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Subject: Re: [PATCH] crypto: x86/twofish-3way - Fix %rbp usage
To: Ingo Molnar <mingo@kernel.org>, Eric Biggers <ebiggers3@gmail.com>
Cc: linux-crypto@vger.kernel.org, Herbert Xu <herbert@gondor.apana.org.au>,
        "David S . Miller" <davem@davemloft.net>,
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        Jussi Kivilinna <jussi.kivilinna@iki.fi>, x86@kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        Eric Biggers <ebiggers@google.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
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 <20171219004026.170565-1-ebiggers3@gmail.com>
 <20171219075443.tdpt2l72eelhpi7j@gmail.com>
From: Juergen Gross <jgross@suse.com>
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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