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

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Message-ID: <CAHk-=wg34q=Mp7abB1zgL1GnXJ9Z_dOoZiBxqyXRtHipCpuyiQ@mail.gmail.com>
Date: Mon, 28 Apr 2025 19:22:59 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Andrew Cooper <andrew.cooper3@...rix.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 Mon, 28 Apr 2025 at 19:00, H. Peter Anvin <hpa@...or.com> wrote:
>
> Now, question: can we just use __builtin_*() for these? I think gcc should always generate inline code for these on x86.

Yeah, I think we can just use __builtin_ffs() directly and get rid of
all the games.

Not all gcc builtins are great: I did a bugzilla about gcc ctzl some time ago:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106471

but for ffs() that does sound like it's the simplest thing to do.

               Linus