[libcxx] Introduce an externally-threaded libc++ variant.

This patch further decouples libc++ from pthread, allowing libc++ to be built
against other threading systems. There are two main use cases:

- Building libc++ against a thread library other than pthreads.

- Building libc++ with an "external" thread API, allowing a separate library to
  provide the implementation of that API.

The two use cases are quite similar, the second one being sligtly more
de-coupled than the first. The cmake option LIBCXX_HAS_EXTERNAL_THREAD_API
enables both kinds of builds. One needs to place an <__external_threading>
header file containing an implementation of the "libc++ thread API" declared
in the <__threading_support> header.

For the second use case, the implementation of the libc++ thread API can
delegate to a custom "external" thread API where the implementation of this
external API is provided in a seperate library. This mechanism allows toolchain
vendors to distribute a build of libc++ with a custom thread-porting-layer API
(which is the "external" API above), platform vendors (recipients of the
toolchain/libc++) are then required to provide their implementation of this API
to be linked with (end-user) C++ programs.

Note that the second use case still requires establishing the basic types that
get passed between the external thread library and the libc++ library
(e.g. __libcpp_mutex_t). These cannot be opaque pointer types (libc++ sources
won't compile otherwise). It should also be noted that the second use case can
have a slight performance penalty; as all the thread constructs need to cross a
library boundary through an additional function call.

When the header <__external_threading> is omitted, libc++ is built with the
"libc++ thread API" (declared in <__threading_support>) as the "external" thread
API (basic types are pthread based). An implementation (pthread based) of this
API is provided in test/support/external_threads.cpp, which is built into a
separate DSO and linked in when running the libc++ test suite. A test run
therefore demonstrates the second use case (less the intermediate custom API).

Differential revision: https://reviews.llvm.org/D21968

Reviewers: bcraig, compnerd, EricWF, mclow.lists

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@281179 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -140,6 +140,9 @@ option(LIBCXX_ENABLE_MONOTONIC_CLOCK
    This option may only be set to OFF when LIBCXX_ENABLE_THREADS=OFF." ON)
 option(LIBCXX_HAS_MUSL_LIBC "Build libc++ with support for the Musl C library" OFF)
 option(LIBCXX_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
+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)
 
 # Misc options ----------------------------------------------------------------
 # FIXME: Turn -pedantic back ON. It is currently off because it warns
@@ -197,6 +200,11 @@ if(LIBCXX_HAS_PTHREAD_API AND NOT LIBCXX_ENABLE_THREADS)
                       " when LIBCXX_ENABLE_THREADS is also set to ON.")
 endif()
 
+if(LIBCXX_HAS_EXTERNAL_THREAD_API AND 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()
+
 # Ensure LLVM_USE_SANITIZER is not specified when LIBCXX_GENERATE_COVERAGE
 # is ON.
 if (LLVM_USE_SANITIZER AND LIBCXX_GENERATE_COVERAGE)
@@ -371,6 +379,17 @@ if (NOT LIBCXX_ENABLE_RTTI)
   add_compile_flags_if_supported(-fno-rtti)
 endif()
 
+# Threading flags =============================================================
+if (LIBCXX_HAS_EXTERNAL_THREAD_API 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()
+
 # Assertion flags =============================================================
 define_if(LIBCXX_ENABLE_ASSERTIONS -UNDEBUG)
 define_if_not(LIBCXX_ENABLE_ASSERTIONS -DNDEBUG)
@@ -432,6 +451,7 @@ config_define_if_not(LIBCXX_ENABLE_MONOTONIC_CLOCK _LIBCPP_HAS_NO_MONOTONIC_CLOC
 config_define_if_not(LIBCXX_ENABLE_THREAD_UNSAFE_C_FUNCTIONS _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS)
 
 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_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
 
 if (LIBCXX_NEEDS_SITE_CONFIG)

include/__config

@@ -831,7 +831,9 @@ extern "C" void __sanitizer_annotate_contiguous_container(
 #endif
 
 // Thread API
-#if !defined(_LIBCPP_HAS_NO_THREADS) && !defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+#if !defined(_LIBCPP_HAS_NO_THREADS) && \
+    !defined(_LIBCPP_HAS_THREAD_API_PTHREAD) && \
+    !defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
 # if defined(__FreeBSD__) || \
     defined(__NetBSD__) || \
     defined(__linux__) || \
@@ -849,6 +851,11 @@ extern "C" void __sanitizer_annotate_contiguous_container(
          _LIBCPP_HAS_NO_THREADS is not defined.
 #endif
 
+#if defined(_LIBCPP_HAS_NO_THREADS) && defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
+#  error _LIBCPP_HAS_EXTERNAL_THREAD_API may not be defined when \
+         _LIBCPP_HAS_NO_THREADS is defined.
+#endif
+
 #if defined(_LIBCPP_HAS_NO_MONOTONIC_CLOCK) && !defined(_LIBCPP_HAS_NO_THREADS)
 #  error _LIBCPP_HAS_NO_MONOTONIC_CLOCK may only be defined when \
          _LIBCPP_HAS_NO_THREADS is defined.

include/__config_site.in

@@ -20,5 +20,6 @@
 #cmakedefine _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 #cmakedefine _LIBCPP_HAS_MUSL_LIBC
 #cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
+#cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
 
 #endif // _LIBCPP_CONFIG_SITE

include/__threading_support

@@ -19,20 +19,83 @@
 
 #ifndef _LIBCPP_HAS_NO_THREADS
 
-#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+#if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL) && (!defined(__has_include) || __has_include(<__external_threading>))
+#include <__external_threading>
+#else
 #include <pthread.h>
 #include <sched.h>
 #endif
 
-_LIBCPP_BEGIN_NAMESPACE_STD
+#if defined(_LIBCPP_HAS_THREAD_API_EXTERNAL)
+#define _LIBCPP_THREAD_ABI_VISIBILITY _LIBCPP_FUNC_VIS
+#else
+#define _LIBCPP_THREAD_ABI_VISIBILITY inline _LIBCPP_INLINE_VISIBILITY
+#endif
 
-#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+_LIBCPP_BEGIN_NAMESPACE_STD
 
 // Mutex
-#define _LIBCPP_MUTEX_INITIALIZER PTHREAD_MUTEX_INITIALIZER
 typedef pthread_mutex_t __libcpp_mutex_t;
+#define _LIBCPP_MUTEX_INITIALIZER PTHREAD_MUTEX_INITIALIZER
+
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_recursive_mutex_init(__libcpp_mutex_t* __m);
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_mutex_lock(__libcpp_mutex_t* __m);
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_mutex_trylock(__libcpp_mutex_t* __m);
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_mutex_unlock(__libcpp_mutex_t* __m);
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_mutex_destroy(__libcpp_mutex_t* __m);
+
+// Condition variable
+typedef pthread_cond_t __libcpp_condvar_t;
+#define _LIBCPP_CONDVAR_INITIALIZER PTHREAD_COND_INITIALIZER
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_condvar_signal(__libcpp_condvar_t* __cv);
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_condvar_broadcast(__libcpp_condvar_t* __cv);
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_condvar_wait(__libcpp_condvar_t* __cv, __libcpp_mutex_t* __m);
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_condvar_timedwait(__libcpp_condvar_t* __cv, __libcpp_mutex_t* __m, timespec* __ts);
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_condvar_destroy(__libcpp_condvar_t* __cv);
+
+// Thread id
+typedef pthread_t __libcpp_thread_id;
+_LIBCPP_THREAD_ABI_VISIBILITY
+bool __libcpp_thread_id_equal(__libcpp_thread_id t1, __libcpp_thread_id t2);
+_LIBCPP_THREAD_ABI_VISIBILITY
+bool __libcpp_thread_id_less(__libcpp_thread_id t1, __libcpp_thread_id t2);
+
+// Thread
+typedef pthread_t __libcpp_thread_t;
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_thread_create(__libcpp_thread_t* __t, void* (*__func)(void*), void* __arg);
+_LIBCPP_THREAD_ABI_VISIBILITY
+__libcpp_thread_id __libcpp_thread_get_current_id();
+_LIBCPP_THREAD_ABI_VISIBILITY
+__libcpp_thread_id __libcpp_thread_get_id(const __libcpp_thread_t* __t);
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_thread_join(__libcpp_thread_t* __t);
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_thread_detach(__libcpp_thread_t* __t);
+_LIBCPP_THREAD_ABI_VISIBILITY
+void __libcpp_thread_yield();
+
+// Thread local storage
+typedef pthread_key_t __libcpp_tls_key;
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_tls_create(__libcpp_tls_key* __key, void (*__at_exit)(void*));
+_LIBCPP_THREAD_ABI_VISIBILITY
+void* __libcpp_tls_get(__libcpp_tls_key __key);
+_LIBCPP_THREAD_ABI_VISIBILITY
+void __libcpp_tls_set(__libcpp_tls_key __key, void* __p);
+
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD) || defined(_LIBCPP_BUILDING_EXTERNAL_THREADS)
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_recursive_mutex_init(__libcpp_mutex_t* __m)
 {
     pthread_mutexattr_t attr;
@@ -59,144 +122,112 @@ int __libcpp_recursive_mutex_init(__libcpp_mutex_t* __m)
     return 0;
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_mutex_lock(__libcpp_mutex_t* __m)
 {
     return pthread_mutex_lock(__m);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_mutex_trylock(__libcpp_mutex_t* __m)
 {
     return pthread_mutex_trylock(__m);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_mutex_unlock(__libcpp_mutex_t* __m)
 {
     return pthread_mutex_unlock(__m);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_mutex_destroy(__libcpp_mutex_t* __m)
 {
     return pthread_mutex_destroy(__m);
 }
 
 // Condition variable
-#define _LIBCPP_CONDVAR_INITIALIZER PTHREAD_COND_INITIALIZER
-typedef pthread_cond_t __libcpp_condvar_t;
-
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_condvar_signal(__libcpp_condvar_t* __cv)
 {
     return pthread_cond_signal(__cv);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_condvar_broadcast(__libcpp_condvar_t* __cv)
 {
     return pthread_cond_broadcast(__cv);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_condvar_wait(__libcpp_condvar_t* __cv, __libcpp_mutex_t* __m)
 {
     return pthread_cond_wait(__cv, __m);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_condvar_timedwait(__libcpp_condvar_t* __cv, __libcpp_mutex_t* __m, timespec* __ts)
 {
     return pthread_cond_timedwait(__cv, __m, __ts);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_condvar_destroy(__libcpp_condvar_t* __cv)
 {
     return pthread_cond_destroy(__cv);
 }
 
-// Thread id
-typedef pthread_t __libcpp_thread_id;
-
 // Returns non-zero if the thread ids are equal, otherwise 0
-inline _LIBCPP_ALWAYS_INLINE
 bool __libcpp_thread_id_equal(__libcpp_thread_id t1, __libcpp_thread_id t2)
 {
     return pthread_equal(t1, t2) != 0;
 }
 
 // Returns non-zero if t1 < t2, otherwise 0
-inline _LIBCPP_ALWAYS_INLINE
 bool __libcpp_thread_id_less(__libcpp_thread_id t1, __libcpp_thread_id t2)
 {
     return t1 < t2;
 }
 
 // Thread
-typedef pthread_t __libcpp_thread_t;
-
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_thread_create(__libcpp_thread_t* __t, void* (*__func)(void*), void* __arg)
 {
     return pthread_create(__t, 0, __func, __arg);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 __libcpp_thread_id __libcpp_thread_get_current_id()
 {
     return pthread_self();
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 __libcpp_thread_id __libcpp_thread_get_id(const __libcpp_thread_t* __t)
 {
     return *__t;
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_thread_join(__libcpp_thread_t* __t)
 {
     return pthread_join(*__t, 0);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 int __libcpp_thread_detach(__libcpp_thread_t* __t)
 {
     return pthread_detach(*__t);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
 void __libcpp_thread_yield()
 {
     sched_yield();
 }
 
 // Thread local storage
-typedef pthread_key_t __libcpp_tl_key;
-
-inline _LIBCPP_ALWAYS_INLINE
-int __libcpp_tl_create(__libcpp_tl_key* __key, void (*__at_exit)(void*))
+int __libcpp_tls_create(__libcpp_tls_key* __key, void (*__at_exit)(void*))
 {
     return pthread_key_create(__key, __at_exit);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
-void* __libcpp_tl_get(__libcpp_tl_key __key)
+void* __libcpp_tls_get(__libcpp_tls_key __key)
 {
     return pthread_getspecific(__key);
 }
 
-inline _LIBCPP_ALWAYS_INLINE
-void __libcpp_tl_set(__libcpp_tl_key __key, void* __p)
+void __libcpp_tls_set(__libcpp_tls_key __key, void* __p)
 {
     pthread_setspecific(__key, __p);
 }
 
-#else // !_LIBCPP_HAS_THREAD_API_PTHREAD
-  #error "No thread API selected."
-#endif
+#endif // _LIBCPP_HAS_THREAD_API_PTHREAD || _LIBCPP_BUILDING_EXTERNAL_THREADS
 
 _LIBCPP_END_NAMESPACE_STD
 

include/thread

@@ -137,7 +137,7 @@ public:
 template <class _Tp>
 class __thread_specific_ptr
 {
-    __libcpp_tl_key __key_;
+    __libcpp_tls_key __key_;
 
      // Only __thread_local_data() may construct a __thread_specific_ptr
      // and only with _Tp == __thread_struct.
@@ -155,7 +155,7 @@ public:
     ~__thread_specific_ptr();
 
     _LIBCPP_INLINE_VISIBILITY
-    pointer get() const {return static_cast<_Tp*>(__libcpp_tl_get(__key_));}
+    pointer get() const {return static_cast<_Tp*>(__libcpp_tls_get(__key_));}
     _LIBCPP_INLINE_VISIBILITY
     pointer operator*() const {return *get();}
     _LIBCPP_INLINE_VISIBILITY
@@ -173,7 +173,7 @@ __thread_specific_ptr<_Tp>::__at_thread_exit(void* __p)
 template <class _Tp>
 __thread_specific_ptr<_Tp>::__thread_specific_ptr()
 {
-    int __ec = __libcpp_tl_create(
+    int __ec = __libcpp_tls_create(
         &__key_,
         &__thread_specific_ptr::__at_thread_exit);
     if (__ec)
@@ -196,7 +196,7 @@ __thread_specific_ptr<_Tp>::set_pointer(pointer __p)
 {
     _LIBCPP_ASSERT(get() == nullptr,
                    "Attempting to overwrite thread local data");
-    __libcpp_tl_set(__key_, __p);
+    __libcpp_tls_set(__key_, __p);
 }
 
 class _LIBCPP_TYPE_VIS thread;

lib/CMakeLists.txt

@@ -201,6 +201,23 @@ if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
   )
 endif()
 
+if (LIBCXX_HAS_EXTERNAL_THREAD_API)
+  file(GLOB LIBCXX_EXTERNAL_THREADING_SUPPORT_SOURCES ../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"
+  )
+endif()
+
 # Generate a linker script inplace of a libc++.so symlink. Rerun this command
 # after cxx builds.
 if (LIBCXX_ENABLE_SHARED AND LIBCXX_ENABLE_ABI_LINKER_SCRIPT)

test/CMakeLists.txt

@@ -22,6 +22,7 @@ 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)
 
 # The tests shouldn't link to any ABI library when it has been linked into
 # libc++ statically or via a linker script.
@@ -52,10 +53,14 @@ if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
   set(experimental_dep cxx_experimental)
 endif()
 
+if (LIBCXX_HAS_EXTERNAL_THREAD_API)
+  set(external_threads_dep cxx_external_threads)
+endif()
+
 add_lit_testsuite(check-cxx
   "Running libcxx tests"
   ${CMAKE_CURRENT_BINARY_DIR}
-  DEPENDS cxx ${experimental_dep})
+  DEPENDS cxx ${experimental_dep} ${external_threads_dep})
 
 add_custom_target(check-libcxx DEPENDS check-cxx)
 

test/libcxx/test/config.py

@@ -510,6 +510,10 @@ class Configuration(object):
                 self.cxx.link_flags += [abs_path]
             else:
                 self.cxx.link_flags += ['-lc++']
+        # This needs to come after -lc++ as we want its unresolved thread-api symbols
+        # to be picked up from this one.
+        if self.get_lit_bool('libcxx_external_thread_api', default=False):
+            self.cxx.link_flags += ['-lc++external_threads']
 
     def configure_link_flags_abi_library(self):
         cxx_abi = self.get_lit_conf('cxx_abi', 'libcxxabi')

test/libcxx/thread/thread.condition/thread.condition.condvar/native_handle.pass.cpp

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: libcpp-has-no-threads, libcpp-has-thread-api-external
 
 // <condition_variable>
 

test/libcxx/thread/thread.mutex/thread.mutex.requirements/thread.mutex.requirements.mutex/thread.mutex.class/native_handle.pass.cpp

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: libcpp-has-no-threads, libcpp-has-thread-api-external
 
 // <mutex>
 

test/libcxx/thread/thread.mutex/thread.mutex.requirements/thread.mutex.requirements.mutex/thread.mutex.recursive/native_handle.pass.cpp

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: libcpp-has-no-threads, libcpp-has-thread-api-external
 
 // <mutex>
 

test/libcxx/thread/thread.threads/thread.thread.class/thread.thread.member/native_handle.pass.cpp

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: libcpp-has-no-threads, libcpp-has-thread-api-external
 
 // <thread>
 

test/libcxx/thread/thread.threads/thread.thread.class/types.pass.cpp

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// UNSUPPORTED: libcpp-has-no-threads
+// UNSUPPORTED: libcpp-has-no-threads, libcpp-has-thread-api-external
 
 // <thread>
 

test/lit.site.cfg.in

@@ -26,6 +26,7 @@ config.has_libatomic            = "@LIBCXX_HAS_ATOMIC_LIB@"
 config.use_libatomic            = "@LIBCXX_HAVE_CXX_ATOMICS_WITH_LIB@"
 
 config.libcxxabi_shared         = "@LIBCXXABI_ENABLE_SHARED@"
+config.libcxx_external_thread_api  = "@LIBCXX_HAS_EXTERNAL_THREAD_API@"
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")

test/support/external_threads.cpp

@@ -0,0 +1,10 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+#define _LIBCPP_BUILDING_EXTERNAL_THREADS
+#include <__threading_support>