Determine if building llvm is needed

[LegNeato]

We either build llvm or use https://github.com/Rust-GPU/rustc_codegen_nvvm-llvm. This was from before a time when llvm was available via rustup. Rust-GPU uses llvm tools: https://github.com/Rust-GPU/rust-gpu/blob/6e2c84d4fe64e32df4c060c5a7f3e35a32e45421/crates/rustc_codegen_spirv/build.rs#L19.

I'm not sure if we need this for NVVM or if the rust llvm components make this unnecessary. We should investigate.

[Schmiedium]

Are we able to get llvm version 7 from llvm-tools? From the NVIDIA Docs, it looks there two dialects for nvvm, one based on llvm-7.0.1, and a newer one based on llvm-18.1.8. I did have trouble trying to use alternate versions since the includes from llvm change between versions and the rustc_llvm_wrapper from within the codegen breaks with different llvm versions, so that would have to maintained.

https://docs.nvidia.com/cuda/nvvm-ir-spec/index.html

Switching to the newer dialect may be an issue since it's only supported on Blackwell architecture and later, so the most recent releases from NVIDIA.

I'm not sure what the compatibility is between versions of llvm, like if version 18 can produce IR that NVCC would still accept as valid for older cards. I think that's another question to answer

Just some of my thoughts from messing around with various versions of llvm

[LegNeato]

Yeah, I don't think we can only support Blackwell+. I'm also not familiar which past versions of llvm are available to rustc and how it interfaces. It would be nice to pull down a version via rustup, even if we have to write our own glue code.

[jorge-ortega]

It seems to me that rustc llvm-tools component only provides binaries for working with IR like llc, llvm-ar, opt, and friends. It does not include headers or libraries needed by the backend to generate IR. I'm actually not sure if the component is required at all today.

The llvm wrapper in rustc tries to support multiple versions of LLVM, and ours is mostly a copy of that, even though we only need to support for 7 or 18 (whenever we get around to that). LLVMs migration to opaque pointers makes versions 17 or greater incompatible with 7. Trying to find a version in the middle that still supports typed pointers AND still compatible with 7 would unlikely be worth the effort.

[LegNeato]

@jorge-ortega makes sense!