Previously even if a type's metadata was optimized away, we would still
emit a field descriptor, which in turn could reference nominal type
descriptors for other types via symbolic references, etc.
If a type has the same layout as one of the basic integer types, or has a single refcounted pointer representation, we can use prefab value witness tables from the runtime instead of instantiating new ones. This saves quite a bit of code size, particularly in the Apple SDK overlays, where there are lots of swift_newtype wrappers and option set structs.
This makes resolving mangled names to nominal types in the same module more efficient, and for eventual secrecy improvements, also allows types in the same module to be referenced from mangled typerefs without encoding any source-level name information about them.
...instead of the witness table’s linkage.
The WT linkage is derived from the minimum of the type’s and protocol’s visibility.
So if there is a public type conforming to a private protocol, we might not see the type’s metadata escaping (because it could be in another module).
This gives big code size wins for unused types and also for types, which are never used in a generic context.
Also it reduces the amount of symbols in the symbol table.
The size wins heavily depend on the project. I have seen binary size reductions from 0 to 20% on real world projects.
rdar://problem/30119960
