[libc++] Cleanup and document <__threading_support>

Summary:
This patch attempts to clean up the macro configuration mess in `<__threading_support>`, specifically the mess involving external threading variants. Additionally this patch adds design documentation for `<__threading_support>` and the configuration macros it uses.

The primary change in this patch is separating the idea of an "external API" provided by `<__external_threading>` and the idea of having an external threading library. Now `_LIBCPP_HAS_THREAD_API_EXTERNAL` means that libc++ should use `<__external_threading>` and that the header is expected to exist.  Additionally the new macro `_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL` is now used to configure for using an "external library"  with the default threading API.

Reviewers: compnerd, rmaprath

Subscribers: smeenai, cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D28316

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291275 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -169,6 +169,9 @@ option(LIBCXX_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread AP
 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)
 
 # Misc options ----------------------------------------------------------------
 # FIXME: Turn -pedantic back ON. It is currently off because it warns
@@ -230,6 +233,17 @@ 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()
+
+endif()
+
+if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY AND LIBCXX_HAS_EXTERNAL_THREAD_API)
+  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 AND LIBCXX_HAS_EXTERNAL_THREAD_API)
@@ -520,6 +534,7 @@ config_define_if_not(LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS _LIBCPP_HAS_NO_THRE
 
 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_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
 
 # By default libc++ on Windows expects to use a shared library, which requires

docs/DesignDocs/ThreadingSupportAPI.rst

@@ -0,0 +1,70 @@
+=====================
+Threading Support API
+=====================
+
+.. contents::
+   :local:
+
+Overview
+========
+
+Libc++ supports using multiple different threading models and configurations
+to implement the threading parts of libc++, including ``<thread>`` and ``<mutex>``.
+These different models provide entirely different interfaces from each
+other. To address this libc++ wraps the underlying threading API in a new and
+consistent API, which it uses internally to implement threading primitives.
+
+The ``<__threading_support>`` header is where libc++ defines its internal
+threading interface. It contains forward declarations of the internal threading
+interface as well as definitions for the interface.
+
+External Threading API and the ``<__external_threading>`` header
+================================================================
+
+In order to support vendors with custom threading API's libc++ allows the
+entire internal threading interface to be provided by an external,
+vendor provided, header.
+
+When ``_LIBCPP_HAS_THREAD_API_EXTERNAL`` is defined the ``<__threading_support>``
+header simply forwards to the ``<__external_threading>`` header (which must exist).
+It is expected that the ``<__external_threading>`` header provide the exact
+interface normally provided by ``<__threading_support>``.
+
+External Threading Library
+==========================
+
+Normally ``<__threading_support>`` provides inline definitions to each internal
+threading API function it declares. However libc++ also supports using an
+external library to provide the definitions.
+
+When ``_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL`` libc++ does not provide inline
+definitions for the internal API, instead assuming the definitions will be
+provided by an external library.
+
+Threading Configuration Macros
+==============================
+
+**_LIBCPP_HAS_NO_THREADS**
+  This macro is defined when libc++ is built without threading support. It
+  should not be manually defined by the user.
+
+**_LIBCPP_HAS_THREAD_API_EXTERNAL**
+  This macro is defined when libc++ should use the ``<__external_threading>``
+  header to provide the internal threading API. This macro overrides
+  ``_LIBCPP_HAS_THREAD_API_PTHREAD``.
+
+**_LIBCPP_HAS_THREAD_API_PTHREAD**
+  This macro is defined when libc++ should use POSIX 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.

docs/index.rst

@@ -131,7 +131,7 @@ Design Documents
    DesignDocs/CapturingConfigInfo
    DesignDocs/ABIVersioning
    DesignDocs/VisibilityMacros
-
+   DesignDocs/ThreadingSupportAPI
 
 * `<atomic> design <http://libcxx.llvm.org/atomic_design.html>`_
 * `<type_traits> design <http://libcxx.llvm.org/type_traits_design.html>`_

include/__config

@@ -891,9 +891,7 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
 #endif
 
 // Thread API
-#if !defined(_LIBCPP_HAS_NO_THREADS) && \
+#if !defined(_LIBCPP_HAS_NO_THREADS)
-    !defined(_LIBCPP_HAS_THREAD_API_PTHREAD) && \
-    !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 # if defined(__FreeBSD__) || \
     defined(__Fuchsia__) || \
     defined(__NetBSD__) || \
@@ -901,7 +899,9 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
     defined(__APPLE__) || \
     defined(__CloudABI__) || \
     defined(__sun__)
-#  define _LIBCPP_HAS_THREAD_API_PTHREAD
+#   ifndef _LIBCPP_HAS_THREAD_API_PTHREAD
+#     define _LIBCPP_HAS_THREAD_API_PTHREAD
+#   endif
 # else
 #  error "No thread API"
 # endif // _LIBCPP_HAS_THREAD_API

include/__config_site.in

@@ -21,6 +21,7 @@
 #cmakedefine _LIBCPP_HAS_MUSL_LIBC
 #cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
+#cmakedefine _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
 
 #endif // _LIBCPP_CONFIG_SITE

include/__threading_support

@@ -17,39 +17,17 @@
 #pragma GCC system_header
 #endif
 
-#ifndef _LIBCPP_HAS_NO_THREADS
-
-#ifndef __libcpp_has_include
-  #ifndef __has_include
-    #define __libcpp_has_include(x) 0
-  #else
-    #define __libcpp_has_include(x) __has_include(x)
-  #endif
-#endif
-
-#if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL) && \
-    !__libcpp_has_include(<__external_threading>)
-// If the <__external_threading> header is absent, build libc++ against a
-// pthread-oriented thread api but leave out its implementation. This setup
-// allows building+testing of an externally-threaded library variant (on any
-// platform that supports pthreads). Here, an 'externally-threaded' library
-// variant is one where the implementation of the libc++ thread api is provided
-// as a separate library.
-#define _LIBCPP_HAS_THREAD_API_EXTERNAL_PTHREAD
-#endif
-
-#if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL) && \
-    __libcpp_has_include(<__external_threading>)
-#include <__external_threading>
-#else
-
-#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD) || \
-    defined(_LIBCPP_HAS_THREAD_API_EXTERNAL_PTHREAD)
-#include <pthread.h>
-#include <sched.h>
-#endif
-
 #if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
+# include <__external_threading>
+#elif !defined(_LIBCPP_HAS_NO_THREADS)
+
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+# include <pthread.h>
+# include <sched.h>
+#endif
+
+#if defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
+    defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL)
 #define _LIBCPP_THREAD_ABI_VISIBILITY _LIBCPP_FUNC_VIS
 #else
 #define _LIBCPP_THREAD_ABI_VISIBILITY inline _LIBCPP_INLINE_VISIBILITY
@@ -57,8 +35,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD) || \
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
-    defined(_LIBCPP_HAS_THREAD_API_EXTERNAL_PTHREAD)
 // Mutex
 typedef pthread_mutex_t __libcpp_mutex_t;
 #define _LIBCPP_MUTEX_INITIALIZER PTHREAD_MUTEX_INITIALIZER
@@ -175,8 +152,10 @@ void *__libcpp_tls_get(__libcpp_tls_key __key);
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_tls_set(__libcpp_tls_key __key, void *__p);
 
-#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD) || \
+#if !defined(_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL) || \
-    defined(_LIBCPP_BUILDING_THREAD_API_EXTERNAL_PTHREAD)
+    defined(_LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL)
+
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 
 int __libcpp_recursive_mutex_init(__libcpp_recursive_mutex_t *__m)
 {
@@ -344,10 +323,10 @@ int __libcpp_tls_set(__libcpp_tls_key __key, void *__p)
 
 #endif // _LIBCPP_HAS_THREAD_API_PTHREAD
 
+#endif // !_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL || _LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL
+
 _LIBCPP_END_NAMESPACE_STD
 
-#endif // !_LIBCPP_HAS_THREAD_API_EXTERNAL || !__libcpp_has_include(<__external_threading>)
+#endif // !_LIBCPP_HAS_NO_THREADS
-
-#endif // _LIBCPP_HAS_NO_THREADS
 
 #endif // _LIBCPP_THREADING_SUPPORT

lib/CMakeLists.txt

@@ -275,7 +275,7 @@ if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
   )
 endif()
 
-if (LIBCXX_HAS_EXTERNAL_THREAD_API)
+if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
   file(GLOB LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES ../test/support/external_threads.cpp)
 
   if (LIBCXX_ENABLE_SHARED)

test/CMakeLists.txt

@@ -34,7 +34,7 @@ pythonize_bool(LIBCXXABI_ENABLE_SHARED)
 pythonize_bool(LIBCXXABI_USE_LLVM_UNWINDER)
 pythonize_bool(LIBCXX_HAS_ATOMIC_LIB)
 pythonize_bool(LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB)
-pythonize_bool(LIBCXX_HAS_EXTERNAL_THREAD_API)
+pythonize_bool(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
 
 # By default, for non-standalone builds, libcxx and libcxxabi share a library
 # directory.

test/lit.site.cfg.in

@@ -28,7 +28,7 @@ config.has_libatomic            = "@LIBCXX_HAS_ATOMIC_LIB@"
 config.use_libatomic            = "@LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB@"
 
 config.libcxxabi_shared         = "@LIBCXXABI_ENABLE_SHARED@"
-config.cxx_ext_threads          = "@LIBCXX_HAS_EXTERNAL_THREAD_API@"
+config.cxx_ext_threads          = "@LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY@"
 
 # Let the main config do the real work.
 config.loaded_site_config = True

test/support/external_threads.cpp

@@ -6,5 +6,5 @@
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
-#define _LIBCPP_BUILDING_THREAD_API_EXTERNAL_PTHREAD
+#define _LIBCPP_BUILDING_THREAD_LIBRARY_EXTERNAL
 #include <__threading_support>