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

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Message-ID: <CAHk-=wgJfWfWa2NTiTmev+Xr=e8Uo=aFkrXujLAQBVAVN-VigQ@mail.gmail.com>
Date: Mon, 28 Apr 2025 09:14:45 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>, 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, 
	"H. Peter Anvin" <hpa@...or.com>, 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 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