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

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Message-ID: <20180726114537.GA12408@gmail.com>
Date:   Thu, 26 Jul 2018 13:45:37 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Henrique de Moraes Holschuh <hmh@....eng.br>,
        Jan Beulich <JBeulich@...e.com>, 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


* Pavel Machek <pavel@....cz> wrote:

> On Tue 2018-06-26 08:38:22, Henrique de Moraes Holschuh wrote:
> > 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.
> 
> Actually, I believe that should be comment in code.

Agreed - mind sending a patch that adds it?

> But Ingo (?) told me everyone knows about this quirk...

I was wrong.

Thanks,

	Ingo