6.2: [TypeLowering] Record packs used in signatures. #81659
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To determine whether an instruction may require pack metadata, the types of its operands are examined. Previously, the top level type was checked for having a pack in its signature, and the whole type was checked for having a type anywhere in its layout (via TypeLowering). This didn't account for the case where the metadata was required for a resilient type which uses a pack in its signature. Here, during type lowering, a type having a pack in its signature is counted towards the type having a pack. Fixes a compiler crash. rdar://147207926
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@swift-ci please test |
tbkka
approved these changes
May 21, 2025
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Explanation: Fix a compiler crash involving a stored propoerty of variadic type substituted with a non-variadic type.
Metadata packs are stored on the stack as an optimization. These allocations must obey stack discipline. This is enforced via the PackMetadataMarkerInserter pass. That pass inserts
alloc_pack_metadatamarker instructions before each instruction which may allocate an on-stack pack and insertsdealloc_pack_metadatainstructions to indicate where those packs should be deallocated (theStackNestingutility ensures proper nesting, reflowing all stack deallocating instructions as needded). The pass relies on knowing whether an instruction may allocate an on-stack pack. This is determined by a recursive walk over the fields of each type of each operand of the instruction. That walk is done in TypeLowering.Previously, the walk only regarded a type as involving a pack if it contained a pack in its storage. This was sufficient for non-resilient types because the metadata was not needed for types like
S<each T>wheneach Twas not stored (recursively). This doesn't work with resilient types however, whose storage is not known during the walk. The metadata for resilient structS<each T>is still required.Here, this is fixed by extending the condition under which a type is said to have a have a pack. In addition to considering a type which is itself a pack and types which store a pack, now types which have a pack in their signature are considered to have a pack as well.
Scope: Affects variadic generics across resilience boundaries.
Issue: rdar://147207926
Original PR: #81581
Risk: Low, this just makes the de/alloc_pack_metadata marker instructions be emitted slightly more frequently.
Testing: Added test.
Reviewer: Arnold Schwaighofer ( @aschwaighofer )