WIP [esnext] implement async iteration proposal

src/builtins/builtins-async.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 {
+
+Node* AsyncBuiltinsAssembler::Await(

[caitp, 2017/01/10 04:13:41]

I think this thing could be used to refactor the Async Functions await code into
TF code. In the current design, the async generator version pretty much needs to
be implemented in TF in order to get access to the fields of the
AsyncGeneratorRequest objects.

+    Node* context, Node* generator, Node* value, Node* outer_promise,
+    const NodeGenerator1& create_closure_context, int on_reject_context_index,
+    bool is_catchable) {
+  // Let promiseCapability be ! NewPromiseCapability(%Promise%).
+  Node* const wrapped_value = AllocateAndInitJSPromise(context);
+
+  // Perform ! Call(promiseCapability.[[Resolve]], undefined, « promise »).
+  InternalResolvePromise(context, wrapped_value, value);
+
+  Node* const native_context = LoadNativeContext(context);
+
+  Node* const closure_context = create_closure_context(native_context);
+  Node* const map = LoadContextElement(
+      native_context, Context::STRICT_FUNCTION_WITHOUT_PROTOTYPE_MAP_INDEX);
+
+  Node* const on_reject_shared_fun =
+      LoadContextElement(native_context, on_reject_context_index);
+  CSA_SLOW_ASSERT(
+      this, HasInstanceType(on_reject_shared_fun, SHARED_FUNCTION_INFO_TYPE));
+  Node* const on_reject = AllocateFunctionWithMapAndContext(
+      map, on_reject_shared_fun, closure_context);
+
+  Node* const on_resolve_shared_fun =
+      LoadContextElement(native_context, on_reject_context_index + 1);
+  CSA_SLOW_ASSERT(
+      this, HasInstanceType(on_resolve_shared_fun, SHARED_FUNCTION_INFO_TYPE));
+  Node* const on_resolve = AllocateFunctionWithMapAndContext(
+      map, on_resolve_shared_fun, closure_context);
+
+  Node* const throwaway_promise =
+      AllocateAndInitJSPromise(context, wrapped_value);
+
+  // The Promise will be thrown away and not handled, but it shouldn't trigger
+  // unhandled reject events as its work is done
+  PromiseSetHasHandler(throwaway_promise);
+
+  Label do_perform_promise_then(this);
+  GotoUnless(IsDebugActive(), &do_perform_promise_then);
+  {
+    Label common(this);
+    GotoUnless(HasInstanceType(value, JS_PROMISE_TYPE), &common);
+    {
+      // Mark the reject handler callback to be a forwarding edge, rather
+      // than a meaningful catch handler
+      Node* const key =
+          HeapConstant(factory()->promise_forwarding_handler_symbol());
+      CallRuntime(Runtime::kSetProperty, on_reject, key, TrueConstant(),
+                  SmiConstant(STRICT));
+
+      if (!is_catchable) {
+        PromiseSetHasHandler(value);
+      }
+    }
+
+    Goto(&common);
+    Bind(&common);
+    // Mark the dependency to outer Promise in case the throwaway Promise is
+    // found on the Promise stack
+    CSA_SLOW_ASSERT(this, HasInstanceType(outer_promise, JS_PROMISE_TYPE));
+
+    Node* const key = HeapConstant(factory()->promise_handled_by_symbol());
+    CallRuntime(Runtime::kSetProperty, throwaway_promise, key, outer_promise,
+                SmiConstant(STRICT));
+  }
+
+  Goto(&do_perform_promise_then);
+  Bind(&do_perform_promise_then);
+  InternalPerformPromiseThen(context, wrapped_value, on_resolve, on_reject,
+                             throwaway_promise, UndefinedConstant(),
+                             UndefinedConstant());
+
+  return wrapped_value;
+}
+
+}  // namespace internal
+}  // namespace v8