[Bug libstdc++/114865] [13/14/15 Regression] std::atomic<X>::compare_exchange_strong seems to hang under GCC 13 for C++11

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114865

--- Comment #18 from Jonathan Wakely <redi at gcc dot gnu.org> ---
So maybe:

diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index aa7374bb252..662cff39df5 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -956,7 +956,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr bool
       __maybe_has_padding()
       {
-#if ! __has_builtin(__builtin_clear_padding)
+       // We cannot clear padding in the constructor for C++11,
+       // so return false here to disable all code for zeroing padding.
+#if __cplusplus < 201402L || ! __has_builtin(__builtin_clear_padding)
        return false;
 #elif __has_builtin(__has_unique_object_representations)
        return !__has_unique_object_representations(_Tp)
diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index 36ff89a146c..336f27832fc 100644
--- a/libstdc++-v3/include/std/atomic
+++ b/libstdc++-v3/include/std/atomic
@@ -238,6 +238,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION

       constexpr atomic(_Tp __i) noexcept : _M_i(__i)
       {
+       // A constexpr constructor must be empty in C++11,
+       // so we can only clear padding for C++14 and later.
 #if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
        if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>())
          __builtin_clear_padding(std::__addressof(_M_i));