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

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Message-ID: <CAHk-=wiAPQFf-RUr=8ra1sYdfGPGLb4QbV8THE9zY3n820L94g@mail.gmail.com>
Date: Tue, 29 Apr 2025 13:12:48 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, 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 Tue, 29 Apr 2025 at 12:13, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>
> That would improve code generation for 32bit, but generally regress 64bit.
>
> Preloading the destination register with -1 is better than the CMOV form
> emitted by the builtin; BSF's habit of conditionally not writing the
> destination register *is* a CMOV of sorts.

Right you are. So we'd need to do this just for the x86-32 case. Oh
well. Ugly, but still prettier than what we have now, I guess.

         Linus