[5.6,SILOptimizer] Let visitTransitiveEndBorrows take SILValues. #41008
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Previously, visitTransitiveEndBorrows took BorrowedValues. However, there is at least one kind of borrow--namely, unchecked_ownership_conversion insts--that is not currently permitted by the BorrowedValue API. The long term fix is to make BorrowedValue handle such instructions. For now, change visitTransitiveEndBorrows to take SILValues so that unchecked_ownership_conversion can be passed to the API. rdar://87985420
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tbkka
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tbkka
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Jan 26, 2022
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5.6 Summary: Fix compiler crash during dead code elimination when handling patterns involving borrows of
unchecked_ownership_conversions.Explanation: When dealing with nested borrow scopes, dead code elimination must not shorten outer borrow scopes to end prior to the end of inner borrow scopes. To ensure that, the ends of outer borrow scopes are marked live, using the visitTransitiveEndBorrows which sees through reborrows.
Previously, that API took a BorrowedValue. BorrowedValue, however, has not been taught about one particular, exotic form of borrow scopes, namely those begun with
unchecked_ownership_conversioninstructions. The result was that a BorrowedValue couldn't be constructed from such instructions, and the value passed to visitTransitiveEndBorrows contained a null BorrowedValue.Here, rather than teaching BorrowedValue to deal with
unchecked_ownership_conversion(the correct long-term solution), for now, loosen the restrictions on visitTransitiveEndBorrows so that it takes a SILValue rather than a BorrowedValue.Issue: rdar://87985420
Original PR: #41007
Testing: New regression tests, Swift CI
Reviewed by: Andrew Trick
Risk: Low.
Scope: Bug fix