Messages in this thread | | | Date | Thu, 26 May 2022 13:42:35 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH 1/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG_LOOP macro |
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On Thu, May 26, 2022 at 10:14:59PM +1000, Michael Ellerman wrote: > Linus Torvalds <torvalds@linux-foundation.org> writes: > > On Wed, May 25, 2022 at 7:40 AM Uros Bizjak <ubizjak@gmail.com> wrote: > >> > >> Use try_cmpxchg64 instead of cmpxchg64 in CMPXCHG_LOOP macro. > >> x86 CMPXCHG instruction returns success in ZF flag, so this > >> change saves a compare after cmpxchg (and related move instruction > >> in front of cmpxchg). The main loop of lockref_get improves from: > > > > Ack on this one regardless of the 32-bit x86 question. > > > > HOWEVER. > > > > I'd like other architectures to pipe up too, because I think right now > > x86 is the only one that implements that "arch_try_cmpxchg()" family > > of operations natively, and I think the generic fallback for when it > > is missing might be kind of nasty. > > > > Maybe it ends up generating ok code, but it's also possible that it > > just didn't matter when it was only used in one place in the > > scheduler. > > This patch seems to generate slightly *better* code on powerpc. > > I see one register-to-register move that gets shifted slightly later, so > that it's skipped on the path that returns directly via the SUCCESS > case.
FWIW, I see the same on arm64; a register-to-register move gets moved out of the success path. That changes the register allocation, and resulting in one fewer move, but otherwise the code generation is the same.
Thanks, Mark.
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