Support `f16` and `f128`

[tgross35]

Thanks for filing a feature request! Please fill out the TODOs below.

Feature

Add support for 16-bit and 128-bit IEEE floating point numbers.

Benefit

This will allow using these types with rustc_codegen_clif, as described at https://github.com/rust-lang/rustc_codegen_cranelift/issues/1461

Implementation

Unsure

Alternatives

Unsure, maybe it would be possible to stub out these types if adding them is difficult.

[jameysharp]

Adding the types is pretty easy, I think, and extending CLIF floating-point instructions to allow the new types is also pretty easy. Successfully compiling anything that uses the types is the hard part, I imagine.

I think the places to add the types are:

• https://github.com/bytecodealliance/wasmtime/blob/e6f9ca5efa0c4bf59c9cf133d9abeaab63ec07bb/cranelift/codegen/meta/src/cdsl/typevar.rs#L646
• https://github.com/bytecodealliance/wasmtime/blob/e6f9ca5efa0c4bf59c9cf133d9abeaab63ec07bb/cranelift/codegen/meta/src/shared/types.rs#L44-L73
• various places in https://github.com/bytecodealliance/wasmtime/blob/e6f9ca5efa0c4bf59c9cf133d9abeaab63ec07bb/cranelift/codegen/src/ir/types.rs

Once the tests are passing again after that, I think you'll have a compiler that will accept f16/f128 instructions. But if you use them, then it will fail later: during lowering on all backends; in the interpreter; in compiletests and runtests; and in fuzzing.

I think this is a fairly large project, but if rustc needs these types, I think it's reasonable to add the types in one PR and then work on adding implementations for them everywhere over time.