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 3303E3858413; Fri, 17 Sep 2021 01:43:31 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 3303E3858413
From: "hv at crypt dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/102381] New: unexpected -Wmaybe-uninitialized
Date: Fri, 17 Sep 2021 01:43:31 +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: 7.5.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: hv at crypt dot 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: bug_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-102381-4@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: Fri, 17 Sep 2021 01:43:31 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102381

            Bug ID: 102381
           Summary: unexpected -Wmaybe-uninitialized
           Product: gcc
           Version: 7.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hv at crypt dot org
  Target Milestone: ---

This is reduced from perl source code. Reduction was a challenge, so there'=
s a
risk the essence may have been lost.

The following code gives a -Wmaybe-uninitialized warning with each of "gcc-7
(Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0", "gcc-9 (Ubuntu 9.2.1-17ubuntu1~18.04.=
1)
9.2.1 20191102" and self-built "gcc (GCC) 11.2.0".

% cat test.c
extern void fail(void) __attribute__((__noreturn__));
int** f(void);

extern int *ip;
extern int i2, i3;=20

void test(int i1) {
    int **ipp;

    if (i1) {
        ipp =3D f();
    }
    for (int i4 =3D 0; i4 < i2; i4++) {
        if (((i3 & 1) !=3D 1) && ((i3 & 3) !=3D 3))
            fail();
    }
    if (i1) {
        ip =3D *ipp;
    }
    return;
}
% gcc -c -Wmaybe-uninitialized -O1 test.c
test.c: In function 'test':
test.c:18:14: warning: 'ipp' may be used uninitialized in this function
[-Wmaybe-uninitialized]
         ip =3D *ipp;
              ^~~~
%=20

I think it should be clear that i1 does not change within this code, and the
'ip =3D *ipp' therefore cannot be reached without also previously having re=
ached
'ipp =3D f()'. The sensitivity of the intervening code suggests this must b=
e a
bug, eg replacing 'if (((i3 & 1) !=3D 1) && ((i3 & 3) !=3D 3))' with 'if ((=
i3 & 3)
!=3D 3)' makes the warning disappear.=