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Re: [PATCH] x86/bitops: Improve code generation for __scanbit()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 9 Nov 2026 09:46:51 +0100
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- Cc: Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 09 Nov 2026 08:46:56 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 06.11.2026 18:12, Andrew Cooper wrote:
> There's no need for an alternative. Use the same technique as arch_ffs() by
> pre-loading the destination register using the caller chosen "zero case"
> instead of -1.
>
> No functional change.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
> CC: Teddy Astie <teddy.astie@xxxxxxxxxx>
>
> Slightly RFC. I don't particualrly like forward-declaring ffsl(), but I don't
> see another option while maintaining a non-tangled common/arch header split.
Forward-declaring isn't a problem by itself, just that imo that wants putting
in xen/bitops.h, not ...
> --- a/xen/arch/x86/include/asm/bitops.h
> +++ b/xen/arch/x86/include/asm/bitops.h
> @@ -320,18 +320,35 @@ extern unsigned int __find_first_zero_bit(
> extern unsigned int __find_next_zero_bit(
> const unsigned long *addr, unsigned int size, unsigned int offset);
>
> +static attr_const unsigned int ffsl(unsigned long x);
... here. asm/bitops.h already isn't included at the very top of the file, so
putting the decl there looks clean enough to me.
(Another alternative would be to see about doing away with __scanbit().)
With the decl moved:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Jan
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