In-object double fields unboxing (for 64-bit only).

src/globals.h

Index: src/globals.h
diff --git a/src/globals.h b/src/globals.h
index 871164eaefe6059f9ed700beb3dbbdca9d780078..70de6d378569d25f29fd41392cb7d887bcfdaa22 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -56,6 +56,13 @@ namespace internal {
 // Determine whether the architecture uses an out-of-line constant pool.
 #define V8_OOL_CONSTANT_POOL 0
 
+// Determine whether double field unboxing feature is enabled.
+#if (V8_TARGET_ARCH_X64)
+#define V8_DOUBLE_FIELDS_UNBOXING 0
+#else
+#define V8_DOUBLE_FIELDS_UNBOXING 0
+#endif
+
 // Support for alternative bool type. This is only enabled if the code is
 // compiled with USE_MYBOOL defined. This catches some nasty type bugs.
 // For instance, 'bool b = "false";' results in b == true! This is a hidden