Meta tracking issue for branch hints (RFC 1131)

[nikomatsakis]

Tracking and meta discussion for branch and path hinting, including:

• likely/unlikely as accepted in Add an expect intrinsic rfcs#1131 (much has changed since this RFC)
• core::hint::{likely, unlikely} Tracking Issue for likely_unlikely #151619
• core::hint::cold_path Tracking Issue for cold_path #136873
• bool::select_unpredictable Tracking Issue for hint::select_unpredictable #133962
• #[cold]:
  • #[cold] on match arms #120193

[pythonesque]

FWIW, looking through my own code, I actually think the fact that this can't apply to match variants is going to be fairly hard on idiomatic Rust (for example, it means you can't use it when matching on an Option, which is going to be extremely common since it amounts to if (likely(ptr != NULL)) in C).

[Aatch]

@pythonesque once I get this done, I plan on writing a follow-up RFC for a #[likely] attribute you can put on match arms. Since it's already possible to put attributes on arms (to support conditional compilation of them), it should be fairly easy to implement.

[Craig-Macomber]

It might be worth having an "unpredictable" option in addition to likely and unlikely. There is now something to lower it to in LLVM: http://reviews.llvm.org/D12341

[apasel422]

@Aatch Are you still working on this?

[vi]

Shall #[likely] be also on Result?

enum Result<T, E> {
   #[likely]
   Ok(T),
   Err(E)
}

I expect for most code Ok is likely and Err is unlikely. Of course, annotating if or match should override enum variant's annotation.

[Aatch]

@vi I think hinting should be only be used rarely, so it being on the type doesn't seem like a good idea. Result is a bit more general purpose than just error-handling, sot that kind of hinting would impact code where it's inappropriate. Hinting should only really be done at the site itself.

[vi]

@Aatch, Is it a good idea to have likely in every place where something is likely? Can it also serve for self-documentation: follow "likelies" and see happy path?

[Aatch]

@vi is is not. As a general rule, people are pretty bad at knowing what cases are actually likely or not. Branch hinting is a fairly specific optimisation hint, used incorrectly it can actually make code slower. It should also only really be used when the unlikely case is known to be very unlikely.

Basically, you should only use it if it makes your code faster, otherwise it should be there.

[pczarn]

@vi It might be good idea if the likely thing is cheap, and the unlikely thing is expensive. For example, Vec::push is expensive if it causes a reallocation, and very cheap otherwise.

In my opinion, frequent use of likely is either unnecessary or just manual profile-guided optimization.