From mboxrd@z Thu Jan  1 00:00:00 1970
From: Alexey Dobriyan <adobriyan@gmail.com>
Subject: Re: [PATCH] dcache: faster dentry_cmp()
Date: Wed, 15 Feb 2012 11:14:00 +0300
Message-ID: <20120215081400.GA3306@p183.telecom.by>
References: <20120214224526.GA3478@p183.telecom.by>
 <20120214225839.GA23916@ZenIV.linux.org.uk>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: linux-fsdevel@vger.kernel.org, npiggin@kernel.dk
To: Al Viro <viro@ZenIV.linux.org.uk>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
Received: from mail-bk0-f46.google.com ([209.85.214.46]:53215 "EHLO
	mail-bk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754173Ab2BOIOH (ORCPT
	<rfc822;linux-fsdevel@vger.kernel.org>);
	Wed, 15 Feb 2012 03:14:07 -0500
Received: by bkcjm19 with SMTP id jm19so678244bkc.19
        for <linux-fsdevel@vger.kernel.org>; Wed, 15 Feb 2012 00:14:05 -0800 (PST)
Content-Disposition: inline
In-Reply-To: <20120214225839.GA23916@ZenIV.linux.org.uk>
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>

On Tue, Feb 14, 2012 at 10:58:39PM +0000, Al Viro wrote:
> 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...

It was memcmp() at some point and Nick changed it and it became faster.
See 9d55c369bb5e695e629bc35cba2ef607755b3bee
"fs: implement faster dentry memcmp".