Expose experimental LLVM features for automatic differentiation and GPU offloading

[nikomatsakis]

Metadata
Owner(s)	@ZuseZ4
Team(s)	compiler, lang
Goal document	2024h2/Rust-for-SciComp

Summary

Expose experimental LLVM features for automatic differentiation and GPU offloading.

Tasks and status

• Development (@ZuseZ4)
  • upstream the autodiff frontend
  • upstream the autodiff backend
  • upstream the autodiff middle-end
  • upstream the offload frontend
  • enable the offload backend
  • upstream the offload middle-end
• Lang-team experiment (lang )
• Standard reviews (compiler )

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[ZuseZ4]

During the first month, I focused on automatic differentiation. I cleaned up my rustc fork and made my first two upstreaming PRs for the frontend and backend. Once they are merged, I will continue with posting PRs for the remaining middle-end. While waiting for reviews, I have been improving the docs a bit, mainly the pages about debugging Enzyme crashes. I am especially proud that due to those docs we recently got our first enzyme core issue with a full LLVM-IR reproducer from a Rust dev, even though the developer reporting that issue had no previous compiler/LLVM experience. Such detailed issues make fixing bugs for Enzyme core much easier.

On the GPU side, I mainly have to thank nikic, who reliably updates the LLVM backend of Rustc every few weeks or months. Thanks to his latest update rustc now supports a sufficiently new LLVM which ships most of the GPU/Offloading work that I want to expose on the Rust side. Once my first two Autodiff patches have settled, I'll look a bit more into setting up documentation for the GPU feature.

[ZuseZ4]

During the last three weeks, my first autodiff PR for the backend, which includes the enzyme submodule and 13 additional files, got merged! I also got a ton of feedback from reviewers, especially for my frontend PR (thanks to jieyouxu). Now that the backend is merged, I did put up my third PR, covering the changes I made to rustc_codegen_llvm. I am currently at RustConf, so I won't be able to address much of the feedback this week, but I am happy to talk to everyone also visiting and will try to get both PRs ready to merge in the next week.
Once we then have the two open PRs merged, we should have my changes to ~55/85 files upstream, so we're making good progress.

On the GPU side again not many updates due to my current autodiff focus, but thanks to another llvm submodule update we can now use some nicer APIs for our development in rustc, which recently got merged into LLVM.

[ZuseZ4]

And as another short update, my talk "When unsafe code is slow - Automatic Differentiation in Rust" got accepted as a techtalk for the LLVM dev mtg. There I'll present a lot of benchmarks and some analysis comparing Rust-Enzyme with the C++ frontend of Enzyme, and show one application which we had to port from Python/JAX to Rust/Enzyme.
The full program of the dev meeting is available here.
For that, I spent some time trying to fix the benchmark infrastructure in Enzyme core, to make sure everyone can reproduce our benchmarks.

[ZuseZ4]

Thanks to some support from the bootstrap team, dist builds with autodiff support enabled now work.
That allowed us to add Rust to our autodiff fork of the compiler explorer: https://enzyme.mit.edu/explorer/
Unfortunately, we still have some dist issues about finding std in the compiler explorer build, so help here would be appreciated.
Other than that, this morning my PR to add Enzyme/autodiff support to the test infra got merged: rust-lang/rust#131044
This should allow to add this larger frontend PR to the merge queue later today: rust-lang/rust#129458

[ZuseZ4]

I've been travelling a lot for the last two weeks, but hope to be able to get back to work next Monday. Since the last update we got:

1. The Autodiff frontend got merged! This included over 2k LoC and 30 files, so the remaining diff is now much smaller.
2. The Autodiff middle-end as the last missing AD piece is probably getting a re-design. Right now we use Enzyme as a library, which means that we must write ffi wrappers around Enzyme's C/C++ functions and have to differentiate functions one by one. If we switch over to an LLVM pass-based approach instead we can drop a lot of glue code (simplifying the review process) and can get us some features for free, which the pass already handles for us (i.e. differentiate higher-order derivatives in the right order). Julia also just moved over from the library to the pass-based approach. C/C++ always used the pass-based approach which in the past had a few limitations that recently got fixed. Finally, a pass-based approach has reproducibility improvements, since now all information will be in the llvm-ir. As summary, this seems like a good moment to also move Rust over.
3. I opened a tracking issue for the GPU offload feature and made the first PR to enable LLVM's offload feature.
4. I started working with some Enzyme and Bootstrap contributors to get a compiler explorer instance with Rust-AD to work.
5. I am giving one tech talk and two workshop talks at the LLVM Dev Conference, I will share the slides (and videos if possible) afterwards. The three talks are about ML in Rust, GPU-Programming in Rust, and the performance benefits of safe over unsafe code.

Help Wanted:
I would appreciate if someone could look into fixing our Rust dist build used in the Enzyme Compiler explorer. I have spend quite a few hours trying different configurations, but have been unable to get rid of the error

error[E0463]: can't find crate for `std`

Any help would be appreciated, I can share more information if someone has time to investigate further.