Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/deoptimizer.cc

 // Copyright 2013 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 1824 matching lines @@
       Handle<Object> value = MaterializeNextValue();
       array->set(i, *value);
     }
   } else {
     // Dispatch on the instance type of the object to be materialized.
     // We also need to make sure that the representation of all fields
     // in the given object are general enough to hold a tagged value.
     Handle<Map> map = Map::GeneralizeAllFieldRepresentations(
         Handle<Map>::cast(MaterializeNextValue()), Representation::Tagged());
     switch (map->instance_type()) {
+      case MUTABLE_HEAP_NUMBER_TYPE:
       case HEAP_NUMBER_TYPE: {
         // Reuse the HeapNumber value directly as it is already properly
-        // tagged and skip materializing the HeapNumber explicitly.
+        // tagged and skip materializing the HeapNumber explicitly. Turn mutable
+        // heap numbers immutable.
         Handle<Object> object = MaterializeNextValue();
         if (object_index < prev_materialized_count_) {
           materialized_objects_->Add(Handle<Object>(
               previously_materialized_objects_->get(object_index), isolate_));
         } else {
           materialized_objects_->Add(object);
         }
         materialization_value_index_ += kDoubleSize / kPointerSize - 1;
         break;
       }
@@ skipping 40 matching lines @@
     }
   }
 
   return materialized_objects_->at(object_index);
 }
 
 
 Handle<Object> Deoptimizer::MaterializeNextValue() {
   int value_index = materialization_value_index_++;
   Handle<Object> value = materialized_values_->at(value_index);
+  if (value->IsMutableHeapNumber()) {
+    HeapNumber::cast(*value)->set_map(isolate_->heap()->heap_number_map());

[Igor Sheludko, 2014/03/27 11:30:16]

I think it is worth adding a comment here about why mutable heap number turns
immutable.

+  }
   if (*value == isolate_->heap()->arguments_marker()) {
     value = MaterializeNextHeapObject();
   }
   return value;
 }
 
 
 void Deoptimizer::MaterializeHeapObjects(JavaScriptFrameIterator* it) {
   ASSERT_NE(DEBUGGER, bailout_type_);
 
@@ skipping 1421 matching lines @@
         return GetPreviouslyMaterialized(isolate, length);
       }
 
       Handle<Object> map_object = slot_refs_[current_slot_].GetValue(isolate);
       Handle<Map> map = Map::GeneralizeAllFieldRepresentations(
           Handle<Map>::cast(map_object), Representation::Tagged());
       current_slot_++;
       // TODO(jarin) this should be unified with the code in
       // Deoptimizer::MaterializeNextHeapObject()
       switch (map->instance_type()) {
+        case MUTABLE_HEAP_NUMBER_TYPE:
         case HEAP_NUMBER_TYPE: {
           // Reuse the HeapNumber value directly as it is already properly
           // tagged and skip materializing the HeapNumber explicitly.
           Handle<Object> object = GetNext(isolate, lvl + 1);
           materialized_objects_.Add(object);
           // On 32-bit architectures, there is an extra slot there because
           // the escape analysis calculates the number of slots as
           // object-size/pointer-size. To account for this, we read out
           // any extra slots.
           for (int i = 0; i < length - 2; i++) {
@@ skipping 187 matching lines @@
 
 void DeoptimizedFrameInfo::Iterate(ObjectVisitor* v) {
   v->VisitPointer(BitCast<Object**>(&function_));
   v->VisitPointers(parameters_, parameters_ + parameters_count_);
   v->VisitPointers(expression_stack_, expression_stack_ + expression_count_);
 }
 
 #endif  // ENABLE_DEBUGGER_SUPPORT
 
 } }  // namespace v8::internal