From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262173AbTIMTZx (ORCPT ); Sat, 13 Sep 2003 15:25:53 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262174AbTIMTZw (ORCPT ); Sat, 13 Sep 2003 15:25:52 -0400 Received: from mail.jlokier.co.uk ([81.29.64.88]:55186 "EHLO mail.jlokier.co.uk") by vger.kernel.org with ESMTP id S262173AbTIMTZv (ORCPT ); Sat, 13 Sep 2003 15:25:51 -0400 Date: Sat, 13 Sep 2003 20:25:39 +0100 From: Jamie Lokier To: =?iso-8859-1?Q?J=F6rn?= Engel Cc: Ricardo Bugalho , insecure@mail.od.ua, linux-kernel@vger.kernel.org Subject: Re: nasm over gas? Message-ID: <20030913192539.GE7404@mail.jlokier.co.uk> References: <20030904104245.GA1823@leto2.endorphin.org> <200309100034.58742.insecure@mail.od.ua> <200309121826.22936.insecure@mail.od.ua> <1063387648.15891.26.camel@ezquiel.nara.homeip.net> <20030912221717.GB11952@wohnheim.fh-wedel.de> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20030912221717.GB11952@wohnheim.fh-wedel.de> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Jörn Engel wrote: > - Why has Alan measured faster kernels with -Os than with -O2? > > Code size *does* matter. That's not just i-cache pressure. It is partly a GCC problem, and it's possible -Os would run faster than -O2 even with no i-cache. I've observed -Os emitting exactly the same code as -O2 for some trivial functions, except that -O2 has a few extra redundant instructions. Obvious the _intent_ of -O2 is to compile for speed, but it's clear that GCC often emits trivially redundant instructions (like stack adjustments) that don't serve to speed up the program at all. -- Jamie