Tracking Issue for batching

This is a tracking issue for the batching feature of llvm/enzyme.
The feature gate for the issue is #![feature(batching)].

This is the second out of three features that were approved as experiments in a project goal. It allows merging N function calls (e.g. from looping over a call) into a single call, with N being a compile-time constant. This allows better vectorization and other optimizations.

We reuse almost all of the infrastructure from the autodiff macro, with a slightly different syntax both in the user facing macro, as well as the llvm-ir we generate. We intend to later extend the autodiff macro to optionally include the batching feature, however some users might want to use batching without automatic differentiation, which is the motivation for this independent batch macro. It also seems wise to experiment with both independently before trying to merge them.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Get compiler MCP approved.
  • https://github.com/rust-lang/compiler-team/issues/611
• Get lang experiment approved.
  • We approved this in the lang triage meeting on 2024-05-01.
• Land the experimental implementation in nightly.
  • Combined change for reference (wip link): https://github.com/EnzymeAD/rust/pull/192
• Accept an RFC.
• Add documentation to the dev guide.
  • See the instructions.
• Add documentation to the reference.
  • See the instructions.
• Add formatting for new syntax to the style guide.
  • See the nightly style procedure.

Unresolved Questions

XXX --- list all the "unresolved questions" found in the RFC to ensure they are
not forgotten

Implementation history

Start with the batching feature description and the linked Enzyme implementation PR. The remaining checklist points to the stabilization guide, compiler dev guide, Rust reference, and style guide; read their contribution instructions before choosing a documentation or formatting task. Done means the relevant checklist item is completed and the batching feature is documented or formatted as required.