Using and Porting the GNU Compiler Collection (GCC)
These `-m
' options are defined for the i386 family of computers:
-mcpu=cpu type
`i386
' @tab `i486
' @tab `i586
' @tab `i686
'
`pentium
' @tab `pentiumpro
' @tab `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
-mcpu
'. Moreover,
specifying `-march=cpu type
' implies `-mcpu=cpu type
'.
-m386
-m486
-mpentium
-mpentiumpro
-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.
As of revision 2.6.1, these instructions are not generated unless you
also use the `-ffast-math
' 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.
-msvr3-shlib
-mno-svr3-shlib
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
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
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.)
-mreg-alloc=regs
a
allocate EAX; b
allocate EBX;
c
allocate ECX; d
allocate EDX; S
allocate ESI;
D
allocate EDI; B
allocate EBP.
-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.
-malign-loops=num
-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
-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
-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
-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
'.
Using and Porting the GNU Compiler Collection (GCC)
Packaging copyright © 1988-2000 OAR Corporation
Context copyright by each document's author. See Free Software Foundation for information.