From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53158) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZydV9-0000wz-VM for qemu-devel@nongnu.org; Tue, 17 Nov 2015 05:28:40 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZydV5-0007yK-U3 for qemu-devel@nongnu.org; Tue, 17 Nov 2015 05:28:39 -0500 Received: from mx1.redhat.com ([209.132.183.28]:41945) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZydV5-0007yG-Oa for qemu-devel@nongnu.org; Tue, 17 Nov 2015 05:28:35 -0500 References: <1447754381-29882-1-git-send-email-pbonzini@redhat.com> From: Paolo Bonzini Message-ID: <564B0150.7030400@redhat.com> Date: Tue, 17 Nov 2015 11:28:32 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Maydell Cc: QEMU Developers On 17/11/2015 11:19, Peter Maydell wrote: > I think we should only take this patch if you can get a cast-iron > guarantee from both clang and gcc that they will never use this > UB to drive optimizations. As you say gcc already say this more or > less, but clang doesn't, and if they're warning about it that to > me suggests that they will feel freer to rely on the UB in future. If and when this happens we will add "-fno-strict-overflow" for clang, just like we are using "-fno-strict-aliasing" already. > GCC is not our only supported compiler; UB is a real thing that > compilers in general take advantage of; we should be trying to > reduce our reliance on UB, not carving out extra areas where we > feel free to use it. We are already feeling free to use it. Paolo