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

[Juergen Gross]

On 19/12/17 08:54, Ingo Molnar 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.

Exchanging 2 registers can be done without memory access via:

xor reg1, reg2
xor reg2, reg1
xor reg1, reg2


Juergen