Re: [PATCH] dcache: faster dentry_cmp()

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, Feb 15, 2012 at 01:45:27AM +0300, Alexey Dobriyan wrote:
> 1) consistently use "unsigned int" for dentry name length,
> 2) reuse subtraction result for return value, exact value doesn't matter
>    because function is only used in boolean context,
> 3) use *p++ idiom for even better code.
> 
> All of this results in performance speedup of "git diff"
> which is way out of statistical error (0.4% vs 0.15% of 3 sigma):

> -	if (scount != tcount)
> -		return 1;
> +
> +	ret = scount - tcount;
> +	if (ret)
> +		return ret;
>  	do {
> -		ret = (*cs != *ct);
> +		ret = *cs++ - *ct++;
>  		if (ret)
>  			break;
> -		cs++;
> -		ct++;
>  		tcount--;
>  	} while (tcount);
>  	return ret;

I wonder what'll happen if you simply replace that loop with
	return memcmp(cs, ct, tcount);
which is what it really is...