The std::atomic constructor clears padding bits so that compare_exchange
will not fail due to differences in padding bits. But we can only do
that for C++14 and later, because in C++11 a constexpr constructor must
have an empty body. However, the code in compare_exchange_strong assumes
that padding is always cleared, and so it fails in C++11 due to non-zero
padding.
Since we can't clear the padding in C++11 mode, we shouldn't assume it's
been cleared when in C++11 mode. This fixes the reported bug. However,
the fix fails to handle the case where the std::atomic is constructed in
C++11 code (and so doesn't zero padding) but the CAS happens in C++14
code (and so assumes padding has been zeroed). We might need to use the
same loop as atomic_ref::compare_exchange_strong to properly fix this
bug for that case.
Although the mixed C++11 / C++14 case isn't fixed, this is still an
incremental improvement. It fixes the pure-C++11 case and doesn't
preclude a more comprehensive fix later.
libstdc++-v3/ChangeLog:
PR libstdc++/114865
* include/bits/atomic_base.h (__maybe_has_padding): Return false
for C++11.
* include/std/atomic (atomic::atomic(T)): Add comment.
* testsuite/29_atomics/atomic/114865.cc: New test.
---
Tested x86_64-linux.
libstdc++-v3/include/bits/atomic_base.h | 4 +-
libstdc++-v3/include/std/atomic | 2 +
.../testsuite/29_atomics/atomic/114865.cc | 49 +++++++++++++++++++
3 files changed, 54 insertions(+), 1 deletion(-)
create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/114865.cc
diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index 92d1269493f..19fc7a77c1b 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -954,7 +954,9 @@ _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 9b1aca0fc09..949a9017862 100644
--- a/libstdc++-v3/include/std/atomic
+++ b/libstdc++-v3/include/std/atomic
@@ -243,6 +243,8 @@ _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));
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/114865.cc
b/libstdc++-v3/testsuite/29_atomics/atomic/114865.cc
new file mode 100644
index 00000000000..577cd480915
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/114865.cc
@@ -0,0 +1,49 @@
+// { dg-do run { target c++11_only } }
+// { dg-require-atomic-cmpxchg-word "" }
+// { dg-add-options libatomic }
+
+// Bug 114865
+// std::atomic<X>::compare_exchange_strong seems to hang under GCC 13 for C++11
+
+#include <atomic>
+#include <cstdint>
+
+struct type
+{
+ std::uint32_t u32;
+ std::uint16_t u16;
+};
+
+[[gnu::noipa,gnu::noinline,gnu::optimize("O0")]]
+type next(const type& old)
+{
+ auto t = old;
+ ++t.u16;
+ return t;
+}
+
+[[gnu::noipa,gnu::noinline,gnu::optimize("O0")]]
+void
+test_pr116440()
+{
+ constexpr auto mo = std::memory_order_relaxed;
+
+ type t;
+ t.u32 = t.u16 = 0;
+ std::atomic<type> a(t);
+
+ auto old = a.load(mo);
+
+ while (true)
+ {
+ auto t = next(old);
+
+ if (a.compare_exchange_strong(old, t, mo, mo))
+ return;
+ }
+}
+
+int main()
+{
+ test_pr116440();
+}
--
2.49.0
