WIP events: optimize emit()

EventEmitter#emit() can be called with many different arguments, but
there are a few that are most common. Increase performance by optimizing
those most common cases. It also drastically decreases the amount of
output from --trace_deopt.

Author: trevnorris

lib/events.js

@@ -56,38 +56,14 @@ EventEmitter.prototype.getMaxListeners = function getMaxListeners() {
   return $getMaxListeners(this);
 };
 
-EventEmitter.prototype.emit = function emit(type) {
-  var er, handler, len, args, i, listeners;
-
-  if (!this._events)
-    this._events = {};
-
-  // If there is no 'error' event listener then throw.
-  if (type === 'error' && !this._events.error) {
-    er = arguments[1];
-    if (this.domain) {
-      if (!er)
-        er = new Error('Uncaught, unspecified "error" event.');
-      er.domainEmitter = this;
-      er.domain = this.domain;
-      er.domainThrown = false;
-      this.domain.emit('error', er);
-    } else if (er instanceof Error) {
-      throw er; // Unhandled 'error' event
-    } else {
-      throw Error('Uncaught, unspecified "error" event.');
-    }
-    return false;
-  }
+function $emitGeneric(type) {
+  var handler, len, args, i, listeners;
 
   handler = this._events[type];
 
   if (util.isUndefined(handler))
     return false;
 
-  if (this.domain && this !== process)
-    this.domain.enter();
-
   if (util.isFunction(handler)) {
     switch (arguments.length) {
       // fast cases
@@ -120,10 +96,61 @@ EventEmitter.prototype.emit = function emit(type) {
       listeners[i].apply(this, args);
   }
 
+  return true;
+}
+
+function $emitUncaughtError(type, er) {
+  // If there is no 'error' event listener then throw.
+  if (this.domain) {
+    if (!er)
+      er = new Error('Uncaught, unspecified "error" event.');
+    er.domainEmitter = this;
+    er.domain = this.domain;
+    er.domainThrown = false;
+    this.domain.emit('error', er);
+  } else if (er instanceof Error) {
+    throw er; // Unhandled 'error' event
+  } else {
+    throw new Error('Uncaught, unspecified "error" event.');
+  }
+  return false;
+}
+
+function $emitNoArguments(type) {
+  var handler = this._events[type];
+
+  if (typeof handler === 'function')
+    handler.call(this);
+  if (typeof handler !== 'object')
+    return;
+
+  var listeners = handler.slice();
+  for (var i = 0; i < listeners.length; i++)
+    listeners[i].call(this);
+
+  return true;
+}
+
+EventEmitter.prototype.emit = function emit(type) {
+  var ret;
+
+  if (!this._events)
+    this._events = {};
+
+  if (this.domain && this !== process)
+    this.domain.enter();
+
+  if (type === 'error' && !this._events.error)
+    ret = $emitUncaughtError.apply(this, arguments);
+  else if (arguments.length === 1)
+    ret = $emitNoArguments.call(this, type);
+  else
+    ret = $emitGeneric.apply(this, arguments);
+
   if (this.domain && this !== process)
     this.domain.exit();
 
-  return true;
+  return ret;
 };
 
 EventEmitter.prototype.addListener = function addListener(type, listener) {