Don't emit code for instructions that are hiding behind an HSoftDeoptimize

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 300673428257d9fee2d14c36757884433f50b716..94407d5794e03ffbbde138ab7a42f9f28b363d06 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -104,6 +104,7 @@ class LChunkBuilder;
   V(DeleteProperty)                            \
   V(Deoptimize)                                \
   V(Div)                                       \
+  V(DummyUse)                                  \
   V(ElementsKind)                              \
   V(EnterInlined)                              \
   V(FastLiteral)                               \
@@ -1024,6 +1025,26 @@ class HBlockEntry: public HTemplateInstruction<0> {
 };
 
 
+class HDummyUse: public HTemplateInstruction<1> {
+ public:
+  explicit HDummyUse(HValue* value) {
+    SetOperandAt(0, value);
+    set_representation(Representation::Tagged());

[danno, 2013/01/10 12:30:05]

Can you add a comment here that explains why you use HType::Smi, i.e. explain
that you want to minimize the expense of inserted changes?

[Jakob Kummerow, 2013/01/16 11:35:02]

Done.

+    set_type(HType::Smi());
+  }
+
+  HValue* value() { return OperandAt(0); }
+
+  virtual Representation RequiredInputRepresentation(int index) {
+    return Representation::None();
+  }
+
+  virtual void PrintDataTo(StringStream* stream);
+
+  DECLARE_CONCRETE_INSTRUCTION(DummyUse);
+};
+
+
 // We insert soft-deoptimize when we hit code with unknown typefeedback,
 // so that we get a chance of re-optimizing with useful typefeedback.
 // HSoftDeoptimize does not end a basic block as opposed to HDeoptimize.