re: wineport: Use a lookup table to make our slow ffs() a bit faster.

[Dan Kegel]

Do you actually have a system that doesn't have HAVE_FFS set?

On my machine,

int main(int argc, char **argv)
{
   return ffs(argc);
}

compiles to

pushl   %ebp
       movl    %esp, %ebp
       movl    $-1, %edx
       bsfl    8(%ebp), %eax
       cmove   %edx, %eax
       addl    $1, %eax
       popl    %ebp
       ret

which is probably pretty fast.

Large lookup tables can dirty the L1 cache and slow other things down.

See also http://stackoverflow.com/questions/757059/position-of-least-significant-bit-...

wineport: Use a lookup table to make our slow ffs() a bit faster.

[David Laight]

On Fri, Mar 11, 2011 at 05:51:27AM -0800, Dan Kegel wrote:
> Large lookup tables can dirty the L1 cache and slow other things down.

And even small tables can also be a slow data cache miss in the
critical path.
Of course, when benchmarking, the table is all in the cache!
In real life it is likely to be absent (unless you are doing
a lot of lookups together).

David