[clang] [llvm] [ARM] Emit an error when the hard-float ABI is enabled but can't be used. (PR #111334)

[Ralf Jung via llvm-commits]

RalfJung wrote:

As a Rust compiler dev, this part of LLVM feels extremely fragile. If we do anything wrong, LLVM will just use *some* ABI, and things will keep working for some situations, but really we have subtle ABI bugs that will eventually bite our users. The rules we have to follow are not documented anywhere (to my knowledge), and require target-specific expertise -- even if we get the x86 part right, we'll need a PowerPC expert to also get soft-float handling right for that target.

It's not a pleasant experience at all. Having LLVM be more robust, and throw errors rather than being like "yeah let's just continue, I'm sure it's going to be fine", would go a long way towards making LLVM easier to use and more reliable as a backend. It is concerning if attempts at improving the robustness of LLVM in this space are being blocked.

https://github.com/llvm/llvm-project/pull/111334