C2: cyclic IR for JVMS

[Roland Westrelin]

> As my investigations show, it's a leftover from a loop.
> CallStaticJava represents loop counter increment.
> 
> The graph is as follows:
> Phi (Region) (Proj #5 (CallStaticJava ... Phi(backref)...))
> 
> The Region is dead, but RegionNode::is_unreachable_region doesn't detect that. Node::simple_data_loop_check(Phi, Proj) doesn't find the cycle, because most of Proj nodes have Flag_is_dead_loop_safe set [1].
> And I don't understand why it is so. Is the check in Node::is_dead_loop_safe() too weak or Flag_is_dead_loop_safe too "optimistic"? :-)

Isn’t the problem that you try to late inline a call that is dead and that the checks in LateInlineCallGenerator::do_late_inline():

  if (call == NULL || call->outcnt() == 0 ||
      call->in(0) == NULL || call->in(0)->is_top()) {
    return;
  }

  const TypeTuple *r = call->tf()->domain();
  for (int i1 = 0; i1 < method()->arg_size(); i1++) {
    if (call->in(TypeFunc::Parms + i1)->is_top() && r->field_at(TypeFunc::Parms + i1) != Type::HALF) {
      assert(Compile::current()->inlining_incrementally(), "shouldn't happen during parsing");
      return;
    }
  }

  if (call->in(TypeFunc::Memory)->is_top()) {
    assert(Compile::current()->inlining_incrementally(), "shouldn't happen during parsing");
    return;
  }

don’t detect it?
FWIW, I know there are rare cases where those tests are not sufficient. Ideally, you would need  Compile::inline_incrementally() to do:
PhaseIdealLoop ideal_loop( igvn, false, true );
after every inline but that’s very expensive.

Roland.