[async-iteration] implement Async-from-Sync Iterator

Introduce a new Object to allow GetIterator("async") to function when the
iterable does not have a Symbol.asyncIterator method.

This patch has been split out from https://codereview.chromium.org/2622833002/
and incorporates test cases.

BUG=v8:5855, v8:4483
R=jgruber@chromium.org, rmcilroy@chromium.org, neis@chromium.org
TBR=hpayer@chromium.org, bmeurer@chromium.org

Review-Url: https://codereview.chromium.org/2645313003
Cr-Commit-Position: refs/heads/master@{#43419}

BUILD.gn

@@ -930,6 +930,7 @@ v8_source_set("v8_base") {
     "src/builtins/builtins-array.cc",
     "src/builtins/builtins-arraybuffer.cc",
     "src/builtins/builtins-async-function.cc",
+    "src/builtins/builtins-async-iterator.cc",
     "src/builtins/builtins-async.cc",
     "src/builtins/builtins-async.h",
     "src/builtins/builtins-boolean.cc",

src/ast/ast-types.cc

@@ -220,6 +220,7 @@ AstType::bitset AstBitsetType::Lub(i::Map* map) {
     case JS_SET_ITERATOR_TYPE:
     case JS_MAP_ITERATOR_TYPE:
     case JS_STRING_ITERATOR_TYPE:
+    case JS_ASYNC_FROM_SYNC_ITERATOR_TYPE:
 
     case JS_TYPED_ARRAY_KEY_ITERATOR_TYPE:
     case JS_FAST_ARRAY_KEY_ITERATOR_TYPE:

src/bootstrapper.cc

@@ -172,6 +172,7 @@ class Genesis BASE_EMBEDDED {
 
   void CreateStrictModeFunctionMaps(Handle<JSFunction> empty);
   void CreateIteratorMaps(Handle<JSFunction> empty);
+  void CreateAsyncIteratorMaps();
   void CreateAsyncFunctionMaps(Handle<JSFunction> empty);
   void CreateJSProxyMaps();
 
@@ -785,6 +786,50 @@ void Genesis::CreateIteratorMaps(Handle<JSFunction> empty) {
       *generator_object_prototype_map);
 }
 
+void Genesis::CreateAsyncIteratorMaps() {
+  // %AsyncIteratorPrototype%
+  // proposal-async-iteration/#sec-asynciteratorprototype
+  Handle<JSObject> async_iterator_prototype =
+      factory()->NewJSObject(isolate()->object_function(), TENURED);
+
+  Handle<JSFunction> async_iterator_prototype_iterator = SimpleCreateFunction(
+      isolate(), factory()->NewStringFromAsciiChecked("[Symbol.asyncIterator]"),
+      Builtins::kReturnReceiver, 0, true);
+
+  JSObject::AddProperty(async_iterator_prototype,
+                        factory()->async_iterator_symbol(),
+                        async_iterator_prototype_iterator, DONT_ENUM);
+
+  // %AsyncFromSyncIteratorPrototype%
+  // proposal-async-iteration/#sec-%asyncfromsynciteratorprototype%-object
+  Handle<JSObject> async_from_sync_iterator_prototype =
+      factory()->NewJSObject(isolate()->object_function(), TENURED);
+  SimpleInstallFunction(async_from_sync_iterator_prototype,
+                        factory()->next_string(),
+                        Builtins::kAsyncFromSyncIteratorPrototypeNext, 1, true);
+  SimpleInstallFunction(
+      async_from_sync_iterator_prototype, factory()->return_string(),
+      Builtins::kAsyncFromSyncIteratorPrototypeReturn, 1, true);
+  SimpleInstallFunction(
+      async_from_sync_iterator_prototype, factory()->throw_string(),
+      Builtins::kAsyncFromSyncIteratorPrototypeThrow, 1, true);
+
+  JSObject::AddProperty(
+      async_from_sync_iterator_prototype, factory()->to_string_tag_symbol(),
+      factory()->NewStringFromAsciiChecked("Async-from-Sync Iterator"),
+      static_cast<PropertyAttributes>(DONT_ENUM | READ_ONLY));
+
+  JSObject::ForceSetPrototype(async_from_sync_iterator_prototype,
+                              async_iterator_prototype);
+
+  Handle<Map> async_from_sync_iterator_map = factory()->NewMap(
+      JS_ASYNC_FROM_SYNC_ITERATOR_TYPE, JSAsyncFromSyncIterator::kSize);
+  Map::SetPrototype(async_from_sync_iterator_map,
+                    async_from_sync_iterator_prototype);
+  native_context()->set_async_from_sync_iterator_map(
+      *async_from_sync_iterator_map);
+}
+
 void Genesis::CreateAsyncFunctionMaps(Handle<JSFunction> empty) {
   // %AsyncFunctionPrototype% intrinsic
   Handle<JSObject> async_function_prototype =
@@ -1295,6 +1340,16 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
     class_function_map_->SetConstructor(*function_fun);
   }
 
+  {
+    // --- A s y n c F r o m S y n c I t e r a t o r
+    Handle<Code> code = isolate->builtins()->AsyncIteratorValueUnwrap();
+    Handle<SharedFunctionInfo> info =
+        factory->NewSharedFunctionInfo(factory->empty_string(), code, false);
+    info->set_internal_formal_parameter_count(1);
+    info->set_length(1);
+    native_context()->set_async_iterator_value_unwrap_shared_fun(*info);
+  }
+
   {  // --- A r r a y ---
     Handle<JSFunction> array_function =
         InstallFunction(global, "Array", JS_ARRAY_TYPE, JSArray::kSize,
@@ -4761,6 +4816,7 @@ Genesis::Genesis(
     Handle<JSFunction> empty_function = CreateEmptyFunction(isolate);
     CreateStrictModeFunctionMaps(empty_function);
     CreateIteratorMaps(empty_function);
+    CreateAsyncIteratorMaps();
     CreateAsyncFunctionMaps(empty_function);
     Handle<JSGlobalObject> global_object =
         CreateNewGlobals(global_proxy_template, global_proxy);

src/builtins/builtins.h

@@ -846,7 +846,18 @@ class Isolate;
       WasmRuntimeCall, 1)                                                      \
   TFS(ThrowWasmTrapFuncInvalid, BUILTIN, kNoExtraICState, WasmRuntimeCall, 1)  \
   TFS(ThrowWasmTrapFuncSigMismatch, BUILTIN, kNoExtraICState, WasmRuntimeCall, \
-      1)
+      1)                                                                       \
+                                                                               \
+  /* Async-from-Sync Iterator */                                               \
+                                                                               \
+  /* %AsyncFromSyncIteratorPrototype% */                                       \
+  /* (proposal-async-iteration/#sec-%asyncfromsynciteratorprototype%-object)*/ \
+  TFJ(AsyncFromSyncIteratorPrototypeNext, 1)                                   \
+  TFJ(AsyncFromSyncIteratorPrototypeThrow, 1)                                  \
+  TFJ(AsyncFromSyncIteratorPrototypeReturn, 1)                                 \
+                                                                               \
+  /* proposal-async-iteration/#sec-async-iterator-value-unwrap-functions */    \
+  TFJ(AsyncIteratorValueUnwrap, 1)
 
 #define IGNORE_BUILTIN(...)
 

src/code-stub-assembler.cc

@@ -2890,6 +2890,11 @@ Node* CodeStubAssembler::IsJSReceiver(Node* object) {
   return IsJSReceiverInstanceType(LoadInstanceType(object));
 }
 
+Node* CodeStubAssembler::IsJSReceiverMap(Node* map) {
+  STATIC_ASSERT(LAST_JS_OBJECT_TYPE == LAST_TYPE);
+  return IsJSReceiverInstanceType(LoadMapInstanceType(map));
+}
+
 Node* CodeStubAssembler::IsJSObject(Node* object) {
   STATIC_ASSERT(LAST_JS_OBJECT_TYPE == LAST_TYPE);
   return Int32GreaterThanOrEqual(LoadInstanceType(object),

src/code-stub-assembler.h

@@ -692,6 +692,7 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
   Node* IsJSGlobalProxy(Node* object);
   Node* IsJSReceiverInstanceType(Node* instance_type);
   Node* IsJSReceiver(Node* object);
+  Node* IsJSReceiverMap(Node* map);
   Node* IsMap(Node* object);
   Node* IsCallableMap(Node* map);
   Node* IsCallable(Node* object);
@@ -1003,6 +1004,11 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
                          Label* if_found, Variable* var_value,
                          Label* if_not_found, Label* if_bailout);
 
+  Node* GetProperty(Node* context, Node* receiver, Handle<Name> name) {
+    return CallStub(CodeFactory::GetProperty(isolate()), context, receiver,
+                    HeapConstant(name));
+  }
+
   void LoadPropertyFromFastObject(Node* object, Node* map, Node* descriptors,
                                   Node* name_index, Variable* var_details,
                                   Variable* var_value);
@@ -1258,6 +1264,16 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
   void Print(const char* prefix, Node* tagged_value);
   inline void Print(Node* tagged_value) { return Print(nullptr, tagged_value); }
 
+  template <class... TArgs>
+  Node* MakeTypeError(MessageTemplate::Template message, Node* context,
+                      TArgs... args) {
+    STATIC_ASSERT(sizeof...(TArgs) <= 3);
+    Node* const make_type_error = LoadContextElement(
+        LoadNativeContext(context), Context::MAKE_TYPE_ERROR_INDEX);
+    return CallJS(CodeFactory::Call(isolate()), context, make_type_error,
+                  UndefinedConstant(), SmiConstant(message), args...);
+  }
+
  protected:
   void DescriptorLookup(Node* unique_name, Node* descriptors, Node* bitfield3,
                         Label* if_found, Variable* var_name_index,

src/compiler/code-assembler.cc

@@ -188,6 +188,10 @@ Node* CodeAssembler::HeapConstant(Handle<HeapObject> object) {
   return raw_assembler()->HeapConstant(object);
 }
 
+Node* CodeAssembler::CStringConstant(const char* str) {
+  return HeapConstant(factory()->NewStringFromAsciiChecked(str, TENURED));
+}
+
 Node* CodeAssembler::BooleanConstant(bool value) {
   return raw_assembler()->BooleanConstant(value);
 }

src/compiler/code-assembler.h

@@ -216,6 +216,7 @@ class V8_EXPORT_PRIVATE CodeAssembler {
   Node* SmiConstant(Smi* value);
   Node* SmiConstant(int value);
   Node* HeapConstant(Handle<HeapObject> object);
+  Node* CStringConstant(const char* str);
   Node* BooleanConstant(bool value);
   Node* ExternalConstant(ExternalReference address);
   Node* Float64Constant(double value);

src/compiler/types.cc

@@ -225,6 +225,7 @@ Type::bitset BitsetType::Lub(i::Map* map) {
     case JS_SET_ITERATOR_TYPE:
     case JS_MAP_ITERATOR_TYPE:
     case JS_STRING_ITERATOR_TYPE:
+    case JS_ASYNC_FROM_SYNC_ITERATOR_TYPE:
 
     case JS_TYPED_ARRAY_KEY_ITERATOR_TYPE:
     case JS_FAST_ARRAY_KEY_ITERATOR_TYPE:

src/contexts.h

@@ -197,11 +197,14 @@ enum ContextLookupFlags {
   V(ARRAY_BUFFER_FUN_INDEX, JSFunction, array_buffer_fun)                      \
   V(ARRAY_BUFFER_MAP_INDEX, Map, array_buffer_map)                             \
   V(ARRAY_FUNCTION_INDEX, JSFunction, array_function)                          \
+  V(ASYNC_FROM_SYNC_ITERATOR_MAP_INDEX, Map, async_from_sync_iterator_map)     \
   V(ASYNC_FUNCTION_AWAIT_REJECT_SHARED_FUN, SharedFunctionInfo,                \
     async_function_await_reject_shared_fun)                                    \
   V(ASYNC_FUNCTION_AWAIT_RESOLVE_SHARED_FUN, SharedFunctionInfo,               \
     async_function_await_resolve_shared_fun)                                   \
   V(ASYNC_FUNCTION_FUNCTION_INDEX, JSFunction, async_function_constructor)     \
+  V(ASYNC_ITERATOR_VALUE_UNWRAP_SHARED_FUN, SharedFunctionInfo,                \
+    async_iterator_value_unwrap_shared_fun)                                    \
   V(BOOLEAN_FUNCTION_INDEX, JSFunction, boolean_function)                      \
   V(BOUND_FUNCTION_WITH_CONSTRUCTOR_MAP_INDEX, Map,                            \
     bound_function_with_constructor_map)                                       \

src/deoptimizer.cc

@@ -3794,6 +3794,19 @@ Handle<Object> TranslatedState::MaterializeCapturedObjectAt(
       object->set_index(Smi::cast(*next_index)->value());
       return object;
     }
+    case JS_ASYNC_FROM_SYNC_ITERATOR_TYPE: {
+      Handle<JSAsyncFromSyncIterator> object =
+          Handle<JSAsyncFromSyncIterator>::cast(
+              isolate_->factory()->NewJSObjectFromMap(map, NOT_TENURED));
+      slot->value_ = object;
+      Handle<Object> properties = materializer.FieldAt(value_index);
+      Handle<Object> elements = materializer.FieldAt(value_index);
+      Handle<Object> sync_iterator = materializer.FieldAt(value_index);
+      object->set_properties(FixedArray::cast(*properties));
+      object->set_elements(FixedArrayBase::cast(*elements));
+      object->set_sync_iterator(JSReceiver::cast(*sync_iterator));
+      return object;
+    }
     case JS_ARRAY_TYPE: {
       Handle<JSArray> object = Handle<JSArray>::cast(
           isolate_->factory()->NewJSObjectFromMap(map, NOT_TENURED));

src/factory.cc

@@ -1975,6 +1975,16 @@ Handle<JSIteratorResult> Factory::NewJSIteratorResult(Handle<Object> value,
   return js_iter_result;
 }
 
+Handle<JSAsyncFromSyncIterator> Factory::NewJSAsyncFromSyncIterator(
+    Handle<JSReceiver> sync_iterator) {
+  Handle<Map> map(isolate()->native_context()->async_from_sync_iterator_map());
+  Handle<JSAsyncFromSyncIterator> iterator =
+      Handle<JSAsyncFromSyncIterator>::cast(NewJSObjectFromMap(map));
+
+  iterator->set_sync_iterator(*sync_iterator);
+  return iterator;
+}
+
 Handle<JSMap> Factory::NewJSMap() {
   Handle<Map> map(isolate()->native_context()->js_map_map());
   Handle<JSMap> js_map = Handle<JSMap>::cast(NewJSObjectFromMap(map));

src/factory.h

@@ -558,6 +558,8 @@ class V8_EXPORT_PRIVATE Factory final {
                                    size_t byte_offset, size_t byte_length);
 
   Handle<JSIteratorResult> NewJSIteratorResult(Handle<Object> value, bool done);
+  Handle<JSAsyncFromSyncIterator> NewJSAsyncFromSyncIterator(
+      Handle<JSReceiver> sync_iterator);
 
   Handle<JSMap> NewJSMap();
   Handle<JSSet> NewJSSet();

src/heap/objects-visiting.cc

@@ -109,6 +109,7 @@ StaticVisitorBase::VisitorId StaticVisitorBase::GetVisitorId(
     case JS_OBJECT_TYPE:
     case JS_ERROR_TYPE:
     case JS_ARGUMENTS_TYPE:
+    case JS_ASYNC_FROM_SYNC_ITERATOR_TYPE:
     case JS_CONTEXT_EXTENSION_OBJECT_TYPE:
     case JS_GENERATOR_OBJECT_TYPE:
     case JS_MODULE_NAMESPACE_TYPE:

src/interpreter/interpreter-intrinsics.cc

@@ -299,6 +299,44 @@ Node* IntrinsicsHelper::ClassOf(Node* args_reg, Node* arg_count,
   return __ ClassOf(value);
 }
 
+Node* IntrinsicsHelper::CreateAsyncFromSyncIterator(Node* args_reg,
+                                                    Node* arg_count,
+                                                    Node* context) {
+  InterpreterAssembler::Label not_receiver(
+      assembler_, InterpreterAssembler::Label::kDeferred);
+  InterpreterAssembler::Label done(assembler_);
+  InterpreterAssembler::Variable return_value(assembler_,
+                                              MachineRepresentation::kTagged);
+
+  Node* sync_iterator = __ LoadRegister(args_reg);
+
+  __ GotoIf(__ TaggedIsSmi(sync_iterator), &not_receiver);
+  __ GotoIfNot(__ IsJSReceiver(sync_iterator), &not_receiver);
+
+  Node* const native_context = __ LoadNativeContext(context);
+  Node* const map = __ LoadContextElement(
+      native_context, Context::ASYNC_FROM_SYNC_ITERATOR_MAP_INDEX);
+  Node* const iterator = __ AllocateJSObjectFromMap(map);
+
+  __ StoreObjectFieldNoWriteBarrier(
+      iterator, JSAsyncFromSyncIterator::kSyncIteratorOffset, sync_iterator);
+
+  return_value.Bind(iterator);
+  __ Goto(&done);
+
+  __ Bind(&not_receiver);
+  {
+    return_value.Bind(
+        __ CallRuntime(Runtime::kThrowSymbolIteratorInvalid, context));
+
+    // Unreachable due to the Throw in runtime call.
+    __ Goto(&done);
+  }
+
+  __ Bind(&done);
+  return return_value.value();
+}
+
 void IntrinsicsHelper::AbortIfArgCountMismatch(int expected, Node* actual) {
   InterpreterAssembler::Label match(assembler_);
   Node* comparison = __ Word32Equal(actual, __ Int32Constant(expected));

src/interpreter/interpreter-intrinsics.h

@@ -23,21 +23,22 @@ namespace interpreter {
 
 // List of supported intrisics, with upper case name, lower case name and
 // expected number of arguments (-1 denoting argument count is variable).
-#define INTRINSICS_LIST(V)                                \
-  V(Call, call, -1)                                       \
-  V(ClassOf, class_of, 1)                                 \
-  V(CreateIterResultObject, create_iter_result_object, 2) \
-  V(HasProperty, has_property, 2)                         \
-  V(IsArray, is_array, 1)                                 \
-  V(IsJSProxy, is_js_proxy, 1)                            \
-  V(IsJSReceiver, is_js_receiver, 1)                      \
-  V(IsSmi, is_smi, 1)                                     \
-  V(IsTypedArray, is_typed_array, 1)                      \
-  V(SubString, sub_string, 3)                             \
-  V(ToString, to_string, 1)                               \
-  V(ToLength, to_length, 1)                               \
-  V(ToInteger, to_integer, 1)                             \
-  V(ToNumber, to_number, 1)                               \
+#define INTRINSICS_LIST(V)                                           \
+  V(Call, call, -1)                                                  \
+  V(ClassOf, class_of, 1)                                            \
+  V(CreateIterResultObject, create_iter_result_object, 2)            \
+  V(CreateAsyncFromSyncIterator, create_async_from_sync_iterator, 1) \
+  V(HasProperty, has_property, 2)                                    \
+  V(IsArray, is_array, 1)                                            \
+  V(IsJSProxy, is_js_proxy, 1)                                       \
+  V(IsJSReceiver, is_js_receiver, 1)                                 \
+  V(IsSmi, is_smi, 1)                                                \
+  V(IsTypedArray, is_typed_array, 1)                                 \
+  V(SubString, sub_string, 3)                                        \
+  V(ToString, to_string, 1)                                          \
+  V(ToLength, to_length, 1)                                          \
+  V(ToInteger, to_integer, 1)                                        \
+  V(ToNumber, to_number, 1)                                          \
   V(ToObject, to_object, 1)
 
 class IntrinsicsHelper {

src/objects-body-descriptors-inl.h

@@ -467,6 +467,7 @@ ReturnType BodyDescriptorApply(InstanceType type, T1 p1, T2 p2, T3 p3) {
     case JS_OBJECT_TYPE:
     case JS_ERROR_TYPE:
     case JS_ARGUMENTS_TYPE:
+    case JS_ASYNC_FROM_SYNC_ITERATOR_TYPE:
     case JS_PROMISE_CAPABILITY_TYPE:
     case JS_PROMISE_TYPE:
     case JS_CONTEXT_EXTENSION_OBJECT_TYPE:

src/objects-debug.cc

@@ -200,6 +200,9 @@ void HeapObject::HeapObjectVerify() {
     case JS_STRING_ITERATOR_TYPE:
       JSStringIterator::cast(this)->JSStringIteratorVerify();
       break;
+    case JS_ASYNC_FROM_SYNC_ITERATOR_TYPE:
+      JSAsyncFromSyncIterator::cast(this)->JSAsyncFromSyncIteratorVerify();
+      break;
     case JS_WEAK_MAP_TYPE:
       JSWeakMap::cast(this)->JSWeakMapVerify();
       break;
@@ -890,6 +893,12 @@ void JSStringIterator::JSStringIteratorVerify() {
   CHECK_LE(index(), String::kMaxLength);
 }
 
+void JSAsyncFromSyncIterator::JSAsyncFromSyncIteratorVerify() {
+  CHECK(IsJSAsyncFromSyncIterator());
+  JSObjectVerify();
+  VerifyHeapPointer(sync_iterator());
+}
+
 void JSWeakSet::JSWeakSetVerify() {
   CHECK(IsJSWeakSet());
   JSObjectVerify();

src/objects-inl.h

@@ -164,6 +164,7 @@ TYPE_CHECKER(JSPromise, JS_PROMISE_TYPE)
 TYPE_CHECKER(JSRegExp, JS_REGEXP_TYPE)
 TYPE_CHECKER(JSSet, JS_SET_TYPE)
 TYPE_CHECKER(JSSetIterator, JS_SET_ITERATOR_TYPE)
+TYPE_CHECKER(JSAsyncFromSyncIterator, JS_ASYNC_FROM_SYNC_ITERATOR_TYPE)
 TYPE_CHECKER(JSStringIterator, JS_STRING_ITERATOR_TYPE)
 TYPE_CHECKER(JSTypedArray, JS_TYPED_ARRAY_TYPE)
 TYPE_CHECKER(JSValue, JS_VALUE_TYPE)
@@ -656,6 +657,7 @@ CAST_ACCESSOR(JSPromiseCapability)
 CAST_ACCESSOR(JSPromise)
 CAST_ACCESSOR(JSSet)
 CAST_ACCESSOR(JSSetIterator)
+CAST_ACCESSOR(JSAsyncFromSyncIterator)
 CAST_ACCESSOR(JSStringIterator)
 CAST_ACCESSOR(JSArrayIterator)
 CAST_ACCESSOR(JSTypedArray)
@@ -8214,6 +8216,9 @@ ACCESSORS(JSArrayIterator, object, Object, kIteratedObjectOffset)
 ACCESSORS(JSArrayIterator, index, Object, kNextIndexOffset)
 ACCESSORS(JSArrayIterator, object_map, Object, kIteratedObjectMapOffset)
 
+ACCESSORS(JSAsyncFromSyncIterator, sync_iterator, JSReceiver,
+          kSyncIteratorOffset)
+
 ACCESSORS(JSStringIterator, string, String, kStringOffset)
 SMI_ACCESSORS(JSStringIterator, index, kNextIndexOffset)
 

src/objects.cc

@@ -12555,6 +12555,7 @@ bool CanSubclassHaveInobjectProperties(InstanceType instance_type) {
     case JS_API_OBJECT_TYPE:
     case JS_ARRAY_BUFFER_TYPE:
     case JS_ARRAY_TYPE:
+    case JS_ASYNC_FROM_SYNC_ITERATOR_TYPE:
     case JS_CONTEXT_EXTENSION_OBJECT_TYPE:
     case JS_DATA_VIEW_TYPE:
     case JS_DATE_TYPE:

src/objects.h

@@ -398,6 +398,7 @@ const int kStubMinorKeyBits = kSmiValueSize - kStubMajorKeyBits - 1;
   V(JS_PROMISE_TYPE)                                            \
   V(JS_REGEXP_TYPE)                                             \
   V(JS_ERROR_TYPE)                                              \
+  V(JS_ASYNC_FROM_SYNC_ITERATOR_TYPE)                           \
   V(JS_STRING_ITERATOR_TYPE)                                    \
                                                                 \
   V(JS_TYPED_ARRAY_KEY_ITERATOR_TYPE)                           \
@@ -743,6 +744,7 @@ enum InstanceType {
   JS_PROMISE_TYPE,
   JS_REGEXP_TYPE,
   JS_ERROR_TYPE,
+  JS_ASYNC_FROM_SYNC_ITERATOR_TYPE,
   JS_STRING_ITERATOR_TYPE,
 
   JS_TYPED_ARRAY_KEY_ITERATOR_TYPE,
@@ -1039,6 +1041,7 @@ template <class C> inline bool Is(Object* obj);
   V(JSArray)                     \
   V(JSArrayBuffer)               \
   V(JSArrayBufferView)           \
+  V(JSAsyncFromSyncIterator)     \
   V(JSCollection)                \
   V(JSTypedArray)                \
   V(JSArrayIterator)             \
@@ -10437,6 +10440,32 @@ class JSArrayIterator : public JSObject {
   DISALLOW_IMPLICIT_CONSTRUCTORS(JSArrayIterator);
 };
 
+// The [Async-from-Sync Iterator] object
+// (proposal-async-iteration/#sec-async-from-sync-iterator-objects)
+// An object which wraps an ordinary Iterator and converts it to behave
+// according to the Async Iterator protocol.
+// (See https://tc39.github.io/proposal-async-iteration/#sec-iteration)
+class JSAsyncFromSyncIterator : public JSObject {
+ public:
+  DECLARE_CAST(JSAsyncFromSyncIterator)
+  DECLARE_PRINTER(JSAsyncFromSyncIterator)
+  DECLARE_VERIFIER(JSAsyncFromSyncIterator)
+
+  // Async-from-Sync Iterator instances are ordinary objects that inherit
+  // properties from the %AsyncFromSyncIteratorPrototype% intrinsic object.
+  // Async-from-Sync Iterator instances are initially created with the internal
+  // slots listed in Table 4.
+  // (proposal-async-iteration/#table-async-from-sync-iterator-internal-slots)
+  DECL_ACCESSORS(sync_iterator, JSReceiver)
+
+  // Offsets of object fields.
+  static const int kSyncIteratorOffset = JSObject::kHeaderSize;
+  static const int kSize = kSyncIteratorOffset + kPointerSize;
+
+ private:
+  DISALLOW_IMPLICIT_CONSTRUCTORS(JSAsyncFromSyncIterator);
+};
+
 class JSStringIterator : public JSObject {
  public:
   // Dispatched behavior.

src/runtime/runtime-internal.cc

@@ -511,10 +511,18 @@ RUNTIME_FUNCTION(Runtime_AllowDynamicFunction) {
 }
 
 RUNTIME_FUNCTION(Runtime_CreateAsyncFromSyncIterator) {
-  // TODO(caitp): split AsyncFromSyncIterator functionality out of
-  //              https://codereview.chromium.org/2622833002
-  UNREACHABLE();
-  return isolate->heap()->undefined_value();
+  HandleScope scope(isolate);
+  DCHECK_EQ(1, args.length());
+
+  CONVERT_ARG_HANDLE_CHECKED(Object, sync_iterator, 0);
+
+  if (!sync_iterator->IsJSReceiver()) {
+    THROW_NEW_ERROR_RETURN_FAILURE(
+        isolate, NewTypeError(MessageTemplate::kSymbolIteratorInvalid));
+  }
+
+  return *isolate->factory()->NewJSAsyncFromSyncIterator(
+      Handle<JSReceiver>::cast(sync_iterator));
 }
 
 }  // namespace internal

src/v8.gyp

@@ -477,6 +477,7 @@
         'builtins/builtins-arguments.h',
         'builtins/builtins-arraybuffer.cc',
         'builtins/builtins-array.cc',
+        'builtins/builtins-async-iterator.cc',
         'builtins/builtins-async-function.cc',
         'builtins/builtins-async.cc',
         'builtins/builtins-async.h',

test/cctest/interpreter/generate-bytecode-expectations.cc

@@ -43,6 +43,7 @@ class ProgramOptions final {
         module_(false),
         top_level_(false),
         do_expressions_(false),
+        async_iteration_(false),
         verbose_(false) {}
 
   bool Validate() const;
@@ -61,6 +62,7 @@ class ProgramOptions final {
   bool module() const { return module_; }
   bool top_level() const { return top_level_; }
   bool do_expressions() const { return do_expressions_; }
+  bool async_iteration() const { return async_iteration_; }
   bool verbose() const { return verbose_; }
   bool suppress_runtime_errors() const { return rebaseline_ && !verbose_; }
   std::vector<std::string> input_filenames() const { return input_filenames_; }
@@ -77,6 +79,7 @@ class ProgramOptions final {
   bool module_;
   bool top_level_;
   bool do_expressions_;
+  bool async_iteration_;
   bool verbose_;
   std::vector<std::string> input_filenames_;
   std::string output_filename_;
@@ -165,6 +168,8 @@ ProgramOptions ProgramOptions::FromCommandLine(int argc, char** argv) {
       options.top_level_ = true;
     } else if (strcmp(argv[i], "--do-expressions") == 0) {
       options.do_expressions_ = true;
+    } else if (strcmp(argv[i], "--async-iteration") == 0) {
+      options.async_iteration_ = true;
     } else if (strcmp(argv[i], "--verbose") == 0) {
       options.verbose_ = true;
     } else if (strncmp(argv[i], "--output=", 9) == 0) {
@@ -267,6 +272,8 @@ void ProgramOptions::UpdateFromHeader(std::istream& stream) {
       top_level_ = ParseBoolean(line.c_str() + 11);
     } else if (line.compare(0, 16, "do expressions: ") == 0) {
       do_expressions_ = ParseBoolean(line.c_str() + 16);
+    } else if (line.compare(0, 17, "async iteration: ") == 0) {
+      async_iteration_ = ParseBoolean(line.c_str() + 17);
     } else if (line == "---") {
       break;
     } else if (line.empty()) {
@@ -289,6 +296,7 @@ void ProgramOptions::PrintHeader(std::ostream& stream) const {  // NOLINT
   if (module_) stream << "\nmodule: yes";
   if (top_level_) stream << "\ntop level: yes";
   if (do_expressions_) stream << "\ndo expressions: yes";
+  if (async_iteration_) stream << "\nasync iteration: yes";
 
   stream << "\n\n";
 }
@@ -393,6 +401,7 @@ void GenerateExpectationsFile(std::ostream& stream,  // NOLINT
   }
 
   if (options.do_expressions()) i::FLAG_harmony_do_expressions = true;
+  if (options.async_iteration()) i::FLAG_harmony_async_iteration = true;
 
   stream << "#\n# Autogenerated by generate-bytecode-expectations.\n#\n\n";
   options.PrintHeader(stream);
@@ -401,6 +410,7 @@ void GenerateExpectationsFile(std::ostream& stream,  // NOLINT
   }
 
   i::FLAG_harmony_do_expressions = false;
+  i::FLAG_harmony_async_iteration = false;
 }
 
 bool WriteExpectationsFile(const std::vector<std::string>& snippet_list,
@@ -445,6 +455,7 @@ void PrintUsage(const char* exec_path) {
          "Specify the name of the test function.\n"
          "  --top-level   Process top level code, not the top-level function.\n"
          "  --do-expressions  Enable harmony_do_expressions flag.\n"
+         "  --async-iteration  Enable harmony_async_iteration flag.\n"
          "  --output=file.name\n"
          "      Specify the output file. If not specified, output goes to "
          "stdout.\n"

test/cctest/interpreter/test-bytecode-generator.cc

@@ -2371,6 +2371,45 @@ TEST(NewAndSpread) {
                      LoadGolden("NewAndSpread.golden")));
 }
 
+TEST(ForAwaitOf) {
+  bool old_flag = i::FLAG_harmony_async_iteration;
+  i::FLAG_harmony_async_iteration = true;
+  InitializedIgnitionHandleScope scope;
+  BytecodeExpectationsPrinter printer(CcTest::isolate());
+  printer.set_wrap(false);
+  printer.set_test_function_name("f");
+
+  const char* snippets[] = {
+      "async function f() {\n"
+      "  for await (let x of [1, 2, 3]) {}\n"
+      "}\n"
+      "f();\n",
+
+      "async function f() {\n"
+      "  for await (let x of [1, 2, 3]) { return x; }\n"
+      "}\n"
+      "f();\n",
+
+      "async function f() {\n"
+      "  for await (let x of [10, 20, 30]) {\n"
+      "    if (x == 10) continue;\n"
+      "    if (x == 20) break;\n"
+      "  }\n"
+      "}\n"
+      "f();\n",
+
+      "async function f() {\n"
+      "  var x = { 'a': 1, 'b': 2 };\n"
+      "  for (x['a'] of [1,2,3]) { return x['a']; }\n"
+      "}\n"
+      "f();\n"};
+
+  CHECK(CompareTexts(BuildActual(printer, snippets),
+                     LoadGolden("ForAwaitOf.golden")));
+
+  i::FLAG_harmony_async_iteration = old_flag;
+}
+
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8