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Date:   Wed, 2 Sep 2020 11:17:41 +0200
From:   peterz@infradead.org
To:     Uros Bizjak <ubizjak@gmail.com>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Karthik Bhargavan <karthikeyan.bhargavan@inria.fr>,
        Chris.Hawblitzel@microsoft.com,
        Jonathan Protzenko <protz@microsoft.com>,
        Aymeric Fromherz <fromherz@cmu.edu>,
        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>
Subject: Re: [PATCH] crypto/x86: Use XORL r32,32 in curve25519-x86_64.c
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On Wed, Sep 02, 2020 at 07:50:36AM +0200, Uros Bizjak wrote:
> On Tue, Sep 1, 2020 at 9:12 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >
> > 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.
> 
> The patch implements one of x86 target specific optimizations,
> performed by gcc. The optimization results in code size savings of one
> byte, where REX prefix is omitted with legacy registers, but otherwise
> should have no measurable runtime effect. Since gcc applies this
> optimization universally to all integer registers, I took the same
> approach and implemented the same change to legacy and REX registers.
> As measured above, 48 bytes saved is a good result for such a trivial
> optimization.

Could we instead implement this optimization in GAS ? Then we can leave
the code as-is.