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 919d9de6ae99b6c1babb3b237f931847e481145f..e223267b9aa1a690c74b57e34a65e77d96e2b4cf 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -85,6 +85,13 @@ namespace internal {
 #endif
 
 
+// Determine whether double field unboxing feature is enabled.
+#if (V8_TARGET_ARCH_X64 || V8_TARGET_ARCH_ARM64)
+#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