From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-500745-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 30899 invoked by alias); 28 Oct 2015 00:20:18 -0000
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
Received: (qmail 30592 invoked by uid 48); 28 Oct 2015 00:20:14 -0000
From: "bugdal at aerifal dot cx" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/67999] Wrong optimization of pointer comparisons
Date: Wed, 28 Oct 2015 00:20:00 -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: 5.2.0
X-Bugzilla-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: bugdal at aerifal dot cx
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: <bug-67999-4-VqJYCuJrEJ@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-67999-4@http.gcc.gnu.org/bugzilla/>
References: <bug-67999-4@http.gcc.gnu.org/bugzilla/>
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: 2015-10/txt/msg02300.txt.bz2

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67999

--- Comment #23 from Rich Felker <bugdal at aerifal dot cx> ---
I think you can always do the right-shift first. Pointer subtraction is
undefined unless both pointers point to elements of the same array, and the
addresses of elements of an array will inherently be congruent modulo the size
of an element.