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

[Andy Shevchenko]

On Thu, Oct 27, 2022 at 6:29 AM Nathan Moinvaziri <nathan@xxxxxxxxxxx> wrote:
> On 10/25/2022 12:19 PM, Andy Shevchenko wrote:
> > Looks promising, but may I suggest a few things:
> > 1) have you considered the word-at-a-time use (like strscpy() does)?
>
> Only briefly at the beginning of the function to check for an identical
> comparison and the added check hurt performance for strings that were
> not identical.
>
> On 10/25/2022 12:19 PM, Andy Shevchenko wrote:
>
> > 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?
>
> I'm not sure what you mean about the possible combinations of the space
> character. I have not investigated this method.

'a' xor 'A' == 0x20 (same for all the letters.
That's why we have a specific _tolower() in vsprintf.c.

> According to my previous findings the check for c1 != c2 does perform
> better for strings that are at least 25% or more the same. I was able to
> get even more performance out of it by changing tolower() to use a
> different hash table than the one used for the is*() functions. By using
> a pre-generated hash table for both islower() and isupper() it is
> possible to remove the branch where ever those functions are used,
> including in strcasecmp. This method I've seen employed in the Android
> code base and also in cURL. Using it would add additional 2x256 bytes to
> the code size for the tables.

Rasmus raised a good question, where do we actually need the
performant strcasecmp()?

-- 
With Best Regards,
Andy Shevchenko