Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751978AbaKYVYk (ORCPT ); Tue, 25 Nov 2014 16:24:40 -0500 Received: from mail-ob0-f182.google.com ([209.85.214.182]:46375 "EHLO mail-ob0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751781AbaKYVYi (ORCPT ); Tue, 25 Nov 2014 16:24:38 -0500 MIME-Version: 1.0 In-Reply-To: <5474EF0B.4070502@suse.cz> References: <1416595200-22359-1-git-send-email-jistone@redhat.com> <5473A74F.8000709@suse.cz> <5473C362.80308@redhat.com> <5474EF0B.4070502@suse.cz> Date: Tue, 25 Nov 2014 16:24:38 -0500 X-Google-Sender-Auth: uH32zVYP44fFAhKghNET9ZSCX4E Message-ID: Subject: Re: [PATCH RESEND] Kbuild: Add an option to enable GCC VTA From: Josh Boyer To: Michal Marek Cc: Josh Stone , "Linux-Kernel@Vger. Kernel. Org" , "Frank Ch. Eigler" , Jakub Jelinek , Greg Kroah-Hartman , Linus Torvalds , Andrew Morton , Markus Trippelsdorf , =?ISO-8859-1?Q?Michel_D=E4nzer?= Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 25, 2014 at 4:05 PM, Michal Marek wrote: > Dne 25.11.2014 v 00:46 Josh Stone napsal(a): >> On 11/24/2014 01:46 PM, Michal Marek wrote: >>> Dne 21.11.2014 v 19:40 Josh Stone napsal(a): >>>> Due to recent codegen issues, gcc -fvar-tracking-assignments was >>>> unconditionally disabled in commit 2062afb4f804a ("Fix gcc-4.9.0 >>>> miscompilation of load_balance() in scheduler"). However, this reduces >>> [...] >>>> With gcc-4.9.2-1.fc22, I can now build v3.18-rc5 with Fedora's i686 and >>>> x86_64 configs, and this is completely clean with GCC_COMPARE_DEBUG. >>> >>> According to gcc's bug#61801, this is really fixed in 4.9.2 (commit >>> 556537c4 in the git mirror). So how about checking for this minimal >>> version instead of a new Kconfig option? >> >> That's possible, if the new Kconfig option is really undesirable. But >> given that there's a similar DWARF4 option, which this pairs well with, >> I thought a VTA option was a good choice. >> >> Besides 4.9.2 though, this particular fix has also been backported to >> 4.8.4, and who knows what various distro maintainers may do. > > My idea was that if we switch -fvar-tracking-assignments on with gcc > 4.9.2+, then yes, some users with patched compilers will not benefit > from it, but it will eventually work out of the box for "everybody." > With the kconfig option, an uninformed user has no idea whether or not > it is a good idea to switch it on. It should at least mention that it is > safe to enable with 4.9.2+. And BTW > > make KCLFAGS=-fvar-tracking-assignments > > works as well, as the user-supplied flags are applied last. I actually tried this after Kyle McMartin mentioned it, and thought about doing it in the Fedora kernel spec file instead of taking the patch. It's a bit less noticeable when trying to reproduce build environments though, and with it being a cc-option in the Makefile we can rely on that to filter out compilers and arches that don't support the flag. josh -- 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/