From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932381AbXBOV5U (ORCPT ); Thu, 15 Feb 2007 16:57:20 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932833AbXBOV5U (ORCPT ); Thu, 15 Feb 2007 16:57:20 -0500 Received: from smtp.osdl.org ([65.172.181.24]:36011 "EHLO smtp.osdl.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932381AbXBOV5T (ORCPT ); Thu, 15 Feb 2007 16:57:19 -0500 Date: Thu, 15 Feb 2007 13:53:58 -0800 From: Andrew Morton To: Ralf Baechle Cc: Atsushi Nemoto , linux-mips@linux-mips.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] Optimize generic get_unaligned / put_unaligned implementations. Message-Id: <20070215135358.020781dd.akpm@linux-foundation.org> In-Reply-To: <20070215143441.GA18155@linux-mips.org> References: <20050830104056.GA4710@linux-mips.org> <20060306.203218.69025300.nemoto@toshiba-tops.co.jp> <20060306170552.0aab29c5.akpm@osdl.org> <20070214214226.GA17899@linux-mips.org> <20070214203903.8d013170.akpm@linux-foundation.org> <20070215143441.GA18155@linux-mips.org> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.6; i686-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 15 Feb 2007 14:34:41 +0000 Ralf Baechle wrote: > On Wed, Feb 14, 2007 at 08:39:03PM -0800, Andrew Morton wrote: > > > Can someone please tell us how this magic works? (And it does appear to > > work). > > > > It seems to assuming that the compiler will assume that members of packed > > structures can have arbitrary alignment, even if that alignment is obvious. > > > > Which makes sense, but I'd like to see chapter-and-verse from the spec or > > from the gcc docs so we can rely upon it working on all architectures and > > compilers from now until ever more. > > > > IOW: your changlogging sucks ;) > > It was my entry for the next edition of the C Puzzle Book ;-) > > The whole union thing was only needed to get rid of a warning but Marcel's > solution does the same thing by attaching the packed keyword to the entire > structure instead, so this patch is now using his macros but using __packed > instead. How do we know this trick will work as-designed across all versions of gcc and icc (at least) and for all architectures and for all sets of compiler options? Basically, it has to be guaranteed by a C standard. Is it?