Re: [gentoo-performance] gcc optimizations

[Andrea Pretto]

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@×××××.se> 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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