[CanonicalizeOSSALifetime] Extend lexical lifetimes to unreachables. #68608
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nate-chandler
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It was already addressed.
It's useful to be able to form a second copy of liveness which is differently pruned by having more instructions added to it, e.g..
Don't respect deinit barriers when canonicalizing if lexical lifetimes are disabled.
Extracted the new visitUnreachableLifetimeEnds static member of OSSALifetimeCompletion from the preexisting endLifetimeAtUnreachableBlocks which now calls through the former.
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When canonicalizing the lifetime of a lexical value, deinit barriers are respected. This is done by walking backwards from lifetime ends and adding encountered deinit barriers to liveness. Only destroy lifetime ends were walked back from under the assumption that lifetimes would be complete. Without complete OSSA lifetimes, however, it's necessary to also necessary to consider lifetimes that end with unreachables. Unfortunately, we can't simply walk back from those unreachables because there may be instructions which are secretly users of the value being canonicalized (e.g. destroys of `partial_apply`s to which a `begin_borrow` of the value was passed). Such uses don't appear in the use list because lifetime canonicalization expects complete lifetimes and only visits lifetime ends of `begin_borrow`s. Here, instead, the instructions before the relevant unreachables are added to liveness. In order to determine which unreachables are relevant, it's necessary to have a liveness that includes the original destroys. So a copy of liveness is created and those destroys are added to it. rdar://115468707
Such destroys are part of the lifetime of lexical values inserted to prevent shortening lifetimes over deinit baarriers by CanonicalizeOSSALifetime. After OSSALifetimeCompletion is enabled, more instructions will need to be preserved.
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When canonicalizing the lifetime of a lexical value, deinit barriers are respected. This is done by walking backwards from lifetime ends and adding encountered deinit barriers to liveness.
Only destroy lifetime ends were walked back from under the assumption that lifetimes would be complete. Without complete OSSA lifetimes, however, it's necessary to also necessary to consider lifetimes that end with unreachables. Unfortunately, we can't simply walk back from those unreachables because there may be instructions which are secretly users of the value being canonicalized (e.g. destroys of
partial_applys to which abegin_borrowof the value was passed). Such uses don't appear in the use list because lifetime canonicalization expects complete lifetimes and only visits lifetime ends ofbegin_borrows.Here, instead, the instructions before the relevant unreachables are added to liveness. In order to determine which unreachables are relevant, it's necessary to have a liveness that includes the original destroys. So a copy of liveness is created and those destroys are added to it.
rdar://115468707