Eliminate further write barriers for old space allocations.

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 3527 matching lines @@
   bool NotInNewSpace() const {
     return is_not_in_new_space_;
   }
 
   bool ImmortalImmovable() const;
 
   bool IsCell() const {
     return instance_type_ == CELL_TYPE || instance_type_ == PROPERTY_CELL_TYPE;
   }
 
+  bool IsMap() const {
+    return instance_type_ == MAP_TYPE;
+  }
+
   virtual Representation RequiredInputRepresentation(int index) V8_OVERRIDE {
     return Representation::None();
   }
 
   virtual Representation KnownOptimalRepresentation() V8_OVERRIDE {
     if (HasSmiValue() && SmiValuesAre31Bits()) return Representation::Smi();
     if (HasInteger32Value()) return Representation::Integer32();
     if (HasNumberValue()) return Representation::Double();
     if (HasExternalReferenceValue()) return Representation::External();
     return Representation::Tagged();
@@ skipping 2143 matching lines @@
     // Stores to external references require no write barriers
     return false;
   }
   if (object != new_space_dominator) return true;
   if (object->IsAllocate()) {
     // Stores to new space allocations require no write barriers if the object
     // is the new space dominator.
     if (HAllocate::cast(object)->IsNewSpaceAllocation()) {
       return false;
     }
+    // Storing a map or an immortal immovable object requires no write barriers
+    // if the object is the new space dominator.
+    if (value->IsConstant() &&
+        (HConstant::cast(value)->IsMap() ||
+         HConstant::cast(value)->ImmortalImmovable())) {
+      return false;
+    }
     // Likewise we don't need a write barrier if we store a value that
     // originates from the same allocation (via allocation folding).
     while (value->IsInnerAllocatedObject()) {
       value = HInnerAllocatedObject::cast(value)->base_object();
     }
     return object != value;
   }
   return true;
 }
 
@@ skipping 1976 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_