Bug 1538768 - Ensure that RenderRootArray<bool> is properly initialized to false on creation. r=dthayer

Differential Revision: https://phabricator.services.mozilla.com/D24950

--HG--
extra : moz-landing-system : lando

gfx/layers/wr/WebRenderBridgeParent.cpp

@@ -1123,7 +1123,7 @@ mozilla::ipc::IPCResult WebRenderBridgeParent::RecvEmptyTransaction(
   AutoWebRenderBridgeParentAsyncMessageSender autoAsyncMessageSender(
       this, &aToDestroy);
 
-  wr::RenderRootArray<bool> scheduleComposite(false);
+  wr::RenderRootArray<bool> scheduleComposite;
 
   UpdateAPZFocusState(aFocusTarget);
 

gfx/layers/wr/WebRenderLayerManager.cpp

@@ -39,8 +39,7 @@ WebRenderLayerManager::WebRenderLayerManager(nsIWidget* aWidget)
       mIsFirstPaint(false),
       mTarget(nullptr),
       mPaintSequenceNumber(0),
-      mWebRenderCommandBuilder(this),
-      mLastDisplayListSizes{0} {
+      mWebRenderCommandBuilder(this) {
   MOZ_COUNT_CTOR(WebRenderLayerManager);
   for (auto renderRoot : wr::kRenderRoots) {
     mStateManagers[renderRoot].mRenderRoot = renderRoot;

gfx/webrender_bindings/WebRenderTypes.h

@@ -18,6 +18,7 @@
 #include "mozilla/layers/LayersTypes.h"
 #include "mozilla/PodOperations.h"
 #include "mozilla/Range.h"
+#include "mozilla/TypeTraits.h"
 #include "mozilla/Variant.h"
 #include "Units.h"
 #include "nsStyleConsts.h"
@@ -91,12 +92,11 @@ template <typename T>
 class RenderRootArray : public Array<T, kRenderRootCount> {
   typedef Array<T, kRenderRootCount> Super;
  public:
-  RenderRootArray() {}
-
-  explicit RenderRootArray(T aDefault) {
-    for (auto renderRoot : kRenderRoots) {
-      (*this)[renderRoot] = aDefault;
-    }
+  RenderRootArray() {
+    if (IsPod<T>::value) {
+      // Ensure primitive types get initialized to 0/false.
+      PodArrayZero(*this);
+    } // else C++ will default-initialize the array elements for us
   }
 
   T& operator[](wr::RenderRoot aIndex) {