diff --git a/libcxx/include/atomic b/libcxx/include/atomic
--- a/libcxx/include/atomic
+++ b/libcxx/include/atomic
@@ -1451,11 +1451,11 @@
     : _Base(value) {}
 };
 
-#ifdef __linux__
+#if defined(__linux__) || (defined(_AIX) && !defined(__64BIT__))
     using __cxx_contention_t = int32_t;
 #else
     using __cxx_contention_t = int64_t;
-#endif //__linux__
+#endif // __linux__ || (_AIX && !__64BIT__)
 
 using __cxx_atomic_contention_t = __cxx_atomic_impl<__cxx_contention_t>;