[heap] Concurrently free empty slot set buckets.

src/heap/remembered-set.h

Index: src/heap/remembered-set.h
diff --git a/src/heap/remembered-set.h b/src/heap/remembered-set.h
index 919da9ff6f4fd9cc96ebac625d71419c7df48213..74791b926bb7772a4646dcec507abbe561c7f85b 100644
--- a/src/heap/remembered-set.h
+++ b/src/heap/remembered-set.h
@@ -116,10 +116,13 @@ class RememberedSet {
       size_t pages = (chunk->size() + Page::kPageSize - 1) / Page::kPageSize;
       int new_count = 0;
       for (size_t page = 0; page < pages; page++) {
-        new_count += slots[page].Iterate(callback);
+        new_count +=
+            slots[page].Iterate(callback, SlotSet::PREFREE_EMPTY_BUCKETS);
       }
-      if (new_count == 0) {
-        ReleaseSlotSet(chunk);
+      // Only old-to-old slot sets are released eagerly. Old-new-slot sets are
+      // released by the sweeper threads.
+      if (direction == OLD_TO_OLD && new_count == 0) {
+        chunk->ReleaseOldToOldSlots();
       }
     }
   }
@@ -219,14 +222,6 @@ class RememberedSet {
     }
   }
 
-  static void ReleaseSlotSet(MemoryChunk* chunk) {
-    if (direction == OLD_TO_OLD) {
-      chunk->ReleaseOldToOldSlots();
-    } else {
-      chunk->ReleaseOldToNewSlots();
-    }
-  }
-
   static void ReleaseTypedSlotSet(MemoryChunk* chunk) {
     if (direction == OLD_TO_OLD) {
       chunk->ReleaseTypedOldToOldSlots();