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Intel
386 Options
The following ‘-m’
options are defined for Intel’s i386 family of computers.
-m486
-m386
Control whether
or not code is optimized for a 486 instead of an 386. Code generated for
a 486 will run on a 386 and vice versa.
-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 GNU CC. 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
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
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.
-mno-wide-multiply
-mwide-multiply
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.
-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 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 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.
-malign-jumps=num
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.
-malign-functions=num
Align the
start of functions to a 2 raised to 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.