Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932834AbaFQN3F (ORCPT ); Tue, 17 Jun 2014 09:29:05 -0400 Received: from mail-wg0-f44.google.com ([74.125.82.44]:58148 "EHLO mail-wg0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932070AbaFQN3E (ORCPT ); Tue, 17 Jun 2014 09:29:04 -0400 Message-ID: <53A0428C.10200@linaro.org> Date: Tue, 17 Jun 2014 14:28:44 +0100 From: Daniel Thompson User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 MIME-Version: 1.0 To: Russell King - ARM Linux CC: Rob Clark , Nicolas Pitre , Arnd Bergmann , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, patches@linaro.org, linaro-kernel@lists.linaro.org Subject: Re: [PATCH v3] ARM: add get_user() support for 8 byte types References: <1402587755-29245-1-git-send-email-daniel.thompson@linaro.org> <20140612155843.GK23430@n2100.arm.linux.org.uk> <53A015B3.2070809@linaro.org> <20140617110908.GH23430@n2100.arm.linux.org.uk> In-Reply-To: <20140617110908.GH23430@n2100.arm.linux.org.uk> X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 17/06/14 12:09, Russell King - ARM Linux wrote: > On Tue, Jun 17, 2014 at 11:17:23AM +0100, Daniel Thompson wrote: >> ... at this point there is a narrowing cast followed by an implicit >> widening. This results in compiler either ignoring r3 altogether or, if >> spilling to the stack, generating code to set r3 to zero before doing >> the store. > > In actual fact, there's very little difference between the two > implementations in terms of generated code. > > The difference between them is what happens on the 64-bit big endian > narrowing case, where we use __get_user_4 with your version. This > adds one additional instruction. Good point. > and 64-bit narrowed to 32-bit: > > str lr, [sp, #-4]! > - mov ip, r0 > + mov r3, r0 > mov r0, r1 > #APP > @ 275 "t-getuser.c" 1 > - bl __get_user_8 > + bl __get_user_4 > @ 0 "" 2 > - str r2, [ip, #0] > + str r2, [r3, #0] > ldr pc, [sp], #4 The later case avoids allocating r3 for the __get_user_x and should reduce register pressure and, potentially, saves a few instructions elsewhere (one of my rather large test functions does demonstrate this effect). I don't know if we care about that. If we do I'm certainly happy to put a patch together than exploits this (whilst avoiding the add in the big endian case). Daniel. -- 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/