On Tue, Dec 16, 2025 at 5:41 PM Jonathan Wakely <[email protected]> wrote:
> On Fri, 05 Dec 2025 at 09:17 +0100, Tomasz Kamiński wrote:
> >This patch implements the P3860R1 (accepted as DR against C++20) and
> LWG4472.
> >
> >The two constraints on the constructor (that T and U are similar types and
> >is_convertible_v<U*, T*>) are combined into a single check:
> >is_convertible_v<_Up(*)[1], _Tp(*)[1]>. While this check does is not
> equivalent
>
> s/does is not/is not/
>
> >for array of know bound to array of unknown bound conversions (T[N] to
> T[]),
>
> s/know bound/known bound/
>
> >this is irrelevant for atomic_ref, since instantiation with an array type
> is
> >ill-formed (due to the return type of load and other members).
> >
> >The __atomic_ref_base constructor is modified to accept _Tp* instead of
> _Tp&.
>
> Please also mention here that taking a pointer avoids doing ADL for
> *__other._M_ptr
>
I have updated it locally to following, does it sound good?
The __atomic_ref_base constructor is modified to accept _Tp* instead of
_Tp&.
This allows both the atomic_ref(atomic_ref<_Up> __other) and
atomic_ref(_Tp& __t)
constructors to delegate to it. Furthermore, such approach does not require
dereferencing *__other._M_ptr, and thus avoid ADL-lookup for operator* and
issues related to it. The precondition check on alignment is moved
specifically
to the atomic_ref(_Tp&) constructor, preventing redundant checks during
atomic_ref
conversion.
>
>
> OK with those changes to the commit mesage, thanks.
>
> >This allows both the atomic_ref(atomic_ref<_Up>) and atomic_ref(_Tp&)
> >constructors to delegate to it. The precondition check on alignment is
> moved
> >specifically to the atomic_ref(_Tp&) constructor, preventing redundant
> checks
> >during atomic_ref conversion.
> >
> >A deleted atomic_ref(_Tp&&) constructor is introduced (per LWG4472).
> >This prevents the construction of atomic_ref<T> from atomic_ref<volatile
> T>
> >via the conversion operator.
> >
> >libstdc++-v3/ChangeLog:
> >
> > * include/bits/atomic_base.h
> > (__atomic_ref_base<const _Tp>::__atomic_ref_base): Accept
> > pointer instead of reference. Remove precondition check and
> > mark as noexcept.
> > (__atomic_ref_base<_Tp>::__atomic_ref_base): Accept pointer
> > insted of reference, and mark as noexcept.
> > * include/std/atomic (atomic_ref::atomic_ref(_Tp&)): Add
> > precondition check and take address of argument.
> > (atomic_ref::atomic_ref(_Tp&&)): Define as deleted.
> > (atomic_ref::atomic_ref(atomic_ref<_Up>)): Define.
> > * include/bits/shared_ptr_atomic.h (_Sp_atomic::_Atomic_count):
> > Pass address to __atomic_ref constructor.
> > * include/std/barrier (__tree_barrier_base::_M_arrive)
> > (__tree_barrier::arrive): Pass address to __atomic_ref constructor.
> > * testsuite/29_atomics/atomic_ref/ctor.cc: New test.
> >---
> >v3 adjusted Overload set to more closely reflect example from reflector.
> >
> >
> > libstdc++-v3/include/bits/atomic_base.h | 11 +-
> > libstdc++-v3/include/bits/shared_ptr_atomic.h | 16 +-
> > libstdc++-v3/include/std/atomic | 21 +-
> > libstdc++-v3/include/std/barrier | 6 +-
> > .../testsuite/29_atomics/atomic_ref/ctor.cc | 193 ++++++++++++++++++
> > 5 files changed, 228 insertions(+), 19 deletions(-)
> > create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic_ref/ctor.cc
> >
> >diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> >index 90b8df55ede..d07cb0f13c8 100644
> >--- a/libstdc++-v3/include/bits/atomic_base.h
> >+++ b/libstdc++-v3/include/bits/atomic_base.h
> >@@ -1561,11 +1561,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > __atomic_ref_base& operator=(const __atomic_ref_base&) = delete;
> >
> > explicit
> >- __atomic_ref_base(const _Tp& __t)
> >- : _M_ptr(const_cast<_Tp*>(std::addressof(__t)))
> >- {
> >- __glibcxx_assert(((__UINTPTR_TYPE__)_M_ptr % required_alignment)
> == 0);
> >- }
> >+ __atomic_ref_base(const _Tp* __ptr) noexcept
> >+ : _M_ptr(const_cast<_Tp*>(__ptr))
> >+ { }
> >
> > __atomic_ref_base(const __atomic_ref_base&) noexcept = default;
> >
> >@@ -1607,7 +1605,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > using value_type = typename __atomic_ref_base<const
> _Tp>::value_type;
> >
> > explicit
> >- __atomic_ref_base(_Tp& __t) : __atomic_ref_base<const _Tp>(__t)
> >+ __atomic_ref_base(_Tp* __ptr) noexcept
> >+ : __atomic_ref_base<const _Tp>(__ptr)
> > { }
> >
> > value_type
> >diff --git a/libstdc++-v3/include/bits/shared_ptr_atomic.h
> b/libstdc++-v3/include/bits/shared_ptr_atomic.h
> >index cbc4bf621f4..b16d93e6697 100644
> >--- a/libstdc++-v3/include/bits/shared_ptr_atomic.h
> >+++ b/libstdc++-v3/include/bits/shared_ptr_atomic.h
> >@@ -421,7 +421,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >
> > ~_Atomic_count()
> > {
> >- auto __val = _AtomicRef(_M_val).load(memory_order_relaxed);
> >+ auto __val = _AtomicRef(&_M_val).load(memory_order_relaxed);
> > _GLIBCXX_TSAN_MUTEX_DESTROY(&_M_val);
> > __glibcxx_assert(!(__val & _S_lock_bit));
> > if (auto __pi = reinterpret_cast<pointer>(__val))
> >@@ -443,7 +443,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > {
> > // To acquire the lock we flip the LSB from 0 to 1.
> >
> >- _AtomicRef __aref(_M_val);
> >+ _AtomicRef __aref(&_M_val);
> > auto __current = __aref.load(memory_order_relaxed);
> > while (__current & _S_lock_bit)
> > {
> >@@ -476,7 +476,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > unlock(memory_order __o) const noexcept
> > {
> > _GLIBCXX_TSAN_MUTEX_PRE_UNLOCK(&_M_val);
> >- _AtomicRef(_M_val).fetch_sub(1, __o);
> >+ _AtomicRef(&_M_val).fetch_sub(1, __o);
> > _GLIBCXX_TSAN_MUTEX_POST_UNLOCK(&_M_val);
> > }
> >
> >@@ -489,7 +489,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > __o = memory_order_release;
> > auto __x = reinterpret_cast<uintptr_t>(__c._M_pi);
> > _GLIBCXX_TSAN_MUTEX_PRE_UNLOCK(&_M_val);
> >- __x = _AtomicRef(_M_val).exchange(__x, __o);
> >+ __x = _AtomicRef(&_M_val).exchange(__x, __o);
> > _GLIBCXX_TSAN_MUTEX_POST_UNLOCK(&_M_val);
> > __c._M_pi = reinterpret_cast<pointer>(__x & ~_S_lock_bit);
> > }
> >@@ -502,7 +502,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > auto __old_ptr = __ptr;
> > _GLIBCXX_TSAN_MUTEX_PRE_UNLOCK(&_M_val);
> > uintptr_t __old_pi
> >- = _AtomicRef(_M_val).fetch_sub(1, memory_order_relaxed) - 1u;
> >+ = _AtomicRef(&_M_val).fetch_sub(1, memory_order_relaxed) - 1u;
> > _GLIBCXX_TSAN_MUTEX_POST_UNLOCK(&_M_val);
> >
> > // Ensure that the correct value of _M_ptr is visible after
> locking,
> >@@ -528,14 +528,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > // wake up if either of the values changed
> > return __new_pi != __old_pi || __new_ptr != __old_ptr;
> > },
> >- [__o, this] { return _AtomicRef(_M_val).load(__o); });
> >+ [__o, this] { return _AtomicRef(&_M_val).load(__o); });
> > }
> >
> > void
> > notify_one() noexcept
> > {
> > _GLIBCXX_TSAN_MUTEX_PRE_SIGNAL(&_M_val);
> >- _AtomicRef(_M_val).notify_one();
> >+ _AtomicRef(&_M_val).notify_one();
> > _GLIBCXX_TSAN_MUTEX_POST_SIGNAL(&_M_val);
> > }
> >
> >@@ -543,7 +543,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > notify_all() noexcept
> > {
> > _GLIBCXX_TSAN_MUTEX_PRE_SIGNAL(&_M_val);
> >- _AtomicRef(_M_val).notify_all();
> >+ _AtomicRef(&_M_val).notify_all();
> > _GLIBCXX_TSAN_MUTEX_POST_SIGNAL(&_M_val);
> > }
> > #endif
> >diff --git a/libstdc++-v3/include/std/atomic
> b/libstdc++-v3/include/std/atomic
> >index ccb77fa6327..15372ea7994 100644
> >--- a/libstdc++-v3/include/std/atomic
> >+++ b/libstdc++-v3/include/std/atomic
> >@@ -1767,14 +1767,31 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > struct atomic_ref : __atomic_ref<_Tp>
> > {
> > explicit
> >- atomic_ref(_Tp& __t) noexcept : __atomic_ref<_Tp>(__t)
> >- { }
> >+ atomic_ref(_Tp& __t) noexcept
> >+ : __atomic_ref<_Tp>(std::addressof(__t))
> >+ {
> >+ __glibcxx_assert(((__UINTPTR_TYPE__)this->_M_ptr %
> this->required_alignment) == 0);
> >+ }
> >+
> >+ // _GLIBCXX_RESOLVE_LIB_DEFECTS
> >+ // 4472. std::atomic_ref<const T> can be constructed from
> temporaries
> >+ explicit
> >+ atomic_ref(_Tp&&) = delete;
> >+
> >+ template<typename _Up>
> >+ requires is_convertible_v<_Up(*)[1], _Tp(*)[1]>
> >+ atomic_ref(atomic_ref<_Up> __other) noexcept
> >+ : __atomic_ref<_Tp>(__other._M_ptr)
> >+ { }
> >
> > atomic_ref& operator=(const atomic_ref&) = delete;
> >
> > atomic_ref(const atomic_ref&) = default;
> >
> > using __atomic_ref<_Tp>::operator=;
> >+
> >+ template<typename>
> >+ friend struct atomic_ref;
> > };
> > #endif // __cpp_lib_atomic_ref
> >
> >diff --git a/libstdc++-v3/include/std/barrier
> b/libstdc++-v3/include/std/barrier
> >index 56270c99e05..978c12a8067 100644
> >--- a/libstdc++-v3/include/std/barrier
> >+++ b/libstdc++-v3/include/std/barrier
> >@@ -146,7 +146,7 @@ It looks different from literature pseudocode for two
> main reasons:
> > if (__current == __end_node)
> > __current = 0;
> > auto __expect = __old_phase;
> >- __atomic_phase_ref_t __phase(__state[__current]
> >+ __atomic_phase_ref_t __phase(&__state[__current]
> > .__tickets[__round]);
> > if (__current == __last_node && (__current_expected & 1))
> > {
> >@@ -198,7 +198,7 @@ It looks different from literature pseudocode for two
> main reasons:
> >
> > std::hash<std::thread::id> __hasher;
> > size_t __current = __hasher(std::this_thread::get_id());
> >- __atomic_phase_ref_t __phase(_M_phase);
> >+ __atomic_phase_ref_t __phase(&_M_phase);
> > const auto __old_phase = __phase.load(memory_order_relaxed);
> > const auto __cur = static_cast<unsigned char>(__old_phase);
> >
> >@@ -230,7 +230,7 @@ It looks different from literature pseudocode for two
> main reasons:
> > void
> > wait(arrival_token&& __old_phase) const
> > {
> >- __atomic_phase_const_ref_t __phase(_M_phase);
> >+ __atomic_phase_const_ref_t __phase(&_M_phase);
> > __phase.wait(__old_phase, memory_order_acquire);
> > }
> >
> >diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_ref/ctor.cc
> b/libstdc++-v3/testsuite/29_atomics/atomic_ref/ctor.cc
> >new file mode 100644
> >index 00000000000..28ab56fa32b
> >--- /dev/null
> >+++ b/libstdc++-v3/testsuite/29_atomics/atomic_ref/ctor.cc
> >@@ -0,0 +1,193 @@
> >+// { dg-do run { target c++26 } }
> >+// { dg-require-atomic-cmpxchg-word "" }
> >+// { dg-add-options libatomic }
> >+
> >+#include <atomic>
> >+#include <testsuite_hooks.h>
> >+
> >+struct X
> >+{
> >+ X() = default;
> >+ X(int i) : i(i) { }
> >+ int i;
> >+
> >+ friend bool
> >+ operator==(X, X) = default;
> >+};
> >+
> >+template<typename T>
> >+void testTemporary()
> >+{
> >+ static_assert( !std::is_constructible_v<std::atomic_ref<T>, T> );
> >+ static_assert( !std::is_constructible_v<std::atomic_ref<const T>, T> );
> >+ static_assert( !std::is_constructible_v<std::atomic_ref<T>, const T> );
> >+ static_assert( !std::is_constructible_v<std::atomic_ref<const T>,
> const T> );
> >+
> >+ if constexpr (std::atomic_ref<T>::is_always_lock_free)
> >+ {
> >+ static_assert( !std::is_constructible_v<std::atomic_ref<volatile T>,
> T> );
> >+ static_assert( !std::is_constructible_v<std::atomic_ref<volatile T>,
> volatile T> );
> >+ static_assert( !std::is_constructible_v<std::atomic_ref<const
> volatile T>, T> );
> >+ static_assert( !std::is_constructible_v<std::atomic_ref<const
> volatile T>, const volatile T> );
> >+ }
> >+
> >+ struct X { X(T) {} };
> >+ struct Overload
> >+ {
> >+ static int operator()(X) { return 1; }
> >+ static int operator()(std::atomic_ref<T>) { return 2; }
> >+ };
> >+ VERIFY( Overload{}(T()) == 1 );
> >+ static_assert( !requires { Overload{}({T()}); } );
> >+}
> >+
> >+template<typename T>
> >+bool same_address(const std::atomic_ref<T>& t, const
> std::type_identity_t<T>& u)
> >+{
> >+#if (__cplusplus > 202302L)
> >+ return t.address() == &u;
> >+#endif
> >+ return true;
> >+}
> >+
> >+template<typename From, typename To = From>
> >+void
> >+testConv()
> >+{
> >+ alignas(std::atomic_ref<From>::required_alignment) From val{};
> >+ std::atomic_ref<From> src(val);
> >+ std::atomic_ref<const From> csrc(val);
> >+
> >+ std::atomic_ref<const To> d1(src);
> >+ VERIFY( same_address(d1, val) );
> >+ std::atomic_ref<const To> d2(csrc);
> >+ VERIFY( same_address(d2, val) );
> >+ static_assert( !std::is_convertible_v<std::atomic_ref<const From>,
> >+ std::atomic_ref<To>> );
> >+
> >+ if constexpr (std::atomic_ref<From>::is_always_lock_free)
> >+ {
> >+ std::atomic_ref<const volatile To> d4(src);
> >+ VERIFY( same_address(d4, val) );
> >+ std::atomic_ref<const volatile To> d5(csrc);
> >+ VERIFY( same_address(d5, val) );
> >+ if constexpr (std::is_same_v<From, To>)
> >+ {
> >+ std::atomic_ref<volatile To> d3(src);
> >+ VERIFY( same_address(d3, val) );
> >+ }
> >+
> >+ static_assert( !std::is_convertible_v<std::atomic_ref<volatile From>,
> >+ std::atomic_ref<To>> );
> >+ static_assert( !std::is_convertible_v<std::atomic_ref<volatile From>,
> >+ std::atomic_ref<const To>> );
> >+ static_assert( !std::is_convertible_v<std::atomic_ref<const volatile
> From>,
> >+ std::atomic_ref<To>> );
> >+ static_assert( !std::is_convertible_v<std::atomic_ref<const volatile
> From>,
> >+ std::atomic_ref<const To>> );
> >+ }
> >+}
> >+
> >+template<typename From, typename To>
> >+void
> >+testSimilarConv()
> >+{
> >+ testConv<From, To>();
> >+ static_assert( !std::is_convertible_v< To, From> );
> >+ static_assert( !std::is_convertible_v< To, const From> );
> >+ static_assert( !std::is_convertible_v<const To, From> );
> >+ static_assert( !std::is_convertible_v<const To, const From> );
> >+
> >+ if constexpr (std::atomic_ref<From>::is_always_lock_free)
> >+ {
> >+ static_assert( !std::is_convertible_v<volatile To, From> );
> >+ static_assert( !std::is_convertible_v< To, volatile From> );
> >+ static_assert( !std::is_convertible_v<volatile To, volatile From> );
> >+
> >+ static_assert( !std::is_convertible_v<const volatile To,
> From> );
> >+ static_assert( !std::is_convertible_v< To, const
> volatile From> );
> >+ static_assert( !std::is_convertible_v<const volatile To, const
> volatile From> );
> >+
> >+ static_assert( !std::is_convertible_v< To, volatile From> );
> >+ static_assert( !std::is_convertible_v<const To, volatile From> );
> >+ static_assert( !std::is_convertible_v< To, const volatile From>
> );
> >+ static_assert( !std::is_convertible_v<const To, const volatile From>
> );
> >+
> >+ static_assert( !std::is_convertible_v<volatile To, From> );
> >+ static_assert( !std::is_convertible_v<volatile To, const From> );
> >+ static_assert( !std::is_convertible_v<const volatile To, From>
> );
> >+ static_assert( !std::is_convertible_v<const volatile To, const From>
> );
> >+ }
> >+}
> >+
> >+template<typename T, template<typename> typename MakePtr =
> std::add_pointer_t>
> >+void
> >+testPtrConv()
> >+{
> >+ testConv<MakePtr<T>>();
> >+ testSimilarConv<MakePtr<T>, MakePtr<const T>>();
> >+ testSimilarConv<MakePtr<T*>, MakePtr<const T* const>>();
> >+ testSimilarConv<MakePtr<const T*>, MakePtr<const T* const>>();
> >+ testSimilarConv<MakePtr<T* const>, MakePtr<const T* const>>();
> >+
> >+ testSimilarConv<MakePtr<T[2]>, MakePtr<const T[2]>>();
> >+ testSimilarConv<MakePtr<T[]>, MakePtr<const T[]>>();
> >+
> >+ testSimilarConv<MakePtr<T[2]>, MakePtr<T[]>>();
> >+ testSimilarConv<MakePtr<T[2]>, MakePtr<const T[]>>();
> >+ testSimilarConv<MakePtr<const T[2]>, MakePtr<const T[]>>();
> >+
> >+ if constexpr (std::atomic_ref<MakePtr<T>>::is_always_lock_free)
> >+ {
> >+ testSimilarConv<MakePtr<T>, MakePtr<volatile T>>();
> >+ testSimilarConv<MakePtr<T>, MakePtr<const volatile T>>();
> >+ testSimilarConv<MakePtr<volatile T>, MakePtr<const volatile T>>();
> >+ testSimilarConv<MakePtr<T*>, MakePtr<volatile T* const>>();
> >+ testSimilarConv<MakePtr<volatile T*>, MakePtr<volatile T* const>>();
> >+ testSimilarConv<MakePtr<T*>, MakePtr<const volatile T* const>>();
> >+ testSimilarConv<MakePtr<volatile T*>, MakePtr<const volatile T*
> const>>();
> >+ testSimilarConv<MakePtr<volatile T* const>, MakePtr<const volatile
> T* const>>();
> >+
> >+ testSimilarConv<MakePtr<T[2]>, MakePtr<volatile T[2]>>();
> >+ testSimilarConv<MakePtr<T[2]>, MakePtr<const volatile T[2]>>();
> >+ testSimilarConv<MakePtr<const T[2]>, MakePtr<const volatile T[2]>>();
> >+ testSimilarConv<MakePtr<volatile T[2]>, MakePtr<const volatile
> T[2]>>();
> >+
> >+ testSimilarConv<MakePtr<T[]>, MakePtr<volatile T[]>>();
> >+ testSimilarConv<MakePtr<T[]>, MakePtr<const volatile T[]>>();
> >+ testSimilarConv<MakePtr<const T[]>, MakePtr<const volatile T[]>>();
> >+ testSimilarConv<MakePtr<volatile T[]>, MakePtr<const volatile
> T[]>>();
> >+
> >+ testSimilarConv<MakePtr<T[2]>, MakePtr<volatile T[]>>();
> >+ testSimilarConv<MakePtr<volatile T[2]>, MakePtr<volatile T[]>>();
> >+ testSimilarConv<MakePtr<const T[2]>, MakePtr<const volatile T[]>>();
> >+ testSimilarConv<MakePtr<volatile T[2]>, MakePtr<const volatile
> T[]>>();
> >+ testSimilarConv<MakePtr<const volatile T[2]>, MakePtr<const volatile
> T[]>>();
> >+ }
> >+}
> >+
> >+struct D : X {};
> >+static_assert( !std::is_convertible_v<std::atomic_ref<D>,
> std::atomic_ref<X>> );
> >+static_assert( !std::is_convertible_v<std::atomic_ref<D>,
> std::atomic_ref<const X>> );
> >+static_assert( !std::is_convertible_v<std::atomic_ref<D*>,
> std::atomic_ref<const X* const>> );
> >+static_assert( !std::is_convertible_v<std::atomic_ref<const D*>,
> std::atomic_ref<const X* const>> );
> >+
> >+template<typename T>
> >+using member_pointer_t = T X::*;
> >+
> >+int
> >+main()
> >+{
> >+ testTemporary<bool>();
> >+ testTemporary<int>();
> >+ testTemporary<float>();
> >+ testTemporary<int*>();
> >+ testTemporary<X>();
> >+
> >+ testConv<bool>();
> >+ testConv<int>();
> >+ testConv<float>();
> >+ testConv<X>();
> >+ testPtrConv<int>();
> >+ testPtrConv<int, member_pointer_t>();
> >+}
> >--
> >2.52.0
> >
> >
>
>
