(gcc.info) i386 Options
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Intel 386 Options
-----------------
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 `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'. 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'
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'.
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'
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'.
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