add a workaround of V8 deoptimization which causes slightly performan…

…ce degradation of `Promise`, #679

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## Changelog
 ##### Unreleased
 - Added a workaround of V8 deoptimization which causes serious performance degradation (~4x in my tests) of `Array#concat`, [#679](https://github.com/zloirock/core-js/issues/679)
+- Added a workaround of V8 deoptimization which causes slightly performance degradation of `Promise`, [#679](https://github.com/zloirock/core-js/issues/679)
 - Added compat data for Chromium-based Edge
 
 ##### 3.3.4 - 2019.10.25

packages/core-js/modules/es.promise.js

@@ -50,20 +50,27 @@ var UNHANDLED = 2;
 var Internal, OwnPromiseCapability, PromiseWrapper, nativeThen;
 
 var FORCED = isForced(PROMISE, function () {
-  // correct subclassing with @@species support
+  if (typeof PromiseConstructor != 'function') return true;
+  // V8 6.6 (Node 10 and Chrome 66) have a bug with resolving custom thenables
+  // https://bugs.chromium.org/p/chromium/issues/detail?id=830565
+  // We can't detect it synchronously, so just check versions
+  if (V8_VERSION === 66) return true;
+  // Unhandled rejections tracking support, NodeJS Promise without it fails @@species test
+  if (!IS_NODE && typeof PromiseRejectionEvent != 'function') return true;
+  // We need Promise#finally in the pure version for preventing prototype pollution
+  if (IS_PURE && !PromiseConstructor.prototype['finally']) return true;
+  // We can't use @@species feature detection in V8 since it causes
+  // deoptimization and performance degradation
+  // https://github.com/zloirock/core-js/issues/679
+  if (V8_VERSION >= 51) return false;
+  // Detect correctness of subclassing with @@species support
   var promise = PromiseConstructor.resolve(1);
-  var empty = function () { /* empty */ };
-  var FakePromise = (promise.constructor = {})[SPECIES] = function (exec) {
-    exec(empty, empty);
+  var FakePromise = function (exec) {
+    exec(function () { /* empty */ }, function () { /* empty */ });
   };
-  // unhandled rejections tracking support, NodeJS Promise without it fails @@species test
-  return !((IS_NODE || typeof PromiseRejectionEvent == 'function')
-    && (!IS_PURE || promise['finally'])
-    && promise.then(empty) instanceof FakePromise
-    // v8 6.6 (Node 10 and Chrome 66) have a bug with resolving custom thenables
-    // https://bugs.chromium.org/p/chromium/issues/detail?id=830565
-    // we can't detect it synchronously, so just check versions
-    && V8_VERSION !== 66);
+  promise.constructor = {};
+  promise.constructor[SPECIES] = FakePromise;
+  return !(promise.then(function () { /* empty */ }) instanceof FakePromise);
 });
 
 var INCORRECT_ITERATION = FORCED || !checkCorrectnessOfIteration(function (iterable) {