Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -211,14 +211,21 @@
                       " when LIBCXX_ENABLE_THREADS is also set to OFF.")
 endif()
 
-if(LIBCXX_HAS_PTHREAD_API AND NOT LIBCXX_ENABLE_THREADS)
-  message(FATAL_ERROR "LIBCXX_HAS_PTHREAD_API can only be set to ON"
-                      " when LIBCXX_ENABLE_THREADS is also set to ON.")
+if(NOT LIBCXX_ENABLE_THREADS)
+  if(LIBCXX_HAS_PTHREAD_API)
+    message(FATAL_ERROR "LIBCXX_HAS_PTHREAD_API can only be set to ON"
+                        " when LIBCXX_ENABLE_THREADS is also set to ON.")
+  endif()
+  if(LIBCXX_HAS_EXTERNAL_THREAD_API)
+    message(FATAL_ERROR "LIBCXX_HAS_EXTERNAL_THREAD_API can only be set to ON"
+                        " when LIBCXX_ENABLE_THREADS is also set to ON.")
+  endif()
 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.")
+if(LIBCXX_HAS_PTHREAD_API AND LIBCXX_HAS_EXTERNAL_THREAD_API)
+  message(FATAL_ERROR "The options LIBCXX_HAS_EXTERNAL_THREAD_API"
+                      "and LIBCXX_HAS_PTHREAD_API cannot be both"
+                      "set to ON at the same time.")
 endif()
 
 # Ensure LLVM_USE_SANITIZER is not specified when LIBCXX_GENERATE_COVERAGE
Index: include/__threading_support
===================================================================
--- include/__threading_support
+++ include/__threading_support
@@ -71,7 +71,20 @@
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_condvar_destroy(__libcpp_condvar_t* __cv);
 
+// Execute once
+typedef pthread_once_t __libcpp_exec_once_flag;
+#define _LIBCPP_EXEC_ONCE_INITIALIZER PTHREAD_ONCE_INIT
+
+_LIBCPP_THREAD_ABI_VISIBILITY
+int __libcpp_execute_once(__libcpp_exec_once_flag *flag,
+                          void (*init_routine)(void));
+
 // Thread id
+#if defined(__APPLE__) && !defined(__arm__)
+_LIBCPP_THREAD_ABI_VISIBILITY
+mach_port_t __libcpp_thread_get_port();
+#endif
+
 typedef pthread_t __libcpp_thread_id;
 _LIBCPP_THREAD_ABI_VISIBILITY
 bool __libcpp_thread_id_equal(__libcpp_thread_id t1, __libcpp_thread_id t2);
@@ -100,7 +113,7 @@
 _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);
+int __libcpp_tls_set(__libcpp_tls_key __key, void* __p);
 
 #if defined(_LIBCPP_HAS_THREAD_API_PTHREAD) || defined(_LIBCPP_BUILDING_EXTERNAL_THREADS)
 
@@ -176,6 +189,19 @@
     return pthread_cond_destroy(__cv);
 }
 
+// Execute once
+int __libcpp_execute_once(__libcpp_exec_once_flag *flag,
+                          void (*init_routine)(void)) {
+  return pthread_once(flag, init_routine);
+}
+
+// Thread id
+#if defined(__APPLE__) && !defined(__arm__)
+mach_port_t __libcpp_thread_get_port() {
+    return pthread_mach_thread_np(pthread_self());
+}
+#endif
+
 // Returns non-zero if the thread ids are equal, otherwise 0
 bool __libcpp_thread_id_equal(__libcpp_thread_id t1, __libcpp_thread_id t2)
 {
@@ -230,9 +256,9 @@
     return pthread_getspecific(__key);
 }
 
-void __libcpp_tls_set(__libcpp_tls_key __key, void* __p)
+int __libcpp_tls_set(__libcpp_tls_key __key, void* __p)
 {
-    pthread_setspecific(__key, __p);
+    return pthread_setspecific(__key, __p);
 }
 
 #endif // _LIBCPP_HAS_THREAD_API_PTHREAD || _LIBCPP_BUILDING_EXTERNAL_THREADS
Index: test/CMakeLists.txt
===================================================================
--- test/CMakeLists.txt
+++ test/CMakeLists.txt
@@ -56,17 +56,19 @@
   @ONLY)
 
 if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
-  set(experimental_dep cxx_experimental)
+  set(LIBCXX_TEST_DEPS cxx_experimental)
+else()
+  set(LIBCXX_TEST_DEPS "")
 endif()
 
 if (LIBCXX_HAS_EXTERNAL_THREAD_API)
-  set(external_threads_dep cxx_external_threads)
+  list(APPEND LIBCXX_TEST_DEPS cxx_external_threads)
 endif()
 
 add_lit_testsuite(check-cxx
   "Running libcxx tests"
   ${CMAKE_CURRENT_BINARY_DIR}
-  DEPENDS cxx ${experimental_dep} ${external_threads_dep})
+  DEPENDS cxx ${LIBCXX_TEST_DEPS})
 
 add_custom_target(check-libcxx DEPENDS check-cxx)
 
Index: test/libcxx/test/config.py
===================================================================
--- test/libcxx/test/config.py
+++ test/libcxx/test/config.py
@@ -576,10 +576,6 @@
                 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')
@@ -608,6 +604,8 @@
                 'C++ ABI setting %s unsupported for tests' % cxx_abi)
 
     def configure_extra_library_flags(self):
+        if self.get_lit_bool('cxx_ext_threads', default=False):
+            self.cxx.link_flags += ['-lc++external_threads']
         self.target_info.add_cxx_link_flags(self.cxx.link_flags)
 
     def configure_color_diagnostics(self):
Index: test/lit.site.cfg.in
===================================================================
--- test/lit.site.cfg.in
+++ test/lit.site.cfg.in
@@ -28,7 +28,7 @@
 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@"
+config.cxx_ext_threads          = "@LIBCXX_HAS_EXTERNAL_THREAD_API@"
 
 # Let the main config do the real work.
 config.loaded_site_config = True