11 This document contains the LLVM Developer Policy which defines the project's
12 policy towards developers and their contributions. The intent of this policy is
13 to eliminate miscommunication, rework, and confusion that might arise from the
14 distributed nature of LLVM's development. By stating the policy in clear terms,
15 we hope each developer can know ahead of time what to expect when making LLVM
16 contributions. This policy covers all llvm.org subprojects, including Clang,
19 This policy is also designed to accomplish the following objectives:
21 #. Attract both users and developers to the LLVM project.
23 #. Make life as simple and easy for contributors as possible.
25 #. Keep the top of Subversion trees as stable as possible.
27 #. Establish awareness of the project's :ref:`copyright, license, and patent
28 policies <copyright-license-patents>` with contributors to the project.
30 This policy is aimed at frequent contributors to LLVM. People interested in
31 contributing one-off patches can do so in an informal way by sending them to the
32 `llvm-commits mailing list
33 <http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits>`_ and engaging another
34 developer to see it through the process.
39 This section contains policies that pertain to frequent LLVM developers. We
40 always welcome `one-off patches`_ from people who do not routinely contribute to
41 LLVM, but we expect more from frequent contributors to keep the system as
42 efficient as possible for everyone. Frequent LLVM contributors are expected to
43 meet the following requirements in order for LLVM to maintain a high standard of
49 Developers should stay informed by reading at least the "dev" mailing list for
50 the projects you are interested in, such as `llvmdev
51 <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_ for LLVM, `cfe-dev
52 <http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev>`_ for Clang, or `lldb-dev
53 <http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev>`_ for LLDB. If you are
54 doing anything more than just casual work on LLVM, it is suggested that you also
55 subscribe to the "commits" mailing list for the subproject you're interested in,
57 <http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits>`_, `cfe-commits
58 <http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits>`_, or `lldb-commits
59 <http://lists.cs.uiuc.edu/mailman/listinfo/lldb-commits>`_. Reading the
60 "commits" list and paying attention to changes being made by others is a good
61 way to see what other people are interested in and watching the flow of the
64 We recommend that active developers register an email account with `LLVM
65 Bugzilla <http://llvm.org/bugs/>`_ and preferably subscribe to the `llvm-bugs
66 <http://lists.cs.uiuc.edu/mailman/listinfo/llvmbugs>`_ email list to keep track
67 of bugs and enhancements occurring in LLVM. We really appreciate people who are
68 proactive at catching incoming bugs in their components and dealing with them
71 Please be aware that all public LLVM mailing lists are public and archived, and
72 that notices of confidentiality or non-disclosure cannot be respected.
77 Making and Submitting a Patch
78 -----------------------------
80 When making a patch for review, the goal is to make it as easy for the reviewer
81 to read it as possible. As such, we recommend that you:
83 #. Make your patch against the Subversion trunk, not a branch, and not an old
84 version of LLVM. This makes it easy to apply the patch. For information on
85 how to check out SVN trunk, please see the `Getting Started
86 Guide <GettingStarted.html#checkout>`_.
88 #. Similarly, patches should be submitted soon after they are generated. Old
89 patches may not apply correctly if the underlying code changes between the
90 time the patch was created and the time it is applied.
92 #. Patches should be made with ``svn diff``, or similar. If you use a
93 different tool, make sure it uses the ``diff -u`` format and that it
94 doesn't contain clutter which makes it hard to read.
96 #. If you are modifying generated files, such as the top-level ``configure``
97 script, please separate out those changes into a separate patch from the rest
100 Once your patch is ready, submit it by emailing it to the appropriate project's
101 commit mailing list (or commit it directly if applicable). Alternatively, some
102 patches get sent to the project's development list or component of the LLVM bug
103 tracker, but the commit list is the primary place for reviews and should
104 generally be preferred.
106 When sending a patch to a mailing list, it is a good idea to send it as an
107 *attachment* to the message, not embedded into the text of the message. This
108 ensures that your mailer will not mangle the patch when it sends it (e.g. by
109 making whitespace changes or by wrapping lines).
111 *For Thunderbird users:* Before submitting a patch, please open *Preferences >
112 Advanced > General > Config Editor*, find the key
113 ``mail.content_disposition_type``, and set its value to ``1``. Without this
114 setting, Thunderbird sends your attachment using ``Content-Disposition: inline``
115 rather than ``Content-Disposition: attachment``. Apple Mail gamely displays such
116 a file inline, making it difficult to work with for reviewers using that
119 When submitting patches, please do not add confidentiality or non-disclosure
120 notices to the patches themselves. These notices conflict with the `LLVM
121 License`_ and may result in your contribution being excluded.
128 LLVM has a code review policy. Code review is one way to increase the quality of
129 software. We generally follow these policies:
131 #. All developers are required to have significant changes reviewed before they
132 are committed to the repository.
134 #. Code reviews are conducted by email on the relevant project's commit mailing
135 list, or alternatively on the project's development list or bug tracker.
137 #. Code can be reviewed either before it is committed or after. We expect major
138 changes to be reviewed before being committed, but smaller changes (or
139 changes where the developer owns the component) can be reviewed after commit.
141 #. The developer responsible for a code change is also responsible for making
142 all necessary review-related changes.
144 #. Code review can be an iterative process, which continues until the patch is
145 ready to be committed. Specifically, once a patch is sent out for review, it
146 needs an explicit "looks good" before it is submitted. Do not assume silent
147 approval, or request active objections to the patch with a deadline.
149 Sometimes code reviews will take longer than you would hope for, especially for
150 larger features. Accepted ways to speed up review times for your patches are:
152 * Review other people's patches. If you help out, everybody will be more
153 willing to do the same for you; goodwill is our currency.
154 * Ping the patch. If it is urgent, provide reasons why it is important to you to
155 get this patch landed and ping it every couple of days. If it is
156 not urgent, the common courtesy ping rate is one week. Remember that you're
157 asking for valuable time from other professional developers.
158 * Ask for help on IRC. Developers on IRC will be able to either help you
159 directly, or tell you who might be a good reviewer.
160 * Split your patch into multiple smaller patches that build on each other. The
161 smaller your patch, the higher the probability that somebody will take a quick
164 Developers should participate in code reviews as both reviewers and
165 reviewees. If someone is kind enough to review your code, you should return the
166 favor for someone else. Note that anyone is welcome to review and give feedback
167 on a patch, but only people with Subversion write access can approve it.
169 There is a web based code review tool that can optionally be used
170 for code reviews. See :doc:`Phabricator`.
175 The LLVM Project relies on two features of its process to maintain rapid
176 development in addition to the high quality of its source base: the combination
177 of code review plus post-commit review for trusted maintainers. Having both is
178 a great way for the project to take advantage of the fact that most people do
179 the right thing most of the time, and only commit patches without pre-commit
180 review when they are confident they are right.
182 The trick to this is that the project has to guarantee that all patches that are
183 committed are reviewed after they go in: you don't want everyone to assume
184 someone else will review it, allowing the patch to go unreviewed. To solve this
185 problem, we have a notion of an 'owner' for a piece of the code. The sole
186 responsibility of a code owner is to ensure that a commit to their area of the
187 code is appropriately reviewed, either by themself or by someone else. The list
188 of current code owners can be found in the file
189 `CODE_OWNERS.TXT <http://llvm.org/viewvc/llvm-project/llvm/trunk/CODE_OWNERS.TXT?view=markup>`_
190 in the root of the LLVM source tree.
192 Note that code ownership is completely different than reviewers: anyone can
193 review a piece of code, and we welcome code review from anyone who is
194 interested. Code owners are the "last line of defense" to guarantee that all
195 patches that are committed are actually reviewed.
197 Being a code owner is a somewhat unglamorous position, but it is incredibly
198 important for the ongoing success of the project. Because people get busy,
199 interests change, and unexpected things happen, code ownership is purely opt-in,
200 and anyone can choose to resign their "title" at any time. For now, we do not
201 have an official policy on how one gets elected to be a code owner.
203 .. _include a testcase:
208 Developers are required to create test cases for any bugs fixed and any new
209 features added. Some tips for getting your testcase approved:
211 * All feature and regression test cases are added to the ``llvm/test``
212 directory. The appropriate sub-directory should be selected (see the
213 :doc:`Testing Guide <TestingGuide>` for details).
215 * Test cases should be written in `LLVM assembly language <LangRef.html>`_
216 unless the feature or regression being tested requires another language
217 (e.g. the bug being fixed or feature being implemented is in the llvm-gcc C++
218 front-end, in which case it must be written in C++).
220 * Test cases, especially for regressions, should be reduced as much as possible,
221 by `bugpoint <Bugpoint.html>`_ or manually. It is unacceptable to place an
222 entire failing program into ``llvm/test`` as this creates a *time-to-test*
223 burden on all developers. Please keep them short.
225 Note that llvm/test and clang/test are designed for regression and small feature
226 tests only. More extensive test cases (e.g., entire applications, benchmarks,
227 etc) should be added to the ``llvm-test`` test suite. The llvm-test suite is
228 for coverage (correctness, performance, etc) testing, not feature or regression
234 The minimum quality standards that any change must satisfy before being
235 committed to the main development branch are:
237 #. Code must adhere to the `LLVM Coding Standards <CodingStandards.html>`_.
239 #. Code must compile cleanly (no errors, no warnings) on at least one platform.
241 #. Bug fixes and new features should `include a testcase`_ so we know if the
242 fix/feature ever regresses in the future.
244 #. Code must pass the ``llvm/test`` test suite.
246 #. The code must not cause regressions on a reasonable subset of llvm-test,
247 where "reasonable" depends on the contributor's judgement and the scope of
248 the change (more invasive changes require more testing). A reasonable subset
249 might be something like "``llvm-test/MultiSource/Benchmarks``".
251 Additionally, the committer is responsible for addressing any problems found in
252 the future that the change is responsible for. For example:
254 * The code should compile cleanly on all supported platforms.
256 * The changes should not cause any correctness regressions in the ``llvm-test``
257 suite and must not cause any major performance regressions.
259 * The change set should not cause performance or correctness regressions for the
262 * The changes should not cause performance or correctness regressions in code
263 compiled by LLVM on all applicable targets.
265 * You are expected to address any `Bugzilla bugs <http://llvm.org/bugs/>`_ that
266 result from your change.
268 We prefer for this to be handled before submission but understand that it isn't
269 possible to test all of this for every submission. Our build bots and nightly
270 testing infrastructure normally finds these problems. A good rule of thumb is
271 to check the nightly testers for regressions the day after your change. Build
272 bots will directly email you if a group of commits that included yours caused a
273 failure. You are expected to check the build bot messages to see if they are
274 your fault and, if so, fix the breakage.
276 Commits that violate these quality standards (e.g. are very broken) may be
277 reverted. This is necessary when the change blocks other developers from making
278 progress. The developer is welcome to re-commit the change after the problem has
281 Obtaining Commit Access
282 -----------------------
284 We grant commit access to contributors with a track record of submitting high
285 quality patches. If you would like commit access, please send an email to
286 `Chris <mailto:sabre@nondot.org>`_ with the following information:
288 #. The user name you want to commit with, e.g. "hacker".
290 #. The full name and email address you want message to llvm-commits to come
291 from, e.g. "J. Random Hacker <hacker@yoyodyne.com>".
293 #. A "password hash" of the password you want to use, e.g. "``2ACR96qjUqsyM``".
294 Note that you don't ever tell us what your password is; you just give it to
295 us in an encrypted form. To get this, run "``htpasswd``" (a utility that
296 comes with apache) in crypt mode (often enabled with "``-d``"), or find a web
297 page that will do it for you.
299 Once you've been granted commit access, you should be able to check out an LLVM
300 tree with an SVN URL of "https://username@llvm.org/..." instead of the normal
301 anonymous URL of "http://llvm.org/...". The first time you commit you'll have
302 to type in your password. Note that you may get a warning from SVN about an
303 untrusted key; you can ignore this. To verify that your commit access works,
304 please do a test commit (e.g. change a comment or add a blank line). Your first
305 commit to a repository may require the autogenerated email to be approved by a
306 mailing list. This is normal and will be done when the mailing list owner has
309 If you have recently been granted commit access, these policies apply:
311 #. You are granted *commit-after-approval* to all parts of LLVM. To get
312 approval, submit a `patch`_ to `llvm-commits
313 <http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits>`_. When approved,
314 you may commit it yourself.
316 #. You are allowed to commit patches without approval which you think are
317 obvious. This is clearly a subjective decision --- we simply expect you to
318 use good judgement. Examples include: fixing build breakage, reverting
319 obviously broken patches, documentation/comment changes, any other minor
322 #. You are allowed to commit patches without approval to those portions of LLVM
323 that you have contributed or maintain (i.e., have been assigned
324 responsibility for), with the proviso that such commits must not break the
325 build. This is a "trust but verify" policy, and commits of this nature are
326 reviewed after they are committed.
328 #. Multiple violations of these policies or a single egregious violation may
329 cause commit access to be revoked.
331 In any case, your changes are still subject to `code review`_ (either before or
332 after they are committed, depending on the nature of the change). You are
333 encouraged to review other peoples' patches as well, but you aren't required
336 .. _discuss the change/gather consensus:
338 Making a Major Change
339 ---------------------
341 When a developer begins a major new project with the aim of contributing it back
342 to LLVM, s/he should inform the community with an email to the `llvmdev
343 <http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>`_ email list, to the extent
344 possible. The reason for this is to:
346 #. keep the community informed about future changes to LLVM,
348 #. avoid duplication of effort by preventing multiple parties working on the
349 same thing and not knowing about it, and
351 #. ensure that any technical issues around the proposed work are discussed and
352 resolved before any significant work is done.
354 The design of LLVM is carefully controlled to ensure that all the pieces fit
355 together well and are as consistent as possible. If you plan to make a major
356 change to the way LLVM works or want to add a major new extension, it is a good
357 idea to get consensus with the development community before you start working on
360 Once the design of the new feature is finalized, the work itself should be done
361 as a series of `incremental changes`_, not as a long-term development branch.
363 .. _incremental changes:
365 Incremental Development
366 -----------------------
368 In the LLVM project, we do all significant changes as a series of incremental
369 patches. We have a strong dislike for huge changes or long-term development
370 branches. Long-term development branches have a number of drawbacks:
372 #. Branches must have mainline merged into them periodically. If the branch
373 development and mainline development occur in the same pieces of code,
374 resolving merge conflicts can take a lot of time.
376 #. Other people in the community tend to ignore work on branches.
378 #. Huge changes (produced when a branch is merged back onto mainline) are
379 extremely difficult to `code review`_.
381 #. Branches are not routinely tested by our nightly tester infrastructure.
383 #. Changes developed as monolithic large changes often don't work until the
384 entire set of changes is done. Breaking it down into a set of smaller
385 changes increases the odds that any of the work will be committed to the main
388 To address these problems, LLVM uses an incremental development style and we
389 require contributors to follow this practice when making a large/invasive
392 * Large/invasive changes usually have a number of secondary changes that are
393 required before the big change can be made (e.g. API cleanup, etc). These
394 sorts of changes can often be done before the major change is done,
395 independently of that work.
397 * The remaining inter-related work should be decomposed into unrelated sets of
398 changes if possible. Once this is done, define the first increment and get
399 consensus on what the end goal of the change is.
401 * Each change in the set can be stand alone (e.g. to fix a bug), or part of a
402 planned series of changes that works towards the development goal.
404 * Each change should be kept as small as possible. This simplifies your work
405 (into a logical progression), simplifies code review and reduces the chance
406 that you will get negative feedback on the change. Small increments also
407 facilitate the maintenance of a high quality code base.
409 * Often, an independent precursor to a big change is to add a new API and slowly
410 migrate clients to use the new API. Each change to use the new API is often
411 "obvious" and can be committed without review. Once the new API is in place
412 and used, it is much easier to replace the underlying implementation of the
413 API. This implementation change is logically separate from the API
416 If you are interested in making a large change, and this scares you, please make
417 sure to first `discuss the change/gather consensus`_ then ask about the best way
418 to go about making the change.
420 Attribution of Changes
421 ----------------------
423 When contributors submit a patch to an LLVM project, other developers with
424 commit access may commit it for the author once appropriate (based on the
425 progression of code review, etc.). When doing so, it is important to retain
426 correct attribution of contributions to their contributors. However, we do not
427 want the source code to be littered with random attributions "this code written
428 by J. Random Hacker" (this is noisy and distracting). In practice, the revision
429 control system keeps a perfect history of who changed what, and the CREDITS.txt
430 file describes higher-level contributions. If you commit a patch for someone
431 else, please say "patch contributed by J. Random Hacker!" in the commit
432 message. Overall, please do not add contributor names to the source code.
434 Also, don't commit patches authored by others unless they have submitted the
435 patch to the project or you have been authorized to submit them on their behalf
436 (you work together and your company authorized you to contribute the patches,
437 etc.). The author should first submit them to the relevant project's commit
438 list, development list, or LLVM bug tracker component. If someone sends you
439 a patch privately, encourage them to submit it to the appropriate list first.
442 .. _copyright-license-patents:
444 Copyright, License, and Patents
445 ===============================
449 This section deals with legal matters but does not provide legal advice. We
450 are not lawyers --- please seek legal counsel from an attorney.
452 This section addresses the issues of copyright, license and patents for the LLVM
453 project. The copyright for the code is held by the individual contributors of
454 the code and the terms of its license to LLVM users and developers is the
455 `University of Illinois/NCSA Open Source License
456 <http://www.opensource.org/licenses/UoI-NCSA.php>`_ (with portions dual licensed
457 under the `MIT License <http://www.opensource.org/licenses/mit-license.php>`_,
458 see below). As contributor to the LLVM project, you agree to allow any
459 contributions to the project to licensed under these terms.
464 The LLVM project does not require copyright assignments, which means that the
465 copyright for the code in the project is held by its respective contributors who
466 have each agreed to release their contributed code under the terms of the `LLVM
469 An implication of this is that the LLVM license is unlikely to ever change:
470 changing it would require tracking down all the contributors to LLVM and getting
471 them to agree that a license change is acceptable for their contribution. Since
472 there are no plans to change the license, this is not a cause for concern.
474 As a contributor to the project, this means that you (or your company) retain
475 ownership of the code you contribute, that it cannot be used in a way that
476 contradicts the license (which is a liberal BSD-style license), and that the
477 license for your contributions won't change without your approval in the
485 We intend to keep LLVM perpetually open source and to use a liberal open source
486 license. **As a contributor to the project, you agree that any contributions be
487 licensed under the terms of the corresponding subproject.** All of the code in
488 LLVM is available under the `University of Illinois/NCSA Open Source License
489 <http://www.opensource.org/licenses/UoI-NCSA.php>`_, which boils down to
492 * You can freely distribute LLVM.
493 * You must retain the copyright notice if you redistribute LLVM.
494 * Binaries derived from LLVM must reproduce the copyright notice (e.g. in an
495 included readme file).
496 * You can't use our names to promote your LLVM derived products.
497 * There's no warranty on LLVM at all.
499 We believe this fosters the widest adoption of LLVM because it **allows
500 commercial products to be derived from LLVM** with few restrictions and without
501 a requirement for making any derived works also open source (i.e. LLVM's
502 license is not a "copyleft" license like the GPL). We suggest that you read the
503 `License <http://www.opensource.org/licenses/UoI-NCSA.php>`_ if further
504 clarification is needed.
506 In addition to the UIUC license, the runtime library components of LLVM
507 (**compiler_rt, libc++, and libclc**) are also licensed under the `MIT License
508 <http://www.opensource.org/licenses/mit-license.php>`_, which does not contain
509 the binary redistribution clause. As a user of these runtime libraries, it
510 means that you can choose to use the code under either license (and thus don't
511 need the binary redistribution clause), and as a contributor to the code that
512 you agree that any contributions to these libraries be licensed under both
513 licenses. We feel that this is important for runtime libraries, because they
514 are implicitly linked into applications and therefore should not subject those
515 applications to the binary redistribution clause. This also means that it is ok
516 to move code from (e.g.) libc++ to the LLVM core without concern, but that code
517 cannot be moved from the LLVM core to libc++ without the copyright owner's
520 Note that the LLVM Project does distribute llvm-gcc and dragonegg, **which are
521 GPL.** This means that anything "linked" into llvm-gcc must itself be compatible
522 with the GPL, and must be releasable under the terms of the GPL. This implies
523 that **any code linked into llvm-gcc and distributed to others may be subject to
524 the viral aspects of the GPL** (for example, a proprietary code generator linked
525 into llvm-gcc must be made available under the GPL). This is not a problem for
526 code already distributed under a more liberal license (like the UIUC license),
527 and GPL-containing subprojects are kept in separate SVN repositories whose
528 LICENSE.txt files specifically indicate that they contain GPL code.
530 We have no plans to change the license of LLVM. If you have questions or
531 comments about the license, please contact the `LLVM Developer's Mailing
532 List <mailto:llvmdev@cs.uiuc.edu>`_.
537 To the best of our knowledge, LLVM does not infringe on any patents (we have
538 actually removed code from LLVM in the past that was found to infringe). Having
539 code in LLVM that infringes on patents would violate an important goal of the
540 project by making it hard or impossible to reuse the code for arbitrary purposes
541 (including commercial use).
543 When contributing code, we expect contributors to notify us of any potential for
544 patent-related trouble with their changes (including from third parties). If
545 you or your employer own the rights to a patent and would like to contribute
546 code to LLVM that relies on it, we require that the copyright owner sign an
547 agreement that allows any other user of LLVM to freely use your patent. Please
548 contact the `oversight group <mailto:llvm-oversight@cs.uiuc.edu>`_ for more