[swift-dev] Reconsidering the global uniqueness of type metadata and protocol conformance instances

[Erik Eckstein]

Would it be possible that whenever a specialized class is instantiated and the metadata already exists with a generic vtable, it just overwrites the vtable pointer in the metadata with the specialized version?
I didn’t think that through, but maybe the same could be done for witness tables?

> On Jul 28, 2017, at 2:20 PM, Joe Groff via swift-dev <swift-dev at swift.org> wrote:
> 
> The Swift runtime currently maintains globally unique pointer identities for type metadata and protocol conformances. This makes checking type equivalence a trivial pointer equality comparison, but most operations on generic values do not really care about exact type identity and only need to invoke value or protocol witness methods or consult other data in the type metadata structure. I think it's worth reevaluating whether having globally unique type metadata objects is the correct design choice. Maintaining global uniqueness of metadata instances carries a number of costs. Any code that wants type metadata for an instance of a generic type, even a fully concrete one, must make a potentially expensive runtime call to get the canonical metadata instance. This also greatly complicates our ability to emit specializations of type metadata, value witness tables, or protocol witness tables for concrete instances of generic types, since specializations would need to be registered with the runtime as canonical metadata objects, and it would be difficult to do this lazily and still reliably favor specializations over more generic witnesses. The lack of witness table specializations leaves an obnoxious performance cliff for instances of generic types that end up inside existential containers or cross into unspecialized code. The runtime also obligates binaries to provide the canonical metadata for all of their public types, along with all the dependent value witnesses, class methods, and protocol witness tables, meaning a type abstraction can never be completely "zero-cost" across modules.
> 
> On the other hand, if type metadata did not need to be unique, then the compiler would be free to emit specialized type metadata and protocol witness tables for fully concrete non-concrete value types without consulting the runtime. This would let us avoid runtime calls to fetch metadata in specialized code, and would make it much easier for us to implement witness specialization. It would also give us the ability to potentially extend the "inlinable" concept to public fragile types, making it a client's responsibility to emit metadata for the type when needed and keeping the type from affecting its home module's ABI. This could significantly reduce the size and ABI surface area of the standard library, since the standard library contains a lot of generic lightweight adapter types for collections and other abstractions that are intended to be optimized away in most use cases.
> 
> There are of course benefits to globally unique metadata objects that we would lose if we gave up uniqueness. Operations that do check type identity, such as comparison, hashing, and dynamic casting, would have to perform more expensive checks, and nonunique metadata objects would need to carry additional information to enable those checks. It is likely that class objects would have to remain globally unique, if for no other reason than that the Objective-C runtime requires it on Apple platforms. Having multiple equivalent copies of type metadata has the potential to increase the working set of an app in some situations, although it's likely that redundant compiler-emitted copies of value type metadata would at least be able to live in constant pages mapped from disk instead of getting dynamically instantiated by the runtime like everything is today. There could also be subtle source-breaking behavior for code that bitcasts metatype values to integers or pointers and expects bit-level equality to indicate type equality. It's unlikely to me that giving up uniqueness would buy us any simplification to the runtime, since the runtime would still need to be able to instantiate metadata for unspecialized code, and we would still want to unique runtime-instantiated metadata objects as an optimization.
> 
> Overall, my intuition is that the tradeoffs come out in favor for nonunique metadata objects, but what do you all think? Is there anything I'm missing?
> 
> -Joe
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