8319376: ParallelGC: Forwarded objects found during heap inspection

Co-authored-by: Albert Mingkun Yang <ayang@openjdk.org>
Reviewed-by: ayang, tschatzl

src/hotspot/share/gc/parallel/mutableSpace.cpp

@@ -235,7 +235,18 @@ void MutableSpace::oop_iterate(OopIterateClosure* cl) {
 void MutableSpace::object_iterate(ObjectClosure* cl) {
   HeapWord* p = bottom();
   while (p < top()) {
-    cl->do_object(cast_to_oop(p));
+    oop obj = cast_to_oop(p);
+    // When promotion-failure occurs during Young GC, eden/from space is not cleared,
+    // so we can encounter objects with "forwarded" markword.
+    // They are essentially dead, so skipping them
+    if (!obj->is_forwarded()) {
+      cl->do_object(obj);
+    }
+#ifdef ASSERT
+    else {
+      assert(obj->forwardee() != obj, "must not be self-forwarded");
+    }
+#endif
     p += cast_to_oop(p)->size();
   }
 }