tree-ssa: a weird (mis)optimization

[Andrew Haley]

One of the Java test cases is failing because of an incorrect
optimization.


Here's the function in the t18.copyrename1 dump:

;; Function Array_3.baz() (_ZN7Array_33bazEv)

Array_3.baz() ()
{
  int <tmp>;
  int nn;
  struct int[] * x;
  int D.631;
  int D.630;
  int retval.0;

<bb 0>:
  _Jv_InitClass (&Array_3.class);
  x_3 = 0B;
  <tmp>_4 = x_3->length;
  <tmp>_5 = <tmp>_4;
  <tmp>_6 = <tmp>_5;
  nn_7 = <tmp>_6;
  D.631_8 = 5;
  return D.631_8;

}


And here it is in the t19.dce1 dump:

;; Function Array_3.baz() (_ZN7Array_33bazEv)

Array_3.baz() ()
{
  int <tmp>;
  int nn;
  struct int[] * x;
  int D.631;
  int D.630;
  int retval.0;

<bb 0>:
  _Jv_InitClass (&Array_3.class);
  D.631_8 = 5;
  return D.631_8;

}

The problem here is that the line

  <tmp>_4 = x_3->length;

has disappeared.  This should not have happened, because the operation
should have may_trap_p() set -- we're deleting a trapping instruction.

Andrew.