Messages in this thread | | | Date | Tue, 19 Dec 2017 18:35:33 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] crypto: x86/twofish-3way - Fix %rbp usage |
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* Ingo Molnar <mingo@kernel.org> wrote:
> > * Eric Biggers <ebiggers3@gmail.com> wrote: > > > There may be a small overhead caused by replacing 'xchg REG, REG' with > > the needed sequence 'mov MEM, REG; mov REG, MEM; mov REG, REG' once per > > round. But, counterintuitively, when I tested "ctr-twofish-3way" on a > > Haswell processor, the new version was actually about 2% faster. > > (Perhaps 'xchg' is not as well optimized as plain moves.) > > XCHG has implicit LOCK semantics on all x86 CPUs, so that's not a surprising > result I think.
Correction: I think XCHG only implies LOCK if there's a memory operand involved - register-register XCHG should not imply any barriers.
So the result is indeed unintuitive.
Thanks,
Ingo
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