     Means that this operand is both read and written by the
     instruction.

     When the compiler fixes up the operands to satisfy the constraints,
     it needs to know which operands are inputs to the instruction and
     which are outputs from it.  `=' identifies an output; `+'
     identifies an operand that is both input and output; all other
     operands are assumed to be input only.

     If you specify `=' or `+' in a constraint, you put it in the first
     character of the constraint string.

`&'
     Means (in a particular alternative) that this operand is an
     "earlyclobber" operand, which is modified before the instruction is
     finished using the input operands.  Therefore, this operand may
     not lie in a register that is used as an input operand or as part
     of any memory address.

     `&' applies only to the alternative in which it is written.  In
     constraints with multiple alternatives, sometimes one alternative
     requires `&' while others do not.  See, for example, the `movdf'
     insn of the 68000.

     An input operand can be tied to an earlyclobber operand if its only
     use as an input occurs before the early result is written.  Adding
     alternatives of this form often allows GCC to produce better code
     when only some of the inputs can be affected by the earlyclobber.
     See, for example, the `mulsi3' insn of the ARM.

     `&' does not obviate the need to write `='.

`%'
     Declares the instruction to be commutative for this operand and the
     following operand.  This means that the compiler may interchange
     the two operands if that is the cheapest way to make all operands
     fit the constraints.

`#'
     Says that all following characters, up to the next comma, are to be
     ignored as a constraint.  They are significant only for choosing
     register preferences.

`*'
     Says that the following character should be ignored when choosing
     register preferences.  `*' has no effect on the meaning of the
     constraint as a constraint, and no effect on reloading.



File: gcc.info,  Node: Machine Constraints,  Prev: Modifiers,  Up: Constraints

5.36.4 Constraints for Particular Machines
------------------------------------------

Whenever possible, you should use the general-purpose constraint letters
in `asm' arguments, since they will convey meaning more readily to
people reading your code.  Failing that, use the constraint letters
that usually have very similar meanings across architectures.  The most
commonly used constraints are `m' and `r' (for memory and
general-purpose registers respectively; *note Simple Constraints::), and
`I', usually the letter indicating the most common immediate-constant
format.

   For each machine architecture, the `config/MACHINE/MACHINE.h' file
defines additional constraints.  These constraints are used by the
compiler itself for instruction generation, as well as for `asm'
statements; therefore, some of the constraints are not particularly
interesting for `asm'.  The constraints are defined through these
macros:

`REG_CLASS_FROM_LETTER'
     Register class constraints (usually lower case).

`CONST_OK_FOR_LETTER_P'
     Immediate constant constraints, for non-floating point constants of
     word size or smaller precision (usually upper case).

`CONST_DOUBLE_OK_FOR_LETTER_P'
     Immediate constant constraints, for all floating point constants
     and for constants of greater than word size precision (usually
     upper case).

`EXTRA_CONSTRAINT'
     Special cases of registers or memory.  This macro is not required,
     and is only defined for some machines.

   Inspecting these macro definitions in the compiler source for your
machine is the best way to be certain you have the right constraints.
However, here is a summary of the machine-dependent constraints
available on some particular machines.

_ARM family--`arm.h'_

    `f'
          Floating-point register

    `F'
          One of the floating-point constants 0.0, 0.5, 1.0, 2.0, 3.0,
          4.0, 5.0 or 10.0

    `G'
          Floating-point constant that would satisfy the constraint `F'
          if it were negated

    `I'
          Integer that is valid as an immediate operand in a data
          processing instruction.  That is, an integer in the range 0
          to 255 rotated by a multiple of 2

    `J'
          Integer in the range -4095 to 4095

    `K'
          Integer that satisfies constraint `I' when inverted (ones
          complement)

    `L'
          Integer that satisfies constraint `I' when negated (twos
          complement)

    `M'
          Integer in the range 0 to 32

    `Q'
          A memory reference where the exact address is in a single
          register (``m'' is preferable for `asm' statements)

    `R'
          An item in the constant pool

    `S'
          A symbol in the text segment of the current file

_AMD 29000 family--`a29k.h'_

    `l'
          Local register 0

    `b'
          Byte Pointer (`BP') register

    `q'
          `Q' register

    `h'
          Special purpose register

    `A'
          First accumulator register

    `a'
          Other accumulator register

    `f'
          Floating point register

    `I'
          Constant greater than 0, less than 0x100

    `J'
          Constant greater than 0, less than 0x10000

    `K'
          Constant whose high 24 bits are on (1)

    `L'
          16-bit constant whose high 8 bits are on (1)

    `M'
          32-bit constant whose high 16 bits are on (1)

    `N'
          32-bit negative constant that fits in 8 bits

    `O'
          The constant 0x80000000 or, on the 29050, any 32-bit constant
          whose low 16 bits are 0.

    `P'
          16-bit negative constant that fits in 8 bits

    `G'
    `H'
          A floating point constant (in `asm' statements, use the
          machine independent `E' or `F' instead)

_AVR family--`avr.h'_

    `l'
          Registers from r0 to r15

    `a'
          Registers from r16 to r23

    `d'
          Registers from r16 to r31

    `w'
          Registers from r24 to r31.  These registers can be used in
          `adiw' command

    `e'
          Pointer register (r26-r31)

    `b'
          Base pointer register (r28-r31)

    `q'
          Stack pointer register (SPH:SPL)

    `t'
          Temporary register r0

    `x'
          Register pair X (r27:r26)

    `y'
          Register pair Y (r29:r28)

    `z'
          Register pair Z (r31:r30)

    `I'
          Constant greater than -1, less than 64

    `J'
          Constant greater than -64, less than 1

    `K'
          Constant integer 2

    `L'
          Constant integer 0

    `M'
          Constant that fits in 8 bits

    `N'
          Constant integer -1

    `O'
          Constant integer 8, 16, or 24

    `P'
          Constant integer 1

    `G'
          A floating point constant 0.0

_IBM RS6000--`rs6000.h'_

    `b'
          Address base register

    `f'
          Floating point register

    `h'
          `MQ', `CTR', or `LINK' register

    `q'
          `MQ' register

    `c'
          `CTR' register

    `l'
          `LINK' register

    `x'
          `CR' register (condition register) number 0

    `y'
          `CR' register (condition register)

    `z'
          `FPMEM' stack memory for FPR-GPR transfers

    `I'
          Signed 16-bit constant

    `J'
          Unsigned 16-bit constant shifted left 16 bits (use `L'
          instead for `SImode' constants)

    `K'
          Unsigned 16-bit constant

    `L'
          Signed 16-bit constant shifted left 16 bits

    `M'
          Constant larger than 31

    `N'
          Exact power of 2

    `O'
          Zero

    `P'
          Constant whose negation is a signed 16-bit constant

    `G'
          Floating point constant that can be loaded into a register
          with one instruction per word

    `Q'
          Memory operand that is an offset from a register (`m' is
          preferable for `asm' statements)

    `R'
          AIX TOC entry

    `S'
          Constant suitable as a 64-bit mask operand

    `T'
          Constant suitable as a 32-bit mask operand

    `U'
          System V Release 4 small data area reference

_Intel 386--`i386.h'_

    `q'
          `a', `b', `c', or `d' register for the i386.  For x86-64 it
          is equivalent to `r' class. (for 8-bit instructions that do
          not use upper halves)

    `Q'
          `a', `b', `c', or `d' register. (for 8-bit instructions, that
          do use upper halves)

    `R'
          Legacy register--equivalent to `r' class in i386 mode.  (for
          non-8-bit registers used together with 8-bit upper halves in
          a single instruction)

    `A'
          Specifies the `a' or `d' registers.  This is primarily useful
          for 64-bit integer values (when in 32-bit mode) intended to
          be returned with the `d' register holding the most
          significant bits and the `a' register holding the least
          significant bits.

    `f'
          Floating point register

    `t'
          First (top of stack) floating point register

    `u'
          Second floating point register

    `a'
          `a' register

    `b'
          `b' register

    `c'
          `c' register

    `d'
          `d' register

    `D'
          `di' register

    `S'
          `si' register

    `x'
          `xmm' SSE register

    `y'
          MMX register

    `I'
          Constant in range 0 to 31 (for 32-bit shifts)

    `J'
          Constant in range 0 to 63 (for 64-bit shifts)

    `K'
          `0xff'

    `L'
          `0xffff'

    `M'
          0, 1, 2, or 3 (shifts for `lea' instruction)

    `N'
          Constant in range 0 to 255 (for `out' instruction)

    `Z'
          Constant in range 0 to `0xffffffff' or symbolic reference
          known to fit specified range.  (for using immediates in zero
          extending 32-bit to 64-bit x86-64 instructions)

    `e'
          Constant in range -2147483648 to 2147483647 or symbolic
          reference known to fit specified range.  (for using
          immediates in 64-bit x86-64 instructions)

    `G'
          Standard 80387 floating point constant

_Intel 960--`i960.h'_

    `f'
          Floating point register (`fp0' to `fp3')

    `l'
          Local register (`r0' to `r15')

    `b'
          Global register (`g0' to `g15')

    `d'
          Any local or global register

    `I'
          Integers from 0 to 31

    `J'
          0

    `K'
          Integers from -31 to 0

    `G'
          Floating point 0

    `H'
          Floating point 1

_MIPS--`mips.h'_

    `d'
          General-purpose integer register

    `f'
          Floating-point register (if available)

    `h'
          `Hi' register

    `l'
          `Lo' register

    `x'
          `Hi' or `Lo' register

    `y'
          General-purpose integer register

    `z'
          Floating-point status register

    `I'
          Signed 16-bit constant (for arithmetic instructions)

    `J'
          Zero

    `K'
          Zero-extended 16-bit constant (for logic instructions)

    `L'
          Constant with low 16 bits zero (can be loaded with `lui')

    `M'
          32-bit constant which requires two instructions to load (a
          constant which is not `I', `K', or `L')

    `N'
          Negative 16-bit constant

    `O'
          Exact power of two

    `P'
          Positive 16-bit constant

    `G'
          Floating point zero

    `Q'
          Memory reference that can be loaded with more than one
          instruction (`m' is preferable for `asm' statements)

    `R'
          Memory reference that can be loaded with one instruction (`m'
          is preferable for `asm' statements)

    `S'
          Memory reference in external OSF/rose PIC format (`m' is
          preferable for `asm' statements)

_Motorola 680x0--`m68k.h'_

    `a'
          Address register

    `d'
          Data register

    `f'
          68881 floating-point register, if available

    `x'
          Sun FPA (floating-point) register, if available

    `y'
          First 16 Sun FPA registers, if available

    `I'
          Integer in the range 1 to 8

    `J'
          16-bit signed number

    `K'
          Signed number whose magnitude is greater than 0x80

    `L'
          Integer in the range -8 to -1

    `M'
          Signed number whose magnitude is greater than 0x100

    `G'
          Floating point constant that is not a 68881 constant

    `H'
          Floating point constant that can be used by Sun FPA

_Motorola 68HC11 & 68HC12 families--`m68hc11.h'_

    `a'
          Register 'a'

    `b'
          Register 'b'

    `d'
          Register 'd'

    `q'
          An 8-bit register

    `t'
          Temporary soft register _.tmp

    `u'
          A soft register _.d1 to _.d31

    `w'
          Stack pointer register

    `x'
          Register 'x'

    `y'
          Register 'y'

    `z'
          Pseudo register 'z' (replaced by 'x' or 'y' at the end)

    `A'
          An address register: x, y or z

    `B'
          An address register: x or y

    `D'
          Register pair (x:d) to form a 32-bit value

    `L'
          Constants in the range -65536 to 65535

    `M'
          Constants whose 16-bit low part is zero

    `N'
          Constant integer 1 or -1

    `O'
          Constant integer 16

    `P'
          Constants in the range -8 to 2


_SPARC--`sparc.h'_

    `f'
          Floating-point register that can hold 32- or 64-bit values.

    `e'
          Floating-point register that can hold 64- or 128-bit values.

    `I'
          Signed 13-bit constant

    `J'
          Zero

    `K'
          32-bit constant with the low 12 bits clear (a constant that
          can be loaded with the `sethi' instruction)

    `L'
          A constant in the range supported by `movcc' instructions

    `M'
          A constant in the range supported by `movrcc' instructions

    `N'
          Same as `K', except that it verifies that bits that are not
          in the lower 32-bit range are all zero.  Must be used instead
          of `K' for modes wider than `SImode'

    `G'
          Floating-point zero

    `H'
          Signed 13-bit constant, sign-extended to 32 or 64 bits

    `Q'
          Floating-point constant whose integral representation can be
          moved into an integer register using a single sethi
          instruction

    `R'
          Floating-point constant whose integral representation can be
          moved into an integer register using a single mov instruction

    `S'
          Floating-point constant whose integral representation can be
          moved into an integer register using a high/lo_sum
          instruction sequence

    `T'
          Memory address aligned to an 8-byte boundary

    `U'
          Even register

    `W'
          Memory address for `e' constraint registers.


_TMS320C3x/C4x--`c4x.h'_

    `a'
          Auxiliary (address) register (ar0-ar7)

    `b'
          Stack pointer register (sp)

    `c'
          Standard (32-bit) precision integer register

    `f'
          Extended (40-bit) precision register (r0-r11)

    `k'
          Block count register (bk)

    `q'
          Extended (40-bit) precision low register (r0-r7)

    `t'
          Extended (40-bit) precision register (r0-r1)

    `u'
          Extended (40-bit) precision register (r2-r3)

    `v'
          Repeat count register (rc)

    `x'
          Index register (ir0-ir1)

    `y'
          Status (condition code) register (st)

    `z'
          Data page register (dp)

    `G'
          Floating-point zero

    `H'
          Immediate 16-bit floating-point constant

    `I'
          Signed 16-bit constant

    `J'
          Signed 8-bit constant

    `K'
          Signed 5-bit constant

    `L'
          Unsigned 16-bit constant

    `M'
          Unsigned 8-bit constant

    `N'
          Ones complement of unsigned 16-bit constant

    `O'
          High 16-bit constant (32-bit constant with 16 LSBs zero)

    `Q'
          Indirect memory reference with signed 8-bit or index register
          displacement

    `R'
          Indirect memory reference with unsigned 5-bit displacement

    `S'
          Indirect memory reference with 1 bit or index register
          displacement

    `T'
          Direct memory reference

    `U'
          Symbolic address


_S/390 and zSeries--`s390.h'_

    `a'
          Address register (general purpose register except r0)

    `d'
          Data register (arbitrary general purpose register)

    `f'
          Floating-point register

    `I'
          Unsigned 8-bit constant (0-255)

    `J'
          Unsigned 12-bit constant (0-4095)

    `K'
          Signed 16-bit constant (-32768-32767)

    `L'
          Unsigned 16-bit constant (0-65535)

    `Q'
          Memory reference without index register

    `S'
          Symbolic constant suitable for use with the `larl' instruction


_Xstormy16--`stormy16.h'_

    `a'
          Register r0.

    `b'
          Register r1.

    `c'
          Register r2.

    `d'
          Register r8.

    `e'
          Registers r0 through r7.

    `t'
          Registers r0 and r1.

    `y'
          The carry register.

    `z'
          Registers r8 and r9.

    `I'
          A constant between 0 and 3 inclusive.

    `J'
          A constant that has exactly one bit set.

    `K'
          A constant that has exactly one bit clear.

    `L'
          A constant between 0 and 255 inclusive.

    `M'
          A constant between -255 and 0 inclusive.

    `N'
          A constant between -3 and 0 inclusive.

    `O'
          A constant between 1 and 4 inclusive.

    `P'
          A constant between -4 and -1 inclusive.

    `Q'
          A memory reference that is a stack push.

    `R'
          A memory reference that is a stack pop.

    `S'
          A memory reference that refers to an constant address of
          known value.

    `T'
          The register indicated by Rx (not implemented yet).

    `U'
          A constant that is not between 2 and 15 inclusive.


_Xtensa--`xtensa.h'_

    `a'
          General-purpose 32-bit register

    `b'
          One-bit boolean register

    `A'
          MAC16 40-bit accumulator register

    `I'
          Signed 12-bit integer constant, for use in MOVI instructions

    `J'
          Signed 8-bit integer constant, for use in ADDI instructions

    `K'
          Integer constant valid for BccI instructions

    `L'
          Unsigned constant valid for BccUI instructions




File: gcc.info,  Node: Asm Labels,  Next: Explicit Reg Vars,  Prev: Constraints,  Up: C Extensions

5.37 Controlling Names Used in Assembler Code
=============================================

You can specify the name to be used in the assembler code for a C
function or variable by writing the `asm' (or `__asm__') keyword after
the declarator as follows:

     int foo asm ("myfoo") = 2;

This specifies that the name to be used for the variable `foo' in the
assembler code should be `myfoo' rather than the usual `_foo'.

   On systems where an underscore is normally prepended to the name of
a C function or variable, this feature allows you to define names for
the linker that do not start with an underscore.

   It does not make sense to use this feature with a non-static local
variable since such variables do not have assembler names.  If you are
trying to put the variable in a particular register, see *Note Explicit
Reg Vars::.  GCC presently accepts such code with a warning, but will
probably be changed to issue an error, rather than a warning, in the
future.

   You cannot use `asm' in this way in a function _definition_; but you
can get the same effect by writing a declaration for the function
before its definition and putting `asm' there, like this:

     extern func () asm ("FUNC");

     func (x, y)
          int x, y;
     ...

   It is up to you to make sure that the assembler names you choose do
not conflict with any other assembler symbols.  Also, you must not use a
register name; that would produce completely invalid assembler code.
GCC does not as yet have the ability to store static variables in
registers.  Perhaps that will be added.


File: gcc.info,  Node: Explicit Reg Vars,  Next: Alternate Keywords,  Prev: Asm Labels,  Up: C Extensions

5.38 Variables in Specified Registers
=====================================

GNU C allows you to put a few global variables into specified hardware
registers.  You can also specify the register in which an ordinary
register variable should be allocated.

   * Global register variables reserve registers throughout the program.
     This may be useful in programs such as programming language
     interpreters which have a couple of global variables that are
     accessed very often.

   * Local register variables in specific registers do not reserve the
     registers.  The compiler's data flow analysis is capable of
     determining where the specified registers contain live values, and
     where they are available for other uses.  Stores into local
     register variables may be deleted when they appear to be dead
     according to dataflow analysis.  References to local register
     variables may be deleted or moved or simplified.

     These local variables are sometimes convenient for use with the
     extended `asm' feature (*note Extended Asm::), if you want to
     write one output of the assembler instruction directly into a
     particular register.  (This will work provided the register you
     specify fits the constraints specified for that operand in the
     `asm'.)

* Menu:

* Global Reg Vars::
* Local Reg Vars::


File: gcc.info,  Node: Global Reg Vars,  Next: Local Reg Vars,  Up: Explicit Reg Vars

5.38.1 Defining Global Register Variables
-----------------------------------------

You can define a global register variable in GNU C like this:

     register int *foo asm ("a5");

Here `a5' is the name of the register which should be used.  Choose a
register which is normally saved and restored by function calls on your
machine, so that library routines will not clobber it.

   Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type.  The register `a5'
would be a good choice on a 68000 for a variable of pointer type.  On
machines with register windows, be sure to choose a "global" register
that is not affected magically by the function call mechanism.

   In addition, operating systems on one type of cpu may differ in how
they name the registers; then you would need additional conditionals.
For example, some 68000 operating systems call this register `%a5'.

   Eventually there may be a way of asking the compiler to choose a
register automatically, but first we need to figure out how it should
choose and how to enable you to guide the choice.  No solution is
evident.

   Defining a global register variable in a certain register reserves
that register entirely for this use, at least within the current
compilation.  The register will not be allocated for any other purpose
in the functions in the current compilation.  The register will not be
saved and restored by these functions.  Stores into this register are
never deleted even if they would appear to be dead, but references may
be deleted or moved or simplified.

   It is not safe to access the global register variables from signal
handlers, or from more than one thread of control, because the system
library routines may temporarily use the register for other things
(unless you recompile them specially for the task at hand).

   It is not safe for one function that uses a global register variable
to call another such function `foo' by way of a third function `lose'
that was compiled without knowledge of this variable (i.e. in a
different source file in which the variable wasn't declared).  This is
because `lose' might save the register and put some other value there.
For example, you can't expect a global register variable to be
available in the comparison-function that you pass to `qsort', since
`qsort' might have put something else in that register.  (If you are
prepared to recompile `qsort' with the same global register variable,
you can solve this problem.)

   If you want to recompile `qsort' or other source files which do not
actually use your global register variable, so that they will not use
that register for any other purpose, then it suffices to specify the
compiler option `-ffixed-REG'.  You need not actually add a global
register declaration to their source code.

   A function which can alter the value of a global register variable
cannot safely be called from a function compiled without this variable,
because it could clobber the value the caller expects to find there on
return.  Therefore, the function which is the entry point into the part
of the program that uses the global register variable must explicitly
save and restore the value which belongs to its caller.

   On most machines, `longjmp' will restore to each global register
variable the value it had at the time of the `setjmp'.  On some
machines, however, `longjmp' will not change the value of global
register variables.  To be portable, the function that called `setjmp'
should make other arrangements to save the values of the global register
variables, and to restore them in a `longjmp'.  This way, the same
thing will happen regardless of what `longjmp' does.

   All global register variable declarations must precede all function
definitions.  If such a declaration could appear after function
definitions, the declaration would be too late to prevent the register
from being used for other purposes in the preceding functions.

   Global register variables may not have initial values, because an
executable file has no means to supply initial contents for a register.

   On the Sparc, there are reports that g3 ... g7 are suitable
registers, but certain library functions, such as `getwd', as well as
the subroutines for division and remainder, modify g3 and g4.  g1 and
g2 are local temporaries.

   On the 68000, a2 ... a5 should be suitable, as should d2 ... d7.  Of
course, it will not do to use more than a few of those.


File: gcc.info,  Node: Local Reg Vars,  Prev: Global Reg Vars,  Up: Explicit Reg Vars

5.38.2 Specifying Registers for Local Variables
-----------------------------------------------

You can define a local register variable with a specified register like
this:

     register int *foo asm ("a5");

Here `a5' is the name of the register which should be used.  Note that
this is the same syntax used for defining global register variables,
but for a local variable it would appear within a function.

   Naturally the register name is cpu-dependent, but this is not a
problem, since specific registers are most often useful with explicit
assembler instructions (*note Extended Asm::).  Both of these things
generally require that you conditionalize your program according to cpu
type.

   In addition, operating systems on one type of cpu may differ in how
they name the registers; then you would need additional conditionals.
For example, some 68000 operating systems call this register `%a5'.

   Defining such a register variable does not reserve the register; it
remains available for other uses in places where flow control determines
the variable's value is not live.  However, these registers are made
unavailable for use in the reload pass; excessive use of this feature
leaves the compiler too few available registers to compile certain
functions.

   This option does not guarantee that GCC will generate code that has
this variable in the register you specify at all times.  You may not
code an explicit reference to this register in an `asm' statement and
assume it will always refer to this variable.

   Stores into local register variables may be deleted when they appear
to be dead according to dataflow analysis.  References to local
register variables may be deleted or moved or simplified.


File: gcc.info,  Node: Alternate Keywords,  Next: Incomplete Enums,  Prev: Explicit Reg Vars,  Up: C Extensions

5.39 Alternate Keywords
=======================

The option `-traditional' disables certain keywords; `-ansi' and the
various `-std' options disable certain others.  This causes trouble
when you want to use GNU C extensions, or ISO C features, in a
general-purpose header file that should be usable by all programs,
including ISO C programs and traditional ones.  The keywords `asm',
`typeof' and `inline' cannot be used since they won't work in a program
compiled with `-ansi' (although `inline' can be used in a program
compiled with `-std=c99'), while the keywords `const', `volatile',
`signed', `typeof' and `inline' won't work in a program compiled with
`-traditional'.  The ISO C99 keyword `restrict' is only available when
`-std=gnu99' (which will eventually be the default) or `-std=c99' (or
the equivalent `-std=iso9899:1999') is used.

   The way to solve these problems is to put `__' at the beginning and
end of each problematical keyword.  For example, use `__asm__' instead
of `asm', `__const__' instead of `const', and `__inline__' instead of
`inline'.

   Other C compilers won't accept these alternative keywords; if you
want to compile with another compiler, you can define the alternate
keywords as macros to replace them with the customary keywords.  It
looks like this:

     #ifndef __GNUC__
     #define __asm__ asm
     #endif

   `-pedantic' and other options cause warnings for many GNU C
extensions.  You can prevent such warnings within one expression by
writing `__extension__' before the expression.  `__extension__' has no
effect aside from this.


File: gcc.info,  Node: Incomplete Enums,  Next: Function Names,  Prev: Alternate Keywords,  Up: C Extensions

5.40 Incomplete `enum' Types
============================

You can define an `enum' tag without specifying its possible values.
This results in an incomplete type, much like what you get if you write
`struct foo' without describing the elements.  A later declaration
which does specify the possible values completes the type.

   You can't allocate variables or storage using the type while it is
incomplete.  However, you can work with pointers to that type.

   This extension may not be very useful, but it makes the handling of
`enum' more consistent with the way `struct' and `union' are handled.

   This extension is not supported by GNU C++.


File: gcc.info,  Node: Function Names,  Next: Return Address,  Prev: Incomplete Enums,  Up: C Extensions

5.41 Function Names as Strings
==============================

GCC predefines two magic identifiers to hold the name of the current
function.  The identifier `__FUNCTION__' holds the name of the function
as it appears in the source.  The identifier `__PRETTY_FUNCTION__'
holds the name of the function pretty printed in a language specific
fashion.

   These names are always the same in a C function, but in a C++
function they may be different.  For example, this program:

     extern "C" {
     extern int printf (char *, ...);
     }

     class a {
      public:
       sub (int i)
         {
           printf ("__FUNCTION__ = %s\n", __FUNCTION__);
           printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
         }
     };

     int
     main (void)
     {
       a ax;
       ax.sub (0);
       return 0;
     }

gives this output:

     __FUNCTION__ = sub
     __PRETTY_FUNCTION__ = int  a::sub (int)

   The compiler automagically replaces the identifiers with a string
literal containing the appropriate name.  Thus, they are neither
preprocessor macros, like `__FILE__' and `__LINE__', nor variables.
This means that they catenate with other string literals, and that they
can be used to initialize char arrays.  For example

     char here[] = "Function " __FUNCTION__ " in " __FILE__;

   On the other hand, `#ifdef __FUNCTION__' does not have any special
meaning inside a function, since the preprocessor does not do anything
special with the identifier `__FUNCTION__'.

   Note that these semantics are deprecated, and that GCC 3.2 will
handle `__FUNCTION__' and `__PRETTY_FUNCTION__' the same way as
`__func__'.  `__func__' is defined by the ISO standard C99:

     The identifier `__func__' is implicitly declared by the translator
     as if, immediately following the opening brace of each function
     definition, the declaration
          static const char __func__[] = "function-name";

     appeared, where function-name is the name of the lexically-enclosing
     function.  This name is the unadorned name of the function.

   By this definition, `__func__' is a variable, not a string literal.
In particular, `__func__' does not catenate with other string literals.

   In `C++', `__FUNCTION__' and `__PRETTY_FUNCTION__' are variables,
declared in the same way as `__func__'.


File: gcc.info,  Node: Return Address,  Next: Vector Extensions,  Prev: Function Names,  Up: C Extensions

5.42 Getting the Return or Frame Address of a Function
======================================================

These functions may be used to get information about the callers of a
function.

 -- Built-in Function: void * __builtin_return_address (unsigned int
          LEVEL)
     This function returns the return address of the current function,
     or of one of its callers.  The LEVEL argument is number of frames
     to scan up the call stack.  A value of `0' yields the return
     address of the current function, a value of `1' yields the return
     address of the caller of the current function, and so forth.

     The LEVEL argument must be a constant integer.

     On some machines it may be impossible to determine the return
     address of any function other than the current one; in such cases,
     or when the top of the stack has been reached, this function will
     return `0' or a random value. In addition,
     `__builtin_frame_address' may be used to determine if the top of
     the stack has been reached.

     This function should only be used with a nonzero argument for
     debugging purposes.

 -- Built-in Function: void * __builtin_frame_address (unsigned int
          LEVEL)
     This function is similar to `__builtin_return_address', but it
     returns the address of the function frame rather than the return
     address of the function.  Calling `__builtin_frame_address' with a
     value of `0' yields the frame address of the current function, a
     value of `1' yields the frame address of the caller of the current
     function, and so forth.

     The frame is the area on the stack which holds local variables and
     saved registers.  The frame address is normally the address of the
     first word pushed on to the stack by the function.  However, the
     exact definition depends upon the processor and the calling
     convention.  If the processor has a dedicated frame pointer
     register, and the function has a frame, then
     `__builtin_frame_address' will return the value of the frame
     pointer register.

     On some machines it may be impossible to determine the frame
     address of any function other than the current one; in such cases,
     or when the top of the stack has been reached, this function will
     return `0' if the first frame pointer is properly initialized by
     the startup code.

     This function should only be used with a nonzero argument for
     debugging purposes.


File: gcc.info,  Node: Vector Extensions,  Next: Other Builtins,  Prev: Return Address,  Up: C Extensions

5.43 Using vector instructions through built-in functions
=========================================================

On some targets, the instruction set contains SIMD vector instructions
that operate on multiple values contained in one large register at the
same time.  For example, on the i386 the MMX, 3Dnow! and SSE extensions
can be used this way.

   The first step in using these extensions is to provide the necessary
data types.  This should be done using an appropriate `typedef':

     typedef int v4si __attribute__ ((mode(V4SI)));

   The base type `int' is effectively ignored by the compiler, the
actual properties of the new type `v4si' are defined by the
`__attribute__'.  It defines the machine mode to be used; for vector
types these have the form `VNB'; N should be the number of elements in
the vector, and B should be the base mode of the individual elements.
The following can be used as base modes:

`QI'
     An integer that is as wide as the smallest addressable unit,
     usually 8 bits.

`HI'
     An integer, twice as wide as a QI mode integer, usually 16 bits.

`SI'
     An integer, four times as wide as a QI mode integer, usually 32
     bits.

`DI'
     An integer, eight times as wide as a QI mode integer, usually 64
     bits.

`SF'
     A floating point value, as wide as a SI mode integer, usually 32
     bits.

`DF'
     A floating point value, as wide as a DI mode integer, usually 64
     bits.

   Not all base types or combinations are always valid; which modes can
be used is determined by the target machine.  For example, if
targetting the i386 MMX extensions, only `V8QI', `V4HI' and `V2SI' are
allowed modes.

   There are no `V1xx' vector modes - they would be identical to the
corresponding base mode.

   There is no distinction between signed and unsigned vector modes.
This distinction is made by the operations that perform on the vectors,
not by the data type.

   The types defined in this manner are somewhat special, they cannot be
used with most normal C operations (i.e., a vector addition can _not_
be represented by a normal addition of two vector type variables).  You
can declare only variables and use them in function calls and returns,
as well as in assignments and some casts.  It is possible to cast from
one vector type to another, provided they are of the same size (in
fact, you can also cast vectors to and from other datatypes of the same
size).

   A port that supports vector operations provides a set of built-in
functions that can be used to operate on vectors.  For example, a
function to add two vectors and multiply the result by a third could
look like this:

     v4si f (v4si a, v4si b, v4si c)
     {
       v4si tmp = __builtin_addv4si (a, b);
       return __builtin_mulv4si (tmp, c);
     }

