SIL: Lower lifetime dependencies when lowering function types.

[jckarter]

Map the lifetime dependencies described in terms of the formal AST-level parameters to the correct parameter(s) in the lowered SIL function type. There can be 0, 1, or many SIL parameters per formal parameter because of tuple exploding. Also, record which dependencies are on addressable parameters (meaning that the dependency includes not only the value of the parameter, but its specific memory location).

[jckarter]

@swift-ci Please test

[atrick]

Thanks! That was more work than I expected. I think we eventually end up wanting ABI-affecting properties of the function signature to be lowered, so it's good to have it there.

Also, to preserve semantics we wouldn't want to specialize the convention to be direct (although that's not relevant for ABI)

I actually expected addressable parameters to be part of the SIL function independent of lifetime dependencies though. Are you sure we'll never need want that?

[jckarter]

My thinking was that lifetime dependencies are ultimately what force the materialization of the addressable parameter and what dictate its lifetime. To me it seemed useful to track it as part of the dependency, since if the dependent parameter or return value could be eliminated by optimization, that would also naturally remove the barrier to eliminating the memory location when nothing is dependent on it anymore.

We do have the ad-hoc use of @_addressable for C++ imports, but nontrivial C++ types are inherently address-only and non-reabstractable, and it seems like their C++-ness should already be a barrier to passes inserting copies that weren't already there. If it's useful, I could also add it as a SIL parameter attribute; would it still be interesting to track as dependency information in that case?

[jckarter]

@swift-ci Please test

[atrick]

Sorry, I glossed over this in my initial "review". I don't think the parser should have any logic that ties addressable parameters to lifetime descriptors. A lifetime descriptor is an element of the syntax. It doesn't tell you whether a lifetime dependency exists.

[jckarter]

This is purely there to allow the LifetimeDependenceInfo to be parsed back from textual SIL. The LifetimeDependenceInfo still contains the canonical info in the end. This form isn't allowed in Swift code.

[atrick]

Oh thanks. This isSILMode is a pretty big clue there

[jckarter]

@swift-ci Please test