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Re: [PATCH v2 07/13] x86/bitops: Improve arch_ffs() in the general case
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 27 May 2024 15:27:11 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>, "consulting @ bugseng . com" <consulting@xxxxxxxxxxx>, Simone Ballarin <simone.ballarin@xxxxxxxxxxx>, Federico Serafini <federico.serafini@xxxxxxxxxxx>, Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 27 May 2024 13:27:22 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 24.05.2024 22:03, Andrew Cooper wrote:
> --- a/xen/arch/x86/include/asm/bitops.h
> +++ b/xen/arch/x86/include/asm/bitops.h
> @@ -432,12 +432,28 @@ static inline int ffsl(unsigned long x)
>
> static always_inline unsigned int arch_ffs(unsigned int x)
> {
> - int r;
> + unsigned int r;
> +
> + if ( __builtin_constant_p(x > 0) && x > 0 )
> + {
> + /* Safe, when the compiler knows that x is nonzero. */
> + asm ( "bsf %[val], %[res]"
> + : [res] "=r" (r)
> + : [val] "rm" (x) );
> + }
In patch 11 relevant things are all in a single patch, making it easier
to spot that this is dead code: The sole caller already has a
__builtin_constant_p(), hence I don't see how the one here could ever
return true. With that the respective part of the description is then
questionable, too, I'm afraid: Where did you observe any actual effect
from this? Or if you did - what am I missing?
Jan
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