Re: [PATCH] crypto/x86: Use XORL r32,32 in curve25519-x86_64.c

[Uros Bizjak <ubizjak@gmail.com>]

On Wed, Sep 2, 2020 at 11:17 AM <peterz@infradead.org> wrote:
>
> 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.

I don't think that e.g. "xorq %rax,%rax" should universally be
translated to "xorl %eax,%eax" in the assembler. Perhaps the author
expected exactly 3 bytes (to align the code or similar), and the
assembler would change the length to 2 bytes behind his back, breaking
the expectations.

Uros.