linux-kernel - Re: [PATCH] x86-64: use 32-bit XOR to zero registers

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Message-ID: <20180626113822.ch3erlyud5wsxvvg@khazad-dum.debian.net>
Date:   Tue, 26 Jun 2018 08:38:22 -0300
From:   Henrique de Moraes Holschuh <hmh@....eng.br>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     mingo@...e.hu, rdunlap@...radead.org, tglx@...utronix.de,
        hpa@...or.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86-64: use 32-bit XOR to zero registers

On Tue, 26 Jun 2018, Jan Beulich wrote:
> >>> On 25.06.18 at 18:33, <rdunlap@...radead.org> wrote:
> > On 06/25/2018 03:25 AM, Jan Beulich wrote:
> >> Some Intel CPUs don't recognize 64-bit XORs as zeroing idioms - use
> >> 32-bit ones instead.
> > 
> > Hmph.  Is that considered a bug (errata)?
> 
> No.
> 
> > URL/references?
> 
> Intel's Optimization Reference Manual says so (in rev 040 this is in section
> 16.2.2.5 "Zeroing Idioms" as a subsection of the Goldmont/Silvermont
> descriptions).
> 
> > Are these changes really only zeroing the lower 32 bits of the register?
> > and that's all that the code cares about?
> 
> No - like all operations targeting a 32-bit register, the result is zero
> extended to the entire 64-bit destination register.

Missing information that would have been helpful in the commit message:

When the processor can recognize something as a zeroing idiom, it
optimizes that operation on the front-end.  Only 32-bit XOR r,r is
documented as a zeroing idiom according to the Intel optimization
manual.  While a few Intel processors recognize the 64-bit version of
XOR r,r as a zeroing idiom, many won't.

Note that the 32-bit operation extends to the high part of the 64-bit
register, so it will zero the entire 64-bit register.  The 32-bit
instruction is also one byte shorter.

The last sentence is just a reminder, for completeness...

-- 
  Henrique Holschuh