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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 14:40:07 +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 12:40:23 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 24.05.2024 22:03, Andrew Cooper wrote:
> The asm in arch_ffs() is safe but inefficient.
>
> CMOV would be an improvement over a conditional branch, but for 64bit CPUs
> both Intel and AMD have provided enough details about the behaviour for a zero
> input. It is safe to pre-load the destination register with -1 and drop the
> conditional logic.
>
> However, it is common to find ffs() in a context where the optimiser knows
> that x in nonzero even if it the value isn't known precisely, and in that case
> it's safe to drop the preload of -1 too.
>
> There are only a handful of uses of ffs() in the x86 build, and all of them
> improve as a result of this:
>
> add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-31 (-31)
> Function old new delta
> mask_write 114 107 -7
> xmem_pool_alloc 1063 1039 -24
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with one suggestion:
> --- 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 )
__builtin_constant_p(x) surely will do. In fact even the other "> 0" could
in principle be left out here.
Jan
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