Avoid setting / depending on flags when transitioning, and when fields are known to be constant.

src/hydrogen.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 4465 matching lines @@
     BuildCheckPrototypeMaps(
         Handle<JSObject>(JSObject::cast(map->prototype())),
         Handle<JSObject>(JSObject::cast(proto)));
   }
 
   HObjectAccess field_access = HObjectAccess::ForField(map, lookup, name);
   bool transition_to_field = lookup->IsTransitionToField(*map);
 
   HStoreNamedField *instr;
   if (FLAG_track_double_fields && field_access.representation().IsDouble()) {
-    HObjectAccess heap_number_access =
-        field_access.WithRepresentation(Representation::Tagged());
+    HObjectAccess heap_number_access = field_access.WithRepresentation(
+        Representation::Tagged(), HObjectAccess::CONSTANT);
     if (transition_to_field) {
       // The store requires a mutable HeapNumber to be allocated.
       NoObservableSideEffectsScope no_side_effects(this);
       HInstruction* heap_number_size = Add<HConstant>(HeapNumber::kSize);
       HInstruction* heap_number = Add<HAllocate>(heap_number_size,
           HType::HeapNumber(), isolate()->heap()->GetPretenureMode(),
           HEAP_NUMBER_TYPE);
       AddStoreMapConstant(heap_number, isolate()->factory()->heap_number_map());
       Add<HStoreNamedField>(heap_number, HObjectAccess::ForHeapNumberValue(),
                             value);
@@ skipping 883 matching lines @@
   current_block()->FinishExit(new(zone()) HAbnormalExit);
   set_current_block(NULL);
 }
 
 
 HLoadNamedField* HGraphBuilder::BuildLoadNamedField(HValue* object,
                                                     HObjectAccess access) {
   if (FLAG_track_double_fields && access.representation().IsDouble()) {
     // load the heap number
     HLoadNamedField* heap_number = Add<HLoadNamedField>(
-        object, access.WithRepresentation(Representation::Tagged()));
+        object, access.WithRepresentation(
+            Representation::Tagged(), HObjectAccess::CONSTANT));
     heap_number->set_type(HType::HeapNumber());
     // load the double value from it
     return New<HLoadNamedField>(
         heap_number, HObjectAccess::ForHeapNumberValue());
   }
   return New<HLoadNamedField>(object, access);
 }
 
 
 HInstruction* HGraphBuilder::BuildLoadStringLength(HValue* object,
@@ skipping 4464 matching lines @@
   if (ShouldProduceTraceOutput()) {
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 } }  // namespace v8::internal