From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 119868 invoked by alias); 2 Jun 2015 07:21:29 -0000 Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-bugs-owner@gcc.gnu.org Received: (qmail 119813 invoked by uid 48); 2 Jun 2015 07:21:21 -0000 From: "sebastiano.vigna at unimi dot it" To: gcc-bugs@gcc.gnu.org Subject: [Bug c/66348] Simple loop has only lower half of the 64-bit counter initialized correctly Date: Tue, 02 Jun 2015 07:21:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: c X-Bugzilla-Version: 4.9.2 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: sebastiano.vigna at unimi dot it X-Bugzilla-Status: UNCONFIRMED X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-SW-Source: 2015-06/txt/msg00119.txt.bz2 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66348 --- Comment #5 from Sebastiano Vigna --- Fantastic tool! I didn't know about it. But it doesn't fire. There is no undefined behaviour in that code--it's just that the optimizer at -O1 does something wrong. I tried a binary search over the single options induced by -O1, but it turns out it's a combination of things--when I split the options in two half, the problem did not show up with either half. The code is 30-40 lines of assembly--I guess someone proficient with the output of the compiler could easily spot what's going wrong.