These -m
options are defined for the i386 and x86-64 family of
computers:
-mcpu=
cpu-type
i386
, i486
, i586
, i686
,
pentium
, pentium-mmx
, pentiumpro
, pentium2
,
pentium3
, pentium4
, k6
, k6-2
, k6-3
,
athlon
, athlon-tbird
, athlon-4
, athlon-xp
,
athlon-mp
, winchip-c6
, winchip2
and c3
.
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
-mcpu
. Moreover,
specifying -march=
cpu-type
implies -mcpu=
cpu-type
.
-m386
-m486
-mpentium
-mpentiumpro
-mcpu=i386
, -mcpu=i486
,
-mcpu=pentium
, and -mcpu=pentiumpro
respectively.
These synonyms are deprecated.
-mfpmath=
unit
387
-ffloat-store
for more detailed description.
This is the default choice for i386 compiler.
sse
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
-masm=
dialect
intel
or att
(the default one).
-mieee-fp
-mno-ieee-fp
-msoft-float
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
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
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
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
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
long double
to 96 bits as required by the i386
application binary interface. This is the default.
-msvr3-shlib
-mno-svr3-shlib
bss
or data
segments. -msvr3-shlib
places them
into bss
. These options are meaningful only on System V Release 3.
-mrtd
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 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
regparm
.
See 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
-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
See 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
-maccumulate-outgoing-args
-mno-push-args
.
-mthreads
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
-minline-all-stringops
-momit-leaf-frame-pointer
-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
-mno-red-zone
-mno-red-zone
disables this red zone.
-mcmodel=small
-mcmodel=kernel
-mcmodel=medium
-mcmodel=large