6.0: [IRGen] Deadends don't need dealloc_pack_metadata. #73236
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There is a debug-build-only verification that is done for alloc_pack_metadata instructions that checks that there exist paired dealloc_pack_metadata instructions which will be keyed off of to clean up the on-stack variadic metadata packs corresponding to (the instruction after) the alloc_pack_metadata. StackNesting omits the deallocation instruction (as does PackMetadataMarkerInserter) if it would be created in a dead end block. If all blocks in the dominance frontier of the alloc_pack_metadata instruction are dead-end blocks, then the verification will incorrectly fail. It should not fail because it is not necessary to clean up the on-stack pack metadata (or any other stack allocations) in such a case. If all such blocks are dead-end blocks, however, the alloc_pack_metadata's block itself is a dead-end block as well. So during the verification, check whether the alloc_pack_metadata occurs in a dead-end block and do not fail verification if it does. rdar://125265980
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@swift-ci please test |
tbkka
approved these changes
Apr 24, 2024
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Explanation: Fix a false-positive assertion failure for variadic generics.
When a SIL instruction involving variadic generic types is lowered, pack metadata may be allocated. As an optimization, that metadata is allocated on the stack. To correctly nest these on-stack pack allocations, there are matching
alloc_pack_metadataanddealloc_pack_metadatainstructions.To catch issues with bad nesting, some verification is run in asserts builds during IRGen. This patch corrects a false positive that was being flagged by that verification.
Specifically, the verification was checking that for any
alloc_pack_metadatainstruction for which metadata was indeed allocated during IRGen that there was at least one matchingdealloc_pack_metadatainstruction which would clean it up. Such a cleanup instruction need not and does not exist, however, if thealloc_pack_metadatais in a "dead end block" (all paths passing through thealloc_pack_metadatainstruction end inunreachable--caused, for example, by a call tofatalError()). This patch adds a check for whether thealloc_pack_metadatainstruction is in a dead end block and does not require there to be a matchingdealloc_pack_metadataif so.Scope: Affects asserts builds only. Affects variadic generic code.
Issue: rdar://125265980
Original PR: #73191
Risk: Very low. Only affects asserts builds.
Testing: Added regression test.
Reviewer: Arnold Schwaighofer ( @aschwaighofer )