The flag -mmmx ( like -msse -m3dnow etc...) activate
some extensions for C language.
See this:
http://gcc.gnu.org/onlinedocs/gcc-3.3.3/gcc/X86-Built-in-Functions.html#X86%20Built-in%20Functions
So, those flag activate these built-in functions.
--- Paul Ingelbrant <pi@...> ha scritto: >
>
>
> I've been thinking about CFLAGS such as -mmmx...
> Does gcc actually have the
> ability to actually produce mmx instructions?
>
>
> for example:
>
>
> mmx.c:
>
> #include <stdio.h>
>
>
> main()
> {
> short x[100000];
> int i;
>
> for (i=0; i < 100000; i++)
> {
> x[i] += 5;
>
> }
>
>
> }
>
> gcc -mmmx -O3 --save-temps -funroll-all-loops mmx.c
> gives
>
>
> .file "mmx.c"
> .def ___main; .scl 2; .type 32; .endef
> .text
> .align 2
> .p2align 4,,15
> .globl _main
> .def _main; .scl 2; .type 32; .endef
> _main:
> pushl %ebp
> movl $200008, %eax
> movl %esp, %ebp
> call __alloca
> xorl %eax, %eax
> andl $-16, %esp
> call __alloca
> call ___main
> xorl %eax, %eax
> .p2align 4,,7
> L6:
> addw $5, -200008(%ebp,%eax,2)
> addw $5, -200006(%ebp,%eax,2)
> addw $5, -200004(%ebp,%eax,2)
> addw $5, -200002(%ebp,%eax,2)
> addw $5, -200000(%ebp,%eax,2)
> addw $5, -199998(%ebp,%eax,2)
> addw $5, -199996(%ebp,%eax,2)
> addw $5, -199994(%ebp,%eax,2)
> addw $5, -199992(%ebp,%eax,2)
> addw $5, -199990(%ebp,%eax,2)
> addl $10, %eax
> cmpl $99999, %eax
> jle L6
> leave
> ret
>
>
>
>
> Shouldn't take too much imagination to see how that
> could be improved with mmx...
>
> Likewise:
>
>
>
> fu(int *x)
> {
> if (*x == 5)
> *x = 10;
> }
>
>
>
> with gcc -march=athlon-xp -O3 --save-temps
> -funroll-all-loops mmx.c
>
> gives
>
>
>
> _fu:
> pushl %ebp
> movl %esp, %ebp
> movl 8(%ebp), %eax
> cmpl $5, (%eax)
> je L33
> L32:
> leave
> ret
> .p2align 6,,7
> L33:
> movl $10, (%eax)
> jmp L32
>
>
> the -march=athlon-xp parameter should imply cmov
> instruction...(?)
>
>
> (it doesn't even use muliple leave/ret to eliminate
> an extra jmp...)
>
>
>
> Do the gcc developers include these fancy parameters
> just to give us ricers a
> false sense of speed?
>
>
> any thoughts?
>
>
>
> --
> gentoo-performance@g.o mailing list
>
>
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