Improve types for variables generated from stack slots - #4071
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…e `object` This was due to StackType.O doing double-duty as `object` and `other`. While ExpressionBuilder would often improve the type of such locals, the `object` nevertheless ended up used in a couple of places, e.g. via the `typeHint`. This could result in value types being boxed even though the original IL didn't contain any `box` instruction. This is an attempt to use better types for stack slot variables created by ILReader. The idea is: there aren't many IL instructions that produce "other" value types, and `InferType()` already handles pretty much all of them, so we can use that to assign types to our stack slots. It's a bit more tricky if the stack is pushed to on multiple branches that join together before the value is used: here the variable type must be suitable for both assignments. In this case, we go back to the previously-used stacktype.
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Stack slots holding value types previously nevertheless often had type
objectThis was due to StackType.O doing double-duty as
objectandother. While ExpressionBuilder would often improve the type of such locals, theobjectnevertheless ended up used in a couple of places, e.g. via thetypeHint. This could result in value types being boxed even though the original IL didn't contain anyboxinstruction.This is an attempt to use better types for stack slot variables created by ILReader. The idea is: there aren't many IL instructions that produce "other" value types, and
InferType()already handles pretty much all of them, so we can use that to assign types to our stack slots.It's a bit more tricky if the stack is pushed to on multiple branches that join together before the value is used: here the variable type must be suitable for both assignments. In this case, we go back to the previously-used stacktype.
Fixes #4068 (comment) -- the issue there was that the stack slot holding the
(int, bool)had typeobjectand was passing this as a TypeHint to the switch-expression translation. That caused switch-expression translation to box -- but the uses of the variable didn't unbox, so this was a correctness bug.