On Sun, Apr 14, 2024 at 08:46:39PM -0000, tip-bot2 for Uros Bizjak wrote:
> The following commit has been merged into the locking/core branch of tip:
>
> Commit-ID: d26e46f6bf329cfcc469878709baa41d3bfc7cc3
> Gitweb: https://git.kernel.org/tip/d26e46f6bf329cfcc469878709baa41d3bfc7cc3
> Author: Uros Bizjak <ubizjak@xxxxxxxxx>
> AuthorDate: Sun, 14 Apr 2024 18:12:43 +02:00
> Committer: Ingo Molnar <mingo@xxxxxxxxxx>
> CommitterDate: Sun, 14 Apr 2024 22:40:54 +02:00
>
> locking/atomic/x86: Introduce arch_try_cmpxchg64_local()
>
> Introduce arch_try_cmpxchg64_local() for 64-bit and 32-bit targets
> to improve code using cmpxchg64_local(). On 64-bit targets, the
> generated assembly improves from:
>
> 3e28: 31 c0 xor %eax,%eax
> 3e2a: 4d 0f b1 7d 00 cmpxchg %r15,0x0(%r13)
> 3e2f: 48 85 c0 test %rax,%rax
> 3e32: 0f 85 9f 00 00 00 jne 3ed7 <...>
>
> to:
>
> 3e28: 31 c0 xor %eax,%eax
> 3e2a: 4d 0f b1 7d 00 cmpxchg %r15,0x0(%r13)
> 3e2f: 0f 85 9f 00 00 00 jne 3ed4 <...>
>
> where a TEST instruction after CMPXCHG is saved. The improvements
> for 32-bit targets are even more noticeable, because double-word
> compare after CMPXCHG8B gets eliminated.
>
> Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Waiman Long <longman@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/20240414161257.49145-1-ubizjak@xxxxxxxxx
> ---
> arch/x86/include/asm/cmpxchg_32.h | 34 ++++++++++++++++++++++++++++++-
> arch/x86/include/asm/cmpxchg_64.h | 6 +++++-
> 2 files changed, 40 insertions(+)
Ok, maybe I'm missing the point here or maybe the commit message doesn't
explain but how does this big diffstat justify one less insn?
And no, 32-bit doesn't matter.
This looks like too crazy micro-optimization to me to be worth the 40
insertions.
But I could be wrong and I'd gladly read explanations for why I am.
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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