--- old/src/hotspot/share/gc/g1/g1CollectionSet.cpp	2019-11-26 16:18:12.202852028 +0100
+++ new/src/hotspot/share/gc/g1/g1CollectionSet.cpp	2019-11-26 16:18:11.778849925 +0100
@@ -205,6 +205,13 @@
   }
 }
 
+void G1CollectionSet::par_iterate(HeapRegionClosure* cl,
+                                  HeapRegionClaimer* hr_claimer,
+                                  uint worker_id,
+                                  uint total_workers) const {
+  iterate_part_from(cl, hr_claimer, 0, cur_length(), worker_id, total_workers);
+}
+
 void G1CollectionSet::iterate_optional(HeapRegionClosure* cl) const {
   assert_at_safepoint();
 
@@ -219,18 +226,25 @@
                                                     HeapRegionClaimer* hr_claimer,
                                                     uint worker_id,
                                                     uint total_workers) const {
-  assert_at_safepoint();
+  iterate_part_from(cl, hr_claimer, _inc_part_start, increment_length(), worker_id, total_workers);
+}
 
-  size_t len = increment_length();
-  if (len == 0) {
+void G1CollectionSet::iterate_part_from(HeapRegionClosure* cl,
+                                        HeapRegionClaimer* hr_claimer,
+                                        size_t offset,
+                                        size_t lenght,
+                                        uint worker_id,
+                                        uint total_workers) const {
+  assert_at_safepoint();
+  if (lenght == 0) {
     return;
   }
 
-  size_t start_pos = (worker_id * len) / total_workers;
+  size_t start_pos = (worker_id * lenght) / total_workers;
   size_t cur_pos = start_pos;
 
   do {
-    uint region_idx = _collection_set_regions[cur_pos + _inc_part_start];
+    uint region_idx = _collection_set_regions[cur_pos + offset];
     if (hr_claimer == NULL || hr_claimer->claim_region(region_idx)) {
       HeapRegion* r = _g1h->region_at(region_idx);
       bool result = cl->do_heap_region(r);
@@ -238,7 +252,7 @@
     }
 
     cur_pos++;
-    if (cur_pos == len) {
+    if (cur_pos == lenght) {
       cur_pos = 0;
     }
   } while (cur_pos != start_pos);