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

[Peter Smith via cfe-commits]

https://github.com/smithp35 commented:

Apologies for the delay in responding.

I don't have much to add over davemgreen and ostannard.

Leaving aside whether this should be an error in the front or back-end. As mentioned before I haven't worked in this area for a long time, and there are enough test changes here that make me nervous enough that this is going to break some existing use cases. Some do look like sloppy triple selection, although a lot of the tests don't use any floating point so it could be seen as harsh to fault them. If I understood the area better that wouldn't be a problem, but as I'm not enthusiastic about moving a diagnostic to the backend, this isn't a change that I am best placed to approve. I won't object if there's a consensus in the other direction.

One use case I'd want to keep working is where we have two functions in the same object that are hardfp with hardware registers, and soft (with or without hardware registers). Something like:
```
__attribute__((target("arch=cortex-m33"))) __attribute__((pcs("aapcs-vfp"))) float f1(float a, float b) {
  return a + b;
}

__attribute__((target("arch=cortex-m0"))) __attribute__((pcs("aapcs"))) float f2(float a, float b) {
  return a + b;
}
```
There are some source files that do run-time selection based on hardware detection.



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