Untangle slots buffer part 1: Make invalid slots filtering part of MarkCompactCollector.

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index 99a50487f9c457465c8879f9efd20bd08d569eb7..8ab877ef555e82aebfdb12668acc22c4dc1cd418 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -306,12 +306,15 @@ class SlotsBuffer {
     slots_[idx_++] = slot;
   }
 
-  // Should be used for testing only.
+  void Set(intptr_t i, ObjectSlot slot) { slots_[i] = slot; }
+
   ObjectSlot Get(intptr_t i) {
-    DCHECK(i >= 0 && i < kNumberOfElements);
+    DCHECK(i >= 0 && i < idx_);
     return slots_[i];
   }
 
+  size_t Size() const { return idx_; }
+
   enum SlotType {
     EMBEDDED_OBJECT_SLOT,
     OBJECT_SLOT,
@@ -396,19 +399,10 @@ class SlotsBuffer {
                     SlotsBuffer** buffer_address, SlotType type, Address addr,
                     AdditionMode mode);
 
-  // Eliminates all stale entries from the slots buffer, i.e., slots that
-  // are not part of live objects anymore. This method must be called after
-  // marking, when the whole transitive closure is known and must be called
-  // before sweeping when mark bits are still intact.
-  static void RemoveInvalidSlots(Heap* heap, SlotsBuffer* buffer);
-
   // Eliminate all slots that are within the given address range.
   static void RemoveObjectSlots(Heap* heap, SlotsBuffer* buffer,
                                 Address start_slot, Address end_slot);
 
-  // Ensures that there are no invalid slots in the chain of slots buffers.
-  static void VerifySlots(Heap* heap, SlotsBuffer* buffer);
-
   static const int kNumberOfElements = 1021;
 
  private:
@@ -684,8 +678,16 @@ class MarkCompactCollector {
 
   bool WillBeDeoptimized(Code* code);
   void EvictPopularEvacuationCandidate(Page* page);
+
+  // Clears invald store buffer and slots buffer entries. This method must
+  // be called after marking, when the whole transitive closure is known and
+  // must be called before sweeping when mark bits are still intact.
   void ClearInvalidStoreAndSlotsBufferEntries();
 
+  // Eliminates all stale entries from the slots buffer, i.e., slots that
+  // are not part of live objects anymore.
+  void ClearInvalidSlotsBufferEntries(SlotsBuffer* buffer);
+
   void StartSweeperThreads();
 
 #ifdef DEBUG