From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id A936B3858D28; Tue, 14 Dec 2021 13:24:15 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org A936B3858D28 From: "pinskia at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/103699] Reading or writing a constant unaligned value is wrongly optimized causing an incorrect result (GCC-11 and up) Date: Tue, 14 Dec 2021 13:24:15 +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: 11.0 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: pinskia at gcc dot gnu.org X-Bugzilla-Status: RESOLVED X-Bugzilla-Resolution: INVALID 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: quoted-printable X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gcc-bugs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-bugs mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 14 Dec 2021 13:24:15 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103699 --- Comment #16 from Andrew Pinski --- You might want to read https://blog.regehr.org/archives/1307 https://developers.redhat.com/blog/2020/06/03/the-joys-and-perils-of-aliasi= ng-in-c-and-c-part-2 https://developers.redhat.com/blog/2020/06/02/the-joys-and-perils-of-c-and-= c-aliasing-part-1 And gcc had some warnings in the past but so many false negatives that gcc = had cut back on them. C/c++ aliasing rules are simple but using pointers and casting to a different type are a sure way to push towards having undefined behavior in your code.=