Intel
386 options
The following -m
options are defined for Intels i386 family of computers.
- -mcpu=cpu type
- -march=cpu type
- -m386
- -mpentium
- -mpentiumpro
- -mieee-fp
- -mno-ieee-fp
- -msoft-float
- Warning:
- Assume the defaults
for the machine type cpu type when scheduling instructions. The choices
for cpu type
are: i386,
i486,
i586
(pentium),
pentium,
i686
(pentiumpro)
and pentiumpro.
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.
- Generate instructions
for the machine type cpu
type. The choices
for cpu type
are: i386, i486, pentium, and pentiumpro. Specifying -march=cpu type implies -mcpu=cpu type.
- Control whether or not code
is optimized for -mcpu=i386,
-mcpu=i486,
-mcpu=pentium,
and -mcpu=pentiumpro,
respectively.
- Control whether or not the
compiler uses IEEE floating point comparisons. These handle correctly the
case where the result of a comparison is unordered.
- Generate output containing
library calls for floating point.
The requisite libraries are not part of GNU CC. Normally the facilities of the machines usual C compiler are used, but this cant 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
- 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
- -malign-double
- -mno-align-double
- Do not use the FPU registers
for return values of functions.
- 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.
- Control whether GNU CC 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
- -mno-wide-multiply
- -mwide-multiply
- -mrtd
- 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.
- 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
- -mregparm=num
- Control whether GNU CC places
uninitialized locals into bss
or data.
-msvr3-shlib places these locals into bss. These options are meaningful only on System V Release 3.
- Control whether GNU CC 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.
- 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.
- 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.
- Control the default allocation
order of integer registers. The string, regs,
is a series of letters specifying a register. The supported letters use
the following allocations: a
allocates EAX; b
allocates EBX; c
allocates ECX; d
allocates EDX; S
allocates ESI; D
allocates EDI; and B
allocates EBP.
- 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.
- -malign-loops=num
- -malign-jumps=num
- -malign-functions=num
- If -malign-jumps is not specified, the default is 2 if optimizing for a 386, and 4 if optimizing for a 486.
- Align loops to a 2 raised
to a num byte boundary. If -malign-loops
is not specified, the default is 2.
- Align instructions that
are only jumped 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.
- Align the start of functions
to a 2 raised to num
byte boundary.
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