8210557: G1 next bitmap verification at the end of concurrent mark sometimes fails

Removed unnecessary verification that can cause spurious false alarm.

Reviewed-by: sjohanss, kbarrett

src/hotspot/share/gc/g1/g1CollectedHeap.cpp

@@ -1079,9 +1079,10 @@ void G1CollectedHeap::verify_after_full_collection() {
   // the full GC has compacted objects and updated TAMS but not updated
   // the prev bitmap.
   if (G1VerifyBitmaps) {
-    GCTraceTime(Debug, gc)("Clear Bitmap for Verification");
+    GCTraceTime(Debug, gc)("Clear Prev Bitmap for Verification");
     _cm->clear_prev_bitmap(workers());
   }
+  // This call implicitly verifies that the next bitmap is clear after Full GC.
   _verifier->check_bitmaps("Full GC End");
 
   // At this point there should be no regions in the

src/hotspot/share/gc/g1/g1ConcurrentMark.cpp

@@ -716,28 +716,6 @@ void G1ConcurrentMark::clear_prev_bitmap(WorkGang* workers) {
   clear_bitmap(_prev_mark_bitmap, workers, false);
 }
 
-class CheckBitmapClearHRClosure : public HeapRegionClosure {
-  G1CMBitMap* _bitmap;
- public:
-  CheckBitmapClearHRClosure(G1CMBitMap* bitmap) : _bitmap(bitmap) {
-  }
-
-  virtual bool do_heap_region(HeapRegion* r) {
-    // This closure can be called concurrently to the mutator, so we must make sure
-    // that the result of the getNextMarkedWordAddress() call is compared to the
-    // value passed to it as limit to detect any found bits.
-    // end never changes in G1.
-    HeapWord* end = r->end();
-    return _bitmap->get_next_marked_addr(r->bottom(), end) != end;
-  }
-};
-
-bool G1ConcurrentMark::next_mark_bitmap_is_clear() {
-  CheckBitmapClearHRClosure cl(_next_mark_bitmap);
-  _g1h->heap_region_iterate(&cl);
-  return cl.is_complete();
-}
-
 class NoteStartOfMarkHRClosure : public HeapRegionClosure {
 public:
   bool do_heap_region(HeapRegion* r) {

src/hotspot/share/gc/g1/g1ConcurrentMark.hpp

@@ -544,10 +544,6 @@ public:
   // Clear the previous marking bitmap during safepoint.
   void clear_prev_bitmap(WorkGang* workers);
 
-  // Return whether the next mark bitmap has no marks set. To be used for assertions
-  // only. Will not yield to pause requests.
-  bool next_mark_bitmap_is_clear();
-
   // These two methods do the work that needs to be done at the start and end of the
   // initial mark pause.
   void pre_initial_mark();

src/hotspot/share/gc/g1/g1ConcurrentMarkThread.cpp

@@ -381,8 +381,6 @@ void G1ConcurrentMarkThread::run_service() {
       if (!_cm->has_aborted()) {
         G1ConcPhase p(G1ConcurrentPhase::CLEANUP_FOR_NEXT_MARK, this);
         _cm->cleanup_for_next_mark();
-      } else {
-        assert(!G1VerifyBitmaps || _cm->next_mark_bitmap_is_clear(), "Next mark bitmap must be clear");
       }
     }