What is the __DynamicallyInvokable attribute for?

I found that it's used in the Runtime*Info.IsNonW8PFrameworkAPI() suite of internal methods. Having this attribute placed on a member makes IsNonW8PFrameworkAPI() return false for it and thus makes the member available in WinRT applications and shuts up the The API '...' cannot be used on the current platform. exception.

Profiler writers should place this attribute on members emitted by their profiler into framework assemblies, if they want to access them under WinRT.

It is undocumented, but it looks like one of the optimizations in .NET 4.5. It appears to be used to prime the reflection type info cache, making subsequent reflection code on common framework types run faster. There's a comment about it in the Reference Source for System.Reflection.Assembly.cs, RuntimeAssembly.Flags property:

 // Each blessed API will be annotated with a "__DynamicallyInvokableAttribute".
 // This "__DynamicallyInvokableAttribute" is a type defined in its own assembly.
 // So the ctor is always a MethodDef and the type a TypeDef.
 // We cache this ctor MethodDef token for faster custom attribute lookup.
 // If this attribute type doesn't exist in the assembly, it means the assembly
 // doesn't contain any blessed APIs.
 Type invocableAttribute = GetType("__DynamicallyInvokableAttribute", false);
 if (invocableAttribute != null)
 {
     Contract.Assert(((MetadataToken)invocableAttribute.MetadataToken).IsTypeDef);

     ConstructorInfo ctor = invocableAttribute.GetConstructor(Type.EmptyTypes);
     Contract.Assert(ctor != null);

     int token = ctor.MetadataToken;
     Contract.Assert(((MetadataToken)token).IsMethodDef);

     flags |= (ASSEMBLY_FLAGS)token & ASSEMBLY_FLAGS.ASSEMBLY_FLAGS_TOKEN_MASK;
 }

Without further hints what a "blessed API" might mean. Although it is clear from the context that this will only work on types in the framework itself. There ought to be additional code somewhere that checks the attribute applied to types and methods. No idea where that is located, but given that it would have to need to have a view of all .NET types to have a shot at caching, I can only think of Ngen.exe.