6.0: [PrunedLiveness] Branch summary merges to ending. #74836
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A branch instruction that is both a lifetime-ending user and also a non-lifetime-ending of a value should be regarded as a lifetime-ending user. Without this, utilities that rely on PrunedLiveness such as `LinearLiveness` and `InteriorLiveness` both incorrectly report that a branch featuring a value and also a reborrow or guaranteed phi are non-ending. rdar://130427564
Now that the underlying issue (PrunedLiveness' merging of summaries for branch instructions) has been fixed, reinstate lifetime completion and add a test to verify that it behaves correctly. This reverts commit c552b90 ("Temporarily turn off completing lifetimes of block arguments"). rdar://130427564
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@swift-ci please test |
tbkka
approved these changes
Jun 28, 2024
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@swift-ci please test linux platform |
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Explanation: Fix underlying utility and reinstate lifetime completion during move checking.
To address a compiler crash, move-checking's lifetime completion was partially disabled. Here, it is reenabled now that the compiler crash will no longer occur.
The move-only address checker depends on complete lifetimes; without them, it miscompiles code. So, if the checker determines that it will have to transform the function, it completes all (non-reborrow) lifetimes before transforming. That completion was temporarily partially disabled because it was producing invalid SIL. Here, the underlying issue which caused completion to produce invalid SIL is fixed and lifetime completion is reinstated to run as before.
The underlying issue was an incorrect summary calculation done by the
PrunedLivenessutility. Specifically, it determined that branch instructions (e.g.br bbN(%x, %y)) were not lifetime-ending if they had both an owned value and a guaranteed value derived from that owned value as their operands†. The result was ending lifetimes of the value actually consumed by the branch in the block that it branched to. This was invalid in a couple of ways: the value was overconsumed, the destroy might not dominate the def. In noasserts builds, this resulted in a compiler crash during LLVM.Now that the underlying issue is fixed, lifetime completion doesn't produce invalid SIL, and the compiler doesn't crash during LLVM.
†In OSSA, in the target block, the guaranteed block argument is understood to be derived from the owned block argument.Scope: Affects noncopyable code.
Issue: rdar://130427564
Original PR: #74815 , #74835
Risk: Low.
Testing: Added and updated tests. Rebuilt project exhibiting compiler crash without fix.
Reviewer: Andrew Trick ( @atrick ), Meghana Gupta ( @meg-gupta )