Improve par_iter and Parallel

[re0312]

Objective

• bevy usually use Parallel::scope to collect items from par_iter, but scope will be called with every satifified items. it will cause a lot of unnecessary lookup.

Solution

• similar to Rayon ,we introduce for_each_init for par_iter which only be invoked when spawn a task for a group of items.

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Changelog

• added for_each_init

Performance

check_visibility in many_foxes

~40% performance gain in check_visibility.

[P-Asta]

its cool

[superdump]

The rendering parts look fine to me. And the benchmarks show it. I defer the parallel iteration / for each init parts to James :)

[james7132]

This was rejected from the initial Parallel PR because of the potential for surprising results if a thread panics while holding the guard, the guard is not dropped, or if multiple guards are retrieved from the same thread. We may need to switch this to use RefCell instead of Cell to avoid these problems. This should be fine since we don't allow iteration without a &mut Parallel, so the Sync bound on the ThreadLocal inner value isn't required.

[JoJoJet]

I was originally against this style of guard (due to the reasons you listed) -- however if enough people think it would be useful my opinions on it aren't that strong. Using a closure to limit the scope technically doesn't alleviate the problems with a guard I don't think, it just makes you less likely to run into them since the scope is much more explicit.

I do lean towards avoiding a drop impl though.

[re0312]

I am also not a fan of using this drop implementation because it provides a gun to the user. In theory, another method to achieve the same performance is by using par_chunk. It is much safer and clearer, but it requires some refactoring in the QueryParIter .

[james7132]

Other than a few nits, LGTM!

[alice-i-cecile]

Thank you to everyone involved with the authoring or reviewing of this PR! This work is relatively important and needs release notes! Head over to bevyengine/bevy-website#1302 if you'd like to help out.