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, for a more interactive chat, go to our Discord server or the IRC #llvm channel on irc.oftc.net.
If you want to contribute code, please familiarize yourself with the LLVM Developer Policy.
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.
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).
There is a separate process to submit security-related bugs, see How to report a security issue?.
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.
Once you have a patch ready, it is time to submit it. The patch should:
include a small unit test
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/mainbranch, 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
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
% 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.
If you don’t already have
git clang-format installed
on your system, the
clang-format binary will be built alongside clang, and
the git integration can be run from
The LLVM project has migrated to GitHub Pull Requests as its review process. We still have an active Phabricator instance for the duration of the migration. If you want to contribute to LLVM now, please use GitHub. For more information about the workflow of using GitHub Pull Requests see our GitHub documentation.
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. Github will normally suggest some reviewers based on rules or people that have worked on the code before. If you are a new contributor, you will not be able to select reviewers in such a way, in which case you can still get the attention of potential reviewers by CC’ing them in a comment – just @name them.
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 the GitHub PR 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.
Once a patch is reviewed, you can select the “Squash and merge” button in the GitHub web interface. You might need to rebase your change before pushing it to the repo.
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.
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
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.