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 Developer’s List (llvm-dev) 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 beginner 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 create an account for the bug tracker and assign it to yourself, 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 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).

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 send a mail to the LLVM Developer’s mailing list, 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.

To get a patch accepted, it has to be reviewed by the LLVM community. This can be done using LLVM’s Phabricator or the llvm-commits mailing list. Please follow Phabricator#requesting-a-review-via-the-web-interface to request a review using Phabricator.

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.

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.
Subsystem Documentation
A collection of pages documenting various subsystems of LLVM.
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.