Added a dedicated HandleMap

[SarthakSingh31]

This version is nowhere ready to merge. I am mostly looking for people's opinion. Idk if I missed something obvious in the benchmark.

Objective

Add a HandleMap<K, V> to replace HashMap<Handle<K>, V>. Its goal is to increase performance by reducing the amount of data that needs to be hashed to access a value in a hash map.

A limitation of the current implementation of HandleMap<K, V> is that it can't store a strong handle whereas HashMap<Handle<K>, V> can. I don't know how often this capability is required.

Bench

Time taken to call .get 1,000,000 times:

HandleId::Id / Total IDs	0.0	0.2	0.4	0.6	0.8	1.0
HashMap<Handle<K>, V>	21.049 ms	23.680 ms	26.296 ms	28.023 ms	27.600 ms	28.015 ms
HandleMap<K, V>	16.563 ms	18.593 ms	20.135 ms	19.353 ms	16.935 ms	14.077 ms

[alice-i-cecile]

Controversial label for now until we can find some clear use cases :) Cool stuff though!

[nicopap]

This would be useful for #5080. Given it relies on a HashMap<Handle<X>, u32>

[SarthakSingh31]

All these other uses will probably also benefit from the HandleMap

crates/bevy_asset/src/debug_asset_server.rs
54:    pub handles: HashMap<Handle<T>, Handle<T>>,

crates/bevy_ui/src/render/mod.rs
782:    pub values: HashMap<Handle<Image>, BindGroup>,

crates/bevy_scene/src/scene_spawner.rs
33:    spawned_scenes: HashMap<Handle<Scene>, Vec<InstanceId>>,
34:    spawned_dynamic_scenes: HashMap<Handle<DynamicScene>, Vec<InstanceId>>,

crates/bevy_pbr/src/material.rs
586:pub struct RenderMaterials<T: Material>(pub HashMap<Handle<T>, PreparedMaterial<T>>);

crates/bevy_render/src/render_resource/pipeline_cache.rs
129:    data: HashMap<Handle<Shader>, ShaderData>,
130:    shaders: HashMap<Handle<Shader>, Shader>,
214:        shaders: &HashMap<Handle<Shader>, Shader>,

crates/bevy_render/src/render_asset.rs
124:pub struct RenderAssets<A: RenderAsset>(HashMap<Handle<A>, A::PreparedAsset>);

crates/bevy_sprite/src/texture_atlas.rs
23:    pub texture_handles: Option<HashMap<Handle<Image>, usize>>,

crates/bevy_sprite/src/render/mod.rs
480:    values: HashMap<Handle<Image>, BindGroup>,

crates/bevy_sprite/src/mesh2d/material.rs
451:pub struct RenderMaterials2d<T: Material2d>(HashMap<Handle<T>, PreparedMaterial2d<T>>);

[alice-i-cecile]

A limitation of the current implementation of HandleMap<K, V> is that it can't store a strong handle whereas HashMap<Handle, V> can. I don't know how often this capability is required.

I use this in my games (it's nice to create a simple library of assets to load), but I'm 100% fine to just stay with a plain HashMap there.

[SarthakSingh31]

@alice-i-cecile I think I can add storing strong handles. I will make that change and report back the performance difference.

[SarthakSingh31]

@alice-i-cecile I think I can add storing strong handles. I will make that change and report back the performance difference.

[SarthakSingh31]

@alice-i-cecile This adds the ability to store strong handles. New benchmark:

HandleId::Id / Total IDs	0.0	0.2	0.4	0.6	0.8	1.0
HashMap<Handle<K>, V>	28.373 ms	26.712 ms	28.027 ms	31.026 ms	28.168 ms	27.011 ms
HandleMap<K, V>	21.654 ms	24.155 ms	26.356 ms	23.962 ms	20.926 ms	17.914 ms

[bas-ie]

Backlog cleanup: closing due to inactivity. Unlikely to benefit from a tracking issue here, on balance.