Filling out the PrunedLiveness utility for composition with other OSSA utilities #39577
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For use in OwnershipUtils.
A perfectly straightforward liveness check. Used by OSSA utilities to check that an instruction is within a lifetime or scope.
Simple wrapper to compute the boundary of PrunedLiveness. Pervasively useful utility for working with OSSA reference lifetimes and borrow scopes. This can also replace the implementation in CanonicalOSSALifetime. This will greatly simplify that utility's logic, preparing it to handle diagnostics (like move-only SILValue checking) and other features.
Straighforward API to build pruned liveness from a single SSA value. This 3-line function allows PrunedLiveness to be used anywhere ValueLifetimeAnalysis was used in OSSA form. Aside from simplicity and efficiency, the difference is that this composes with other utilities that only care about PrunedLiveBlocks, PrunedLiveness, or PrunesLivenessBounary independent of how those data structures were generated.
Given a computed ValueLifetimeBoundary, visit all the points at which the lifetime needs to be terminated, e.g. via and end_borrow or destroy_value. Especially useful for creating a borrow scope over guaranteed uses. This completely decouples the DeadBlocks analysis from the liveness analysis. It will allow phasing out the complex and bug-prone ValueLifetimeAnalysis::Frontier API.
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This small collection of liveness utilities each does one only thing and makes no unnecessary assumptions about the client.
It is meant to compose cleanly with other utilities. It is flexible enough to represent any SIL lifetime or scope, characterized by a set of dominating points. It can handle a lifetime or scope with multiple SSA values just as efficiently as it handles a single SSA value.
All of the above makes it extremely useful as a building block for simple and efficient OSSA utilities.
This will also allow CanonicalOSSALifetime (CopyPropagation) to be cleaned up and extended for other purposes. It was originally meant to do only one thing, but now has logic for multiple modes embedded within it and would need callbacks for diagnostics. Instead the OSSA analysis and "copy propagation" logic need to be decoupled.