x86-32 float return for 'Rust' ABI: treat all float types consistently

[RalfJung]

This helps with #131819: for our own ABI on x86-32, we want to never use the float registers. The previous logic only considered F32 and F64, but skipped F16 and F128. So I made the logic just apply to all float types.

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[RalfJung]

Cc @tgross35

[workingjubilee]

Huh. I hadn't noticed a bunch of target-specific code has escaped rustc_target.

That will be fixed.

[workingjubilee]

@RalfJung just to confirm: This winds up applying to both i586 and i686, yes?

[RalfJung]

This winds up applying to both i586 and i686, yes?

I assume so. I didn't change that part of the logic.

[RalfJung]

Huh. I hadn't noticed a bunch of target-specific code has escaped rustc_target.

Yeah, rustc_target does the non-Rust-ABI handling, but at some point we had to start making Rust ABI handling target-specific...

[tgross35]

👍 for making this change

[beetrees]

Suggested change

	Abi::Scalar(s) => matches!(s.primitive(), Float(_)),
	Abi::ScalarPair(s1, s2) => {
	matches!(s1.primitive(), Float(_)) || matches!(s2.primitive(), Float(_))
	Abi::Scalar(s) => matches!(s.primitive(), Float(F16 | F32 | F64)),
	Abi::ScalarPair(s1, s2) => {
	matches!(s1.primitive(), Float(F16 | F32 | F64)) || matches!(s2.primitive(), Float(F16 | F32 | F64))

nit: There's no need to include f128 here as the 32-bit x86 ABI already guarantees it gets passed in memory.

[RalfJung]

IMO we should treat all float types uniformly here, that's more future-proof. The result is the same both ways but my code makes it more clear what happens, relying on fewer external assumptions.

We don't care about being consistent with the C ABI here, after all.

[workingjubilee]

Yeah, I kinda agree morally with beetrees but not actually: this seems fine as-is.

I don't think it's worth spending too much time on this. While I do need to be able to read this code to refactor it, I do intend to bulldoze it.

[workingjubilee]

While I do need to be able to read this code to refactor it, I do intend to bulldoze it.

And since this seems fine-ish, assuming CI doesn't find a reason to take issue with it, and I would hate to make RalfJung rebase this a bunch:

@bors r+

[bors]

📌 Commit a9f6fd1 has been approved by workingjubilee

It is now in the queue for this repository.