[llvm-dev] Opaque Pointers Help Wanted

[Kaylor, Andrew via llvm-dev]

Hi Nikita,

We saw the elementtype attribute changes and have been working on some patches to make use of it. This is what led us to the change in https://reviews.llvm.org/D108796, which I plan to commit for Chenyang tomorrow. Apart from that minor problem, it looks like it will work very well for us. Thanks!

-Andy

From: Nikita Popov <nikita.ppv at gmail.com>
Sent: Saturday, September 04, 2021 1:39 AM
To: Kaylor, Andrew <andrew.kaylor at intel.com>
Cc: David Blaikie <dblaikie at gmail.com>; craig.topper at gmail.com; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Opaque Pointers Help Wanted

On Tue, Jun 22, 2021 at 6:16 PM Kaylor, Andrew via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Honestly, I didn’t (and still don’t) understand enough about the intrinsics Craig mentioned to even be sure they were analogous to the case I brought up. It seemed like they might be more interested in debug types than IR types and may or may not be going down the same paths of reasoning, but I just don’t understand what the intrinsics are doing.

We’re scrambling to get our out-of-tree code caught up with the opaque pointer transition. I think the scatter/gather intrinsics are likely to be our first useful overlap with the in-tree work to be done, though now that I’ve thought about it a bit more, I think the type in that can just be inferred from non-pointer arguments. In any case, I guess talking about it and agreeing on an approach is a good first step.

I started trying to mock up some IR for how this might work, but I wasn’t entirely happy with it. Specifically, how do you describe an IR type in metadata. We could use an undef value in the metadata, similar to what Arthur proposed as an argument, but that only moves the ugliness. This is kind of what I was thinking


define void @f() {
  ptr %p = <however we got %p>
  call void @llvm.some.intrinsic(ptr %p, metadata !1)
  <…>
  return void
}

define void @llvm.some.intrinsic(ptr, metadata)

%MyElementTy = type { <whatever> }

!1 = !{ %MyElementTy undef }


I guess that would work, but I don’t really like it. I looked at the way TBAA and debug info describe types, but that’s not describing the IR type and so it seems simultaneously fragile and too verbose. Is there a better way to do this? What about introducing a parameter attribute that would be considered required for the intrinsic? Something like this:


define void @f() {
  ptr %p = <however we got %p>
  call void @llvm.some.intrinsic(ptr elementtype(%MyElementTy) %p)
  <…>
  return void
}

; Requires ‘elementype’ parameter attribute
define void @llvm.some.intrinsic(ptr)

%MyElementTy = type { <whatever> }


Does that feel like a step backward to you? I don’t know how hard it would be for front end’s to generate something like that, but it hasn’t the benefit that the elementtype would have intentional and well-defined semantic meaning.

Thanks,
Andy

For the record, we did end up adding the elementtype() attribute for use by intrinsics in https://reviews.llvm.org/D105407 / https://reviews.llvm.org/D106008 and a sample usage for BPF intrinsics in https://reviews.llvm.org/D106184.

Would be great if you can confirm whether this works for your use-case as well!

Regards,
Nikita
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