Make AstConsString::length constant-time instead of O(N)

src/ast/ast-value-factory.h

 // Copyright 2014 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 44 matching lines @@
     DCHECK(!string_.is_null());
     return string_;
   }
 
  protected:
   // This is null until the string is internalized.
   Handle<String> string_;
 };
 
 
-class AstRawString : public AstString {
+class AstRawString final : public AstString {
  public:
   int length() const override {
     if (is_one_byte_)
       return literal_bytes_.length();
     return literal_bytes_.length() / 2;
   }
 
   int byte_length() const { return literal_bytes_.length(); }
 
   void Internalize(Isolate* isolate) override;
@@ skipping 32 matching lines @@
         hash_(0) {}
 
   bool is_one_byte_;
 
   // Points to memory owned by Zone.
   Vector<const byte> literal_bytes_;
   uint32_t hash_;
 };
 
 
-class AstConsString : public AstString {
+class AstConsString final : public AstString {
  public:
   AstConsString(const AstString* left, const AstString* right)
-      : left_(left),
-        right_(right) {}
+      : length_(left->length() + right->length()), left_(left), right_(right) {}
 
-  int length() const override { return left_->length() + right_->length(); }
+  int length() const override { return length_; }
 
   void Internalize(Isolate* isolate) override;
 
  private:
-  friend class AstValueFactory;
-
+  const int length_;
   const AstString* left_;
   const AstString* right_;
 };
 
 
 // AstValue is either a string, a number, a string array, a boolean, or a
 // special value (null, undefined, the hole).
 class AstValue : public ZoneObject {
  public:
   bool IsString() const {
@@ skipping 224 matching lines @@
   OTHER_CONSTANTS(F)
 #undef F
 };
 }  // namespace internal
 }  // namespace v8
 
 #undef STRING_CONSTANTS
 #undef OTHER_CONSTANTS
 
 #endif  // V8_AST_AST_VALUE_FACTORY_H_