Align double array backing store during compaction and mark-sweep promotion.

src/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1428 matching lines @@
   MUST_USE_RESULT MaybeObject* CreateInternalizedString(
       const char* str, int length, int hash);
   MUST_USE_RESULT MaybeObject* CreateInternalizedString(String* str);
 
   // Write barrier support for address[offset] = o.
   INLINE(void RecordWrite(Address address, int offset));
 
   // Write barrier support for address[start : start + len[ = o.
   INLINE(void RecordWrites(Address address, int start, int len));
 
+  // Return true if object on heap have to be double aligned.
+  INLINE(static bool MustBeDoubleAligned(HeapObject* object));
+
+  // Ensure object is 8-byte aligned after raw allocation.
+  // Return aligned address. One word in allocated space is prefilled to
+  // provide padding, so value of the size argument is amount of allocated
+  // memory for the object that must be one word larger than actual size of
+  // the object.
+  INLINE(HeapObject* EnsureDoubleAligned(HeapObject* object, int size));
+
   enum HeapState { NOT_IN_GC, SCAVENGE, MARK_COMPACT };
   inline HeapState gc_state() { return gc_state_; }
 
   inline bool IsInGCPostProcessing() { return gc_post_processing_depth_ > 0; }
 
 #ifdef DEBUG
   void set_allocation_timeout(int timeout) {
     allocation_timeout_ = timeout;
   }
 
@@ skipping 1615 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_H_