Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932510AbaFQNg3 (ORCPT ); Tue, 17 Jun 2014 09:36:29 -0400 Received: from gw-1.arm.linux.org.uk ([78.32.30.217]:60997 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754462AbaFQNg2 (ORCPT ); Tue, 17 Jun 2014 09:36:28 -0400 Date: Tue, 17 Jun 2014 14:36:20 +0100 From: Russell King - ARM Linux To: Daniel Thompson 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 Message-ID: <20140617133620.GJ23430@n2100.arm.linux.org.uk> 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> <53A0428C.10200@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <53A0428C.10200@linaro.org> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 17, 2014 at 02:28:44PM +0100, Daniel Thompson wrote: > 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). No need - the + case is your version, the - case is my version. So your version wins on this point. :) -- FTTC broadband for 0.8mile line: now at 9.7Mbps down 460kbps up... slowly improving, and getting towards what was expected from it. -- 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/