Fix representation inference for mutable double boxes.

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HYDROGEN_INSTRUCTIONS_H_
 #define V8_HYDROGEN_INSTRUCTIONS_H_
 
 #include "v8.h"
 
 #include "allocation.h"
@@ skipping 696 matching lines @@
       ClearFlag(kFlexibleRepresentation);
     }
   }
   virtual void AssumeRepresentation(Representation r);
 
   virtual Representation KnownOptimalRepresentation() {
     Representation r = representation();
     if (r.IsTagged()) {
       HType t = type();
       if (t.IsSmi()) return Representation::Smi();
+      // TODO(mstarzinger): This is not correct for mutable HeapNumbers.
       if (t.IsHeapNumber()) return Representation::Double();
       if (t.IsHeapObject()) return r;
       return Representation::None();
     }
     return r;
   }
 
   HType type() const { return type_; }
   void set_type(HType new_type) {
     ASSERT(new_type.IsSubtypeOf(type_));
@@ skipping 4769 matching lines @@
   HValue* context() { return OperandAt(0); }
   HValue* size() { return OperandAt(1); }
 
   virtual Representation RequiredInputRepresentation(int index) V8_OVERRIDE {
     if (index == 0) {
       return Representation::Tagged();
     } else {
       return Representation::Integer32();
     }
   }
+  // TODO(mstarzinger): Workaround until we track mutable HeapNumber types.
+  virtual Representation KnownOptimalRepresentation() V8_OVERRIDE {
+    return representation();
+  }
 
   virtual Handle<Map> GetMonomorphicJSObjectMap() {
     return known_initial_map_;
   }
 
   void set_known_initial_map(Handle<Map> known_initial_map) {
     known_initial_map_ = known_initial_map;
   }
 
   bool IsNewSpaceAllocation() const {
@@ skipping 2239 matching lines @@
   virtual bool IsDeletable() const V8_OVERRIDE { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_