improve bench by looking at gc.stat

bench/dune

@@ -19,6 +19,8 @@
  (modules once)
  (libraries
   unix
+  lwt
+  lwt.unix
   server-reason-react.react
   server-reason-react.reactDom
   demo_shared_native)

bench/once.ml

@@ -0,0 +1,55 @@
+let measure_alloc title f =
+  let before = Gc.stat () in
+  let result = f () in
+  let after = Gc.stat () in
+  Printf.printf "\n=== %s ===\n" title;
+  Printf.printf "Minor words: %f\n" (after.minor_words -. before.minor_words);
+  Printf.printf "Major words: %f\n" (after.major_words -. before.major_words);
+  Printf.printf "Minor collections: %d\n" (after.minor_collections - before.minor_collections);
+  result
+
+let loop n f =
+  for _ = 1 to n do
+    f ()
+  done
+
+let main () =
+  let filter_map_style () =
+    let element =
+      React.createElement "div"
+        (Stdlib.List.filter_map Fun.id
+           (List.init 50 (fun i ->
+                Some
+                  (React.JSX.String (Printf.sprintf "prop%d" i, Printf.sprintf "prop%d" i, Printf.sprintf "value%d" i)))))
+        []
+    in
+    let _ = ReactDOM.renderToStaticMarkup element in
+    ()
+  in
+
+  let direct_style () =
+    let props =
+      List.init 50 (fun i ->
+          React.JSX.String (Printf.sprintf "prop%d" i, Printf.sprintf "prop%d" i, Printf.sprintf "value%d" i))
+    in
+    let element = React.createElement "div" props [] in
+    let _ = ReactDOM.renderToStaticMarkup element in
+    ()
+  in
+
+  let render_hello_world () =
+    let _ = ReactDOM.renderToStaticMarkup (HelloWorld.make ()) in
+    ()
+  in
+  let render_app () =
+    let _ = ReactDOM.renderToStaticMarkup (App.make ()) in
+    ()
+  in
+
+  measure_alloc "Use filter_map" (fun () -> loop 10000 filter_map_style);
+  measure_alloc "Use list direct style" (fun () -> loop 10000 direct_style);
+  measure_alloc "Render <HelloWorld />" (fun () -> loop 10000 render_hello_world);
+  measure_alloc "Render <App />" (fun () -> loop 10000 render_app);
+  Lwt.return ()
+
+let () = Lwt_main.run (main ())