From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 29384 invoked by alias); 3 Sep 2011 18:50:24 -0000 Received: (qmail 29375 invoked by uid 22791); 3 Sep 2011 18:50:23 -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; Sat, 03 Sep 2011 18:49:55 +0000 From: "rafael.espindola at gmail dot com" To: gcc-bugs@gcc.gnu.org Subject: [Bug c/50284] New: possible miscompilation with -fstrict-aliasing Date: Sat, 03 Sep 2011 18:50:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: new X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: c X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: rafael.espindola at gmail dot com X-Bugzilla-Status: UNCONFIRMED X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Changed-Fields: Message-ID: 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: 2011-09/txt/msg00230.txt.bz2 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50284 Bug #: 50284 Summary: possible miscompilation with -fstrict-aliasing Classification: Unclassified Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned@gcc.gnu.org ReportedBy: rafael.espindola@gmail.com I am not sure if the attached program is valid, but I think it is covered by c99 6.5 p7. On irc pinkia points out that it might be invalid. His arguments are * upcast is undefined in general, 6.5 p7 is trying to allow downcasting. * upcasting is defined when the type was originally that type. Two followup observations are that * If we read "z->data.XXX" as an access to the member (an not the full structure), all the access in the program are of the correct type. * On the implementation side, this "bug" show up when main is in a another translation unit too, a case where gcc could not know if "the type was originally that type". Philip Taylor pointed me at http://davmac.wordpress.com/2010/02/26/c99-revisited/ which has an interesting discussion about "Does accessing an object constitute access to the containing object"? This bug is "fixed" on trunk by 160947, but since that is an optimization change, it probably has just deactivated the code path that caused this behavior. For some context, this testcase is a reduction from: http://hg.mozilla.org/mozilla-central/file/a351ae35f2c4/js/src/jscntxtinlines.h#l179