[gcc r16-7471] libstdc++: Clear padding bits in std::atomic ctor in C++11 [PR114865]

[Patrick Palka]

https://gcc.gnu.org/g:48f2e8aa6ddad72955781728bdf515eb50411d24

commit r16-7471-g48f2e8aa6ddad72955781728bdf515eb50411d24
Author: Patrick Palka <ppalka@redhat.com>
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 <tkaminsk@redhat.com>
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

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 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/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 0d6579b5ab6e..ad48d66de3a4 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic/cons/zero_padding.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/cons/zero_padding.cc
@@ -57,15 +57,11 @@ 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});