Implement [Generator].prototype.return.

src/js/generator.js

Index: src/js/generator.js
diff --git a/src/js/generator.js b/src/js/generator.js
index 7f43656ebcf5c1fb4cfb5eca311e8381082f5c33..c88b15d2b46bb8a5a9d05679638b305def175a70 100644
--- a/src/js/generator.js
+++ b/src/js/generator.js
@@ -26,6 +26,23 @@ utils.Import(function(from) {
 // 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,
@@ -44,7 +61,35 @@ function GeneratorObjectNext(value) {
     }
   } 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);
@@ -78,9 +123,11 @@ function GeneratorObjectThrow(exn) {
 
 // ----------------------------------------------------------------------------
 
-// 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.
@@ -88,6 +135,7 @@ var GeneratorObjectPrototype = GeneratorFunctionPrototype.prototype;
 utils.InstallFunctions(GeneratorObjectPrototype,
                        DONT_ENUM,
                       ["next", GeneratorObjectNext,
+                       "return", GeneratorObjectReturn,
                        "throw", GeneratorObjectThrow]);
 
 %AddNamedProperty(GeneratorObjectPrototype, "constructor",