Messages in this thread | | | Subject | Re: [PATCH] crypto: x86/twofish-3way - Fix %rbp usage | From | Juergen Gross <> | Date | Tue, 19 Dec 2017 09:04:56 +0100 |
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On 19/12/17 08:54, Ingo Molnar 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.
Exchanging 2 registers can be done without memory access via:
xor reg1, reg2 xor reg2, reg1 xor reg1, reg2
Juergen
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