Re: [PATCH] Optimise memset on i386

[Christian Franke <Christian.Franke@t-online.de>]

richardvoigt wrote:
>
> might I suggest:
>
> unsigned long patternl = pattern8;
> patternl |= patternl << 8;
> patternl |= patternl << 16;
> patternl |= patternl << 32;
> patternl |= patternl << 64;
>
> O(lg N) instead of O(N), no loop, no branches, and the compiler should 
> be smart enough to optimize away the last two lines on systems with 
> narrower long.

The latter is unfortunately not the case. At least gcc 4.5.0 prints a 
warning but still produces code.

$ cat <<EOF >f.c
unsigned long f(unsigned long x)
{
   x |= x << 32;
   x |= x << 64;
   return x;
}

$ gcc -O3 -S f.c
x.c: In function ‘f’:
x.c:3: warning: left shift count >= width of type
x.c:4: warning: left shift count >= width of type

$ cat f.s
...
         pushl   %ebp
         movl    $32, %ecx
         movl    %esp, %ebp
         movl    8(%ebp), %eax
         popl    %ebp
         movl    %eax, %edx
         sall    %cl, %edx
         movl    $64, %ecx
         orl     %eax, %edx
         movl    %edx, %eax
         sall    %cl, %eax
         orl     %edx, %eax
         ret


-- 
Regards,
Christian Franke