Re: [PATCH 3/3] lib/uuid: avoid double traversal in __uuid_parse()

[Andy Shevchenko]

On Thu, Mar 12, 2026 at 06:41:13PM +0000, Josh Law wrote:
> __uuid_parse() calls uuid_is_valid() to walk all 36 characters for
> format validation, then walks the string a second time to parse the
> hex bytes.  Combine both passes into one: validate each hex digit
> inline via hex_to_bin() return value and check the four dash positions
> after the loop.
> 
> uuid_is_valid() remains exported unchanged for callers that only need
> validation without parsing.

...

> -	if (!uuid_is_valid(uuid))
> -		return -EINVAL;

In case if valid string it heats up the caches, no?

>  	for (i = 0; i < 16; i++) {
>  		int hi = hex_to_bin(uuid[si[i] + 0]);
>  		int lo = hex_to_bin(uuid[si[i] + 1]);

> +		if (hi < 0 || lo < 0)
> +			return -EINVAL;

Here we add a branch (or two, depending on the code generation)

>  		b[ei[i]] = (hi << 4) | lo;
>  	}

> +	if (uuid[8] != '-' || uuid[13] != '-' ||
> +	    uuid[18] != '-' || uuid[23] != '-')
> +		return -EINVAL;

And one more here.

>  	return 0;
>  }

If you a really into performance, please, provide the tests and the results to
show the benefit. Numbers will tell for themselves.

-- 
With Best Regards,
Andy Shevchenko