Fabric: Using small_vector for SharedShadowNodeList

Summary:
SharedShadowNodeList is one of the core collections in Fabric. Every ShadowNode has one, it's used intensivly during diffing and so on.
Usually, nodes have a very few children, so using `small_vector` instead of regular `vector` should save us a lot of memory allocations.

Reviewed By: JoshuaGross

Differential Revision: D14249201

fbshipit-source-id: 53297caa027a74099d0a1ed4a3cce78f8feb651b

ReactCommon/better/small_vector.h

@@ -0,0 +1,39 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <better/better.h>
+
+// `folly::small_vector` is broken on some versions of Android.
+#if defined(BETTER_USE_FOLLY_CONTAINERS) && !defined(ANDROID)
+
+#include <folly/small_vector.h>
+
+#else
+
+#include <vector>
+
+#endif
+
+namespace facebook {
+namespace better {
+
+#if defined(BETTER_USE_FOLLY_CONTAINERS) && !defined(ANDROID)
+
+template <typename T, std::size_t Size, typename... Ts>
+using small_vector = folly::small_vector<T, Size, Ts...>;
+
+#else
+
+template <typename T, std::size_t Size, typename... Ts>
+using small_vector = std::vector<T, Ts...>;
+
+#endif
+
+} // namespace better
+} // namespace facebook

ReactCommon/fabric/core/shadownode/ShadowNode.h

@@ -11,6 +11,7 @@
 #include <string>
 #include <vector>
 
+#include <better/small_vector.h>
 #include <react/core/EventEmitter.h>
 #include <react/core/LocalData.h>
 #include <react/core/Props.h>
@@ -22,6 +23,8 @@
 namespace facebook {
 namespace react {
 
+static constexpr const int kShadowNodeChildrenSmallVectorSize = 8;
+
 class ComponentDescriptor;
 struct ShadowNodeFragment;
 
@@ -30,7 +33,8 @@ class ShadowNode;
 using SharedShadowNode = std::shared_ptr<const ShadowNode>;
 using WeakShadowNode = std::weak_ptr<const ShadowNode>;
 using UnsharedShadowNode = std::shared_ptr<ShadowNode>;
-using SharedShadowNodeList = std::vector<SharedShadowNode>;
+using SharedShadowNodeList =
+    better::small_vector<SharedShadowNode, kShadowNodeChildrenSmallVectorSize>;
 using SharedShadowNodeSharedList = std::shared_ptr<const SharedShadowNodeList>;
 using SharedShadowNodeUnsharedList = std::shared_ptr<SharedShadowNodeList>;
 

ReactCommon/fabric/core/tests/ShadowNodeTest.cpp

@@ -267,8 +267,8 @@ TEST(ShadowNodeTest, handleBacktracking) {
       },
       componentDescriptor);
 
-  auto nodeABChildren = std::make_shared<std::vector<SharedShadowNode>>(
-      std::vector<SharedShadowNode>{nodeABA, nodeABB, nodeABC});
+  auto nodeABChildren = std::make_shared<SharedShadowNodeList>(
+      SharedShadowNodeList{nodeABA, nodeABB, nodeABC});
   auto nodeAB = std::make_shared<TestShadowNode>(
       ShadowNodeFragment{
           /* .tag = */ ShadowNodeFragment::tagPlaceholder(),
@@ -289,8 +289,8 @@ TEST(ShadowNodeTest, handleBacktracking) {
       },
       componentDescriptor);
 
-  auto nodeAChildren = std::make_shared<std::vector<SharedShadowNode>>(
-      std::vector<SharedShadowNode>{nodeAA, nodeAB, nodeAC});
+  auto nodeAChildren = std::make_shared<SharedShadowNodeList>(
+      SharedShadowNodeList{nodeAA, nodeAB, nodeAC});
   auto nodeA = std::make_shared<TestShadowNode>(
       ShadowNodeFragment{
           /* .tag = */ ShadowNodeFragment::tagPlaceholder(),