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Subject: Re: [PATCH] lib/string: Bring optimized memcmp from glibc
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Dave Chinner <david@fromorbit.com>
References: <20210721135926.602840-1-nborisov@suse.com>
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 <CAHk-=wgMyXh3gGuSzj_Dgw=Gn_XPxGSTPq6Pz7dEyx6JNuAh9g@mail.gmail.com>
From:   Nikolay Borisov <nborisov@suse.com>
Message-ID: <80b4bbe8-2394-cfbb-ace1-9402169f5131@suse.com>
Date:   Thu, 22 Jul 2021 11:28:56 +0300
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On 21.07.21 г. 21:45, Linus Torvalds wrote:
> On Wed, Jul 21, 2021 at 11:17 AM Nikolay Borisov <nborisov@suse.com> wrote:
>>
>> I find it somewhat arbitrary that we choose to align the 2nd pointer and
>> not the first.
> 
> Yeah, that's a bit odd, but I don't think it matters.
> 
> The hope is obviously that they are mutually aligned, and in that case
> it doesn't matter which one you aim to align.
> 
>> So you are saying that the current memcmp could indeed use improvement
>> but you don't want it to be based on the glibc's code due to the ugly
>> misalignment handling?
> 
> Yeah. I suspect that this (very simple) patch gives you the same
> performance improvement that the glibc code does.

You suspect correctly, perf profile:

30.44%    -29.38%  [kernel.vmlinux]         [k] memcmp

This is only on x86-64 as I don't have other arch handy. But this one is
definitely good.