Index: include/__refstring
===================================================================
--- include/__refstring
+++ include/__refstring
@@ -83,7 +83,11 @@
     : __imp_(s.__imp_)
 {
     if (__uses_refcount())
+#ifdef _LIBCPP_HAS_NO_THREADS
+        ++rep_from_data(__imp_)->count;
+#else
         __sync_add_and_fetch(&rep_from_data(__imp_)->count, 1);
+#endif
 }
 
 inline
@@ -95,7 +99,11 @@
         __sync_add_and_fetch(&rep_from_data(__imp_)->count, 1);
     if (adjust_old_count)
     {
+#ifdef _LIBCPP_HAS_NO_THREADS
+        if ((old_rep->count -=  count_t(-1)) < 0)
+#else
         if (__sync_add_and_fetch(&old_rep->count, count_t(-1)) < 0)
+#endif
         {
             ::operator delete(old_rep);
         }
@@ -107,7 +115,11 @@
 __libcpp_refstring::~__libcpp_refstring() {
     if (__uses_refcount()) {
         _Rep_base* rep = rep_from_data(__imp_);
+#ifdef _LIBCPP_HAS_NO_THREADS
+        if ((rep->count -= count_t(-1)) < 0) {
+#else
         if (__sync_add_and_fetch(&rep->count, count_t(-1)) < 0) {
+#endif
             ::operator delete(rep);
         }
     }
Index: src/locale.cpp
===================================================================
--- src/locale.cpp
+++ src/locale.cpp
@@ -667,7 +667,11 @@
 void
 locale::id::__init()
 {
+#ifdef _LIBCPP_HAS_NO_THREADS
+    __id_ = ++__next_id;
+#else
     __id_ = __sync_add_and_fetch(&__next_id, 1);
+#endif
 }
 
 // template <> class collate_byname<char>
Index: src/support/runtime/exception_fallback.ipp
===================================================================
--- src/support/runtime/exception_fallback.ipp
+++ src/support/runtime/exception_fallback.ipp
@@ -20,13 +20,23 @@
 unexpected_handler
 set_unexpected(unexpected_handler func) _NOEXCEPT
 {
+#ifdef _LIBCPP_HAS_NO_THREADS
+  unexpected_handler old = __unexpected_handler;
+  __unexpected_handler = func;
+  return old;
+#else
   return __sync_lock_test_and_set(&__unexpected_handler, func);
+#endif
 }
 
 unexpected_handler
 get_unexpected() _NOEXCEPT
 {
+#ifdef _LIBCPP_HAS_NO_THREADS
+  return __unexpected_handler;
+#else
   return __sync_fetch_and_add(&__unexpected_handler, (unexpected_handler)0);
+#endif
 
 }
 
@@ -41,14 +51,23 @@
 terminate_handler
 set_terminate(terminate_handler func) _NOEXCEPT
 {
+#ifdef _LIBCPP_HAS_NO_THREADS
+  terminate_handler old = __terminate_handler;
+  __terminate_handler = func;
+  return old;
+#else
   return __sync_lock_test_and_set(&__terminate_handler, func);
+#endif
 }
 
 terminate_handler
 get_terminate() _NOEXCEPT
 {
+#ifdef _LIBCPP_HAS_NO_THREADS
+  return __terminate_handler;
+#else
   return __sync_fetch_and_add(&__terminate_handler, (terminate_handler)0);
-
+#endif
 }
 
 #ifndef __EMSCRIPTEN__ // We provide this in JS
Index: src/support/runtime/new_handler_fallback.ipp
===================================================================
--- src/support/runtime/new_handler_fallback.ipp
+++ src/support/runtime/new_handler_fallback.ipp
@@ -15,13 +15,23 @@
 new_handler
 set_new_handler(new_handler handler) _NOEXCEPT
 {
+#ifdef _LIBCPP_HAS_NO_THREADS
+    auto old = __new_handler;
+    __new_handler = handler;
+    return old;
+#else
     return __sync_lock_test_and_set(&__new_handler, handler);
+#endif
 }
 
 new_handler
 get_new_handler() _NOEXCEPT
 {
+#ifdef _LIBCPP_HAS_NO_THREADS
+    return __new_handler;
+#else
     return __sync_fetch_and_add(&__new_handler, nullptr);
+#endif
 }
 
 } // namespace std