[OSSACanonicalizeOwned] Record traversed defs and don't traverse copies of guaranteed values. #77968
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Add a type which distinguishes among the types of defs that are pushed onto the "def-use worklist". Note that it's not possible to rely on the kind of value because the root may itself be a copy_value. For now, the distinction is discarded as soon as the def is visited.
When rewriting uses, determine how that rewriting should proceed based on the kind of def whose use is being visited. Direct uses of reborrows and borrowed-froms never need to be rewritten because every such use is a use of a guaranteed value and can never require a copy.
Add a local variable that shadows the `defUseWorklist` instance member, enabling further constraints (e.g. `const`ness) to eventually be imposed.
In preparation for only recording the defs once, replace the GraphNodeWorklist of defs with a SetVector. Preserve the current visitation order by creating a worklist of indices to be visited.
The field is no longer a worklist, just a list of discovered defs.
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The utility performs two def-use traversals. The first determines liveness from uses. The second rewrites copies. Previously, the defs whose uses were analyzed were discovered twice, once during each traversal. The non-triviality of the discovery logic (i.e. the logic determining when to walk into the values produced by the instructions which were the users of visited uses) opened the possibility for a divergence between the two discoveries. This possibility had indeed been realized--the two traversals didn't visit exactly the same uses, and issues ensue. Here, the defs whose uses are analyzed are discovered only once (and not discarded as their uses are analyzed) during the first traversal. The second traversal reuses the defs discovered in the first traversal, eliminating the possibility of a def discovery difference. The second traversal is now done in a different order. This results in perturbing the SIL in certain cases.
Because discovery of defs walks into reborrows and borrowed-from instructions, copies may be seen whose underlying value is a guaranteed value (namely, a reborrow or a borrowed-from instruction). Such copies may be used beyond the lifetime end of such guaranteed values, so it's not allowed to sink copies to their consuming uses. Such canonicalization is the responsibility of the OSSACanonicalizeGuaranteed utility. rdar://139842132
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@swift-ci please test |
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@swift-ci please apple silicon benchmark |
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@swift-ci please clean test linux platform |
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@swift-ci please test linux platform |
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@swift-ci please test macos platform |
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The utility performs two def-use traversals. The first determines liveness from uses. The second rewrites copies.
Previously, the defs whose uses are analyzed were discovered twice, once during each traversal. The non-triviality of the discovery logic (i.e. the logic determining when to walk into the values produced by the instructions which were the users of visited uses) opened the possibility for a divergence between the two discoveries. This possibility had indeed been realized--the two discoveries didn't find exactly the same defs, the two traversals didn't visit exactly the same uses, and issues ensued.
Here, the defs whose uses are analyzed are discovered only once during the first traversal. The second "traversal" now reuses the defs discovered in the first traversal, eliminating the possibility of a def discovery difference.
Extra care is taken to preserve the traversal order of the first traversal (as compared to the order before this change). Evidently diagnostics depend on it. The second traversal's order is changed, which results in slight SIL perturbations in certain cases (e.g. an instruction which consumes a value and its copy may see the order of those operands reversed).
Because discovery of defs walks into reborrows and borrowed-from instructions, copies are processed whose underlying value is a reborrow or borrowed-from. Such copies cannot generally be sunk to consuming uses because the lifetime of their underlying value (the reborrow or borrowed-from) may not extend to the consuming use. Eliminating such copies is outside the scope of OSSACanonicalizeOwned--it's the responsibility of a separate OSSACanonicalizeGuaranteed utility. For this reason, when visiting copies during the first traversal, the copies are only walked into (and added to the list of defs) if their operand is not a reborrow or borrowed-from.
rdar://139842132