diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -721,24 +721,34 @@
     static const size_type __endian_factor = 2;
 #endif
 
-    struct __long
+_LIBCPP_PACKED_BYTE_FOR_AIX
+    struct __long_impl
     {
         size_type __is_long_ : 1;
         size_type __cap_ : sizeof(size_type) * CHAR_BIT - 1;
         size_type __size_;
+        char
+            __padding_[sizeof(pointer) *
+                           ((2 * sizeof(size_type) - 1) / sizeof(pointer) + 1) -
+                       2 * sizeof(size_type)];
         pointer   __data_;
     };
+_LIBCPP_PACKED_BYTE_FOR_AIX_END
+    struct alignas(size_type) alignas(pointer) __long : __long_impl {};
 
     enum {__min_cap = (sizeof(__long) - 1)/sizeof(value_type) > 2 ?
                       (sizeof(__long) - 1)/sizeof(value_type) : 2};
 
-    struct __short
+_LIBCPP_PACKED_BYTE_FOR_AIX
+    struct __short_impl
     {
         unsigned char __is_long_ : 1;
         unsigned char __size_ : 7;
         char __padding_[sizeof(value_type) - 1];
         value_type __data_[__min_cap];
     };
+_LIBCPP_PACKED_BYTE_FOR_AIX_END
+    struct alignas(value_type) __short : __short_impl {};
 
 #endif // _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT