From aaa64b9647ac443917213038fb4bd6881b77a1b5 Mon Sep 17 00:00:00 2001
From: Roy Tam <roytam@gmail.com>
Date: Sat, 16 May 2020 06:52:54 +0800
Subject: [PATCH] Reverted "Issue #1507 - Remove nsRefreshDriver's use of high
 res system timers. (7630b1c56)"

This helps resolving constant stutter when scrolling web pages on XP.
---
 layout/base/nsRefreshDriver.cpp | 98 +++++++++++++++++++++++++++++++++
 layout/base/nsRefreshDriver.h   |  5 ++
 2 files changed, 103 insertions(+)

diff --git a/layout/base/nsRefreshDriver.cpp b/layout/base/nsRefreshDriver.cpp
index c0d09d2a0b..5ea7a01881 100644
--- a/layout/base/nsRefreshDriver.cpp
+++ b/layout/base/nsRefreshDriver.cpp
@@ -851,6 +851,12 @@ NS_IMPL_ISUPPORTS(VsyncChildCreateCallback, nsIIPCBackgroundChildCreateCallback)
 static StaticRefPtr<RefreshDriverTimer> sRegularRateTimer;
 static StaticRefPtr<InactiveRefreshDriverTimer> sThrottledRateTimer;
 
+#ifdef XP_WIN
+static int32_t sHighPrecisionTimerRequests = 0;
+// a bare pointer to avoid introducing a static constructor
+static nsITimer *sDisableHighPrecisionTimersTimer = nullptr;
+#endif
+
 static void
 CreateContentVsyncRefreshTimer(void*)
 {
@@ -928,6 +934,16 @@ nsRefreshDriver::Shutdown()
 
   sRegularRateTimer = nullptr;
   sThrottledRateTimer = nullptr;
+
+#ifdef XP_WIN
+  if (sDisableHighPrecisionTimersTimer) {
+    sDisableHighPrecisionTimersTimer->Cancel();
+    NS_RELEASE(sDisableHighPrecisionTimersTimer);
+    timeEndPeriod(1);
+  } else if (sHighPrecisionTimerRequests) {
+    timeEndPeriod(1);
+  }
+#endif
 }
 
 /* static */ int32_t
@@ -1046,6 +1062,7 @@ nsRefreshDriver::nsRefreshDriver(nsPresContext* aPresContext)
     mNeedToRecomputeVisibility(false),
     mTestControllingRefreshes(false),
     mViewManagerFlushIsPending(false),
+    mRequestedHighPrecision(false),
     mInRefresh(false),
     mWaitingForTransaction(false),
     mSkippedPaints(false),
@@ -1283,6 +1300,83 @@ nsRefreshDriver::StopTimer()
 
   mActiveTimer->RemoveRefreshDriver(this);
   mActiveTimer = nullptr;
+
+  if (mRequestedHighPrecision) {
+    SetHighPrecisionTimersEnabled(false);
+  }
+}
+
+#ifdef XP_WIN
+static void
+DisableHighPrecisionTimersCallback(nsITimer *aTimer, void *aClosure)
+{
+  timeEndPeriod(1);
+  NS_RELEASE(sDisableHighPrecisionTimersTimer);
+}
+#endif
+
+void
+nsRefreshDriver::ConfigureHighPrecision()
+{
+  bool haveUnthrottledFrameRequestCallbacks =
+    mFrameRequestCallbackDocs.Length() > 0;
+
+  // if the only change that's needed is that we need high precision,
+  // then just set that
+  if (!mThrottled && !mRequestedHighPrecision &&
+      haveUnthrottledFrameRequestCallbacks) {
+    SetHighPrecisionTimersEnabled(true);
+  } else if (mRequestedHighPrecision && !haveUnthrottledFrameRequestCallbacks) {
+    SetHighPrecisionTimersEnabled(false);
+  }
+}
+
+void
+nsRefreshDriver::SetHighPrecisionTimersEnabled(bool aEnable)
+{
+  LOG("[%p] SetHighPrecisionTimersEnabled (%s)", this, aEnable ? "true" : "false");
+
+  if (aEnable) {
+    NS_ASSERTION(!mRequestedHighPrecision, "SetHighPrecisionTimersEnabled(true) called when already requested!");
+#ifdef XP_WIN
+    if (++sHighPrecisionTimerRequests == 1) {
+      // If we had a timer scheduled to disable it, that means that it's already
+      // enabled; just cancel the timer.  Otherwise, really enable it.
+      if (sDisableHighPrecisionTimersTimer) {
+        sDisableHighPrecisionTimersTimer->Cancel();
+        NS_RELEASE(sDisableHighPrecisionTimersTimer);
+      } else {
+        timeBeginPeriod(1);
+      }
+    }
+#endif
+    mRequestedHighPrecision = true;
+  } else {
+    NS_ASSERTION(mRequestedHighPrecision, "SetHighPrecisionTimersEnabled(false) called when not requested!");
+#ifdef XP_WIN
+    if (--sHighPrecisionTimerRequests == 0) {
+      // Don't jerk us around between high precision and low precision
+      // timers; instead, only allow leaving high precision timers
+      // after 90 seconds.  This is arbitrary, but hopefully good
+      // enough.
+      NS_ASSERTION(!sDisableHighPrecisionTimersTimer, "We shouldn't have an outstanding disable-high-precision timer !");
+
+      nsCOMPtr<nsITimer> timer = do_CreateInstance(NS_TIMER_CONTRACTID);
+      if (timer) {
+        timer.forget(&sDisableHighPrecisionTimersTimer);
+        sDisableHighPrecisionTimersTimer->InitWithFuncCallback(DisableHighPrecisionTimersCallback,
+                                                               nullptr,
+                                                               90 * 1000,
+                                                               nsITimer::TYPE_ONE_SHOT);
+      } else {
+        // might happen if we're shutting down XPCOM; just drop the time period down
+        // immediately
+        timeEndPeriod(1);
+      }
+    }
+#endif
+    mRequestedHighPrecision = false;
+  }
 }
 
 uint32_t
@@ -1899,6 +1993,8 @@ nsRefreshDriver::Tick(int64_t aNowEpoch, TimeStamp aNowTime)
     observer->DidRefresh();
   }
 
+  ConfigureHighPrecision();
+
   NS_ASSERTION(mInRefresh, "Still in refresh");
 
   if (mPresContext->IsRoot() && XRE_IsContentProcess() && gfxPrefs::AlwaysPaint()) {
@@ -2151,6 +2247,7 @@ nsRefreshDriver::ScheduleFrameRequestCallbacks(nsIDocument* aDocument)
   }
 
   // make sure that the timer is running
+  ConfigureHighPrecision();
   EnsureTimerStarted();
 }
 
@@ -2159,6 +2256,7 @@ nsRefreshDriver::RevokeFrameRequestCallbacks(nsIDocument* aDocument)
 {
   mFrameRequestCallbackDocs.RemoveElement(aDocument);
   mThrottledFrameRequestCallbackDocs.RemoveElement(aDocument);
+  ConfigureHighPrecision();
   // No need to worry about restarting our timer in slack mode if it's already
   // running; that will happen automatically when it fires.
 }
diff --git a/layout/base/nsRefreshDriver.h b/layout/base/nsRefreshDriver.h
index 4e839e7641..b2dd9be4be 100644
--- a/layout/base/nsRefreshDriver.h
+++ b/layout/base/nsRefreshDriver.h
@@ -450,6 +450,7 @@ private:
   bool mNeedToRecomputeVisibility;
   bool mTestControllingRefreshes;
   bool mViewManagerFlushIsPending;
+  bool mRequestedHighPrecision;
   bool mInRefresh;
 
   // True if the refresh driver is suspended waiting for transaction
@@ -501,6 +502,10 @@ private:
 
   friend class mozilla::RefreshDriverTimer;
 
+  // turn on or turn off high precision based on various factors
+  void ConfigureHighPrecision();
+  void SetHighPrecisionTimersEnabled(bool aEnable);
+
   static void Shutdown();
 
   // `true` if we are currently in jank-critical mode.