add a workaround of V8 deoptimization which causes serious performanc…

…e degradation of some `RegExp`-related methods like `String#split`, #306

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## Changelog
 ##### Unreleased
+- Added a workaround of V8 deoptimization which causes serious performance degradation (~20x in my tests) of some `RegExp`-related methods like `String#split`, [#306](https://github.com/zloirock/core-js/issues/306)
 - Fixed work of `fetch` with polyfilled `URLSearchParams`, [#674](https://github.com/zloirock/core-js/issues/674)
 - `package-lock.json` no longer generated in libraries
 

packages/core-js/internals/fix-regexp-well-known-symbol-logic.js

@@ -44,15 +44,22 @@ module.exports = function (KEY, length, exec, sham) {
     // Symbol-named RegExp methods call .exec
     var execCalled = false;
     var re = /a/;
-    re.exec = function () { execCalled = true; return null; };
 
     if (KEY === 'split') {
+      // We can't use real regex here since it will cause deoptimization in V8
+      // and serious performance degradation
+      // https://github.com/zloirock/core-js/issues/306
+      re = {};
       // RegExp[@@split] doesn't call the regex's exec method, but first creates
       // a new one. We need to return the patched regex when creating the new one.
       re.constructor = {};
       re.constructor[SPECIES] = function () { return re; };
+      re.flags = '';
+      re[SYMBOL] = /./[SYMBOL];
     }
 
+    re.exec = function () { execCalled = true; return null; };
+
     re[SYMBOL]('');
     return !execCalled;
   });