From: Ingo Molnar <mingo@kernel.org>
To: Pavel Machek <pavel@ucw.cz>
Cc: Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
Jan Beulich <JBeulich@suse.com>,
mingo@elte.hu, rdunlap@infradead.org, tglx@linutronix.de,
hpa@zytor.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86-64: use 32-bit XOR to zero registers
Date: Thu, 26 Jul 2018 13:45:37 +0200 [thread overview]
Message-ID: <20180726114537.GA12408@gmail.com> (raw)
In-Reply-To: <20180726091916.GA23471@amd>
* Pavel Machek <pavel@ucw.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@infradead.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
next prev parent reply other threads:[~2018-07-26 11:45 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-25 10:25 [PATCH] x86-64: use 32-bit XOR to zero registers Jan Beulich
2018-06-25 16:33 ` Randy Dunlap
2018-06-25 16:49 ` hpa
2018-06-26 6:32 ` Jan Beulich
2018-06-26 11:38 ` Henrique de Moraes Holschuh
2018-07-26 9:19 ` Pavel Machek
2018-07-26 11:45 ` Ingo Molnar [this message]
2018-07-26 18:17 ` Pavel Machek
2018-07-26 19:06 ` Ingo Molnar
2018-06-26 7:17 ` Ingo Molnar
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