.. _developer_policy:

=====================
LLVM Developer Policy
=====================

.. contents::
   :local:

Introduction
============

This document contains the LLVM Developer Policy which defines the project's
policy towards developers and their contributions. The intent of this policy is
to eliminate miscommunication, rework, and confusion that might arise from the
distributed nature of LLVM's development.  By stating the policy in clear terms,
we hope each developer can know ahead of time what to expect when making LLVM
contributions.  This policy covers all llvm.org subprojects, including Clang,
LLDB, libc++, etc.

This policy is also designed to accomplish the following objectives:

#. Attract both users and developers to the LLVM project.

#. Make life as simple and easy for contributors as possible.

#. Keep the top of tree as stable as possible.

#. Establish awareness of the project's :ref:`copyright, license, and patent
   policies <copyright-license-patents>` with contributors to the project.

This policy is aimed at frequent contributors to LLVM. People interested in
contributing one-off patches can do so in an informal way by sending them to the
`llvm-commits mailing list
<http://lists.llvm.org/mailman/listinfo/llvm-commits>`_ and engaging another
developer to see it through the process.

Developer Policies
==================

This section contains policies that pertain to frequent LLVM developers.  We
always welcome `one-off patches`_ from people who do not routinely contribute to
LLVM, but we expect more from frequent contributors to keep the system as
efficient as possible for everyone.  Frequent LLVM contributors are expected to
meet the following requirements in order for LLVM to maintain a high standard of
quality.

Stay Informed
-------------

Developers should stay informed by reading the `LLVM Discourse forums`_ and subscribing
to the categories of interest for notifications.

Paying attention to changes being made by others is a good way to see what other people
are interested in and watching the flow of the project as a whole.

Contibutions to the project are made through :ref:`GitHub Pull Requests <github-reviews>`.
You can subscribe to notification for areas of the codebase by joining
one of the `pr-subscribers-* <https://github.com/orgs/llvm/teams?query=pr-subscribers>`_
GitHub teams. This `mapping <https://github.com/llvm/llvm-project/blob/main/.github/new-prs-labeler.yml>`_
indicates which team is associated with a particular paths in the repository.

You can also subscribe to the "commits" mailing list for a subproject you're interested in,
such as `llvm-commits
<http://lists.llvm.org/mailman/listinfo/llvm-commits>`_, `cfe-commits
<http://lists.llvm.org/mailman/listinfo/cfe-commits>`_, or `lldb-commits
<http://lists.llvm.org/mailman/listinfo/lldb-commits>`_.

Missing features and bugs are tracked through our `GitHub issue tracker <https://github.com/llvm/llvm-project/issues>`_
and assigned labels. We recommend that active developers monitor incoming issues.
You can subscribe for notification for specific components by joining
one of the `issue-subscribers-* <https://github.com/orgs/llvm/teams?query=issue-subscribers>`_
teams.
You may also subscribe to the `llvm-bugs
<http://lists.llvm.org/mailman/listinfo/llvm-bugs>`_ email list to keep track
of bugs and enhancements occurring in the entire project.  We really appreciate people
who are proactive at catching incoming bugs in their components and dealing with them
promptly.

Please be aware that all public LLVM mailing lists and discourse forums are public and archived, and
that notices of confidentiality or non-disclosure cannot be respected.

.. _patch:
.. _one-off patches:

Making and Submitting a Patch
-----------------------------

When making a patch for review, the goal is to make it as easy for the reviewer
to read it as possible.  As such, we recommend that you:

#. Make your patch against git main, not a branch, and not an old version
   of LLVM.  This makes it easy to apply the patch.  For information on how to
   clone from git, please see the :ref:`Getting Started Guide <sources>`.

#. Similarly, patches should be submitted soon after they are generated.  Old
   patches may not apply correctly if the underlying code changes between the
   time the patch was created and the time it is applied.

#. Once you have created your patch, create a
   :ref:`GitHub Pull Request <github-reviews>` for
   it (or commit it directly if applicable).

When submitting patches, please do not add confidentiality or non-disclosure
notices to the patches themselves.  These notices conflict with the LLVM
licensing terms and may result in your contribution being excluded.


.. _code review:

Code Reviews
------------

LLVM has a code-review policy. Code review is one way to increase the quality of
software. Please see :doc:`CodeReview` for more information on LLVM's code-review
process.

.. _breaking:

Making Potentially Breaking Changes
-----------------------------------

Please help notify users and vendors of potential disruptions when upgrading to
a newer version of a tool. For example, deprecating a feature that is expected
to be removed in the future, removing an already-deprecated feature, upgrading a
diagnostic from a warning to an error, switching important default behavior, or
any other potentially disruptive situation thought to be worth raising
awareness of. For such changes, the following should be done:

.. warning::

  Phabricator is deprecated is available in read-only mode,
  for new code contributions use :ref:`GitHub Pull Requests <github-reviews>`.
  This section contains old information that needs to be updated.

* When performing the code review for the change, please add any applicable
  "vendors" group to the review for their awareness. The purpose of these
  groups is to give vendors early notice that potentially disruptive changes
  are being considered but have not yet been accepted. Vendors can give early
  testing feedback on the changes to alert us to unacceptable breakages. The
  current list of vendor groups is:

  * `Clang vendors <https://reviews.llvm.org/project/members/113/>`_
  * `libc++ vendors <https://reviews.llvm.org/project/members/109/>`_

  People interested in joining the vendors group can do so by clicking the
  "Join Project" link on the vendor's "Members" page in Phabricator.

* When committing the change to the repository, add appropriate information
  about the potentially breaking changes to the ``Potentially Breaking Changes``
  section of the project's release notes. The release note should have
  information about what the change is, what is potentially disruptive about
  it, as well as any code examples, links, and motivation that is appropriate
  to share with users. This helps users to learn about potential issues with
  upgrading to that release.

* After the change has been committed to the repository, the potentially
  disruptive changes described in the release notes should be posted to the
  `Announcements <https://discourse.llvm.org/c/announce/>`_ channel on
  Discourse. The post should be tagged with the ``potentially-breaking`` label
  and a label specific to the project (such as ``clang``, ``llvm``, etc). This
  is another mechanism by which we can give pre-release notice to users about
  potentially disruptive changes. It is a lower-traffic alternative to the
  joining "vendors" group. To automatically be notified of new announcements
  with the ``potentially-breaking`` label, go to your user preferences page in
  Discourse, and add the label to one of the watch categories under
  ``Notifications->Tags``.

.. _code owners:

Code Owners
-----------

The LLVM Project relies on two features of its process to maintain rapid
development in addition to the high quality of its source base: the combination
of code review plus post-commit review for trusted maintainers.  Having both is
a great way for the project to take advantage of the fact that most people do
the right thing most of the time, and only commit patches without pre-commit
review when they are confident they are right.

The trick to this is that the project has to guarantee that all patches that are
committed are reviewed after they go in: you don't want everyone to assume
someone else will review it, allowing the patch to go unreviewed.  To solve this
problem, we have a notion of an 'owner' for a piece of the code.  The sole
responsibility of a code owner is to ensure that a commit to their area of the
code is appropriately reviewed, either by themself or by someone else.  The list
of current code owners can be found in the file `CODE_OWNERS.TXT
<https://github.com/llvm/llvm-project/blob/main/llvm/CODE_OWNERS.TXT>`_ in the
root of the LLVM source tree.

Note that code ownership is completely different than reviewers: anyone can
review a piece of code, and we welcome code review from anyone who is
interested.  Code owners are the "last line of defense" to guarantee that all
patches that are committed are actually reviewed.

Being a code owner is a somewhat unglamorous position, but it is incredibly
important for the ongoing success of the project.  Because people get busy,
interests change, and unexpected things happen, code ownership is purely opt-in,
and anyone can choose to resign their "title" at any time. For now, we do not
have an official policy on how one gets elected to be a code owner.

.. _include a testcase:

Test Cases
----------

Developers are required to create test cases for any bugs fixed and any new
features added.  Some tips for getting your testcase approved:

* All feature and regression test cases are added to the ``llvm/test``
  directory. The appropriate sub-directory should be selected (see the
  :doc:`Testing Guide <TestingGuide>` for details).

* Test cases should be written in :doc:`LLVM assembly language <LangRef>`.

* Test cases, especially for regressions, should be reduced as much as possible,
  by :doc:`bugpoint <Bugpoint>` or manually. It is unacceptable to place an
  entire failing program into ``llvm/test`` as this creates a *time-to-test*
  burden on all developers. Please keep them short.

* Avoid adding links to resources that are not available to the entire
  community, such as links to private bug trackers, internal corporate
  documentation, etc. Instead, add sufficient comments to the test to provide
  the context behind such links.

Note that llvm/test and clang/test are designed for regression and small feature
tests only. More extensive test cases (e.g., entire applications, benchmarks,
etc) should be added to the ``llvm-test`` test suite.  The llvm-test suite is
for coverage (correctness, performance, etc) testing, not feature or regression
testing.

Release Notes
-------------

Many projects in LLVM communicate important changes to users through release
notes, typically found in ``docs/ReleaseNotes.rst`` for the project. Changes to
a project that are user-facing, or that users may wish to know about, should be
added to the project's release notes at the author's or code reviewer's
discretion, preferably as part of the commit landing the changes. Examples of
changes that would typically warrant adding a release note (this list is not
exhaustive):

* Adding, removing, or modifying command-line options.
* Adding, removing, or regrouping a diagnostic.
* Fixing a bug that potentially has significant user-facing impact (please link
  to the issue fixed in the bug database).
* Adding or removing optimizations that have widespread impact or enables new
  programming paradigms.
* Modifying a C stable API.
* Notifying users about a potentially disruptive change expected to be made in
  a future release, such as removal of a deprecated feature. In this case, the
  release note should be added to a ``Potentially Breaking Changes`` section of
  the notes with sufficient information and examples to demonstrate the
  potential disruption. Additionally, any new entries to this section should be
  announced in the `Announcements <https://discourse.llvm.org/c/announce/>`_
  channel on Discourse. See :ref:`breaking` for more details.

Code reviewers are encouraged to request a release note if they think one is
warranted when performing a code review.

Quality
-------

The minimum quality standards that any change must satisfy before being
committed to the main development branch are:

#. Code must adhere to the `LLVM Coding Standards <CodingStandards.html>`_.

#. Code must compile cleanly (no errors, no warnings) on at least one platform.

#. Bug fixes and new features should `include a testcase`_ so we know if the
   fix/feature ever regresses in the future.

#. Code must pass the ``llvm/test`` test suite.

#. The code must not cause regressions on a reasonable subset of llvm-test,
   where "reasonable" depends on the contributor's judgement and the scope of
   the change (more invasive changes require more testing). A reasonable subset
   might be something like "``llvm-test/MultiSource/Benchmarks``".

#. Ensure that links in source code and test files point to publicly available
   resources and are used primarily to add additional information rather than
   to supply critical context. The surrounding comments should be sufficient
   to provide the context behind such links.

Additionally, the committer is responsible for addressing any problems found in
the future that the change is responsible for.  For example:

* The code should compile cleanly on all supported platforms.

* The changes should not cause any correctness regressions in the ``llvm-test``
  suite and must not cause any major performance regressions.

* The change set should not cause performance or correctness regressions for the
  LLVM tools.

* The changes should not cause performance or correctness regressions in code
  compiled by LLVM on all applicable targets.

* You are expected to address any `GitHub Issues <https://github.com/llvm/llvm-project/issues>`_ that
  result from your change.

We prefer for this to be handled before submission but understand that it isn't
possible to test all of this for every submission.  Our build bots and nightly
testing infrastructure normally finds these problems.  A good rule of thumb is
to check the nightly testers for regressions the day after your change.  Build
bots will directly email you if a group of commits that included yours caused a
failure.  You are expected to check the build bot messages to see if they are
your fault and, if so, fix the breakage.

Commits that violate these quality standards (e.g. are very broken) may be
reverted. This is necessary when the change blocks other developers from making
progress. The developer is welcome to re-commit the change after the problem has
been fixed.

.. _commit messages:

Commit messages
---------------

Although we don't enforce the format of commit messages, we prefer that
you follow these guidelines to help review, search in logs, email formatting
and so on. These guidelines are very similar to rules used by other open source
projects.

Most importantly, the contents of the message should be carefully written to
convey the rationale of the change (without delving too much in detail). It
also should avoid being vague or overly specific. For example, "bits were not
set right" will leave the reviewer wondering about which bits, and why they
weren't right, while "Correctly set overflow bits in TargetInfo" conveys almost
all there is to the change.

Below are some guidelines about the format of the message itself:

* Separate the commit message into title and body separated by a blank line.

* If you're not the original author, ensure the 'Author' property of the commit is
  set to the original author and the 'Committer' property is set to yourself.
  You can use a command similar to
  ``git commit --amend --author="John Doe <jdoe@llvm.org>"`` to correct the
  author property if it is incorrect. See `Attribution of Changes`_ for more
  information including the method we used for attribution before the project
  migrated to git.

  In the rare situation where there are multiple authors, please use the `git
  tag 'Co-authored-by:' to list the additional authors
  <https://github.blog/2018-01-29-commit-together-with-co-authors/>`_.

* The title should be concise. Because all commits are emailed to the list with
  the first line as the subject, long titles are frowned upon.  Short titles
  also look better in `git log`.

* When the changes are restricted to a specific part of the code (e.g. a
  back-end or optimization pass), it is customary to add a tag to the
  beginning of the line in square brackets.  For example, "[SCEV] ..."
  or "[OpenMP] ...". This helps email filters and searches for post-commit
  reviews.

* The body, if it exists, should be separated from the title by an empty line.

* The body should be concise, but explanatory, including a complete
  reasoning.  Unless it is required to understand the change, examples,
  code snippets and gory details should be left to bug comments, web
  review or the mailing list.

* Text formatting and spelling should follow the same rules as documentation
  and in-code comments, ex. capitalization, full stop, etc.

* If the commit is a bug fix on top of another recently committed patch, or a
  revert or reapply of a patch, include the git commit hash of the prior
  related commit. This could be as simple as "Revert commit NNNN because it
  caused PR#".

* If the patch has been reviewed, add a link to its review page, as shown
  `here <https://www.llvm.org/docs/Phabricator.html#committing-a-change>`__.
  If the patch fixes a bug in GitHub Issues, we encourage adding a reference to
  the issue being closed, as described
  `here <https://llvm.org/docs/BugLifeCycle.html#resolving-closing-bugs>`__.

* It is also acceptable to add other metadata to the commit message to automate
  processes, including for downstream consumers. This metadata can include
  links to resources that are not available to the entire community. However,
  such links and/or metadata should not be used in place of making the commit
  message self-explanatory. Note that such non-public links should not be
  included in the submitted code.

For minor violations of these recommendations, the community normally favors
reminding the contributor of this policy over reverting. Minor corrections and
omissions can be handled by sending a reply to the commits mailing list.

.. _revert_policy:

Patch reversion policy
----------------------

As a community, we strongly value having the tip of tree in a good state while
allowing rapid iterative development.  As such, we tend to make much heavier
use of reverts to keep the tree healthy than some other open source projects,
and our norms are a bit different.

How should you respond if someone reverted your change?

* Remember, it is normal and healthy to have patches reverted.  Having a patch
  reverted does not necessarily mean you did anything wrong.
* We encourage explicitly thanking the person who reverted the patch for doing
  the task on your behalf.
* If you need more information to address the problem, please follow up in the
  original commit thread with the reverting patch author.

When should you revert your own change?

* Any time you learn of a serious problem with a change, you should revert it.
  We strongly encourage "revert to green" as opposed to "fixing forward".  We
  encourage reverting first, investigating offline, and then reapplying the
  fixed patch - possibly after another round of review if warranted.
* If you break a buildbot in a way which can't be quickly fixed, please revert.
* If a test case that demonstrates a problem is reported in the commit thread,
  please revert and investigate offline.
* If you receive substantial :ref:`post-commit review <post_commit_review>`
  feedback, please revert and address said feedback before recommitting.
  (Possibly after another round of review.)
* If you are asked to revert by another contributor, please revert and discuss
  the merits of the request offline (unless doing so would further destabilize
  tip of tree).

When should you revert someone else's change?

* In general, if the author themselves would revert the change per these
  guidelines, we encourage other contributors to do so as a courtesy to the
  author.  This is one of the major cases where our norms differ from others;
  we generally consider reverting a normal part of development.  We don't
  expect contributors to be always available, and the assurance that a
  problematic patch will be reverted and we can return to it at our next
  opportunity enables this.

What are the expectations around a revert?

* Use your best judgment. If you're uncertain, please start an email on
  the commit thread asking for assistance.  We aren't trying to enumerate
  every case, but rather give a set of guidelines.
* You should be sure that reverting the change improves the stability of tip
  of tree.  Sometimes reverting one change in a series can worsen things
  instead of improving them.  We expect reasonable judgment to ensure that
  the proper patch or set of patches is being reverted.
* The commit message for the reverting commit should explain why patch
  is being reverted.
* It is customary to respond to the original commit email mentioning the
  revert.  This serves as both a notice to the original author that their
  patch was reverted, and helps others following llvm-commits track context.
* Ideally, you should have a publicly reproducible test case ready to share.
  Where possible, we encourage sharing of test cases in commit threads, or
  in PRs.  We encourage the reverter to minimize the test case and to prune
  dependencies where practical.  This even applies when reverting your own
  patch; documenting the reasons for others who might be following along
  is critical.
* It is not considered reasonable to revert without at least the promise to
  provide a means for the patch author to debug the root issue.  If a situation
  arises where a public reproducer can not be shared for some reason (e.g.
  requires hardware patch author doesn't have access to, sharp regression in
  compile time of internal workload, etc.), the reverter is expected to be
  proactive about working with the patch author to debug and test candidate
  patches.
* Reverts should be reasonably timely.  A change submitted two hours ago
  can be reverted without prior discussion.  A change submitted two years ago
  should not be.  Where exactly the transition point is is hard to say, but
  it's probably in the handful of days in tree territory.  If you are unsure,
  we encourage you to reply to the commit thread, give the author a bit to
  respond, and then proceed with the revert if the author doesn't seem to be
  actively responding.
* When re-applying a reverted patch, the commit message should be updated to
  indicate the problem that was addressed and how it was addressed.

.. _obtaining_commit_access:

Obtaining Commit Access
-----------------------

We grant commit access to contributors that can provide a valid justification.
If you would like commit access, please send an email to
`Chris <mailto:clattner@llvm.org>`_ with your GitHub username.  This is true
for former contributors with SVN access as well as new contributors. If
approved, a GitHub invitation will be sent to your GitHub account. In case you
don't get notification from GitHub, go to
`Invitation Link <https://github.com/orgs/llvm/invitation>`_ directly. Once
accept the invitation, you'll get commit access.

Prior to obtaining commit access, it is common practice to request that
someone with commit access commits on your behalf. When doing so, please
provide the name and email address you would like to use in the Author
property of the commit.

For external tracking purposes, committed changes are automatically reflected
on a commits mailing list soon after the commit lands (e.g. llvm-commits_).
Note that these mailing lists are moderated, and it is not unusual for a large
commit to require a moderator to approve the email, so do not be concerned if a
commit does not immediately appear in the archives.

If you have recently been granted commit access, these policies apply:

#. You are granted *commit-after-approval* to all parts of LLVM. For
   information on how to get approval for a patch, please see :doc:`CodeReview`.
   When approved, you may commit it yourself.

#. You are allowed to commit patches without approval which you think are
   obvious. This is clearly a subjective decision --- we simply expect you to
   use good judgement.  Examples include: fixing build breakage, reverting
   obviously broken patches, documentation/comment changes, any other minor
   changes. Avoid committing formatting- or whitespace-only changes outside of
   code you plan to make subsequent changes to. Also, try to separate
   formatting or whitespace changes from functional changes, either by
   correcting the format first (ideally) or afterward. Such changes should be
   highly localized and the commit message should clearly state that the commit
   is not intended to change functionality, usually by stating it is
   :ref:`NFC <nfc>`.

#. You are allowed to commit patches without approval to those portions of LLVM
   that you have contributed or maintain (i.e., have been assigned
   responsibility for), with the proviso that such commits must not break the
   build.  This is a "trust but verify" policy, and commits of this nature are
   reviewed after they are committed.

#. Multiple violations of these policies or a single egregious violation may
   cause commit access to be revoked.

In any case, your changes are still subject to `code review`_ (either before or
after they are committed, depending on the nature of the change).  You are
encouraged to review other peoples' patches as well, but you aren't required
to do so.

.. _discuss the change/gather consensus:

Making a Major Change
---------------------

When a developer begins a major new project with the aim of contributing it back
to LLVM, they should inform the community with a post to the `LLVM Discourse forums`_, to the extent
possible. The reason for this is to:

#. keep the community informed about future changes to LLVM,

#. avoid duplication of effort by preventing multiple parties working on the
   same thing and not knowing about it, and

#. ensure that any technical issues around the proposed work are discussed and
   resolved before any significant work is done.

The design of LLVM is carefully controlled to ensure that all the pieces fit
together well and are as consistent as possible. If you plan to make a major
change to the way LLVM works or want to add a major new extension, it is a good
idea to get consensus with the development community before you start working on
it.

Once the design of the new feature is finalized, the work itself should be done
as a series of `incremental changes`_, not as a long-term development branch.

.. _incremental changes:

Incremental Development
-----------------------

In the LLVM project, we do all significant changes as a series of incremental
patches.  We have a strong dislike for huge changes or long-term development
branches.  Long-term development branches have a number of drawbacks:

#. Branches must have mainline merged into them periodically.  If the branch
   development and mainline development occur in the same pieces of code,
   resolving merge conflicts can take a lot of time.

#. Other people in the community tend to ignore work on branches.

#. Huge changes (produced when a branch is merged back onto mainline) are
   extremely difficult to `code review`_.

#. Branches are not routinely tested by our nightly tester infrastructure.

#. Changes developed as monolithic large changes often don't work until the
   entire set of changes is done.  Breaking it down into a set of smaller
   changes increases the odds that any of the work will be committed to the main
   repository.

To address these problems, LLVM uses an incremental development style and we
require contributors to follow this practice when making a large/invasive
change.  Some tips:

* Large/invasive changes usually have a number of secondary changes that are
  required before the big change can be made (e.g. API cleanup, etc).  These
  sorts of changes can often be done before the major change is done,
  independently of that work.

* The remaining inter-related work should be decomposed into unrelated sets of
  changes if possible.  Once this is done, define the first increment and get
  consensus on what the end goal of the change is.

* Each change in the set can be stand alone (e.g. to fix a bug), or part of a
  planned series of changes that works towards the development goal.

* Each change should be kept as small as possible. This simplifies your work
  (into a logical progression), simplifies code review and reduces the chance
  that you will get negative feedback on the change. Small increments also
  facilitate the maintenance of a high quality code base.

* Often, an independent precursor to a big change is to add a new API and slowly
  migrate clients to use the new API.  Each change to use the new API is often
  "obvious" and can be committed without review.  Once the new API is in place
  and used, it is much easier to replace the underlying implementation of the
  API.  This implementation change is logically separate from the API
  change.

If you are interested in making a large change, and this scares you, please make
sure to first `discuss the change/gather consensus`_ then ask about the best way
to go about making the change.

Attribution of Changes
----------------------

When contributors submit a patch to an LLVM project, other developers with
commit access may commit it for the author once appropriate (based on the
progression of code review, etc.). When doing so, it is important to retain
correct attribution of contributions to their contributors. However, we do not
want the source code to be littered with random attributions "this code written
by J. Random Hacker" (this is noisy and distracting). In practice, the revision
control system keeps a perfect history of who changed what, and the CREDITS.txt
file describes higher-level contributions. If you commit a patch for someone
else, please follow the attribution of changes in the simple manner as outlined
by the `commit messages`_ section. Overall, please do not add contributor names
to the source code.

Also, don't commit patches authored by others unless they have submitted the
patch to the project or you have been authorized to submit them on their behalf
(you work together and your company authorized you to contribute the patches,
etc.). The author should first submit them to the relevant project's commit
list, development list, or LLVM bug tracker component. If someone sends you
a patch privately, encourage them to submit it to the appropriate list first.

Our previous version control system (subversion) did not distinguish between the
author and the committer like git does. As such, older commits used a different
attribution mechanism. The previous method was to include "Patch by John Doe."
in a separate line of the commit message and there are automated processes that
rely on this format.

Bans
----

The goal of a ban is to protect people in the community from having to interact
with people who are consistently not respecting the
:ref:`LLVM Community Code of Conduct` in LLVM project spaces. Contributions of
any variety (pull requests, issue reports, forum posts, etc.) require
interacting with the community. Therefore, we do not accept any form of direct
contribution from a banned individual.

Indirect contributions are permissible only by someone taking full ownership of
such a contribution and they are responsible for all related interactions with
the community regarding that contribution.

When in doubt how to act in a specific instance, please reach out to
conduct@llvm.org for advice.


.. _IR backwards compatibility:

IR Backwards Compatibility
--------------------------

When the IR format has to be changed, keep in mind that we try to maintain some
backwards compatibility. The rules are intended as a balance between convenience
for llvm users and not imposing a big burden on llvm developers:

* The textual format is not backwards compatible. We don't change it too often,
  but there are no specific promises.

* Additions and changes to the IR should be reflected in
  ``test/Bitcode/compatibility.ll``.

* The current LLVM version supports loading any bitcode since version 3.0.

* After each X.Y release, ``compatibility.ll`` must be copied to
  ``compatibility-X.Y.ll``. The corresponding bitcode file should be assembled
  using the X.Y build and committed as ``compatibility-X.Y.ll.bc``.

* Newer releases can ignore features from older releases, but they cannot
  miscompile them. For example, if nsw is ever replaced with something else,
  dropping it would be a valid way to upgrade the IR.

* Debug metadata is special in that it is currently dropped during upgrades.

* Non-debug metadata is defined to be safe to drop, so a valid way to upgrade
  it is to drop it. That is not very user friendly and a bit more effort is
  expected, but no promises are made.

C API Changes
-------------

* Stability Guarantees: The C API is, in general, a "best effort" for stability.
  This means that we make every attempt to keep the C API stable, but that
  stability will be limited by the abstractness of the interface and the
  stability of the C++ API that it wraps. In practice, this means that things
  like "create debug info" or "create this type of instruction" are likely to be
  less stable than "take this IR file and JIT it for my current machine".

* Release stability: We won't break the C API on the release branch with patches
  that go on that branch, with the exception that we will fix an unintentional
  C API break that will keep the release consistent with both the previous and
  next release.

* Testing: Patches to the C API are expected to come with tests just like any
  other patch.

* Including new things into the API: If an LLVM subcomponent has a C API already
  included, then expanding that C API is acceptable. Adding C API for
  subcomponents that don't currently have one needs to be discussed on the
  `LLVM Discourse forums`_ for design and maintainability feedback prior to implementation.

* Documentation: Any changes to the C API are required to be documented in the
  release notes so that it's clear to external users who do not follow the
  project how the C API is changing and evolving.

.. _toolchain:

Updating Toolchain Requirements
-------------------------------

We intend to require newer toolchains as time goes by. This means LLVM's
codebase can use newer versions of C++ as they get standardized. Requiring newer
toolchains to build LLVM can be painful for those building LLVM; therefore, it
will only be done through the following process:

  * It is a general goal to support LLVM and GCC versions from the last 3 years
    at a minimum. This time-based guideline is not strict: we may support much
    older compilers, or decide to support fewer versions.

  * An RFC is sent to the `LLVM Discourse forums`_

    - Detail upsides of the version increase (e.g. which newer C++ language or
      library features LLVM should use; avoid miscompiles in particular compiler
      versions, etc).
    - Detail downsides on important platforms (e.g. Ubuntu LTS status).

  * Once the RFC reaches consensus, update the CMake toolchain version checks as
    well as the :doc:`getting started<GettingStarted>` guide.  This provides a
    softer transition path for developers compiling LLVM, because the
    error can be turned into a warning using a CMake flag. This is an important
    step: LLVM still doesn't have code which requires the new toolchains, but it
    soon will. If you compile LLVM but don't read the forums, we should
    tell you!

  * Ensure that at least one LLVM release has had this soft-error. Not all
    developers compile LLVM top-of-tree. These release-bound developers should
    also be told about upcoming changes.

  * Turn the soft-error into a hard-error after said LLVM release has branched.

  * Update the :doc:`coding standards<CodingStandards>` to allow the new
    features we've explicitly approved in the RFC.

  * Start using the new features in LLVM's codebase.

Here's a `sample RFC
<https://discourse.llvm.org/t/rfc-migrating-past-c-11/50943>`_ and the
`corresponding change <https://reviews.llvm.org/D57264>`_.

.. _ci-usage:

Working with the CI system
--------------------------

The main continuous integration (CI) tool for the LLVM project is the
`LLVM Buildbot <https://lab.llvm.org/buildbot/>`_. It uses different *builders*
to cover a wide variety of sub-projects and configurations. The builds are
executed on different *workers*. Builders and workers are configured and
provided by community members.

The Buildbot tracks the commits on the main branch and the release branches.
This means that patches are built and tested after they are merged to the these
branches (aka post-merge testing). This also means it's okay to break the build
occasionally, as it's unreasonable to expect contributors to build and test
their patch with every possible configuration.

*If your commit broke the build:*

* Fix the build as soon as possible as this might block other contributors or
  downstream users.
* If you need more time to analyze and fix the bug, please revert your change to
  unblock others.

*If someone else broke the build and this blocks your work*

* Comment on the code review in `GitHub <https://github.com/llvm/llvm-project/pulls>`_
  (if available) or email the author, explain the problem and how this impacts
  you. Add a link to the broken build and the error message so folks can
  understand the problem.
* Revert the commit if this blocks your work, see revert_policy_ .

*If a build/worker is permanently broken*

* 1st step: contact the owner of the worker. You can find the name and contact
  information for the *Admin* of worker on the page of the build in the
  *Worker* tab:

  .. image:: buildbot_worker_contact.png

* 2nd step: If the owner does not respond or fix the worker, please escalate
  to Galina Kostanova, the maintainer of the BuildBot master.
* 3rd step: If Galina could not help you, please escalate to the
  `Infrastructure Working Group <mailto:iwg@llvm.org>`_.

.. _new-llvm-components:

Introducing New Components into LLVM
====================================

The LLVM community is a vibrant and exciting place to be, and we look to be
inclusive of new projects and foster new communities, and increase
collaboration across industry and academia.

That said, we need to strike a balance between being inclusive of new ideas and
people and the cost of ongoing maintenance that new code requires.  As such, we
have a general :doc:`support policy<SupportPolicy>` for introducing major new
components into the LLVM world, depending on the degree of detail and
responsibility required. *Core* projects need a higher degree of scrutiny
than *peripheral* projects, and the latter may have additional differences.

However, this is really only intended to cover common cases
that we have seen arise: different situations are different, and we are open
to discussing unusual cases as well - just start an RFC thread on the
`LLVM Discourse forums`_.

Adding a New Target
-------------------

LLVM is very receptive to new targets, even experimental ones, but a number of
problems can appear when adding new large portions of code, and back-ends are
normally added in bulk. New targets need the same level of support as other
*core* parts of the compiler, so they are covered in the *core tier* of our
:doc:`support policy<SupportPolicy>`.

We have found that landing large pieces of new code and then trying to fix
emergent problems in-tree is problematic for a variety of reasons. For these
reasons, new targets are *always* added as *experimental* until they can be
proven stable, and later moved to non-experimental.

The differences between both classes are:

* Experimental targets are not built by default (they need to be explicitly
  enabled at CMake time).

* Test failures, bugs, and build breakages that only appear when the
  experimental target is enabled, caused by changes unrelated to the target, are
  the responsibility of the community behind the target to fix.

The basic rules for a back-end to be upstreamed in **experimental** mode are:

* Every target must have a :ref:`code owner<code owners>`. The `CODE_OWNERS.TXT`
  file has to be updated as part of the first merge. The code owner makes sure
  that changes to the target get reviewed and steers the overall effort.

* There must be an active community behind the target. This community
  will help maintain the target by providing buildbots, fixing
  bugs, answering the LLVM community's questions and making sure the new
  target doesn't break any of the other targets, or generic code. This
  behavior is expected to continue throughout the lifetime of the
  target's code.

* The code must be free of contentious issues, for example, large
  changes in how the IR behaves or should be formed by the front-ends,
  unless agreed by the majority of the community via refactoring of the
  (:doc:`IR standard<LangRef>`) **before** the merge of the new target changes,
  following the :ref:`IR backwards compatibility`.

* The code conforms to all of the policies laid out in this developer policy
  document, including license, patent, and coding standards.

* The target should have either reasonable documentation on how it
  works (ISA, ABI, etc.) or a publicly available simulator/hardware
  (either free or cheap enough) - preferably both.  This allows
  developers to validate assumptions, understand constraints and review code
  that can affect the target.

In addition, the rules for a back-end to be promoted to **official** are:

* The target must have addressed every other minimum requirement and
  have been stable in tree for at least 3 months. This cool down
  period is to make sure that the back-end and the target community can
  endure continuous upstream development for the foreseeable future.

* The target's code must have been completely adapted to this policy
  as well as the :doc:`coding standards<CodingStandards>`. Any exceptions that
  were made to move into experimental mode must have been fixed **before**
  becoming official.

* The test coverage needs to be broad and well written (small tests,
  well documented). The build target ``check-all`` must pass with the
  new target built, and where applicable, the ``test-suite`` must also
  pass without errors, in at least one configuration (publicly
  demonstrated, for example, via buildbots).

* Public buildbots need to be created and actively maintained, unless
  the target requires no additional buildbots (ex. ``check-all`` covers
  all tests). The more relevant and public the new target's CI infrastructure
  is, the more the LLVM community will embrace it.

To **continue** as a supported and official target:

* The maintainer(s) must continue following these rules throughout the lifetime
  of the target. Continuous violations of aforementioned rules and policies
  could lead to complete removal of the target from the code base.

* Degradation in support, documentation or test coverage will make the target as
  nuisance to other targets and be considered a candidate for deprecation and
  ultimately removed.

In essence, these rules are necessary for targets to gain and retain their
status, but also markers to define bit-rot, and will be used to clean up the
tree from unmaintained targets.

Those wishing to add a new target to LLVM must follow the procedure below:

1. Read this section and make sure your target follows all requirements. For
   minor issues, your community will be responsible for making all necessary
   adjustments soon after the initial merge.
2. Send a request for comment (RFC) to the `LLVM Discourse forums`_ describing
   your target and how it follows all the requirements and what work has been
   done and will need to be done to accommodate the official target requirements.
   Make sure to expose any and all controversial issues, changes needed in the
   base code, table gen, etc.
3. Once the response is positive, the LLVM community can start reviewing the
   actual patches (but they can be prepared before, to support the RFC). Create
   a sequence of N patches, numbered '1/N' to 'N/N' (make sure N is an actual
   number, not the letter 'N'), that completes the basic structure of the target.
4. The initial patch should add documentation, code owners and triple support in
   clang and LLVM. The following patches add TableGen infrastructure to describe
   the target and lower instructions to assembly. The final patch must show that
   the target can lower correctly with extensive LIT tests (IR to MIR, MIR to
   ASM, etc).
5. Some patches may be approved before others, but only after *all* patches are
   approved that the whole set can be merged in one go. This is to guarantee
   that all changes are good as a single block.
6. After the initial merge, the target community can stop numbering patches and
   start working asynchronously on the target to complete support. They should
   still seek review from those who helped them in the initial phase, to make
   sure the progress is still consistent.
7. Once all official requirements have been fulfilled (as above), the code owner
   should request the target to be enabled by default by sending another RFC to
   the `LLVM Discourse forums`_.

Adding an Established Project To the LLVM Monorepo
--------------------------------------------------

The `LLVM monorepo <https://github.com/llvm/llvm-project>`_ is the centerpoint
of development in the LLVM world, and has all of the primary LLVM components,
including the LLVM optimizer and code generators, Clang, LLDB, etc.  `Monorepos
in general <https://en.wikipedia.org/wiki/Monorepo>`_ are great because they
allow atomic commits to the project, simplify CI, and make it easier for
subcommunities to collaborate.

Like new targets, most projects already in the monorepo are considered to be in
the *core tier* of our :doc:`support policy<SupportPolicy>`. The burden to add
things to the LLVM monorepo needs to be very high - code that is added to this
repository is checked out by everyone in the community.  As such, we hold
components to a high bar similar to "official targets", they:

 * Must be generally aligned with the mission of the LLVM project to advance
   compilers, languages, tools, runtimes, etc.
 * Must conform to all of the policies laid out in this developer policy
   document, including license, patent, coding standards, and code of conduct.
 * Must have an active community that maintains the code, including established
   code owners.
 * Should have reasonable documentation about how it works, including a high
   quality README file.
 * Should have CI to catch breakage within the project itself or due to
   underlying LLVM dependencies.
 * Should have code free of issues the community finds contentious, or be on a
   clear path to resolving them.
 * Must be proposed through the LLVM RFC process, and have its addition approved
   by the LLVM community - this ultimately mediates the resolution of the
   "should" concerns above.

If you have a project that you think would make sense to add to the LLVM
monorepo, please start an RFC topic on the `LLVM Discourse forums`_ to kick off
the discussion.  This process can take some time and iteration - please don’t
be discouraged or intimidated by that!

If you have an earlier stage project that you think is aligned with LLVM, please
see the "Incubating New Projects" section.

Incubating New Projects
-----------------------

The burden to add a new project to the LLVM monorepo is intentionally very high,
but that can have a chilling effect on new and innovative projects.  To help
foster these sorts of projects, LLVM supports an "incubator" process that is
much easier to get started with.  It provides space for potentially valuable,
new top-level and sub-projects to reach a critical mass before they have enough
code to prove their utility and grow a community.  This also allows
collaboration between teams that already have permissions to make contributions
to projects under the LLVM umbrella.

Projects which can be considered for the LLVM incubator meet the following
criteria:

 * Must be generally aligned with the mission of the LLVM project to advance
   compilers, languages, tools, runtimes, etc.
 * Must conform to the license, patent, and code of conduct policies laid out
   in this developer policy document.
 * Must have a documented charter and development plan, e.g. in the form of a
   README file, mission statement, and/or manifesto.
 * Should conform to coding standards, incremental development process, and
   other expectations.
 * Should have a sense of the community that it hopes to eventually foster, and
   there should be interest from members with different affiliations /
   organizations.
 * Should have a feasible path to eventually graduate as a dedicated top-level
   or sub-project within the `LLVM monorepo
   <https://github.com/llvm/llvm-project>`_.
 * Should include a notice (e.g. in the project README or web page) that the
   project is in ‘incubation status’ and is not included in LLVM releases (see
   suggested wording below).
 * Must be proposed through the LLVM RFC process, and have its addition
   approved by the LLVM community - this ultimately mediates the resolution of
   the "should" concerns above.

That said, the project need not have any code to get started, and need not have
an established community at all!  Furthermore, incubating projects may pass
through transient states that violate the "Should" guidelines above, or would
otherwise make them unsuitable for direct inclusion in the monorepo (e.g.
dependencies that have not yet been factored appropriately, leveraging
experimental components or APIs that are not yet upstream, etc).

When approved, the llvm-admin group can grant the new project:
 * A new repository in the LLVM Github Organization - but not the LLVM monorepo.
 * New mailing list, discourse forum, and/or discord chat hosted with other LLVM
   forums.
 * Other infrastructure integration can be discussed on a case-by-case basis.

Graduation to the mono-repo would follow existing processes and standards for
becoming a first-class part of the monorepo.  Similarly, an incubating project
may be eventually retired, but no process has been established for that yet.  If
and when this comes up, please start an RFC discussion on the `LLVM Discourse forums`_.

This process is very new - please expect the details to change, it is always
safe to ask on the `LLVM Discourse forums`_ about this.

Suggested disclaimer for the project README and the main project web page:

::

   This project is participating in the LLVM Incubator process: as such, it is
   not part of any official LLVM release.  While incubation status is not
   necessarily a reflection of the completeness or stability of the code, it
   does indicate that the project is not yet endorsed as a component of LLVM.

.. _copyright-license-patents:

Copyright, License, and Patents
===============================

.. note::

   This section deals with legal matters but does not provide legal advice.  We
   are not lawyers --- please seek legal counsel from a licensed attorney.

This section addresses the issues of copyright, license and patents for the LLVM
project.  The copyright for the code is held by the contributors of
the code.  The code is licensed under permissive `open source licensing terms`_,
namely the Apache-2.0 with LLVM-exception license, which includes a copyright
and `patent license`_.  When you contribute code to the LLVM project, you
license it under these terms.

In certain circumstances, code licensed under other licenses can be added
to the codebase.  However, this may only be done with approval of the LLVM
Foundation Board of Directors, and contributors should plan for the approval
process to take at least 4-6 weeks.  If you would like to contribute code
under a different license, please create a pull request with the code
you want to contribute and email board@llvm.org requesting a review.

If you have questions or comments about these topics, please ask on the
`LLVM Discourse forums`_.  However,
please realize that most compiler developers are not lawyers, and therefore you
will not be getting official legal advice.

Copyright
---------

The LLVM project does not collect copyright assignments, which means that the
copyright for the code in the project is held by the respective contributors.
Because you (or your company)
retain ownership of the code you contribute, you know it may only be used under
the terms of the open source license you contributed it under: the license for
your contributions cannot be changed in the future without your approval.

Because the LLVM project does not require copyright assignments, changing the
LLVM license requires tracking down the
contributors to LLVM and getting them to agree that a license change is
acceptable for their contributions.  We feel that a high burden for relicensing
is good for the project, because contributors do not have to fear that their
code will be used in a way with which they disagree.

Relicensing
-----------

The last paragraph notwithstanding, the LLVM Project is in the middle of a large
effort to change licenses, which aims to solve several problems:

* The old licenses made it difficult to move code from (e.g.) the compiler to
  runtime libraries, because runtime libraries used a different license from the
  rest of the compiler.
* Some contributions were not submitted to LLVM due to concerns that
  the patent grant required by the project was overly broad.
* The patent grant was unique to the LLVM Project, not written by a lawyer, and
  was difficult to determine what protection was provided (if any).

The scope of relicensing is all code that is considered part of the LLVM
project, including the main LLVM repository, runtime libraries (compiler_rt,
OpenMP, etc), Polly, and all other subprojects.  There are a few exceptions:

* Code imported from other projects (e.g. Google Test, Autoconf, etc) will
  remain as it is.  This code isn't developed as part of the LLVM project, it
  is used by LLVM.
* Some subprojects are impractical or uninteresting to relicense (e.g. llvm-gcc
  and dragonegg). These will be split off from the LLVM project (e.g. to
  separate GitHub projects), allowing interested people to continue their
  development elsewhere.

To relicense LLVM, we will be seeking approval from all of the copyright holders
of code in the repository, or potentially remove/rewrite code if we cannot.
This is a large
and challenging project which will take a significant amount of time to
complete.

Starting on 2024-06-01 (first of June 2024), new contributions only need to
be covered by the new LLVM license, i.e. Apache-2.0 WITH LLVM-exception.
Before this date, the project required all contributions to be made under
both the new license and the legacy license.

If you are a contributor to LLVM with contributions committed before 2019-01-19
and have not done so already, please do follow the instructions at
https://foundation.llvm.org/docs/relicensing/, under section "Individual
Relicensing Agreement" to relicense your contributions under the new license.


.. _open source licensing terms:

New LLVM Project License Framework
----------------------------------

Contributions to LLVM are licensed under the `Apache License, Version 2.0
<https://www.apache.org/licenses/LICENSE-2.0>`_, with two limited
exceptions intended to ensure that LLVM is very permissively licensed.
Collectively, the name of this license is "Apache 2.0 License with LLVM
exceptions".  The exceptions read:

::

   ---- LLVM Exceptions to the Apache 2.0 License ----

   As an exception, if, as a result of your compiling your source code, portions
   of this Software are embedded into an Object form of such source code, you
   may redistribute such embedded portions in such Object form without complying
   with the conditions of Sections 4(a), 4(b) and 4(d) of the License.

   In addition, if you combine or link compiled forms of this Software with
   software that is licensed under the GPLv2 ("Combined Software") and if a
   court of competent jurisdiction determines that the patent provision (Section
   3), the indemnity provision (Section 9) or other Section of the License
   conflicts with the conditions of the GPLv2, you may retroactively and
   prospectively choose to deem waived or otherwise exclude such Section(s) of
   the License, but only in their entirety and only with respect to the Combined
   Software.


We intend to keep LLVM perpetually open source and available under a permissive
license - this fosters the widest adoption of LLVM by
**allowing commercial products to be derived from LLVM** with few restrictions
and without a requirement for making any derived works also open source.  In
particular, LLVM's license is not a "copyleft" license like the GPL.

The "Apache 2.0 License with LLVM exceptions" allows you to:

* freely download and use LLVM (in whole or in part) for personal, internal, or
  commercial purposes.
* include LLVM in packages or distributions you create.
* combine LLVM with code licensed under every other major open source
  license (including BSD, MIT, GPLv2, GPLv3...).
* make changes to LLVM code without being required to contribute it back
  to the project - contributions are appreciated though!

However, it imposes these limitations on you:

* You must retain the copyright notice if you redistribute LLVM: You cannot
  strip the copyright headers off or replace them with your own.
* Binaries that include LLVM must reproduce the copyright notice (e.g. in an
  included README file or in an "About" box), unless the LLVM code was added as
  a by-product of compilation.  For example, if an LLVM runtime library like
  compiler_rt or libc++ was automatically included into your application by the
  compiler, you do not need to attribute it.
* You can't use our names to promote your products (LLVM derived or not) -
  though you can make truthful statements about your use of the LLVM code,
  without implying our sponsorship.
* There's no warranty on LLVM at all.

We want LLVM code to be widely used, and believe that this provides a model that
is great for contributors and users of the project.  For more information about
the Apache 2.0 License, please see the `Apache License FAQ
<http://www.apache.org/foundation/license-faq.html>`_, maintained by the
Apache Project.

.. _patent license:

Patents
-------

Section 3 of the Apache 2.0 license is a patent grant under which
contributors of code to the project contribute the rights to use any of
their patents that would otherwise be infringed by that code contribution
(protecting uses of that code).  Further, the patent grant is revoked
from anyone who files a patent lawsuit about code in LLVM - this protects the
community by providing a "patent commons" for the code base and reducing the
odds of patent lawsuits in general.

The license specifically scopes which patents are included with code
contributions.  To help explain this, the `Apache License FAQ
<http://www.apache.org/foundation/license-faq.html>`_ explains this scope using
some questions and answers, which we reproduce here for your convenience (for
reference, the "ASF" is the Apache Software Foundation, the guidance still
holds though)::

   Q1: If I own a patent and contribute to a Work, and, at the time my
   contribution is included in that Work, none of my patent's claims are subject
   to Apache's Grant of Patent License, is there a way any of those claims would
   later become subject to the Grant of Patent License solely due to subsequent
   contributions by other parties who are not licensees of that patent.

   A1: No.

   Q2: If at any time after my contribution, I am able to license other patent
   claims that would have been subject to Apache's Grant of Patent License if
   they were licensable by me at the time of my contribution, do those other
   claims become subject to the Grant of Patent License?

   A2: Yes.

   Q3: If I own or control a licensable patent and contribute code to a specific
   Apache product, which of my patent claims are subject to Apache's Grant of
   Patent License?

   A3:  The only patent claims that are licensed to the ASF are those you own or
   have the right to license that read on your contribution or on the
   combination of your contribution with the specific Apache product to which
   you contributed as it existed at the time of your contribution. No additional
   patent claims become licensed as a result of subsequent combinations of your
   contribution with any other software. Note, however, that licensable patent
   claims include those that you acquire in the future, as long as they read on
   your original contribution as made at the original time. Once a patent claim
   is subject to Apache's Grant of Patent License, it is licensed under the
   terms of that Grant to the ASF and to recipients of any software distributed
   by the ASF for any Apache software product whatsoever.

.. _legacy:

Legacy License Structure
------------------------

.. note::
   The code base was previously licensed under the Terms described here.
   We are in the middle of relicensing to a new approach (described above).
   More than 99% of all contributions made to LLVM are covered by the Apache-2.0
   WITH LLVM-exception license. A small portion of LLVM code remains exclusively
   covered by the legacy license. Contributions after 2024-06-01 are covered
   exclusively by the new license._

We intend to keep LLVM perpetually open source and to use a permissive open
source license.  The code in
LLVM is available under the `University of Illinois/NCSA Open Source License
<http://www.opensource.org/licenses/UoI-NCSA.php>`_, which boils down to
this:

* You can freely distribute LLVM.
* You must retain the copyright notice if you redistribute LLVM.
* Binaries derived from LLVM must reproduce the copyright notice (e.g. in an
  included README file).
* You can't use our names to promote your LLVM derived products.
* There's no warranty on LLVM at all.

We believe this fosters the widest adoption of LLVM because it **allows
commercial products to be derived from LLVM** with few restrictions and without
a requirement for making any derived works also open source (i.e. LLVM's
license is not a "copyleft" license like the GPL). We suggest that you read the
`License <http://www.opensource.org/licenses/UoI-NCSA.php>`_ if further
clarification is needed.

In addition to the UIUC license, the runtime library components of LLVM
(**compiler_rt, libc++, and libclc**) are also licensed under the `MIT License
<http://www.opensource.org/licenses/mit-license.php>`_, which does not contain
the binary redistribution clause.  As a user of these runtime libraries, it
means that you can choose to use the code under either license (and thus don't
need the binary redistribution clause), and as a contributor to the code that
you agree that any contributions to these libraries be licensed under both
licenses.  We feel that this is important for runtime libraries, because they
are implicitly linked into applications and therefore should not subject those
applications to the binary redistribution clause. This also means that it is ok
to move code from (e.g.)  libc++ to the LLVM core without concern, but that code
cannot be moved from the LLVM core to libc++ without the copyright owner's
permission.

.. _LLVM Discourse forums: https://discourse.llvm.org