[libc++] Documents details of the pre-commit CI.

This documentation aims to make it cleare how the libc++ pre-commit CI
works. For libc++ developers and other LLVM projects whose changes can
affect libc++.

This was discusses with @aaron.ballman as a follow on some unclearities
for the Clang communitee how the libc++ pre-commit CI works.

Note some parts depend on patches under review as commented in the
documentation.

Reviewed By: ldionne, #libc, philnik

Differential Revision: https://reviews.llvm.org/D133249

clang/www/hacking.html

@@ -30,6 +30,7 @@
         <li><a href="#testingNonWindows">Testing on Unix-like Systems</a></li>
         <li><a href="#testingWindows">Testing using Visual Studio on Windows</a></li>
         <li><a href="#testingCommands">Testing on the Command Line</a></li>
+        <li><a href="#testingLibc++">Testing changes affecting libc++</a></li>
       </ul>
       </li>
       <li><a href="#patches">Creating Patch Files</a></li>
@@ -271,6 +272,41 @@ Testing Time: 81.52s
 
   <p>The statistic, "Failed" (not shown if all tests pass), is the important one.</p>
 
+  <!--=====================================================================-->
+  <h3 id="testingLibc++">Testing changes affecting libc++</h3>
+  <!--=====================================================================-->
+
+  <p>Some changes in Clang affect <a href="https://libcxx.llvm.org">libc++</a>,
+  for example:</p>
+  <ul>
+      <li>Changing the output of Clang's diagnostics.</li>
+      <li>Changing compiler builtins, especially the builtins used for type traits
+      or replacements of library functions like <tt>std::move</tt> or
+      <tt>std::forward</tt>.</li>
+  </ul>
+
+  <p>After adjusting libc++ to work with the changes, the next revision will be
+  tested by libc++'s
+  <a href="https://buildkite.com/llvm-project/libcxx-ci">pre-commit CI</a>.
+
+  <p>For most configurations, the pre-commit CI uses a recent
+  <a href="https://apt.llvm.org/">nightly build</a> of Clang from LLVM's main
+  branch. These configurations do <em>not</em> use the Clang changes in the
+  patch. They only use the libc++ changes.</p>
+
+  <p>The &quot;Bootstrapping build&quot; builds Clang and uses it to build and
+  test libc++. This build <em>does</em> use the Clang changes in the patch.</p>
+
+  <p>Libc++ supports multiple versions of Clang. Therefore when a patch changes
+  the diagnostics it might be required to use a regex in the
+  &quot;expected&quot; tests to make it pass the CI.</p>
+
+  <p>Libc++ has more
+  <a href="https://libcxx.llvm.org/Contributing.html#pre-commit-ci">
+  documentation</a> about the pre-commit CI. For questions regarding
+  libc++, the best place to ask is the <tt>#libcxx</tt> channel on
+  <a href="https://discord.gg/jzUbyP26tQ">LLVM's Discord server</a>.</p>
+
   <!--=====================================================================-->
   <h2 id="patches">Creating Patch Files</h2>
   <!--=====================================================================-->

libcxx/docs/Contributing.rst

@@ -84,5 +84,136 @@ updated list from the failed build at
 Look for the failed build and select the ``artifacts`` tab. There, download the
 abilist for the platform, e.g.:
 
-* C++20 for the Linux platform.
+* C++<version>.
-* MacOS C++20 for the Apple platform.
+* MacOS X86_64 and MacOS arm64 for the Apple platform.
+
+
+Pre-commit CI
+=============
+
+Introduction
+------------
+
+Unlike most parts of the LLVM project, libc++ uses a pre-commit CI [#]_. This
+CI is hosted on `Buildkite <https://buildkite.com/llvm-project/libcxx-ci>`__ and
+the build results are visible in the review on Phabricator. Please make sure
+the CI is green before committing a patch.
+
+The CI tests libc++ for all :ref:`supported platforms <SupportedPlatforms>`.
+The build is started for every diff uploaded to Phabricator. A complete CI run
+takes approximately one hour. To reduce the load:
+
+* The build is cancelled when a new diff for the same revision is uploaded.
+* The build is done in several stages and cancelled when a stage fails.
+
+Typically, the libc++ jobs use a Ubuntu Docker image. This image contains
+recent `nightly builds <https://apt.llvm.org>`__ of all supported versions of
+Clang and the current version of the ``main`` branch. These versions of Clang
+are used to build libc++ and execute its tests.
+
+Unless specified otherwise, the configurations:
+
+* use a nightly build of the ``main`` branch of Clang,
+* execute the tests using the language C++<latest>. This is the version
+  "developed" by the C++ committee.
+
+.. note:: Updating the Clang nightly builds in the Docker image is a manual
+   process and is done at an irregular interval on purpose. When you need to
+   have the latest nightly build to test recent Clang changes, ask in the
+   ``#libcxx`` channel on `LLVM's Discord server
+   <https://discord.gg/jzUbyP26tQ>`__.
+
+.. [#] There's `LLVM Dev Meeting talk <https://www.youtube.com/watch?v=B7gB6van7Bw>`__
+   explaining the benefits of libc++'s pre-commit CI.
+
+Builds
+------
+
+Below is a short description of the most interesting CI builds [#]_:
+
+* ``Format`` runs ``clang-format`` and uploads its output as an artifact. At the
+  moment this build is a soft error and doesn't fail the build.
+* ``Generated output`` runs the ``libcxx-generate-files`` build target and
+  tests for non-ASCII characters in libcxx. Some files are excluded since they
+  use Unicode, mainly tests. The output of these commands are uploaded as
+  artifact.
+* ``Documentation`` builds the documentation. (This is done early in the build
+  process since it is cheap to run.)
+* ``C++<version>`` these build steps test the various C++ versions, making sure all
+  C++ language versions work with the changes made.
+* ``Clang <version>`` these build steps test whether the changes work with all
+  supported Clang versions.
+* ``Booststrapping build`` builds Clang using the revision of the patch and
+  uses that Clang version to build and test libc++. This validates the current
+  Clang and lib++ are compatible.
+
+  When a crash occurs in this build, the crash reproducer is available as an
+  artifact.
+
+* ``Modular build`` tests libc++ using Clang modules [#]_.
+* ``GCC <version>`` tests libc++ with the latest stable GCC version. Only C++11
+  and the latest C++ version are tested.
+* ``Santitizers`` tests libc++ using the Clang sanitizers.
+* ``Parts disabled`` tests libc++ with certain libc++ features disabled.
+* ``Windows`` tests libc++ using MinGW and clang-cl.
+* ``Apple`` tests libc++ on MacOS.
+* ``ARM`` tests libc++ on various Linux ARM platforms.
+* ``AIX`` tests libc++ on AIX.
+
+.. [#] Not all all steps are listed: steps are added and removed when the need
+   arises.
+.. [#] Clang modules are not the same as C++20's modules.
+
+Infrastructure
+--------------
+
+All files of the CI infrastructure are in the directory ``libcxx/utils/ci``.
+Note that quite a bit of this infrastructure is heavily Linux focused. This is
+the platform used by most of libc++'s Buildkite runners and developers.
+
+Dockerfile
+~~~~~~~~~~
+
+Contains the Docker image for the Ubuntu CI. Because the same Docker image is
+used for the ``main`` and ``release`` branch, it should contain no hard-coded
+versions.  It contains the used versions of Clang, various clang-tools,
+GCC, and CMake.
+
+.. note:: This image is pulled from Docker hub and not rebuild when changing
+   the Dockerfile.
+
+run-buildbot-container
+~~~~~~~~~~~~~~~~~~~~~~
+
+Helper script that pulls and runs the Docker image. This image mounts the LLVM
+monorepo at ``/llvm``. This can be used to test with compilers not available on
+your system.
+
+run-buildbot
+~~~~~~~~~~~~
+
+Contains the buld script executed on Buildkite. This script can be executed
+locally or inside ``run-buildbot-container``. The script must be called with
+the target to test. For example, ``run-buildbot generic-cxx20`` will build
+libc++ and test it using C++20.
+
+.. warning:: This script will overwrite the directory ``<llvm-root>/build/XX``
+  where ``XX`` is the target of ``run-buildbot``.
+
+This script contains as little version information as possible. This makes it
+easy to use the script with a different compiler. This allows testing a
+combination not in the libc++ CI. It can be used to add a new (temporary)
+job to the CI. For example, testing the C++17 build with Clang-14 can be done
+like:
+
+.. code-block:: bash
+
+  CC=clang-14 CXX=clang++-14 run-buildbot generic-cxx17
+
+buildkite-pipeline.yml
+~~~~~~~~~~~~~~~~~~~~~~
+
+Contains the jobs executed in the CI. This file contains the version
+information of the jobs being executed. Since this script differs between the
+``main`` and ``release`` branch, both branches can use different compiler
+versions.

libcxx/docs/index.rst

@@ -93,6 +93,7 @@ reasons, but some of the major ones are:
   Further, both projects are apparently abandoned: STLport 5.2.1 was
   released in Oct'08, and STDCXX 4.2.1 in May'08.
 
+.. _SupportedPlatforms:
 
 Platform and Compiler Support
 =============================