node: improve performance of nextTick

Couple micro optimizations to improve performance of process.nextTick().
Removes ~60ns of execution time.

Also added small threshold to test that allows timer to fire early on
the order if microseconds.

PR-URL: https://github.com/iojs/io.js/pull/985
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>

src/node.js

@@ -340,8 +340,7 @@
     function _tickCallback() {
       var callback, threw, tock;
 
-      scheduleMicrotasks();
+      do {
-
         while (tickInfo[kIndex] < tickInfo[kLength]) {
           tock = nextTickQueue[tickInfo[kIndex]++];
           callback = tock.callback;
@@ -356,15 +355,16 @@
           if (1e4 < tickInfo[kIndex])
             tickDone();
         }
-
         tickDone();
+        _runMicrotasks();
+        emitPendingUnhandledRejections();
+      } while (tickInfo[kLength] !== 0);
     }
 
     function _tickDomainCallback() {
       var callback, domain, threw, tock;
 
-      scheduleMicrotasks();
+      do {
-
         while (tickInfo[kIndex] < tickInfo[kLength]) {
           tock = nextTickQueue[tickInfo[kIndex]++];
           callback = tock.callback;
@@ -384,8 +384,15 @@
           if (domain)
             domain.exit();
         }
-
         tickDone();
+        _runMicrotasks();
+        emitPendingUnhandledRejections();
+      } while (tickInfo[kLength] !== 0);
+    }
+
+    function TickObject(c) {
+      this.callback = c;
+      this.domain = process.domain || null;
     }
 
     function nextTick(callback) {
@@ -393,12 +400,7 @@
       if (process._exiting)
         return;
 
-      var obj = {
+      nextTickQueue.push(new TickObject(callback));
-        callback: callback,
-        domain: process.domain || null
-      };
-
-      nextTickQueue.push(obj);
       tickInfo[kLength]++;
     }
 

test/parallel/test-timers-first-fire.js

@@ -8,5 +8,5 @@ setTimeout(function() {
   var ms = (hr[0] * 1e3) + (hr[1] / 1e6);
   var delta = ms - TIMEOUT;
   console.log('timer fired in', delta);
-  assert.ok(delta > 0, 'Timer fired early');
+  assert.ok(delta > -0.5, 'Timer fired early');
 }, TIMEOUT);