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

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Message-ID: <20180726181750.GA4404@amd>
Date:   Thu, 26 Jul 2018 20:17:50 +0200
From:   Pavel Machek <pavel@....cz>
To:     Ingo Molnar <mingo@...nel.org>
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

On Thu 2018-07-26 13:45:37, Ingo Molnar wrote:
> 
> * 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?

Ok. Would /* write to low 32 bits clears high 32 bits, too */ be
reasonable comment?

Thanks,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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