[turbofan] Don't need any write barrier when storing maps from the root set.

src/compiler/simplified-lowering.cc

 // Copyright 2014 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.
 
 #include "src/compiler/simplified-lowering.h"
 
 #include <limits>
 
 #include "src/address-map.h"
 #include "src/base/bits.h"
@@ skipping 715 matching lines @@
       if (field_type->Is(Type::BooleanOrNullOrUndefined()) ||
           value_type->Is(Type::BooleanOrNullOrUndefined())) {
         // Write barriers are not necessary when storing true, false, null or
         // undefined, because these special oddballs are always in the root set.
         return kNoWriteBarrier;
       }
       if (value_type->IsConstant() &&
           value_type->AsConstant()->Value()->IsHeapObject()) {
         Handle<HeapObject> value_object =
             Handle<HeapObject>::cast(value_type->AsConstant()->Value());
-        if (value_object->IsMap()) {
-          // Write barriers for storing maps are cheaper.
-          return kMapWriteBarrier;
-        }
         RootIndexMap root_index_map(jsgraph_->isolate());
         int root_index = root_index_map.Lookup(*value_object);
         if (root_index != RootIndexMap::kInvalidRootIndex &&
             jsgraph_->isolate()->heap()->RootIsImmortalImmovable(root_index)) {
           // Write barriers are unnecessary for immortal immovable roots.
           return kNoWriteBarrier;
         }
+        if (value_object->IsMap()) {
+          // Write barriers for storing maps are cheaper.
+          return kMapWriteBarrier;
+        }
       }
       if (field_type->Is(Type::TaggedPointer()) ||
           value_type->Is(Type::TaggedPointer())) {
         // Write barriers for heap objects are cheaper.
         return kPointerWriteBarrier;
       }
       NumberMatcher m(value);
       if (m.HasValue()) {
         if (IsSmiDouble(m.Value())) {
           // Storing a smi doesn't need a write barrier.
@@ skipping 1703 matching lines @@
         isolate(), graph()->zone(), callable.descriptor(), 0, flags,
         Operator::kNoProperties);
     to_number_operator_.set(common()->Call(desc));
   }
   return to_number_operator_.get();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8