[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 {
+
+// Describe fields of Context associated with the AsyncIterator unwrap closure.
+class ValueUnwrapContext {

[Benedikt Meurer, 2017/02/16 05:16:38]

Can we wrap these helpers into an anonymous namespace?

[caitp, 2017/02/16 14:25:11]

Done.

+ public:
+  enum Fields { kDoneSlot = Context::MIN_CONTEXT_SLOTS, kLength };
+};
+
+class AsyncFromSyncBuiltinsAssembler : public AsyncBuiltinsAssembler {
+ public:
+  explicit AsyncFromSyncBuiltinsAssembler(CodeAssemblerState* state)
+      : AsyncBuiltinsAssembler(state) {}
+
+  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* native_context, Node* 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);
+
+  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 =
+        CallStub(CodeFactory::GetProperty(isolate()), context, iter_result,
+                 HeapConstant(factory()->value_string()));
+    GotoIfException(value, if_exception, var_exception);
+
+    // Let nextDone be IteratorComplete(nextResult).
+    // IfAbruptRejectPromise(nextDone, promiseCapability).
+    Node* const done =
+        CallStub(CodeFactory::GetProperty(isolate()), context, iter_result,
+                 HeapConstant(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(IsBooleanMap(LoadMap(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,
+                                                          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);
+}
+
+// https://tc39.github.io/proposal-async-iteration/
+// Section #sec-%asyncfromsynciteratorprototype%.next
+TF_BUILTIN(AsyncFromSyncIteratorPrototypeNext, AsyncFromSyncBuiltinsAssembler) {
+  const char* method_name = "[Async-from-Sync Iterator].prototype.next";
+
+  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_parent(this, MachineRepresentation::kTagged);
+  Variable var_exception(this, MachineRepresentation::kTagged);
+  var_exception.Bind(UndefinedConstant());
+  Label reject_promise(this, Label::kDeferred);
+  Label if_receiverisincompatible(this, Label::kDeferred);
+
+  GotoIf(TaggedIsSmi(iterator), &if_receiverisincompatible);
+  GotoUnless(HasInstanceType(iterator, JS_ASYNC_FROM_SYNC_ITERATOR_TYPE),
+             &if_receiverisincompatible);
+
+  Node* const sync_iterator =
+      LoadObjectField(iterator, JSAsyncFromSyncIterator::kSyncIteratorOffset);
+
+  // Let nextResult be IteratorNext(syncIterator, value).
+  // IfAbruptRejectPromise(nextResult, promiseCapability).
+  Node* next_method =
+      CallStub(CodeFactory::GetProperty(isolate()), context, sync_iterator,
+               HeapConstant(factory()->next_string()));
+  Node* const iter_result = CallJS(CodeFactory::Call(isolate()), context,
+                                   next_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,
+  //                « 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);
+
+  // TODO(caitp): Use TailCallStub once it's possible for a JS-linkage builtin

[Benedikt Meurer, 2017/02/16 05:16:38]

Is the TailCall required by the specification? Because if it's for performance
reasons, then we shouldn't do it, especially since we measured in the past that
tail call from JS to CodeStub wasn't more efficient.

[caitp, 2017/02/16 14:25:11]

Not required by the spec. My reasoning is, it should be a stub call to make
these builtins smaller rather than duplicating a lot of code, and a tailcall
should help reduce odds of a stack overflow. Those are the only reasons for it.

+  // to tailcall a CS-linkage builtin when an arguments adaptor frame is used.
+  Return(promise);
+
+  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), iterator);
+
+    // Perform ! Call(promiseCapability.[[Reject]], undefined,
+    //                « badIteratorError »).
+    var_exception.Bind(error);
+    Goto(&reject_promise);
+  }
+
+  Bind(&reject_promise);
+  {
+    Node* const promise = AllocateAndInitJSPromise(context);
+    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) {
+  const char* method_name = "[Async-from-Sync Iterator].prototype.return";
+
+  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);
+  var_exception.Bind(UndefinedConstant());
+  Label reject_promise(this, Label::kDeferred);
+  Label if_receiverisincompatible(this, Label::kDeferred);
+
+  GotoIf(TaggedIsSmi(iterator), &if_receiverisincompatible);
+  GotoUnless(HasInstanceType(iterator, JS_ASYNC_FROM_SYNC_ITERATOR_TYPE),
+             &if_receiverisincompatible);
+
+  Node* const sync_iterator =
+      LoadObjectField(iterator, JSAsyncFromSyncIterator::kSyncIteratorOffset);
+
+  // Let return be GetMethod(syncIterator, "return").
+  // IfAbruptRejectPromise(return, promiseCapability).
+  Node* const return_method =
+      CallStub(CodeFactory::GetProperty(isolate()), context, sync_iterator,
+               HeapConstant(factory()->return_string()));
+  GotoIfException(return_method, &reject_promise, &var_exception);
+
+  Variable var_value(this, MachineRepresentation::kTagged);
+  Variable var_done(this, MachineRepresentation::kTagged);
+
+  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);
+  }
+
+  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);
+  }
+
+  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), iterator);
+
+    // Perform ! Call(promiseCapability.[[Reject]], undefined,
+    //                « badIteratorError »).
+    var_exception.Bind(error);
+    Goto(&reject_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%.throw
+TF_BUILTIN(AsyncFromSyncIteratorPrototypeThrow,
+           AsyncFromSyncBuiltinsAssembler) {
+  const char* method_name = "[Async-from-Sync Iterator].prototype.throw";
+
+  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);
+  var_exception.Bind(parameter);
+  Variable var_value(this, MachineRepresentation::kTagged);
+  Variable var_done(this, MachineRepresentation::kTagged);
+  Label reject_promise(this, Label::kDeferred);
+  Label if_receiverisincompatible(this, Label::kDeferred);
+
+  GotoIf(TaggedIsSmi(iterator), &if_receiverisincompatible);
+  GotoUnless(HasInstanceType(iterator, JS_ASYNC_FROM_SYNC_ITERATOR_TYPE),
+             &if_receiverisincompatible);
+
+  Node* const sync_iterator =
+      LoadObjectField(iterator, JSAsyncFromSyncIterator::kSyncIteratorOffset);
+
+  // Let throw be GetMethod(syncIterator, "throw").
+  // IfAbruptRejectPromise(nextResult, promiseCapability).
+  // IfAbruptRejectPromise(throw, promiseCapability).
+  Node* const throw_method =
+      CallStub(CodeFactory::GetProperty(isolate()), context, sync_iterator,

[Benedikt Meurer, 2017/02/16 05:16:38]

How about introducing a helper method on the CSA for this? Like

  GetProperty(Node* context, Node* object, Handle<Name> name)

We have this all over the code base now.

[caitp, 2017/02/16 14:25:11]

Done.

+               HeapConstant(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(&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), iterator);
+
+    // Perform ! Call(promiseCapability.[[Reject]], undefined,
+    //                « badIteratorError »).
+    var_exception.Bind(error);
+    Goto(&reject_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);
+  Node* const unwrapped_value = CallStub(
+      CodeFactory::CreateIterResultObject(isolate()), context, value, done);
+
+  Return(unwrapped_value);
+}
+
+Node* AsyncBuiltinsAssembler::AllocateAsyncIteratorValueUnwrapContext(
+    Node* native_context, Node* done) {
+  Node* const context =
+      CreatePromiseContext(native_context, ValueUnwrapContext::kLength);
+  StoreContextElementNoWriteBarrier(context, ValueUnwrapContext::kDoneSlot,
+                                    done);
+  return context;
+}
+
+}  // namespace internal
+}  // namespace v8