Initial implementation of a @_cdecl attribute to export top-level functions to C

[jckarter]

There's an immediate need for this in the core libs, and we have most of the necessary pieces on hand to make it easy to implement. This is an unpolished initial implementation, with the following limitations, among others:

• It doesn't support bridging error conventions,
• It relies on ObjC interop,
• It doesn't check for symbol name collisions,
• It has an underscored name with required symbol name @cdecl("symbol_name"), awaiting official bikeshed painting.

[parkera]

Thanks so much for looking into this. I just wanted to point out a few requirements we have for this in the core libraries area, for your future work in this area.

We currently (in Foundation) have three places where we need this:

• _NSCFConstantString class, which we need to know at link time so we can fix up references to constant strings. We currently alias the mangled name in our linker options to make this work.
• A function call to _CFSwiftGetBase class, which happens during __CFInitialize and returns the class we need to use as the base classes for CF types:

internal func __CFSwiftGetBaseClass() -> AnyObject.Type {
    return __NSCFType.self
}

We currently work around not having a cdecl like this:

#ifndef __CFSwiftGetBaseClass
#if TARGET_OS_LINUX
#define __CFSwiftGetBaseClass _TF10Foundation21__CFSwiftGetBaseClassFT_PMPs9AnyObject_
#elif TARGET_OS_MAC
#define __CFSwiftGetBaseClass _TF15SwiftFoundation21__CFSwiftGetBaseClassFT_PMPs9AnyObject_
#endif
#endif
extern uintptr_t __CFSwiftGetBaseClass();

uintptr_t NSCFType = __CFSwiftGetBaseClass();

• Another callout from __CFInitialize to internal func __CFInitializeSwift() { /* ... */ }.

As you can imagine I'd much rather avoid hard coding mangling into our source code...

Obviously we'll need to lift the requirement of ObjC interop for us to use this as well.

[gottesmm]

Joe, is this pull request still being used? (I am looking through old pull requests).

[jckarter]

@swift-ci Please test

[jckarter]

Updated and merged as 013aad1, so this doesn't bitrot if anyone wants to take up the mantle.