Switch out asserts for llvm_unreachable

[airspeedswift]

#16237 introduced fall through warnings when building without asserts, this replaces them with llvm_unreachable

[airspeedswift]

@swift-ci please test

[davezarzycki]

The "problem" with llvm_unreachable is that debug builds can fail because not every library links against the requisite LLVM support library, even though they include this file (perhaps indirectly). Please test the debug build first because this failed for my configuration.

[airspeedswift]

We can't live with this number of asserts for very long – it's not just an annoyance, it also fills up CI logs. If a quick fix like this isn't enough, we'd probably have to revert the other patch.

edit: sorry, meant this number of warnings not asserts

[davezarzycki]

Let me make another attempt at fixing CMake files when building debug. Otherwise we'll need to use LLVM_BUILTIN_UNREACHABLE.

[davezarzycki]

Do I have permission to push to this branch if I fix the CMake files?

[airspeedswift]

sure, I think the right boxes are checked

[jrose-apple]

Just to be clear, please don't push to the branch directly. You should always be going through PR testing.

[davezarzycki]

So I have llvm_unreachable working, but I can't fix the remote mirror CMakeLists.txt "correctly". For some reason, LLVM_COMPONENT_DEPENDS is being ignored, but if I force add -lLLVMSupport to LINK_FLAGS, then the build works. What do you want to do?

[davezarzycki]

Oh wait, never mind, this seems intentional. TARGET_LIRARY disables stuff like LLVM_COMPONENT_DEPENDS. Time to try plan B: LLVM_BUILTIN_UNREACHABLE

[davezarzycki]

It looks like pushing to your branch isn't worth the trouble for a hot fix like this. I've testing #17772 now. That solves the accidental fall through warning and avoids using llvm_unreachable in stdlib code.

[airspeedswift]

@jrose-apple I think he meant push to my fork's PR branch – which GitHub should allow.

[davezarzycki]

Hi @airspeedswift – Yes, that is what I meant, but as I quickly discovered, that would have meant temporarily adding your repo/branch as a remote on my repo, and that doesn't seem to be worth the trouble for a hot fix.

[airspeedswift]

Closing as superseded by #17772

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - b284a80