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

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Message-ID: <916BD58C-E6A7-495E-9A60-722E130AC7A7@zytor.com>
Date: Mon, 28 Apr 2025 19:00:12 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
CC: Arnd Bergmann <arnd@...db.de>, Arnd Bergmann <arnd@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Alexander Usyskin <alexander.usyskin@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mateusz Jończyk <mat.jonczyk@...pl>,
        Mike Rapoport <rppt@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C

On April 28, 2025 5:12:13 PM PDT, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>On 28/04/2025 10:38 pm, H. Peter Anvin wrote:
>> On April 28, 2025 9:14:45 AM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>>> On Mon, 28 Apr 2025 at 00:05, Ingo Molnar <mingo@...nel.org> 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.