timers: optimize `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()
- save the arguments and do not build closures for the callbacks

PR-URL: https://github.com/nodejs/node/pull/6436
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/internal/linkedlist.js

@@ -8,7 +8,7 @@ exports.init = init;
 
 // create a new linked list
 function create() {
-  var list = { _idleNext: null, _idlePrev: null };
+  const list = { _idleNext: null, _idlePrev: null };
   init(list);
   return list;
 }

lib/timers.js

@@ -506,7 +506,7 @@ var immediateQueue = L.create();
 
 
 function processImmediate() {
-  var queue = immediateQueue;
+  const queue = immediateQueue;
   var domain, immediate;
 
   immediateQueue = L.create();
@@ -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)) {
@@ -552,14 +557,36 @@ function tryOnImmediate(immediate, queue) {
   }
 }
 
+function runCallback(timer) {
+  const argv = timer._argv;
+  const 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 than 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 Immediate() {
+  // assigning the callback here can cause optimize/deoptimize thrashing
+  // so have caller annotate the object (node v6.0.0, v8 5.0.71.35)
+  this._idleNext = null;
+  this._idlePrev = null;
+  this._callback = null;
+  this._argv = null;
+  this._onImmediate = null;
+  this.domain = process.domain;
+}
 
 exports.setImmediate = function(callback, arg1, arg2, arg3) {
   if (typeof callback !== 'function') {
@@ -567,52 +594,40 @@ exports.setImmediate = function(callback, arg1, arg2, arg3) {
   }
 
   var i, args;
-  var len = arguments.length;
-  var immediate = new Immediate();
 
-  L.init(immediate);
+  switch (arguments.length) {
-
-  switch (len) {
     // fast cases
     case 0:
     case 1:
-      immediate._onImmediate = callback;
       break;
     case 2:
-      immediate._onImmediate = function() {
+      args = [arg1];
-        callback.call(immediate, arg1);
-      };
       break;
     case 3:
-      immediate._onImmediate = function() {
+      args = [arg1, arg2];
-        callback.call(immediate, arg1, arg2);
-      };
       break;
     case 4:
-      immediate._onImmediate = function() {
+      args = [arg1, arg2, arg3];
-        callback.call(immediate, arg1, arg2, arg3);
-      };
       break;
     // slow case
     default:
-      args = new Array(len - 1);
+      args = [arg1, arg2, arg3];
-      for (i = 1; i < len; i++)
+      for (i = 4; i < arguments.length; i++)
+        // extend array dynamically, makes .apply run much faster in v6.0.0
         args[i - 1] = arguments[i];
-
-      immediate._onImmediate = function() {
-        callback.apply(immediate, args);
-      };
       break;
   }
+  // declaring it `const immediate` causes v6.0.0 to deoptimize this function
+  var immediate = new Immediate();
+  immediate._callback = callback;
+  immediate._argv = args;
+  immediate._onImmediate = callback;
 
   if (!process._needImmediateCallback) {
     process._needImmediateCallback = true;
     process._immediateCallback = processImmediate;
   }
 
-  if (process.domain)
-    immediate.domain = process.domain;
-
   L.append(immediateQueue, immediate);
 
   return immediate;

test/parallel/test-timers-immediate.js

@@ -5,6 +5,7 @@ var assert = require('assert');
 let immediateA = false;
 let immediateB = false;
 let immediateC = [];
+let immediateD = [];
 
 setImmediate(function() {
   try {
@@ -25,8 +26,13 @@ setImmediate(function(x, y, z) {
   immediateC = [x, y, z];
 }, 1, 2, 3);
 
+setImmediate(function(x, y, z, a, b) {
+  immediateD = [x, y, z, a, b];
+}, 1, 2, 3, 4, 5);
+
 process.on('exit', function() {
   assert.ok(immediateA, 'Immediate should happen after normal execution');
   assert.notStrictEqual(immediateB, true, 'immediateB should not fire');
   assert.deepStrictEqual(immediateC, [1, 2, 3], 'immediateC args should match');
+  assert.deepStrictEqual(immediateD, [1, 2, 3, 4, 5], '5 args should match');
 });

test/parallel/test-timers-linked-list.js

@@ -103,8 +103,8 @@ assert.equal(C, L.shift(list));
 // list
 assert.ok(L.isEmpty(list));
 
-var list2 = L.create();
+const list2 = L.create();
-var list3 = L.create();
+const list3 = L.create();
 assert.ok(L.isEmpty(list2));
 assert.ok(L.isEmpty(list3));
 assert.ok(list2 != list3);