Entity cmp::Eq improvement

[tower120]

What problem does this solve or what need does it fill?

Current Entity comparison relies on rust's #derive(Eq, ...)

It now actually makes two comparisons (see asm code)

https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=38662860685c22c6cbaf0a14a7e38b8b

What solution would you like?

If treat whole Entity struct as u64 (through pointer cast on comparison, or even by store generation+id as u64) this could become one bitwise comparison.

https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=3351c4c203e76073de253b3301ed8d13

[NathanSWard]

I'm surprised the compiler didn't optimize this 😢

[mockersf]

Bevy very rarely rely on entity equality so I don't think it would change much in performances

[NathanSWard]

Bevy very rarely rely on entity equality so I don't think it would change much in performances

Bevy may not, but that doesn't mean users won't use it a lot :)

[NathanSWard]

This also seems to greatly affect the assembly for the Ord trait as well :(
https://godbolt.org/z/GKqYnee4h

[alice-i-cecile]

#2372 can and should be revived if this is worth pushing forward.

FWIW, I'm in favor of those changes; I tend to use Entity equality quite a bit in my code, and suspect that it will be important for relation-like features.