Mark await expressions as caught or uncaught

src/js/harmony-async-await.js

 // 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.
 
 (function(global, utils, extrasUtils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
 
 var AsyncFunctionNext;
 var AsyncFunctionThrow;
 var GlobalPromise;
+var IsPromise;
 var NewPromiseCapability;
 var PerformPromiseThen;
-var PromiseCastResolved;
+var PromiseCreate;
 var RejectPromise;
+var ResolvePromise;
 
 utils.Import(function(from) {
   AsyncFunctionNext = from.AsyncFunctionNext;
   AsyncFunctionThrow = from.AsyncFunctionThrow;
+  IsPromise = from.IsPromise;
   GlobalPromise = from.GlobalPromise;
   NewPromiseCapability = from.NewPromiseCapability;
-  PromiseCastResolved = from.PromiseCastResolved;
+  PromiseCreate = from.PromiseCreate;
   PerformPromiseThen = from.PerformPromiseThen;
   RejectPromise = from.RejectPromise;
+  ResolvePromise = from.ResolvePromise;
 });
 
+var promiseAwaitHandlerSymbol = utils.ImportNow("promise_await_handler_symbol");
+var promiseHandledHintSymbol =
+    utils.ImportNow("promise_handled_hint_symbol");
+
 // -------------------------------------------------------------------
 
-function AsyncFunctionAwait(generator, value) {
-  // Promise.resolve(value).then(
+function PromiseCastResolved(value) {
+  if (IsPromise(value)) {
+    return value;
+  } else {
+    var promise = PromiseCreate();
+    ResolvePromise(promise, value);
+    return promise;
+  }
+}
+
+// ES#abstract-ops-async-function-await
+// AsyncFunctionAwait ( value )
+// Shared logic for the core of await. The parser desugars

[jgruber, 2016/09/12 09:29:00]

Thanks, these comments are very helpful.

+//   await awaited
+// into
+//   yield AsyncFunctionAwait{Caught,Uncaught}(.generator, awaited)
+// The 'awaited' parameter is the value; the generator stands in
+// for the asyncContext, and mark is metadata for debugging
+function AsyncFunctionAwait(generator, awaited, mark) {
+  // Promise.resolve(awaited).then(
   //     value => AsyncFunctionNext(value),
   //     error => AsyncFunctionThrow(error)
   // );
-  var promise = PromiseCastResolved(value);
+  var promise = PromiseCastResolved(awaited);
 
   var onFulfilled =
       (sentValue) => %_Call(AsyncFunctionNext, generator, sentValue);
   var onRejected =
       (sentError) => %_Call(AsyncFunctionThrow, generator, sentError);
 
-  // false debugEvent to avoid redundant ExceptionEvents
+  if (mark && DEBUG_IS_ACTIVE && IsPromise(awaited)) {
+    // Mark the reject handler callback such that it does not influence
+    // catch prediction.
+    SET_PRIVATE(onRejected, promiseAwaitHandlerSymbol, true);
+  }
+
+  // Just forwarding the exception, so no debugEvent for throwawayCapability
   var throwawayCapability = NewPromiseCapability(GlobalPromise, false);
   return PerformPromiseThen(promise, onFulfilled, onRejected,
                             throwawayCapability);
 }
 
+// Called by the parser from the desugaring of 'await' when catch
+// prediction indicates no locally surrounding catch block
+function AsyncFunctionAwaitUncaught(generator, awaited) {
+  // TODO(littledan): Install a dependency edge from awaited to outerPromise
+  return AsyncFunctionAwait(generator, awaited, true);
+}
+
+// Called by the parser from the desugaring of 'await' when catch
+// prediction indicates that there is a locally surrounding catch block
+function AsyncFunctionAwaitCaught(generator, awaited) {
+  if (DEBUG_IS_ACTIVE && IsPromise(awaited)) {
+    SET_PRIVATE(awaited, promiseHandledHintSymbol, true);
+  }
+  return AsyncFunctionAwait(generator, awaited, false);
+}
+
 // How the parser rejects promises from async/await desugaring
 function RejectPromiseNoDebugEvent(promise, reason) {
   return RejectPromise(promise, reason, false);
 }
 
 %InstallToContext([
-  "async_function_await", AsyncFunctionAwait,
+  "async_function_await_caught", AsyncFunctionAwaitCaught,
+  "async_function_await_uncaught", AsyncFunctionAwaitUncaught,
   "reject_promise_no_debug_event", RejectPromiseNoDebugEvent,
 ]);
 
 })