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 56B143858C39; Thu,  7 Oct 2021 08:21:08 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 56B143858C39
From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/102631] -Wmaybe-uninitialized cannot see
 through a series of PHIs
Date: Thu, 07 Oct 2021 08:21:08 +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: 12.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: aldyh at gcc dot gnu.org
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-102631-4-Bd7VFa94Og@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-102631-4@http.gcc.gnu.org/bugzilla/>
References: <bug-102631-4@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: Thu, 07 Oct 2021 08:21:08 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102631

--- Comment #4 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
For the calls-aarch64.ii testcase, there's some additional information in t=
he
upstream thread.  Quoted here for convenience:

There's some missing context.

The only way to get to BB101->BB102 is through:

   <bb 100>
   if (_6711 !=3D 0)
     goto <bb 101>; [5.50%]
   else
     goto <bb 293>; [94.50%]

And there's an implicit relation between _6711 and _811:

<bb 86>
...
   if (_6711 !=3D 0)
     goto <bb 287>; [5.50%]
   else
     goto <bb 87>; [94.50%]

   <bb 287> [local count: 17344687]:
   goto <bb 88>; [100.00%]

   <bb 87> [local count: 298013267]:

   <bb 88> [local count: 315357954]:
   # _881 =3D PHI <1(87), 0(287)>

That is, _6711 =3D=3D !_881.

Presumably the threader shuffled things sufficiently so that the above
relationship is difficult to devise.  It seemsthat  it's having a hard time
noticing that the _6711 and _811 guards are inversely related.=