Add support for adding an external and a tagged pointer

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index b7e9903f05aa078e458654341a77b464fc231e33..b133efd0f39e902d89d0db47d425289ebf2707e4 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -4886,6 +4886,10 @@ class HAdd final : public HArithmeticBinaryOperation {
   HAdd(HValue* context, HValue* left, HValue* right, Strength strength)
       : HArithmeticBinaryOperation(context, left, right, strength) {
     SetFlag(kCanOverflow);
+    if (left->representation().IsExternal() &&
+        right->representation().IsTagged()) {

[Jarin, 2015/07/20 08:52:32]

This looks quite brittle, perhaps we should introduce some other flag that
differentiates the special HAdd from ordinary ones.

You could then use the flag to do the right thing in
HAdd::RequiredInputRepresentation.

[Michael Starzinger, 2015/07/20 08:56:38]

+1

+      SetDependsOnFlag(kNewSpacePromotion);
+    }
   }
 };