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

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

* Eric Biggers [off-list ref] wrote:

quoted

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