[Bug tree-optimization/54014] New: Value Range propagation bug

["ferrandi at elet dot polimi.it" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54014

             Bug #: 54014
           Summary: Value Range propagation bug
    Classification: Unclassified
           Product: gcc
           Version: 4.5.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ferrandi@elet.polimi.it


Created attachment 27822
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27822
sightly modified version of arith-rand.c

Dears,

I've found a problem with a sightly modified version of a gcc regression test
(arith-rand.c).
The regression test has been modified in order to abort on a specific
condition. The test correctly aborts when no optimization is performed but it
does not fail when -O2 is passed. The problem seems to be in how the VRP work,
in fact once the -fno-tree-vrp is passed the test correctly fails.

The line I've added to arith-rand.c is the following

if((unsigned int) ABS (yy)==2147483648u) abort();

The VRP claims that (unsigned int) ABS (yy) is in the range [1, 2147483647] but
the actual range should be [1, 2147483648]. Everything seems to be related to
the managing of -(-2147483648).

The test does not abort on these versions on gcc-4.5.3 (Ubuntu 12.04), 4.6.3
(Ubuntu 12.04) and on gcc-4.7.1 (debian wheezy/sid) with the following command
line:

gcc-4.5 arith-rand.c   -O2 #(Ubuntu 12.04)
gcc-4.5 arith-rand.c   -O2 #(Ubuntu 12.04)
gcc-4.7 arith-rand.c  -O2 #(debian wheezy/sid)

while it correctly aborts once the option -fno-tree-vrp is passed

Cheers,

Fabrizio