From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756731Ab2BCOw2 (ORCPT ); Fri, 3 Feb 2012 09:52:28 -0500 Received: from mga11.intel.com ([192.55.52.93]:26214 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756034Ab2BCOw1 (ORCPT ); Fri, 3 Feb 2012 09:52:27 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.71,315,1320652800"; d="scan'208";a="120328416" Message-ID: <4F2BF4A7.2070603@linux.intel.com> Date: Fri, 03 Feb 2012 06:52:23 -0800 From: Arjan van de Ven User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:9.0) Gecko/20111222 Thunderbird/9.0.1 MIME-Version: 1.0 To: Chinmay V S CC: Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org, cvs268@gmail.com, tglx@linutronix.de, jeff.chua.linux@gmail.com Subject: Re: [RFC] [PATCH] [timer] Optimise apply_slack() for size and speed. References: <1328002652-7700-1-git-send-email-chinmay.v.s@pathpartnertech.com> <20120202225917.GA1189@kibibi> In-Reply-To: X-Enigmail-Version: 1.3.5 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2/2/2012 10:31 PM, Chinmay V S wrote: > Here is the dis-assembly (build for x86) : > > Original code snippet : > mov %eax,%ecx > mov $0x1,%edx > shl %cl,%edx > sub $0x1,%edx > mov %edx,-0x10(%ebp) > not %edx > and %esi,%edx > mov %edx,-0x14(%ebp) > > Patched code snippet : > mov %eax,%ecx > shrl %cl,-0x14(%ebp) > shll %cl,-0x14(%ebp) > > As is evident, we are able to reduce 5 instructions by using a bit-shift > logic (compared to a masking logic). > this code is so not performance critical to this level, that we should optimize for readability, not for the output of a compiler.