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[libc++] Remove _LIBCPP_ATOMIC_ONLY_USE_BUILTINS (#82000)
As discussed in #76647, _LIBCPP_ATOMIC_ONLY_USE_BUILTINS is a questionable configuration option. It makes our implementation of std::atomic even more complicated than it already is for a limited benefit. Indeed, the original goal of that setting was to decouple libc++ from libraries like compiler-rt and libatomic in Freestanding mode. We didn't have a clear understanding of goals and non-goals of Freestanding back then, but nowadays we do have a better understanding that removing all dependencies of libc++ in Freestanding is a non-goal. We should still be able to depend on builtins like those defined in compiler-rt for implementing our atomic operations in Freestanding. Freestanding means that there is no underlying operating system, not that there is no toolchain available. This patch removes the configuration option. This should have a very limited fallout since that configuration was only enabled with -ffreestanding, and libc++ basically doesn't work out of the box on Freestanding platforms today. The benefits are a slightly simpler implementation of std::atomic, getting rid of one of the ABI-incompatible representations of std::atomic, and clearing the way for proper Freestanding support to eventually land in the library. Fixes #81286
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‎libcxx/docs/ReleaseNotes/19.rst

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Original file line numberDiff line numberDiff line change
@@ -73,6 +73,11 @@ Deprecations and Removals
7373

7474
- The ``_LIBCPP_INLINE_VISIBILITY`` and ``_VSTD`` macros have been removed in LLVM 19.
7575

76+
- The ``_LIBCPP_ATOMIC_ONLY_USE_BUILTINS`` configuration option has been removed in LLVM 19. This should not affect
77+
many users, except perhaps users using the library with ``-ffreestanding`` with a toolchain where compiler-rt or
78+
libatomic is not available. If you are one such user, please reach out to the libc++ developers so we can collaborate
79+
on a path for supporting atomics properly on freestanding platforms.
80+
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7782
Upcoming Deprecations and Removals
7883
----------------------------------

‎libcxx/include/__atomic/aliases.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -92,7 +92,7 @@ using __largest_lock_free_type = short;
9292
# elif ATOMIC_CHAR_LOCK_FREE == 2
9393
using __largest_lock_free_type = char;
9494
# else
95-
# define _LIBCPP_NO_LOCK_FREE_TYPES // There are no lockfree types (this can happen in freestanding)
95+
# define _LIBCPP_NO_LOCK_FREE_TYPES // There are no lockfree types (this can happen on unusual platforms)
9696
# endif
9797

9898
# ifndef _LIBCPP_NO_LOCK_FREE_TYPES

‎libcxx/include/__atomic/cxx_atomic_impl.h

Lines changed: 1 addition & 290 deletions
Original file line numberDiff line numberDiff line change
@@ -9,24 +9,21 @@
99
#ifndef _LIBCPP___ATOMIC_CXX_ATOMIC_IMPL_H
1010
#define _LIBCPP___ATOMIC_CXX_ATOMIC_IMPL_H
1111

12-
#include <__atomic/is_always_lock_free.h>
1312
#include <__atomic/memory_order.h>
1413
#include <__config>
1514
#include <__memory/addressof.h>
16-
#include <__type_traits/conditional.h>
1715
#include <__type_traits/is_assignable.h>
1816
#include <__type_traits/is_trivially_copyable.h>
1917
#include <__type_traits/remove_const.h>
2018
#include <cstddef>
21-
#include <cstring>
2219

2320
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
2421
# pragma GCC system_header
2522
#endif
2623

2724
_LIBCPP_BEGIN_NAMESPACE_STD
2825

29-
#if defined(_LIBCPP_HAS_GCC_ATOMIC_IMP) || defined(_LIBCPP_ATOMIC_ONLY_USE_BUILTINS)
26+
#if defined(_LIBCPP_HAS_GCC_ATOMIC_IMP)
3027

3128
// [atomics.types.generic]p1 guarantees _Tp is trivially copyable. Because
3229
// the default operator= in an object is not volatile, a byte-by-byte copy
@@ -44,10 +41,6 @@ _LIBCPP_HIDE_FROM_ABI void __cxx_atomic_assign_volatile(_Tp volatile& __a_value,
4441
*__to++ = *__from++;
4542
}
4643

47-
#endif
48-
49-
#if defined(_LIBCPP_HAS_GCC_ATOMIC_IMP)
50-
5144
template <typename _Tp>
5245
struct __cxx_atomic_base_impl {
5346
_LIBCPP_HIDE_FROM_ABI
@@ -529,289 +522,7 @@ __cxx_atomic_fetch_xor(__cxx_atomic_base_impl<_Tp>* __a, _Tp __pattern, memory_o
529522

530523
#endif // _LIBCPP_HAS_GCC_ATOMIC_IMP, _LIBCPP_HAS_C_ATOMIC_IMP
531524

532-
#ifdef _LIBCPP_ATOMIC_ONLY_USE_BUILTINS
533-
534-
template <typename _Tp>
535-
struct __cxx_atomic_lock_impl {
536-
_LIBCPP_HIDE_FROM_ABI __cxx_atomic_lock_impl() _NOEXCEPT : __a_value(), __a_lock(0) {}
537-
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR explicit __cxx_atomic_lock_impl(_Tp value) _NOEXCEPT
538-
: __a_value(value),
539-
__a_lock(0) {}
540-
541-
_Tp __a_value;
542-
mutable __cxx_atomic_base_impl<_LIBCPP_ATOMIC_FLAG_TYPE> __a_lock;
543-
544-
_LIBCPP_HIDE_FROM_ABI void __lock() const volatile {
545-
while (1 == __cxx_atomic_exchange(&__a_lock, _LIBCPP_ATOMIC_FLAG_TYPE(true), memory_order_acquire))
546-
/*spin*/;
547-
}
548-
_LIBCPP_HIDE_FROM_ABI void __lock() const {
549-
while (1 == __cxx_atomic_exchange(&__a_lock, _LIBCPP_ATOMIC_FLAG_TYPE(true), memory_order_acquire))
550-
/*spin*/;
551-
}
552-
_LIBCPP_HIDE_FROM_ABI void __unlock() const volatile {
553-
__cxx_atomic_store(&__a_lock, _LIBCPP_ATOMIC_FLAG_TYPE(false), memory_order_release);
554-
}
555-
_LIBCPP_HIDE_FROM_ABI void __unlock() const {
556-
__cxx_atomic_store(&__a_lock, _LIBCPP_ATOMIC_FLAG_TYPE(false), memory_order_release);
557-
}
558-
_LIBCPP_HIDE_FROM_ABI _Tp __read() const volatile {
559-
__lock();
560-
_Tp __old;
561-
__cxx_atomic_assign_volatile(__old, __a_value);
562-
__unlock();
563-
return __old;
564-
}
565-
_LIBCPP_HIDE_FROM_ABI _Tp __read() const {
566-
__lock();
567-
_Tp __old = __a_value;
568-
__unlock();
569-
return __old;
570-
}
571-
_LIBCPP_HIDE_FROM_ABI void __read_inplace(_Tp* __dst) const volatile {
572-
__lock();
573-
__cxx_atomic_assign_volatile(*__dst, __a_value);
574-
__unlock();
575-
}
576-
_LIBCPP_HIDE_FROM_ABI void __read_inplace(_Tp* __dst) const {
577-
__lock();
578-
*__dst = __a_value;
579-
__unlock();
580-
}
581-
};
582-
583-
template <typename _Tp>
584-
_LIBCPP_HIDE_FROM_ABI void __cxx_atomic_init(volatile __cxx_atomic_lock_impl<_Tp>* __a, _Tp __val) {
585-
__cxx_atomic_assign_volatile(__a->__a_value, __val);
586-
}
587-
template <typename _Tp>
588-
_LIBCPP_HIDE_FROM_ABI void __cxx_atomic_init(__cxx_atomic_lock_impl<_Tp>* __a, _Tp __val) {
589-
__a->__a_value = __val;
590-
}
591-
592-
template <typename _Tp>
593-
_LIBCPP_HIDE_FROM_ABI void __cxx_atomic_store(volatile __cxx_atomic_lock_impl<_Tp>* __a, _Tp __val, memory_order) {
594-
__a->__lock();
595-
__cxx_atomic_assign_volatile(__a->__a_value, __val);
596-
__a->__unlock();
597-
}
598-
template <typename _Tp>
599-
_LIBCPP_HIDE_FROM_ABI void __cxx_atomic_store(__cxx_atomic_lock_impl<_Tp>* __a, _Tp __val, memory_order) {
600-
__a->__lock();
601-
__a->__a_value = __val;
602-
__a->__unlock();
603-
}
604-
605-
template <typename _Tp>
606-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_load(const volatile __cxx_atomic_lock_impl<_Tp>* __a, memory_order) {
607-
return __a->__read();
608-
}
609-
template <typename _Tp>
610-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_load(const __cxx_atomic_lock_impl<_Tp>* __a, memory_order) {
611-
return __a->__read();
612-
}
613-
614-
template <typename _Tp>
615-
_LIBCPP_HIDE_FROM_ABI void
616-
__cxx_atomic_load(const volatile __cxx_atomic_lock_impl<_Tp>* __a, _Tp* __dst, memory_order) {
617-
__a->__read_inplace(__dst);
618-
}
619-
template <typename _Tp>
620-
_LIBCPP_HIDE_FROM_ABI void __cxx_atomic_load(const __cxx_atomic_lock_impl<_Tp>* __a, _Tp* __dst, memory_order) {
621-
__a->__read_inplace(__dst);
622-
}
623-
624-
template <typename _Tp>
625-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_exchange(volatile __cxx_atomic_lock_impl<_Tp>* __a, _Tp __value, memory_order) {
626-
__a->__lock();
627-
_Tp __old;
628-
__cxx_atomic_assign_volatile(__old, __a->__a_value);
629-
__cxx_atomic_assign_volatile(__a->__a_value, __value);
630-
__a->__unlock();
631-
return __old;
632-
}
633-
template <typename _Tp>
634-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_exchange(__cxx_atomic_lock_impl<_Tp>* __a, _Tp __value, memory_order) {
635-
__a->__lock();
636-
_Tp __old = __a->__a_value;
637-
__a->__a_value = __value;
638-
__a->__unlock();
639-
return __old;
640-
}
641-
642-
template <typename _Tp>
643-
_LIBCPP_HIDE_FROM_ABI bool __cxx_atomic_compare_exchange_strong(
644-
volatile __cxx_atomic_lock_impl<_Tp>* __a, _Tp* __expected, _Tp __value, memory_order, memory_order) {
645-
_Tp __temp;
646-
__a->__lock();
647-
__cxx_atomic_assign_volatile(__temp, __a->__a_value);
648-
bool __ret = (std::memcmp(&__temp, __expected, sizeof(_Tp)) == 0);
649-
if (__ret)
650-
__cxx_atomic_assign_volatile(__a->__a_value, __value);
651-
else
652-
__cxx_atomic_assign_volatile(*__expected, __a->__a_value);
653-
__a->__unlock();
654-
return __ret;
655-
}
656-
template <typename _Tp>
657-
_LIBCPP_HIDE_FROM_ABI bool __cxx_atomic_compare_exchange_strong(
658-
__cxx_atomic_lock_impl<_Tp>* __a, _Tp* __expected, _Tp __value, memory_order, memory_order) {
659-
__a->__lock();
660-
bool __ret = (std::memcmp(&__a->__a_value, __expected, sizeof(_Tp)) == 0);
661-
if (__ret)
662-
std::memcpy(&__a->__a_value, &__value, sizeof(_Tp));
663-
else
664-
std::memcpy(__expected, &__a->__a_value, sizeof(_Tp));
665-
__a->__unlock();
666-
return __ret;
667-
}
668-
669-
template <typename _Tp>
670-
_LIBCPP_HIDE_FROM_ABI bool __cxx_atomic_compare_exchange_weak(
671-
volatile __cxx_atomic_lock_impl<_Tp>* __a, _Tp* __expected, _Tp __value, memory_order, memory_order) {
672-
_Tp __temp;
673-
__a->__lock();
674-
__cxx_atomic_assign_volatile(__temp, __a->__a_value);
675-
bool __ret = (std::memcmp(&__temp, __expected, sizeof(_Tp)) == 0);
676-
if (__ret)
677-
__cxx_atomic_assign_volatile(__a->__a_value, __value);
678-
else
679-
__cxx_atomic_assign_volatile(*__expected, __a->__a_value);
680-
__a->__unlock();
681-
return __ret;
682-
}
683-
template <typename _Tp>
684-
_LIBCPP_HIDE_FROM_ABI bool __cxx_atomic_compare_exchange_weak(
685-
__cxx_atomic_lock_impl<_Tp>* __a, _Tp* __expected, _Tp __value, memory_order, memory_order) {
686-
__a->__lock();
687-
bool __ret = (std::memcmp(&__a->__a_value, __expected, sizeof(_Tp)) == 0);
688-
if (__ret)
689-
std::memcpy(&__a->__a_value, &__value, sizeof(_Tp));
690-
else
691-
std::memcpy(__expected, &__a->__a_value, sizeof(_Tp));
692-
__a->__unlock();
693-
return __ret;
694-
}
695-
696-
template <typename _Tp, typename _Td>
697-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_fetch_add(volatile __cxx_atomic_lock_impl<_Tp>* __a, _Td __delta, memory_order) {
698-
__a->__lock();
699-
_Tp __old;
700-
__cxx_atomic_assign_volatile(__old, __a->__a_value);
701-
__cxx_atomic_assign_volatile(__a->__a_value, _Tp(__old + __delta));
702-
__a->__unlock();
703-
return __old;
704-
}
705-
template <typename _Tp, typename _Td>
706-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_fetch_add(__cxx_atomic_lock_impl<_Tp>* __a, _Td __delta, memory_order) {
707-
__a->__lock();
708-
_Tp __old = __a->__a_value;
709-
__a->__a_value += __delta;
710-
__a->__unlock();
711-
return __old;
712-
}
713-
714-
template <typename _Tp, typename _Td>
715-
_LIBCPP_HIDE_FROM_ABI _Tp*
716-
__cxx_atomic_fetch_add(volatile __cxx_atomic_lock_impl<_Tp*>* __a, ptrdiff_t __delta, memory_order) {
717-
__a->__lock();
718-
_Tp* __old;
719-
__cxx_atomic_assign_volatile(__old, __a->__a_value);
720-
__cxx_atomic_assign_volatile(__a->__a_value, __old + __delta);
721-
__a->__unlock();
722-
return __old;
723-
}
724-
template <typename _Tp, typename _Td>
725-
_LIBCPP_HIDE_FROM_ABI _Tp* __cxx_atomic_fetch_add(__cxx_atomic_lock_impl<_Tp*>* __a, ptrdiff_t __delta, memory_order) {
726-
__a->__lock();
727-
_Tp* __old = __a->__a_value;
728-
__a->__a_value += __delta;
729-
__a->__unlock();
730-
return __old;
731-
}
732-
733-
template <typename _Tp, typename _Td>
734-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_fetch_sub(volatile __cxx_atomic_lock_impl<_Tp>* __a, _Td __delta, memory_order) {
735-
__a->__lock();
736-
_Tp __old;
737-
__cxx_atomic_assign_volatile(__old, __a->__a_value);
738-
__cxx_atomic_assign_volatile(__a->__a_value, _Tp(__old - __delta));
739-
__a->__unlock();
740-
return __old;
741-
}
742-
template <typename _Tp, typename _Td>
743-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_fetch_sub(__cxx_atomic_lock_impl<_Tp>* __a, _Td __delta, memory_order) {
744-
__a->__lock();
745-
_Tp __old = __a->__a_value;
746-
__a->__a_value -= __delta;
747-
__a->__unlock();
748-
return __old;
749-
}
750-
751-
template <typename _Tp>
752-
_LIBCPP_HIDE_FROM_ABI _Tp
753-
__cxx_atomic_fetch_and(volatile __cxx_atomic_lock_impl<_Tp>* __a, _Tp __pattern, memory_order) {
754-
__a->__lock();
755-
_Tp __old;
756-
__cxx_atomic_assign_volatile(__old, __a->__a_value);
757-
__cxx_atomic_assign_volatile(__a->__a_value, _Tp(__old & __pattern));
758-
__a->__unlock();
759-
return __old;
760-
}
761-
template <typename _Tp>
762-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_fetch_and(__cxx_atomic_lock_impl<_Tp>* __a, _Tp __pattern, memory_order) {
763-
__a->__lock();
764-
_Tp __old = __a->__a_value;
765-
__a->__a_value &= __pattern;
766-
__a->__unlock();
767-
return __old;
768-
}
769-
770-
template <typename _Tp>
771-
_LIBCPP_HIDE_FROM_ABI _Tp
772-
__cxx_atomic_fetch_or(volatile __cxx_atomic_lock_impl<_Tp>* __a, _Tp __pattern, memory_order) {
773-
__a->__lock();
774-
_Tp __old;
775-
__cxx_atomic_assign_volatile(__old, __a->__a_value);
776-
__cxx_atomic_assign_volatile(__a->__a_value, _Tp(__old | __pattern));
777-
__a->__unlock();
778-
return __old;
779-
}
780-
template <typename _Tp>
781-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_fetch_or(__cxx_atomic_lock_impl<_Tp>* __a, _Tp __pattern, memory_order) {
782-
__a->__lock();
783-
_Tp __old = __a->__a_value;
784-
__a->__a_value |= __pattern;
785-
__a->__unlock();
786-
return __old;
787-
}
788-
789-
template <typename _Tp>
790-
_LIBCPP_HIDE_FROM_ABI _Tp
791-
__cxx_atomic_fetch_xor(volatile __cxx_atomic_lock_impl<_Tp>* __a, _Tp __pattern, memory_order) {
792-
__a->__lock();
793-
_Tp __old;
794-
__cxx_atomic_assign_volatile(__old, __a->__a_value);
795-
__cxx_atomic_assign_volatile(__a->__a_value, _Tp(__old ^ __pattern));
796-
__a->__unlock();
797-
return __old;
798-
}
799-
template <typename _Tp>
800-
_LIBCPP_HIDE_FROM_ABI _Tp __cxx_atomic_fetch_xor(__cxx_atomic_lock_impl<_Tp>* __a, _Tp __pattern, memory_order) {
801-
__a->__lock();
802-
_Tp __old = __a->__a_value;
803-
__a->__a_value ^= __pattern;
804-
__a->__unlock();
805-
return __old;
806-
}
807-
808-
template <typename _Tp,
809-
typename _Base = typename conditional<__libcpp_is_always_lock_free<_Tp>::__value,
810-
__cxx_atomic_base_impl<_Tp>,
811-
__cxx_atomic_lock_impl<_Tp> >::type>
812-
#else
813525
template <typename _Tp, typename _Base = __cxx_atomic_base_impl<_Tp> >
814-
#endif //_LIBCPP_ATOMIC_ONLY_USE_BUILTINS
815526
struct __cxx_atomic_impl : public _Base {
816527
static_assert(is_trivially_copyable<_Tp>::value, "std::atomic<T> requires that 'T' be a trivially copyable type");
817528

‎libcxx/include/__config

Lines changed: 0 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -1200,9 +1200,6 @@ __sanitizer_verify_double_ended_contiguous_container(const void*, const void*, c
12001200
# ifndef _LIBCPP_ATOMIC_FLAG_TYPE
12011201
# define _LIBCPP_ATOMIC_FLAG_TYPE bool
12021202
# endif
1203-
# ifdef _LIBCPP_FREESTANDING
1204-
# define _LIBCPP_ATOMIC_ONLY_USE_BUILTINS
1205-
# endif
12061203
# endif
12071204

12081205
# if defined(__FreeBSD__) && defined(__clang__) && __has_attribute(__no_thread_safety_analysis__)

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