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

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Message-ID: <20220812190631.GA1347@bug>
Date:   Fri, 12 Aug 2022 21:06:31 +0200
From:   Pavel Machek <pavel@....cz>
To:     Yu-Jen Chang <arthurchang09@...il.com>
Cc:     andy@...nel.org, akinobu.mita@...il.com, jserv@...s.ncku.edu.tw,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Optimize memchr()

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?

> 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