Unblock Swift Calling Convention Mangling on Windows

[troughton]

Summary

Currently, when Clang is trying to mangle a function that uses the Swift calling convention, it crashes in an llvm_unreachable statement. This blocks the Swift calling convention from being used on Windows.

Following discussion on the swift-dev mailing list and on a pull request for swift-clang, unblock support for the Swift calling convention on targets using Microsoft mangling by mangling the name as per the __cdecl calling convention.

Also in this PR, replace llvm_unreachable with an error diagnostic since this code path can be hit.

[smeenai]

Patch is missing context.

lib/AST/MicrosoftMangle.cpp

2133

You still need the default label, right?

[troughton]

Add back a mistakenly removed default case.

[rjmccall]

Reid, David, do you have a recommendation about the right way to support non-MS-supported CCs here?

[rnk]

@compnerd's suggestion is still a decent one: https://reviews.llvm.org/D31372

Which, I think is just adding something wacky like @swiftcc@__Swift@@ which would demangle as __Swift::swiftcc if the demangler expected an NNS there. Of course, it doesn't, so it won't demangle, but at least you can overload between cdecl and swiftcc then.

lib/AST/MicrosoftMangle.cpp

2133

Surely we can emit a real custom diagnostic, similar to CodeGenModule::ErrorUnsupported.

[troughton]

Which, I think is just adding something wacky like @swiftcc@__Swift@@ which would demangle as __Swift::swiftcc if the demangler expected an NNS there. Of course, it doesn't, so it won't demangle, but at least you can overload between cdecl and swiftcc then.

Could we do a combination of these? Use the A prefix for the CC slot but then append the @swiftcc@__Swift@@ at the end of the qualified name? That way you could still distinguish a SwiftCC function and have it correctly demangle with existing tools.

As for emitting a proper diagnostic: we definitely can do something better here. I wasn't sure what to base it off so went with the simplest option, but given that CodeGenModule::ErrorUnsupported example I'll take a look and try to implement something similar.

lib/AST/MicrosoftMangle.cpp

2133

Oops, yes. Thanks. I've added it back.

[compnerd]

D42768 adds the __swift_cc in the right location and detangles correctly with undname as well.

lib/AST/MicrosoftMangle.cpp

   // them.)
 
   switch (CC) {
-    default:
-      llvm_unreachable("Unsupported CC for mangling");
+      llvm::errs() << "Unsupported CC for mangling: " << CC << ".\n";
     case CC_Win64:
     case CC_X86_64SysV:
+    // NOTE: SwiftCC should have its own mangling specifier.
+    // For now, don't do anything special and treat SwiftCC like __cdecl.
+    case CC_Swift:
     case CC_C: Out << 'A'; break;
     case CC_X86Pascal: Out << 'C'; break;
     case CC_X86ThisCall: Out << 'E'; break;