Optionally use 31-bits SMI value for 64-bit system

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 311ad434de8516dff174558f15dfe8ae45ad8179..2534e7112f29faa495336a5c4e3eb2a0891cbffd 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -5417,7 +5417,15 @@ template <> struct SmiTagging<8> {
   }
 };
 
+#if V8_USE_31_BIT_SMI_VALUE
+typedef SmiTagging<4> PlatformSmiTagging;
+V8_INLINE(static bool SmiValuesAre31Bits()) { return true; }
+V8_INLINE(static bool SmiValuesAre32Bits()) { return false; }
+#else
 typedef SmiTagging<kApiPointerSize> PlatformSmiTagging;
+V8_INLINE(static bool SmiValuesAre31Bits()) { return (kApiPointerSize == 4); }
+V8_INLINE(static bool SmiValuesAre32Bits()) { return (kApiPointerSize == 8); }
+#endif
 const int kSmiShiftSize = PlatformSmiTagging::kSmiShiftSize;
 const int kSmiValueSize = PlatformSmiTagging::kSmiValueSize;