Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756176AbXFPLef (ORCPT ); Sat, 16 Jun 2007 07:34:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753610AbXFPLe0 (ORCPT ); Sat, 16 Jun 2007 07:34:26 -0400 Received: from canuck.infradead.org ([209.217.80.40]:35455 "EHLO canuck.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755490AbXFPLeU (ORCPT ); Sat, 16 Jun 2007 07:34:20 -0400 Subject: Re: [PATCH] Introduce compat_u64 and compat_s64 types From: David Woodhouse To: Arnd Bergmann Cc: Benjamin Herrenschmidt , Linux Kernel Mailing List , Dave Airlie , linux-arch@vger.kernel.org, Andrew Morton In-Reply-To: <200706161321.35940.arnd@arndb.de> References: <200706150159.l5F1xNgM000459@hera.kernel.org> <1181986686.25228.639.camel@pmac.infradead.org> <200706161226.40875.arnd@arndb.de> <200706161321.35940.arnd@arndb.de> Content-Type: text/plain Date: Sat, 16 Jun 2007 12:34:11 +0100 Message-Id: <1181993652.25228.690.camel@pmac.infradead.org> Mime-Version: 1.0 X-Mailer: Evolution 2.10.1 (2.10.1-17.fc7.dwmw2.1) Content-Transfer-Encoding: 7bit X-SRS-Rewrite: SMTP reverse-path rewritten from by canuck.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1418 Lines: 33 On Sat, 2007-06-16 at 13:21 +0200, Arnd Bergmann wrote: > On Saturday 16 June 2007, Arnd Bergmann wrote: > > On Saturday 16 June 2007, David Woodhouse wrote: > > > Will GCC know that it needs to emit code to handle that (mis)alignment? > > > > I've tested this with gcc-4.0.3, and it does the right thing, which > > is to split a 4 byte aligned 64 bit load/store into two 32 bit accesses, > > if you pass -mstrict-align. > > I just realized this was correct but slightly misleading. On powerpc, we > don't set the 'attribute((aligned(4)))' on compat_64, so there is never > a reason to handle the misalignment, even though it would work. You're right. My question was probably not relevant -- all these 64-bit architectures cope with misaligned loads anyway. If we ever have to deal with 32-bit compat on a 64-bit architecture which can't handle misalignment, I'm just going to hide under my desk and never come out. > On x86_64, misaligned loads are always ok, so gcc never needs to > care about this, even attribute((packed)) does not cause byte access > here. IA64 too, but it'll be handled there too -- either naturally or by fixups; it doesn't matter. -- dwmw2 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/