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 06F9A3861026; Tue, 11 Aug 2020 17:10:11 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 06F9A3861026
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1597165811;
 bh=7r0XtpkX1eosfsxUyVG7M1bYA3ySf6oFchC0wMdLKMU=;
 h=From:To:Subject:Date:In-Reply-To:References:From;
 b=xLgn1yl/n2bRSrlGz5n1PO7CFXixLbctrAm4lwxWYzz+A03BgMri/n4k0pO6x/m3Q
 jUUZA/yXPw9yBIPLAjoKXRPV3Y6bctjdlGVWADQLosm8AxY5lMPavPsDHDdi2Nl64d
 TiLkTxUBnEx8sByyO1arYF59AdR/0bL9WTJ2SzJU=
From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/96564] [11 Regression] New maybe use of
 uninitialized variable warning since r11-959
Date: Tue, 11 Aug 2020 17:10:10 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: middle-end
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: alias, diagnostic, missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: msebor at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: 11.0
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: see_also keywords
Message-ID: <bug-96564-4-dRlaTr71p0@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-96564-4@http.gcc.gnu.org/bugzilla/>
References: <bug-96564-4@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, 11 Aug 2020 17:10:11 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96564

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=3D87313
           Keywords|                            |alias, diagnostic,
                   |                            |missed-optimization

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
r11-959 enabled -Wuninitialized for allocated objects (alloca/VLA and mallo=
c).=20
The underlying problem is the failure to take advantage of the non-aliasing
guarantee of the return value of these functions.  One report that describes
this limitation is pr87313.  A slightly simpler but even more obvious test =
case
is:

$ cat pr96564.c && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=3D/dev/st=
dout
pr96564.c
void f (int *x)
{
  int a[*x];

  if (a !=3D x)
    *a =3D 0;

  *x =3D *a;
}

pr96564.c: In function =E2=80=98f=E2=80=99:
pr96564.c:8:6: warning: =E2=80=98*(<unknown>)[0]=E2=80=99 may be used unini=
tialized
[-Wmaybe-uninitialized]
    8 |   *x =3D *a;
      |   ~~~^~~~

;; Function f (f, funcdef_no=3D0, decl_uid=3D1931, cgraph_uid=3D1, symbol_o=
rder=3D0)

Removing basic block 5
f (int * x)
{
  int[0:D.1936] * a.0;
  sizetype _2;
  int _8;
  sizetype _9;
  int prephitmp_18;
  int pretmp_20;

  <bb 2> [local count: 1073741824]:
  _8 =3D *x_1(D);
  _2 =3D (sizetype) _8;
  _9 =3D _2 * 4;
  a.0_11 =3D __builtin_alloca_with_align (_9, 32);
  if (x_1(D) !=3D a.0_11)
    goto <bb 4>; [70.00%]
  else
    goto <bb 3>; [30.00%]

  <bb 3> [local count: 322122544]:
  pretmp_20 =3D (*a.0_11)[0];

  <bb 4> [local count: 1073741824]:
  # prephitmp_18 =3D PHI <pretmp_20(3), 0(2)>
  *x_1(D) =3D prephitmp_18;
  return;

}=