From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 14403 invoked by alias); 19 Sep 2014 16:29:00 -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 13697 invoked by uid 48); 19 Sep 2014 16:28:53 -0000 From: "mikulas at artax dot karlin.mff.cuni.cz" To: gcc-bugs@gcc.gnu.org Subject: [Bug c/63303] Pointer subtraction is broken when using -fsanitize=undefined Date: Fri, 19 Sep 2014 16:29:00 -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: 4.9.1 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: mikulas at artax dot karlin.mff.cuni.cz X-Bugzilla-Status: UNCONFIRMED 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: 2014-09/txt/msg01971.txt.bz2 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63303 --- Comment #9 from mikulas at artax dot karlin.mff.cuni.cz --- > See what I wrote, any object size bigger than half of address space really > isn't supportable, because then (char *) (P) - (char *) (Q) might not fit into > ptrdiff_t. There is no point slowing down all pointer subtractions (other than > char/signed char/unsigned char pointers) for something that really wouldn't > work reliably anyway. But the code in comment 4 doesn't perform (char *)P - (char *)Q. It performs (short *)P - (short *)Q. And that result clearly fits into the signed ptrdiff_t type. If the code in comment 4 performed (char *)b - (char *)a, that operation would be invalid because of overflow. But it doesn't.