https://gcc.gnu.org/g:283f85bc232a173ed9b7a7030e7027afdc85a357
commit r15-10875-g283f85bc232a173ed9b7a7030e7027afdc85a357 Author: Patrick Palka <[email protected]> Date: Wed Feb 11 21:35:21 2026 -0500 libstdc++: Clear padding bits in std::atomic ctor in C++11 [PR114865] 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 now 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/compare_exchange_padding.cc: Enable this test in earlier modes, including C++11. * testsuite/29_atomics/atomic/cons/zero_padding.cc [C++11]: Enable tests verifying cleared padding bits for a non-static-init std::atomic object. Reviewed-by: Tomasz Kamiński <[email protected]> Reviewed-by: Jonathan Wakely <[email protected]> (cherry picked from commit 48f2e8aa6ddad72955781728bdf515eb50411d24) Diff: --- libstdc++-v3/include/std/atomic | 5 ++++- libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc | 2 +- libstdc++-v3/testsuite/29_atomics/atomic/cons/zero_padding.cc | 4 ---- 3 files changed, 5 insertions(+), 6 deletions(-) diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic index 98576234b6ec..0127d7dbc3d5 100644 --- a/libstdc++-v3/include/std/atomic +++ b/libstdc++-v3/include/std/atomic @@ -241,14 +241,17 @@ _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/compare_exchange_padding.cc b/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc index a6081968ca86..d9c7b90c1a5c 100644 --- a/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc +++ b/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc @@ -1,4 +1,4 @@ -// { dg-do run { target c++20 } } +// { dg-do run { target c++11 } } // { dg-require-atomic-cmpxchg-word "" } // { dg-add-options libatomic } // { dg-additional-options "-fno-tree-sra" } diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/cons/zero_padding.cc b/libstdc++-v3/testsuite/29_atomics/atomic/cons/zero_padding.cc index d57846d60205..f85ac4859ec5 100644 --- a/libstdc++-v3/testsuite/29_atomics/atomic/cons/zero_padding.cc +++ b/libstdc++-v3/testsuite/29_atomics/atomic/cons/zero_padding.cc @@ -56,15 +56,11 @@ 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 std::atomic<T>* h = new std::atomic<T>(T{1, 2}); std::memcpy(&t, &zp, sizeof(T)); -#if __cplusplus >= 201402L // Remove once PR114865 is fixed VERIFY( h->compare_exchange_strong(t, d) ); -#endif delete h; constexpr std::atomic<T> cl(T{1, 2});
