[libcxx-commits] [libcxx] [libc++] Add tombstone traits and use them in optional (PR #98498)
via libcxx-commits
libcxx-commits at lists.llvm.org
Fri Nov 8 04:26:01 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: Nikolas Klauser (philnik777)
<details>
<summary>Changes</summary>
This adds tombstone traits, which describe how (and whether) the bit representation of a given type can be checked to know whether there is a valid object of that type in a memory location. This can be used to optimize the size of sum types like `optional` and `variant`. This patch only optimizes `optional` to keep it as small as possible.
---
Patch is 33.42 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/98498.diff
16 Files Affected:
- (modified) libcxx/include/CMakeLists.txt (+1)
- (modified) libcxx/include/__configuration/abi.h (+5)
- (modified) libcxx/include/__functional/reference_wrapper.h (+10-1)
- (modified) libcxx/include/__locale (+6)
- (modified) libcxx/include/__memory/shared_ptr.h (+17-2)
- (added) libcxx/include/__memory/tombstone_traits.h (+277)
- (modified) libcxx/include/__memory/unique_ptr.h (+11)
- (modified) libcxx/include/__type_traits/enable_if.h (+3)
- (modified) libcxx/include/__utility/pair.h (+19-1)
- (modified) libcxx/include/__vector/vector.h (+8)
- (modified) libcxx/include/module.modulemap (+4)
- (modified) libcxx/include/optional (+65-3)
- (modified) libcxx/include/string (+14)
- (modified) libcxx/include/string_view (+8)
- (modified) libcxx/test/libcxx/utilities/optional/optional.object/optional_size.pass.cpp (+29)
- (added) libcxx/test/std/utilities/optional/tombstone_types.pass.cpp (+68)
``````````diff
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index ae2e8bcb32aaa4..aca8ea182ec511 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -560,6 +560,7 @@ set(files
__memory/swap_allocator.h
__memory/temp_value.h
__memory/temporary_buffer.h
+ __memory/tombstone_traits.h
__memory/uninitialized_algorithms.h
__memory/unique_ptr.h
__memory/unique_temporary_buffer.h
diff --git a/libcxx/include/__configuration/abi.h b/libcxx/include/__configuration/abi.h
index c6ef6fdcdf96e6..124e349e09024b 100644
--- a/libcxx/include/__configuration/abi.h
+++ b/libcxx/include/__configuration/abi.h
@@ -124,6 +124,8 @@
// This setting disables the addition of such artificial padding, leading to a more optimal
// representation for several types.
# define _LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING
+// Use __tombstone_traits to optimize the memory layout of std::optional.
+# define _LIBCPP_ABI_OPTIONAL_USE_TOMBSTONE_TRAITS
#elif _LIBCPP_ABI_VERSION == 1
# if !(defined(_LIBCPP_OBJECT_FORMAT_COFF) || defined(_LIBCPP_OBJECT_FORMAT_XCOFF))
// Enable compiling copies of now inline methods into the dylib to support
@@ -141,6 +143,9 @@
# if defined(__FreeBSD__) && __FreeBSD__ < 14
# define _LIBCPP_DEPRECATED_ABI_DISABLE_PAIR_TRIVIAL_COPY_CTOR
# endif
+
+// TODO: This shouldn't be in the final commit - this just to test the changes across all the different configurations
+# define _LIBCPP_ABI_OPTIONAL_USE_TOMBSTONE_TRAITS
#endif
// We had some bugs where we use [[no_unique_address]] together with construct_at,
diff --git a/libcxx/include/__functional/reference_wrapper.h b/libcxx/include/__functional/reference_wrapper.h
index a4a66a50cf84ca..c150bbc7736db0 100644
--- a/libcxx/include/__functional/reference_wrapper.h
+++ b/libcxx/include/__functional/reference_wrapper.h
@@ -13,8 +13,10 @@
#include <__compare/synth_three_way.h>
#include <__concepts/boolean_testable.h>
#include <__config>
+#include <__cstddef/size_t.h>
#include <__functional/weak_result_type.h>
#include <__memory/addressof.h>
+#include <__memory/tombstone_traits.h>
#include <__type_traits/enable_if.h>
#include <__type_traits/invoke.h>
#include <__type_traits/is_const.h>
@@ -22,6 +24,7 @@
#include <__type_traits/void_t.h>
#include <__utility/declval.h>
#include <__utility/forward.h>
+#include <cstdint>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
@@ -120,7 +123,13 @@ class _LIBCPP_TEMPLATE_VIS reference_wrapper : public __weak_result_type<_Tp> {
#if _LIBCPP_STD_VER >= 17
template <class _Tp>
reference_wrapper(_Tp&) -> reference_wrapper<_Tp>;
-#endif
+
+template <class _Tp>
+struct __tombstone_traits<reference_wrapper<_Tp>> {
+ static constexpr uintptr_t __disengaged_value_ = 0;
+ static constexpr size_t __is_disengaged_offset_ = 0;
+};
+#endif // _LIBCPP_STD_VER >= 17
template <class _Tp>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference_wrapper<_Tp> ref(_Tp& __t) _NOEXCEPT {
diff --git a/libcxx/include/__locale b/libcxx/include/__locale
index b07b9f3329f42c..3a40dc24ad1116 100644
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -13,6 +13,7 @@
#include <__config>
#include <__locale_dir/locale_base_api.h>
#include <__memory/shared_ptr.h> // __shared_count
+#include <__memory/tombstone_traits.h>
#include <__mutex/once_flag.h>
#include <__type_traits/make_unsigned.h>
#include <__utility/no_destroy.h>
@@ -114,6 +115,11 @@ private:
friend const _Facet& use_facet(const locale&);
};
+#if _LIBCPP_STD_VER >= 17
+template <>
+struct __tombstone_traits<locale> : __tombstone_traits_assume_aligned_pointer {};
+#endif
+
class _LIBCPP_EXPORTED_FROM_ABI locale::facet : public __shared_count {
protected:
_LIBCPP_HIDE_FROM_ABI explicit facet(size_t __refs = 0) : __shared_count(static_cast<long>(__refs) - 1) {}
diff --git a/libcxx/include/__memory/shared_ptr.h b/libcxx/include/__memory/shared_ptr.h
index 5a84c2ce9bfe17..1e88e38b6804c9 100644
--- a/libcxx/include/__memory/shared_ptr.h
+++ b/libcxx/include/__memory/shared_ptr.h
@@ -30,6 +30,7 @@
#include <__memory/compressed_pair.h>
#include <__memory/construct_at.h>
#include <__memory/pointer_traits.h>
+#include <__memory/tombstone_traits.h>
#include <__memory/uninitialized_algorithms.h>
#include <__memory/unique_ptr.h>
#include <__type_traits/add_lvalue_reference.h>
@@ -831,7 +832,14 @@ template <class _Tp>
shared_ptr(weak_ptr<_Tp>) -> shared_ptr<_Tp>;
template <class _Tp, class _Dp>
shared_ptr(unique_ptr<_Tp, _Dp>) -> shared_ptr<_Tp>;
-#endif
+
+template <class _Tp>
+struct __tombstone_traits<shared_ptr<_Tp>> {
+ static constexpr auto __disengaged_value_ = __tombstone_traits_assume_aligned_pointer::__disengaged_value_;
+ static constexpr size_t __is_disengaged_offset_ =
+ sizeof(void*) + __tombstone_traits_assume_aligned_pointer::__is_disengaged_offset_;
+};
+#endif // _LIBCPP_STD_VER >= 17
//
// std::allocate_shared and std::make_shared
@@ -1381,7 +1389,14 @@ class _LIBCPP_SHARED_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS weak_ptr {
#if _LIBCPP_STD_VER >= 17
template <class _Tp>
weak_ptr(shared_ptr<_Tp>) -> weak_ptr<_Tp>;
-#endif
+
+template <class _Tp>
+struct __tombstone_traits<weak_ptr<_Tp>> {
+ static constexpr auto __disengaged_value_ = __tombstone_traits_assume_aligned_pointer::__disengaged_value_;
+ static constexpr size_t __is_disengaged_offset_ =
+ sizeof(void*) + __tombstone_traits_assume_aligned_pointer::__is_disengaged_offset_;
+};
+#endif // _LIBCPP_STD_VER >= 17
template <class _Tp>
inline _LIBCPP_CONSTEXPR weak_ptr<_Tp>::weak_ptr() _NOEXCEPT : __ptr_(nullptr), __cntrl_(nullptr) {}
diff --git a/libcxx/include/__memory/tombstone_traits.h b/libcxx/include/__memory/tombstone_traits.h
new file mode 100644
index 00000000000000..4cb05ab3612ee9
--- /dev/null
+++ b/libcxx/include/__memory/tombstone_traits.h
@@ -0,0 +1,277 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_DISENGAGED_TRAITS_H
+#define _LIBCPP___TYPE_TRAITS_DISENGAGED_TRAITS_H
+
+#include <__assert>
+#include <__config>
+#include <__cstddef/size_t.h>
+#include <__memory/construct_at.h>
+#include <__type_traits/datasizeof.h>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/is_constant_evaluated.h>
+#include <__type_traits/is_fundamental.h>
+#include <__type_traits/is_integral.h>
+#include <__type_traits/is_trivial.h>
+#include <__type_traits/is_trivially_destructible.h>
+#include <__type_traits/remove_cv.h>
+#include <__type_traits/void_t.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+#include <__utility/piecewise_construct.h>
+#include <cstdint>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+# pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class>
+struct __tombstone_traits;
+
+// bools have always exactly one bit set. If there is more than one set it's disengaged.
+template <>
+struct __tombstone_traits<bool> {
+ static constexpr uint8_t __disengaged_value_ = 3;
+ static constexpr size_t __is_disengaged_offset_ = 0;
+};
+
+struct __tombstone_traits_assume_aligned_pointer {
+ static constexpr uint8_t __disengaged_value_ = 1;
+#ifdef _LIBCPP_LITTLE_ENDIAN
+ static constexpr size_t __is_disengaged_offset_ = 0;
+#else
+ static constexpr size_t __is_disengaged_offset_ = sizeof(void*) - 1;
+#endif
+};
+
+// TODO: Look into
+// - filesystem::directory_iterator
+// - vector<T> with alignof(T) == 1
+// - string_view (basic_string_view<T> works with alignof(T) >= 2)
+
+// This is constrained on fundamental types because we might not always know the alignment of a user-defined type.
+// For example, in one TU there may only be a forward declaration and in another there is already the definition
+// available. If we made this optimization conditional on the completeness of the type this would result in a non-benign
+// ODR violation.
+template <class _Tp>
+struct __tombstone_traits<__enable_specialization_if<is_fundamental_v<_Tp> && alignof(_Tp) >= 2, _Tp*>>
+ : __tombstone_traits_assume_aligned_pointer {};
+
+template <class _Tp>
+struct __tombstone_traits<_Tp**> : __tombstone_traits_assume_aligned_pointer {
+ static_assert(alignof(_Tp*) >= 2, "alignment of a pointer isn't at least 2!?");
+};
+
+inline constexpr struct __init_engaged_t {
+} __init_engaged;
+inline constexpr struct __init_disengaged_t {
+} __init_disengaged;
+
+template <class _Tp, class _Payload, bool = __tombstone_traits<_Tp>::__is_disengaged_offset_ == 0>
+struct __tombstone_is_disengaged {
+ using _TombstoneLayout = __tombstone_traits<_Tp>;
+ using _IsDisengagedT = remove_cv_t<decltype(_TombstoneLayout::__disengaged_value_)>;
+
+ char __padding_[_TombstoneLayout::__is_disengaged_offset_];
+ _IsDisengagedT __is_disengaged_;
+};
+
+template <class _Tp, class _Payload>
+struct __tombstone_is_disengaged<_Tp, _Payload, true> {
+ using _TombstoneLayout = __tombstone_traits<_Tp>;
+ using _IsDisengagedT = remove_cv_t<decltype(_TombstoneLayout::__disengaged_value_)>;
+
+ _IsDisengagedT __is_disengaged_;
+};
+
+template <class _Tp, class _Payload, bool = __tombstone_traits<_Tp>::__is_disengaged_offset_ == 0>
+struct __tombstone_data {
+ using _TombstoneLayout = __tombstone_traits<_Tp>;
+ using _IsDisengagedT = remove_cv_t<decltype(_TombstoneLayout::__disengaged_value_)>;
+
+ static_assert(is_trivial<_IsDisengagedT>::value, "disengaged type has to be trivial!");
+ static_assert(_TombstoneLayout::__is_disengaged_offset_ >= __datasizeof_v<_Payload>);
+
+ _LIBCPP_NO_UNIQUE_ADDRESS _Payload __payload_;
+ char __padding_[_TombstoneLayout::__is_disengaged_offset_ - __datasizeof_v<_Payload>];
+ _IsDisengagedT __is_disengaged_;
+
+ template <class... _Args>
+ _LIBCPP_HIDE_FROM_ABI constexpr __tombstone_data(_Args&&... __args)
+ : __payload_(std::forward<_Args>(__args)...), __is_disengaged_(_TombstoneLayout::__disengaged_value_) {}
+};
+
+template <class _Tp, class _Payload>
+struct __tombstone_data<_Tp, _Payload, true> {
+ using _TombstoneLayout = __tombstone_traits<_Tp>;
+ using _IsDisengagedT = remove_cv_t<decltype(_TombstoneLayout::__disengaged_value_)>;
+
+ _IsDisengagedT __is_disengaged_;
+ _LIBCPP_NO_UNIQUE_ADDRESS _Payload __payload_;
+
+ template <class... _Args>
+ _LIBCPP_HIDE_FROM_ABI constexpr __tombstone_data(_Args&&... __args)
+ : __is_disengaged_(_TombstoneLayout::__disengaged_value_), __payload_(std::forward<_Args>(__args)...) {}
+};
+
+template <class _Tp, class _Payload>
+struct __tombstoned_value final {
+ using _TombstoneLayout = __tombstone_traits<_Tp>;
+ using _TombstoneData = __tombstone_data<_Tp, _Payload>;
+
+ template <bool = is_trivially_destructible_v<_Tp> && is_trivially_destructible_v<_Payload>>
+ union _MaybeTombstone {
+ _Tp __value_;
+ _TombstoneData __tombstone_;
+
+ template <class... _Args>
+ constexpr _MaybeTombstone(__init_disengaged_t, _Args&&... __args) : __tombstone_(std::forward<_Args>(__args)...) {}
+
+ template <class... _Args>
+ constexpr _MaybeTombstone(__init_engaged_t, _Args&&... __args) : __value_(std::forward<_Args>(__args)...) {}
+
+ _MaybeTombstone(const _MaybeTombstone&) = default;
+ _MaybeTombstone(_MaybeTombstone&&) = default;
+ _MaybeTombstone& operator=(const _MaybeTombstone&) = default;
+ _MaybeTombstone& operator=(_MaybeTombstone&&) = default;
+
+ _LIBCPP_HIDE_FROM_ABI constexpr bool __is_engaged() const noexcept {
+ if (__libcpp_is_constant_evaluated())
+ return !__builtin_constant_p(__tombstone_.__is_disengaged_ == _TombstoneLayout::__disengaged_value_);
+ __tombstone_is_disengaged<_Tp, _Payload> __is_disengaged;
+ static_assert(sizeof(__tombstone_is_disengaged<_Tp, _Payload>) <= sizeof(_MaybeTombstone));
+ __builtin_memcpy(&__is_disengaged, this, sizeof(__tombstone_is_disengaged<_Tp, _Payload>));
+
+ return __is_disengaged.__is_disengaged_ != _TombstoneLayout::__disengaged_value_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 ~_MaybeTombstone() {
+ if (__is_engaged()) {
+ std::destroy_at(&__value_);
+ } else {
+ std::destroy_at(&__tombstone_);
+ }
+ }
+ };
+
+ template <>
+ union _MaybeTombstone<true> {
+ _Tp __value_;
+ _TombstoneData __tombstone_;
+
+ template <class... _Args>
+ constexpr _MaybeTombstone(__init_disengaged_t, _Args&&... __args) : __tombstone_(std::forward<_Args>(__args)...) {}
+
+ template <class... _Args>
+ constexpr _MaybeTombstone(__init_engaged_t, _Args&&... __args) : __value_(std::forward<_Args>(__args)...) {}
+
+ _MaybeTombstone(const _MaybeTombstone&) = default;
+ _MaybeTombstone(_MaybeTombstone&&) = default;
+ _MaybeTombstone& operator=(const _MaybeTombstone&) = default;
+ _MaybeTombstone& operator=(_MaybeTombstone&&) = default;
+
+ _LIBCPP_HIDE_FROM_ABI constexpr bool __is_engaged() const noexcept {
+ if (__libcpp_is_constant_evaluated())
+ return !__builtin_constant_p(__tombstone_.__is_disengaged_ == _TombstoneLayout::__disengaged_value_);
+ __tombstone_is_disengaged<_Tp, _Payload> __is_disengaged;
+ static_assert(sizeof(__tombstone_is_disengaged<_Tp, _Payload>) <= sizeof(_MaybeTombstone));
+ __builtin_memcpy(&__is_disengaged, this, sizeof(__tombstone_is_disengaged<_Tp, _Payload>));
+
+ return __is_disengaged.__is_disengaged_ != _TombstoneLayout::__disengaged_value_;
+ }
+
+ _LIBCPP_CONSTEXPR_SINCE_CXX20 ~_MaybeTombstone() = default;
+ };
+
+ _MaybeTombstone<> __data_;
+
+ static_assert(sizeof(__tombstone_data<_Tp, _Payload>) <= sizeof(_Tp));
+ static_assert(is_integral_v<decltype(_TombstoneLayout::__disengaged_value_)>);
+ static_assert(__builtin_offsetof(_TombstoneData, __is_disengaged_) == _TombstoneLayout::__is_disengaged_offset_);
+
+ template <class... _Args>
+ _LIBCPP_HIDE_FROM_ABI constexpr __tombstoned_value(__init_disengaged_t, _Args&&... __args)
+ : __data_(__init_disengaged, std::forward<_Args>(__args)...) {}
+
+ template <class... _Args>
+ _LIBCPP_HIDE_FROM_ABI constexpr __tombstoned_value(__init_engaged_t, _Args&&... __args)
+ : __data_(__init_engaged, std::forward<_Args>(__args)...) {}
+
+ _LIBCPP_HIDE_FROM_ABI constexpr bool __is_engaged() const noexcept { return __data_.__is_engaged(); }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto&& __get_value() & noexcept {
+ _LIBCPP_ASSERT_INTERNAL(__self.__is_engaged(), "Trying to get the value of a disenagaged tombstoned value");
+ return __data_.__value_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto&& __get_value() const & noexcept {
+ _LIBCPP_ASSERT_INTERNAL(__self.__is_engaged(), "Trying to get the value of a disenagaged tombstoned value");
+ return __data_.__value_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto&& __get_value() && noexcept {
+ _LIBCPP_ASSERT_INTERNAL(__self.__is_engaged(), "Trying to get the value of a disenagaged tombstoned value");
+ return std::move(__data_.__value_);
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto&& __get_value() const && noexcept {
+ _LIBCPP_ASSERT_INTERNAL(__self.__is_engaged(), "Trying to get the value of a disenagaged tombstoned value");
+ return std::move(__data_.__value_);
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto&& __get_payload() & noexcept {
+ _LIBCPP_ASSERT_INTERNAL(!__self.__is_engaged(), "Trying to get the payload of an enagaged tombstoned value");
+ return __data_.__tombstone_.__payload_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto&& __get_payload() const & noexcept {
+ _LIBCPP_ASSERT_INTERNAL(!__self.__is_engaged(), "Trying to get the payload of an enagaged tombstoned value");
+ return __data_.__tombstone_.__payload_;
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto&& __get_payload() && noexcept {
+ _LIBCPP_ASSERT_INTERNAL(!__self.__is_engaged(), "Trying to get the payload of an enagaged tombstoned value");
+ return std::move(__data_.__tombstone_.__payload_);
+ }
+
+ _LIBCPP_HIDE_FROM_ABI constexpr auto&& __get_payload() const && noexcept {
+ _LIBCPP_ASSERT_INTERNAL(!__self.__is_engaged(), "Trying to get the payload of an enagaged tombstoned value");
+ return std::move(__data_.__tombstone_.__payload_);
+ }
+
+ template <class... _Args>
+ _LIBCPP_HIDE_FROM_ABI constexpr void __engage(piecewise_construct_t, _Args&&... __args) {
+ _LIBCPP_ASSERT_INTERNAL(!__self.__is_engaged(), "Trying to enage a already engaged tombstoned value");
+ std::destroy_at(&__data_.__tombstone_);
+ std::__construct_at(&__data_.__value_, std::forward<_Args>(__args)...);
+ }
+
+ template <class... _Args>
+ _LIBCPP_HIDE_FROM_ABI constexpr void __disengage(piecewise_construct_t, _Args&&... __args) {
+ _LIBCPP_ASSERT_INTERNAL(!__self.__is_engaged(), "Trying to disenage a disengaged tombstoned value");
+ std::destroy_at(&__data_.__value_);
+ std::__construct_at(&__data_.__tombstone_, std::forward<_Args>(__args)...);
+ }
+};
+
+template <class _Tp, class = void>
+inline constexpr bool __has_tombstone_v = false;
+
+template <class _Tp>
+inline constexpr bool __has_tombstone_v<_Tp, void_t<decltype(sizeof(__tombstone_traits<_Tp>))>> = true;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___TYPE_TRAITS_DISENGAGED_TRAITS_H
diff --git a/libcxx/include/__memory/unique_ptr.h b/libcxx/include/__memory/unique_ptr.h
index 28c62e13566e24..965395e0e79626 100644
--- a/libcxx/include/__memory/unique_ptr.h
+++ b/libcxx/include/__memory/unique_ptr.h
@@ -24,6 +24,7 @@
#include <__memory/auto_ptr.h>
#include <__memory/compressed_pair.h>
#include <__memory/pointer_traits.h>
+#include <__memory/tombstone_traits.h>
#include <__type_traits/add_lvalue_reference.h>
#include <__type_traits/common_type.h>
#include <__type_traits/conditional.h>
@@ -405,6 +406,16 @@ struct __unique_ptr_array_bounds_stored {
size_t __size_;
};
+#if _LIBCPP_STD_VER >= 17
+template <class _Tp>
+struct __tombstone_traits<__enable_specialization_if<__has_tombstone_v<_Tp*>, unique_ptr<_Tp>>>
+ : __tombstone_traits<_Tp*> {};
+
+template <class _Tp>
+struct __tombstone_traits<__enable_specialization_if<__has_tombstone_v<_Tp*>, unique_ptr<_Tp[]>>>
+ : __tombstone_traits<_Tp*> {};
+#endif // _LIBCPP_STD_VER >= 17
+
template <class _Tp, class _Dp>
class _LIBCPP_UNIQUE_PTR_TRIVIAL_ABI _LIBCPP_TEMPLATE_VIS unique_ptr<_Tp[], _Dp> {
public:
diff --git a/libcxx/include/__type_traits/enable_if.h b/libcxx/include/__type_traits/enable_if.h
index 77da9622ca28fc..67c3f7ffa24c14 100644
--- a/libcxx/include/__type_traits/enable_if.h
+++ b/libcxx/include/__type_traits/enable_if.h
@@ -32,6 +32,9 @@ template <bool _Bp, class _Tp = void>
using enable_if_t = typename enable_if<_Bp, _Tp>::type;
#endif
+template <bool _Bp, class _Tp, class = __enable_if_t<_Bp> >
+using __enable_specialization_if = _Tp;
+
_LIBCPP_END_NAMESPACE_STD
#endif // _LIBCPP___TYPE_TRAITS_ENABLE_IF_H
diff --git a/libcxx/include/__utility/pair.h b/libcxx/include/__utility/pair.h
index f9d0f4e4723113..965b29bbae27f3 100644
--- a/libcxx/include/__utility/pair.h
+++ b/libcxx/include/__utility/pair.h
@@ -17,6 +17,7 @@
#include <__fwd/array.h>
#include <__fwd/pair.h>
#include <__fwd/tuple.h>
+#include <__memory/tombstone_traits.h>
#include <__tuple/tuple_indices.h>
#include <__tuple/tuple_like_no_subrange.h>
#include <__tuple/tuple_size.h>
@@ -444,7 +445,24 @@ struct _LIBCPP_TEMPLATE_VIS pair
#if _LIBCPP_STD_VER >= 17
template <class _T1, class _T2>
pair(_T1, _T2) -> pair<_T1, _T2>;
-#endif
+
+template <class _Tp, class _Up, bool __first_tombstone>
+struct __tombstone_traits_pair;
+
+template <class _Tp, class _Up>
+struct __tombstone_traits_pair<_Tp, _Up, true> : __tombstone_traits<_Tp> {};
+
+template <class _Tp, class _Up>
+struct __tombstone_traits_pair<_Tp, _Up, false> {
+ static constexpr auto __disengaged_value_ = __tombstone_traits<_Up>::__disengaged_value_;
+ static constexpr size_t __is_disengaged_offset_ = sizeof(_Tp) + __tombstone_traits<_Up>::__is_disengaged_offset_;
+};
+
+templa...
[truncated]
``````````
</details>
https://github.com/llvm/llvm-project/pull/98498
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