These ‘-m’ options are defined for the x86 family of computers.
-march=cpu-type ¶Generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type.
The choices for cpu-type are:
This selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set.
Original Intel i386 CPU.
Intel i486 CPU. (No scheduling is implemented for this chip.)
Intel Pentium CPU with no MMX support.
Intel Lakemont MCU, based on Intel Pentium CPU.
Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
Intel Pentium Pro CPU.
When used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with -mtune, it has the same meaning as ‘generic’.
Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set support.
Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE instruction set support.
Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks.
Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.
Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support.
Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support.
Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support.
Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and PCLMUL instruction set support.
Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and F16C instruction set support.
Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX and PREFETCHW instruction set support.
Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and XSAVES instruction set support.
Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL and RDRND instruction set support.
Intel Knight’s Landing CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support.
Intel Skylake Server CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support.
AMD K6 CPU with MMX instruction set support.
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.
Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.)
Improved versions of AMD K8 cores with SSE3 instruction set support.
CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCL_MUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.
AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCL_MUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.
CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)
CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCL_MUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.
IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.
VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
AMD Geode embedded processor with MMX and 3DNow! instruction set support.
-mtune=cpu-type ¶Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. While picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type unless you use a -march=cpu-type option. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines.
The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:
Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But, if you do not know exactly what CPU users of your application will have, then you should use this option.
As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released.
There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized.
Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option.
As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released.
There is no -march=intel option because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized.
-mcpu=cpu-type ¶A deprecated synonym for -mtune.
-mfpmath=unit ¶Generate floating-point arithmetic for selected unit unit. The choices for unit are:
Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the 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 x86-32 targets.
Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too.
For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default.
The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits.
This is the default choice for the x86-64 compiler.
Attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance.
-masm=dialect ¶Output assembly instructions using selected dialect. Also affects
which dialect is used for basic asm (see Basic Asm — Assembler Instructions Without Operands) and
extended asm (see Extended Asm - Assembler Instructions with C Expression Operands). Supported choices (in dialect
order) are ‘att’ or ‘intel’. The default is ‘att’. Darwin does
not support ‘intel’.
-mieee-fp ¶-mno-ieee-fpControl whether or not the compiler uses IEEE floating-point comparisons. These correctly handle 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
OpenBSD and NetBSD. This option is overridden when -march
indicates that the target CPU always has an FPU and so the
instruction does not need emulation. These
instructions are not generated unless you also use the
-funsafe-math-optimizations switch.
-malign-double ¶-mno-align-doubleControl 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
produces code that runs somewhat faster on a Pentium at the
expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch.
-m96bit-long-double ¶-m128bit-long-doubleThese switches control the size of long double type. The x86-32
application binary interface specifies the size to be 96 bits,
so -m96bit-long-double is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer long double
to be aligned to an 8- or 16-byte boundary. In arrays or structures
conforming to the ABI, this is not possible. So specifying
-m128bit-long-double aligns long double
to a 16-byte boundary by padding the long double with an additional
32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as
its ABI specifies that long double is aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87
standard of 80 bits for a long double.
Warning: if you override the default value for your target ABI, this
changes the size of
structures and arrays containing long double variables,
as well as modifying the function calling convention for functions taking
long double. Hence they are not binary-compatible
with code compiled without that switch.
-mlong-double-64 ¶-mlong-double-80-mlong-double-128These switches control the size of long double type. A size
of 64 bits makes the long double type equivalent to the double
type. This is the default for 32-bit Bionic C library. A size
of 128 bits makes the long double type equivalent to the
__float128 type. This is the default for 64-bit Bionic C library.
Warning: if you override the default value for your target ABI, this
changes the size of
structures and arrays containing long double variables,
as well as modifying the function calling convention for functions taking
long double. Hence they are not binary-compatible
with code compiled without that switch.
-malign-data=type ¶Control how GCC aligns variables. Supported values for type are ‘compat’ uses increased alignment value compatible uses GCC 4.8 and earlier, ‘abi’ uses alignment value as specified by the psABI, and ‘cacheline’ uses increased alignment value to match the cache line size. ‘compat’ is the default.
-mlarge-data-threshold=threshold ¶When -mcmodel=medium is specified, data objects larger than threshold are placed in the large data section. This value must be the same across all objects linked into the binary, and defaults to 65535.
-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. See Declaring Attributes of Functions.
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 is generated for calls to those
functions.
In addition, seriously incorrect code results 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.
See Declaring Attributes of Functions.
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.
-msseregparm ¶Use SSE register passing conventions for float and double arguments
and return values. You can control this behavior for a specific
function by using the function attribute sseregparm.
See Declaring Attributes of Functions.
Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.
-mvect8-ret-in-mem ¶Return 8-byte vectors in memory instead of MMX registers. This is the default on Solaris 8 and 9 and VxWorks to match the ABI of the Sun Studio compilers until version 12. Later compiler versions (starting with Studio 12 Update 1) follow the ABI used by other x86 targets, which is the default on Solaris 10 and later. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC.
-mpc32 ¶-mpc64-mpc80Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called “catastrophic cancellation”, when this option is used to set the precision to less than extended precision.
-mstackrealign ¶Realign the stack at entry. On the x86, the -mstackrealign
option generates an alternate prologue and epilogue that realigns the
run-time stack if necessary. This supports mixing legacy codes that keep
4-byte stack alignment with modern codes that keep 16-byte stack alignment for
SSE compatibility. See also the attribute force_align_arg_pointer,
applicable to individual functions.
-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).
Warning: When generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules.
-mincoming-stack-boundary=num ¶Assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.
On Pentium and Pentium Pro, 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 may not work
properly 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 most likely misaligns 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 ¶-msse-msse2-msse3-mssse3-msse4-msse4a-msse4.1-msse4.2-mavx-mavx2-mavx512f-mavx512pf-mavx512er-mavx512cd-mavx512vl-mavx512bw-mavx512dq-mavx512ifma-mavx512vbmi-msha-maes-mpclmul-mclfushopt-mfsgsbase-mrdrnd-mf16c-mfma-mfma4-mprefetchwt1-mxop-mlwp-m3dnow-mpopcnt-mabm-mbmi-mbmi2-mlzcnt-mfxsr-mxsave-mxsaveopt-mxsavec-mxsaves-mrtm-mtbm-mmpx-mmwaitx-mclzero-mpkuThese switches enable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, AVX, AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD, SHA, AES, PCLMUL, FSGSBASE, RDRND, F16C, FMA, SSE4A, FMA4, XOP, LWP, ABM, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA AVX512VBMI, BMI, BMI2, FXSR, XSAVE, XSAVEOPT, LZCNT, RTM, MPX, MWAITX, PKU or 3DNow! extended instruction sets. Each has a corresponding -mno- option to disable use of these instructions.
These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches.
To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed.
These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options.
-mdump-tune-features ¶This option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in -mtune-ctrl=feature-list.
-mtune-ctrl=feature-list ¶This option is used to do fine grain control of x86 code generation features. feature-list is a comma separated list of feature names. See also -mdump-tune-features. When specified, the feature is turned on if it is not preceded with ‘^’, otherwise, it is turned off. -mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors.
-mno-default ¶This option instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features.
-mcld ¶This option instructs GCC to emit a cld instruction in the prologue
of functions that use string instructions. String instructions depend on
the DF flag to select between autoincrement or autodecrement mode. While the
ABI specifies the DF flag to be cleared on function entry, some operating
systems violate this specification by not clearing the DF flag in their
exception dispatchers. The exception handler can be invoked with the DF flag
set, which leads to wrong direction mode when string instructions are used.
This option can be enabled by default on 32-bit x86 targets by configuring
GCC with the --enable-cld configure option. Generation of cld
instructions can be suppressed with the -mno-cld compiler option
in this case.
-mvzeroupper ¶This option instructs GCC to emit a vzeroupper instruction
before a transfer of control flow out of the function to minimize
the AVX to SSE transition penalty as well as remove unnecessary zeroupper
intrinsics.
-mprefer-avx128 ¶This option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.
-mcx16 ¶This option enables GCC to generate CMPXCHG16B instructions.
CMPXCHG16B allows for atomic operations on 128-bit double quadword
(or oword) data types.
This is useful for high-resolution counters that can be updated
by multiple processors (or cores). This instruction is generated as part of
atomic built-in functions: see Legacy __sync Built-in Functions for Atomic Memory Access or
Built-in Functions for Memory Model Aware Atomic Operations for details.
-msahf ¶This option enables generation of SAHF instructions in 64-bit code.
Early Intel Pentium 4 CPUs with Intel 64 support,
prior to the introduction of Pentium 4 G1 step in December 2005,
lacked the LAHF and SAHF instructions
which are supported by AMD64.
These are load and store instructions, respectively, for certain status flags.
In 64-bit mode, the SAHF instruction is used to optimize fmod,
drem, and remainder built-in functions;
see Other Built-in Functions Provided by GCC for details.
-mmovbe ¶This option enables use of the movbe instruction to implement
__builtin_bswap32 and __builtin_bswap64.
-mcrc32 ¶This option enables built-in functions __builtin_ia32_crc32qi,
__builtin_ia32_crc32hi, __builtin_ia32_crc32si and
__builtin_ia32_crc32di to generate the crc32 machine instruction.
-mrecip ¶This option enables use of RCPSS and RSQRTSS instructions
(and their vectorized variants RCPPS and RSQRTPS)
with an additional Newton-Raphson step
to increase precision instead of DIVSS and SQRTSS
(and their vectorized
variants) for single-precision floating-point arguments. These instructions
are generated only when -funsafe-math-optimizations is enabled
together with -ffinite-math-only and -fno-trapping-math.
Note that while the throughput of the sequence is higher than the throughput
of the non-reciprocal instruction, the precision of the sequence can be
decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
Note that GCC implements 1.0f/sqrtf(x) in terms of RSQRTSS
(or RSQRTPS) already with -ffast-math (or the above option
combination), and doesn’t need -mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson step
for vectorized single-float division and vectorized sqrtf(x)
already with -ffast-math (or the above option combination), and
doesn’t need -mrecip.
-mrecip=opt ¶This option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ‘!’ to invert the option:
Enable all estimate instructions.
Enable the default instructions, equivalent to -mrecip.
Disable all estimate instructions, equivalent to -mno-recip.
Enable the approximation for scalar division.
Enable the approximation for vectorized division.
Enable the approximation for scalar square root.
Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root.
-mveclibabi=type ¶Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are ‘svml’ for the Intel short vector math library and ‘acml’ for the AMD math core library. To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time.
GCC currently emits calls to vmldExp2,
vmldLn2, vmldLog102, vmldLog102, vmldPow2,
vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2,
vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2,
vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2,
vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104,
vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4,
vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4,
vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4,
vmlsCos4, vmlsAcosh4 and vmlsAcos4 for corresponding
function type when -mveclibabi=svml is used, and __vrd2_sin,
__vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2,
__vrd2_log10, __vrs4_sinf, __vrs4_cosf,
__vrs4_expf, __vrs4_logf, __vrs4_log2f,
__vrs4_log10f and __vrs4_powf for the corresponding function type
when -mveclibabi=acml is used.
-mabi=name ¶Generate code for the specified calling convention. Permissible values
are ‘sysv’ for the ABI used on GNU/Linux and other systems, and
‘ms’ for the Microsoft ABI. The default is to use the Microsoft
ABI when targeting Microsoft Windows and the SysV ABI on all other systems.
You can control this behavior for specific functions by
using the function attributes ms_abi and sysv_abi.
See Declaring Attributes of Functions.
-mtls-dialect=type ¶Generate code to access thread-local storage using the ‘gnu’ or ‘gnu2’ conventions. ‘gnu’ is the conservative default; ‘gnu2’ is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems.
-mpush-args ¶-mno-push-argsUse 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 is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the 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 MinGW. Programs that rely 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.
-mms-bitfields ¶-mno-ms-bitfieldsEnable/disable bit-field layout compatible with the native Microsoft Windows compiler.
If packed is used on a structure, or if bit-fields are used,
it may be that the Microsoft ABI lays out the structure differently
than the way GCC normally does. Particularly when moving packed
data between functions compiled with GCC and the native Microsoft compiler
(either via function call or as data in a file), it may be necessary to access
either format.
This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by use of variable or type attributes. For more information, see x86 Variable Attributes and x86 Type Attributes.
The Microsoft structure layout algorithm is fairly simple with the exception of the bit-field packing. The padding and alignment of members of structures and whether a bit-field can straddle a storage-unit boundary are determine by these rules:
aligned attribute or the pack pragma),
whichever is less. For structures, unions, and arrays,
the alignment requirement is the largest alignment requirement of its members.
Every object is allocated an offset so that:
offset % alignment_requirement == 0
MSVC interprets zero-length bit-fields in the following ways:
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;
The size of t1 is 8 bytes with the zero-length bit-field. If the
zero-length bit-field were removed, t1’s size would be 4 bytes.
foo, and the
alignment of the zero-length bit-field is greater than the member that follows it,
bar, bar is aligned as the type of the zero-length bit-field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;
For t2, bar is placed at offset 2, rather than offset 1.
Accordingly, the size of t2 is 4. For t3, the zero-length
bit-field does not affect the alignment of bar or, as a result, the size
of the structure.
Taking this into account, it is important to note the following:
t2 has a size of 4 bytes, since the zero-length bit-field follows a
normal bit-field, and is of type short.
struct
{
char foo : 6;
long : 0;
} t4;
Here, t4 takes up 4 bytes.
struct
{
char foo;
long : 0;
char bar;
} t5;
Here, t5 takes up 2 bytes.
-mno-align-stringops ¶Do not align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn’t know about it.
-minline-all-stringops ¶By default GCC inlines string operations only when the destination is
known to be aligned to least a 4-byte boundary.
This enables more inlining and increases code
size, but may improve performance of code that depends on fast
memcpy, strlen,
and memset for short lengths.
-minline-stringops-dynamically ¶For string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks.
-mstringop-strategy=alg ¶Override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:
Expand using i386 rep prefix of the specified size.
Expand into an inline loop.
Always use a library call.
-mmemcpy-strategy=strategy ¶Override the internal decision heuristic to decide if __builtin_memcpy
should be inlined and what inline algorithm to use when the expected size
of the copy operation is known. strategy
is a comma-separated list of alg:max_size:dest_align triplets.
alg is specified in -mstringop-strategy, max_size specifies
the max byte size with which inline algorithm alg is allowed. For the last
triplet, the max_size must be -1. The max_size of the triplets
in the list must be specified in increasing order. The minimal byte size for
alg is 0 for the first triplet and max_size + 1 of the
preceding range.
-mmemset-strategy=strategy ¶The option is similar to -mmemcpy-strategy= except that it is to control
__builtin_memset expansion.
-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-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder.
-mtls-direct-seg-refs ¶-mno-tls-direct-seg-refsControls whether TLS variables may be accessed with offsets from the
TLS segment register (%gs for 32-bit, %fs for 64-bit),
or whether the thread base pointer must be added. Whether or not this
is valid depends on the operating system, and whether it maps the
segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
-msse2avx ¶-mno-sse2avxSpecify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by default.
-mfentry ¶-mno-fentryIf profiling is active (-pg), put the profiling
counter call before the prologue.
Note: On x86 architectures the attribute ms_hook_prologue
isn’t possible at the moment for -mfentry and -pg.
-mrecord-mcount ¶-mno-record-mcountIf profiling is active (-pg), generate a __mcount_loc section that contains pointers to each profiling call. This is useful for automatically patching and out calls.
-mnop-mcount ¶-mno-nop-mcountIf profiling is active (-pg), generate the calls to the profiling functions as nops. This is useful when they should be patched in later dynamically. This is likely only useful together with -mrecord-mcount.
-mskip-rax-setup ¶-mno-skip-rax-setupWhen generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can be used to skip setting up RAX register when there are no variable arguments passed in vector registers.
Warning: Since RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable arguments, the impacts of this option are callees may waste some stack space, misbehave or jump to a random location. GCC 4.4 or newer don’t have those issues, regardless the RAX register value.
-m8bit-idiv ¶-mno-8bit-idivOn some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide.
-mavx256-split-unaligned-load ¶-mavx256-split-unaligned-storeSplit 32-byte AVX unaligned load and store.
-mstack-protector-guard=guard ¶Generate stack protection code using canary at guard. Supported locations are ‘global’ for global canary or ‘tls’ for per-thread canary in the TLS block (the default). This option has effect only when -fstack-protector or -fstack-protector-all is specified.
-mmitigate-rop ¶Try to avoid generating code sequences that contain unintended return opcodes, to mitigate against certain forms of attack. At the moment, this option is limited in what it can do and should not be relied on to provide serious protection.
These ‘-m’ switches are supported in addition to the above on x86-64 processors in 64-bit environments.
-m32 ¶-m64-mx32-m16-miamcuGenerate code for a 16-bit, 32-bit or 64-bit environment.
The -m32 option sets int, long, and pointer types
to 32 bits, and
generates code that runs on any i386 system.
The -m64 option sets int to 32 bits and long and pointer
types to 64 bits, and generates code for the x86-64 architecture.
For Darwin only the -m64 option also turns off the -fno-pic
and -mdynamic-no-pic options.
The -mx32 option sets int, long, and pointer types
to 32 bits, and
generates code for the x86-64 architecture.
The -m16 option is the same as -m32, except for that
it outputs the .code16gcc assembly directive at the beginning of
the assembly output so that the binary can run in 16-bit mode.
The -miamcu option generates code which conforms to Intel MCU psABI. It requires the -m32 option to be turned on.
-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 is not 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. Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked.
-mcmodel=large ¶Generate code for the large model. This model makes no assumptions about addresses and sizes of sections.
-maddress-mode=long ¶Generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments.
-maddress-mode=short ¶Generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments.