[async-await] Don't create resolving callbacks for throwaway promises

src/js/promise.js

Index: src/js/promise.js
diff --git a/src/js/promise.js b/src/js/promise.js
index d7e25124708737f8c5031e8d86d0d20b31176be1..ff4358a81086939309a872e8dbbeccc0ec66ad27 100644
--- a/src/js/promise.js
+++ b/src/js/promise.js
@@ -293,10 +293,22 @@ function DoRejectPromise(promise, reason) {
 
 // ES#sec-newpromisecapability
 // NewPromiseCapability ( C )
-function NewPromiseCapability(C, debugEvent) {
+function NewPromiseCapability(C, debugEvent, doNotCreateCallbacks) {

[Dan Ehrenberg, 2016/11/18 08:05:38]

Style suggestion: Rather than making this extra boolean flag (which I think
makes the code harder to read), what if you refactored the true case into a
separate function that either returns PromiseCreate() or falls back to
NewPromiseCapability().promise for other constructors?

[gsathya, 2016/11/18 08:34:28]

Do you mean a function like --
NewPromiseCapabilityWithoutCallbacks() { return { promise: PromiseCreate(),
resolve: UNDEFINED, reject: UNDEFINED} }

This would make code in PromiseThen look a little more verbose --
if (constructor === GlobalPromise) {
  resultCapability = NewPromiseCapabilityWithoutCallbacks();
} else {
  resultCapability = NewPromiseCapability(constructor, false);
}

Do you think this is better?

[Dan Ehrenberg, 2016/11/18 08:55:37]

I was imagining that all of that would be part of the body of
NewPromiseCapabilityWithoutCallbacks. In general, boolean flags are really
difficult for readability/maintainability, and I think it's worth it to add a
tiny bit more verbose code to avoid them.

   if (C === GlobalPromise) {
     // Optimized case, avoid extra closure.
     var promise = PromiseCreate();
+
+    // The resultCapability.promise is only ever fulfilled internally,
+    // so we don't need the closures to protect against accidentally
+    // calling them multiple times.

[caitp, 2016/11/18 05:47:15]

I like that this is all documented so nicely. Since this behaviour is specific
to await expressions, I think we will want to move this out of
NewPromiseCapability, maybe to a code-stub (or just inlined),  since this
internal-ness is always known statically. The documentation in this CL should
help make that transition easier.

LGTM, but would like assurance that it's not visibly different in the debugger

[gsathya, 2016/11/18 06:02:28]

This is used by PromiseThen as well which is why I didn't inline it.
 
There shouldn't be anything visibly different in the debugger since its the same
callbacks (without the alreadyResolved check) that's being run.

[Dan Ehrenberg, 2016/11/18 08:05:38]

Could you mention this in the commit message? Does this benchmark change
Bluebird perf?
This is a decent reason for making it a separate function. The new function does
not need to take debugEvent as an argument, as that argument is just used for
making the callbacks do the right thing.

[gsathya, 2016/11/18 08:34:27]

This isn't a new change. This piece of code was specific to PromiseThen, I
refactored it out to be in NewPromiseCapability.
Yeah, agreed, but it makes code more verbose in other places as mentioned above.

+    if (doNotCreateCallbacks) {
+      return {
+        promise: promise,
+        resolve: UNDEFINED,
+        reject: UNDEFINED
+      };
+    }
+
     // TODO(gsathya): Remove container for callbacks when this is
     // moved to CPP/TF.
     var callbacks = %create_resolving_functions(promise, debugEvent);
@@ -380,23 +392,12 @@ function PromiseThen(onResolve, onReject) {
   }
 
   var constructor = SpeciesConstructor(this, GlobalPromise);
-  var resultCapability;
-
-  // The resultCapability.promise is only ever fulfilled internally,
-  // so we don't need the closures to protect against accidentally
-  // calling them multiple times.
-  if (constructor === GlobalPromise) {
-    // TODO(gsathya): Combine this into NewPromiseCapability.
-    resultCapability = {
-      promise: PromiseCreate(),
-      resolve: UNDEFINED,
-      reject: UNDEFINED
-    };
-  } else {
-    // Pass false for debugEvent so .then chaining does not trigger
-    // redundant ExceptionEvents.
-    resultCapability = NewPromiseCapability(constructor, false);
-  }
+
+  // Pass false for debugEvent so .then chaining does not trigger
+  // redundant ExceptionEvents.
+  var resultCapability = NewPromiseCapability(
+      constructor, false, constructor === GlobalPromise);

[Dan Ehrenberg, 2016/11/18 08:05:38]

Not sure why you do this comparison. Seems like avoiding creating the functions
is only done on GlobalPromise anyway.

[gsathya, 2016/11/18 08:34:27]

Yes, but this comparison is required to not create the functions. You could
create functions in the the GlobalPromise case too (like in Promise.all and
Promise.race).

[Dan Ehrenberg, 2016/11/18 08:55:37]

I'd suggest factoring all this logic into NewPromiseCapabilityWithoutCallbacks.
If C is not GlobalPromise, create the functions. The logic only applies to the
callers of this special variant, so it wouldn't affect Promise.all.

+
   return PerformPromiseThen(this, onResolve, onReject, resultCapability);
 }
 
@@ -423,8 +424,10 @@ function PromiseResolve(x) {
     return promise;
   }
 
-  // debugEvent is not so meaningful here as it will be resolved
-  var promiseCapability = NewPromiseCapability(this, true);
+  // debugEvent is not so meaningful here as it will be resolved.
+  // We have already addressed the case where we don't need callbacks created,
+  // so we pass false to create the callbacks.
+  var promiseCapability = NewPromiseCapability(this, true, false);
   %_Call(promiseCapability.resolve, UNDEFINED, x);
   return promiseCapability.promise;
 }
@@ -438,7 +441,8 @@ function PromiseAll(iterable) {
 
   // false debugEvent so that forwarding the rejection through all does not
   // trigger redundant ExceptionEvents
-  var deferred = NewPromiseCapability(this, false);
+  // We need to create the resolving callbacks for the promise.
+  var deferred = NewPromiseCapability(this, false, false);
   var resolutions = new InternalArray();
   var count;
 
@@ -502,7 +506,8 @@ function PromiseRace(iterable) {
 
   // false debugEvent so that forwarding the rejection through race does not
   // trigger redundant ExceptionEvents
-  var deferred = NewPromiseCapability(this, false);
+  // We need to create the resolving callbacks for the promise.
+  var deferred = NewPromiseCapability(this, false, false);
 
   // For catch prediction, don't treat the .then calls as handling it;
   // instead, recurse outwards.