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 7fa43174d4a857a9895a753e0a579dc1e3998f07..dd49ef3b17fcf8c5d55df5ad689d887cfdeb60dc 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -81,6 +81,13 @@ 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