Optimize away updatable_queue_set_

third_party/WebKit/Source/platform/scheduler/base/work_queue.cc

Index: third_party/WebKit/Source/platform/scheduler/base/work_queue.cc
diff --git a/third_party/WebKit/Source/platform/scheduler/base/work_queue.cc b/third_party/WebKit/Source/platform/scheduler/base/work_queue.cc
index 0b7d0cd9b753d9850c3bf77138b99b39d16e49ed..3a8210698d7e41b0a2f549e550931f822ef28d18 100644
--- a/third_party/WebKit/Source/platform/scheduler/base/work_queue.cc
+++ b/third_party/WebKit/Source/platform/scheduler/base/work_queue.cc
@@ -10,12 +10,15 @@ namespace blink {
 namespace scheduler {
 namespace internal {
 
-WorkQueue::WorkQueue(TaskQueueImpl* task_queue, const char* name)
+WorkQueue::WorkQueue(TaskQueueImpl* task_queue,
+                     const char* name,
+                     QueueType queue_type)
     : work_queue_sets_(nullptr),
       task_queue_(task_queue),
       work_queue_set_index_(0),
       name_(name),
-      fence_(0) {}
+      fence_(0),
+      queue_type_(queue_type) {}
 
 void WorkQueue::AsValueInto(base::trace_event::TracedValue* state) const {
   for (const TaskQueueImpl::Task& task : work_queue_) {
@@ -85,11 +88,13 @@ void WorkQueue::PopTaskForTest() {
   work_queue_.pop_front();
 }
 
-void WorkQueue::SwapLocked(WTF::Deque<TaskQueueImpl::Task>& incoming_queue) {
+void WorkQueue::ReloadEmptyImmediateQueue() {
   DCHECK(work_queue_.empty());
-  work_queue_.swap(incoming_queue);
+
+  work_queue_ = task_queue_->TakeImmediateIncomingQueue();
   if (work_queue_.empty())
     return;
+
   // If we hit the fence, pretend to WorkQueueSets that we're empty.
   if (work_queue_sets_ && !BlockedByFence())
     work_queue_sets_->OnPushQueue(this);
@@ -106,6 +111,11 @@ TaskQueueImpl::Task WorkQueue::TakeTaskFromWorkQueue() {
   }
 
   TaskQueueImpl::Task pending_task = work_queue_.takeFirst();
+  // NB immediate tasks have a different pipeline to delayed ones.
+  if (queue_type_ == QueueType::IMMEDIATE && work_queue_.empty())
+    work_queue_ = task_queue_->TakeImmediateIncomingQueue();

[Sami, 2017/01/18 17:57:04]

Not sure why we're not calling OnPushQueue here? Could we just call
ReloadEmptyImmediateQueue()?

[alex clarke (OOO till 29th), 2017/01/19 08:50:53]

No, we'd have to do that after OnPopQueue() (because OnPushQueue assumes the
work_queue_ is empty).  It's more efficient to do this.

[Sami, 2017/01/19 11:57:38]

Okay, I think I understand now: we're short-circuiting the queue reload so that
OnPopQueue does the right thing. Might be worth adding some comment to that
effect here.

(N.B. it feels like OnPushQueue/OnPopQueue could use better names since they
aren't quite symmetric. Maybe something like OnTaskPushedToEmptyQueue and
OnTaskPoppedFromQueue? Up to you.)

[alex clarke (OOO till 29th), 2017/01/19 12:34:31]

Done.

+  // OnPopQueue calls GetFrontTaskEnqueueOrder which checks BlockedByFence() so
+  // we don't need to here.
   work_queue_sets_->OnPopQueue(this);
   task_queue_->TraceQueueSize(false);
   return pending_task;