[swift-dev] Emitting redundant protocol conformances to support type lookup

[Luke Howard]

I made an experimental patch to IRGen to emit a table of type metadata, independent of the protocol conformance table.

The entry format is a subset of a protocol conformance record:

struct TypeMetadataRecord {
private:
  // Some description of the type that is resolvable at runtime.
  union {
    /// A direct reference to the metadata.
    RelativeIndirectablePointer<Metadata> DirectType;
    
    /// An indirect reference to the metadata.
    RelativeIndirectablePointer<const ClassMetadata *> IndirectClass;
  };

  /// Flags describing the type metadata record.
  TypeMetadataRecordFlags Flags;
}

Otherwise the implementation is pretty similar. They are emitted in irgen::emitXXXMetadata. Mappings for generic types are at runtime when swift_getGenericMetadata() is called.

It seems to work OK and removes the limitation that the conformance table approach had, where one could only resolve the names of types with explicit protocol conformances.

Haven’t tested on Linux yet.

— Luke
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