[heap] Refactor updating of marking deque after scavenge.

src/heap/sequential-marking-deque.h

 // Copyright 2017 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_SEQUENTIAL_MARKING_DEQUE_
 #define V8_HEAP_SEQUENTIAL_MARKING_DEQUE_
 
 #include <deque>
 
 #include "src/base/platform/mutex.h"
@@ skipping 77 matching lines @@
 
   template <typename Callback>
   void Iterate(Callback callback) {
     int i = bottom_;
     while (i != top_) {
       callback(array_[i]);
       i = (i + 1) & mask_;
     }
   }
 
-  HeapObject** array() { return array_; }
-  int bottom() { return bottom_; }
+  // Calls the specified callback on each element of the deque and replaces
+  // the element with the result of the callback. If the callback returns
+  // nullptr then the element is removed from the deque.
+  // The callback must accept HeapObject* and return HeapObject*.
+  template <typename Callback>
+  void Update(Callback callback) {
+    int i = bottom_;
+    int new_top = bottom_;
+    while (i != top_) {
+      HeapObject* object = callback(array_[i]);
+      if (object) {
+        array_[new_top] = object;
+        new_top = (new_top + 1) & mask_;
+      }
+      i = (i + 1) & mask_;
+    }
+    top_ = new_top;
+  }
+
   int top() { return top_; }
-  int mask() { return mask_; }
-  void set_top(int top) { top_ = top; }
 
  private:
   // This task uncommits the marking_deque backing store if
   // markin_deque->in_use_ is false.
   class UncommitTask : public CancelableTask {
    public:
     explicit UncommitTask(Isolate* isolate,
                           SequentialMarkingDeque* marking_deque)
         : CancelableTask(isolate), marking_deque_(marking_deque) {}
 
@@ skipping 41 matching lines @@
   bool uncommit_task_pending_;
   Heap* heap_;
 
   DISALLOW_COPY_AND_ASSIGN(SequentialMarkingDeque);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SEQUENTIAL_MARKING_DEQUE_