Bug 1771718, nsRefreshDriver::IsInHighRateMode(), r=mstange

Differential Revision: https://phabricator.services.mozilla.com/D147642

layout/base/nsRefreshDriver.cpp

@@ -155,6 +155,8 @@ class AutoRecordPhase {
 
 namespace mozilla {
 
+static TimeStamp sMostRecentHighRateVsync;
+
 /*
  * The base class for all global refresh driver timers.  It takes care
  * of managing the list of refresh drivers attached to them and
@@ -780,6 +782,12 @@ class VsyncRefreshDriverTimer : public RefreshDriverTimer {
 
     TimeStamp tickStart = TimeStamp::Now();
 
+    TimeDuration rate = GetTimerRate();
+    if (TimeDuration::FromMilliseconds(nsRefreshDriver::DefaultInterval() / 2) >
+        rate) {
+      sMostRecentHighRateVsync = tickStart;
+    }
+
     // On 32-bit Windows we sometimes get times where TimeStamp::Now() is not
     // monotonic because the underlying system apis produce non-monontonic
     // results. (bug 1306896)
@@ -812,7 +820,6 @@ class VsyncRefreshDriverTimer : public RefreshDriverTimer {
     // (aVsyncTimestamp) and the next tick needs to be at least the amount of
     // work normal tasks and RefreshDrivers did together (minus short grace
     // period).
-    TimeDuration rate = GetTimerRate();
     TimeDuration gracePeriod = rate / int64_t(100);
 
     if (shouldGiveNonVSyncTasksMoreTime) {
@@ -1187,6 +1194,23 @@ int32_t nsRefreshDriver::DefaultInterval() {
   return NSToIntRound(1000.0 / gfxPlatform::GetDefaultFrameRate());
 }
 
+/* static */
+bool nsRefreshDriver::IsInHighRateMode() {
+  // We're in high rate mode if we've gotten a fast rate during the last
+  // DefaultInterval().
+  bool inHighRateMode =
+      !gfxPlatform::IsInLayoutAsapMode() &&
+      StaticPrefs::layout_expose_high_rate_mode_from_refreshdriver() &&
+      !sMostRecentHighRateVsync.IsNull() &&
+      (sMostRecentHighRateVsync +
+       TimeDuration::FromMilliseconds(DefaultInterval())) > TimeStamp::Now();
+  if (!inHighRateMode) {
+    // Clear the timestamp so that the next call is faster.
+    sMostRecentHighRateVsync = TimeStamp();
+  }
+  return inHighRateMode;
+}
+
 // Compute the interval to use for the refresh driver timer, in milliseconds.
 // outIsDefault indicates that rate was not explicitly set by the user
 // so we might choose other, more appropriate rates (e.g. vsync, etc)

layout/base/nsRefreshDriver.h

@@ -322,6 +322,12 @@ class nsRefreshDriver final : public mozilla::layers::TransactionIdAllocator,
    */
   static int32_t DefaultInterval();
 
+  /**
+   * Returns true if a recent vsync interval has been less than a half of
+   * DefaultInterval.
+   */
+  static bool IsInHighRateMode();
+
   bool IsInRefresh() { return mInRefresh; }
 
   void SetIsResizeSuppressed() { mResizeSuppressed = true; }

modules/libpref/init/StaticPrefList.yaml

@@ -8382,6 +8382,12 @@
   value: true
   mirror: always
 
+# Controls whether nsRefreshDriver::IsInHighRateMode() may ever return true.
+- name: layout.expose_high_rate_mode_from_refreshdriver
+  type: bool
+  value: true
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "mathml."
 #---------------------------------------------------------------------------