Index: lib/Support/Unix/Path.inc
===================================================================
--- lib/Support/Unix/Path.inc
+++ lib/Support/Unix/Path.inc
@@ -492,7 +492,12 @@
   // If we have posix_fallocate use it. Unlike ftruncate it always allocates
   // space, so we get an error if the disk is full.
   if (int Err = ::posix_fallocate(FD, 0, Size)) {
-    if (Err != EINVAL && Err != EOPNOTSUPP)
+#ifdef _AIX
+    constexpr int NotSupportedError = ENOTSUP;
+#else
+    constexpr int NotSupportedError = EOPNOTSUPP;
+#endif
+    if (Err != EINVAL && Err != NotSupportedError)
       return std::error_code(Err, std::generic_category());
   }
 #endif