On Tue, 10 Feb 2026 at 21:55, Patrick Palka <[email protected]> wrote:
>
> Tested on x86_64-pc-linux-gnu with GLIBCXX_TESTSUITE_STDS=11,14,20,
> does this look OK for trunk and backports (after r16-7199 gets
> backported)? Also tested with Clang 21.
Yes for trunk and backports - thanks!
>
> -- >8 --
>
> After the front end change r16-7199 both GCC and Clang allow
> non-empty constexpr constructor bodies in C++11 as an extension,
> so we can unconditionally enable the __builtin_clear_padding logic
> in std::atomic's constructor.
>
> PR libstdc++/114865
>
> libstdc++-v3/ChangeLog:
>
> * include/std/atomic (atomic<_Tp>::atomic(_Tp))
> [C++11]: Enable __builtin_clear_padding logic.
> * testsuite/29_atomics/atomic/cons/static_zero_padding.cc
> [C++11]: Enable test verifying cleared padding bits for a
> std::atomic local variable.
> ---
> libstdc++-v3/include/std/atomic | 5 ++++-
> .../testsuite/29_atomics/atomic/cons/static_zero_padding.cc | 2 --
> 2 files changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
> index 56dbe7bf5b95..306667e3c633 100644
> --- a/libstdc++-v3/include/std/atomic
> +++ b/libstdc++-v3/include/std/atomic
> @@ -247,14 +247,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> atomic& operator=(const atomic&) = delete;
> atomic& operator=(const atomic&) volatile = delete;
>
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wc++14-extensions" // constexpr ctor body
> constexpr atomic(_Tp __i) noexcept : _M_i(__i)
> {
> -#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> +#if __has_builtin(__builtin_clear_padding)
> if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>())
> if (!std::__is_constant_evaluated())
> __builtin_clear_padding(std::__addressof(_M_i));
> #endif
> }
> +#pragma GCC diagnostic pop
>
> operator _Tp() const noexcept
> { return load(); }
> diff --git
> a/libstdc++-v3/testsuite/29_atomics/atomic/cons/static_zero_padding.cc
> b/libstdc++-v3/testsuite/29_atomics/atomic/cons/static_zero_padding.cc
> index 506513647c6c..e3594fb1c8ea 100644
> --- a/libstdc++-v3/testsuite/29_atomics/atomic/cons/static_zero_padding.cc
> +++ b/libstdc++-v3/testsuite/29_atomics/atomic/cons/static_zero_padding.cc
> @@ -57,9 +57,7 @@ void test_struct(std::atomic<T>& g, const T& zp)
>
> std::atomic<T> l(T{1, 2});
> std::memcpy(&t, &zp, sizeof(T));
> -#if __cplusplus >= 201402L // Remove once PR114865 is fixed
> VERIFY( l.compare_exchange_strong(t, d) );
> -#endif
>
> constexpr std::atomic<T> cl(T{1, 2});
> }
> --
> 2.53.0.68.g864f55e190
>
