1// -*- C++ -*-
2//===----------------------------------------------------------------------===//
3//
4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5// See https://llvm.org/LICENSE.txt for license information.
6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef _LIBCPP___MEMORY_COMPRESSED_PAIR_H
11#define _LIBCPP___MEMORY_COMPRESSED_PAIR_H
12
13#include <__config>
14#include <__cstddef/size_t.h>
15#include <__type_traits/datasizeof.h>
16#include <__type_traits/is_empty.h>
17#include <__type_traits/is_final.h>
18
19#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
20# pragma GCC system_header
21#endif
22
23_LIBCPP_BEGIN_NAMESPACE_STD
24
25// ================================================================================================================== //
26// The utilites here are for staying ABI compatible with the legacy `__compressed_pair`. They should not be used //
27// for new data structures. Use `_LIBCPP_NO_UNIQUE_ADDRESS` for new data structures instead (but make sure you //
28// understand how it works). //
29// ================================================================================================================== //
30
31// The first member is aligned to the alignment of the second member to force padding in front of the compressed pair
32// in case there are members before it.
33//
34// For example:
35// (assuming x86-64 linux)
36// class SomeClass {
37// uint32_t member1;
38// _LIBCPP_COMPRESSED_PAIR(uint32_t, member2, uint64_t, member3);
39// }
40//
41// The layout with __compressed_pair is:
42// member1 - offset: 0, size: 4
43// padding - offset: 4, size: 4
44// member2 - offset: 8, size: 4
45// padding - offset: 12, size: 4
46// member3 - offset: 16, size: 8
47//
48// If the [[gnu::aligned]] wasn't there, the layout would instead be:
49// member1 - offset: 0, size: 4
50// member2 - offset: 4, size: 4
51// member3 - offset: 8, size: 8
52//
53// Furthermore, that alignment must be the same as what was used in the old __compressed_pair layout, so we must
54// handle reference types specially since alignof(T&) == alignof(T).
55// See https://github.com/llvm/llvm-project/issues/118559.
56
57#ifndef _LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING
58
59template <class _Tp>
60inline const size_t __compressed_pair_alignment = _LIBCPP_ALIGNOF(_Tp);
61
62template <class _Tp>
63inline const size_t __compressed_pair_alignment<_Tp&> = _LIBCPP_ALIGNOF(void*);
64
65template <class _ToPad>
66inline const bool __is_reference_or_unpadded_object =
67 (is_empty<_ToPad>::value && !__libcpp_is_final<_ToPad>::value) || sizeof(_ToPad) == __datasizeof_v<_ToPad>;
68
69template <class _Tp>
70inline const bool __is_reference_or_unpadded_object<_Tp&> = true;
71
72template <class _Tp>
73inline const bool __is_reference_or_unpadded_object<_Tp&&> = true;
74
75template <class _ToPad, bool _Empty = __is_reference_or_unpadded_object<_ToPad> >
76class __compressed_pair_padding {
77 char __padding_[sizeof(_ToPad) - __datasizeof_v<_ToPad>] = {};
78};
79
80template <class _ToPad>
81class __compressed_pair_padding<_ToPad, true> {};
82
83# define _LIBCPP_COMPRESSED_PAIR(T1, Initializer1, T2, Initializer2) \
84 _LIBCPP_NO_UNIQUE_ADDRESS __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>))) T1 Initializer1; \
85 _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _); \
86 _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2; \
87 _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _)
88
89# define _LIBCPP_COMPRESSED_TRIPLE(T1, Initializer1, T2, Initializer2, T3, Initializer3) \
90 _LIBCPP_NO_UNIQUE_ADDRESS \
91 __attribute__((__aligned__(::std::__compressed_pair_alignment<T2>), \
92 __aligned__(::std::__compressed_pair_alignment<T3>))) T1 Initializer1; \
93 _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _); \
94 _LIBCPP_NO_UNIQUE_ADDRESS T2 Initializer2; \
95 _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T2> _LIBCPP_CONCAT3(__padding2_, __LINE__, _); \
96 _LIBCPP_NO_UNIQUE_ADDRESS T3 Initializer3; \
97 _LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T3> _LIBCPP_CONCAT3(__padding3_, __LINE__, _)
98
99#else
100# define _LIBCPP_COMPRESSED_PAIR(T1, Name1, T2, Name2) \
101 _LIBCPP_NO_UNIQUE_ADDRESS T1 Name1; \
102 _LIBCPP_NO_UNIQUE_ADDRESS T2 Name2
103
104# define _LIBCPP_COMPRESSED_TRIPLE(T1, Name1, T2, Name2, T3, Name3) \
105 _LIBCPP_NO_UNIQUE_ADDRESS T1 Name1; \
106 _LIBCPP_NO_UNIQUE_ADDRESS T2 Name2; \
107 _LIBCPP_NO_UNIQUE_ADDRESS T3 Name3
108#endif // _LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING
109
110_LIBCPP_END_NAMESPACE_STD
111
112#endif // _LIBCPP___MEMORY_COMPRESSED_PAIR_H
113