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# 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.
50 lines
1.2 KiB
TOML
50 lines
1.2 KiB
TOML
[package]
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name = "bevy_ecs"
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version = "0.14.0-dev"
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edition = "2021"
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description = "Bevy Engine's entity component system"
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homepage = "https://bevyengine.org"
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repository = "https://github.com/bevyengine/bevy"
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license = "MIT OR Apache-2.0"
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keywords = ["ecs", "game", "bevy"]
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categories = ["game-engines", "data-structures"]
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[features]
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trace = []
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multi-threaded = ["bevy_tasks/multi-threaded"]
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bevy_debug_stepping = []
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default = ["bevy_reflect", "bevy_debug_stepping"]
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[dependencies]
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bevy_ptr = { path = "../bevy_ptr", version = "0.14.0-dev" }
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bevy_reflect = { path = "../bevy_reflect", version = "0.14.0-dev", optional = true }
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bevy_tasks = { path = "../bevy_tasks", version = "0.14.0-dev" }
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bevy_utils = { path = "../bevy_utils", version = "0.14.0-dev" }
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bevy_ecs_macros = { path = "macros", version = "0.14.0-dev" }
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bitflags = "2.3"
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concurrent-queue = "2.4.0"
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fixedbitset = "0.4.2"
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rustc-hash = "1.1"
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serde = "1"
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thiserror = "1.0"
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nonmax = "0.5"
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[dev-dependencies]
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rand = "0.8"
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static_assertions = "1.1.0"
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[[example]]
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name = "events"
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path = "examples/events.rs"
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[[example]]
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name = "resources"
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path = "examples/resources.rs"
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[[example]]
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name = "change_detection"
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path = "examples/change_detection.rs"
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[lints]
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workspace = true
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