linux-kernel - Re: [PATCH] lib/string.c: Improve strcasecmp speed by not lowering if chars match

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Date:   Tue, 25 Oct 2022 22:19:22 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Nathan Moinvaziri <nathan@...hanm.com>
Cc:     Andy Shevchenko <andy@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/string.c: Improve strcasecmp speed by not lowering if
 chars match

On Tue, Oct 25, 2022 at 8:53 PM Nathan Moinvaziri <nathan@...hanm.com> wrote:
>
> Hi Andy,
>
> I appreciate your quick feedback!
>
> I have done as you suggested and published my results this time using Google benchmark:
> https://github.com/nmoinvaz/strcasecmp

Thank you for sharing!

Looks promising, but may I suggest a few things:
1) have you considered the word-at-a-time use (like strscpy() does)?
2) instead of using tolower() on both sides,  have you considered
(with the above in mind) to use XOR over words and if they are not 0,
check if the result is one of possible combinations of 0x20 and then
by excluding the non-letters from the range you may find the
difference?

So, I think it's a good exercise for the twiddling of bits.

> After you review it, and if you still think the patch is worthwhile then I can fix the other problems you mentioned for the original patch. If you think it is not worth it, then I understand.

P.S. Avoid top-posting in the Linux kernel mailing lists!

> -----Original Message-----
> From: Andy Shevchenko <andy@...nel.org>
> Sent: Tuesday, October 25, 2022 2:04 AM
> To: Nathan Moinvaziri <nathan@...hanm.com>
> Cc: linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] lib/string.c: Improve strcasecmp speed by not lowering if chars match
>
> On Tue, Oct 25, 2022 at 11:00:36AM +0300, Andy Shevchenko wrote:
> > On Tue, Oct 25, 2022 at 4:46 AM Nathan Moinvaziri <nathan@...hanm.com> wrote:
>
> ...
>
> > > When running tests using Quick Benchmark with two matching 256
> > > character strings these changes result in anywhere between ~6-9x speed improvement.
> > >
> > > * We use unsigned char instead of int similar to strncasecmp.
> > > * We only subtract c1 - c2 when they are not equal.
>
> ...
>
> > You tell us that this is more preformant, but have not provided the
> > numbers. Can we see those, please?
>
> So, I have read carefully and see the reference to some QuickBenchmark I have no idea about. What I meant here is to have numbers provided by an (open
> source) tool (maybe even in-kernel test case) that anybody can test on their machines. You also missed details about how you run, what the data set has been used, etc.
>
> > 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.


-- 
With Best Regards,
Andy Shevchenko