struct CaseSet: Optimize by matching on len once per set_ctx
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In C, `case_set` works by `switch`ing once on `len`, once per `set_ctx`, but my `CaseSet` implementation in Rust accidentally switched to `match`ing on `buf.len()` each time. This goes back to the original, optimal behavior. To do this, `set_ctx` is made a rank-2 polymorphic "closure". This does not exist in Rust, so it's emulated through a generic trait with a generic method. This inner generic over `trait CaseSetter` is what allows `fn CaseSet::one` to select the correct `CaseSetterN` at compile time. However, this means that closures can't be used anymore, which is very annoying. To partially remedy this, I added the `set_ctx!` macro, which emulates the `set_ctx` closure as much as possible. All captures (up vars in `rustc`) and their types must be declared.
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I'm working on validating performance for this. It's not making as much of a difference as expected, and I think some checks are not getting elided. |
😭 I didn't get a chance to look at the asm yet, though. Let me know if there's anything simple we can do there. I can look a bit too to see which things aren't elided. |
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I have a very different way of doing this that is a lot less complicated, I think, so don't spend time on it for now. |
Oh interesting, hope it works! |
kkysen
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struct CaseSet: Optimize by matching on len one per set_ctxstruct CaseSet: Optimize by matching on len once per set_ctx
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In C,
case_setworks byswitching once onlen, once perset_ctx, but myCaseSetimplementation in Rust accidentally switched tomatching onbuf.len()each time. This goes back to the original, optimal behavior.To do this,
set_ctxis made a rank-2 polymorphic "closure". This does not exist in Rust, so it's emulated through a generic trait with a generic method. This inner generic overtrait CaseSetteris what allowsfn CaseSet::oneto select the correctCaseSetterNat compile time.However, this means that closures can't be used anymore, which is very annoying. To partially remedy this, I added the
set_ctx!macro, which emulates theset_ctxclosure as much as possible. All captures (up vars inrustc) and their types must be declared.I didn't actually benchmark this or look at the asm yet, though, if anyone wants to do that.