From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-409383-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 4974 invoked by alias); 10 Dec 2012 14:14:39 -0000
Received: (qmail 4864 invoked by uid 48); 10 Dec 2012 14:14:13 -0000
From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/55079] [4.8 regression] false positive -Warray-bounds (also seen at -O3 bootstrap)
Date: Mon, 10 Dec 2012 14:14:00 -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-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: rguenth at gcc dot gnu.org
X-Bugzilla-Status: ASSIGNED
X-Bugzilla-Priority: P1
X-Bugzilla-Assigned-To: rguenth at gcc dot gnu.org
X-Bugzilla-Target-Milestone: 4.8.0
X-Bugzilla-Changed-Fields:
Message-ID: <bug-55079-4-EeQLpWZYlz@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-55079-4@http.gcc.gnu.org/bugzilla/>
References: <bug-55079-4@http.gcc.gnu.org/bugzilla/>
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
X-SW-Source: 2012-12/txt/msg00991.txt.bz2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D55079

--- Comment #13 from Richard Biener <rguenth at gcc dot gnu.org> 2012-12-10=
 14:14:07 UTC ---
(In reply to comment #9)
> This is reduced testcase from gcov.c
> int a[8];
> int
> t (void)
> {
>   int ix =3D 0;
>   int k;
>   int b =3D 0;
>   int curr =3D 0;
>   for (k =3D 0; k < 2; k++)
>     {
>       b =3D ix * 32;
>       curr =3D a[ix++];
>       if (!(ix <=3D 8))

See below.

>         abort ();
>=20
>       while (curr)
>         {
>           b =3D ix * 32;
>           curr =3D a[ix++];
>           if (!(ix <=3D 8))

This is a test after the fact.  For ix =3D=3D 8 we will still enter the
next loop iteration (GCC can't know anything about 'curr') and thus
access a[8] which is out-of-bounds.

Fixing the tests to test < 8 instead fixes the warnings.

This testcase is invalid.

>             abort ();
>         }
>     }
>   return curr + b;
> }
>=20
> jan@linux-e0ml:~/trunk/build/gcc> ./xgcc -B ./ -O2 -fprofile-use t.c
> -Warray-bounds -S -S -fdump-tree-cunroll-details  -fdump-tree-all-details=
=20=20
> t.c: In function =E2=80=98t=E2=80=99:
> t.c:14:2: warning: incompatible implicit declaration of built-in function
> =E2=80=98abort=E2=80=99 [enabled by default]
>   abort ();
>   ^
> t.c:25:1: note: file /home/jan/trunk/build/gcc/t.gcda not found, execution
> counts estimated
>  }
>  ^
> t.c:19:15: warning: array subscript is above array bounds [-Warray-bounds]
>        curr =3D a[ix++];
>                ^
> t.c:19:15: warning: array subscript is above array bounds [-Warray-bounds]
>=20
> Obivously here unroller does not know that bv_ix is at least 1