[libc++] Fix warnings on AIX 32 bit

[ldionne]

Summary

The compiler complains about large atomic operation potentially incurring
a performance penalty with -Watomic-alignment, which is turned into an
error.

Note that I don't really intend to commit this patch as-is -- I mostly
want to have a discussion to find out why this warning begun. I don't
intend that the right fix is simply going to silence the warning, at
least not from within the headers.

[mstorsjo]

See D124519 for a different fix for the warning. As for why -Werror started working here, I don’t really know - earlier, it was only unblocked by D124375, and I’ve held off applying that one into these warnings were fixed.

[mstorsjo]

I bisected it - it seems that D120719 in combination with D110005 makes the -Werror flag test succeed now, even before D124375 had been landed.

[ldionne]

Thanks, I had not seen D124519 yet. I think D124519 makes more sense than this.

libcxx/include/atomic

 void __cxx_atomic_store(__cxx_atomic_base_impl<_Tp> * __a, _Tp __val, memory_order __order) _NOEXCEPT {
     __c11_atomic_store(&__a->__a_value, __val, static_cast<__memory_order_underlying_t>(__order));
 }
 
 template<class _Tp>
 _LIBCPP_INLINE_VISIBILITY
 _Tp __cxx_atomic_load(__cxx_atomic_base_impl<_Tp> const volatile* __a, memory_order __order) _NOEXCEPT {
     using __ptr_type = typename remove_const<decltype(__a->__a_value)>::type*;
+#ifdef _AIX
+    _LIBCPP_DIAGNOSTIC_PUSH
+    _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(-Watomic-alignment)
+#endif
     return __c11_atomic_load(const_cast<__ptr_type>(&__a->__a_value), static_cast<__memory_order_underlying_t>(__order));
+#ifdef _AIX
+    _LIBCPP_DIAGNOSTIC_POP
+#endif
 }
 template<class _Tp>
 _LIBCPP_INLINE_VISIBILITY
 _Tp __cxx_atomic_load(__cxx_atomic_base_impl<_Tp> const* __a, memory_order __order) _NOEXCEPT {
     using __ptr_type = typename remove_const<decltype(__a->__a_value)>::type*;
     return __c11_atomic_load(const_cast<__ptr_type>(&__a->__a_value), static_cast<__memory_order_underlying_t>(__order));
 }
 
bool __cxx_atomic_compare_exchange_weak(__cxx_atomic_base_impl<_Tp> volatile* __a, _Tp* __expected, _Tp __value, memory_order __success, memory_order __failure) _NOEXCEPT {
     return __c11_atomic_compare_exchange_weak(&__a->__a_value, __expected, __value, static_cast<__memory_order_underlying_t>(__success), static_cast<__memory_order_underlying_t>(__to_failure_order(__failure)));
 }
 template<class _Tp>
 _LIBCPP_INLINE_VISIBILITY
 bool __cxx_atomic_compare_exchange_weak(__cxx_atomic_base_impl<_Tp> * __a, _Tp* __expected, _Tp __value, memory_order __success, memory_order __failure) _NOEXCEPT {
     return __c11_atomic_compare_exchange_weak(&__a->__a_value, __expected, __value,  static_cast<__memory_order_underlying_t>(__success), static_cast<__memory_order_underlying_t>(__to_failure_order(__failure)));
 }
 
+#ifdef _AIX
+  _LIBCPP_DIAGNOSTIC_PUSH
+  _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(-Watomic-alignment)
+#endif
 template<class _Tp>
 _LIBCPP_INLINE_VISIBILITY
 _Tp __cxx_atomic_fetch_add(__cxx_atomic_base_impl<_Tp> volatile* __a, _Tp __delta, memory_order __order) _NOEXCEPT {
     return __c11_atomic_fetch_add(&__a->__a_value, __delta, static_cast<__memory_order_underlying_t>(__order));
 }
 template<class _Tp>
 _LIBCPP_INLINE_VISIBILITY
 _Tp __cxx_atomic_fetch_add(__cxx_atomic_base_impl<_Tp> * __a, _Tp __delta, memory_order __order) _NOEXCEPT {
     return __c11_atomic_fetch_add(&__a->__a_value, __delta, static_cast<__memory_order_underlying_t>(__order));
 }
+#ifdef _AIX
+  _LIBCPP_DIAGNOSTIC_POP
+#endif
 
 template<class _Tp>
 _LIBCPP_INLINE_VISIBILITY
 _Tp* __cxx_atomic_fetch_add(__cxx_atomic_base_impl<_Tp*> volatile* __a, ptrdiff_t __delta, memory_order __order) _NOEXCEPT {
     return __c11_atomic_fetch_add(&__a->__a_value, __delta, static_cast<__memory_order_underlying_t>(__order));
 }
 template<class _Tp>
 _LIBCPP_INLINE_VISIBILITY
 _Tp* __cxx_atomic_fetch_add(__cxx_atomic_base_impl<_Tp*> * __a, ptrdiff_t __delta, memory_order __order) _NOEXCEPT {
     return __c11_atomic_fetch_add(&__a->__a_value, __delta, static_cast<__memory_order_underlying_t>(__order));
 }
 
 template<class _Tp>
 _LIBCPP_INLINE_VISIBILITY
 _Tp __cxx_atomic_fetch_sub(__cxx_atomic_base_impl<_Tp> volatile* __a, _Tp __delta, memory_order __order) _NOEXCEPT {
+#ifdef _AIX
+    _LIBCPP_DIAGNOSTIC_PUSH
+    _LIBCPP_CLANG_DIAGNOSTIC_IGNORED(-Watomic-alignment)
+#endif
     return __c11_atomic_fetch_sub(&__a->__a_value, __delta, static_cast<__memory_order_underlying_t>(__order));
+#ifdef _AIX
+    _LIBCPP_DIAGNOSTIC_POP
+#endif
 }
 template<class _Tp>
 _LIBCPP_INLINE_VISIBILITY
 _Tp __cxx_atomic_fetch_sub(__cxx_atomic_base_impl<_Tp> * __a, _Tp __delta, memory_order __order) _NOEXCEPT {
     return __c11_atomic_fetch_sub(&__a->__a_value, __delta, static_cast<__memory_order_underlying_t>(__order));
 }
 template<class _Tp>
 _LIBCPP_INLINE_VISIBILITY