Updating LLVM

The Rust compiler uses LLVM as its primary codegen backend today, and naturally we want to at least occasionally update this dependency! Currently we do not have a strict policy about when to update LLVM or what it can be updated to, but a few guidelines are applied:

  • We try to always support the latest released version of LLVM
  • We try to support the "last few" versions of LLVM (how many is changing over time)
  • We allow moving to arbitrary commits during development.
  • Strongly prefer to upstream all patches to LLVM before including them in rustc.

This policy may change over time (or may actually start to exist as a formal policy!), but for now these are rough guidelines!

Why update LLVM?

There are two primary reasons nowadays that we want to update LLVM in one way or another:

  • First, a bug could have been fixed! Often we find bugs in the compiler and fix them upstream in LLVM. We'll want to pull fixes back to the compiler itself as they're merged upstream.

  • Second, a new feature may be available in LLVM that we want to use in rustc, but we don't want to wait for a full LLVM release to test it out.

Each of these reasons has a different strategy for updating LLVM, and we'll go over both in detail here.

Bugfix Updates

For updates of LLVM that typically just update a bug, we cherry-pick the bugfix to the branch we're already using. The steps for this are:

  1. Make sure the bugfix is in upstream LLVM.
  2. Identify the branch that rustc is currently using. The src/llvm-project submodule is always pinned to a branch of the rust-lang/llvm-project repository.
  3. Fork the rust-lang/llvm-project repository
  4. Check out the appropriate branch (typically named rustc/a.b-yyyy-mm-dd)
  5. Cherry-pick the upstream commit onto the branch
  6. Push this branch to your fork
  7. Send a Pull Request to rust-lang/llvm-project to the same branch as before
  8. Wait for the PR to be merged
  9. Send a PR to rust-lang/rust updating the src/llvm-project submodule with your bugfix
  10. Wait for PR to be merged

The tl;dr; is that we can cherry-pick bugfixes at any time and pull them back into the rust-lang/llvm-project branch that we're using, and getting it into the compiler is just updating the submodule via a PR!

Example PRs look like: #59089

Feature updates

Note that this is all information as applies to the current day in age. This process for updating LLVM changes with practically all LLVM updates, so this may be out of date!

Unlike bugfixes, updating to pick up a new feature of LLVM typically requires a lot more work. This is where we can't reasonably cherry-pick commits backwards so we need to do a full update. There's a lot of stuff to do here, so let's go through each in detail.

  1. Create a new branch in the rust-lang/llvm-project repository. This branch should be named rustc/a.b-yyyy-mm-dd where a.b is the current version number of LLVM in-tree at the time of the branch and the remaining part is today's date.

  2. Apply Rust-specific patches to the llvm-project repository. All features and bugfixes are upstream, but there's often some weird build-related patches that don't make sense to upstream which we have on our repositories. These patches are around the latest patches in the rust-lang/llvm-project branch that rustc is currently using.

  3. Update the compiler-rt submodule in the rust-lang-nursery/compiler-builtins repository. Push this update to the same branch name of the llvm-project submodule to the of the rust-lang/compiler-rt repository. Then push this update to a branch of compiler-builtins with the same-named branch. Note that this step is frequently optional since we may not need to update compiler-rt.

  4. Prepare a commit to rust-lang/rust

  • Update src/llvm-project
  • Update compiler-builtins crate in Cargo.lock (if necessary)
  1. Build your commit. Make sure you've committed the previous changes to ensure submodule updates aren't reverted. Some commands you should execute are:

    • ./x.py build src/llvm - test that LLVM still builds
    • ./x.py build src/tools/lld - same for LLD
    • ./x.py build - build the rest of rustc

    You'll likely need to update src/rustllvm/*.cpp to compile with updated LLVM bindings. Note that you should use #ifdef and such to ensure that the bindings still compile on older LLVM versions.

  2. Test for regressions across other platforms. LLVM often has at least one bug for non-tier-1 architectures, so it's good to do some more testing before sending this to bors! If you're low on resources you can send the PR as-is now to bors, though, and it'll get tested anyway.

    Ideally, build LLVM and test it on a few platforms:

    • Linux
    • OSX
    • Windows

    and afterwards run some docker containers that CI also does:

    • ./src/ci/docker/run.sh wasm32-unknown
    • ./src/ci/docker/run.sh arm-android
    • ./src/ci/docker/run.sh dist-various-1
    • ./src/ci/docker/run.sh dist-various-2
    • ./src/ci/docker/run.sh armhf-gnu
  3. Send a PR! Hopefully it's smooth sailing from here :).

For prior art, previous LLVM updates look like #55835 #47828

Caveats and gotchas

Ideally the above instructions are pretty smooth, but here's some caveats to keep in mind while going through them:

  • LLVM bugs are hard to find, don't hesitate to ask for help! Bisection is definitely your friend here (yes LLVM takes forever to build, yet bisection is still your friend)
  • Updating LLDB has some Rust-specific patches currently that aren't upstream. If you have difficulty @tromey can likely help out.
  • If you've got general questions, @alexcrichton can help you out.
  • Creating branches is a privileged operation on GitHub, so you'll need someone with write access to create the branches for you most likely.