src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp

rev 47984 : Check for NULL in is-alive-closures

*** 1830,1839 ****
--- 1830,1848 ----
    _heap(ShenandoahHeap::heap_no_check()) {
  }
  
  bool ShenandoahForwardedIsAliveClosure::do_object_b(oop obj) {
    assert(_heap != NULL, "sanity");
+   if (_heap->is_concurrent_traversal_in_progress() && oopDesc::is_null(obj)) {
+     // NOTE:
+     // The situation might arise only with traversal GC, because it can mark through the Reference object
+     // right after it is created, but fields not yet initialized. In this case, referent is still NULL, but
+     // it can already be visible to the GC. A subsequent putfield to store the referent will do the right
+     // thing (i.e. put the referent in the traversal work queue, and mark it live). Returning 'is-alive'
+     // for the NULL is the best we can do.
+     return true;
+   }
    assert(oopDesc::unsafe_equals(obj, ShenandoahBarrierSet::resolve_oop_static_not_null(obj)) ||
           (!_heap->is_marked_next(obj)), "from-space obj must not be marked");
  
    obj = ShenandoahBarrierSet::resolve_oop_static_not_null(obj);
  #ifdef ASSERT
*** 1849,1858 ****
--- 1858,1876 ----
    _heap(ShenandoahHeap::heap_no_check()) {
  }
  
  bool ShenandoahIsAliveClosure::do_object_b(oop obj) {
    assert(_heap != NULL, "sanity");
+   if (_heap->is_concurrent_traversal_in_progress() && oopDesc::is_null(obj)) {
+     // NOTE:
+     // The situation might arise only with traversal GC, because it can mark through the Reference object
+     // right after it is created, but fields not yet initialized. In this case, referent is still NULL, but
+     // it can already be visible to the GC. A subsequent putfield to store the referent will do the right
+     // thing (i.e. put the referent in the traversal work queue, and mark it live). Returning 'is-alive'
+     // for the NULL is the best we can do.
+     return true;
+   }
    assert(!oopDesc::is_null(obj), "null");
    assert(oopDesc::unsafe_equals(obj, ShenandoahBarrierSet::resolve_oop_static_not_null(obj)), "only query to-space");
    return _heap->is_marked_next(obj);
  }