events: improve arrayClone performance

benchmark/events/ee-emit.js

@@ -10,6 +10,7 @@ const bench = common.createBenchmark(main, {
 
 function main({ n, argc, listeners }) {
   const ee = new EventEmitter();
+  ee.setMaxListeners(listeners + 1);
 
   for (let k = 0; k < listeners; k += 1)
     ee.on('dummy', () => {});

benchmark/events/ee-listeners.js

@@ -2,20 +2,34 @@
 const common = require('../common.js');
 const EventEmitter = require('events').EventEmitter;
 
-const bench = common.createBenchmark(main, { n: [5e6] });
+const bench = common.createBenchmark(main, {
+  n: [5e6],
+  listeners: [5, 50],
+  raw: ['true', 'false']
+});
 
-function main({ n }) {
+function main({ n, listeners, raw }) {
   const ee = new EventEmitter();
+  ee.setMaxListeners(listeners * 2 + 1);
 
-  for (let k = 0; k < 5; k += 1) {
+  for (let k = 0; k < listeners; k += 1) {
     ee.on('dummy0', () => {});
     ee.on('dummy1', () => {});
   }
 
-  bench.start();
-  for (let i = 0; i < n; i += 1) {
-    const dummy = (i % 2 === 0) ? 'dummy0' : 'dummy1';
-    ee.listeners(dummy);
+  if (raw === 'true') {
+    bench.start();
+    for (let i = 0; i < n; i += 1) {
+      const dummy = (i % 2 === 0) ? 'dummy0' : 'dummy1';
+      ee.rawListeners(dummy);
+    }
+    bench.end(n);
+  } else {
+    bench.start();
+    for (let i = 0; i < n; i += 1) {
+      const dummy = (i % 2 === 0) ? 'dummy0' : 'dummy1';
+      ee.listeners(dummy);
+    }
+    bench.end(n);
   }
-  bench.end(n);
 }

lib/events.js

@@ -322,7 +322,7 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
     }
   } else {
     const len = handler.length;
-    const listeners = arrayClone(handler, len);
+    const listeners = arrayClone(handler);
     for (let i = 0; i < len; ++i) {
       const result = ReflectApply(listeners[i], this, args);
 
@@ -560,7 +560,7 @@ function _listeners(target, type, unwrap) {
     return unwrap ? [evlistener.listener || evlistener] : [evlistener];
 
   return unwrap ?
-    unwrapListeners(evlistener) : arrayClone(evlistener, evlistener.length);
+    unwrapListeners(evlistener) : arrayClone(evlistener);
 }
 
 EventEmitter.prototype.listeners = function listeners(type) {
@@ -599,11 +599,17 @@ EventEmitter.prototype.eventNames = function eventNames() {
   return this._eventsCount > 0 ? ReflectOwnKeys(this._events) : [];
 };
 
-function arrayClone(arr, n) {
-  const copy = new Array(n);
-  for (let i = 0; i < n; ++i)
-    copy[i] = arr[i];
-  return copy;
+function arrayClone(arr) {
+  // At least since V8 8.3, this implementation is faster than the previous
+  // which always used a simple for-loop
+  switch (arr.length) {
+    case 2: return [arr[0], arr[1]];
+    case 3: return [arr[0], arr[1], arr[2]];
+    case 4: return [arr[0], arr[1], arr[2], arr[3]];
+    case 5: return [arr[0], arr[1], arr[2], arr[3], arr[4]];
+    case 6: return [arr[0], arr[1], arr[2], arr[3], arr[4], arr[5]];
+  }
+  return arr.slice();
 }
 
 function unwrapListeners(arr) {