[libc++] Remove internal "build-with-external-thread-library" configuration

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.

Differential Revision: https://reviews.llvm.org/D154466

6 files changed, 1 insertions, 87 deletions

diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index 0aac341..86bc1eb 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -272,9 +272,6 @@ option(LIBCXX_HAS_WIN32_THREAD_API "Ignore auto-detection and force use of win32
 option(LIBCXX_HAS_EXTERNAL_THREAD_API
   "Build libc++ with an externalized threading API.
    This option may only be set to ON when LIBCXX_ENABLE_THREADS=ON." OFF)
-option(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY
-    "Build libc++ with an externalized threading library.
-     This option may only be set to ON when LIBCXX_ENABLE_THREADS=ON" OFF)
 
 if (LIBCXX_ENABLE_THREADS)
   set(LIBCXX_PSTL_CPU_BACKEND "std_thread" CACHE STRING "Which PSTL CPU backend to use")
@@ -327,10 +324,6 @@ if(NOT LIBCXX_ENABLE_THREADS)
     message(FATAL_ERROR "LIBCXX_HAS_EXTERNAL_THREAD_API can only be set to ON"
                         " when LIBCXX_ENABLE_THREADS is also set to ON.")
   endif()
-  if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
-    message(FATAL_ERROR "LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY can only be set "
-                        "to ON when LIBCXX_ENABLE_THREADS is also set to ON.")
-  endif()
   if (LIBCXX_HAS_WIN32_THREAD_API)
     message(FATAL_ERROR "LIBCXX_HAS_WIN32_THREAD_API can only be set to ON"
                         " when LIBCXX_ENABLE_THREADS is also set to ON.")
@@ -339,11 +332,6 @@ if(NOT LIBCXX_ENABLE_THREADS)
 endif()
 
 if (LIBCXX_HAS_EXTERNAL_THREAD_API)
-  if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
-    message(FATAL_ERROR "The options LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY and "
-                        "LIBCXX_HAS_EXTERNAL_THREAD_API cannot both be ON at "
-                        "the same time")
-  endif()
   if (LIBCXX_HAS_PTHREAD_API)
     message(FATAL_ERROR "The options LIBCXX_HAS_EXTERNAL_THREAD_API"
                         "and LIBCXX_HAS_PTHREAD_API cannot be both"
@@ -573,17 +561,6 @@ function(cxx_add_rtti_flags target)
   endif()
 endfunction()
 
-# Threading flags =============================================================
-if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY AND LIBCXX_ENABLE_SHARED)
-  # Need to allow unresolved symbols if this is to work with shared library builds
-  if (APPLE)
-    add_link_flags("-undefined dynamic_lookup")
-  else()
-    # Relax this restriction from HandleLLVMOptions
-    string(REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
-  endif()
-endif()
-
 # Modules flags ===============================================================
 # FIXME The libc++ sources are fundamentally non-modular. They need special
 # versions of the headers in order to provide C++03 and legacy ABI definitions.
@@ -764,7 +741,6 @@ endif()
 config_define_if(LIBCXX_HAS_PTHREAD_API _LIBCPP_HAS_THREAD_API_PTHREAD)
 config_define_if(LIBCXX_HAS_EXTERNAL_THREAD_API _LIBCPP_HAS_THREAD_API_EXTERNAL)
 config_define_if(LIBCXX_HAS_WIN32_THREAD_API _LIBCPP_HAS_THREAD_API_WIN32)
-config_define_if(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
 config_define_if(LIBCXX_NO_VCRUNTIME _LIBCPP_NO_VCRUNTIME)
 config_define_if_not(LIBCXX_ENABLE_FILESYSTEM _LIBCPP_HAS_NO_FILESYSTEM)
@@ -860,10 +836,6 @@ endif()
 
 set(LIBCXX_TEST_DEPS "cxx_experimental")
 
-if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
-  list(APPEND LIBCXX_TEST_DEPS cxx_external_threads)
-endif()
-
 if (LIBCXX_ENABLE_CLANG_TIDY)
   list(APPEND LIBCXX_TEST_DEPS cxx-tidy)
 endif()
diff --git a/libcxx/docs/DesignDocs/ThreadingSupportAPI.rst b/libcxx/docs/DesignDocs/ThreadingSupportAPI.rst
index 330ce74..752a9d4 100644
--- a/libcxx/docs/DesignDocs/ThreadingSupportAPI.rst
+++ b/libcxx/docs/DesignDocs/ThreadingSupportAPI.rst
@@ -42,14 +42,6 @@ On a production setting, this would be achieved through a custom
 ``<__external_threading>`` header, which declares the libc++ internal threading
 API but leaves out the implementation.
 
-The ``-DLIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY`` option allows building libc++ in
-such a configuration while allowing it to be tested on a platform that supports
-any of the threading systems (e.g. pthread) supported in ``__threading_support``
-header. Therefore, the main purpose of this option is to allow testing of this
-particular configuration of the library without being tied to a vendor-specific
-threading system. This option is only meant to be used by libc++ library
-developers.
-
 Threading Configuration Macros
 ==============================
 
@@ -69,15 +61,3 @@ Threading Configuration Macros
 **_LIBCPP_HAS_THREAD_API_WIN32**
   This macro is defined when libc++ should use Win32 threads to implement the
   internal threading API.
-
-**_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL**
-  This macro is defined when libc++ expects the definitions of the internal
-  threading API to be provided by an external library. When defined
-  ``<__threading_support>`` will only provide the forward declarations and
-  typedefs for the internal threading API.
-
-**_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL**
-  This macro is used to build an external threading library using the
-  ``<__threading_support>``. Specifically it exposes the threading API
-  definitions in ``<__threading_support>`` as non-inline definitions meant to
-  be compiled into a library.
diff --git a/libcxx/include/__config_site.in b/libcxx/include/__config_site.in
index 8602e0b..8ec3722 100644
--- a/libcxx/include/__config_site.in
+++ b/libcxx/include/__config_site.in
@@ -19,7 +19,6 @@
 #cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
 #cmakedefine _LIBCPP_HAS_THREAD_API_WIN32
-#cmakedefine _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
 #cmakedefine _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS
 #cmakedefine _LIBCPP_NO_VCRUNTIME
diff --git a/libcxx/include/__threading_support b/libcxx/include/__threading_support
index 2e6bfbd..97df18b 100644
--- a/libcxx/include/__threading_support
+++ b/libcxx/include/__threading_support
@@ -36,9 +36,7 @@
 # include <threads.h>
 #endif
 
-#if defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
-    defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL) || \
-    defined(_LIBCPP_HAS_THREAD_API_WIN32)
+#if defined(_LIBCPP_HAS_THREAD_API_WIN32)
 #define _LIBCPP_THREAD_ABI_VISIBILITY _LIBCPP_EXPORTED_FROM_ABI
 #else
 #define _LIBCPP_THREAD_ABI_VISIBILITY inline _LIBCPP_INLINE_VISIBILITY
@@ -248,9 +246,6 @@ int __libcpp_tls_set(__libcpp_tls_key __key, void *__p);
 
 #endif // !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 
-#if (!defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
-     defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL))
-
 #if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 
 int __libcpp_recursive_mutex_init(__libcpp_recursive_mutex_t *__m)
@@ -585,8 +580,6 @@ int __libcpp_tls_set(__libcpp_tls_key __key, void *__p)
 
 #endif
 
-#endif // !_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL || _LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL
-
 #endif // !_LIBCPP_HAS_NO_THREADS
 
 _LIBCPP_END_NAMESPACE_STD
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
index dc6b591..9091311 100644
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -346,27 +346,6 @@ set_target_properties(cxx_experimental
 cxx_add_common_build_flags(cxx_experimental)
 target_compile_options(cxx_experimental PUBLIC -D_LIBCPP_ENABLE_EXPERIMENTAL)
 
-
-if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
-  set(LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES
-      "${CMAKE_CURRENT_SOURCE_DIR}/../test/support/external_threads.cpp")
-
-  if (LIBCXX_ENABLE_SHARED)
-    add_library(cxx_external_threads SHARED ${LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES})
-  else()
-    add_library(cxx_external_threads STATIC ${LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES})
-  endif()
-
-  set_target_properties(cxx_external_threads
-    PROPERTIES
-      LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
-      COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
-      OUTPUT_NAME   "c++external_threads"
-  )
-
-  target_link_libraries(cxx_external_threads PRIVATE cxx-headers)
-endif()
-
 if (LIBCXX_INSTALL_SHARED_LIBRARY)
   install(TARGETS cxx_shared
     ARCHIVE DESTINATION ${LIBCXX_INSTALL_LIBRARY_DIR} COMPONENT cxx
diff --git a/libcxx/test/support/external_threads.cpp b/libcxx/test/support/external_threads.cpp
deleted file mode 100644
index 3ffce1a..0000000
--- a/libcxx/test/support/external_threads.cpp
+++ /dev/null
@@ -1,9 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-#define _LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL
-#include <__threading_support>