timer: Improve performance of callbacks

setImmediate, setTimeout, and setInterval were called in an inefficient
way, especially in the presence of arguments.  This optimization
improves their performance, with special cases for up to 4 arguments.
Performance of setImmediate increases with 35%, setInterval with 60%,
setTimeout with 70%.

PR-URL: https://github.com/iojs/io.js/pull/406
Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-by: Christian Tellnes <christian@tellnes.com>

lib/timers.js

@@ -173,8 +173,9 @@ exports.active = function(item) {
  */
 
 
-exports.setTimeout = function(callback, after) {
-  var timer;
+exports.setTimeout = function(callback, after, arg1, arg2, arg3) {
+  var timer, i, args;
+  var len = arguments.length;
 
   after *= 1; // coalesce to number or NaN
 
@@ -184,22 +185,38 @@ exports.setTimeout = function(callback, after) {
 
   timer = new Timeout(after);
 
-  if (arguments.length <= 2) {
-    timer._onTimeout = callback;
-  } else {
-    /*
-     * Sometimes setTimeout is called with arguments, EG
-     *
-     *   setTimeout(callback, 2000, "hello", "world")
-     *
-     * If that's the case we need to call the callback with
-     * those args. The overhead of an extra closure is not
-     * desired in the normal case.
-     */
-    var args = Array.prototype.slice.call(arguments, 2);
-    timer._onTimeout = function() {
-      callback.apply(timer, args);
-    };
+  switch (len) {
+    // fast cases
+    case 0:
+    case 1:
+    case 2:
+      timer._onTimeout = callback;
+      break;
+    case 3:
+      timer._onTimeout = function() {
+        callback.call(timer, arg1);
+      };
+      break;
+    case 4:
+      timer._onTimeout = function() {
+        callback.call(timer, arg1, arg2);
+      };
+      break;
+    case 5:
+      timer._onTimeout = function() {
+        callback.call(timer, arg1, arg2, arg3);
+      };
+      break;
+    // slow case
+    default:
+      args = new Array(len - 2);
+      for (i = 2; i < len; i++)
+        args[i - 2] = arguments[i];
+
+      timer._onTimeout = function() {
+        callback.apply(timer, args);
+      };
+      break;
   }
 
   if (process.domain) timer.domain = process.domain;
@@ -222,17 +239,24 @@ exports.clearTimeout = function(timer) {
 };
 
 
-exports.setInterval = function(callback, repeat) {
+exports.setInterval = function(callback, repeat, arg1, arg2, arg3) {
   repeat *= 1; // coalesce to number or NaN
 
   if (!(repeat >= 1 && repeat <= TIMEOUT_MAX)) {
     repeat = 1; // schedule on next tick, follows browser behaviour
   }
 
+  var args, i;
   var timer = new Timeout(repeat);
-  var args = Array.prototype.slice.call(arguments, 2);
+  var len = arguments.length - 2;
   timer._onTimeout = wrapper;
   timer._repeat = true;
+  // Initialize args once for repeated invocation of slow case below
+  if (len > 3) {
+    args = new Array(len);
+    for (i = 0; i < len; i++)
+      args[i] = arguments[i + 2];
+  }
 
   if (process.domain) timer.domain = process.domain;
   exports.active(timer);
@@ -240,7 +264,25 @@ exports.setInterval = function(callback, repeat) {
   return timer;
 
   function wrapper() {
-    callback.apply(this, args);
+    switch (len) {
+      // fast cases
+      case 0:
+        callback.call(this);
+        break;
+      case 1:
+        callback.call(this, arg1);
+        break;
+      case 2:
+        callback.call(this, arg1, arg2);
+        break;
+      case 3:
+        callback.call(this, arg1, arg2, arg3);
+        break;
+      // slow case
+      default:
+        callback.apply(this, args);
+        break;
+    }
     // If callback called clearInterval().
     if (timer._repeat === false) return;
     // If timer is unref'd (or was - it's permanently removed from the list.)
@@ -361,22 +403,44 @@ Immediate.prototype._idleNext = undefined;
 Immediate.prototype._idlePrev = undefined;
 
 
-exports.setImmediate = function(callback) {
+exports.setImmediate = function(callback, arg1, arg2, arg3) {
+  var i, args;
+  var len = arguments.length;
   var immediate = new Immediate();
-  var args, index;
 
   L.init(immediate);
 
-  immediate._onImmediate = callback;
-
-  if (arguments.length > 1) {
-    args = [];
-    for (index = 1; index < arguments.length; index++)
-      args.push(arguments[index]);
-
-    immediate._onImmediate = function() {
-      callback.apply(immediate, args);
-    };
+  switch (len) {
+    // fast cases
+    case 0:
+    case 1:
+      immediate._onImmediate = callback;
+      break;
+    case 2:
+      immediate._onImmediate = function() {
+        callback.call(immediate, arg1);
+      };
+      break;
+    case 3:
+      immediate._onImmediate = function() {
+        callback.call(immediate, arg1, arg2);
+      };
+      break;
+    case 4:
+      immediate._onImmediate = function() {
+        callback.call(immediate, arg1, arg2, arg3);
+      };
+      break;
+    // slow case
+    default:
+      args = new Array(len - 1);
+      for (i = 1; i < len; i++)
+        args[i - 1] = arguments[i];
+
+      immediate._onImmediate = function() {
+        callback.apply(immediate, args);
+      };
+      break;
   }
 
   if (!process._needImmediateCallback) {