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

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Message-ID: <D843E4F2-0983-4D67-B346-A77134608196@zytor.com>
Date: Tue, 29 Apr 2025 15:10:29 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
CC: Ingo Molnar <mingo@...nel.org>, 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 29, 2025 3:04:30 PM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Tue, 29 Apr 2025 at 14:59, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>>
>> do_variable_ffs() doesn't quite work.
>>
>> REP BSF is LZCNT, and unconditionally writes it's output operand, and
>> defeats the attempt to preload with -1.
>>
>> Drop the REP prefix, and it should work as intended.
>
>Bah. That's what I get for just doing it blindly without actually
>looking at the kernel source. I just copied the __ffs() thing - and
>there the 'rep' is not for the zero case - which we don't care about -
>but because lzcnt performs better on newer CPUs.
>
>So you're obviously right.
>
>            Linus

Yeah, the encoding of lzcnt was a real mistake, because the outputs are different (so you still need instruction-specific postprocessing.)