--- old/src/hotspot/share/gc/g1/g1RemSet.cpp	2019-11-19 11:06:12.158983095 +0100
+++ new/src/hotspot/share/gc/g1/g1RemSet.cpp	2019-11-19 11:06:11.718980913 +0100
@@ -196,30 +196,6 @@
     }
   };
 
-  // Creates a snapshot of the current _top values at the start of collection to
-  // filter out card marks that we do not want to scan.
-  class G1ResetScanTopClosure : public HeapRegionClosure {
-    G1RemSetScanState* _scan_state;
-
-  public:
-    G1ResetScanTopClosure(G1RemSetScanState* scan_state) : _scan_state(scan_state) { }
-
-    virtual bool do_heap_region(HeapRegion* r) {
-      uint hrm_index = r->hrm_index();
-      if (r->in_collection_set()) {
-        // Young regions had their card table marked as young at their allocation;
-        // we need to make sure that these marks are cleared at the end of GC, *but*
-        // they should not be scanned for cards.
-        // So directly add them to the "all_dirty_regions".
-        // Same for regions in the (initial) collection set: they may contain cards from
-        // the log buffers, make sure they are cleaned.
-        _scan_state->add_all_dirty_region(hrm_index);
-       } else if (r->is_old_or_humongous_or_archive()) {
-        _scan_state->set_scan_top(hrm_index, r->top());
-       }
-       return false;
-     }
-  };
   // For each region, contains the maximum top() value to be used during this garbage
   // collection. Subsumes common checks like filtering out everything but old and
   // humongous regions outside the collection set.
@@ -328,16 +304,8 @@
   }
 
   void prepare() {
-    for (size_t i = 0; i < _max_regions; i++) {
-      _collection_set_iter_state[i] = false;
-      clear_scan_top((uint)i);
-    }
-
     _all_dirty_regions = new G1DirtyRegions(_max_regions);
     _next_dirty_regions = new G1DirtyRegions(_max_regions);
-
-    G1ResetScanTopClosure cl(this);
-    G1CollectedHeap::heap()->heap_region_iterate(&cl);
   }
 
   void prepare_for_merge_heap_roots() {
@@ -430,6 +398,10 @@
     } while (cur != start_pos);
   }
 
+  void reset_region_claim(uint region_idx) {
+    _collection_set_iter_state[region_idx] = false;
+  }
+
   // Attempt to claim the given region in the collection set for iteration. Returns true
   // if this call caused the transition from Unclaimed to Claimed.
   inline bool claim_collection_set_region(uint region) {
@@ -909,6 +881,26 @@
   }
 }
 
+void G1RemSet::prepare_region_for_scan(HeapRegion* region) {
+  uint hrm_index = region->hrm_index();
+
+  _scan_state->reset_region_claim(hrm_index);
+  if (region->in_collection_set()) {
+    // Young regions had their card table marked as young at their allocation;
+    // we need to make sure that these marks are cleared at the end of GC, *but*
+    // they should not be scanned for cards.
+    // So directly add them to the "all_dirty_regions".
+    // Same for regions in the (initial) collection set: they may contain cards from
+    // the log buffers, make sure they are cleaned.
+    _scan_state->clear_scan_top(hrm_index);
+    _scan_state->add_all_dirty_region(hrm_index);
+  } else if (region->is_old_or_humongous_or_archive()) {
+    _scan_state->set_scan_top(hrm_index, region->top());
+  } else {
+    assert(region->is_free(), "Should only be free region at this point %s", region->get_type_str());
+  }
+}
+
 void G1RemSet::prepare_for_scan_heap_roots() {
   G1DirtyCardQueueSet& dcqs = G1BarrierSet::dirty_card_queue_set();
   dcqs.concatenate_logs();
@@ -1236,7 +1228,7 @@
   }
 }
 
-void G1RemSet::prepare_for_scan_heap_roots(uint region_idx) {
+void G1RemSet::exclude_region_from_scan(uint region_idx) {
   _scan_state->clear_scan_top(region_idx);
 }