linux-kernel - RE: [PATCH 0/2] Optimize memchr()

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Message-ID: <e234b9e1314b48ac9940644616a9757a@AcuMS.aculab.com>
Date:   Mon, 15 Aug 2022 10:59:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Pavel Machek' <pavel@....cz>,
        Yu-Jen Chang <arthurchang09@...il.com>
CC:     "andy@...nel.org" <andy@...nel.org>,
        "akinobu.mita@...il.com" <akinobu.mita@...il.com>,
        "jserv@...s.ncku.edu.tw" <jserv@...s.ncku.edu.tw>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 0/2] Optimize memchr()

From: Pavel Machek
> Sent: 12 August 2022 20:07
> 
> Hi!
> 
> > This patche series optimized "memchr()" and add a macro for
> > "memchr_inv()" so that both funtions can use it to generate bit mask.
> >
> > The original implementaion of "memchr()" is based on byte-wise comparison,
> > which do not fully use 64-bit or 32-bit register in CPU. We implement a
> > word-wise comparison so that at least 4 bytes can be compared at the same
> > time. The optimized "memchr()" is nearly 4x faster than the original one
> > for long strings. In Linux Kernel, we find that the length of the string
> 
> Well... how much slower is it for short strings?

And cold cache??

	David

> > searched by "memchr()" is up to 512 bytes in drivers/misc/lkdtm/heap.c.
> > In our test, the optimized version is about 20% faster if the target
> > character is at the end of the string when going through a 512-byte
> > string.
> 
> "What is the average length passed to memchr" would be more useful question.
> 
> Best regards,
> 								Pavel

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