Reduce the cost of get_scene

[InnocentusLime]

Synopsis

get_scene is called in quite a few places. The most notable place where it gets called is get_frame_time.

It has been discovered, that calling get_scene is quite costly. Which by transitivity makes get_frame_time costly.

This is because in get_scene, SCENE.get_or_insert(Scene::new()) is used. This causes a temporary Scene struct to be constructed each time the get_scene call is made as Rust eagerly evaluates function arguments.

This is a problem, because Scene::new() calls Arena::new(). Arena::new() calls vec![Vec::with_capacity(ARENA_BLOCK)] (aka allocates memory). This essentially causes get_scene (and get_frame_time) to constantly allocate and deallocate memory.

The Fix

The fix is to simply replace get_or_insert with get_or_insert_with, which allows Scene::new() to be lazily computed.