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

From: Ingo Molnar
Date: Tue Dec 19 2017 - 12:36:10 EST

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> * Eric Biggers <ebiggers3@xxxxxxxxx> 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.