Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On April 28, 2025 9:14:45 AM PDT, Linus Torvalds 
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>On Mon, 28 Apr 2025 at 00:05, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>>
>> And once we remove 486, I think we can do the optimization below to
>> just assume the output doesn't get clobbered by BS*L in the zero-case,
>> right?
>
>We probably can't, because who knows what "Pentium" CPU's are out there.
>
>Or even if Pentium really does get it right. I doubt we have any
>developers with an original Pentium around.
>
>So just leave the "we don't know what the CPU result is for zero"
>unless we get some kind of official confirmation.
>
>          Linus

If anyone knows for sure, it is probably Christian Ludloff. However, there was 
a *huge* tightening of the formal ISA when the i686 was introduced (family=6) 
and I really believe this was part of it.

I also really don't trust that family=5 really means conforms to undocumented 
P5 behavior, e.g. for Quark.