5.10: [CanonicalizeOSSALifetime] Extend lexical lifetimes to unreachables. #68691
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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.
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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@swift-ci please test |
tbkka
approved these changes
Sep 22, 2023
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Description: Work around incomplete lifetimes to extend lifetimes to unreachables.
When a non-lexical value's lifetime is canonicalized, copies are propagated to consuming uses and destroys to last non-consuming uses. Shortening a lexical value's lifetime in that way isn't allowed: it doesn't respect deinit barriers.
The original lifetime of a value ends at its original lifetime-ending uses: destroys, last consumes, and--without complete OSSA lifetimes enabled--
unreachableinstructions. For a lexical value, the lifetime ends after canonicalization can be hoisted but not over any instructions that are deinit barriers. For destroys, this is handled via a backwards walk.Here, because complete OSSA lifetimes aren't enabled, handling is added for unreachables. The approach is to add the instructions before the
unreachableinstructions to liveness and then skip creating adestroy_valueif the next instruction is anunreachable.To find the relevant
unreachableinstructions, a copy ofPrunedLivenessthat canonicalization uses is created and the destroys are added to it. (Destroys are not in the original instance of liveness and should not be added to it because having them there would result in extra copies.) That copy is then passed to some utility code shared with OSSALifetimeCompletion to find theunreachableinstructions that terminate the dead-end blocks found by walking forward from the boundary of liveness. Once thoseunreachables are found, the instructions prior to them are added to the original copy of liveness.Risk: Low. Uses two preexisting utilities (PrunedLiveness, OSSALifetimeCompletion)
Scope: Narrow. Only affects lifetimes in dead end blocks.
Original PR: #68608
Reviewed By: Andrew Trick ( @atrick )
Testing: Added tests.
Resolves: rdar://115468707