From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 32587 invoked by alias); 20 Jan 2012 19:32:30 -0000 Received: (qmail 32579 invoked by uid 22791); 20 Jan 2012 19:32:29 -0000 X-SWARE-Spam-Status: No, hits=-2.9 required=5.0 tests=ALL_TRUSTED,AWL,BAYES_00 X-Spam-Check-By: sourceware.org Received: from localhost (HELO gcc.gnu.org) (127.0.0.1) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Fri, 20 Jan 2012 19:32:17 +0000 From: "redi at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/51885] g++ compiler options -O2 and -O3 modifies program results Date: Fri, 20 Jan 2012 20:06:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: c++ X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: redi at gcc dot gnu.org X-Bugzilla-Status: RESOLVED X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated Content-Type: text/plain; charset="UTF-8" MIME-Version: 1.0 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 X-SW-Source: 2012-01/txt/msg02359.txt.bz2 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51885 --- Comment #5 from Jonathan Wakely 2012-01-20 19:31:48 UTC --- (In reply to comment #4) > i am quite sure there is something strange : reinterpret_cast is used to cast a > float pointer to a uint32_t pointer > > these two types have the same size (4 bytes) > > therefore, the swap function (following the reinterpret_cast) should work > properly ... No, the C++ standard says your code has undefined behaviour. Please read the section "Casting does not work as expected when optimization is turned on" at http://gcc.gnu.org/bugs/#nonbugs_c and the article it links to, http://mail-index.netbsd.org/tech-kern/2003/08/11/0001.html