RFR(XS): 8211451: ~2.5% regression on compression benchmark starting with 12-b11

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8211451/webrev.00/

The Compressor::output() method has a loop for which the exit condition
is transformed by code added for 8209544:

  if ((_test._test == BoolTest::eq || _test._test == BoolTest::ne) && 
      cop == Op_CmpI && 
      cmp1_op == Op_AddI && 
      cmp1->in(2) != NULL && 
      phase->type(cmp1->in(2))->isa_int() && 
      phase->type(cmp1->in(2))->is_int()->is_con() && 
      cmp2_type == TypeInt::ZERO && 
      !is_counted_loop_cmp(cmp) // modifying the exit test of a counted loop messes the counted loop shape 
      ) { 
    const TypeInt* cmp1_in2 = phase->type(cmp1->in(2))->is_int(); 
    Node *ncmp = phase->transform( new CmpINode(cmp1->in(1),phase->intcon(-cmp1_in2->_hi))); 
    return new BoolNode( ncmp, _test._test ); 
  } 

That loop is then not transformed into a counted loop because of the
following test in PhaseIdealLoop::is_counted_loop():

  if (phi_incr != NULL) { 
    // check if there is a possiblity of IV overflowing after the first increment 
    if (stride_con > 0) { 
      if (init_t->_hi > max_jint - stride_con) { 
        return false; 
      } 
    } else { 
      if (init_t->_lo < min_jint - stride_con) { 
        return false; 
      } 
    } 
  } 

I think the logic above does not apply when the exit condition is an
equality test because there's no risk of overflow leading to an
incorrect test result.

Roland.