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From: "rjones at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/52560] New: if (r == -1) causes 'assuming signed overflow does not occur when simplifying conditional to constant'
Date: Mon, 12 Mar 2012 11:19:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D52560

             Bug #: 52560
           Summary: if (r =3D=3D -1) causes 'assuming signed overflow does =
not
                    occur when simplifying conditional to constant'
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rjones@redhat.com


Here is the reproducer:

wget 'http://oirase.annexia.org/strict-overflow-warning.i.xz'
unxz strict-overflow-warning.i.xz
gcc -std=3Dgnu99 -O2 -Wstrict-overflow -c strict-overflow-warning.i

The warning is:

  inspect_fs_unix.c: In function =E2=80=98check_fstab=E2=80=99:
  inspect_fs_unix.c:1075:6: warning: assuming signed overflow does not occur
when simplifying conditional to constant [-Wstrict-overflow]

However the code doesn't look like anything should be simplified,
or a warning:

  n_app_md_devices =3D map_app_md_devices (g, &app_map);
  if (n_app_md_devices =3D=3D -1) goto error;

where map_app_md_devices is a function that returns an int:

  static int map_app_md_devices (guestfs_h *g, Hash_table **map);

and n_app_md_devices is also an int.

I've tried this on several versions of gcc:

gcc (GCC) 4.7.0 20120308 (Red Hat 4.7.0-0.19)
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

(for further build info, go to:
http://koji.fedoraproject.org/koji/buildinfo?buildID=3D305760
and click 'build logs')

Same thing with this gcc from Ubuntu 11.10:

$ gcc --version
gcc (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

We think this first started happening in gcc 4.5.1.