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

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 86 matching lines @@
   V(CompareGeneric)                            \
   V(CompareObjectEqAndBranch)                  \
   V(CompareMap)                                \
   V(CompareConstantEqAndBranch)                \
   V(Constant)                                  \
   V(Context)                                   \
   V(DeclareGlobals)                            \
   V(DeleteProperty)                            \
   V(Deoptimize)                                \
   V(Div)                                       \
+  V(DummyUse)                                  \
   V(ElementsKind)                              \
   V(EnterInlined)                              \
   V(FastLiteral)                               \
   V(FixedArrayBaseLength)                      \
   V(ForceRepresentation)                       \
   V(FunctionLiteral)                           \
   V(GetCachedArrayIndex)                       \
   V(GlobalObject)                              \
   V(GlobalReceiver)                            \
   V(Goto)                                      \
@@ skipping 900 matching lines @@
 class HBlockEntry: public HTemplateInstruction<0> {
  public:
   virtual Representation RequiredInputRepresentation(int index) {
     return Representation::None();
   }
 
   DECLARE_CONCRETE_INSTRUCTION(BlockEntry)
 };
 
 
+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.
 class HSoftDeoptimize: public HTemplateInstruction<0> {
  public:
   virtual Representation RequiredInputRepresentation(int index) {
     return Representation::None();
   }
 
   DECLARE_CONCRETE_INSTRUCTION(SoftDeoptimize)
@@ skipping 4381 matching lines @@
   virtual bool IsDeletable() const { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_