[esnext] prototype runtime implementation for async functions

src/ast/ast.h

 // Copyright 2012 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_AST_AST_H_
 #define V8_AST_AST_H_
 
 #include "src/assembler.h"
 #include "src/ast/ast-value-factory.h"
 #include "src/ast/modules.h"
@@ skipping 2520 matching lines @@
 class Yield final : public Expression {
  public:
   DECLARE_NODE_TYPE(Yield)
 
   Expression* generator_object() const { return generator_object_; }
   Expression* expression() const { return expression_; }
 
   void set_generator_object(Expression* e) { generator_object_ = e; }
   void set_expression(Expression* e) { expression_ = e; }
 
+  // AsyncFunction's first yield must suspend execution before starting, to
+  // prevent 'already executing' error. This avoids complexities of introducing
+  // an internal generator function.
+  bool is_async_function_start() const { return is_async_function_start_; }
+
  protected:
   Yield(Zone* zone, Expression* generator_object, Expression* expression,
-        int pos)
+        bool is_async_function_start, int pos)
       : Expression(zone, pos),
         generator_object_(generator_object),
-        expression_(expression) {}
+        expression_(expression),
+        is_async_function_start_(is_async_function_start) {}
 
  private:
   Expression* generator_object_;
   Expression* expression_;
+  bool is_async_function_start_;
 };
 
-
 class Throw final : public Expression {
  public:
   DECLARE_NODE_TYPE(Throw)
 
   Expression* exception() const { return exception_; }
   void set_exception(Expression* e) { exception_ = e; }
 
  protected:
   Throw(Zone* zone, Expression* exception, int pos)
       : Expression(zone, pos), exception_(exception) {}
@@ skipping 831 matching lines @@
       assign->binary_operation_ =
           NewBinaryOperation(assign->binary_op(), target, value, pos + 1);
     }
     return assign;
   }
 
   Yield* NewYield(Expression *generator_object,
                   Expression* expression,
                   int pos) {
     if (!expression) expression = NewUndefinedLiteral(pos);
-    return new (local_zone_)
-        Yield(local_zone_, generator_object, expression, pos);
+    const bool is_async_function_start = false;
+    return new (local_zone_) Yield(local_zone_, generator_object, expression,
+                                   is_async_function_start, pos);
+  }
+
+  Yield* NewAsyncFunctionStart(Expression* generator_object,
+                               Expression* expression, int pos) {
+    DCHECK_NOT_NULL(expression);
+    // Starting an AsyncFunction emits a yield which suspends before evaluating
+    // operand.
+    const bool is_async_function_start = true;
+    return new (local_zone_) Yield(local_zone_, generator_object, expression,
+                                   is_async_function_start, pos);
   }
 
   Throw* NewThrow(Expression* exception, int pos) {
     return new (local_zone_) Throw(local_zone_, exception, pos);
   }
 
   FunctionLiteral* NewFunctionLiteral(
       const AstRawString* name, Scope* scope, ZoneList<Statement*>* body,
       int materialized_literal_count, int expected_property_count,
       int parameter_count,
@@ skipping 137 matching lines @@
                                     : NULL;                                   \
   }
 AST_NODE_LIST(DECLARE_NODE_FUNCTIONS)
 #undef DECLARE_NODE_FUNCTIONS
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_AST_AST_H_