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

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

On April 28, 2025 5:12:13 PM PDT, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
wrote:
>On 28/04/2025 10:38 pm, H. Peter Anvin wrote:
>> 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.
>
>https://www.sandpile.org/x86/flags.htm
>
>That's a lot of "can't even characterise the result" in the P5.
>
>Looking at P4 column, that is clearly what the latest SDM has
>retroactively declared to be architectural.
>
>~Andrew

Yes, but it wasn't about flags here. 

Now, question: can we just use __builtin_*() for these? I think gcc should 
always generate inline code for these on x86.