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

src/builtins/builtins-async-iterator.cc

+// Copyright 2017 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.
+
+#include "src/builtins/builtins-async.h"
+#include "src/builtins/builtins-utils.h"
+#include "src/builtins/builtins.h"
+#include "src/code-factory.h"
+#include "src/code-stub-assembler.h"
+#include "src/frames-inl.h"
+
+namespace v8 {
+namespace internal {
+
+namespace {
+
+// Describe fields of Context associated with the AsyncIterator unwrap closure.
+class ValueUnwrapContext {
+ public:
+  enum Fields { kDoneSlot = Context::MIN_CONTEXT_SLOTS, kLength };
+};
+
+class AsyncFromSyncBuiltinsAssembler : public AsyncBuiltinsAssembler {
+ public:
+  explicit AsyncFromSyncBuiltinsAssembler(CodeAssemblerState* state)
+      : AsyncBuiltinsAssembler(state) {}
+
+  void ThrowIfNotAsyncFromSyncIterator(Node* const context, Node* const object,
+                                       Label* if_exception,
+                                       Variable* var_exception,
+                                       const char* method_name);
+
+  // Load "value" and "done" from an iterator result object. If an exception
+  // is thrown at any point, jumps to te `if_exception` label with exception
+  // stored in `var_exception`.
+  //
+  // Returns a Pair of Nodes, whose first element is the value of the "value"
+  // property, and whose second element is the value of the "done" property,
+  // converted to a Boolean if needed.
+  std::pair<Node*, Node*> LoadIteratorResult(Node* const context,
+                                             Node* const native_context,
+                                             Node* const iter_result,
+                                             Label* if_exception,
+                                             Variable* var_exception);
+
+  Node* CreateUnwrapClosure(Node* const native_context, Node* const done);
+};
+
+void AsyncFromSyncBuiltinsAssembler::ThrowIfNotAsyncFromSyncIterator(
+    Node* const context, Node* const object, Label* if_exception,
+    Variable* var_exception, const char* method_name) {
+  Label if_receiverisincompatible(this, Label::kDeferred), done(this);
+
+  GotoIf(TaggedIsSmi(object), &if_receiverisincompatible);
+  Branch(HasInstanceType(object, JS_ASYNC_FROM_SYNC_ITERATOR_TYPE), &done,
+         &if_receiverisincompatible);
+
+  Bind(&if_receiverisincompatible);
+  {
+    // If Type(O) is not Object, or if O does not have a [[SyncIterator]]
+    // internal slot, then
+
+    // Let badIteratorError be a new TypeError exception.
+    Node* const error =
+        MakeTypeError(MessageTemplate::kIncompatibleMethodReceiver, context,
+                      CStringConstant(method_name), object);
+
+    // Perform ! Call(promiseCapability.[[Reject]], undefined,
+    //                « badIteratorError »).
+    var_exception->Bind(error);
+    Goto(if_exception);
+  }
+
+  Bind(&done);
+}
+
+std::pair<Node*, Node*> AsyncFromSyncBuiltinsAssembler::LoadIteratorResult(
+    Node* const context, Node* const native_context, Node* const iter_result,
+    Label* if_exception, Variable* var_exception) {
+  Label if_fastpath(this), if_slowpath(this), merge(this), to_boolean(this),
+      done(this);
+  GotoIf(TaggedIsSmi(iter_result), &if_slowpath);

[neis, 2017/02/20 11:27:59]

I think LoadIteratorResult should either assume that iter_result is a JSReceiver
or throw a TypeError if it isn't.  See also the comment in .next.

+
+  Node* const fast_iter_result_map =
+      LoadContextElement(native_context, Context::ITERATOR_RESULT_MAP_INDEX);
+  Node* const iter_result_map = LoadMap(iter_result);
+
+  Variable var_value(this, MachineRepresentation::kTagged);
+  Variable var_done(this, MachineRepresentation::kTagged);
+  Branch(WordEqual(iter_result_map, fast_iter_result_map), &if_fastpath,
+         &if_slowpath);
+
+  Bind(&if_fastpath);
+  {
+    var_value.Bind(
+        LoadObjectField(iter_result, JSIteratorResult::kValueOffset));
+    var_done.Bind(LoadObjectField(iter_result, JSIteratorResult::kDoneOffset));
+    Goto(&merge);
+  }
+
+  Bind(&if_slowpath);
+  {
+    // Let nextValue be IteratorValue(nextResult).
+    // IfAbruptRejectPromise(nextValue, promiseCapability).
+    Node* const value =
+        GetProperty(context, iter_result, factory()->value_string());
+    GotoIfException(value, if_exception, var_exception);
+
+    // Let nextDone be IteratorComplete(nextResult).
+    // IfAbruptRejectPromise(nextDone, promiseCapability).
+    Node* const done =
+        GetProperty(context, iter_result, factory()->done_string());
+    GotoIfException(done, if_exception, var_exception);
+
+    var_value.Bind(value);
+    var_done.Bind(done);
+    Goto(&merge);
+  }
+
+  Bind(&merge);
+  // Ensure `iterResult.done` is a Boolean.
+  GotoIf(TaggedIsSmi(var_done.value()), &to_boolean);
+  Branch(IsBoolean(var_done.value()), &done, &to_boolean);
+
+  Bind(&to_boolean);
+  {
+    Node* const result =
+        CallStub(CodeFactory::ToBoolean(isolate()), context, var_done.value());
+    var_done.Bind(result);
+    Goto(&done);
+  }
+
+  Bind(&done);
+  return std::make_pair(var_value.value(), var_done.value());
+}
+
+Node* AsyncFromSyncBuiltinsAssembler::CreateUnwrapClosure(Node* native_context,

[neis, 2017/02/20 11:27:59]

It's funny that they call this "unwrap" function.

+                                                          Node* done) {
+  Node* const map = LoadContextElement(
+      native_context, Context::STRICT_FUNCTION_WITHOUT_PROTOTYPE_MAP_INDEX);
+  Node* const on_resolve_shared = LoadContextElement(
+      native_context, Context::ASYNC_ITERATOR_VALUE_UNWRAP_SHARED_FUN);
+  CSA_ASSERT(this,
+             HasInstanceType(on_resolve_shared, SHARED_FUNCTION_INFO_TYPE));
+  Node* const closure_context =
+      AllocateAsyncIteratorValueUnwrapContext(native_context, done);
+  return AllocateFunctionWithMapAndContext(map, on_resolve_shared,
+                                           closure_context);
+}
+}  // namespace
+
+// https://tc39.github.io/proposal-async-iteration/
+// Section #sec-%asyncfromsynciteratorprototype%.next
+TF_BUILTIN(AsyncFromSyncIteratorPrototypeNext, AsyncFromSyncBuiltinsAssembler) {
+  Node* const iterator = Parameter(0);
+  Node* const parameter = Parameter(1);

[neis, 2017/02/20 11:27:59]

nit: call this "value", then it will also match the spec comments.

[caitp, 2017/02/20 17:09:35]

I opted to call it `parameter` so that I could use the `Node* const` convention
everyhere, as `value` is used below for the iter_result property name.

How about `sent_value`?

[neis, 2017/02/21 12:48:08]

Fine too.

+  Node* const context = Parameter(4);
+
+  Node* const native_context = LoadNativeContext(context);
+  Node* const promise = AllocateAndInitJSPromise(context);
+
+  Variable var_exception(this, MachineRepresentation::kTagged,
+                         UndefinedConstant());
+  Label reject_promise(this, Label::kDeferred);
+
+  ThrowIfNotAsyncFromSyncIterator(context, iterator, &reject_promise,
+                                  &var_exception,
+                                  "[Async-from-Sync Iterator].prototype.next");
+
+  Node* const sync_iterator =
+      LoadObjectField(iterator, JSAsyncFromSyncIterator::kSyncIteratorOffset);
+
+  // Let nextResult be IteratorNext(syncIterator, value).
+  // IfAbruptRejectPromise(nextResult, promiseCapability).
+  Node* next_method =
+      GetProperty(context, sync_iterator, factory()->next_string());
+
+  Node* const iter_result = CallJS(CodeFactory::Call(isolate()), context,
+                                   next_method, sync_iterator, parameter);
+  GotoIfException(iter_result, &reject_promise, &var_exception);
+

[neis, 2017/02/20 11:27:58]

This seems to be missing the Object check of IteratorNext.  (Same in .throw and
.return I think.)  See also the comment in LoadIteratorResult.

[caitp, 2017/02/20 17:09:35]

Added the object check in LoadIteratorResult().

+  Node* value;
+  Node* done;
+  std::tie(value, done) = LoadIteratorResult(
+      context, native_context, iter_result, &reject_promise, &var_exception);
+
+  // Let valueWrapperCapability be ! NewPromiseCapability(%Promise%).
+  Node* const wrapper = AllocateAndInitJSPromise(context);
+
+  // Perform ! Call(valueWrapperCapability.[[Resolve]], undefined,
+  //                « nextValue »).
+  InternalResolvePromise(context, wrapper, value);
+
+  // Let onFulfilled be a new built-in function object as defined in
+  // Async Iterator Value Unwrap Functions.
+  // Set onFulfilled.[[Done]] to nextDone.
+  Node* const on_resolve = CreateUnwrapClosure(native_context, done);

[neis, 2017/02/20 11:27:59]

nit: s/on_resolve/on_fulfilled/

[caitp, 2017/02/20 17:09:35]

done

+
+  // Perform ! PerformPromiseThen(valueWrapperCapability.[[Promise]],
+  //                              onFulfilled, undefined, promiseCapability).
+  Node* const undefined = UndefinedConstant();
+  InternalPerformPromiseThen(context, wrapper, on_resolve, undefined, promise,
+                             undefined, undefined);
+  Return(promise);
+
+  Bind(&reject_promise);
+  {
+    Node* const exception = var_exception.value();
+    InternalPromiseReject(context, promise, exception, TrueConstant());
+
+    // Return promiseCapability.[[Promise]].
+    Return(promise);
+  }
+}
+
+// https://tc39.github.io/proposal-async-iteration/
+// Section #sec-%asyncfromsynciteratorprototype%.return
+TF_BUILTIN(AsyncFromSyncIteratorPrototypeReturn,
+           AsyncFromSyncBuiltinsAssembler) {
+  Node* const iterator = Parameter(0);
+  Node* const parameter = Parameter(1);
+  Node* const context = Parameter(4);
+
+  Node* const native_context = LoadNativeContext(context);
+  Node* const promise = AllocateAndInitJSPromise(context);
+
+  Variable var_exception(this, MachineRepresentation::kTagged,
+                         UndefinedConstant());
+  Label reject_promise(this, Label::kDeferred);
+
+  ThrowIfNotAsyncFromSyncIterator(
+      context, iterator, &reject_promise, &var_exception,
+      "[Async-from-Sync Iterator].prototype.return");
+
+  Node* const sync_iterator =
+      LoadObjectField(iterator, JSAsyncFromSyncIterator::kSyncIteratorOffset);
+
+  // Let return be GetMethod(syncIterator, "return").
+  // IfAbruptRejectPromise(return, promiseCapability).
+  Node* const return_method =
+      GetProperty(context, sync_iterator, factory()->return_string());
+  GotoIfException(return_method, &reject_promise, &var_exception);
+
+  Variable var_value(this, MachineRepresentation::kTagged);
+  Variable var_done(this, MachineRepresentation::kTagged);

[neis, 2017/02/20 11:27:58]

These seem unnecessary, see below.

+
+  Label if_isundefined(this), if_isnotundefined(this), resolve_promise(this);
+
+  Branch(IsUndefined(return_method), &if_isundefined, &if_isnotundefined);
+  Bind(&if_isundefined);
+  {
+    // If return is undefined, then
+    // Let iterResult be ! CreateIterResultObject(value, true)
+    Node* const iter_result =
+        CallStub(CodeFactory::CreateIterResultObject(isolate()), context,
+                 parameter, TrueConstant());
+
+    // Perform ! Call(promiseCapability.[[Resolve]], undefined, « iterResult »).
+    // IfAbruptRejectPromise(nextDone, promiseCapability).
+    // Return promiseCapability.[[Promise]].
+    PromiseFulfill(context, promise, iter_result, v8::Promise::kFulfilled);
+    Return(promise);
+  }
+
+  Bind(&if_isnotundefined);
+  {
+    // Let returnResult be Call(return, syncIterator, « value »).
+    Node* const iter_result = CallJS(CodeFactory::Call(isolate()), context,
+                                     return_method, sync_iterator, parameter);
+    // IfAbruptRejectPromise(returnResult, promiseCapability).
+    GotoIfException(iter_result, &reject_promise, &var_exception);
+
+    Node* value;
+    Node* done;
+    std::tie(value, done) = LoadIteratorResult(
+        context, native_context, iter_result, &reject_promise, &var_exception);
+
+    var_value.Bind(value);
+    var_done.Bind(done);
+    Goto(&resolve_promise);
+  }
+

[neis, 2017/02/20 11:27:59]

You should get rid of var_value, var_done, resolve_promise, and then merge the
two blocks.

[caitp, 2017/02/20 17:09:35]

Done.

+  Bind(&resolve_promise);
+  {
+    // Let valueWrapperCapability be ! NewPromiseCapability(%Promise%).
+    Node* const wrapper = AllocateAndInitJSPromise(context);
+    Node* const value = var_value.value();
+    Node* const done = var_done.value();
+
+    // Perform ! Call(valueWrapperCapability.[[Resolve]], undefined,
+    //                « nextValue »).
+    InternalResolvePromise(context, wrapper, value);
+
+    // Let onFulfilled be a new built-in function object as defined in
+    // Async Iterator Value Unwrap Functions.
+    // Set onFulfilled.[[Done]] to nextDone.
+    Node* const on_resolve = CreateUnwrapClosure(native_context, done);
+
+    // Perform ! PerformPromiseThen(valueWrapperCapability.[[Promise]],
+    //                              onFulfilled, undefined, promiseCapability).
+    Node* const undefined = UndefinedConstant();
+    InternalPerformPromiseThen(context, wrapper, on_resolve, undefined, promise,
+                               undefined, undefined);
+    Return(promise);
+  }

[neis, 2017/02/20 11:27:59]

Could you try to factor out this code?  Seems to basically be the same in all
three functions.

[caitp, 2017/02/20 17:09:35]

Done.

+
+  Bind(&reject_promise);
+  {
+    Node* const exception = var_exception.value();
+    InternalPromiseReject(context, promise, exception, TrueConstant());
+
+    // Return promiseCapability.[[Promise]].
+    Return(promise);
+  }
+}
+
+// https://tc39.github.io/proposal-async-iteration/
+// Section #sec-%asyncfromsynciteratorprototype%.throw
+TF_BUILTIN(AsyncFromSyncIteratorPrototypeThrow,
+           AsyncFromSyncBuiltinsAssembler) {
+  Node* const iterator = Parameter(0);
+  Node* const parameter = Parameter(1);
+  Node* const context = Parameter(4);
+
+  Node* const native_context = LoadNativeContext(context);
+  Node* const promise = AllocateAndInitJSPromise(context);
+
+  Variable var_exception(this, MachineRepresentation::kTagged, parameter);
+  Variable var_value(this, MachineRepresentation::kTagged);
+  Variable var_done(this, MachineRepresentation::kTagged);
+  Label reject_promise(this, Label::kDeferred);
+
+  ThrowIfNotAsyncFromSyncIterator(context, iterator, &reject_promise,
+                                  &var_exception,
+                                  "[Async-from-Sync Iterator].prototype.throw");
+
+  Node* const sync_iterator =
+      LoadObjectField(iterator, JSAsyncFromSyncIterator::kSyncIteratorOffset);
+
+  // Let throw be GetMethod(syncIterator, "throw").
+  // IfAbruptRejectPromise(nextResult, promiseCapability).

[neis, 2017/02/20 11:27:59]

This shouldn't be here.

+  // IfAbruptRejectPromise(throw, promiseCapability).
+  Node* const throw_method =
+      GetProperty(context, sync_iterator, factory()->throw_string());
+  GotoIfException(throw_method, &reject_promise, &var_exception);
+
+  Label if_isnotundefined(this), resolve_promise(this);
+
+  // If throw is undefined, then
+  // Perform ! Call(promiseCapability.[[Reject]], undefined, « value »).
+  Branch(IsUndefined(throw_method), &reject_promise, &if_isnotundefined);
+
+  Bind(&if_isnotundefined);
+  {
+    // Let throwResult be Call(throw, syncIterator, « value »).
+    Node* const iter_result = CallJS(CodeFactory::Call(isolate()), context,
+                                     throw_method, sync_iterator, parameter);
+    GotoIfException(iter_result, &reject_promise, &var_exception);
+
+    Node* value;
+    Node* done;
+    std::tie(value, done) = LoadIteratorResult(
+        context, native_context, iter_result, &reject_promise, &var_exception);
+
+    // Let valueWrapperCapability be ! NewPromiseCapability(%Promise%).
+    Node* const wrapper = AllocateAndInitJSPromise(context);
+
+    // Perform ! Call(valueWrapperCapability.[[Resolve]], undefined, «
+    // throwValue »).
+    InternalResolvePromise(context, wrapper, value);
+
+    // Let onFulfilled be a new built-in function object as defined in
+    // Async Iterator Value Unwrap Functions.
+    // Set onFulfilled.[[Done]] to throwDone.
+    Node* const on_resolve = CreateUnwrapClosure(native_context, done);
+
+    // Perform ! PerformPromiseThen(valueWrapperCapability.[[Promise]],
+    //     onFulfilled, undefined, promiseCapability).
+    Node* const undefined = UndefinedConstant();
+    InternalPerformPromiseThen(context, wrapper, on_resolve, undefined, promise,
+                               undefined, undefined);
+    Return(promise);
+  }
+
+  Bind(&reject_promise);
+  {
+    Node* const exception = var_exception.value();
+    InternalPromiseReject(context, promise, exception, TrueConstant());
+
+    // Return promiseCapability.[[Promise]].
+    Return(promise);
+  }
+}
+
+TF_BUILTIN(AsyncIteratorValueUnwrap, AsyncBuiltinsAssembler) {
+  Node* const value = Parameter(1);
+  Node* const context = Parameter(4);
+
+  Node* const done = LoadContextElement(context, ValueUnwrapContext::kDoneSlot);
+  CSA_ASSERT(this, IsBoolean(done));
+
+  Node* const unwrapped_value = CallStub(
+      CodeFactory::CreateIterResultObject(isolate()), context, value, done);
+
+  Return(unwrapped_value);
+}
+
+Node* AsyncBuiltinsAssembler::AllocateAsyncIteratorValueUnwrapContext(
+    Node* native_context, Node* done) {
+  CSA_ASSERT(this, IsNativeContext(native_context));
+  CSA_ASSERT(this, IsBoolean(done));
+
+  Node* const context =
+      CreatePromiseContext(native_context, ValueUnwrapContext::kLength);
+  StoreContextElementNoWriteBarrier(context, ValueUnwrapContext::kDoneSlot,
+                                    done);
+  return context;
+}
+
+}  // namespace internal
+}  // namespace v8