[turbofan] Optimize string "length" property access based on types.

src/compiler/js-type-feedback-lowering.h

Index: src/compiler/js-type-feedback-lowering.h
diff --git a/src/compiler/js-type-feedback-lowering.h b/src/compiler/js-type-feedback-lowering.h
new file mode 100644
index 0000000000000000000000000000000000000000..19f4df0ffa1845ede72d94c37ce1a3bf1d55e486
--- /dev/null
+++ b/src/compiler/js-type-feedback-lowering.h
@@ -0,0 +1,62 @@
+// Copyright 2015 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_COMPILER_JS_TYPE_FEEDBACK_LOWERING_H_
+#define V8_COMPILER_JS_TYPE_FEEDBACK_LOWERING_H_
+
+#include "src/base/flags.h"
+#include "src/compiler/graph-reducer.h"
+#include "src/compiler/simplified-operator.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+// Forward declarations.
+class CommonOperatorBuilder;
+class JSGraph;
+class MachineOperatorBuilder;
+
+
+// Lowers JS-level operators to simplified operators based on type feedback.
+class JSTypeFeedbackLowering final : public AdvancedReducer {
+ public:
+  // Various configuration flags to control the operation of this lowering.
+  enum Flag {
+    kNoFlags = 0,
+    kDeoptimizationEnabled = 1 << 0,
+  };
+  typedef base::Flags<Flag> Flags;
+
+  JSTypeFeedbackLowering(Editor* editor, Flags flags, JSGraph* jsgraph);
+  ~JSTypeFeedbackLowering() final {}
+
+  Reduction Reduce(Node* node) final;
+
+ private:
+  Reduction ReduceJSLoadNamed(Node* node);
+
+  Factory* factory() const;
+  Flags flags() const { return flags_; }
+  Graph* graph() const;
+  Isolate* isolate() const;
+  JSGraph* jsgraph() const { return jsgraph_; }
+  CommonOperatorBuilder* common() const;
+  MachineOperatorBuilder* machine() const;
+  SimplifiedOperatorBuilder* simplified() { return &simplified_; }
+
+  Flags const flags_;
+  JSGraph* const jsgraph_;
+  SimplifiedOperatorBuilder simplified_;
+
+  DISALLOW_COPY_AND_ASSIGN(JSTypeFeedbackLowering);
+};
+
+DEFINE_OPERATORS_FOR_FLAGS(JSTypeFeedbackLowering::Flags)
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_COMPILER_JS_TYPE_FEEDBACK_LOWERING_H_