James Liu 57733bbec3 Use NonMaxUsize for non-component SparseSets (#12083) ... # Objective Adoption of #2104 and #11843. The `Option<usize>` wastes 3-7 bytes of memory per potential entry, and represents a scaling memory overhead as the ID space grows. The goal of this PR is to reduce memory usage without significantly impacting common use cases. Co-Authored By: @NathanSWard Co-Authored By: @tygyh ## Solution Replace `usize` in `SparseSet`'s sparse array with `nonmax::NonMaxUsize`. NonMaxUsize wraps a NonZeroUsize, and applies a bitwise NOT to the value when accessing it. This allows the compiler to niche the value and eliminate the extra padding used for the `Option` inside the sparse array, while moving the niche value from 0 to usize::MAX instead. Checking the [diff in x86 generated assembly](6e4da653cc), this change actually results in fewer instructions generated. One potential downside is that it seems to have moved a load before a branch, which means we may be incurring a cache miss even if the element is not there. Note: unlike #2104 and #11843, this PR only targets the metadata stores for the ECS and not the component storage itself. Due to #9907 targeting `Entity::generation` instead of `Entity::index`, `ComponentSparseSet` storing only up to `u32::MAX` elements would become a correctness issue. This will come with a cost when inserting items into the SparseSet, as now there is a potential for a panic. These cost are really only incurred when constructing a new Table, Archetype, or Resource that has never been seen before by the World. All operations that are fairly cold and not on any particular hotpath, even for command application. --- ## Changelog Changed: `SparseSet` now can only store up to `usize::MAX - 1` elements instead of `usize::MAX`. Changed: `SparseSet` now uses 33-50% less memory overhead per stored item.		2024-03-03 14:55:27 +00:00
..
entity	Document instability of Entity's internal representation (#12249)	2024-03-02 18:37:52 +00:00
identifier	fix some typos (#12038)	2024-02-22 18:55:22 +00:00
query	remove repetitive code (#12270)	2024-03-03 07:58:22 +00:00
reflect	Move commands module into bevy::ecs::world (#12234)	2024-03-02 23:13:45 +00:00
schedule	Replace FromWorld requirement on ReflectResource and reflect Resource for State<S> (#12136)	2024-02-27 15:49:39 +00:00
storage	Use NonMaxUsize for non-component SparseSets (#12083)	2024-03-03 14:55:27 +00:00
system	Move commands module into bevy::ecs::world (#12234)	2024-03-02 23:13:45 +00:00
world	remove repetitive code (#12270)	2024-03-03 07:58:22 +00:00
archetype.rs	Component Lifecycle Hooks and a Deferred World (#10756)	2024-03-01 14:59:22 +00:00
bundle.rs	remove repetitive code (#12270)	2024-03-03 07:58:22 +00:00
change_detection.rs	bevy_ecs address trivial cases of unsafe_op_in_unsafe_fn (#11861)	2024-02-22 00:04:38 +00:00
component.rs	Replace init_component_info with register_component_hooks (#12244)	2024-03-02 05:27:48 +00:00
event.rs	Fix bug where events are not being dropped (#11528)	2024-02-02 21:14:54 +00:00
lib.rs	Component Lifecycle Hooks and a Deferred World (#10756)	2024-03-01 14:59:22 +00:00
removal_detection.rs	Docs reflect that RemovalDetection also yields despawned entities (#11795)	2024-02-10 11:18:05 +00:00