ABI checks: add support for loongarch

[heiher]

LoongArch psABI¹ specifies that LSX vector types are passed via general-purpose registers, while LASX vector types are passed indirectly through the stack.

This patch addresses the following warnings:

warning: this function call uses a SIMD vector type that is not currently supported with the chosen ABI
    --> .../library/core/src/../../stdarch/crates/core_arch/src/loongarch64/lsx/generated.rs:3695:5
     |
3695 |     __lsx_vreplgr2vr_b(a)
     |     ^^^^^^^^^^^^^^^^^^^^^ function called here
     |
     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     = note: for more information, see issue #116558 <https://github.com/rust-lang/rust/issues/116558>
     = note: `#[warn(abi_unsupported_vector_types)]` on by default

r? @workingjubilee

Footnotes

1. https://github.com/loongson/la-abi-specs/blob/release/lapcs.adoc ↩

[workingjubilee]

lsx? lasx? oh dear.

[heiher]

lsx? lasx? oh dear.

LSX is a 128-bit vector extension, and LASX is a 256-bit vector extension. Parameters and return values for neither of these data types are passed through vector registers. 😃

[workingjubilee]

Then why is the lint triggering?

[workingjubilee]

Here is the uses_vector_registers check.

It looks fairly dubious to me. https://github.com/rust-lang/rust/blame/bfe809d93c67de03e3a84b80549927fd828ec5d0/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs#L43

[workingjubilee]

This is not the correct fix, the correct fix is to fix the lint itself.

[heiher]

Here is the uses_vector_registers check.

It looks fairly dubious to me. https://github.com/rust-lang/rust/blame/bfe809d93c67de03e3a84b80549927fd828ec5d0/compiler/rustc_monomorphize/src/mono_checks/abi_check.rs#L43

I see it. But I'm not sure how to make it more accurate.

Here's a case:

#![feature(stdarch_loongarch)]
use std::arch::loongarch64::*;

pub unsafe fn simd(s: i32) -> u32 {
    lsx_vpickve2gr_bu::<0>(lsx_vreplgr2vr_b(s))
}

and the PassMode is

Direct(ArgAttributes { regular: , arg_ext: None, pointee_size: Size(0 bytes), pointee_align: None })

BackendRepr is

Vector { element: Initialized { value: Int(I8, true), valid_range: 0..=255 }, count: 16 }

[heiher]

Is it true that the Rust frontend cannot determine whether a parameter is passed through a vector register or a general-purpose register based solely on PassMode and BackendRepr? @veluca93 This decision is made by the LLVM target backend. If so, adding ABI checks to LoongArch is also a valid solution, and at least harmless.

[workingjubilee]

My understanding is the frontend technically cannot precisely control it, no.