Avoid heap allocation with uncoalesced touch events

Each enqueued event in the browser-side touch queue maintains a vector
of events that must be ack'ed, allowing each enqueued event to coalesce
with subsequent events delivered from the platform. Currently, the
vector is always populated with the first event, even if no coalescing
occurs. Instead, defer population until coalescing occurs, optimizing
the common case for most events by avoiding an extra heap allocation for
every touch. This can save ~5-10us per forwarded touch event on a typical
Android device.

BUG=384562

Review URL: https://codereview.chromium.org/565033002

Cr-Commit-Position: refs/heads/master@{#295998}

2 files changed, 40 insertions, 38 deletions

diff --git a/content/browser/renderer_host/input/touch_event_queue.cc b/content/browser/renderer_host/input/touch_event_queue.cc
index d98655a..8b03c88 100644
--- a/content/browser/renderer_host/input/touch_event_queue.cc
+++ b/content/browser/renderer_host/input/touch_event_queue.cc
@@ -253,15 +253,9 @@ class TouchEventQueue::TouchMoveSlopSuppressor {
 // the Client receives the event with their original timestamp.
 class CoalescedWebTouchEvent {
  public:
-  // Events for which |async| is true will not be ack'ed to the client after the
-  // corresponding ack is received following dispatch.
-  CoalescedWebTouchEvent(const TouchEventWithLatencyInfo& event, bool async)
-      : coalesced_event_(event) {
-    if (async)
-      coalesced_event_.event.cancelable = false;
-    else
-      events_to_ack_.push_back(event);
-
+  CoalescedWebTouchEvent(const TouchEventWithLatencyInfo& event,
+                         bool suppress_client_ack)
+      : coalesced_event_(event), suppress_client_ack_(suppress_client_ack) {
     TRACE_EVENT_ASYNC_BEGIN0("input", "TouchEventQueue::QueueEvent", this);
   }
 
@@ -273,42 +267,48 @@ class CoalescedWebTouchEvent {
   // the event was coalesced.
   bool CoalesceEventIfPossible(
       const TouchEventWithLatencyInfo& event_with_latency) {
-    if (!WillDispatchAckToClient())
+    if (suppress_client_ack_)
       return false;
 
     if (!coalesced_event_.CanCoalesceWith(event_with_latency))
       return false;
 
+    // Addition of the first event to |uncoaleseced_events_to_ack_| is deferred
+    // until the first coalesced event, optimizing the (common) case where the
+    // event is not coalesced at all.
+    if (uncoaleseced_events_to_ack_.empty())
+      uncoaleseced_events_to_ack_.push_back(coalesced_event_);
+
     TRACE_EVENT_INSTANT0(
         "input", "TouchEventQueue::MoveCoalesced", TRACE_EVENT_SCOPE_THREAD);
     coalesced_event_.CoalesceWith(event_with_latency);
-    events_to_ack_.push_back(event_with_latency);
+    uncoaleseced_events_to_ack_.push_back(event_with_latency);
+    DCHECK_GE(uncoaleseced_events_to_ack_.size(), 2U);
     return true;
   }
 
-  void UpdateLatencyInfoForAck(const ui::LatencyInfo& renderer_latency_info) {
-    if (!WillDispatchAckToClient())
-      return;
-
-    for (WebTouchEventWithLatencyList::iterator iter = events_to_ack_.begin(),
-                                                end = events_to_ack_.end();
-         iter != end;
-         ++iter) {
-      iter->latency.AddNewLatencyFrom(renderer_latency_info);
-    }
-  }
-
   void DispatchAckToClient(InputEventAckState ack_result,
+                           const ui::LatencyInfo* optional_latency_info,
                            TouchEventQueueClient* client) {
     DCHECK(client);
-    if (!WillDispatchAckToClient())
+    if (suppress_client_ack_)
       return;
 
-    for (WebTouchEventWithLatencyList::const_iterator
-             iter = events_to_ack_.begin(),
-             end = events_to_ack_.end();
+    if (uncoaleseced_events_to_ack_.empty()) {
+      if (optional_latency_info)
+        coalesced_event_.latency.AddNewLatencyFrom(*optional_latency_info);
+      client->OnTouchEventAck(coalesced_event_, ack_result);
+      return;
+    }
+
+    DCHECK_GE(uncoaleseced_events_to_ack_.size(), 2U);
+    for (WebTouchEventWithLatencyList::iterator
+             iter = uncoaleseced_events_to_ack_.begin(),
+             end = uncoaleseced_events_to_ack_.end();
          iter != end;
          ++iter) {
+      if (optional_latency_info)
+        iter->latency.AddNewLatencyFrom(*optional_latency_info);
       client->OnTouchEventAck(*iter, ack_result);
     }
   }
@@ -318,15 +318,16 @@ class CoalescedWebTouchEvent {
   }
 
  private:
-  bool WillDispatchAckToClient() const { return !events_to_ack_.empty(); }
-
   // This is the event that is forwarded to the renderer.
   TouchEventWithLatencyInfo coalesced_event_;
 
   // This is the list of the original events that were coalesced, each requiring
   // future ack dispatch to the client.
+  // Note that this will be empty if no coalescing has occurred.
   typedef std::vector<TouchEventWithLatencyInfo> WebTouchEventWithLatencyList;
-  WebTouchEventWithLatencyList events_to_ack_;
+  WebTouchEventWithLatencyList uncoaleseced_events_to_ack_;
+
+  bool suppress_client_ack_;
 
   DISALLOW_COPY_AND_ASSIGN(CoalescedWebTouchEvent);
 };
@@ -695,20 +696,19 @@ void TouchEventQueue::FlushQueue() {
 }
 
 void TouchEventQueue::PopTouchEventToClient(InputEventAckState ack_result) {
-  AckTouchEventToClient(ack_result, PopTouchEvent());
+  AckTouchEventToClient(ack_result, PopTouchEvent(), NULL);
 }
 
 void TouchEventQueue::PopTouchEventToClient(
     InputEventAckState ack_result,
     const LatencyInfo& renderer_latency_info) {
-  scoped_ptr<CoalescedWebTouchEvent> acked_event = PopTouchEvent();
-  acked_event->UpdateLatencyInfoForAck(renderer_latency_info);
-  AckTouchEventToClient(ack_result, acked_event.Pass());
+  AckTouchEventToClient(ack_result, PopTouchEvent(), &renderer_latency_info);
 }
 
 void TouchEventQueue::AckTouchEventToClient(
     InputEventAckState ack_result,
-    scoped_ptr<CoalescedWebTouchEvent> acked_event) {
+    scoped_ptr<CoalescedWebTouchEvent> acked_event,
+    const ui::LatencyInfo* optional_latency_info) {
   DCHECK(acked_event);
   DCHECK(!dispatching_touch_ack_);
   UpdateTouchAckStates(acked_event->coalesced_event().event, ack_result);
@@ -717,7 +717,7 @@ void TouchEventQueue::AckTouchEventToClient(
   // to the renderer, or touch-events being queued.
   base::AutoReset<const CoalescedWebTouchEvent*> dispatching_touch_ack(
       &dispatching_touch_ack_, acked_event.get());
-  acked_event->DispatchAckToClient(ack_result, client_);
+  acked_event->DispatchAckToClient(ack_result, optional_latency_info, client_);
 }
 
 scoped_ptr<CoalescedWebTouchEvent> TouchEventQueue::PopTouchEvent() {
diff --git a/content/browser/renderer_host/input/touch_event_queue.h b/content/browser/renderer_host/input/touch_event_queue.h
index 0e24073..a45e60f 100644
--- a/content/browser/renderer_host/input/touch_event_queue.h
+++ b/content/browser/renderer_host/input/touch_event_queue.h
@@ -160,9 +160,11 @@ class CONTENT_EXPORT TouchEventQueue {
   void PopTouchEventToClient(InputEventAckState ack_result,
                              const ui::LatencyInfo& renderer_latency_info);
 
-  // Ack all coalesced events in |acked_event| to the client with |ack_result|.
+  // Ack all coalesced events in |acked_event| to the client with |ack_result|,
+  // updating the acked events with |optional_latency_info| if it exists.
   void AckTouchEventToClient(InputEventAckState ack_result,
-                             scoped_ptr<CoalescedWebTouchEvent> acked_event);
+                             scoped_ptr<CoalescedWebTouchEvent> acked_event,
+                             const ui::LatencyInfo* optional_latency_info);
 
   // Safely pop the head of the queue.
   scoped_ptr<CoalescedWebTouchEvent> PopTouchEvent();