process: slight refinements to nextTick

Remove length prop on NextTickQueue class. We technically don't need to
keep track of the length of the queue in two places as we already have
tickInfo doing that work (between the index & the length we have enough
data for everything).

Store asyncId in a const within the _tickCallback function. Accessing
Symbol properties seems to be quite a bit more expensive than string
keys so this actually has a decent performance impact.

PR-URL: #17421
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/process/next_tick.js

@@ -14,35 +14,31 @@ class NextTickQueue {
   constructor() {
     this.head = null;
     this.tail = null;
-    this.length = 0;
   }
 
   push(v) {
     const entry = { data: v, next: null };
-    if (this.length > 0)
+    if (this.tail !== null)
       this.tail.next = entry;
     else
       this.head = entry;
     this.tail = entry;
-    ++this.length;
   }
 
   shift() {
-    if (this.length === 0)
+    if (this.head === null)
       return;
     const ret = this.head.data;
-    if (this.length === 1)
+    if (this.head === this.tail)
       this.head = this.tail = null;
     else
       this.head = this.head.next;
-    --this.length;
     return ret;
   }
 
   clear() {
     this.head = null;
     this.tail = null;
-    this.length = 0;
   }
 }
 
@@ -90,7 +86,7 @@ function setupNextTick() {
         nextTickQueue.clear();
         tickInfo[kLength] = 0;
       } else {
-        tickInfo[kLength] = nextTickQueue.length;
+        tickInfo[kLength] -= tickInfo[kIndex];
       }
     }
     tickInfo[kIndex] = 0;
@@ -124,8 +120,6 @@ function setupNextTick() {
     }
   }
 
-  // Run callbacks that have no domain.
-  // Using domains will cause this to be overridden.
   function _tickCallback() {
     do {
       while (tickInfo[kIndex] < tickInfo[kLength]) {
@@ -137,7 +131,8 @@ function setupNextTick() {
         // CHECK(Number.isSafeInteger(tock[trigger_async_id_symbol]))
         // CHECK(tock[trigger_async_id_symbol] > 0)
 
-        emitBefore(tock[async_id_symbol], tock[trigger_async_id_symbol]);
+        const asyncId = tock[async_id_symbol];
+        emitBefore(asyncId, tock[trigger_async_id_symbol]);
         // emitDestroy() places the async_id_symbol into an asynchronous queue
         // that calls the destroy callback in the future. It's called before
         // calling tock.callback so destroy will be called even if the callback
@@ -148,15 +143,15 @@ function setupNextTick() {
         // any async hooks are enabled during the callback's execution then
         // this tock's after hook will be called, but not its destroy hook.
         if (async_hook_fields[kDestroy] > 0)
-          emitDestroy(tock[async_id_symbol]);
+          emitDestroy(asyncId);
 
         const callback = tock.callback;
         if (tock.args === undefined)
           callback();
         else
           Reflect.apply(callback, undefined, tock.args);
 
-        emitAfter(tock[async_id_symbol]);
+        emitAfter(asyncId);
 
         if (kMaxCallbacksPerLoop < tickInfo[kIndex])
           tickDone();