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 4F228386186E; Sun, 27 Dec 2020 01:39:52 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 4F228386186E
From: "alexmudmiko at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/80635] [8/9/10/11 regression] std::optional
 and bogus -Wmaybe-uninitialized warning
Date: Sun, 27 Dec 2020 01:39:47 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 8.2.0
X-Bugzilla-Keywords: diagnostic, patch
X-Bugzilla-Severity: normal
X-Bugzilla-Who: alexmudmiko at gmail dot com
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P2
X-Bugzilla-Assigned-To: law at redhat dot com
X-Bugzilla-Target-Milestone: 8.5
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: cc
Message-ID: <bug-80635-4-lAAL6bFm47@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-80635-4@http.gcc.gnu.org/bugzilla/>
References: <bug-80635-4@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: Sun, 27 Dec 2020 01:39:53 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D80635

Miko Vich <alexmudmiko at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |alexmudmiko at gmail dot c=
om

--- Comment #56 from Miko Vich <alexmudmiko at gmail dot com> ---
I think Jeff is correct. While I have no experience with writing compilers,
I've been wrestling with this "feature" for about 3 years now.  What's odd =
is
that it doesn't always manifest.  It also seems to be restricted (at least =
in
my cases) to using std::optional with POD or enum types.  Also, the volatile
thingy doesn't solve the issue if sizeof(T) is less than 16 bytes. (Again my
experience.)

Anyway... some findings which I think support Jeff's comment:  about a year=
 ago
I went so far as to memset to zero the memory in std::optional for sizeof(T=
).=20
This made the warning go away... defeats the purpose of optional of course,=
 but
no more warning.  But on inspecting the assembly generated, there was no
zero-ing of the memory (or any other assignment to 0). So the optimizer no
longer error'd on maybe-uninitialized, but knew enough to elide away the
memset.

So... is this maybe a case of the warning simply being emitted too soon in =
the
passes, i.e., before its completely finished optimizing?  Because even with=
out
the memset change, the final assembly generated under -O3 never has a path
where the value in the optional is accessed when uninitialized.=