* Ingo Molnar <[email protected]> wrote:
>
> * Eric Biggers <[email protected]> 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
