`simd_shuffle!` behaves strangely on `aarch64_be-unknown-linux-gnu`

[folkertdev]

I'm writing this up here so that we have a place to discuss it. Consider this program:

#![feature(portable_simd, core_intrinsics, repr_simd)]

use std::simd::Simd;

#[allow(unused)]
#[repr(simd)]
pub(crate) struct SimdShuffleIdx<const LEN: usize>(pub(crate) [u32; LEN]);

#[allow(unused)]
macro_rules! simd_shuffle {
    ($x:expr, $y:expr, $idx:expr $(,)?) => {{
        std::intrinsics::simd::simd_shuffle($x, $y, const { SimdShuffleIdx($idx) })
    }};
}

#[unsafe(no_mangle)]
extern "C" fn foo(elements: [i16; 8]) -> Simd<i16, 4> {
    unsafe {
        simd_shuffle!(
            Simd::from_array(elements),
            Simd::from_array([0i16; 8]),
            [0u32, 2, 4, 6]
        )
    }
}

#[unsafe(no_mangle)]
extern "C" fn bar(elements: [i16; 8]) -> Simd<i16, 4> {
    unsafe {
        simd_shuffle!(
            Simd::from_array(elements),
            Simd::from_array([0i16; 8]),
            [1u32, 3, 5, 7]
        )
    }
}

fn main() {
    let input = [0i16, 4, 1, 5, 2, 6, 3, 7];

    unsafe {
        dbg!(foo(input));
        dbg!(bar(input));
    }
}

The expected output here is

> cargo +nightly run --target aarch64-unknown-linux-gnu
warning: `playground` (bin "playground") generated 6 warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.18s
     Running `qemu-aarch64 -L /usr/aarch64-linux-gnu target/aarch64-unknown-linux-gnu/debug/playground`
[src/main.rs:42:9] foo(input) = [ 0, 1, 2, 3, ]
[src/main.rs:43:9] bar(input) = [ 4, 5, 6, 7, ]

but with aarch64_be-unknown-linux-gnu I get

> cargo +nightly run --target aarch64_be-unknown-linux-gnu -Zbuild-std=std,panic_abort
warning: `playground` (bin "playground") generated 6 warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.42s
     Running `qemu-aarch64_be -cpu max -L /home/folkertdev/Downloads/arm-gnu-toolchain-14.2.rel1-x86_64-aarch64_be-none-linux-gnu/aarch64_be-none-linux-gnu/libc target/aarch64_be-unknown-linux-gnu/debug/playground`
[src/main.rs:42:9] foo(input) = [ 4, 5, 6, 7, ]
[src/main.rs:43:9] bar(input) = [ 4, 5, 6, 7, ]

Miri does get it right

> cargo +nightly miri run --target aarch64_be-unknown-linux-gnu
[src/main.rs:42:9] foo(input) = [ 0, 1, 2, 3, ]
[src/main.rs:43:9] bar(input) = [ 4, 5, 6, 7, ]

It might be an LLVM issue, though the assembly for foo and bar is different.

foo:
	fmov d0, x1
	fmov d1, x0
	rev64 v0.4h, v0.4h
	rev64 v1.4h, v1.4h
	uzp1 v0.4h, v1.4h, v0.4h
	rev64 v0.4h, v0.4h
	ret

bar:
	fmov d0, x1
	fmov d1, x0
	rev64 v0.4h, v0.4h
	rev64 v1.4h, v1.4h
	uzp2 v0.4h, v1.4h, v0.4h
	rev64 v0.4h, v0.4h
	ret

So perhaps the actual problem is with qemu-aarch64_be? It's still present with a fresh build

> /home/folkertdev/c/qemu/build/qemu-aarch64_be --version
qemu-aarch64_be version 10.2.94 (v11.0.0-rc4)
Copyright (c) 2003-2026 Fabrice Bellard and the QEMU Project developers

Or maybe all of this is expected on aarch64_be-unknown-linux-gnu? but then miri would be wrong.

cc @adamgemmell @davidtwco