`PassMode` carries `ArgAttributes` only as `Opaque`; can we expose a typed mirror?

[nihalpasham]

This is more of a question, and if it makes sense, a proposal.

Motivation

cuda-oxide is a rustc codegen backend for NVIDIA GPUs; its GPU-side translation consumes rustc_public only. I'd like kernel parameters to carry the same LLVM parameter attributes rustc emits for the host (noalias, readonly, nonnull, align, dereferenceable).

Why: on a field-wise struct-copy kernel, adding noalias (and readonly) by hand to the emitted IR turned 4 x 32-bit loads + 4 x 32-bit stores into one 128-bit load (read-only cache path) + one 128-bit store: 1.33x faster for the streaming version, 4.89x faster for the cache-resident version.

Today I can see the pointer kind (RigidTy::Ref) and the layout, but not whether the pointee is Freeze / Unpin, so I cannot tell &u32 from &Cell<u32>. Re-deriving rustc's rules is not an option: they depend on those auto traits and the opt-level gate, and they change.

Two things I want to confirm before proposing anything:

1. Instance::fn_abi() returns a FnAbi whose modes are PassMode::Direct(Opaque), Pair(Opaque, Opaque) and Indirect { attrs: Opaque, .. }; the bridge wraps rustc_target::callconv::ArgAttributes with opaque(attr) (unstable/convert/stable/abi.rs:164). So the attribute flags are reachable only by parsing the Debug string.
2. I don't see a way to ask whether a type is Freeze or Unpin. Both are auto traits, so trait_impls cannot answer it either.

I see #58 listed "Expand PassMode attributes" as a task when the ABI module landed, and it was closed without it. Was that dropped for a reason, or just never picked up?

Proposal

Replace Opaque in Direct, Pair and Indirect with a typed mirror of rustc_target::callconv::ArgAttributes, reusing rustc_public's existing Size and Align:

pub struct ArgAttributes {
    pub regular: ArgAttribute,   // flags: NoAlias, ReadOnly, NonNull, NoFree, NoUndef,
                                 //        InReg, Writable, CapturesNone/Address/ReadOnly
    pub arg_ext: ArgExtension,   // None | Zext | Sext
    pub pointee_size: Size,
    pub pointee_align: Option<Align>,
}

Cast { cast: Opaque } can stay opaque. Before / after for fn f(a: &u32):

today   PassMode::Direct(Opaque("ArgAttributes { regular: NoAlias | NonNull | ReadOnly | NoFree | NoUndef, .. }"))
after   PassMode::Direct(ArgAttributes { regular: NoAlias | NonNull | ReadOnly | NoFree | NoUndef,
                                         arg_ext: None, pointee_size: 4 bytes, pointee_align: Some(4) })

Would this make sense?

[makai410]

Hi @nihalpasham , we have an open PR that‘s probably related to this: rust-lang/rust#159359, is that what you need?

[nihalpasham]

Thanks - read through rust-lang/rust#159359. It's adjacent: it types arg_ext, pointee_size and pointee_align, but leaves regular out on purpose, and regular is the part I need (NoAlias, ReadOnly, NonNull, NoFree, NoUndef).

I don't think those are LLVM-specific. They're decided in rustc_ty_utils/src/abi.rs from Freeze/Unpin/UnsafeUnpin and the scalar validity range, and ArgAttributes::eq_abi says InReg is the only regular flag that affects the call ABI. cg_gcc already reads NoAlias/NonNull out of regular too.

Exposing pointee_size without NoFree also looks like a trap: cg_llvm emits dereferenceable only when NoFree is set, and on main pointee_size is 4 for both &u32 and &Cell<u32>.

In my case the consumer is a codegen backend that emits LLVM IR and runs llc, so I want to copy rustc's finished per-argument decision onto kernel parameters rather than re-derive it.

@makai410 , @celinval - Would a small PR that only types ArgAttributes (all four fields, Cast stays opaque) be acceptable, or would you rather this go into #159359?

[celinval]

@nihalpasham Let me merge the original PR and we can have a follow up one to add the part missing. Would you be up to creating the follow up PR?

[nihalpasham]

Thanks @celinval, sure, I can take on the follow-up here. I see rust-lang/rust#159359 has landed.

I’ll follow up with typed access to the missing regular flags, following the existing ArgAttributes API, and clarify how they should be interpreted alongside pointee_size.

P.S. this will be my first PR to rust-lang/rust, so I’d appreciate some guidance along the way to make sure I get everything right.

[makai410]

I can review, feel free to r? me whenever you have a pr then :)

[rustbot]

Error: r? is only allowed on PRs.

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #triagebot on Zulip.