Introduce v8.execute tracing category with RunMicrotasks event.

src/isolate.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/isolate.h"
 
 #include <stdlib.h>
 
 #include <fstream>  // NOLINT(readability/streams)
 #include <sstream>
@@ skipping 2939 matching lines @@
     queue = factory()->CopyFixedArrayAndGrow(queue, num_tasks);
     heap()->set_microtask_queue(*queue);
   }
   DCHECK(queue->get(num_tasks)->IsUndefined(this));
   queue->set(num_tasks, *microtask);
   set_pending_microtask_count(num_tasks + 1);
 }
 
 
 void Isolate::RunMicrotasks() {
-  // Increase call depth to prevent recursive callbacks.
-  v8::Isolate::SuppressMicrotaskExecutionScope suppress(
-      reinterpret_cast<v8::Isolate*>(this));
-  is_running_microtasks_ = true;
-  RunMicrotasksInternal();
-  is_running_microtasks_ = false;
+  if (pending_microtask_count()) {
+    TRACE_EVENT0("v8.execute", "RunMicrotasks");

[caseq, 2016/08/30 16:16:14]

let's move down to RunMicrotasksInternal?

[alph, 2016/08/30 17:06:04]

Done.
Also I noticed that originally FireMicrotasksCompletedCallback were guarded
inside SuppressMicrotaskExecutionScope. Not sure if it's important, but moved
the code to keep the logic.

+    // Increase call depth to prevent recursive callbacks.
+    v8::Isolate::SuppressMicrotaskExecutionScope suppress(
+        reinterpret_cast<v8::Isolate*>(this));
+    is_running_microtasks_ = true;
+    RunMicrotasksInternal();
+    is_running_microtasks_ = false;
+  }
   FireMicrotasksCompletedCallback();
 }
 
 
 void Isolate::RunMicrotasksInternal() {
+  DCHECK(pending_microtask_count() > 0);
   while (pending_microtask_count() > 0) {
     HandleScope scope(this);
     int num_tasks = pending_microtask_count();
     Handle<FixedArray> queue(heap()->microtask_queue(), this);
     DCHECK(num_tasks <= queue->length());
     set_pending_microtask_count(0);
     heap()->set_microtask_queue(heap()->empty_fixed_array());
 
     Isolate* isolate = this;
     FOR_WITH_HANDLE_SCOPE(isolate, int, i = 0, i, i < num_tasks, i++, {
@@ skipping 194 matching lines @@
   // Then check whether this scope intercepts.
   if ((flag & intercept_mask_)) {
     intercepted_flags_ |= flag;
     return true;
   }
   return false;
 }
 
 }  // namespace internal
 }  // namespace v8