Bug 1850056. Disable willReadFrequently on Windows until perf issues can be sorted out. r=gfx-reviewers,lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D186830

dom/canvas/CanvasRenderingContext2D.cpp

@@ -1425,7 +1425,7 @@ bool CanvasRenderingContext2D::BorrowTarget(const IntRect& aPersistedRect,
   // acceleration, then we skip trying to use this provider so that it will be
   // recreated by EnsureTarget later.
   if (!mBufferProvider || mBufferProvider->RequiresRefresh() ||
-      (mBufferProvider->IsAccelerated() && mWillReadFrequently)) {
+      (mBufferProvider->IsAccelerated() && GetEffectiveWillReadFrequently())) {
     return false;
   }
   mTarget = mBufferProvider->BorrowDrawTarget(aPersistedRect);
@@ -1647,7 +1647,7 @@ bool CanvasRenderingContext2D::TryAcceleratedTarget(
   }
   // Don't try creating an accelerate DrawTarget if either acceleration failed
   // previously or if the application expects acceleration to be slow.
-  if (!mAllowAcceleration || mWillReadFrequently) {
+  if (!mAllowAcceleration || GetEffectiveWillReadFrequently()) {
     return false;
   }
   aOutDT = DrawTargetWebgl::Create(GetSize(), GetSurfaceFormat());
@@ -1685,7 +1685,7 @@ bool CanvasRenderingContext2D::TrySharedTarget(
 
   aOutProvider = renderer->CreatePersistentBufferProvider(
       GetSize(), GetSurfaceFormat(),
-      !mAllowAcceleration || mWillReadFrequently);
+      !mAllowAcceleration || GetEffectiveWillReadFrequently());
 
   if (!aOutProvider) {
     return false;
@@ -6340,6 +6340,11 @@ void CanvasRenderingContext2D::SetWriteOnly() {
   }
 }
 
+bool CanvasRenderingContext2D::GetEffectiveWillReadFrequently() const {
+  return StaticPrefs::gfx_canvas_willreadfrequently_enabled_AtStartup() &&
+         mWillReadFrequently;
+}
+
 NS_IMPL_CYCLE_COLLECTION_WRAPPERCACHE(CanvasPath, mParent)
 
 CanvasPath::CanvasPath(nsISupports* aParent) : mParent(aParent) {

dom/canvas/CanvasRenderingContext2D.h

@@ -779,6 +779,8 @@ class CanvasRenderingContext2D : public nsICanvasRenderingContextInternal,
     return CurrentState().font;
   }
 
+  bool GetEffectiveWillReadFrequently() const;
+
   // Member vars
   int32_t mWidth, mHeight;
 

modules/libpref/init/StaticPrefList.yaml

@@ -5694,6 +5694,16 @@
 #endif
   mirror: always
 
+- name: gfx.canvas.willreadfrequently.enabled
+  type: bool
+#if defined(XP_WIN)
+  value: false
+#else
+  value: true
+#endif
+  mirror: once
+
+
 - name: gfx.color_management.display_profile
   type: DataMutexString
   value: ""