LLVM Developer Policy — LLVM 21.0.0git documentation (original) (raw)

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:

  1. Attract both users and developers to the LLVM project.
  2. Make life as simple and easy for contributors as possible.
  3. Keep the top of tree as stable as possible.
  4. Establish awareness of the project’s copyright, license, and patent policies 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 thellvm-commits mailing list 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 GitHub Pull Requests. You can subscribe to notification for areas of the codebase by joining one of the pr-subscribers-*GitHub teams. This mappingindicates 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, cfe-commits, or lldb-commits.

Missing features and bugs are tracked through our GitHub issue trackerand 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-*teams. You may also subscribe to the 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.

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:

  1. 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 Getting Started Guide.
  2. 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.
  3. Once you have created your patch, create aGitHub Pull Request 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.

Email Addresses

The LLVM project uses email to communicate to contributors outside of the GitHub platform about their past contributions. Primarily, our buildbot infrastructure uses emails to contact contributors about build and test failures.

Therefore, the LLVM community requires contributors to have a public email address associated with their GitHub commits, so please ensure that “Keep my email addresses private” is disabled in youraccount settings.

Code Reviews

LLVM has a code-review policy. Code review is one way to increase the quality of software. Please see LLVM Code-Review Policy and Practices for more information on LLVM’s code-review process.

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 and is available in read-only mode, for new code contributions use GitHub Pull Requests. This section contains old information that needs to be updated.

Maintainers

The LLVM Project aims to evolve features quickly while continually being in a release-ready state. In order to accomplish this, the project needs volunteers willing to do the less glamorous work to ensure we produce robust, high-quality products.

Maintainers are those volunteers; they are regular contributors who volunteer to take on additional community responsibilities beyond code contributions. Community members can find active and inactive maintainers for a project in theMaintainers.rst file at the root directory of the individual project.

Maintainers are volunteering to take on the following shared responsibilities within an area of a project:

Each top-level project in the monorepo will specify one or more lead maintainers who are responsible for ensuring community needs are met for that project. This role is like any other maintainer role, except the responsibilities span the project rather than a limited area within the project. If you cannot reach a maintainer or don’t know which maintainer to reach out to, a lead maintainer is always a good choice to reach out to. If a project has no active lead maintainers, it may be a reasonable candidate for removal from the monorepo. A discussion should be started on Discourse to find a new, active lead maintainer or whether the project should be discontinued.

All contributors with commit access to the LLVM Project are eligible to be a maintainer. However, we are looking for people who can commit to:

We recognize that priorities shift, job changes happen, burnout is real, extended vacations are a blessing, and people’s lives are generally complex. Therefore, we want as little friction as possible for someone to become a maintainer or to step down as a maintainer.

To become a new maintainer, you can volunteer yourself by posting a PR which adds yourself to the area(s) you are volunteering for. Alternatively, an existing maintainer can nominate you by posting a PR, but the nominee must explicitly accept the PR so that it’s clear they agree to volunteer within the proposed area(s). The PR will be accepted so long as at least one maintainer in the same project vouches for their ability to perform the responsibilities and there are no explicit objections raised by the community.

To step down as a maintainer, you can move your name to the “inactive maintainers” section of the Maintainers.rst file for the project, or remove your name entirely; no PR review is necessary. Additionally, any maintainer who has not been actively performing their responsibilities over an extended period of time can be moved to the “inactive maintainers” section by another active maintainer within that project with agreement from one other active maintainer within that project. If there is only one active maintainer for a project, please post on Discourse to solicit wider community feedback about the removal and future direction for the project. However, please discuss the situation with the inactive maintainer before such removal to avoid accidental miscommunications. If the inactive maintainer is unreachable, no discussion with them is required. Stepping down or being removed as a maintainer is normal and does not prevent someone from resuming their activities as a maintainer in the future.

To resume activities as a maintainer, you can post a PR moving your name from the “inactive maintainers” section of the Maintainers.rst file to the active maintainers list. Because the volunteer was already previously accepted, they will be re-accepted so long as at least one maintainer in the same project approves the PR and there are no explicit objections raised by the community.

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:

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):

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:

  1. Code must adhere to the LLVM Coding Standards.
  2. Code must compile cleanly (no errors, no warnings) on at least one platform.
  3. Bug fixes and new features should include a testcase so we know if the fix/feature ever regresses in the future.
  4. Code must pass the llvm/test test suite.
  5. 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”.
  6. 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:

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

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:

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.

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?

When should you revert your own change?

When should you revert someone else’s change?

What are the expectations around a revert?

Obtaining Commit Access

We grant commit access to contributors that can provide a valid justification. If you would like commit access, please use this link to file an issue and request commit access. Replace the string in the title with your github username, and explain why you are requesting commit access in the issue description. If approved, a GitHub invitation will be sent to your GitHub account. In case you don’t get notification from GitHub, go toInvitation Link directly. Once you 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:

  1. You are granted commit-after-approval to all parts of LLVM. For information on how to get approval for a patch, please see LLVM Code-Review Policy and Practices. When approved, you may commit it yourself.
  2. 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 isNFC.
  3. 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.
  4. 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.

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:

  1. keep the community informed about future changes to LLVM,
  2. avoid duplication of effort by preventing multiple parties working on the same thing and not knowing about it, and
  3. 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 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:

  1. 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.
  2. Other people in the community tend to ignore work on branches.
  3. Huge changes (produced when a branch is merged back onto mainline) are extremely difficult to code review.
  4. Branches are not routinely tested by our nightly tester infrastructure.
  5. 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:

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 theLLVM 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 toconduct@llvm.org for advice.

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:

C API Changes

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:

Here’s a sample RFC and thecorresponding change.

Working with the CI system

The main continuous integration (CI) tool for the LLVM project is theLLVM 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:

If someone else broke the build and this blocks your work

If a build/worker is permanently broken

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 support policy 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 theLLVM 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 oursupport policy.

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:

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

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

To continue as a supported and official target:

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, maintainers, 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 maintainers 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 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 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 support policy. 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:

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:

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:

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.

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 theLLVM 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.

The LLVM project does not accept contributions that include in-source copyright notices except where such notices are part of a larger external project being added as a vendored dependency.

LLVM source code lives for a long time and is edited by many people, the best way to track contributions is through revision control history. See the Attribution of Changes section for more information about attributing changes to authors other than the committer.

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 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:

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 athttps://foundation.llvm.org/docs/relicensing/, under section “Individual Relicensing Agreement” to relicense your contributions under the new license.

New LLVM Project License Framework

Contributions to LLVM are licensed under the Apache License, Version 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 byallowing 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:

However, it imposes these limitations on you:

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, maintained by the Apache Project.

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 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 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, 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 theLicense 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, 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.

AI generated contributions

Artificial intelligence systems raise many questions around copyright that have yet to be answered. Our policy on AI tools is guided by our copyright policy: Contributors are responsible for ensuring that they have the right to contribute code under the terms of our license, typically meaning that either they, their employer, or their collaborators hold the copyright. Using AI tools to regenerate copyrighted material does not remove the copyright, and contributors are responsible for ensuring that such material does not appear in their contributions.

As such, the LLVM policy is that contributors are permitted to use artificial intelligence tools to produce contributions, provided that they have the right to license that code under the project license. Contributions found to violate this policy will be removed just like any other offending contribution.

While the LLVM project has a liberal policy on AI tool use, contributors are considered responsible for their contributions. We encourage contributors to review all generated code before sending it for review to verify its correctness and to understand it so that they can answer questions during code review. Reviewing and maintaining generated code that the original contributor does not understand is not a good use of limited project resources.