Re: [RFC,PATCH] Avoid manual shift-and-test logic in AllocSetFreeIndex - Mailing list pgsql-hackers

Hi Atsushi,

> If size <= 8, fls((size - 1) >> ALLOC_MINBITS) is fls(0).
> The result of fls(0) is undefined.

Yep, got caught out by this because my previous fls() supported zero.

> I think we have to never call fls(0) from AllocSetFreeIndex().
> My proposal code:
>
>      if (size > (1 << ALLOC_MINBITS))
>      {
>          idx = fls((size - 1) >> ALLOC_MINBITS);
>          Assert(idx < ALLOCSET_NUM_FREELISTS);
>      }

Looks good, I'll send an updated patch.

Also, are you still seeing the same improvement with the __builtin_clz 
as your inline asm implementation?

Cheers,


Jeremy