[compiler] Delete extra map check in StringIterator.

src/compiler/js-builtin-reducer.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/js-builtin-reducer.h"
 
 #include "src/compilation-dependencies.h"
 #include "src/compiler/access-builder.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/node-matchers.h"
@@ skipping 78 matching lines @@
     // Skip first (i.e. callee) and second (i.e. receiver) operand.
     return NodeProperties::GetValueInput(node_, index + 2);
   }
 
  private:
   Node* node_;
 };
 
 JSBuiltinReducer::JSBuiltinReducer(Editor* editor, JSGraph* jsgraph,
                                    Flags flags,
-                                   CompilationDependencies* dependencies)
+                                   CompilationDependencies* dependencies,
+                                   Handle<Context> native_context)
     : AdvancedReducer(editor),
       dependencies_(dependencies),
       flags_(flags),
       jsgraph_(jsgraph),
+      native_context_(native_context),
       type_cache_(TypeCache::Get()) {}
 
 namespace {
 
 MaybeHandle<Map> GetMapWitness(Node* node) {
   Node* receiver = NodeProperties::GetValueInput(node, 1);
   Node* effect = NodeProperties::GetEffectInput(node);
   // Check if the {node} is dominated by a CheckMaps with a single map
   // for the {receiver}, and if so use that map for the lowering below.
   for (Node* dominator = effect;;) {
@@ skipping 912 matching lines @@
     }
   }
 
   return NoChange();
 }
 
 Reduction JSBuiltinReducer::ReduceStringIterator(Node* node) {
   if (Node* receiver = GetStringWitness(node)) {
     Node* effect = NodeProperties::GetEffectInput(node);
     Node* control = NodeProperties::GetControlInput(node);
-    Node* context = NodeProperties::GetContextInput(node);
 
-    Node* native_context = effect = graph()->NewNode(
-        javascript()->LoadContext(0, Context::NATIVE_CONTEXT_INDEX, true),
-        context, context, effect);
-    Node* map = effect = graph()->NewNode(
-        javascript()->LoadContext(0, Context::STRING_ITERATOR_MAP_INDEX, true),
-        native_context, native_context, effect);
+    Node* map = jsgraph()->HeapConstant(
+        handle(native_context()->string_iterator_map(), isolate()));
 
     // allocate new iterator
     effect = graph()->NewNode(
         common()->BeginRegion(RegionObservability::kNotObservable), effect);
     Node* value = effect = graph()->NewNode(
         simplified()->Allocate(NOT_TENURED),
         jsgraph()->Int32Constant(JSStringIterator::kSize), effect, control);
     effect = graph()->NewNode(simplified()->StoreField(AccessBuilder::ForMap()),
                               value, map, effect, control);
     effect = graph()->NewNode(
@@ skipping 392 matching lines @@
   return jsgraph()->simplified();
 }
 
 JSOperatorBuilder* JSBuiltinReducer::javascript() const {
   return jsgraph()->javascript();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8