Optimize exact casts better in Unsubtyping #7930
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With the upcoming stricter validation rules for descriptor types, we can no longer use the placeholder describee trick in GlobalTypeOptimization, so its ability to optimize out unneeded descriptors will be significantly restricted. Because of the increasing coupling between the validation for subtyping and descriptor relationships, it makes most sense to optimize both at the same time in Unsubtyping. Update Unsubtyping to start the analysis assuming no descriptors are necessary, then add them in as it discovers that they are indeed necessary either to preserve behavior or validitiy. Discovering new required descriptors can imply that new subtypings are necessary and vice versa.
Casts generally need special treatment in Unsubtyping because any subtype of the destination type could flow into the source type and be successfully cast, meaning all original subtypes of the destination type that are still subtypes of the source types need to remain subtypes of the destination type as well. However, exact casts do not have this property because strict subtypes of the destination type will fail the cast whether or not they remain subtypes of the destination type in the optimized module. Update the analysis so that exact casts require only simple subtyping between the destination and source types.
kripken
approved these changes
Sep 29, 2025
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Casts generally need special treatment in Unsubtyping because any
subtype of the destination type could flow into the source type and be
successfully cast, meaning all original subtypes of the destination type
that are still subtypes of the source types need to remain subtypes of
the destination type as well. However, exact casts do not have this
property because strict subtypes of the destination type will fail the
cast whether or not they remain subtypes of the destination type in the
optimized module.
Update the analysis so that exact casts require only simple subtyping
between the destination and source types.