[Constant Evaluator] Enable SILConstants::setIndexedElement function to work with aggregates containing unknown values #65003
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to work with aggregates containing unknown values. Such aggregates can be generated when an instruction is skipped during constant evaluation and its results are used to create a struct.
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@swift-ci smoke test |
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| %6 = struct_element_addr %5 : $*StructWithSkippedElements, #StructWithSkippedElements.unknownValue | ||
| %7 = struct_element_addr %6 : $*UnknownStruct, #UnknownStruct.value | ||
| store %1 to %7 : $*Int64 |
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Trying to understand the new behavior better. Now that this known value has been set in the location of the unknown value, what is the expected behavior? The code below appears to be simply accessing the known value in the first field of StructWithSkippedElements, but what if we were to now try to access %5.unknownValue.value again now that we've tried to set it? Would we get an unknown because the set would not actually have occurred, or would it have occurred and we'd be left with 301? I'm guessing the first option (we'd get an unknown).
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@matthewseaman that's a great question. Yes, unknown. It is a conservative but safe option. It is certainly possible to teach the constant evaluator to again track the constant value being assigned to the property, but it will be bit more complex to correctly implement it, and also we don't need that capability for any application at this point.
Such aggregates can be generated when an instruction is skipped during constant evaluation and its results are used to create a struct.
rdar://107344820