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Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C
- To: "H. Peter Anvin" <hpa@xxxxxxxxx>
- From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 28 Apr 2025 19:22:59 -0700
- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxx>, Arnd Bergmann <arnd@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, x86@xxxxxxxxxx, Juergen Gross <jgross@xxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Alexander Usyskin <alexander.usyskin@xxxxxxxxx>, Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>, Mateusz Jończyk <mat.jonczyk@xxxxx>, Mike Rapoport <rppt@xxxxxxxxxx>, Ard Biesheuvel <ardb@xxxxxxxxxx>, Peter Zijlstra <peterz@xxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 29 Apr 2025 02:28:42 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Mon, 28 Apr 2025 at 19:00, H. Peter Anvin <hpa@xxxxxxxxx> 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
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