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

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 8e4952e8a83fb5e2d577a42f50eef38af99c16e9..5067344fc00e8b7c71af3741b006116159f92b3a 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -741,9 +741,11 @@ class Heap {
 
   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 (SWEEPING_IS_OFF),
+  // - or from within GC (SWEEPING_STARTED),
+  // - or mutator code (SWEEPING_STARTED).
+  enum InvocationMode { SWEEPING_IS_OFF, SWEEPING_STARTED };

[Hannes Payer (out of office), 2015/03/26 16:27:24]

Can we call it SEQUENTIAL_TO_SWEEPER and CONCURRENT_TO_SWEEPER.

 
   // Maintain consistency of live bytes during incremental marking.
   void AdjustLiveBytes(Address address, int by, InvocationMode mode);