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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Tue, 1 Sep 2020 21:12:12 +0200
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Subject: Re: [PATCH] crypto/x86: Use XORL r32,32 in curve25519-x86_64.c
To:     Uros Bizjak <ubizjak@gmail.com>,
        Karthik Bhargavan <karthikeyan.bhargavan@inria.fr>,
        Chris.Hawblitzel@microsoft.com,
        Jonathan Protzenko <protz@microsoft.com>,
        Aymeric Fromherz <fromherz@cmu.edu>
Cc:     Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        X86 ML <x86@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        Ard Biesheuvel <ardb@kernel.org>
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On Tue, Sep 1, 2020 at 8:13 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> operands are the same. Also, have you seen any measurable differences
> when benching this? I can stick it into kbench9000 to see if you
> haven't looked yet.

On a Skylake server (Xeon Gold 5120), I'm unable to see any measurable
difference with this, at all, no matter how much I median or mean or
reduce noise by disabling interrupts.

One thing that sticks out is that all the replacements of r8-r15 by
their %r8d-r15d counterparts still have the REX prefix, as is
necessary to access those registers. The only ones worth changing,
then, are the legacy registers, and on a whole, this amounts to only
48 bytes of difference.