aarch64 softfloat target: always pass floats in int registers

[RalfJung]

This is a part of #131058: on softfloat aarch64 targets, the float registers may be unavailable. And yet, LLVM will happily use them to pass float types if the corresponding target features are enabled. That's a problem as it means enabling/disabling neon instructions can change the ABI.

Other targets have a soft-float target feature that forces the use of the soft-float ABI no matter whether float registers are enabled or not; aarch64 has nothing like that.

So we follow the aarch64 softfloat ABI and treat floats like integers for extern "C" functions. For the "Rust" ABI, we do the same for scalars, and then just do something reasonable for ScalarPair that avoids the pointer indirection.

Cc @workingjubilee

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[RalfJung]

Ah, we also have to do something for ScalarPair types. The easiest thing to do is pass them indirectly, so that's what I did for now.

[Nadrieril]

r? compiler

[workingjubilee]

Isn't this the second target we are manufacturing a by-integer ABI for?

[RalfJung]

Is it? I am not aware of another.

[workingjubilee]

x87 Rust ABI?

[RalfJung]

There we are returning floats by-ptr, I think? Ah maybe we only do that for floats larger than a ptr.

[workingjubilee]

I am mostly musing that I should probably cross-check those cases to see if there's something that can be factored out before accepting this.

[RalfJung]

For x86-32 we only do something with the return value, since arguments anyway never use the x87 stack.

But yeah we could probably use the same logic for both. I am just not sure what is the best way to share that logic.

[RalfJung]

OTOH we might want to change the x87 thing to use SSE registers if we can, so sharing the code might not be such a good idea.

[wesleywiser]

Overall looks good to me, just a few comments.

[davidtwco]

I think this makes sense

[RalfJung]

@bors r=davidtwco,wesleywiser

[bors]

📌 Commit 666bcbd has been approved by davidtwco,wesleywiser

It is now in the queue for this repository.