Intel 386 OptionsContents|Index|Previous|Next Intel
386 options The following ‘-m’
options are defined for Intel’s 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
(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.
-march=cpu type
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.
-m486
-m386
-mpentium
-mpentiumpro
Control whether or not code
is optimized for -mcpu=i386,
-mcpu=i486,
-mcpu=pentium,
and -mcpu=pentiumpro,
respectively.
-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 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.
-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.
