diff --git a/libcxx/include/__format/parser_std_format_spec.h b/libcxx/include/__format/parser_std_format_spec.h
--- a/libcxx/include/__format/parser_std_format_spec.h
+++ b/libcxx/include/__format/parser_std_format_spec.h
@@ -52,7 +52,7 @@
  * * The format-type filtering needs to be done post parsing in the parser
  *   derived from @ref __parser_std.
  */
-class _LIBCPP_TYPE_VIS _Flags {
+class _LIBCPP_TYPE_VIS __attribute__((__packed__)) _Flags {
 public:
   enum class _LIBCPP_ENUM_VIS _Alignment : uint8_t {
     /**
diff --git a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_bool.pass.cpp b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_bool.pass.cpp
--- a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_bool.pass.cpp
+++ b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_bool.pass.cpp
@@ -8,7 +8,6 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-format
-// XFAIL: LIBCXX-AIX-FIXME
 
 // <format>
 
diff --git a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_char.pass.cpp b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_char.pass.cpp
--- a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_char.pass.cpp
+++ b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_char.pass.cpp
@@ -8,7 +8,6 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-format
-// XFAIL: LIBCXX-AIX-FIXME
 
 // <format>
 
diff --git a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_integer.pass.cpp b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_integer.pass.cpp
--- a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_integer.pass.cpp
+++ b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_integer.pass.cpp
@@ -8,7 +8,6 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-format
-// XFAIL: LIBCXX-AIX-FIXME
 
 // <format>
 
diff --git a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string.pass.cpp b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string.pass.cpp
--- a/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string.pass.cpp
+++ b/libcxx/test/libcxx/utilities/format/format.string/format.string.std/std_format_spec_string.pass.cpp
@@ -8,7 +8,6 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 // UNSUPPORTED: libcpp-no-concepts
 // UNSUPPORTED: libcpp-has-no-incomplete-format
-// XFAIL: LIBCXX-AIX-FIXME
 
 // <format>