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 is also designed to accomplish the following objectives:
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 and engaging another developer to see it through the process.
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.
Developers should stay informed by reading at least the llvmdev email list. If you are doing anything more than just casual work on LLVM, it is suggested that you also subscribe to the llvm-commits list and pay attention to changes being made by others.
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:
svn diff -x -u
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.
LLVM has a code review policy. Code review is one way to increase the quality of software. We generally follow these policies:
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.
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 current code owners are:
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.
Developers are required to create test cases for any bugs fixed and any new features added. Some tips for getting your testcase approved:
Note that llvm/test is 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.
The minimum quality standards that any change must satisfy before being committed to the main development branch are:
Additionally, the committer is responsible for addressing any problems found in the future that the change is responsible for. For example:
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 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.
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.
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 with the following information:
Once you've been granted commit access, you should be able to check out an LLVM tree with an SVN URL of "https://email@example.com/..." 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 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:
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.
When a developer begins a major new project with the aim of contributing it back to LLVM, s/he should inform the community with an email to the llvmdev email list, to the extent possible. The reason for this is to:
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.
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:
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:
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.
We believe in 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 say "patch contributed by J. Random Hacker!" in the commit message.
Overall, please do not add contributor names to the source code.
This section addresses the issues of copyright, license and patents for the LLVM project. Currently, the University of Illinois is the LLVM copyright holder and the terms of its license to LLVM users and developers is the University of Illinois/NCSA Open Source License.
NOTE: This section deals with legal matters but does not provide legal advice. We are not lawyers, please seek legal counsel from an attorney.
For consistency and ease of management, the project requires the copyright for all LLVM software to be held by a single copyright holder: the University of Illinois (UIUC).
Although UIUC may eventually reassign the copyright of the software to another entity (e.g. a dedicated non-profit "LLVM Organization") the intent for the project is to always have a single entity hold the copyrights to LLVM at any given time.
We believe that having a single copyright holder is in the best interests of all developers and users as it greatly reduces the managerial burden for any kind of administrative or technical decisions about LLVM. The goal of the LLVM project is to always keep the code open and licensed under a very liberal license.
We intend to keep LLVM perpetually open source and to use a liberal open source license. The current license is the University of llinois/NCSA Open Source License, which boils down to this:
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 if further clarification is needed.
Note that the LLVM Project does distribute llvm-gcc, which is GPL. This means that anything "linked" into llvm-gcc must itself be compatible with the GPL, and must be releasable under the terms of the GPL. This implies that any code linked into llvm-gcc and distributed to others may be subject to the viral aspects of the GPL (for example, a proprietary code generator linked into llvm-gcc must be made available under the GPL). This is not a problem for code already distributed under a more liberal license (like the UIUC license), and does not affect code generated by llvm-gcc. It may be a problem if you intend to base commercial development on llvm-gcc without redistributing your source code.
We have no plans to change the license of LLVM. If you have questions or comments about the license, please contact the LLVM Oversight Group.
To the best of our knowledge, LLVM does not infringe on any patents (we have actually removed code from LLVM in the past that was found to infringe). Having code in LLVM that infringes on patents would violate an important goal of the project by making it hard or impossible to reuse the code for arbitrary purposes (including commercial use).
When contributing code, we expect contributors to notify us of any potential for patent-related trouble with their changes. If you or your employer own the rights to a patent and would like to contribute code to LLVM that relies on it, we require that the copyright owner sign an agreement that allows any other user of LLVM to freely use your patent. Please contact the oversight group for more details.
With regards to the LLVM copyright and licensing, developers agree to assign their copyrights to UIUC for any contribution made so that the entire software base can be managed by a single copyright holder. This implies that any contributions can be licensed under the license that the project uses.
When contributing code, you also affirm that you are legally entitled to grant this copyright, personally or on behalf of your employer. If the code belongs to some other entity, please raise this issue with the oversight group before the code is committed.