Store offset between fixed typed array base and data start in object

src/objects.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_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 
 #include "src/allocation.h"
@@ skipping 4518 matching lines @@
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(ExternalFloat64Array);
 };
 
 
 class FixedTypedArrayBase: public FixedArrayBase {
  public:
   // [base_pointer]: For now, points to the FixedTypedArrayBase itself.
   DECL_ACCESSORS(base_pointer, Object)
 
+  // [external_pointer]: For now, contains the offset between base_pointer and
+  // the start of the data.
+  DECL_ACCESSORS(external_pointer, void)
+
   // Dispatched behavior.
   inline void FixedTypedArrayBaseIterateBody(ObjectVisitor* v);
 
   template <typename StaticVisitor>
   inline void FixedTypedArrayBaseIterateBody();
 
   DECLARE_CAST(FixedTypedArrayBase)
 
-  static const int kBasePointerOffset =
-      FixedArrayBase::kHeaderSize + kPointerSize;
-  static const int kHeaderSize = kBasePointerOffset + kPointerSize;
+  static const int kBasePointerOffset = FixedArrayBase::kHeaderSize;
+  static const int kExternalPointerOffset = kBasePointerOffset + kPointerSize;
+  static const int kHeaderSize =
+      DOUBLE_POINTER_ALIGN(kExternalPointerOffset + kPointerSize);
 
-  static const int kDataOffset = DOUBLE_POINTER_ALIGN(kHeaderSize);
+  static const int kDataOffset = kHeaderSize;
 
   inline int size();
 
   static inline int TypedArraySize(InstanceType type, int length);
   inline int TypedArraySize(InstanceType type);
 
   // Use with care: returns raw pointer into heap.
   inline void* DataPtr();
 
   inline int DataSize();
@@ skipping 6233 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_