events: remove emit micro-optimizations

With improvements in V8, using separate emit functions is no longer
necessary and can instead be replaced by the spread operator.

improvement                             confidence        p.value
events/ee-emit.js n=2000000                 2.98 %        0.09852489
events/ee-emit-2-args.js n=2000000          4.19 %    *** 0.0001914216
events/ee-emit-6-args.js n=2000000         61.69 %    *** 6.611964e-35
events/ee-emit-diff-args.js n=2000000      -0.36 %        0.305069
events/ee-once.js n=20000000                6.42 %    *** 1.27831e-06

PR-URL: https://github.com/nodejs/node/pull/16869
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Benedikt Meurer <benedikt.meurer@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>

benchmark/events/ee-emit-multi-args.js

@@ -1,20 +0,0 @@
-'use strict';
-const common = require('../common.js');
-const EventEmitter = require('events').EventEmitter;
-
-const bench = common.createBenchmark(main, { n: [2e6] });
-
-function main(conf) {
-  const n = conf.n | 0;
-
-  const ee = new EventEmitter();
-
-  for (var k = 0; k < 10; k += 1)
-    ee.on('dummy', function() {});
-
-  bench.start();
-  for (var i = 0; i < n; i += 1) {
-    ee.emit('dummy', 5, true);
-  }
-  bench.end(n);
-}

benchmark/events/ee-emit.js

@@ -2,19 +2,51 @@
 const common = require('../common.js');
 const EventEmitter = require('events').EventEmitter;
 
-const bench = common.createBenchmark(main, { n: [2e6] });
+const bench = common.createBenchmark(main, {
+  n: [2e6],
+  argc: [0, 2, 4, 10],
+  listeners: [1, 5, 10],
+});
 
 function main(conf) {
   const n = conf.n | 0;
+  const argc = conf.argc | 0;
+  const listeners = Math.max(conf.listeners | 0, 1);
 
   const ee = new EventEmitter();
 
-  for (var k = 0; k < 10; k += 1)
+  for (var k = 0; k < listeners; k += 1)
     ee.on('dummy', function() {});
 
+  var i;
+  switch (argc) {
+    case 2:
       bench.start();
-  for (var i = 0; i < n; i += 1) {
+      for (i = 0; i < n; i += 1) {
+        ee.emit('dummy', true, 5);
+      }
+      bench.end(n);
+      break;
+    case 4:
+      bench.start();
+      for (i = 0; i < n; i += 1) {
+        ee.emit('dummy', true, 5, 10, false);
+      }
+      bench.end(n);
+      break;
+    case 10:
+      bench.start();
+      for (i = 0; i < n; i += 1) {
+        ee.emit('dummy', true, 5, 10, false, 5, 'string', true, false, 11, 20);
+      }
+      bench.end(n);
+      break;
+    default:
+      bench.start();
+      for (i = 0; i < n; i += 1) {
         ee.emit('dummy');
       }
       bench.end(n);
+      break;
+  }
 }

lib/events.js

@@ -105,63 +105,6 @@ EventEmitter.prototype.getMaxListeners = function getMaxListeners() {
   return $getMaxListeners(this);
 };
 
-// These standalone emit* functions are used to optimize calling of event
-// handlers for fast cases because emit() itself often has a variable number of
-// arguments and can be deoptimized because of that. These functions always have
-// the same number of arguments and thus do not get deoptimized, so the code
-// inside them can execute faster.
-function emitNone(handler, isFn, self) {
-  if (isFn)
-    handler.call(self);
-  else {
-    var len = handler.length;
-    var listeners = arrayClone(handler, len);
-    for (var i = 0; i < len; ++i)
-      listeners[i].call(self);
-  }
-}
-function emitOne(handler, isFn, self, arg1) {
-  if (isFn)
-    handler.call(self, arg1);
-  else {
-    var len = handler.length;
-    var listeners = arrayClone(handler, len);
-    for (var i = 0; i < len; ++i)
-      listeners[i].call(self, arg1);
-  }
-}
-function emitTwo(handler, isFn, self, arg1, arg2) {
-  if (isFn)
-    handler.call(self, arg1, arg2);
-  else {
-    var len = handler.length;
-    var listeners = arrayClone(handler, len);
-    for (var i = 0; i < len; ++i)
-      listeners[i].call(self, arg1, arg2);
-  }
-}
-function emitThree(handler, isFn, self, arg1, arg2, arg3) {
-  if (isFn)
-    handler.call(self, arg1, arg2, arg3);
-  else {
-    var len = handler.length;
-    var listeners = arrayClone(handler, len);
-    for (var i = 0; i < len; ++i)
-      listeners[i].call(self, arg1, arg2, arg3);
-  }
-}
-
-function emitMany(handler, isFn, self, args) {
-  if (isFn)
-    handler.apply(self, args);
-  else {
-    var len = handler.length;
-    var listeners = arrayClone(handler, len);
-    for (var i = 0; i < len; ++i)
-      listeners[i].apply(self, args);
-  }
-}
-
 EventEmitter.prototype.emit = function emit(type, ...args) {
   let doError = (type === 'error');
 
@@ -212,22 +155,13 @@ EventEmitter.prototype.emit = function emit(type, ...args) {
     needDomainExit = true;
   }
 
-  const isFn = typeof handler === 'function';
+  if (typeof handler === 'function') {
-  switch (args.length) {
+    handler.apply(this, args);
-    case 0:
+  } else {
-      emitNone(handler, isFn, this);
+    const len = handler.length;
-      break;
+    const listeners = arrayClone(handler, len);
-    case 1:
+    for (var i = 0; i < len; ++i)
-      emitOne(handler, isFn, this, args[0]);
+      listeners[i].apply(this, args);
-      break;
-    case 2:
-      emitTwo(handler, isFn, this, args[0], args[1]);
-      break;
-    case 3:
-      emitThree(handler, isFn, this, args[0], args[1], args[2]);
-      break;
-    default:
-      emitMany(handler, isFn, this, args);
   }
 
   if (needDomainExit)
@@ -313,23 +247,11 @@ EventEmitter.prototype.prependListener =
       return _addListener(this, type, listener, true);
     };
 
-function onceWrapper() {
+function onceWrapper(...args) {
   if (!this.fired) {
     this.target.removeListener(this.type, this.wrapFn);
     this.fired = true;
-    switch (arguments.length) {
+    this.listener.apply(this.target, args);
-      case 0:
-        return this.listener.call(this.target);
-      case 1:
-        return this.listener.call(this.target, arguments[0]);
-      case 2:
-        return this.listener.call(this.target, arguments[0], arguments[1]);
-      case 3:
-        return this.listener.call(this.target, arguments[0], arguments[1],
-                                  arguments[2]);
-      default:
-        this.listener.apply(this.target, arguments);
-    }
   }
 }
 

test/message/stdin_messages.out

@@ -9,10 +9,10 @@ SyntaxError: Strict mode code may not include a with statement
     at Module._compile (module.js:*:*)
     at evalScript (bootstrap_node.js:*:*)
     at Socket.<anonymous> (bootstrap_node.js:*:*)
-    at emitNone (events.js:*:*)
     at Socket.emit (events.js:*:*)
     at endReadableNT (_stream_readable.js:*:*)
     at _combinedTickCallback (internal/process/next_tick.js:*:*)
+    at process._tickCallback (internal/process/next_tick.js:*:*)
 42
 42
 [stdin]:1
@@ -27,9 +27,9 @@ Error: hello
     at Module._compile (module.js:*:*)
     at evalScript (bootstrap_node.js:*:*)
     at Socket.<anonymous> (bootstrap_node.js:*:*)
-    at emitNone (events.js:*:*)
     at Socket.emit (events.js:*:*)
     at endReadableNT (_stream_readable.js:*:*)
+    at _combinedTickCallback (internal/process/next_tick.js:*:*)
 [stdin]:1
 throw new Error("hello")
 ^
@@ -42,9 +42,9 @@ Error: hello
     at Module._compile (module.js:*:*)
     at evalScript (bootstrap_node.js:*:*)
     at Socket.<anonymous> (bootstrap_node.js:*:*)
-    at emitNone (events.js:*:*)
     at Socket.emit (events.js:*:*)
     at endReadableNT (_stream_readable.js:*:*)
+    at _combinedTickCallback (internal/process/next_tick.js:*:*)
 100
 [stdin]:1
 var x = 100; y = x;
@@ -58,9 +58,9 @@ ReferenceError: y is not defined
     at Module._compile (module.js:*:*)
     at evalScript (bootstrap_node.js:*:*)
     at Socket.<anonymous> (bootstrap_node.js:*:*)
-    at emitNone (events.js:*:*)
     at Socket.emit (events.js:*:*)
     at endReadableNT (_stream_readable.js:*:*)
+    at _combinedTickCallback (internal/process/next_tick.js:*:*)
 
 [stdin]:1
 var ______________________________________________; throw 10