8035326: Assume non-NULL references in G1CollectedHeap::in_cset_fast_test

Summary: Remove the assumption that G1CollectedHeap::in_cset_fast_test needs to check for NULL references. Most of the time this is not required, making the code doing this check multiple times.
Reviewed-by: stefank, mgerdin, jmasa

src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp

@@ -4794,11 +4794,16 @@ void G1ParCopyHelper::do_klass_barrier(T* p, oop new_obj) {
 template <G1Barrier barrier, bool do_mark_object>
 template <class T>
 void G1ParCopyClosure<barrier, do_mark_object>::do_oop_work(T* p) {
-  oop obj = oopDesc::load_decode_heap_oop(p);
+  T heap_oop = oopDesc::load_heap_oop(p);
+
+  if (oopDesc::is_null(heap_oop)) {
+    return;
+  }
+
+  oop obj = oopDesc::decode_heap_oop_not_null(heap_oop);
 
   assert(_worker_id == _par_scan_state->queue_num(), "sanity");
 
-  // here the null check is implicit in the cset_fast_test() test
   if (_g1->in_cset_fast_test(obj)) {
     oop forwardee;
     if (obj->is_forwarded()) {
@@ -4821,12 +4826,12 @@ void G1ParCopyClosure<barrier, do_mark_object>::do_oop_work(T* p) {
     // The object is not in collection set. If we're a root scanning
     // closure during an initial mark pause (i.e. do_mark_object will
     // be true) then attempt to mark the object.
-    if (do_mark_object && _g1->is_in_g1_reserved(obj)) {
+    if (do_mark_object) {
       mark_object(obj);
     }
   }
 
-  if (barrier == G1BarrierEvac && obj != NULL) {
+  if (barrier == G1BarrierEvac) {
     _par_scan_state->update_rs(_from, p, _worker_id);
   }
 }

src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp

@@ -698,11 +698,11 @@ public:
   }
 
   // This is a fast test on whether a reference points into the
-  // collection set or not. It does not assume that the reference
-  // points into the heap; if it doesn't, it will return false.
+  // collection set or not. Assume that the reference
+  // points into the heap.
   bool in_cset_fast_test(oop obj) {
     assert(_in_cset_fast_test != NULL, "sanity");
-    if (_g1_committed.contains((HeapWord*) obj)) {
+    assert(_g1_committed.contains((HeapWord*) obj), err_msg("Given reference outside of heap, is "PTR_FORMAT, (HeapWord*)obj));
     // no need to subtract the bottom of the heap from obj,
     // _in_cset_fast_test is biased
     uintx index = cast_from_oop<uintx>(obj) >> HeapRegion::LogOfHRGrainBytes;
@@ -712,9 +712,6 @@ public:
     assert( ret || !obj_in_cs(obj), "sanity");
     assert(!ret ||  obj_in_cs(obj), "sanity");
     return ret;
-    } else {
-      return false;
-    }
   }
 
   void clear_cset_fast_test() {