==================================
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`_.

If you want to contribute code, please familiarize yourself with the :doc:`DeveloperPolicy`.

.. contents::
  :local:


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
:doc:`HowToSubmitABug`  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 :doc:`GettingStarted` 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 :ref:`report-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.

.. _submit_patch:

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 :doc:`CodingStandards`. 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, up-to-date with the upstream ``origin/main`` branch, and don't have merges.

.. _format patches:

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:

.. code-block:: console

  % git clang-format HEAD~1

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

.. code-block:: console

  % 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``.

The LLVM project has migrated to GitHub Pull Requests as its review process.
For more information about the workflow of using GitHub Pull Requests see our
:ref:`GitHub <github-reviews>` documentation. We still have an read-only
`LLVM's Phabricator <https://reviews.llvm.org>`_ instance.

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 maintainers of the project you are modifying, and
anyone else working in the area your patch touches. To find maintainers, look for
the ``Maintainers.md`` or ``Maintainers.rst`` file in the root of the project's
sub-directory. For example, LLVM's is ``llvm/Maintainers.md`` and Clang's is
``clang/Maintainers.rst``.

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.

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" in a comment. The common courtesy 'ping' rate
is once a week. Please remember that you are asking for valuable time from
other professional developers.

After your PR is approved, ensure that:

  * The PR title and description describe the final changes. These will be used
    as the title and message of the final squashed commit. The titles and
    messages of commits in the PR will **not** be used.
  * You have set a valid email address in your GitHub account, see :ref:`github-email-address`.

Now you can merge your PR. If you do not have the ability to merge the PR, ask your
reviewers to merge it on your behalf. You must do this explicitly, as reviewers'
default assumption is that you are able to merge your own PR.

For more information on LLVM's code-review process, please see
:doc:`CodeReview`.

.. _commit_from_git:

For developers to commit changes from Git
-----------------------------------------

Once a patch is reviewed, you can select the "Squash and merge" button in the
GitHub web interface.

When pushing directly from the command-line to the ``main`` branch, you will need
to rebase your change. LLVM has a linear-history policy, which means
that merge commits are not allowed and the ``main`` branch is configured to reject
pushes that include merges.

GitHub will display a message that looks like this:

.. code-block:: console

  remote: Bypassed rule violations for refs/heads/main:
  remote:
  remote: - Required status check “buildkite/github-pull-requests” is expected.

This can seem scary, but this is just an artifact of the GitHub setup: it is
intended as a warning for people merging pull-requests with failing CI. We can't
disable it for people pushing on the command-line.

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

.. _git_pre_push_hook:

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:

.. code-block:: console

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

Helpful Information About LLVM
==============================
:doc:`LLVM's documentation <index>` 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:

:doc:`GettingStarted`
   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.

:doc:`LangRef`
  Defines the LLVM intermediate representation.

:doc:`ProgrammersManual`
  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.

  .. __: http://www.cs.cornell.edu/~asampson/blog/llvm.html

`Intro to LLVM`__
  Book chapter providing a compiler hacker's introduction to LLVM.

  .. __: http://www.aosabook.org/en/llvm.html

.. _Forum: https://discourse.llvm.org
.. _Discord server: https://discord.gg/xS7Z362
.. _irc.oftc.net: irc://irc.oftc.net/llvm
.. _good first issue: https://github.com/llvm/llvm-project/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22
.. _bug tracker: https://github.com/llvm/llvm-project/issues
.. _clang-format-diff.py: https://github.com/llvm/llvm-project/blob/main/clang/tools/clang-format/clang-format-diff.py
.. _git-clang-format: https://github.com/llvm/llvm-project/blob/main/clang/tools/clang-format/git-clang-format
.. _LLVM's GitHub: https://github.com/llvm/llvm-project
.. _LLVM's Phabricator (read-only): https://reviews.llvm.org/
.. _LLVM's Open Projects page: https://llvm.org/OpenProjects.html#what
