[Bug middle-end/70308] memset generates rep stosl instead of rep stosq

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70308

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Uroš Bizjak from comment #3)
> Minimized testcase:
> 
> --cut here--
> extern int scanf (const char *, ...);
> extern void *memset (void *, int, __SIZE_TYPE__);
> 
> int
> main () {
>   char buf[128];
> 
>   if (scanf("%s", buf) != 1)
>     return 42;
> 
>   memset(buf,0, 128);
>   asm volatile("": : :"memory");
>   return 0;
> }
> --cut here--
> 
> gcc -O3, generic tuning (-mtune=x86-64):
> 
> .L7:
>         xorl    %eax, %eax
>         movq    %rsp, %rdi
>         movl    $32, %ecx
>         rep; stosl
>         xorl    %edx, %edx
>         jmp     .L1
> 
> Changing the name of "main" to e.g "foo":
> 
> .L7:
>         xorl    %eax, %eax
>         movq    %rsp, %rdi
>         movl    $16, %ecx
>         rep; stosq
>         xorl    %edx, %edx
>         jmp     .L1
> 
> Interesting...

That's probably because for main() we can predict the call to be cold
(main is called exactly once).