From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756470Ab3BLHNf (ORCPT ); Tue, 12 Feb 2013 02:13:35 -0500 Received: from terminus.zytor.com ([198.137.202.10]:46784 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751307Ab3BLHNd (ORCPT ); Tue, 12 Feb 2013 02:13:33 -0500 Message-ID: <5119EB71.6080508@zytor.com> Date: Mon, 11 Feb 2013 23:12:49 -0800 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130110 Thunderbird/17.0.2 MIME-Version: 1.0 To: Linus Torvalds CC: Ingo Molnar , Linux Kernel Mailing List , Jamie Lokier , ville.syrjala@linux.intel.com, Borislav Petkov , Russell King - ARM Linux , Thomas Gleixner , linux-tip-commits@vger.kernel.org, "H.J. Lu" Subject: Re: [tip:x86/mm] x86, mm: Use a bitfield to mask nuisance get_user() warnings References: <20130209110031.GA17833@n2100.arm.linux.org.uk> <5119C34B.70207@zytor.com> In-Reply-To: X-Enigmail-Version: 1.5 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Just to be sure, I re-ran the comparison using gcc 4.6.3 instead of gcc 4.7.2. With gcc 4.6.3 I consistently get a few hundred bytes longer with the bitfield variant than with the pre-get_user() baseline. I looked at some of the diffed disassembly, and the differences seem to be in the code generated downstream of __get_user_1 and __get_user_2, which I guess is to be expected, mostly in the form of padding. Annoyingly enough in *both* cases I found unnecessary instructions like: +c12f6fbb: 0f b7 d2 movzwl %dx,%edx -hpa