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

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Message-ID: <565ac06a-005f-4b77-930e-d63285c025b9@citrix.com>
Date: Tue, 29 Apr 2025 22:59:52 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 "H. Peter Anvin" <hpa@...or.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 29/04/2025 10:53 pm, Linus Torvalds wrote:
> I just did
>
>     gcc -m32 -O2 -S -fomit-frame-pointer t.c
>
> (with, and without that -m32) and looked at the result to see that it
> looks sane. No *actual* testing.

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.

~Andrew