Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752710AbbDFBAa (ORCPT ); Sun, 5 Apr 2015 21:00:30 -0400 Received: from hofr.at ([212.69.189.236]:47236 "EHLO mail.hofr.at" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752590AbbDFBA2 (ORCPT ); Sun, 5 Apr 2015 21:00:28 -0400 Date: Mon, 6 Apr 2015 03:00:25 +0200 From: Nicholas Mc Guire To: Joe Perches Cc: Nicholas Mc Guire , Michal Marek , Masahiro Yamada , Sam Ravnborg , Thomas Gleixner , "H. Peter Alvin" , John Stultz , Andrew Hunter , Paul Turner , linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] time: allow gcc to fold constants when using msecs_to_jiffies Message-ID: <20150406010025.GA5956@opentech.at> References: <1428218636-3780-1-git-send-email-hofrat@osadl.org> <1428218636-3780-3-git-send-email-hofrat@osadl.org> <1428278627.2775.75.camel@perches.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1428278627.2775.75.camel@perches.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4142 Lines: 103 On Sun, 05 Apr 2015, Joe Perches wrote: > On Sun, 2015-04-05 at 09:23 +0200, Nicholas Mc Guire wrote: > > The majority of the msecs_to_jiffies() users in the kernel are passing in > > constants which would allow gcc to do constant folding by checking with > > __builtin_constant_p() in msecs_to_jiffies(). > > > > The original msecs_to_jiffies is renamed to __msecs_to_jiffies and aside > > from the removal of the check for negative values being moved out, is > > unaltered. > > At least for gcc 4.9, this doesn't allow the compiler > to optimize / precalculation msecs_to_jiffies calls > with a constant. > > This does: (on top of your patch x86-64 defconfig) > > $ size vmlinux.o.* > text data bss dec hex filename > 11770523 1505971 1018454 14294948 da1fa4 vmlinux.o.next-b0a12fb5bc8 > 11770530 1505971 1018454 14294955 da1fab vmlinux.o.next-b0a12fb5bc8-inline > 11768734 1505971 1018454 14293159 da18a7 vmlinux.o.next-b0a12fb5bc8-macro > > I think this should still move the if (m) < 0 back into the > original __msecs_to_jiffies function. > could you check if you can reproduce the results below ? my assumption was that gcc would always optimize out an if(CONST < 0) return CONST; reducing it to the return CONST; only and thus this should not make any difference but Im not that familiar with gcc. gcc versions here are: for x86 gcc version 4.7.2 (Debian 4.7.2-5) for powerpc it is a gcc version 4.9.2 (crosstool-NG 1.20.0) for arm gcc version 4.9.2 20140904 (prerelease) (crosstool-NG linaro-1.13.1-4.9-2014.09 - Linaro GCC 4.9-2014.09) Procedure used: root@debian:~/linux-next# make distclean root@debian:~/linux-next# make defconfig root@debian:~/linux-next# make drivers/net/wireless/p54/p54usb.lst root@debian:~/linux-next# make drivers/net/wireless/p54/p54usb.s same setup in unpatched /usr/src/linux-next/ e.g: root@debian:/usr/src/linux-next# grep msecs_to_jiffies drivers/net/wireless/p54/p54usb.c timeout = jiffies + msecs_to_jiffies(1000); timeout = jiffies + msecs_to_jiffies(1000); So both calls are constants and should be optimized out if it works as expected. without the patch applied: root@debian:/usr/src/linux-next# grep msecs_to_jiffies drivers/net/wireless/p54/p54usb.s call msecs_to_jiffies # call msecs_to_jiffies # root@debian:/usr/src/linux-next# grep msecs_to_jiffies drivers/net/wireless/p54/p54usb.lst timeout = jiffies + msecs_to_jiffies(1000); e19: R_X86_64_PC32 msecs_to_jiffies+0xfffffffffffffffc timeout = jiffies + msecs_to_jiffies(1000); timeout = jiffies + msecs_to_jiffies(1000); fd8: R_X86_64_PC32 msecs_to_jiffies+0xfffffffffffffffc timeout = jiffies + msecs_to_jiffies(1000); with the patch applied this then gives me: root@debian:~/linux-next# grep msecs_to_jiffies drivers/net/wireless/p54/p54usb.s root@debian:~/linux-next# grep msecs_to_jiffies drivers/net/wireless/p54/p54usb.lst timeout = jiffies + msecs_to_jiffies(1000); timeout = jiffies + msecs_to_jiffies(1000); timeout = jiffies + msecs_to_jiffies(1000); timeout = jiffies + msecs_to_jiffies(1000); Conversely in kernel/sched/core.c the msecs_to_jiffies is not a constant and the result is that it calls __msecs_to_jiffies patched: root@debian:~/linux-next# grep msecs_to_jiffies kernel/sched/core.s call __msecs_to_jiffies # unpatched: root@debian:/usr/src/linux-next# grep msecs_to_jiffies kernel/sched/core.s call msecs_to_jiffies # Could you check if you get these results for this test-case ? If this really were compiler dependant that would be very bad. I did move the < 0 check - but that did not change the situation here. but it well may be that there are some cases where this does make a difference thx! hofrat -- 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/