disable f16 tests for the cranelift backend

[folkertdev]

fixes #519

@bjorn3 does this look like it would work?

[bjorn3]

It still fails: https://github.com/rust-lang/rustc_codegen_cranelift/actions/runs/24515088390/job/71656650354

[folkertdev]

So actually it looks like the failure is in proptest? And there we certainly don't want to cfg on f16 math given that we want to stabilize f16 soon.

The libc implementation seems doable to implement, actually?

https://github.com/llvm/llvm-project/blob/b96818f2e0118fcbf908b0a168ca1d23190ac295/libc/src/__support/FPUtil/NearestIntegerOperations.h#L54-L105

[bjorn3]

And there we certainly don't want to cfg on f16 math given that we want to stabilize f16 soon.

There are still a bunch of cases where the LLVM backend also says it doesn't support f16 math. So either way cfg(target_has_reliable_f16_math) is necessary to make it possible to test portable-simd on those targets at all.

[folkertdev]

I believe that is because we still support older LLVM versions?

https://github.com/rust-lang/rust/blob/e8e4541ff19649d95afab52fdde2c2eaa6829965/compiler/rustc_codegen_llvm/src/llvm_util.rs#L373-L389

With LLVM 22 f16 should work everywhere as far as I know. Unless we're running into this case here

        (Arch::X86_64, Os::Windows) if *target_env == Env::Gnu && *target_abi != CfgAbi::Llvm => {

[bjorn3]

To unblock the cg_clif rustc update, I have for now added a patch that remove all f16 support in portable-simd again.

[tgross35]

I'd drop this comment, it's not a cranelift-specific issue and the target_has_reliable_f16_math is effectively already a fixme.

View changes since the review

[tgross35]

At this time I'd still consider it more or less a requirement to gate anything that may codegen behind cfg(target_has_reliable_f16). We still have these edge cases and they're kind of bootstrapped by the fact that the cfg is used pretty consistently (out of necessity) like here. It will be at least ~4 months until our minimum LLVM version is high enough to drop the cfg completely.