Avoid emitting mountitems for default values

Summary:
Noticed that we emit a large amount of (admittedly cheap) mountitems as part of node creation for values that are all zero (e.g. padding, overflowinset), which we can assume to be already initialised with these values on the native side.

There's a further opportunity to do this for State as well, as ReactImageComponentState exports just empty maps to Java.

Changelog: [Internal]

Reviewed By: genkikondo

Differential Revision: D36345402

fbshipit-source-id: 8d776ca124bdb9e1cd4de57a04e2785a9a0f918c

Author: javache

ReactAndroid/src/main/java/com/facebook/react/fabric/jni/FabricMountingManager.cpp

@@ -421,20 +421,23 @@ void FabricMountingManager::executeMount(
             // are updated in the view. This is necessary to ensure that events
             // (resulting from layout changes) are dispatched with the correct
             // padding information.
-            cppUpdatePaddingMountItems.push_back(
-                CppMountItem::UpdatePaddingMountItem(
-                    mutation.newChildShadowView));
+            if (newChildShadowView.layoutMetrics.contentInsets !=
+                EdgeInsets::ZERO) {
+              cppUpdatePaddingMountItems.push_back(
+                  CppMountItem::UpdatePaddingMountItem(newChildShadowView));
+            }
 
             // Layout
             cppUpdateLayoutMountItems.push_back(
-                CppMountItem::UpdateLayoutMountItem(
-                    mutation.newChildShadowView));
+                CppMountItem::UpdateLayoutMountItem(newChildShadowView));
 
             // OverflowInset: This is the values indicating boundaries including
             // children of the current view. The layout of current view may not
             // change, and we separate this part from layout mount items to not
             // pack too much data there.
-            if (useOverflowInset_) {
+            if (useOverflowInset_ &&
+                newChildShadowView.layoutMetrics.overflowInset !=
+                    EdgeInsets::ZERO) {
               cppUpdateOverflowInsetMountItems.push_back(
                   CppMountItem::UpdateOverflowInsetMountItem(
                       newChildShadowView));

ReactCommon/react/renderer/graphics/RectangleEdges.h

@@ -39,8 +39,13 @@ struct RectangleEdges {
   bool isUniform() const noexcept {
     return left == top && left == right && left == bottom;
   }
+
+  static RectangleEdges<T> const ZERO;
 };
 
+template <typename T>
+constexpr RectangleEdges<T> const RectangleEdges<T>::ZERO = {};
+
 template <typename T>
 RectangleEdges<T> operator+(
     RectangleEdges<T> const &lhs,