Bringup on a New OS or Architecture

Building the libc

An OS specific config directory

If you are starting to bring up LLVM’s libc on a new operating system, the first step is to add a directory for that OS in the libc/config directory. Both Linux and Windows, the two operating systems on which LLVM’s libc is being actively developed, have their own config directory.

Note

Windows development is not as active as the development on Linux. There is a Darwin config also which is in a similar state as Windows.

Note

LLVM’s libc is being brought up on the Fuchsia operating system also. However, there is no config directory for Fuchsia as the bring up is being done in the Fuchsia source tree.

Architecture Subdirectory

There are parts of the libc which are implemented differently for different architectures. The simplest example of this is the syscall function and its internal implementation - its Linux implementation differs for different architectures. Since a large part of the libc makes use of syscalls (or an equivalent on non-Linux like platforms), it might be simpler and convenient to bring up the libc for one architecture at a time. In such cases, wherein the support surface of LLVM’s libc differs for each target architecture, one will have to add a subdirectory (within the config directory of the operating system) for each target architecture, and list the relevant config information separately in those subdirectories. For example, for Linux, the x86_64 and aarch64 configs are in separate directories, named x86_64 and aarch64. The libc CMake machinery looks for subdirectories named after the target architecture.

The entrypoints.txt file

One of the important pieces of config information is listed in a file named entrypoints.txt. This file lists the targets for the entrypoints (see Entrypoints in LLVM libc) you want to include in the static archive of the libc (for the Overlay Mode and/or the Full Host Build.) If you are doing an architecture specific bring up, then an entrypoints.txt file should be created in the architecture subdirectory for each architecture. Else, having a single entrypoints.txt in the operating system directory is sufficient.

The Linux config has an entrypoint.txt for each individual target architecture separately: aarch64, arm32 and x86_64. On the other hand, the Windows config has a single entrypoints.txt file.

A typical bring up procedure will normally bring up a small group of entrypoints at a time. The usual practice is to progressively add the targets for those entrypoints to the entrypoints.txt file as they are being brought up. The same is the case if one is implementing a new entrypoint - the target for the new entrypoint should be added to the relevant entrypoints.txt file. If the implementation of the new entrypoint supports multiple operating systems and target architectures, then multiple entrypoints.txt files will have to be updated.

The headers.txt file

Another important piece of config information is listed in a file named headers.txt. It lists the targets for the set of public headers that are provided by the libc. This is relevant only if the libc is to be used in the Full Host Build on the target operating system and architecture. As with the entrypoints.txt file, one headers.txt file should be listed for each individual target architecture if you are doing an architecture specific bring up. The Linux config has headers.txt file listed separately for the aarch64 config and the x86_64 config.

Upstreaming

Adding a target to the main LLVM-libc has some requirements to ensure that the targets stay in usable condition. LLVM-libc is under active development and without active maintenance targets will become stale and may be sunset.

Maintenance

To add a target there must be one or more people whose responsibility it is to keep the target up to date or push it forwards if it’s not complete. Those people are the maintainers, and they are responsible for keeping their target in good shape. This means fixing their target when it breaks, reviewing patches related to their target, and keeping the target’s CI running.

Maintainers are listed in libc/maintainers.rst and must follow LLVM’s maintainer policy.

CI builders

Every target needs at least one CI builder. These are used to check when the target breaks, and to help people who don’t have access to the specific architecture fix their bugs. LLVM-libc has both presubmit CI on github and postsubmit CI on the LLVM buildbot. For instructions on contributing a postsubmit buildbot read the LLVM documentation and for presubmit tests read the best practices

The test configurations are at these links:

• Linux Postsubmit

• Windows Postsubmit

• Fullbuild Presubmit

• Overlay Presubmit

Sunsetting

Sunsetting is the process through which targets can be removed from LLVM-libc. If a target is broken or stale it may be sunset. It is the responsibility of the target’s maintainers to keep it from being sunset. The target’s maintainers may be marked inactive if their target is sunset.

Broken targets are ones where the target’s CI has been failing for at least 30 days. After 30 days have passed the CI should be changed so it does not block commits and does not notify people when it fails. If the target remains broken for another 90 days it may be sunset.

Stale targets are ones with no active maintainers or no contributions. If a target has 0 specific contributions between two major LLVM releases, or if it has no active maintainers, then it may be marked “deprecated” in the next major release. If there are still no contributions or no maintainers after the next major release it may be sunset.

To sunset a target, all specific references to that target in the code and build system should be removed. All buildbots for that target should be shut down, or otherwise removed.

To restart a target that was previously sunset, the new maintainers are encouraged to look at the commit(s) removing the target to provide a starting point.