LLVM Community Support Policy

As a compilation infrastructure, LLVM has multiple types of users, both downstream and upstream, of many combinations of its projects, tools and libraries.

There is a core part of it that encompass the implementation of the compiler (front/middle/back ends), run-time libraries (RT, C++, OpenMP, etc) and associated tools (debugger, linker, object file manipulation, etc). These components are present in the public release on our supported architectures and operating systems and the whole community must maintain and care about.

There are, however, other components within the main repository that either cater to a specific sub-community of LLVM (upstream or downstream) or help parts of the community to integrate LLVM into their own development tools or external projects. Those parts of the main repository don’t always have rigorous testing like the core parts, nor are they validated and shipped with our public upstream releases.

Even not being a core part of the project, we have enough sub-communities needing those changes with enough overlap that having them in the main repository is beneficial to minimise the repetition of those changes in all the external repositories that need them.

But the maintenance costs of such diverse ecosystem is non trivial, so we divide the level of support in two tiers: core and peripheral, with two different levels of impact and responsibilities. Those tiers refer only to the main repository (llvm-project) and not the other repositories in our git project, unless explicitly stated.

Regardless of the tier, all code must follow the existing policies on quality, reviews, style, etc.

Core Tier

The core tier encompasses all of the code in the main repository that is in production, is actively tested and released in a regular schedule, including core LLVM APIs and infrastructure, front/middle/back-ends, run-time libraries, tools, etc.

It is the responsibility of every LLVM developer to care for the core tier regardless of where their work is applied to.

What is covered

The core tier is composed of:
  • Core code (llvm-project) present in official releases and buildbots: compiler, debugger, linker, libraries, etc, including infrastructure code (table-gen, lit, file-check, unit-tests, etc).
  • Build infrastructure that creates releases and buildbots (CMake, scripts).
  • Phabricator and buildbot infrastructure.
  • The test-suite.

Requirements

Code in this tier must:
  • Keep official buildbots green, with warnings on breakages being emailed to all affected developers. Those must be fixed as soon as possible or patches must be reverted, as per review policy.
  • Bit-rot of a component in the core tier will result in that component being downgraded to the peripheral tier or being removed. Sub-communities can avoid this by fixing all raised issues in a timely manner.

Peripheral Tier

The peripheral tier encompass the parts of LLVM that cater to a specific sub-community and which don’t usually affect the core components directly.

This includes experimental back-ends, disabled-by-default options and alternative paths (work-in-progress replacements) in the same repository, as well as separate efforts to integrate LLVM development with local practices.

It is the responsibility of each sub-community to care about their own parts and the intersection of that with the core tier and other peripheral parts.

There are three main groups of code that fit in this category:
  • Code that is making its way into LLVM, via the experimental roadmap or similar efforts.
  • Code that is making its way out of LLVM, via deprecation, replacement or bit-rot, and will be removed if the sub-community that cares about it cannot maintain it.
  • Code that isn’t meant to be in LLVM core and can coexist with the code in the core tier (and others in the peripheral tier) long term, without causing breakages or disturbances.

What is covered

The peripheral tier is composed of:
  • Experimental targets and options that haven’t been enable by default yet.
  • Main repository projects that don’t get released or regularly tested.
  • Legacy tools and scripts that aren’t used in upstream validation.
  • Alternative build systems (ex. GN, Bazel) and related infrastructure.
  • Tools support (ex. gdb scripts, editor configuration, helper scripts).

Requirements

Code in this tier must:
  • Have a clear benefit for residing in the main repository, catering to an active sub-community (upstream or downstream).
  • Be actively maintained by such sub-community and have its problems addressed in a timely manner.
Code in this tier must not:
  • Break or invalidate core tier code or infrastructure. If that happens accidentally, reverting functionality and working on the issues offline is the only acceptable course of action.
  • Negatively affect development of core tier code, with the sub-community involved responsible for making changes to address specific concerns.
  • Negatively affect other peripheral tier code, with the sub-communities involved tasked to resolve the issues, still making sure the solution doesn’t break or invalidate the core tier.
  • Impose sub-optimal implementation strategies on core tier components as a result of idiosyncrasies in the peripheral component.
  • Have build infrastructure that spams all developers about their breakages.
  • Fall into disrepair. This is a reflection of lack of an active sub-community and will result in removal.
Code in this tier should:
  • Have infrastructure to test, whenever meaningful, with either no warnings or notification contained within the sub-community.
  • Have support and testing that scales with the complexity and resilience of the component, with the bar for simple and gracefully-degrading components (such as editor bindings) much lower than for complex components that must remain fresh with HEAD (such as experimental back-ends or alternative build systems).
  • Have a document making clear the status of implementation, level of support available, who the sub-community is and, if applicable, roadmap for inclusion into the core tier.
  • Be restricted to a specific directory or have a consistent pattern (ex. unique file suffix), making it easy to remove when necessary.

Inclusion Policy

To add a new peripheral component, send an RFC to the appropriate dev list proposing its addition and explaining how it will meet the support requirements listed above. Different types of components could require different levels of detail. when in doubt, ask the community what’s the best approach.

Inclusion must reach consensus in the RFC by the community and the approval of the corresponding review (by multiple members of the community) is the official note of acceptance.

After merge, there often is a period of transition, where teething issues on existing buildbots are discovered and fixed. If those cannot be fixed straight away, the sub-community is responsible for tracking and reverting all the pertinent patches and retrying the inclusion review.

Once the component is stable in tree, it must follow this policy and the deprecation rules below apply.

Due to the uncertain nature of inclusion, it’s advisable that new components are not added too close to a release branch. The time will depend on the size and complexity of the component, so adding release and testing managers on the RFC and review is strongly advisable.

Deprecation Policy

The LLVM code base has a number of files that aren’t being actively maintained. But not all of those files are obstructing the development of the project and so it remains in the repository with the assumption that it could still be useful for downstream users.

For code to remain in the repository, its presence must not impose an undue burden on maintaining other components (core or peripheral).

Warnings

There are multiple types of issues that might trigger a request for deprecation, including (but not limited to):

  • Changes in a component consistently break other areas of the project.
  • Components go broken for long periods of time (weeks or more).
  • Clearly superior alternatives are in use and maintenance is painful.
  • Builds and tests are harder / take longer, increasing the cost of maintenance, overtaking the perceived benefits.

If the maintenance cost is higher than it is acceptable by the majority of developers, it means that either the sub-community is too small (and the extra cost should be paid locally), or not active enough (and the problems won’t be fixed any time soon). In either case, removal of such problematic component is justified.

Steps for removal

However clear the needs for removal are, we should take an incremental approach to deprecating code, especially when there’s still a sub-community that cares about it. In that sense, code will never be removed outright without a series of steps are taken.

A minimum set of steps should be:
  1. A proposal for removal / deactivation should be made to the developers’ mailing lists (llvm-dev, cfe-dev, lldb-dev, etc), with a clear statement of the maintenance costs imposed and the alternatives, if applicable.
  2. There must be enough consensus on the list that removal is warranted, and no pending proposals to fix the situation from a sub-community.
  3. An announcement for removal must be made on the same lists, with ample time for downstream users to take action on their local infrastructure. The time will depend on what is being removed.
    1. If a script or documents are to be removed, they can always be pulled from previous revision, and can be removed within days.
    2. if a whole target is removed, we need to first announce publicly, and potentially mark as deprecated in one release, only to remove on the next release.
    3. Everything else will fall in between those two extremes.
  4. The removal is made by either the proposer or the sub-community that used to maintain it, with replacements and arrangements made atomically on the same commit.

If a proposal for removal is delayed by the promise a sub-community will take care of the code affected, the sub-community will have a time to fix all the issues (depending on each case, as above), and if those are not fixed in time, a subsequent request for removal should be made and the community may elect to eject the component without further attempts to fix.

Reinstatement

If a component is removed from LLVM, it may, at a later date, request inclusion of a modified version, with evidence that all of the issues were fixed and that there is a clear sub-community that will maintain it.

By consequence, the pressure on such sub-community will be higher to keep overall maintenance costs to a minimum and will need to show steps to mitigate all of the issues that were listed as reasons for its original removal.

Failing on those again, will lead to become a candidate for removal yet again.