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 4A886386EC68; Tue, 12 Jan 2021 10:13:58 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 4A886386EC68
From: "tilps at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/98632] New: Warn about unspecified expression ordering for
 atomics with non-relaxed memory ordering.
Date: Tue, 12 Jan 2021 10:13:58 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: unknown
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: tilps at hotmail dot com
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: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-98632-4@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: <https://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: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Tue, 12 Jan 2021 10:13:58 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98632

            Bug ID: 98632
           Summary: Warn about unspecified expression ordering for atomics
                    with non-relaxed memory ordering.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tilps at hotmail dot com
  Target Milestone: ---

C++ defines that execution ordering for expressions is largely unspecified.=
  In
cases where there are multiple atomic operations in an expression for which
there is no standard required ordering, and if those atomic operations are
marked with a non-relaxed memory ordering, it would be useful to have a
warning. Since the compiler is technically free to reorder them in-spite of=
 the
memory ordering indicating that the user cares about the specific ordering.

While it might be able to be argued that the warning should fire for any
expression involving just a single atomic and some other expression compone=
nt
that would be unable to be reordered if the sub-expressions had been assign=
ed
to locals first, it seems that would be likely to have vastly more false
positives than expressions that involve multiple atomic operations. So I wo=
uld
suggest only triggering for expressions involving multiple atomic operation=
s.=