RE: [PATCH] lib/string.c: Improve strcasecmp speed by not lowering if chars match

[Nathan Moinvaziri]

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

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. 

Thanks again,
Nathan

-----Original Message-----
From: Andy Shevchenko <andy@xxxxxxxxxx> 
Sent: Tuesday, October 25, 2022 2:04 AM
To: Nathan Moinvaziri <nathan@xxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
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@xxxxxxxxxxx> 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