[libcxx-commits] [PATCH] D154466: [libc++] Remove internal "build-with-external-thread-library" configuration

[Louis Dionne via Phabricator via libcxx-commits]

ldionne created this revision.
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Our threading support layer is currently a huge mess. There are too many
configurations with too many confusing names, and none of them are tested
in the usual CI. Here's a list of names related to these configurations:

  LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY
  _LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL
  
  LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY
  _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL
  
  LIBCXX_HAS_EXTERNAL_THREAD_API
  _LIBCPP_HAS_THREAD_API_EXTERNAL

This patch cleans this up by removing the ability to build libc++ with
an "external" threading library for testing purposes, removing 4 out of
6 "names" above. That setting was meant to be used by libc++ developers,
but we don't use it in-tree and it's not part of our CI.

I know the ability to use an external threading API is used by some folks
out-of-tree, and this patch doesn't change that. This only changes the
way they will have to test their external threading support. After this
patch, the intent would be for them to set `-DLIBCXX_HAS_EXTERNAL_THREAD_API=ON`
when building the library, and to provide their usual `<__external_threading>`
header when they are testing the library. This can be done easily now
that we support custom lit configuration files in test suites.

The motivation for this patch is that our threading support layer is
basically unmaintainable -- anything beyond adding a new "backend" in
the slot designed for it requires incredible attention. The complexity
added by this setting just doesn't pull its weigh considering the
available alternatives.

Concretely, this will also allow future patches to clean up
`<__threading_support>` significantly.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D154466

Files:
  libcxx/CMakeLists.txt
  libcxx/docs/DesignDocs/ThreadingSupportAPI.rst
  libcxx/include/__config_site.in
  libcxx/include/__threading_support
  libcxx/src/CMakeLists.txt
  libcxx/test/support/external_threads.cpp
  libcxxabi/CMakeLists.txt
  libcxxabi/test/CMakeLists.txt

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