linux-kernel - RE: [PATCH v4] arch/x86: Improve 'rep movs{b|q}' usage in memmove

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Date:   Wed, 17 Nov 2021 22:31:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Noah Goldstein' <goldstein.w.n@...il.com>
CC:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v4] arch/x86: Improve 'rep movs{b|q}' usage in
 memmove_64.S

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?

Did they find a different corner case??

	David

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