RFR(XS): 8227384: C2 compilation fails with "graph should be schedulable" when running with -XX:-EliminateLocks

[Roland Westrelin]

Hi Vladimir,

> My concern is that your loading from Klass::prototype_header_offset (or make constant) instead of 
> loading from object's mark word in LoadNod Ideal() and Value() methods bypassing Store into mark 
> word or possible access to object from other thread before this load. It is okay if 
> object/allocation is not escaping but not in general case I think.

LoadNode::is_new_object_mark_load() checks the memory edge of the load:

1565     if (alloc != NULL && mem->is_Proj() &&
1566         mem->in(0) != NULL &&
1567         mem->in(0) == alloc->initialization() &&

So it can't bypass a store unless the store was captured but stores to
headers are not captured (see below).

Roland.

int InitializeNode::captured_store_insertion_point(intptr_t start,
                                                   int size_in_bytes,
                                                   PhaseTransform* phase) {
  const int FAIL = 0, MAX_STORE = BytesPerLong;

  if (is_complete())
    return FAIL;                // arraycopy got here first; punt

  assert(allocation() != NULL, "must be present");

  // no negatives, no header fields:
  if (start < (intptr_t) allocation()->minimum_header_size())  return FAIL;