Well, yes, but I'm unsure why you think that the calling convention has anything to do with the mangled name. That is just a name after all.
The name above is the name used to link the functions. The calling convention says how such functions end up being called and is directly represented by the machine code the compiler generates. For example, in which registers arguments are passed, or whether they are passed on the stack and in which order, etc.
The C `__cdecl` traditionally just specifies the calling convention, not the link name which is specified by `extern C`. Looks like the Swift `@_cdecl` combines the two.
I think in contrast the `@_silgen_name` just specified the link name and not the calling convention. (the result is exactly the same like your example). This is precisely the issue with using `@_silgen_name` - you have to be lucky that calling conventions happen to be the same for simple setups, like `(Void)->Void` or `(OpaquePointer!)->Void` - which seems to be the case at the moment.
"P.S.: I'm unsure why anyone would think that this has anything to do with the Objective-C runtime. I guess it may if you pass any kind of ARC objects."
Because the comment on the commit that adds this to the swift source states that "It relies on ObjC interop".
