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SubjectRe: [PATCH] crypto: x86/twofish-3way - Fix %rbp usage

* 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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