linux-kernel - Re: [PATCH] un-improve strrchr()

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Message-ID: <CACVxJT-XK1yBQroNXa=BKFTkmgDMCsfZAuskAR4WLNdxfYEoKA@mail.gmail.com>
Date:	Wed, 1 Jul 2015 17:07:40 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Chris Rorvick <chris@...vick.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH] un-improve strrchr()

On Wed, Jul 1, 2015 at 2:52 AM, Chris Rorvick <chris@...vick.com> wrote:
> [ resending w/o HTML formatting ]
>
> On Sun, Jun 28, 2015 at 11:44 AM, Alexey Dobriyan <adobriyan@...il.com> wrote:
>> Previous code did 1 branch per character + 1 branch for every character
>> in the last path component. Current code does 2 branches per characher
>> regardless.
>
> Shouldn't that be "+ 2 branches for every character in the last path
> component"?  The structure of the loop is basically the same; you're
> just performing fewer iterations if the character is found when
> searching from the end.

Yes, changelog is inaccurate.

It is "1 branch per character + 2 branches per character in
the last path component" vs "2 branches per character".

Rasmus posted benchmark (obvious rdtsc/strrchr/rdtsc) in private.

Speed highly depends on -O2/-Os setting and current mainline code
is not uniformly faster at least for me. I'll probably resend with new
and improved changelog.
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