Implement [Generator].prototype.return.

src/js/generator.js

 // Copyright 2013 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.
 
 (function(global, utils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
 
 var GeneratorFunctionPrototype = utils.ImportNow("GeneratorFunctionPrototype");
 var GeneratorFunction = utils.ImportNow("GeneratorFunction");
 var GlobalFunction = global.Function;
 var MakeTypeError;
 var toStringTagSymbol = utils.ImportNow("to_string_tag_symbol");
 
 utils.Import(function(from) {
   MakeTypeError = from.MakeTypeError;
 });
 
 // ----------------------------------------------------------------------------
 
 // Generator functions and objects are specified by ES6, sections 15.19.3 and
 // 15.19.4.
 
+// Explanation of the use of %GeneratorClose:
+//
+// There are two situations in which resuming a generator must result in
+// closing it (in ES6 terms: setting [[GeneratorState]] to "completed").
+// - When the generator throws.  We close it by calling %GeneratorClose in the
+//   catch blocks below.
+// - When the generator returns a "done" iterator result.
+//   * For GeneratorObjectNext and GeneratorObjectThrow this can only happen
+//     when the generator hits a "final" yield (as introduced by the
+//     parser).  A final yield already takes care of closing the generator
+//     itself, so we don't need to do it here.
+//   * For GeneratorObjectReturn this can also happen if the triggered return is
+//     not "caught" and terminates the generator.  We handle this here by just
+//     always calling %GeneratorClose when the received result is "done".  This
+//     may include cases where the generator has already been closed but doing
+//     it twice is harmless.
+
 function GeneratorObjectNext(value) {
   if (!IS_GENERATOR(this)) {
     throw MakeTypeError(kIncompatibleMethodReceiver,
                         '[Generator].prototype.next', this);
   }
 
   var continuation = %GeneratorGetContinuation(this);
   if (continuation > 0) {
     // Generator is suspended.
     DEBUG_PREPARE_STEP_IN_IF_STEPPING(this);
     try {
       return %_GeneratorNext(this, value);
     } catch (e) {
       %GeneratorClose(this);
       throw e;
     }
   } else if (continuation == 0) {
     // Generator is already closed.
-    return { value: void 0, done: true };
+    return { value: UNDEFINED, done: true };

[Dan Ehrenberg, 2016/01/20 23:48:21]

Not that this should be done in this patch, but shouldn't we make sure that
these objects are of the IterationResult map by calling
%_CreateIterResultObject? (Probably slower in fullcodegen for now, but more
optimizable.)

[Benedikt Meurer, 2016/01/21 05:09:02]

Nice catch! Yes that should use %_CreateIterResultObject, which is BTW.
optimized in fullcodegen as well.

   } else {
     // Generator is running.
     throw MakeTypeError(kGeneratorRunning);
+  }
+}
+
+
+function GeneratorObjectReturn(value) {
+  if (!IS_GENERATOR(this)) {
+    throw MakeTypeError(kIncompatibleMethodReceiver,
+                        '[Generator].prototype.return', this);
+  }
+
+  var continuation = %GeneratorGetContinuation(this);
+  if (continuation > 0) {
+    // Generator is suspended.
+    DEBUG_PREPARE_STEP_IN_IF_STEPPING(this);
+    try {
+      let result = %_GeneratorReturn(this, value);
+      if (result.done) %GeneratorClose(this);
+      return result;
+    } catch (e) {
+      %GeneratorClose(this);
+      throw e;
+    }
+  } else if (continuation == 0) {
+    // Generator is already closed.
+    return { value: value, done: true };

[Benedikt Meurer, 2016/01/21 05:09:02]

Use %_CreateIterResultObject, as explained above.

+  } else {
+    // Generator is running.
+    throw MakeTypeError(kGeneratorRunning);
   }
 }
 
 
 function GeneratorObjectThrow(exn) {
   if (!IS_GENERATOR(this)) {
     throw MakeTypeError(kIncompatibleMethodReceiver,
                         '[Generator].prototype.throw', this);
   }
 
@@ skipping 10 matching lines @@
     // Generator is already closed.
     throw exn;
   } else {
     // Generator is running.
     throw MakeTypeError(kGeneratorRunning);
   }
 }
 
 // ----------------------------------------------------------------------------
 
-// Both Runtime_GeneratorNext and Runtime_GeneratorThrow are supported by
-// neither Crankshaft nor TurboFan, disable optimization of wrappers here.
+// None of the three resume operations (Runtime_GeneratorNext,
+// Runtime_GeneratorReturn, Runtime_GeneratorThrow) is supported by
+// Crankshaft or TurboFan.  Disable optimization of wrappers here.
 %NeverOptimizeFunction(GeneratorObjectNext);
+%NeverOptimizeFunction(GeneratorObjectReturn);
 %NeverOptimizeFunction(GeneratorObjectThrow);
 
 // Set up non-enumerable functions on the generator prototype object.
 var GeneratorObjectPrototype = GeneratorFunctionPrototype.prototype;
 utils.InstallFunctions(GeneratorObjectPrototype,
                        DONT_ENUM,
                       ["next", GeneratorObjectNext,
+                       "return", GeneratorObjectReturn,
                        "throw", GeneratorObjectThrow]);
 
 %AddNamedProperty(GeneratorObjectPrototype, "constructor",
     GeneratorFunctionPrototype, DONT_ENUM | READ_ONLY);
 %AddNamedProperty(GeneratorObjectPrototype,
     toStringTagSymbol, "Generator", DONT_ENUM | READ_ONLY);
 %InternalSetPrototype(GeneratorFunctionPrototype, GlobalFunction.prototype);
 %AddNamedProperty(GeneratorFunctionPrototype,
     toStringTagSymbol, "GeneratorFunction", DONT_ENUM | READ_ONLY);
 %AddNamedProperty(GeneratorFunctionPrototype, "constructor",
     GeneratorFunction, DONT_ENUM | READ_ONLY);
 %InternalSetPrototype(GeneratorFunction, GlobalFunction);
 
 })