From:   David Laight <David.Laight@ACULAB.COM>
To:     'Noah Goldstein' <goldstein.w.n@gmail.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
Thread-Topic: [PATCH v4] arch/x86: Improve 'rep movs{b|q}' usage in
 memmove_64.S
Thread-Index: AQHX2/aBIpegHmlt3EiJPKc4pU+GYawITRxw
Date:   Wed, 17 Nov 2021 22:31:21 +0000
Message-ID: <bc0297a1b97a4b129fa3ea1b155f6062@AcuMS.aculab.com>
References: <20211101044955.2295495-1-goldstein.w.n@gmail.com>
 <20211117210245.843374-1-goldstein.w.n@gmail.com>
In-Reply-To: <20211117210245.843374-1-goldstein.w.n@gmail.com>
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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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