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From: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
To: Andy Shevchenko <andy.shevchenko@gmail.com>,
	Nathan Moinvaziri <nathan@nathanm.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Andy Shevchenko <andy@kernel.org>
Subject: Re: [PATCH] lib/string.c: Improve strcasecmp speed by not lowering if chars match
Date: Tue, 25 Oct 2022 21:55:26 +0200	[thread overview]
Message-ID: <7695dbae-0fc6-d53c-5b9f-b3bd7e7fe8f8@prevas.dk> (raw)
In-Reply-To: <CAHp75Vd4Vk0v-T3kfxApGHb-H26KTHgH59DGP3Wm4qBgunDt6A@mail.gmail.com>

On 25/10/2022 10.00, Andy Shevchenko wrote:
> On Tue, Oct 25, 2022 at 4:46 AM Nathan Moinvaziri <nathan@nathanm.com> wrote:
>>
>> From fcb0159ee74908f92adc34143657d8ca56e9a811 Mon Sep 17 00:00:00 2001
>> From: Nathan Moinvaziri <nathan@nathanm.com>
>> Date: Mon, 24 Oct 2022 16:37:59 -0700
>> Subject: [PATCH] lib/string.c: Improve strcasecmp speed by not lowering if
>>  chars match.
> 
> Why is the above in the commit message?
> 
>> With strings where many characters match exactly each character is needlessly
>> converted to lowercase before comparing. This patch improves the comparison
>> by only converting to lowercase after checking that the characters don't match.
>>
>> The more characters that match exactly the better performance we expect versus
>> the old function.

> You tell us that this is more preformant, but have not provided the
> numbers. Can we see those, please?
> 
> Note, that you basically trash CPU cache lines when characters are not
> equal, and before doing that you have a branching. I'm unsure that
> your way is more performant than the original one.
> 

Are there any code paths in the kernel where strcasecmp performance
matters? strcmp, sure, but strcasecmp or strncasecmp? I don't think so.
If anything, we should nuke the complication in strncasecmp(), and then
make strcasecmp() simply do strncasecmp(a, b, SIZE_MAX).

Rasmus

  parent reply	other threads:[~2022-10-25 19:55 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-10-25  1:46 [PATCH] lib/string.c: Improve strcasecmp speed by not lowering if chars match Nathan Moinvaziri
2022-10-25  8:00 ` Andy Shevchenko
2022-10-25  9:03   ` Andy Shevchenko
2022-10-25 17:53     ` Nathan Moinvaziri
2022-10-25 19:19       ` Andy Shevchenko
2022-10-27  3:29         ` Nathan Moinvaziri
2022-10-27  6:31           ` Andy Shevchenko
2022-10-25 19:32       ` Christophe JAILLET
2022-10-25 23:27         ` Nathan Moinvaziri
2022-10-25 19:55   ` Rasmus Villemoes [this message]
2022-10-25 22:37     ` Nathan Moinvaziri

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