Index: libcxx/include/memory
===================================================================
--- libcxx/include/memory
+++ libcxx/include/memory
@@ -3681,6 +3681,44 @@
 
 #endif // _LIBCPP_STD_VER > 14
 
+// NOTE: Relaxed and acq/rel atomics (for increment and decrement respectively)
+// should be sufficient for thread safety.
+// See https://llvm.org/bugs/show_bug.cgi?id=22803
+#if defined(__clang__) && __has_builtin(__atomic_load_n)             \
+                       && __has_builtin(__atomic_store_n)            \
+                       && __has_builtin(__atomic_add_fetch)          \
+                       && __has_builtin(__atomic_compare_exchange_n) \
+                       && defined(__ATOMIC_RELAXED)                  \
+                       && defined(__ATOMIC_CONSUME)                  \
+                       && defined(__ATOMIC_ACQUIRE)                  \
+                       && defined(__ATOMIC_RELEASE)                  \
+                       && defined(__ATOMIC_ACQ_REL)                  \
+                       && defined(__ATOMIC_SEQ_CST)
+#   define _LIBCPP_HAS_ATOMIC_BUILTINS
+#endif
+
+template <class T>
+inline T
+__libcpp_atomic_refcount_increment(T& t) _NOEXCEPT
+{
+#if defined(_LIBCPP_HAS_ATOMIC_BUILTINS) && !defined(_LIBCPP_HAS_NO_THREADS)
+    return __atomic_add_fetch(&t, 1, __ATOMIC_RELAXED);
+#else
+    return t += 1;
+#endif
+}
+
+template <class T>
+inline T
+__libcpp_atomic_refcount_decrement(T& t) _NOEXCEPT
+{
+#if defined(_LIBCPP_HAS_ATOMIC_BUILTINS) && !defined(_LIBCPP_HAS_NO_THREADS)
+    return __atomic_add_fetch(&t, -1, __ATOMIC_ACQ_REL);
+#else
+    return t -= 1;
+#endif
+}
+
 class _LIBCPP_EXCEPTION_ABI bad_weak_ptr
     : public std::exception
 {
@@ -3717,8 +3755,23 @@
     explicit __shared_count(long __refs = 0) _NOEXCEPT
         : __shared_owners_(__refs) {}
 
-    void __add_shared() _NOEXCEPT;
-    bool __release_shared() _NOEXCEPT;
+#ifdef _LIBCPP_BUILDING_MEMORY
+    void _LIBCPP_FUNC_VIS __add_shared() _NOEXCEPT;
+    bool _LIBCPP_FUNC_VIS __release_shared() _NOEXCEPT;
+#else
+    _LIBCPP_INLINE_VISIBILITY
+    void __add_shared() _NOEXCEPT {
+      __libcpp_atomic_refcount_increment(__shared_owners_);
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    bool __release_shared() _NOEXCEPT {
+      if (__libcpp_atomic_refcount_decrement(__shared_owners_) == -1) {
+        __on_zero_shared();
+        return true;
+      }
+      return false;
+    }
+#endif
     _LIBCPP_INLINE_VISIBILITY
     long use_count() const _NOEXCEPT {
         return __libcpp_relaxed_load(&__shared_owners_) + 1;
@@ -3739,9 +3792,25 @@
     virtual ~__shared_weak_count();
 
 public:
-    void __add_shared() _NOEXCEPT;
-    void __add_weak() _NOEXCEPT;
-    void __release_shared() _NOEXCEPT;
+#ifdef _LIBCPP_BUILDING_MEMORY
+    void _LIBCPP_FUNC_VIS __add_shared() _NOEXCEPT;
+    void _LIBCPP_FUNC_VIS __add_weak() _NOEXCEPT;
+    void _LIBCPP_FUNC_VIS __release_shared() _NOEXCEPT;
+#else
+    _LIBCPP_INLINE_VISIBILITY
+    void __add_shared() _NOEXCEPT {
+      __shared_count::__add_shared();
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void __add_weak() _NOEXCEPT {
+      __libcpp_atomic_refcount_increment(__shared_weak_owners_);
+    }
+    _LIBCPP_INLINE_VISIBILITY
+    void __release_shared() _NOEXCEPT {
+      if (__shared_count::__release_shared())
+        __release_weak();
+    }
+#endif
     void __release_weak() _NOEXCEPT;
     _LIBCPP_INLINE_VISIBILITY
     long use_count() const _NOEXCEPT {return __shared_count::use_count();}
Index: libcxx/src/memory.cpp
===================================================================
--- libcxx/src/memory.cpp
+++ libcxx/src/memory.cpp
@@ -17,28 +17,6 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-namespace
-{
-
-// NOTE: Relaxed and acq/rel atomics (for increment and decrement respectively)
-// should be sufficient for thread safety.
-// See https://llvm.org/bugs/show_bug.cgi?id=22803
-template <class T>
-inline T
-increment(T& t) _NOEXCEPT
-{
-    return __libcpp_atomic_add(&t, 1, _AO_Relaxed);
-}
-
-template <class T>
-inline T
-decrement(T& t) _NOEXCEPT
-{
-    return __libcpp_atomic_add(&t, -1, _AO_Acq_Rel);
-}
-
-}  // namespace
-
 const allocator_arg_t allocator_arg = allocator_arg_t();
 
 bad_weak_ptr::~bad_weak_ptr() _NOEXCEPT {}
@@ -53,16 +31,20 @@
 {
 }
 
+__shared_weak_count::~__shared_weak_count()
+{
+}
+
 void
 __shared_count::__add_shared() _NOEXCEPT
 {
-    increment(__shared_owners_);
+    __libcpp_atomic_refcount_increment(__shared_owners_);
 }
 
 bool
 __shared_count::__release_shared() _NOEXCEPT
 {
-    if (decrement(__shared_owners_) == -1)
+    if (__libcpp_atomic_refcount_decrement(__shared_owners_) == -1)
     {
         __on_zero_shared();
         return true;
@@ -70,10 +52,6 @@
     return false;
 }
 
-__shared_weak_count::~__shared_weak_count()
-{
-}
-
 void
 __shared_weak_count::__add_shared() _NOEXCEPT
 {
@@ -83,7 +61,7 @@
 void
 __shared_weak_count::__add_weak() _NOEXCEPT
 {
-    increment(__shared_weak_owners_);
+    __libcpp_atomic_refcount_increment(__shared_weak_owners_);
 }
 
 void
@@ -124,7 +102,7 @@
         //__libcpp_atomic_store(&__shared_weak_owners_, -1, _AO_Release);
         __on_zero_shared_weak();
     }
-    else if (decrement(__shared_weak_owners_) == -1)
+    else if (__libcpp_atomic_refcount_decrement(__shared_weak_owners_) == -1)
         __on_zero_shared_weak();
 }