Runtime: add UnknownObject on non-ObjC builds

[compnerd]

The UnknownObject type is used for the block convention block pointers.
This is used in libdispatch and we cannot build for Windows without it.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

CC: @jckarter @mikeash @gparker42

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 5ee3566

[jckarter]

This shouldn't be necessary. How does libdispatch work on Linux? You may need the value witness table, but the type should not have to exist.

[compnerd]

@jckarter pretty sure that it is getting away since not all symbols need to be fulfilled by default on ELF (-z defs is not the default).

[troughton]

FWIW, when I built the Swift Dispatch overlays on Windows against DrPizza's libdispatch port I didn't need to make any compiler changes, although I did need to enable support for and link in the Blocks runtime to the libdispatch library. I'm not sure what's different to cause you to need changes here.

[compnerd]

@troughton this is specifically the SDK overlay which is not part of that port I believe.

@jckarter btw, this is a smaller reduced case:

class C {
  var _block: @convention(block) () -> Void
  init(block: @escaping @convention(block) () -> Void) {
    self._block = block
  }
}

You're correct in that Linux for some reason does not generate that reference! The reference is generated in the type metadata completion function for C. I don't see that being generated on Linux for some reason?

[troughton]

If it helps to narrow down the problem:

By the Dispatch overlays I mean these files, along with the C stubs reimplemented in DispatchStubs.cc.

When linked against the Windows SDK and built for x64 Windows this did not require any compiler changes for a Swift 4.1-era built-from-master toolchain.

[compnerd]

@jckarter I wonder if this is a result of the lazy initialized metadata ... which may now emit the completion function for the initialization?

[compnerd]

Okay, verified, this is the result of the lazy initialization of the type metadata. This in fact would be needed on ELF if we moved to the lazy initialization of the type metadata.

[jckarter]

@compnerd Are you sure it's not only looking for the value witness table? We really want to eliminate the UnknownObject builtin type even for Darwin targets.

[compnerd]

@jckarter yes, it is looking for the VWT. The changes here enable the emission of the metadata into the KnownMetadata.cpp.

[jckarter]

I see. It would be nice to only include the value witness table, but that's splitting hairs a bit.

[compnerd]

@swift-ci please test macOS platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 5ee3566

[compnerd]

@swift-ci please clean test macOS platform