timers: 3x faster setImmediate

Save the setImmediate callback arguments into an array instead of a
closure, and invoke the callback on the arguments from an optimizable
function.

  60% faster setImmediate with 0 args (15% if self-recursive)
  4x faster setImmediate with 1-3 args, 2x with > 3
  seems to be faster with less memory pressure when memory is tight

Changes:
- use L.create() to build faster lists
- use runCallback() from within tryOnImmediate
- create immediate timers with a function instead of new
- just save the arguments and not build closures for the callbacks

lib/timers.js

@@ -515,9 +515,13 @@ function processImmediate() {
     immediate = L.shift(queue);
     domain = immediate.domain;
 
+    if (!immediate._onImmediate)
+      continue;
+
     if (domain)
       domain.enter();
 
+    immediate._callback = immediate._onImmediate;
     tryOnImmediate(immediate, queue);
 
     if (domain)
@@ -538,7 +542,8 @@ function processImmediate() {
 function tryOnImmediate(immediate, queue) {
   var threw = true;
   try {
-    immediate._onImmediate();
+    // make the actual call outside the try/catch to allow it to be optimized
+    runCallback(immediate);
     threw = false;
   } finally {
     if (threw && !L.isEmpty(queue)) {
@@ -552,56 +557,66 @@ function tryOnImmediate(immediate, queue) {
   }
 }
 
+function runCallback(timer) {
+  var argv = timer._argv;
+  var argc = argv ? argv.length : 0;
+  switch (argc) {
+    // fast-path callbacks with 0-3 arguments
+    case 0:
+      return timer._callback();
+    case 1:
+      return timer._callback(argv[0]);
+    case 2:
+      return timer._callback(argv[0], argv[1]);
+    case 3:
+      return timer._callback(argv[0], argv[1], argv[2]);
+    // more then 3 arguments run slower with .apply
+    default:
+      return timer._callback.apply(timer, argv);
+  }
+}
 
-function Immediate() { }
-
-Immediate.prototype.domain = undefined;
-Immediate.prototype._onImmediate = undefined;
-Immediate.prototype._idleNext = undefined;
-Immediate.prototype._idlePrev = undefined;
 
+function createImmediate(callback) {
+  return {
+    _idleNext: null,
+    _idlePrev: null,
+    _callback: callback,
+    _argv: null,
+    _onImmediate: callback,
+    domain: process.domain,
+  };
+}
 
 exports.setImmediate = function(callback, arg1, arg2, arg3) {
   if (typeof callback !== 'function') {
     throw new TypeError('"callback" argument must be a function');
   }
 
   var i, args;
-  var len = arguments.length;
-  var immediate = new Immediate();
+  var immediate = createImmediate(callback);
 
-  L.init(immediate);
-
-  switch (len) {
+  switch (arguments.length) {
     // fast cases
     case 0:
     case 1:
-      immediate._onImmediate = callback;
       break;
     case 2:
-      immediate._onImmediate = function() {
-        callback.call(immediate, arg1);
-      };
+      immediate._argv = [arg1];
       break;
     case 3:
-      immediate._onImmediate = function() {
-        callback.call(immediate, arg1, arg2);
-      };
+      immediate._argv = [arg1, arg2];
       break;
     case 4:
-      immediate._onImmediate = function() {
-        callback.call(immediate, arg1, arg2, arg3);
-      };
+      immediate._argv = [arg1, arg2, arg3];
       break;
     // slow case
     default:
+      var len = arguments.length;
       args = new Array(len - 1);
       for (i = 1; i < len; i++)
         args[i - 1] = arguments[i];
-
-      immediate._onImmediate = function() {
-        callback.apply(immediate, args);
-      };
+      immediate._argv = args;
       break;
   }
 
@@ -610,9 +625,6 @@ exports.setImmediate = function(callback, arg1, arg2, arg3) {
     process._immediateCallback = processImmediate;
   }
 
-  if (process.domain)
-    immediate.domain = process.domain;
-
   L.append(immediateQueue, immediate);
 
   return immediate;