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Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 29 Apr 2025 11:05:18 -0700
- Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>, 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 18:05:49 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue, 29 Apr 2025 at 07:38, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> I tried that. (The thread started as a question around
> __builtin_constant_p() but did grow to cover __builtin_ffs().)
Maybe we could do something like
#define ffs(x) \
(statically_true((x) != 0) ? __ffs(x)+1 : __builtin_ffs(x))
which uses our "statically_true()" helper that is actually fairly good
at the whole "let the compiler tell us that it knows that value cannot
be zero"
I didn't check what code that generated, but I've seen gcc do well on
that statically_true() thing in the past.
Then we can just remove our current variable_ffs() thing entirely,
because we now depend on our (good) __ffs() and the builtin being
"good enough" for the bad case.
(And do the same thing for fls() too, of course)
Linus
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