2.14.15 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'. See section 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 section 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 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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