Patching LLVM and rustc

This post is just an aside; As I mentioned in my last post, there is a compiler bug in LLVM that is resulting in incorrect code being generated for AVR with Rust. I made some vague assertions in a footnote about “just follow the directions and you’ll be fine” because I couldn’t remember how to do it, and then I ended up having to figure out how to rebuild my toolchain today. So I’m mostly posting these instructions for my own benefit, but maybe someone else will find them useful too.

1. Download the patch file
2. If this is your first time building rustc, run git clone https://github.com/rust-lang/rust; otherwise, go into your checked-out version of Rust and git pull.
3. git submodule update --init --recursive
4. cd src/llvm-project
5. git apply patch.diff, where patch.diff is the file you downloaded in step 1.
6. mkdir build && cd build
7. cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release ../llvm
8. cmake --build . -- -j24 (or whatever number of build processes you want to run in parallel instead of 24)
9. cd ../../..
10. ./x.py build --stage 2 library/core library/proc_macro
11. rustup toolchain link stage2 build/x86_64-unknown-linux-gnu/stage2

And then you’re done! In your project directory, run rustup override set stage2 to use the compiler you just built.