NEON intrinsics are broken on big-endian

[Amanieu]

These are currently broken because the order of elements inside vectors is reversed on big-endian systems: the ARM ABI requires that element 0 is located at the highest address of the vector type. However LLVM intrinsics expect element 0 to be located at the lowest address.

See https://llvm.org/docs/BigEndianNEON.html and arm_neon.h in Clang for more details.

[RalfJung]

the ARM ABI requires that element 0 is located at the highest address of the vector type. However LLVM intrinsics expect element 0 to be located at the lowest address.

What exactly does this mean? Is there a bug in LLVM? If so, where is it tracked?

Or is the problem that Rust stdarch wants to expose the intrinsics the way they work on hardware, but LLVM doesn't provide those semantics? If so, could that be fixed by doing appropriate translation of indices before calling the intrinsics?

[Amanieu]

The short answer is that, on big-endian, LLVM portable vectors have a different element ordering than the one in the vector types used by the NEON intrinsics.

The C intrinsics work around this by reversing the element ordering in vectors before & after each intrinsic. We need to do the same in stdarch.

[RalfJung]

Oh I see, so this is a mismatch about the simd_x intrinsics vs vendor-specific intrinsics? Okay makes sense.

OTOH this is good news for portable-simd, seems like there we'll be getting consistent behavior across platforms without extra work then.

[workingjubilee]

@he32

I currently appear unable to find the actual connecting tissue between library/stdarch/crates/core_arch/src/aarch64/neon/mod.rs and LLVM

The actual place that intrinsics themselves are handled is in two places: if it's an architecture-specific intrinsic, it uses link_llvm_intrinsics, which effectively specifies a lowering directly to LLVM textual IR. Otherwise, if it's one of rustc's "portable" intrinsics (simd_add and the like), the primary definition is in rustc_codegen_llvm: https://github.com/rust-lang/rust/blob/master/compiler/rustc_codegen_llvm/src/intrinsic.rs