On Thu, Jan 29, 2026 at 9:05 PM Tomasz Kamiński <[email protected]> wrote:
> Currently for the types T that contains padding bits, std::atomic<T>(T)
> constructor was not usable at compile-time in C++14 or later modes. This
> regression caused by use of __builtin_clear_padding introduced in
> r13-2548-g157236dbd62164.
>
> This leads to two regressions when switching from C++11 to C++14
> standard (or switching from GCC-12 to later version for C++14 standard),
> where for type X that contains padding
> * constexpr std::atomic<X> cx(X(...)) becomes ill-formed,
> * std::atomic<X> gx(X(...)) with static storage duration, switch from
> static to dynamic initialization.
> The later breakage is silent and may introduced very hard to localize
> order of initialization issues.
>
> This patch mitigates above issue by not invoking the
> __builtin_clear_padding,
> during constant initialization (std::__is_constant_evaluated() is false).
> This is considered to be safe, as:
> * for objects with static storage duration, padding bits are already
> cleared by zero-initialization
> * for contexpr objects with static storage duration, there is no
> API that would allow user to observe padding bits on const atomic objects
>
> To elaborate on the second point, values of padding bits in atomic can
> be observed by:
> * The compapre_exchange_weak/compare_exchange_strong operations are
> mutating,
> so cannot be invoked on const objects.
> * As atomic<X> is not required to store actual object of type X,
> observing it's object representation does (via bitcast, memcpy), does
> not provide values of object representation of X. Furthermore, the
> operations are defined only for trivally_copyable types, and atomic
> specializations meets above requirement only due to bug in libstdc++
> (see PR67572).
>
> Note that above will no longer hold, and the solution will need to
> revisited during implementation of C++26 paper P3309R3: constexpr
> atomic and atomic_ref (it will be possible to call compare_exchange
> during constant evaluation).
>
> PR libstdc++/123875
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/atomic_base.h (__atomic_impl::__clear_padding):
> Use if constexpr unconditionally.
> (__atomic_float<_Fp>::__atomic_float(_Fp)): Skip __clear_padding
> call for constant evaluation.
> * include/std/atomic (atomic<_Tp>::atomic(_Tp)): Likewise.
> * testsuite/29_atomics/atomic/cons/static_zero_padding.cc: New
> test.
> ---
> Tested on x86_64-linux. OK for trunk?
> Then 13/14/15?
>
> libstdc++-v3/include/bits/atomic_base.h | 13 ++-
> libstdc++-v3/include/std/atomic | 3 +-
> .../atomic/cons/static_zero_padding.cc | 106 ++++++++++++++++++
> 3 files changed, 116 insertions(+), 6 deletions(-)
> create mode 100644
> libstdc++-v3/testsuite/29_atomics/atomic/cons/static_zero_padding.cc
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> index bc4778f08cb..79c1b31ccc4 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -1003,21 +1003,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #endif
> }
>
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wc++17-extensions"
> +
> template<typename _Tp>
> _GLIBCXX_ALWAYS_INLINE _GLIBCXX14_CONSTEXPR _Tp*
> __clear_padding(_Tp& __val) noexcept
> {
> auto* __ptr = std::__addressof(__val);
> #if __has_builtin(__builtin_clear_padding)
> - if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>())
> + if constexpr (__atomic_impl::__maybe_has_padding<_Tp>())
> __builtin_clear_padding(__ptr);
> #endif
> return __ptr;
> }
>
> -#pragma GCC diagnostic push
> -#pragma GCC diagnostic ignored "-Wc++17-extensions"
> -
> template<bool _AtomicRef = false, typename _Tp>
> _GLIBCXX_ALWAYS_INLINE bool
> __compare_exchange(_Tp& __val, _Val<_Tp>& __e, _Val<_Tp>& __i,
> @@ -1392,7 +1392,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr
> __atomic_float(_Fp __t) : _M_fp(__t)
> - { __atomic_impl::__clear_padding(_M_fp); }
> + {
> + if (!std::__is_constant_evaluated())
> + __atomic_impl::__clear_padding(_M_fp);
> + }
>
> __atomic_float(const __atomic_float&) = delete;
> __atomic_float& operator=(const __atomic_float&) = delete;
> diff --git a/libstdc++-v3/include/std/atomic
> b/libstdc++-v3/include/std/atomic
> index 46d2bbb509e..56dbe7bf5b9 100644
> --- a/libstdc++-v3/include/std/atomic
> +++ b/libstdc++-v3/include/std/atomic
> @@ -251,7 +251,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> {
> #if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>())
> - __builtin_clear_padding(std::__addressof(_M_i));
> + if (!std::__is_constant_evaluated())
> + __builtin_clear_padding(std::__addressof(_M_i));
> #endif
> }
>
> 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
> new file mode 100644
> index 00000000000..3866795a422
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/29_atomics/atomic/cons/static_zero_padding.cc
> @@ -0,0 +1,106 @@
> +// { dg-do run { target c++11 } }
> +// { dg-require-thread-fence "" }
> +// { dg-add-options no_pch }
> +
> +#include <atomic>
> +#include <cstring>
> +#include <testsuite_hooks.h>
> +
> +struct TailPadding { int i; char c; };
> +TailPadding ztail{1, 2}; // zeroed-padding
> +constexpr std::atomic<TailPadding> ctail(TailPadding{1,2});
> +std::atomic<TailPadding> gtail(TailPadding{1,2});
> +
> +struct MidPadding { char c; int x; };
> +MidPadding zmid{1, 2}; // zeroed-padding
> +constexpr std::atomic<MidPadding> cmid(MidPadding{1,2});
> +std::atomic<MidPadding> gmid(MidPadding{1,2});
> +
> +struct BitPadding { int : 4; int i : 5; int : 4; int j : 5; int : 4; };
> +BitPadding zbit{1, 2}; // zeroed-padding
> +constexpr std::atomic<BitPadding> cbit(BitPadding{1,2});
> +std::atomic<BitPadding> gbit(BitPadding{1,2});
> +
> +struct Ctor
> +{
> + Ctor() = default;
> +
> + constexpr Ctor(char pc, char pi)
> + : c(pc), i(pi), d(pc)
> + {}
> +
> + char c;
> + int i;
> + char d;
> +};
> +
> +Ctor zctor{1, 2}; // zeroed-padding
> +constexpr std::atomic<Ctor> cctor(Ctor{1,2});
> +std::atomic<Ctor> gctor(Ctor{1,2});
> +
> +template<typename T>
> +void test_struct(std::atomic<T>& g, const T& zp)
> +{
> + T const d{3, 4};
> + T t;
> +
> + std::memcpy(&t, &zp, sizeof(T));
> + VERIFY( g.compare_exchange_strong(t, d) );
> +
> + static std::atomic<T> st(T{1, 2});
> + std::memcpy(&t, &zp, sizeof(T));
> + VERIFY( st.compare_exchange_strong(t, d) );
> +
> + thread_local std::atomic<T> tl(T{1, 2});
> + std::memcpy(&t, &zp, sizeof(T));
> + VERIFY( tl.compare_exchange_strong(t, d) );
> +
> + std::atomic<T> l(T{1, 2});
> + std::memcpy(&t, &zp, sizeof(T));
> +#if __cplusplus >= 202002L // Remove once PR114865 is fixed
>
This should be checking C++14 or later (201402L), not C++20, changed
locally, and 29_atomics/atomic/* tests are still passing in all supported
standard modes.
> + VERIFY( l.compare_exchange_strong(t, d) );
> +#endif
> +
> + constexpr std::atomic<T> cl(T{1, 2});
> +}
> +
> +#if __cplusplus >= 202002L
> +long double zld(10.5);
> +constexpr std::atomic<long double> cld(10.5);
> +std::atomic<long double> gld(10.5);
> +
> +template<typename T>
> +void test_floating(std::atomic<T>& g, const T& zp)
> +{
> + T const d = T(7.5);
> + T t;
> +
> + std::memcpy(&t, &zp, sizeof(T));
> + VERIFY( g.compare_exchange_strong(t, d) );
> +
> + static std::atomic<T> st(T(10.5));
> + std::memcpy(&t, &zp, sizeof(T));
> + VERIFY( st.compare_exchange_strong(t, d) );
> +
> + thread_local std::atomic<T> tl(T(10.5));
> + std::memcpy(&t, &zp, sizeof(T));
> + VERIFY( tl.compare_exchange_strong(t, d) );
> +
> + std::atomic<T> l(T(10.5));
> + std::memcpy(&t, &zp, sizeof(T));
> + VERIFY( l.compare_exchange_strong(t, d) );
> +
> + constexpr std::atomic<T> cl(T(10.5));
> +}
> +#endif
> +
> +int main()
> +{
> + test_struct(gtail, ztail);
> + test_struct(gmid, zmid);
> + test_struct(gbit, zbit);
> + test_struct(gctor, zctor);
> +#if __cplusplus >= 202002L
> + test_floating(gld, zld);
> +#endif
> +}
> --
> 2.52.0
>
>
