Autodiff Upstreaming - rustc_codegen_llvm changes

[ZuseZ4]

Now that the autodiff/Enzyme backend is merged, this is an upstream PR for the rustc_codegen_llvm changes.
It also includes small changes to three files under compiler/rustc_ast, which overlap with my frontend PR.
Here I only include minimal definitions of structs and enums to be able to build this backend code.
The same goes for minimal changes to compiler/rustc_codegen_ssa, the majority of changes there will be in another PR, once either this or the frontend gets merged.

We currently have 68 files left to merge, 19 in the frontend PR, 21 (+3 from the frontend) in this PR, and then ~30 in the middle-end.

To already highlight the things which reviewers might want to discuss:

1. enzyme_ffi.rs: I do have a fallback module to make sure that we don't link rustc against Enzyme when we build rustc without autodiff support.

2. add_panic_msg_to_global was a pain to write and I currently can't even use it. Enzyme writes gradients into shadow memory. Pass in one float scalar? We'll allocate and return an extra float telling you how this float affected the output. Pass in a slice of floats? We'll let you allocate the vector and pass in a mutable reference to a float slice, we'll then write the gradient into that slice. It should be at least as large as your original slice, so we check that and panic if not. Currently we panic silently, but I already generate a nicer panic message with this function. I just don't know how to print it to the user. yet. I discussed this with a few rustc devs and the best we could come up with (for now), was to look for mangled panic calls in the IR and pick one, which works surprising reliably. If someone knows a good way to clean this up and print the panic message I'm all in, otherwise I can remove the code that writes the nicer panic message and keep the silent panic, since it's enough for soundness. Especially since this PR is already a bit larger.

3. SanitizeHWAddress: When differentiating C++, Enzyme can use TBAA to "understand" enums/unions, but for Rust we don't have this information. LLVM might to speculative loads which (without TBAA) confuse Enzyme, so we disable those with this attribute. This attribute is only set during the first opt run before Enzyme differentiates code. We then remove it again once we are done with autodiff and run the opt pipeline a second time. Since enums are everywhere in Rust, support for them is crucial, but if this looks too cursed I can remove these ~100 lines and keep them in my fork for now, we can then discuss them separately to make this PR simpler?

4. Duplicated llvm-opt runs: Differentiating already optimized code (and being able to do additional optimizations on the fly, e.g. for GPU code) is the reason why Enzyme is so fast, so the compile time is acceptable for autodiff users: https://enzyme.mit.edu/talks/Publications/ (There are also algorithmic issues in Enzyme core which are more serious than running opt twice).

5. I assume that if we merge these minimal cg_ssa changes here already, I also need to fix the other backends (GCC and cliff) to have dummy implementations, correct?

6. I'm happy to split this PR up further if reviewers have recommendations on how to.

For the full implementation, see: rust-lang#129175

Tracking:

• Tracking Issue for autodiff rust-lang/rust#124509