[libc++] Config `pointer_safety` type to be backward compatible on AIX

[jasonliu]

Summary

The libc++ AIX already shipped contains a fix that's a bit different for pointer_safety on other platform.
It used enum class for pointer_safety without specifying the underlying type (which means the underlying type is "int").

[ldionne]

libcxx/include/memory

4079

This code is already only used if the unstable ABI is used, so I don't understand why we're concerned about breaking that ABI.

[jasonliu]

libcxx/include/memory

4079

We shipped a downstream version libc++ with essentially this change to customer around November 2019. (Sorry for not being able to upstream before shipping due to time constraints we had back then)
This is part of the effort to unwind it.

[jasonliu]

Fix failed test case from pre-commit CI.

[ldionne]

As discussed offline, my preferred way forward would be https://reviews.llvm.org/D100410. Nobody cares about std::pointer_safety, let's not add any more complexity to the library to salvage it.

[jasonliu]

Abandoning this patch in favour of the approach in https://reviews.llvm.org/D100410.

libcxx/include/__config

 // Don't use a nullptr_t simulation type in C++03 instead using C++11 nullptr
 // provided under the alternate keyword __nullptr, which changes the mangling
 // of nullptr_t. This option is ABI incompatible with GCC in C++03 mode.
 #  define _LIBCPP_ABI_ALWAYS_USE_CXX11_NULLPTR
 // Define the `pointer_safety` enum as a C++11 strongly typed enumeration
 // instead of as a class simulating an enum. If this option is enabled
 // `pointer_safety` and `get_pointer_safety()` will no longer be available
 // in C++03.
-#  define _LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE
+// The underlying type of the typed enumeration is "unsigned char".
+#  define _LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE unsigned char
 // Define a key function for `bad_function_call` in the library, to centralize
 // its vtable and typeinfo to libc++ rather than having all other libraries
 // using that class define their own copies.
 #  define _LIBCPP_ABI_BAD_FUNCTION_CALL_KEY_FUNCTION
 // Enable optimized version of __do_get_(un)signed which avoids redundant copies.
 #  define _LIBCPP_ABI_OPTIMIZED_LOCALE_NUM_GET
 // Use the smallest possible integer type to represent the index of the variant.
 // Previously libc++ used "unsigned int" exclusively.

 // conflict with the dllexport-emitted copy, so we disable it.
 #    define _LIBCPP_DEPRECATED_ABI_LEGACY_LIBRARY_DEFINITIONS_FOR_INLINE_FUNCTIONS
 #  endif
 // Feature macros for disabling pre ABI v1 features. All of these options
 // are deprecated.
 #  if defined(__FreeBSD__)
 #    define _LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR
 #  endif
+
+#  if defined(_AIX)
+// AIX used C++11 strongly typed enumeration with underlying type as "int"
+// for `pointer_safety`.
+#    define _LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE int
+#  endif
 #endif
 
 #if defined(_LIBCPP_BUILDING_LIBRARY) || defined(_LIBCPP_ABI_UNSTABLE) || _LIBCPP_ABI_VERSION >= 2
 // Enable additional explicit instantiations of iostreams components. This
 // reduces the number of weak definitions generated in programs that use
 // iostreams by providing a single strong definition in the shared library.
 # define _LIBCPP_ABI_ENABLE_ADDITIONAL_IOSTREAM_EXPLICIT_INSTANTIATIONS_1
 #endif

libcxx/include/memory

atomic_compare_exchange_weak_explicit(shared_ptr<_Tp>* __p, shared_ptr<_Tp>* __v,
     return atomic_compare_exchange_weak(__p, __v, __w);
 }
 
 #endif  // !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 
 //enum class
 #if defined(_LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE)
 # ifndef _LIBCPP_CXX03_LANG
-enum class pointer_safety : unsigned char {
+enum class pointer_safety : _LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE {
   relaxed,
   preferred,
   strict
 };
 # endif
 #else
 struct _LIBCPP_TYPE_VIS pointer_safety
 {

libcxx/test/libcxx/utilities/memory/util.dynamic.safety/get_pointer_safety_new_abi.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // <memory>
 
 // pointer_safety get_pointer_safety();
 
 // The pointer_safety interface is no longer provided in C++03 in the new ABI.
 // XFAIL: c++03
 
-// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE="unsigned char"
 
 #include <memory>
 #include <cassert>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {