libcxx/include/__memory/compressed_pair.h

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