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# Objective - Implements change described in https://github.com/bevyengine/bevy/issues/3022 - Goal is to allow Entity to benefit from niche optimization, especially in the case of Option<Entity> to reduce memory overhead with structures with empty slots ## Discussion - First PR attempt: https://github.com/bevyengine/bevy/pull/3029 - Discord: https://discord.com/channels/691052431525675048/1154573759752183808/1154573764240093224 ## Solution - Change `Entity::generation` from u32 to NonZeroU32 to allow for niche optimization. - The reason for changing generation rather than index is so that the costs are only encountered on Entity free, instead of on Entity alloc - There was some concern with generations being used, due to there being some desire to introduce flags. This was more to do with the original retirement approach, however, in reality even if generations were reduced to 24-bits, we would still have 16 million generations available before wrapping and current ideas indicate that we would be using closer to 4-bits for flags. - Additionally, another concern was the representation of relationships where NonZeroU32 prevents us using the full address space, talking with Joy it seems unlikely to be an issue. The majority of the time these entity references will be low-index entries (ie. `ChildOf`, `Owes`), these will be able to be fast lookups, and the remainder of the range can use slower lookups to map to the address space. - It has the additional benefit of being less visible to most users, since generation is only ever really set through `from_bits` type methods. - `EntityMeta` was changed to match - On free, generation now explicitly wraps: - Originally, generation would panic in debug mode and wrap in release mode due to using regular ops. - The first attempt at this PR changed the behavior to "retire" slots and remove them from use when generations overflowed. This change was controversial, and likely needs a proper RFC/discussion. - Wrapping matches current release behaviour, and should therefore be less controversial. - Wrapping also more easily migrates to the retirement approach, as users likely to exhaust the exorbitant supply of generations will code defensively against aliasing and that defensive code is less likely to break than code assuming that generations don't wrap. - We use some unsafe code here when wrapping generations, to avoid branch on NonZeroU32 construction. It's guaranteed safe due to how we perform wrapping and it results in significantly smaller ASM code. - https://godbolt.org/z/6b6hj8PrM ## Migration - Previous `bevy_scene` serializations have a high likelihood of being broken, as they contain 0th generation entities. ## Current Issues - `Entities::reserve_generations` and `EntityMapper` wrap now, even in debug - although they technically did in release mode already so this probably isn't a huge issue. It just depends if we need to change anything here? --------- Co-authored-by: Natalie Baker <natalie.baker@advancednavigation.com> |
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