[ARM] MVE compare vector splat combine

[llvm-complete.git] / docs / DeveloperPolicy.rst

=====================
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 at least the "dev" mailing list for
the projects you are interested in, such as `llvm-dev
<http://lists.llvm.org/mailman/listinfo/llvm-dev>`_ for LLVM, `cfe-dev
<http://lists.llvm.org/mailman/listinfo/cfe-dev>`_ for Clang, or `lldb-dev
<http://lists.llvm.org/mailman/listinfo/lldb-dev>`_ for LLDB.  If you are
doing anything more than just casual work on LLVM, it is suggested that you also
subscribe to the "commits" mailing list for the 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>`_.  Reading the
"commits" list and 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.

We recommend that active developers register an email account with `LLVM
Bugzilla <https://bugs.llvm.org/>`_ and preferably subscribe to the `llvm-bugs
<http://lists.llvm.org/mailman/listinfo/llvm-bugs>`_ email list to keep track
of bugs and enhancements occurring in LLVM.  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 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 master, 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
   <checkout>`.

#. 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.

#. Patches should be made with ``git format-patch``, or similar. If you use a
   different tool, make sure it uses the ``diff -u`` format and that it
   doesn't contain clutter which makes it hard to read.

Once your patch is ready, submit it by emailing it to the appropriate project's
commit mailing list (or commit it directly if applicable). Alternatively, some
patches get sent to the project's development list or component of the LLVM bug
tracker, but the commit list is the primary place for reviews and should
generally be preferred.

When sending a patch to a mailing list, it is a good idea to send it as an
*attachment* to the message, not embedded into the text of the message.  This
ensures that your mailer will not mangle the patch when it sends it (e.g. by
making whitespace changes or by wrapping lines).

*For Thunderbird users:* Before submitting a patch, please open *Preferences >
Advanced > General > Config Editor*, find the key
``mail.content_disposition_type``, and set its value to ``1``. Without this
setting, Thunderbird sends your attachment using ``Content-Disposition: inline``
rather than ``Content-Disposition: attachment``. Apple Mail gamely displays such
a file inline, making it difficult to work with for reviewers using that
program.

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. We generally follow these policies:

#. All developers are required to have significant changes reviewed before they
   are committed to the repository.

#. Code reviews are conducted by email on the relevant project's commit mailing
   list, or alternatively on the project's development list or bug tracker.

#. Code can be reviewed either before it is committed or after.  We expect major
   changes to be reviewed before being committed, but smaller changes (or
   changes where the developer owns the component) can be reviewed after commit.

#. The developer responsible for a code change is also responsible for making
   all necessary review-related changes.

#. Code review can be an iterative process, which continues until the patch is
   ready to be committed. Specifically, once a patch is sent out for review, it
   needs an explicit "looks good" before it is submitted. Do not assume silent
   approval, or request active objections to the patch with a deadline.

Sometimes code reviews will take longer than you would hope for, especially for
larger features. Accepted ways to speed up review times for your patches are:

* Review other people's patches. If you help out, everybody will be more
  willing to do the same for you; goodwill is our currency.
* Ping the patch. If it is urgent, provide reasons why it is important to you to
  get this patch landed and ping it every couple of days. If it is
  not urgent, the common courtesy ping rate is one week. Remember that you're
  asking for valuable time from other professional developers.
* Ask for help on IRC. Developers on IRC will be able to either help you
  directly, or tell you who might be a good reviewer.
* Split your patch into multiple smaller patches that build on each other. The
  smaller your patch, the higher the probability that somebody will take a quick
  look at it.

Developers should participate in code reviews as both reviewers and
reviewees. If someone is kind enough to review your code, you should return the
favor for someone else.  Note that anyone is welcome to review and give feedback
on a patch, but only people with Subversion write access can approve it.

There is a web based code review tool that can optionally be used
for code reviews. See :doc:`Phabricator`.

.. _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/master/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.

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.

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``".

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 `Bugzilla bugs <https://bugs.llvm.org/>`_ 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, body and, if you're not the original
  author, a "Patch by" attribution line (see below).

* 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.

* If the patch fixes a bug in bugzilla, please include the PR# in the message.

* `Attribution of Changes`_ should be in a separate line, after the end of
  the body, as simple as "Patch by John Doe.". This is how we officially
  handle attribution, and there are automated processes that rely on this
  format.

* 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 svn revision number of the prior
  related commit. This could be as simple as "Revert rNNNN because it caused
  PR#".

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.

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

We grant commit access to contributors with a track record of submitting high
quality patches.  If you would like commit access, please send an email to
`Chris <mailto:clattner@llvm.org>`_ with the following information:

#. The user name you want to commit with, e.g. "hacker".

#. The full name and email address you want message to llvm-commits to come
   from, e.g. "J. Random Hacker <hacker@yoyodyne.com>".

#. A "password hash" of the password you want to use, e.g. "``2ACR96qjUqsyM``".
   Note that you don't ever tell us what your password is; you just give it to
   us in an encrypted form.  To get this, run "``htpasswd``" (a utility that
   comes with apache) in *crypt* mode (often enabled with "``-d``"), or find a web
   page that will do it for you.  Note that our system does not work with MD5
   hashes.  These are significantly longer than a crypt hash - e.g.
   "``$apr1$vea6bBV2$Z8IFx.AfeD8LhqlZFqJer0``", we only accept the shorter crypt hash.

Once you've been granted commit access, you should be able to check out an LLVM
tree with an SVN URL of "https://username@llvm.org/..." instead of the normal
anonymous URL of "http://llvm.org/...".  The first time you commit you'll have
to type in your password.  Note that you may get a warning from SVN about an
untrusted key; you can ignore this.  To verify that your commit access works,
please do a test commit (e.g. change a comment or add a blank line).  Your first
commit to a repository may require the autogenerated email to be approved by a
moderator of the mailing list.
This is normal and will be done when the mailing list owner has time.

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

#. You are granted *commit-after-approval* to all parts of LLVM.  To get
   approval, submit a `patch`_ to `llvm-commits
   <http://lists.llvm.org/mailman/listinfo/llvm-commits>`_. 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 an email to the `llvm-dev
<http://lists.llvm.org/mailman/listinfo/llvm-dev>`_ email list, 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.


.. _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
  mailing list 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.

New Targets
-----------

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.  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 difference
between both classes is that experimental targets are not built by default
(need to be added to -DLLVM_TARGETS_TO_BUILD at CMake time).

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 essences, 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.

.. _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:

  * Generally, try 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-dev mailing list <http://lists.llvm.org/mailman/listinfo/llvm-dev>`_

    - 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. We want to
    soft-error when developers compile LLVM. We say "soft-error" 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 mailing list, 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
<http://lists.llvm.org/pipermail/llvm-dev/2019-January/129452.html>`_ and the
`corresponding change <https://reviews.llvm.org/D57264>`_.

.. _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 license, which includes a copyright and `patent license`_.
When you contribute code to the LLVM project, you license it under these terms.

If you have questions or comments about these topics, please contact the
`LLVM Developer's Mailing List <mailto:llvm-dev@lists.llvm.org>`_.  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.  In the interim, **all contributions to the project will be made under
the terms of both the new license and the legacy license scheme** (each of which
is described below).  The exception to this is the legacy patent grant, which
will not be required for new contributions.

When all of the code in the project has been converted to the new license or
removed, we will drop the requirement to contribute under the legacy license.
This will achieve the goal of having
a single standardized license for the entire codebase.

If you are a prior contributor to LLVM and have not done so already, please do
*TODO* to allow us to use your code. *Add a link to a separate page here, which
is probably a click through web form or something like that.  Details to be
determined later*.


.. _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.


.. note::

   The LLVM Project includes some really old subprojects (dragonegg,
   llvm-gcc-4.0, and llvm-gcc-4.2), which are licensed under **GPL
   licenses**.  This code is not actively maintained - it does not even
   build successfully.  This code is cleanly separated into distinct SVN
   repositories from the rest of LLVM, and the LICENSE.txt files specifically
   indicate that they contain GPL code.  When LLVM transitions from SVN to Git,
   we plan to drop these code bases from the new repository structure.


.. _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 licenseable 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), but
   until this effort is complete, the code is also still available under these
   terms.  Once we finish the relicensing project, new versions of the code will
   not be available under these terms.  However, nothing takes away your right
   to use old versions under the licensing terms under which they were
   originally released.

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.