[Bug tree-optimization/21367] New: VRP does not fold "if (a == b)" in a certain situation

[kazu at cs dot umass dot edu]

Consider

int
foo (int a, int b)
{
  if (a == b)
    if (a == b)
      return 123;

  return 0;
}

With -fno-tree-dominator-opts -fdump-tree-vrp-details, VRP does not fold
the second "if" statement.  Here is what I get from VRP.

Value ranges after VRP:

D.1234_1: VARYING
a_2: VARYING
b_3: VARYING
<retval>_4: VARYING
D.1234_5: [0, 0]
D.1234_6: [123, 123]
a_7: [b_3, b_3]
a_8: ~[b_3, b_3]


foo (a, b)
{
  int D.1234;

<bb 0>:
  if (a_2 == b_3) goto <L0>; else goto <L5>;

<L0>:;
  a_7 = b_3;
  if (a_7 == b_3) goto <L1>; else goto <L2>;

<L1>:;
  D.1234_6 = 123;
  goto <bb 5> (<L4>);

<L5>:;
  a_8 = a_2;

<L2>:;
  D.1234_5 = 0;

  # D.1234_1 = PHI <D.1234_6(2), D.1234_5(4)>;
<L4>:;
  return D.1234_1;

}

Note that we have

  b_3: VARYING
  a_7: [b_3, b_3]

so we have enough information to fold the second "if" statement despite
the fact that b_3 is VARYING.

-- 
           Summary: VRP does not fold "if (a == b)" in a certain situation
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at cs dot umass dot edu
                CC: dnovillo at redhat dot com,gcc-bugs at gcc dot gnu dot
                    org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21367