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
..
blob_vec.rs	bevy_ecs address trivial cases of unsafe_op_in_unsafe_fn (#11861)	2024-02-22 00:04:38 +00:00
mod.rs	Improve doc formatting. (#9840)	2023-09-18 19:43:56 +00:00
resource.rs	bevy_ecs address trivial cases of unsafe_op_in_unsafe_fn (#11861)	2024-02-22 00:04:38 +00:00
sparse_set.rs	Use NonMaxUsize for non-component SparseSets (#12083)	2024-03-03 14:55:27 +00:00
table.rs	Use immutable key for HashMap and HashSet (#12086)	2024-02-26 16:27:40 +00:00