reuse placeholder and empty nodes on each thread to reduce allocation (#2217)

packages/core/src/nodes.rs

@@ -167,8 +167,14 @@ impl VNode {
 
     /// Create a template with no nodes that will be skipped over during diffing
     pub fn empty() -> Element {
-        Some(Self {
+        use std::cell::OnceCell;
-            vnode: Rc::new(VNodeInner {
+        // We can reuse all placeholders across the same thread to save memory
+        thread_local! {
+            static EMPTY_VNODE: OnceCell<Rc<VNodeInner>> = const { OnceCell::new() };
+        }
+        let vnode = EMPTY_VNODE.with(|cell| {
+            cell.get_or_init(move || {
+                Rc::new(VNodeInner {
                     key: None,
                     dynamic_nodes: Box::new([]),
                     dynamic_attrs: Box::new([]),
@@ -178,15 +184,26 @@ impl VNode {
                         node_paths: &[],
                         attr_paths: &[],
                     }),
-            }),
+                })
+            })
+            .clone()
+        });
+        Some(Self {
+            vnode,
             mount: Default::default(),
         })
     }
 
     /// Create a template with a single placeholder node
     pub fn placeholder() -> Self {
-        Self {
+        use std::cell::OnceCell;
-            vnode: Rc::new(VNodeInner {
+        // We can reuse all placeholders across the same thread to save memory
+        thread_local! {
+            static PLACEHOLDER_VNODE: OnceCell<Rc<VNodeInner>> = const { OnceCell::new() };
+        }
+        let vnode = PLACEHOLDER_VNODE.with(|cell| {
+            cell.get_or_init(move || {
+                Rc::new(VNodeInner {
                     key: None,
                     dynamic_nodes: Box::new([DynamicNode::Placeholder(Default::default())]),
                     dynamic_attrs: Box::new([]),
@@ -196,7 +213,12 @@ impl VNode {
                         node_paths: &[&[]],
                         attr_paths: &[],
                     }),
-            }),
+                })
+            })
+            .clone()
+        });
+        Self {
+            vnode,
             mount: Default::default(),
         }
     }