From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-405750-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 20475 invoked by alias); 5 Nov 2012 20:21:34 -0000
Received: (qmail 19810 invoked by uid 48); 5 Nov 2012 20:21:15 -0000
From: "mattiase at acm dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/55217] New: False -Wstrict-overflow warning
Date: Mon, 05 Nov 2012 20:21:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: mattiase at acm dot org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Changed-Fields:
Message-ID: <bug-55217-4@http.gcc.gnu.org/bugzilla/>
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
Content-Type: text/plain; charset="UTF-8"
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: 2012-11/txt/msg00412.txt.bz2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55217

             Bug #: 55217
           Summary: False -Wstrict-overflow warning
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mattiase@acm.org


This code yields a false -Wstrict-overflow warning in gcc 4.7.2:

void h(int *s);
void f(int n, int s)
{
        int r = 1;
        for (int i = 1; i < n; i++)
                if (r)
                        r++;
        if (r * s >= s + 3)       // warning here
                for (int j = 0; j < r; j++)
                        h(&s);
}

beta.c:8:12: warning: assuming signed overflow does not occur when assuming
that (X + c) < X is always false [-Wstrict-overflow]

The condition is not on the form X+c<X, and can legitimately go either way.

Here is a variant with s hard-wired to 0. The call to g() seems necessary:

void g(void);
void h(int *s);
void f(int n)
{
        int s = 0;
        int r = 1;
        g();
        for (int i = 1; i < n; i++)
                if (r)
                        r++;
        if (r * s >= s + 3)      // warning here
                for (int j = 0; j < r; j++)
                        h(&s);
}