6.0: [MoveOnlyAddressChecker] Fix empty deiniting struct representation. #73178
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kavon
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Apr 22, 2024
tbkka
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Apr 22, 2024
An empty struct without a deinit gets a single bit which is used to track the struct's liveness. Previously, an empty struct with a deinit also only got a single bit. Consequently, when discarding the struct (i.e. dropping the deinit), there was no bit left to represent the struct. This resulted in a failure to track liveness for the value. rdar://126863003
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Explanation: Fix MoveOnlyAddressChecker miscompile caused by not representing empty struct with deinit.
The checker represents each type as a bitfield. An empty struct without a deinit gets a single bit--that bit corresponds to the whole struct. That allows the checker to track uses of the whole struct as a use of some bit.
Previously, the checker only represented an empty struct with a deinit as a single bit too. The single bit in this case corresponds to the deinit. When the deinit is dropped via discard, there is still an empty struct whose lifetime needs representation in bits--it should have the same number of bits as an empty struct without a deinit--i.e. two.
Here, the problem is fixed by giving an empty struct with a deinit two bits.
Scope: Affects noncopyable code.
Issue: rdar://126863003
Original PR: #73174
Risk: Low.
Testing: Added and updated unit tests.
Reviewer: @kavon