1325254 - optimize TimerThread data structures

1325254 P1 Make TimerThread::mTimers store RefPtr<nsTimerImpl> objects.

1325254 P2 Make TimerThread list store an entry struct and just drop nsTimerImpl ref on cancel.

1325254 P3 Sort TimerThread list as a binary heap.

1325254 P4 Dynamically allocate Entry structs stored in TimerThread::mTimers.

1325254 P5 Make nsITimer::Cancel() O(c).

xpcom/threads/nsTimerImpl.h

@@ -32,12 +32,18 @@ extern mozilla::LogModule* GetTimerLog();
     {0x84, 0x27, 0xfb, 0xab, 0x44, 0xf2, 0x9b, 0xc8} \
 }
 
+class nsTimerImplHolder;
+
 // TimerThread, nsTimerEvent, and nsTimer have references to these. nsTimer has
 // a separate lifecycle so we can Cancel() the underlying timer when the user of
 // the nsTimer has let go of its last reference.
 class nsTimerImpl
 {
-  ~nsTimerImpl() {}
+  ~nsTimerImpl()
+  {
+    MOZ_ASSERT(!mHolder);
+  }
+
 public:
   typedef mozilla::TimeStamp TimeStamp;
 
@@ -158,6 +164,8 @@ public:
            mType == nsITimer::TYPE_REPEATING_SLACK_LOW_PRIORITY;
   }
 
+  void SetHolder(nsTimerImplHolder* aHolder);
+
   nsCOMPtr<nsIEventTarget> mEventTarget;
 
   void LogFiring(const Callback& aCallback, uint8_t aType, uint32_t aDelay);
@@ -168,6 +176,10 @@ public:
                                       uint32_t aType,
                                       Callback::Name aName);
 
+  // This weak reference must be cleared by the nsTimerImplHolder by calling
+  // SetHolder(nullptr) before the holder is destroyed.
+  nsTimerImplHolder*    mHolder;
+
   // These members are set by the initiating thread, when the timer's type is
   // changed and during the period where it fires on that thread.
   uint8_t               mType;
@@ -219,4 +231,39 @@ private:
   RefPtr<nsTimerImpl> mImpl;
 };
 
+// A class that holds on to an nsTimerImpl.  This lets the nsTimerImpl object
+// directly instruct its holder to forget the timer, avoiding list lookups.
+class nsTimerImplHolder
+{
+public:
+  explicit nsTimerImplHolder(nsTimerImpl* aTimerImpl)
+    : mTimerImpl(aTimerImpl)
+  {
+    if (mTimerImpl) {
+      mTimerImpl->SetHolder(this);
+    }
+  }
+
+  ~nsTimerImplHolder()
+  {
+    if (mTimerImpl) {
+      mTimerImpl->SetHolder(nullptr);
+    }
+  }
+
+  void
+  Forget(nsTimerImpl* aTimerImpl)
+  {
+    if (MOZ_UNLIKELY(!mTimerImpl)) {
+      return;
+    }
+    MOZ_ASSERT(aTimerImpl == mTimerImpl);
+    mTimerImpl->SetHolder(nullptr);
+    mTimerImpl = nullptr;
+  }
+
+protected:
+  RefPtr<nsTimerImpl> mTimerImpl;
+};
+
 #endif /* nsTimerImpl_h___ */