WIP [esnext] implement async iteration proposal

src/builtins/builtins-async-generator.cc

+// Copyright 2016 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 AsyncGeneratorAwait resume
+// closures.
+class AwaitContext {
+ public:
+  enum Fields { kGeneratorSlot = Context::MIN_CONTEXT_SLOTS, kLength };
+};
+
+}  // anonymous namespace
+
+// https://tc39.github.io/proposal-async-iteration/
+// Section #sec-asyncgenerator-prototype-next
+TF_BUILTIN(AsyncGeneratorPrototypeNext, AsyncBuiltinsAssembler) {
+  Node* const generator = Parameter(0);
+  Node* const value = Parameter(1);
+  Node* const context = Parameter(4);
+  AsyncGeneratorEnqueue(context, generator, value,
+                        JSAsyncGeneratorObject::kNext,
+                        "[AsyncGenerator].prototype.next");
+}
+
+// https://tc39.github.io/proposal-async-iteration/
+// Section #sec-asyncgenerator-prototype-return
+TF_BUILTIN(AsyncGeneratorPrototypeReturn, AsyncBuiltinsAssembler) {
+  Node* generator = Parameter(0);
+  Node* value = Parameter(1);
+  Node* context = Parameter(4);
+  AsyncGeneratorEnqueue(context, generator, value,
+                        JSAsyncGeneratorObject::kReturn,
+                        "[AsyncGenerator].prototype.return");
+}
+
+// https://tc39.github.io/proposal-async-iteration/
+// Section #sec-asyncgenerator-prototype-throw
+TF_BUILTIN(AsyncGeneratorPrototypeThrow, AsyncBuiltinsAssembler) {
+  Node* generator = Parameter(0);
+  Node* value = Parameter(1);
+  Node* context = Parameter(4);
+  AsyncGeneratorEnqueue(context, generator, value,
+                        JSAsyncGeneratorObject::kThrow,
+                        "[AsyncGenerator].prototype.throw");
+}
+
+TF_BUILTIN(AsyncGeneratorYield, AsyncBuiltinsAssembler) {
+  Node* const generator = Parameter(0);
+  Node* const value = Parameter(1);
+  Node* const context = Parameter(4);
+
+  CSA_ASSERT_JS_ARGC_EQ(this, 1);
+  CSA_SLOW_ASSERT(this,
+                  HasInstanceType(generator, JS_ASYNC_GENERATOR_OBJECT_TYPE));
+
+  Node* request = TakeFirstAsyncGeneratorRequestFromQueue(generator);
+
+  Node* const promise =
+      LoadObjectField(request, AsyncGeneratorRequest::kPromiseOffset);
+
+  // The request Promise must still be pending at this point.
+  CSA_SLOW_ASSERT(this,
+                  WordEqual(LoadObjectField(promise, JSPromise::kStatusOffset),
+                            SmiConstant(v8::Promise::kPending)));
+
+  // If a yield expression is reached, we must not be awaiting any value.
+  CSA_SLOW_ASSERT(this,
+                  IsUndefined(LoadObjectField(
+                      request, AsyncGeneratorRequest::kAwaitedPromiseOffset)));
+
+  Node* const iter_result = AllocateJSIteratorResult(context, value, false);
+
+  InternalResolvePromise(context, promise, iter_result);
+
+  AsyncGeneratorResumeNext(context, generator);
+
+  Return(UndefinedConstant());
+}
+
+TF_BUILTIN(AsyncGeneratorRawYield, AsyncBuiltinsAssembler) {
+  Node* const generator = Parameter(0);
+  Node* const value = Parameter(1);
+  Node* const context = Parameter(4);
+
+  CSA_ASSERT_JS_ARGC_EQ(this, 1);
+  CSA_SLOW_ASSERT(this,
+                  HasInstanceType(generator, JS_ASYNC_GENERATOR_OBJECT_TYPE));
+
+  Node* request = TakeFirstAsyncGeneratorRequestFromQueue(generator);
+
+  Node* const promise =
+      LoadObjectField(request, AsyncGeneratorRequest::kPromiseOffset);
+
+  // The request Promise must still be pending at this point.
+  CSA_SLOW_ASSERT(this,
+                  WordEqual(LoadObjectField(promise, JSPromise::kStatusOffset),
+                            SmiConstant(v8::Promise::kPending)));
+
+  // If a yield expression is reached, we must not be awaiting any value.
+  CSA_SLOW_ASSERT(this,
+                  IsUndefined(LoadObjectField(
+                      request, AsyncGeneratorRequest::kAwaitedPromiseOffset)));
+
+  InternalResolvePromise(context, promise, value);
+
+  AsyncGeneratorResumeNext(context, generator);
+
+  Return(UndefinedConstant());
+}
+
+TF_BUILTIN(AsyncGeneratorAwaitResolveClosure, AsyncBuiltinsAssembler) {
+  Node* value = Parameter(1);
+  Node* context = Parameter(4);
+
+  AsyncGeneratorAwaitResumeClosure(context, value,
+                                   JSAsyncGeneratorObject::kNext);
+  Return(UndefinedConstant());
+}
+
+TF_BUILTIN(AsyncGeneratorAwaitRejectClosure, AsyncBuiltinsAssembler) {
+  Node* value = Parameter(1);
+  Node* context = Parameter(4);
+
+  AsyncGeneratorAwaitResumeClosure(context, value,
+                                   JSAsyncGeneratorObject::kThrow);
+  Return(UndefinedConstant());
+}
+
+TF_BUILTIN(AsyncGeneratorAwaitUncaught, AsyncBuiltinsAssembler) {
+  const bool kIsCatchable = false;
+  AsyncGeneratorAwait(kIsCatchable);
+}
+
+TF_BUILTIN(AsyncGeneratorAwaitCaught, AsyncBuiltinsAssembler) {
+  const bool kIsCatchable = true;
+  AsyncGeneratorAwait(kIsCatchable);
+}
+
+// Shared implementation for the 3 Async Iterator protocol methods of Async
+// Generators.
+void AsyncBuiltinsAssembler::AsyncGeneratorEnqueue(
+    Node* context, Node* generator, Node* value,
+    JSAsyncGeneratorObject::ResumeMode resume_mode, const char* method_name) {
+  // AsyncGeneratorEnqueue produces a new Promise, and appends it to the list
+  // of async generator requests to be executed. If the generator is not
+  // presently executing, then this method will loop through, processing each
+  // request from front to back.
+  // This loop is resides in AsyncGeneratorResumeNext.
+  Node* promise = AllocateAndInitJSPromise(context);
+
+  Label enqueue(this), if_receiverisincompatible(this, Label::kDeferred);
+
+  GotoIf(TaggedIsSmi(generator), &if_receiverisincompatible);
+  Branch(HasInstanceType(generator, JS_ASYNC_GENERATOR_OBJECT_TYPE), &enqueue,
+         &if_receiverisincompatible);
+
+  Bind(&enqueue);
+  {
+    Label done(this);
+    Node* const req =
+        AllocateAsyncGeneratorRequest(resume_mode, value, promise);
+
+    AddAsyncGeneratorRequestToQueue(generator, req);
+
+    // Let state be generator.[[AsyncGeneratorState]]
+    // If state is not "executing", then
+    //     Perform AsyncGeneratorResumeNext(Generator)
+    // Check if the {receiver} is running or already closed.
+    Node* continuation =
+        LoadObjectField(generator, JSAsyncGeneratorObject::kContinuationOffset);
+
+    GotoIf(SmiEqual(continuation,
+                    SmiConstant(JSAsyncGeneratorObject::kGeneratorExecuting)),
+           &done);
+
+    AsyncGeneratorResumeNext(context, generator, continuation);
+
+    Goto(&done);
+    Bind(&done);
+    Return(promise);
+  }
+
+  Bind(&if_receiverisincompatible);
+  {
+    Node* native_context = LoadNativeContext(context);
+    Node* make_type_error =
+        LoadContextElement(native_context, Context::MAKE_TYPE_ERROR_INDEX);
+    Handle<String> method =
+        factory()->NewStringFromAsciiChecked(method_name, TENURED);
+    Node* error =
+        CallJS(CodeFactory::Call(isolate()), context, make_type_error,
+               UndefinedConstant(),
+               SmiConstant(MessageTemplate::kIncompatibleMethodReceiver),
+               HeapConstant(method), generator);
+
+    CallRuntime(Runtime::kPromiseReject, context, promise, error,
+                TrueConstant());
+    Return(promise);
+  }
+}
+
+void AsyncBuiltinsAssembler::AsyncGeneratorResumeNext(Node* context,
+                                                      Node* generator,
+                                                      Node* continuation) {
+  // Loop through each queued request, from first to last, and resuming the
+  // generator. Will not resume generator if resumed with an abrupt completion
+  // such as via the .throw() or .return() methods, or if an Await is
+  // in progress.
+  Variable var_request(this, MachineRepresentation::kTagged);
+  Variable var_continuation(this, MachineRepresentation::kTaggedSigned);
+  var_continuation.Bind(continuation);
+
+  Variable* loop_vars[] = {&var_request, &var_continuation};
+  Label loop(this, arraysize(loop_vars),
+             reinterpret_cast<Variable **>(loop_vars)),
+      done(this);
+
+  var_request.Bind(
+      LoadObjectField(generator, JSAsyncGeneratorObject::kQueueOffset));
+  Branch(WordEqual(var_request.value(), UndefinedConstant()), &done, &loop);
+  Bind(&loop);
+  {
+    AsyncGeneratorResumeNextStep(context, generator, var_request.value(),
+                                 var_continuation.value(), &done);
+
+    var_continuation.Bind(LoadObjectField(
+        generator, JSAsyncGeneratorObject::kContinuationOffset));
+    var_request.Bind(
+        LoadObjectField(generator, JSAsyncGeneratorObject::kQueueOffset));
+
+    // Continue loop if there is another request queued up.
+    Branch(WordEqual(var_request.value(), UndefinedConstant()), &done, &loop);
+  }
+
+  Bind(&done);
+}
+
+void AsyncBuiltinsAssembler::AsyncGeneratorResumeNextStep(Node* context,
+                                                          Node* generator,
+                                                          Node* request,
+                                                          Node* continuation,
+                                                          Label* exit_loop) {
+  Variable var_return(this, MachineRepresentation::kTagged);
+  var_return.Bind(UndefinedConstant());
+  CSA_SLOW_ASSERT(this, HasInstanceType(request, ASYNC_GENERATOR_REQUEST_TYPE));
+
+  Label start(this), loop_next(this);
+
+  // Stop consuming the AsyncGeneratorRequest queue and resuming the generator
+  // if an Await is in progress.
+  Node* awaited =
+      LoadObjectField(request, AsyncGeneratorRequest::kAwaitedPromiseOffset);
+  Branch(IsUndefined(awaited), &start, exit_loop);
+
+  Bind(&start);
+
+  // Let completion be next.[[Completion]]
+  Node* resume_mode =
+      LoadObjectField(request, AsyncGeneratorRequest::kResumeModeOffset);
+  Node* value = LoadObjectField(request, AsyncGeneratorRequest::kValueOffset);
+
+  // If completion is an abrupt completion, then
+  Label if_abruptcompletion(this), check_ifcompleted(this),
+      resume_generator(this), if_return(this, &var_return), if_throw(this);
+  Branch(SmiEqual(resume_mode, SmiConstant(JSAsyncGeneratorObject::kNext)),
+         &check_ifcompleted, &if_abruptcompletion);
+
+  Bind(&if_abruptcompletion);
+  {
+    Label check_completed(this), if_completed(this);
+
+    // If state is "suspendedStart", then
+    //     Set generator.[[AsyncGeneratorState]] to "completed"
+    //     Set state to "completed"
+    GotoIf(WordNotEqual(continuation, SmiConstant(0)), &check_completed);
+    StoreObjectField(generator, JSAsyncGeneratorObject::kContinuationOffset,
+                     SmiConstant(JSAsyncGeneratorObject::kGeneratorClosed));
+    Goto(&if_completed);
+
+    Bind(&check_completed);
+    Branch(WordEqual(continuation,
+                     SmiConstant(JSAsyncGeneratorObject::kGeneratorClosed)),
+           &if_completed, &resume_generator);
+
+    Bind(&if_completed);
+    {
+      // If state is "completed", then
+      //   If completion.[[Type]] is return, then return
+      //       ! AsyncGeneratorResolve(generator, completion.[[Value]], true).
+      //   Else, return ! AsyncGeneratorReject(generator,
+      //   completion.[[Value]])
+      var_return.Bind(value);
+      Branch(
+          WordEqual(resume_mode, SmiConstant(JSAsyncGeneratorObject::kReturn)),
+          &if_return, &if_throw);
+    }
+  }
+
+  Bind(&check_ifcompleted);
+  {
+    // Else if state is "completed", then return !
+    // AsyncGeneratorResolve(generator, undefined, true).
+    Branch(WordEqual(continuation,
+                     SmiConstant(JSAsyncGeneratorObject::kGeneratorClosed)),
+           &if_return, &resume_generator);
+  }
+
+  Bind(&resume_generator);
+  {
+    CSA_ASSERT(this, SmiAboveOrEqual(continuation, SmiConstant(0)));
+    Node* result = CallStub(CodeFactory::ResumeGenerator(isolate()), context,
+                            value, generator, resume_mode);
+    Goto(&loop_next);
+  }
+
+  Bind(&if_return);
+  {
+    Node* const request = TakeFirstAsyncGeneratorRequestFromQueue(generator);
+    Node* const promise =
+        LoadObjectField(request, AsyncGeneratorRequest::kPromiseOffset);
+    Node* const result =
+        AllocateJSIteratorResult(context, var_return.value(), true);
+    PromiseFulfill(context, promise, result, v8::Promise::kFulfilled);
+    Goto(&loop_next);
+  }
+
+  Bind(&if_throw);
+  {
+    Node* request = TakeFirstAsyncGeneratorRequestFromQueue(generator);
+
+    // Let promiseCapability be next.[[Capability]].
+    Node* promise =
+        LoadObjectField(request, AsyncGeneratorRequest::kPromiseOffset);
+
+    // Perform ! Call(promiseCapability.[[Reject]], undefined, « exception »).
+    CallRuntime(Runtime::kPromiseReject, context, promise, value,
+                TrueConstant());
+    Goto(&loop_next);
+  }
+
+  Bind(&loop_next);
+}
+
+Node* AsyncBuiltinsAssembler::AllocateAsyncGeneratorRequest(
+    JSAsyncGeneratorObject::ResumeMode resume_mode, Node* resume_value,
+    Node* promise) {
+  CSA_SLOW_ASSERT(this, HasInstanceType(promise, JS_PROMISE_TYPE));
+  Node* request = Allocate(AsyncGeneratorRequest::kSize);
+  StoreMapNoWriteBarrier(request, Heap::kAsyncGeneratorRequestMapRootIndex);
+  StoreObjectFieldNoWriteBarrier(request, AsyncGeneratorRequest::kNextOffset,
+                                 UndefinedConstant());
+  StoreObjectFieldNoWriteBarrier(request,
+                                 AsyncGeneratorRequest::kResumeModeOffset,
+                                 SmiConstant(resume_mode));
+  StoreObjectFieldNoWriteBarrier(request, AsyncGeneratorRequest::kValueOffset,
+                                 resume_value);
+  StoreObjectFieldNoWriteBarrier(request, AsyncGeneratorRequest::kPromiseOffset,
+                                 promise);
+  StoreObjectFieldNoWriteBarrier(request,
+                                 AsyncGeneratorRequest::kAwaitedPromiseOffset,
+                                 UndefinedConstant());
+  return request;
+}
+
+void AsyncBuiltinsAssembler::AsyncGeneratorAwaitResumeClosure(
+    Node* context, Node* value,
+    JSAsyncGeneratorObject::ResumeMode resume_mode) {
+  Label done(this);
+  Node* const generator =
+      LoadContextElement(context, AwaitContext::kGeneratorSlot);
+  CSA_SLOW_ASSERT(this,
+                  HasInstanceType(generator, JS_ASYNC_GENERATOR_OBJECT_TYPE));
+
+  Node* const request =
+      LoadObjectField(generator, JSAsyncGeneratorObject::kQueueOffset);
+  CSA_SLOW_ASSERT(this, HasInstanceType(request, ASYNC_GENERATOR_REQUEST_TYPE));
+
+  Node* const promise =
+      LoadObjectField(request, AsyncGeneratorRequest::kAwaitedPromiseOffset);
+  CSA_SLOW_ASSERT(this, HasInstanceType(promise, JS_PROMISE_TYPE));
+  CSA_SLOW_ASSERT(
+      this, WordNotEqual(LoadObjectField(promise, JSPromise::kStatusOffset),
+                         SmiConstant(v8::Promise::kPending)));
+
+  StoreObjectField(request, AsyncGeneratorRequest::kAwaitedPromiseOffset,
+                   UndefinedConstant());
+
+  Node* const continuation =
+      LoadObjectField(generator, JSAsyncGeneratorObject::kContinuationOffset);
+
+  GotoUnless(SmiAboveOrEqual(continuation, SmiConstant(0)), &done);
+  CallStub(CodeFactory::ResumeAwaitedGenerator(isolate()), context, value,
+           generator, SmiConstant(resume_mode));
+
+  Goto(&done);
+  Bind(&done);
+  AsyncGeneratorResumeNext(context, generator);
+}
+
+void AsyncBuiltinsAssembler::AsyncGeneratorAwait(bool is_catchable) {
+  Node* generator = Parameter(1);
+  Node* value = Parameter(2);
+  Node* context = Parameter(5);
+
+  CSA_ASSERT_JS_ARGC_EQ(this, 2);
+
+  CSA_SLOW_ASSERT(this,
+                  HasInstanceType(generator, JS_ASYNC_GENERATOR_OBJECT_TYPE));
+
+  Node* const request =
+      LoadObjectField(generator, JSAsyncGeneratorObject::kQueueOffset);
+  CSA_SLOW_ASSERT(this, WordNotEqual(request, UndefinedConstant()));
+
+  NodeGenerator1 closure_context = [&](Node* native_context) -> Node* {
+    Node* const context =
+        CreatePromiseContext(native_context, AwaitContext::kLength);
+    StoreContextElementNoWriteBarrier(context, AwaitContext::kGeneratorSlot,
+                                      generator);
+    return context;
+  };
+
+  Node* outer_promise =
+      LoadObjectField(request, AsyncGeneratorRequest::kPromiseOffset);
+
+  const int reject_index = Context::ASYNC_GENERATOR_AWAIT_REJECT_SHARED_FUN;
+  STATIC_ASSERT((Context::ASYNC_GENERATOR_AWAIT_REJECT_SHARED_FUN + 1) ==
+                Context::ASYNC_GENERATOR_AWAIT_RESOLVE_SHARED_FUN);

[Dan Ehrenberg, 2017/01/14 00:48:38]

Oh sorry I had previously missed this--I guess you have the safety covered.
Still, I'm wondering if things would all be simpler if you passed in two Node*s
which will evaluate to JSFunctions, rather than closure_context and
reject_index, and handled setting that up in the caller.

+
+  Node* promise = Await(context, generator, value, outer_promise,
+                        closure_context, reject_index, is_catchable);
+
+  CSA_SLOW_ASSERT(this,
+                  IsUndefined(LoadObjectField(
+                      request, AsyncGeneratorRequest::kAwaitedPromiseOffset)));
+  StoreObjectField(request, AsyncGeneratorRequest::kAwaitedPromiseOffset,
+                   promise);
+  Return(UndefinedConstant());
+}
+
+void AsyncBuiltinsAssembler::AddAsyncGeneratorRequestToQueue(Node* generator,
+                                                             Node* request) {

[Dan Ehrenberg, 2017/01/14 00:48:38]

+gsathya would it make sense to share this (admittedly very simple) queue
implementation with the future implementation of the microtask queue?

+  Variable var_current(this, MachineRepresentation::kTagged);
+  Label empty(this), loop(this, &var_current), done(this);
+
+  var_current.Bind(
+      LoadObjectField(generator, JSAsyncGeneratorObject::kQueueOffset));
+  Branch(IsUndefined(var_current.value()), &empty, &loop);
+
+  Bind(&empty);
+  {
+    StoreObjectField(generator, JSAsyncGeneratorObject::kQueueOffset, request);
+    Goto(&done);
+  }
+
+  Bind(&loop);
+  {
+    Label loop_next(this), next_empty(this);
+    Node* current = var_current.value();
+    Node* next = LoadObjectField(current, AsyncGeneratorRequest::kNextOffset);
+
+    Branch(IsUndefined(next), &next_empty, &loop_next);
+    Bind(&next_empty);
+    {
+      StoreObjectField(current, AsyncGeneratorRequest::kNextOffset, request);
+      Goto(&done);
+    }
+
+    Bind(&loop_next);
+    {
+      var_current.Bind(next);
+      Goto(&loop);
+    }
+  }
+  Bind(&done);
+}
+
+Node* AsyncBuiltinsAssembler::TakeFirstAsyncGeneratorRequestFromQueue(
+    Node* generator) {
+  // Removes and returns the first AsyncGeneratorRequest from a
+  // JSAsyncGeneratorObject's queue. Asserts that the queue is not empty.

[Dan Ehrenberg, 2017/01/14 00:48:38]

Ignorable style suggestion: For a comment that describes what a function does,
put it above the function, rather than before a line (because then it looks like
it describes what that line does).

+  CSA_SLOW_ASSERT(this,
+                  HasInstanceType(generator, JS_ASYNC_GENERATOR_OBJECT_TYPE));
+
+  Node* request =
+      LoadObjectField(generator, JSAsyncGeneratorObject::kQueueOffset);
+  CSA_SLOW_ASSERT(this, WordNotEqual(request, UndefinedConstant()));
+
+  Node* next = LoadObjectField(request, AsyncGeneratorRequest::kNextOffset);
+
+  StoreObjectField(generator, JSAsyncGeneratorObject::kQueueOffset, next);
+  return request;
+}
+
+}  // namespace internal
+}  // namespace v8