On Tue, 16 Dec 2025 at 18:04, Tomasz Kaminski <[email protected]> wrote:
>
>
>
> 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.
That's great - thanks.
>>
>>
>>
>> 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
>> >
>> >
>>
