Re: [discuss] [PATCH] x86-64: memset optimization

[Jan Hubicka]

>  > > The problem is with the optimization flags: passing -Os causes the compiler
>  > > to be stupid and not inline any memset/memcpy functions.
>  > 
>  > you get what you ask for.. if you don't want that then don't ask for
>  > it ;)
> 
> Well, the compiler is really being dumb about -Os and in fact it's
> giving bigger code, so I'm not really getting what I ask for.  
> 
> With my gcc at least (x86_64, gcc (GCC) 4.1.3 20070812 (prerelease)
> (Ubuntu 4.1.2-15ubuntu2)) and Andi's example:
> 
> #include <string.h>
> 
> f(char x[6]) {
>         memset(x, 1, 6);
> }
> 
> compiling with -O2 gives
> 
> 0000000000000000 <f>:
>    0:	c7 07 01 01 01 01    	movl   $0x1010101,(%rdi)
>    6:	66 c7 47 04 01 01    	movw   $0x101,0x4(%rdi)
>    c:	c3                   	retq   

GCC mainline (ie future GCC4.3.0) now give:
0000000000000000 <f>:
   0:   b0 01                   mov    $0x1,%al
   2:   b9 06 00 00 00          mov    $0x6,%ecx
   7:   f3 aa                   rep stos %al,%es:(%rdi)
   9:   c3                      retq 
That is smallest, definitly not fastest.
GCC up to 4.3.0 won't be able to inline memset with non-0 operand...

Honza
> 
> and compiling with -Os gives
> 
> 0000000000000000 <f>:
>    0:	48 83 ec 08          	sub    $0x8,%rsp
>    4:	ba 06 00 00 00       	mov    $0x6,%edx
>    9:	be 01 00 00 00       	mov    $0x1,%esi
>    e:	e8 00 00 00 00       	callq  13 <f+0x13>
>   13:	5a                   	pop    %rdx
>   14:	c3                   	retq   
> 
> so the code gets bigger and worse in every way.
> 
>  - R.
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