These `-m' options are defined for the i386 family of computers:
-mcpu=cpu type
Assume the defaults for the machine type cpu type when scheduling
instructions. The choices for cpu type are:
`i386'
`i486'
`i586'
`i686'
`pentium'
`pentiumpro'
`k6'
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' is the AMD chip 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
Synonyms for -mcpu=i386, -mcpu=i486, -mcpu=pentium, and -mcpu=pentiumpro
respectively. These synonyms are deprecated.
-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.
As of revision 2.6.1, these instructions are not generated unless you
also use the `-ffast-math' 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.
-msvr3-shlib
-mno-svr3-shlib
Control whether GCC places uninitialized locals into bss or
data. `-msvr3-shlib' places these locals into bss.
These options are meaningful only on System V Release 3.
-mno-wide-multiply
-mwide-multiply
Control whether GCC uses the mul and imul that produce
64 bit results in eax:edx from 32 bit operands to do long
long multiplies and 32-bit division by constants.
-mrtd
Use a different function-calling convention, in which functions that
take a fixed number of arguments return with the retnum
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 section 4.23 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 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.)
-mreg-alloc=regs
Control the default allocation order of integer registers. The
string regs is a series of letters specifying a register. The
supported letters are: a allocate EAX; b allocate EBX;
c allocate ECX; d allocate EDX; S allocate ESI;
D allocate EDI; B allocate EBP.
-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 section 4.23 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.
-malign-loops=num
Align loops to a 2 raised to a num byte boundary. If
`-malign-loops' is not specified, the default is 2 unless
gas 2.8 (or later) is being used in which case the default is
to align the loop on a 16 byte boundary if it is less than 8
bytes away.
-malign-jumps=num
Align instructions that are only jumped to to a 2 raised to a num
byte boundary. If `-malign-jumps' is not specified, the default is
2 if optimizing for a 386, and 4 if optimizing for a 486 unless
gas 2.8 (or later) is being used in which case the default is
to align the instruction on a 16 byte boundary if it is less
than 8 bytes away.
-malign-functions=num
Align the start of functions to a 2 raised to num byte boundary.
If `-malign-functions' is not specified, the default is 2 if optimizing
for a 386, and 4 if optimizing for a 486.
-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).
The stack is required to be aligned on a 4 byte boundary. 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 Extention (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. 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'.