Use atomic operation to read the length of a fixed array.

src/heap/heap.h

 // Copyright 2012 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_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 
 #include "src/allocation.h"
@@ skipping 711 matching lines @@
   // Finalizes an external string by deleting the associated external
   // data and clearing the resource pointer.
   inline void FinalizeExternalString(String* string);
 
   // Initialize a filler object to keep the ability to iterate over the heap
   // when introducing gaps within pages.
   void CreateFillerObjectAt(Address addr, int size);
 
   bool CanMoveObjectStart(HeapObject* object);
 
-  // Indicates whether live bytes adjustment is triggered from within the GC
-  // code or from mutator code.
-  enum InvocationMode { FROM_GC, FROM_MUTATOR };
+  // Indicates whether live bytes adjustment is triggered
+  // - from within the GC code before sweeping started (SEQUENTIAL_TO_SWEEPER),
+  // - or from within GC (CONCURRENT_TO_SWEEPER),
+  // - or mutator code (CONCURRENT_TO_SWEEPER).
+  enum InvocationMode { SEQUENTIAL_TO_SWEEPER, CONCURRENT_TO_SWEEPER };
 
   // Maintain consistency of live bytes during incremental marking.
   void AdjustLiveBytes(Address address, int by, InvocationMode mode);
 
   // Trim the given array from the left. Note that this relocates the object
   // start and hence is only valid if there is only a single reference to it.
   FixedArrayBase* LeftTrimFixedArray(FixedArrayBase* obj, int elements_to_trim);
 
   // Trim the given array from the right.
   template<Heap::InvocationMode mode>
@@ skipping 1861 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_HEAP_H_