Index: include/memory
===================================================================
--- include/memory
+++ include/memory
@@ -5464,9 +5464,8 @@
 basic_ostream<_CharT, _Traits>&
 operator<<(basic_ostream<_CharT, _Traits>& __os, shared_ptr<_Yp> const& __p);
 
-// TODO(EricWF): Enable this for both Clang and GCC. Currently it is only
-// enabled with clang.
-#if defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
+
+#if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 
 class _LIBCPP_TYPE_VIS __sp_mut
 {
@@ -5595,7 +5594,7 @@
     return atomic_compare_exchange_weak(__p, __v, __w);
 }
 
-#endif  // defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
+#endif  // !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 
 //enum class
 struct _LIBCPP_TYPE_VIS pointer_safety
Index: src/memory.cpp
===================================================================
--- src/memory.cpp
+++ src/memory.cpp
@@ -124,7 +124,7 @@
 
 #endif  // _LIBCPP_NO_RTTI
 
-#if defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
+#if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 
 static const std::size_t __sp_mut_count = 16;
 static __libcpp_mutex_t mut_back_imp[__sp_mut_count] =
@@ -177,7 +177,7 @@
     return muts[hash<const void*>()(p) & (__sp_mut_count-1)];
 }
 
-#endif // defined(_LIBCPP_HAS_C_ATOMIC_IMP) && !defined(_LIBCPP_HAS_NO_THREADS)
+#endif // !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 
 void
 declare_reachable(void*)