Promises: Short circuit promise resolution procedure

src/js/promise.js

 // Copyright 2012 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();
 
@@ skipping 232 matching lines @@
   if (resolution === promise) {
     return RejectPromise(promise, MakeTypeError(kPromiseCyclic, resolution));
   }
   if (IS_RECEIVER(resolution)) {
     // 25.4.1.3.2 steps 8-12
     try {
       var then = resolution.then;
     } catch (e) {
       return RejectPromise(promise, e);
     }
+
+    // Resolution is a native promise and if it's already resolved or
+    // rejected, shortcircuit the resolution procedure by directly
+    // reusing the value from the promise.
+    if (IsPromise(resolution) && then === PromiseThen) {
+      var thenableState = GET_PRIVATE(resolution, promiseStateSymbol);
+      if (thenableState === kFulfilled) {
+        // This goes inside the if-else to save one symbol lookup in
+        // the slow path.
+        var thenableValue = GET_PRIVATE(resolution, promiseResultSymbol);
+        FulfillPromise(promise, kFulfilled, thenableValue,
+                       promiseFulfillReactionsSymbol);
+        SET_PRIVATE(promise, promiseHasHandlerSymbol, true);
+        return;
+      } else if (thenableState === kRejected) {
+        var thenableValue = GET_PRIVATE(resolution, promiseResultSymbol);
+        if (!HAS_DEFINED_PRIVATE(resolution, promiseHasHandlerSymbol)) {
+          // Promise has already been rejected, but had no handler.
+          // Revoke previously triggered reject event.
+          %PromiseRevokeReject(resolution);
+        }
+        RejectPromise(promise, thenableValue);
+        SET_PRIVATE(resolution, promiseHasHandlerSymbol, true);
+        return;
+      }
+    }
+
     if (IS_CALLABLE(then)) {
       // PromiseResolveThenableJob
       var id, name, instrumenting = DEBUG_IS_ACTIVE;
       %EnqueueMicrotask(function() {
         if (instrumenting) {
           %DebugAsyncTaskEvent({ type: "willHandle", id: id, name: name });
         }
         var callbacks = CreateResolvingFunctions(promise);
         try {
           %_Call(then, resolution, callbacks.resolve, callbacks.reject);
@@ skipping 321 matching lines @@
   to.PromiseChain = PromiseChain;
   to.PromiseDefer = PromiseDefer;
   to.PromiseAccept = PromiseAccept;
 
   to.PromiseCreateRejected = PromiseCreateRejected;
   to.PromiseCreateResolved = PromiseCreateResolved;
   to.PromiseThen = PromiseThen;
 });
 
 })