AllocBoxToStack: Remove bodies of closure functions left unused after specialization. #67031
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… specialization. We can't remove the functions at this point in case they might have other function passes enqueued to run on them, but we can at least remove the function contents that are now unnecessary. We need to do this in cases when move-only types are involved, since the semantics of the move checker rely on unescaped captures being promoted before the pass runs, and we leave behind invalid SIL in the unpromoted code. rdar://110675352
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@swift-ci Please test |
gottesmm
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Jun 30, 2023
atrick
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Jun 30, 2023
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| // TODO: Erase from module if there are no more uses. | ||
| // If the function has no remaining references, it should eventually | ||
| // be deleted. We can't do that from a function pass, since the function |
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@eeckstein I'm surprised that we can't mark functions deleted (which should automatically remove the function body). We should even have a SILFunction::deleteIfUnused API. Do you remember why we never did that?
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Well, it's not that easy to see if a function is unused. There is no use list for functions. We have Refcount, but that also includes references from utilities, like the cache in the Deserializer. Also, in case of cycles the refcount would be useless.
So the strategy is to keep unused functions around until the DeadFunctionElimination pass deletes them eventually.
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…originals removed by MoveOnlyChecker. This is an improvement of swiftlang#67031 which avoids deleting the closure function body during AllocBoxToStack, which still breaks pass invariants by modifying functions other than the currently-analyzed function. As a function pass, AllocBoxToStack also doesn't really know with certainty whether the original closure function is unused after stack promotion or not. We still want to eliminate the original when it may contain invalid SIL for move-only values that rely on the escape analysis for correct semantics, so rather than mark the original function to be *ignored* during move-only checking, mark it to be *deleted* by move-only checking if the function is in fact unused at that point. If the marked function is still used, we let it pass through move-only checking normally, which may cause redundant diagnostics but is the right thing to do since code is still potentially using the closure with escaping semantics. We should rearrange things to make this situation impossible in the future. rdar://110675352
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Thanks for the feedback @atrick and @eeckstein. I'll revert this and try another approach: #67205 |
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…originals removed by MoveOnlyChecker. This is an improvement of swiftlang#67031 which avoids deleting the closure function body during AllocBoxToStack, which still breaks pass invariants by modifying functions other than the currently-analyzed function. As a function pass, AllocBoxToStack also doesn't really know with certainty whether the original closure function is unused after stack promotion or not. We still want to eliminate the original when it may contain invalid SIL for move-only values that rely on the escape analysis for correct semantics, so rather than mark the original function to be *ignored* during move-only checking, mark it to be *deleted* by move-only checking if the function is in fact unused at that point. If the marked function is still used, we let it pass through move-only checking normally, which may cause redundant diagnostics but is the right thing to do since code is still potentially using the closure with escaping semantics. We should rearrange things to make this situation impossible in the future. rdar://110675352
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We can't remove the functions at this point in case they might have other function passes enqueued to run on them, but we can at least remove the function contents that are now unnecessary. We need to do this in cases when move-only types are involved, since the semantics of the move checker rely on unescaped captures being promoted before the pass runs, and we leave behind invalid SIL in the unpromoted code. rdar://110675352