Bug 1690653 - Don't create a CATransaction if nothing changed. r=mattwoodrow

This avoids some overhead from committing the transaction.

Differential Revision: https://phabricator.services.mozilla.com/D104085

gfx/layers/NativeLayerCA.h

@@ -244,6 +244,7 @@ class NativeLayerCA : public NativeLayer {
   typedef NativeLayerRootCA::WhichRepresentation WhichRepresentation;
   CALayer* UnderlyingCALayer(WhichRepresentation aRepresentation);
   void ApplyChanges(WhichRepresentation aRepresentation);
+  bool HasUpdate(WhichRepresentation aRepresentation);
   void SetBackingScale(float aBackingScale);
 
   // Invalidates the specified region in all surfaces that are tracked by this
@@ -298,6 +299,13 @@ class NativeLayerCA : public NativeLayer {
                       gfx::SamplingFilter aSamplingFilter,
                       CFTypeRefPtr<IOSurfaceRef> aFrontSurface);
 
+    // Return whether any aspects of this layer representation have been mutated
+    // since the last call to ApplyChanges, i.e. whether ApplyChanges needs to
+    // be called.
+    // This is used to optimize away a CATransaction commit if no layers have
+    // changed.
+    bool HasUpdate();
+
     // Lazily initialized by first call to ApplyChanges. mWrappingLayer is the
     // layer that applies the intersection of mDisplayRect and mClipRect (if
     // set), and mContentCALayer is the layer that hosts the IOSurface. We do

gfx/layers/NativeLayerCA.mm

@@ -283,6 +283,14 @@ NativeLayerRootCA::Representation::~Representation() {
 
 void NativeLayerRootCA::Representation::Commit(WhichRepresentation aRepresentation,
                                                const nsTArray<RefPtr<NativeLayerCA>>& aSublayers) {
+  if (!mMutated &&
+      std::none_of(aSublayers.begin(), aSublayers.end(), [=](const RefPtr<NativeLayerCA>& layer) {
+        return layer->HasUpdate(aRepresentation);
+      })) {
+    // No updates, skip creating the CATransaction altogether.
+    return;
+  }
+
   AutoCATransaction transaction;
 
   // Call ApplyChanges on our sublayers first, and then update the root layer's
@@ -805,6 +813,11 @@ void NativeLayerCA::ApplyChanges(WhichRepresentation aRepresentation) {
                     mSurfaceIsFlipped, mSamplingFilter, surface);
 }
 
+bool NativeLayerCA::HasUpdate(WhichRepresentation aRepresentation) {
+  MutexAutoLock lock(mMutex);
+  return GetRepresentation(aRepresentation).HasUpdate();
+}
+
 CALayer* NativeLayerCA::UnderlyingCALayer(WhichRepresentation aRepresentation) {
   MutexAutoLock lock(mMutex);
   return GetRepresentation(aRepresentation).UnderlyingCALayer();
@@ -961,6 +974,16 @@ void NativeLayerCA::Representation::ApplyChanges(
   mMutatedSamplingFilter = false;
 }
 
+bool NativeLayerCA::Representation::HasUpdate() {
+  if (!mWrappingCALayer) {
+    return true;
+  }
+
+  return mMutatedPosition || mMutatedTransform || mMutatedDisplayRect || mMutatedClipRect ||
+         mMutatedBackingScale || mMutatedSize || mMutatedSurfaceIsFlipped || mMutatedFrontSurface ||
+         mMutatedSamplingFilter;
+}
+
 // Called when mMutex is already being held by the current thread.
 Maybe<NativeLayerCA::SurfaceWithInvalidRegion> NativeLayerCA::GetUnusedSurfaceAndCleanUp(
     const MutexAutoLock&) {