From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S261674AbUB0BGJ (ORCPT ); Thu, 26 Feb 2004 20:06:09 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S261548AbUB0BEK (ORCPT ); Thu, 26 Feb 2004 20:04:10 -0500 Received: from waste.org ([209.173.204.2]:10907 "EHLO waste.org") by vger.kernel.org with ESMTP id S261672AbUB0BCb (ORCPT ); Thu, 26 Feb 2004 20:02:31 -0500 Date: Thu, 26 Feb 2004 19:01:52 -0600 From: Matt Mackall To: Andrew Morton Cc: Denis Vlasenko , linux-kernel@vger.kernel.org Subject: Re: [PATCH] __cacheline_aligned always in own section Message-ID: <20040227010151.GI3883@waste.org> References: <20040226064551.1E44B2C57E@lists.samba.org> <200402270009.26768.vda@port.imtp.ilyichevsk.odessa.ua> <20040226152139.5b881a12.akpm@osdl.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20040226152139.5b881a12.akpm@osdl.org> User-Agent: Mutt/1.3.28i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 26, 2004 at 03:21:39PM -0800, Andrew Morton wrote: > Denis Vlasenko wrote: > > > > I compile my kernels for 486 but buils system aligns > > functions and labels to 16 bytes, with results like this: > > > > 00000730 : > > 730: 55 push %ebp > > 731: 89 e5 mov %esp,%ebp > > 733: 8b 45 08 mov 0x8(%ebp),%eax > > 736: 8b 40 64 mov 0x64(%eax),%eax > > 739: 05 14 03 00 00 add $0x314,%eax > > 73e: ff 40 14 incl 0x14(%eax) > > 741: 5d pop %ebp > > 742: c3 ret > > 743: 90 nop > > 744: 90 nop > > 745: 90 nop > > 746: 90 nop > > 747: 90 nop > > 748: 90 nop > > 749: 90 nop > > 74a: 90 nop > > 74b: 90 nop > > 74c: 90 nop > > 74d: 90 nop > > 74e: 90 nop > > 74f: 90 nop > > > > Losing on average 15/2 bytes to alignment, my kernel lose > > # echo $((`cat System.map | grep '0 ' | wc -l`*15/2)) > > 149632 > > bytes only due to function alignment, not counting jump target > > alighment. > > > > Is there any way to prevent this? > > Yes, there are ways of turning off a lot of this alignment padding and it > makes a significant different in code size. You need to dig around in the > gcc documentation for the several -*align* options. > > IIRC, not all of the padding could be turned off (the gcc options were not > complete) but it's a couple of years since I investigated this. Denis, there's a patch in -tiny to let you experiment with overriding CFLAGS. See http://selenic.com/tiny-about/ You probably want something like: -falign-functions=1 -falign-jumps=1 -falign-labels=1 -falign-loops=1 -- Matt Mackall : http://www.selenic.com : Linux development and consulting