Contributing to LLVM

Thank you for your interest in contributing to LLVM! There are multiple ways to contribute, and we appreciate all contributions. In case you have questions, you can either use the Forum or the #llvm channel on irc.oftc.net.

If you want to contribute code, please familiarize yourself with the LLVM Developer Policy.

Ways to Contribute

Bug Reports

If you are working with LLVM and run into a bug, we definitely want to know about it. Please let us know and follow the instructions in How to submit an LLVM bug report to create a bug report.

Bug Fixes

If you are interested in contributing code to LLVM, bugs labeled with the good first issue keyword in the bug tracker are a good way to get familiar with the code base. If you are interested in fixing a bug please comment on it to let people know you are working on it.

Then try to reproduce and fix the bug with upstream LLVM. Start by building LLVM from source as described in Getting Started with the LLVM System and use the built binaries to reproduce the failure described in the bug. Use a debug build (-DCMAKE_BUILD_TYPE=Debug) or a build with assertions (-DLLVM_ENABLE_ASSERTIONS=On, enabled for Debug builds).

Reporting a Security Issue

There is a separate process to submit security-related bugs, see How to report a security issue?.

Bigger Pieces of Work

In case you are interested in taking on a bigger piece of work, a list of interesting projects is maintained at the LLVM’s Open Projects page. In case you are interested in working on any of these projects, please post on the Forum, so that we know the project is being worked on.

How to Submit a Patch

Once you have a patch ready, it is time to submit it. The patch should:

  • include a small unit test
  • conform to the LLVM Coding Standards. You can use the clang-format-diff.py or git-clang-format tools to automatically format your patch properly.
  • not contain any unrelated changes
  • be an isolated change. Independent changes should be submitted as separate patches as this makes reviewing easier.
  • have a single commit (unless stacked on another Differential), up-to-date with the upstream origin/main branch, and don’t have merges.

Before sending a patch for review, please also try to ensure it is formatted properly. We use clang-format for this, which has git integration through the git-clang-format script. On some systems, it may already be installed (or be installable via your package manager). If so, you can simply run it – the following command will format only the code changed in the most recent commit:

% git clang-format HEAD~1

Note that this modifies the files, but doesn’t commit them – you’ll likely want to run

% git commit --amend -a

in order to update the last commit with all pending changes.

Note

If you don’t already have clang-format or git clang-format installed on your system, the clang-format binary will be built alongside clang, and the git integration can be run from clang/tools/clang-format/git-clang-format.

We don’t currently accept GitHub pull requests, and you’ll need to send patches via Phabricator#phabricator-reviews. (We used to allow patches on the llvm-commits mailing list, but the mailing lists have been deprecated.)

To make sure the right people see your patch, please select suitable reviewers and add them to your patch when requesting a review. Suitable reviewers are the code owner (see CODE_OWNERS.txt) and other people doing work in the area your patch touches. If you are using Phabricator, add them to the Reviewers field when creating a review and if you are using llvm-commits, add them to the CC of your email.

A reviewer may request changes or ask questions during the review. If you are uncertain on how to provide test cases, documentation, etc., feel free to ask for guidance during the review. Please address the feedback and re-post an updated version of your patch. This cycle continues until all requests and comments have been addressed and a reviewer accepts the patch with a Looks good to me or LGTM. Once that is done the change can be committed. If you do not have commit access, please let people know during the review and someone should commit it on your behalf.

If you have received no comments on your patch for a week, you can request a review by ‘ping’ing a patch by responding to the email thread containing the patch, or the Phabricator review with “Ping.” The common courtesy ‘ping’ rate is once a week. Please remember that you are asking for valuable time from other professional developers.

For more information on LLVM’s code-review process, please see LLVM Code-Review Policy and Practices.

For developers to commit changes from Git

Once a patch is reviewed, you should rebase it, re-test locally, and commit the changes to LLVM’s main branch. This is done using git push if you have the required access rights. See committing a change for Phabricator based commits or obtaining commit access for commit access.

Here is an example workflow using git. This workflow assumes you have an accepted commit on the branch named branch-with-change.

# Pull changes from the upstream main branch.
% git checkout main && git pull
# Rebase your change onto main.
% git rebase --onto main --root branch-with-change
# Rerun the appropriate tests if needed.
% ninja check-$whatever
# Check that the list of commits about to be pushed is correct.
% git log origin/main...HEAD --oneline
# Push to Github.
% git push origin HEAD:main

LLVM currently has a linear-history policy, which means that merge commits are not allowed. The llvm-project repo on github is configured to reject pushes that include merges, so the git rebase step above is required.

Please ask for help if you’re having trouble with your particular git workflow.

Git pre-push hook

We include an optional pre-push hook that run some sanity checks on the revisions you are about to push and ask confirmation if you push multiple commits at once. You can set it up (on Unix systems) by running from the repository root:

% ln -sf ../../llvm/utils/git/pre-push.py .git/hooks/pre-push

Helpful Information About LLVM

LLVM’s documentation provides a wealth of information about LLVM’s internals as well as various user guides. The pages listed below should provide a good overview of LLVM’s high-level design, as well as its internals:

Getting Started with the LLVM System
Discusses how to get up and running quickly with the LLVM infrastructure. Everything from unpacking and compilation of the distribution to execution of some tools.
LLVM Language Reference Manual
Defines the LLVM intermediate representation.
LLVM Programmer’s Manual
Introduction to the general layout of the LLVM sourcebase, important classes and APIs, and some tips & tricks.
LLVM for Grad Students
This is an introduction to the LLVM infrastructure by Adrian Sampson. While it has been written for grad students, it provides a good, compact overview of LLVM’s architecture, LLVM’s IR and how to write a new pass.
Intro to LLVM
Book chapter providing a compiler hacker’s introduction to LLVM.