From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 7649B389365C; Fri, 24 Apr 2020 19:59:28 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 7649B389365C
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1587758368;
 bh=5eQ4r/c/yqQtqDjc2O1MGeKvbroc/d6uotBWit9UwQ8=;
 h=From:To:Subject:Date:In-Reply-To:References:From;
 b=ukTLLgVQMCLMCSHzWA7EUIlDHFLVE26637BC82rrl+GWAxGi7b+L4YO3RxN8gu6Rn
 AX8UIrVAQfa2R7cM8FA31Vc/0sZLsIoBOo0FWt6qVLuWCMqeqDwjlWTVdM3HkdCg0T
 IWlRrVzoKruCFl0KGqb87zYuhDttoCQZj8O4fNgU=
From: "nathan at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/94747] Confusing code in libsupc++/dyncast.cc
Date: Fri, 24 Apr 2020 19:59:28 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 7.5.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: nathan at gcc dot gnu.org
X-Bugzilla-Status: ASSIGNED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: nathan at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: cf_reconfirmed_on short_desc cc bug_status
 assigned_to resolution everconfirmed
Message-ID: <bug-94747-4-bliNg9Dris@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-94747-4@http.gcc.gnu.org/bugzilla/>
References: <bug-94747-4@http.gcc.gnu.org/bugzilla/>
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 <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <http://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <http://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Fri, 24 Apr 2020 19:59:28 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94747

Nathan Sidwell <nathan at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-04-24
            Summary|Undefined behavior: integer |Confusing code in
                   |overflow in                 |libsupc++/dyncast.cc
                   |libsupc++/dyncast.cc        |
                 CC|                            |nathan at gcc dot gnu.org
             Status|RESOLVED                    |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |nathan at gcc dot g=
nu.org
         Resolution|INVALID                     |---
     Ever confirmed|0                           |1

--- Comment #3 from Nathan Sidwell <nathan at gcc dot gnu.org> ---
While the code is well formed, it does not convey intent well.  I wrote that
code, and probably even considered spelling as -ptrdiff_t (offsetof ...). b=
ut
didn't because it causes a line wrap and has no semantic difference.

negating unsigned numbers often shows programmer confusion, and can cause b=
ugs
on I32LP64 systems, where the negation happens at a lower precision than
expected.=20=20

That's not the case here. If size_t has fewer bits than a pointer, there ca=
nnot
be overflow from the top size_t bit to the pointer arithmetic -- that break=
s C
& C++ semantics.

Something for stage 1=