# Objective
Optimize the cloning process for Access-related structs in the ECS
system, specifically targeting the `clone_from` method.
Previously, profiling showed that 1% of CPU time was spent in
`FixedBitSet`'s `drop_in_place`, due to the default `clone_from`
implementation:
```rust
fn clone_from(&mut self, source: &Self) {
*self = source.clone()
}
```
This implementation causes unnecessary allocations and deallocations.
However, [FixedBitSet provides a more optimized clone_from
method](https://github.com/petgraph/fixedbitset/blob/master/src/lib.rs#L1445-L1465)
that avoids these allocations and utilizes SIMD instructions for better
performance.
This PR aims to leverage the optimized clone_from method of FixedBitSet
and implement custom clone_from methods for Access-related structs to
take full advantage of this optimization. By doing so, we expect to
significantly reduce CPU time spent on cloning operations and improve
overall system performance.
## Solution
- Implemented custom `clone` and `clone_from` methods for `Access`,
`FilteredAccess`, `AccessFilters`, and `FilteredAccessSet` structs.
- Removed `#[derive(Clone)]` and manually implemented `Clone` trait to
use optimized `clone_from` method from `FixedBitSet`.
- Added unit tests for cloning and `clone_from` methods to ensure
correctness.
## Testing
- Conducted performance testing comparing the original and optimized
versions.
- Measured CPU time consumption for the `clone_from` method:
- Original version: 1.34% of CPU time
- Optimized version: 0.338% of CPU time
- Compared FPS before and after the changes (results may vary depending
on the run):
Before optimization:
```
2024-07-28T12:49:11.864019Z INFO bevy diagnostic: fps : 213.489463 (avg 214.502488)
2024-07-28T12:49:11.864037Z INFO bevy diagnostic: frame_time : 4.704746ms (avg 4.682251ms)
2024-07-28T12:49:11.864042Z INFO bevy diagnostic: frame_count: 7947.000000 (avg 7887.500000)
```
After optimization:
```
2024-07-28T12:29:42.705738Z INFO bevy diagnostic: fps : 220.273721 (avg 220.912227)
2024-07-28T12:29:42.705762Z INFO bevy diagnostic: frame_time : 4.559127ms (avg 4.544905ms)
2024-07-28T12:29:42.705769Z INFO bevy diagnostic: frame_count: 7596.000000 (avg 7536.500000)
```
---
Reviewers can test these changes by running `cargo run --release
--example ssr`
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