5.10: [OSSALifetimeCompletion] Handle unavailable blocks in dead-end regions. #68997
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OSSALifetimeCompletion needs to insert not at unreachable instructions that appear after the non-lifetime-ending boundary of a value but rather at the terminators of the availability boundary of the value within that region. Once it does so, it will no longer be sufficient to check whether the insertion point is an unreachable because such terminators may be another terminator that appears on the availability boundary. Prepare for that by recording the instructions that were found and checking whether the destroy insertion point is such an instruction before bailing rather than specifically checking for `unreachable`.
tbkka
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Not every block in a region which begins with the non-lifetime-ending boundary of a value and ending with unreachable-terminated blocks has the value available. If the unreachable-terminated blocks in this boundary are not available, it is incorrect to insert destroys of the value in them: it is an overconsume on some paths. Previously, however, destroys were simply being inserted at the unreachable. Here, this is fixed by finding the boundary of availability within that region and inserting destroys before the terminators of the blocks on that boundary. rdar://116255254
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@swift-ci please test |
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Description: Fix overconsume on unreachable-terminated paths by handling conditional availability of values in dead-end regions in lifetime completion.
The function
OSSALifetimeCompletion::visitUnreachableLifetimeEndsis used (1) to complete OSSA lifetimes and--while OSSA lifetime completion remains disabled--(2) to canonicalize lexical OSSA lifetimes. The function visits the instructions at which the lifetime implicitly ends--implicitly because with incomplete lifetimes, it's valid for a value not to be destroyed on paths that terminate inunreachable.Previously, its determination for those ends was incorrect in the face of unavailable blocks in dead-end regions. Specifically, it simply walked forward from the non-lifetime-ending boundary until finding
unreachableinstructions. (Suchunreachableinstructions are guaranteed to exist because the lifetime is "partially complete" by precondition.) That walk alone is insufficient, however: It's possible that the value isn't available at thoseunreachableinstructions. For example, if a value isn't destroyed on one path to the unreachable but is destroyed on another path to it, then the value isn't available at the unreachable. Inserting a destroy at theunreachablewould result in an overconsume on the path on which the value was destroyed.Here, the determination is fixed by computing availability within the region discovered by that initial walk. This is done via a forward iterative dataflow within the region, propagating availability. The visited instructions are then the terminators of the last blocks in the region in which the value is available. In the preceeding example, rather than visiting the
unreachableinstruction, the terminator of the last block on the path to it where the value has not yet been destroyed would be visited.Risk: Low. The fix uses a well-known dataflow implementation to propagate availability.
Scope: Narrow. Only affects lifetimes in dead end blocks.
Original PR: #68975
Reviewed By: Andrew Trick ( @atrick )
Testing: Added tests.
Resolves: rdar://116255254