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From: Noah Goldstein <goldstein.w.n@gmail.com>
To: David Laight <David.Laight@aculab.com>
Cc: "tglx@linutronix.de" <tglx@linutronix.de>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"bp@alien8.de" <bp@alien8.de>, "x86@kernel.org" <x86@kernel.org>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"luto@kernel.org" <luto@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4] arch/x86: Improve 'rep movs{b|q}' usage in memmove_64.S
Date: Wed, 17 Nov 2021 16:45:13 -0600	[thread overview]
Message-ID: <CAFUsyfJTuFjVXHMgYi0uggVNW=1WW1uVYa7avVjW5VBb2cmAkQ@mail.gmail.com> (raw)
In-Reply-To: <bc0297a1b97a4b129fa3ea1b155f6062@AcuMS.aculab.com>

On Wed, Nov 17, 2021 at 4:31 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Noah Goldstein
> > Sent: 17 November 2021 21:03
> >
> > Add check for "short distance movsb" for forwards FSRM usage and
> > entirely remove backwards 'rep movsq'. Both of these usages hit "slow
> > modes" that are an order of magnitude slower than usual.
> >
> > 'rep movsb' has some noticeable VERY slow modes that the current
> > implementation is either 1) not checking for or 2) intentionally
> > using.
>
> How does this relate to the decision that glibc made a few years
> ago to use backwards 'rep movs' for non-overlapping copies?

GLIBC doesn't use backwards `rep movs`.  Since the regions are
non-overlapping it just uses forward copy. Backwards `rep movs` is
from setting the direction flag (`std`) and is a very slow byte
copy. For overlapping regions where backwards copy is necessary GLIBC
uses 4x vec copy loop.

>
>
> Did they find a different corner case??
>
>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>

  reply	other threads:[~2021-11-17 22:45 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-01  4:49 [PATCH v1] " Noah Goldstein
2021-11-01 18:59 ` [PATCH v2] " Noah Goldstein
2021-11-02 23:15 ` [PATCH v3] " Noah Goldstein
2021-11-17 21:02 ` [PATCH v4] " Noah Goldstein
2021-11-17 22:31   ` David Laight
2021-11-17 22:45     ` Noah Goldstein [this message]
2021-11-19 22:31       ` David Laight
2021-11-20  0:05         ` Noah Goldstein
2021-12-10 18:35           ` Noah Goldstein
2022-01-12  3:13             ` Noah Goldstein
2022-02-10  9:08               ` Noah Goldstein
2022-03-16 14:15                 ` Noah Goldstein
2022-03-16 16:23   ` Borislav Petkov

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