libstdc++
align.h
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1 // align implementation -*- C++ -*-
2 
3 // Copyright (C) 2014-2021 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the
7 // terms of the GNU General Public License as published by the
8 // Free Software Foundation; either version 3, or (at your option)
9 // any later version.
10 
11 // This library is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 // GNU General Public License for more details.
15 
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19 
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
24 
25 /** @file bits/align.h
26  * This is an internal header file, included by other library headers.
27  * Do not attempt to use it directly. @headername{memory}
28  */
29 
30 #ifndef _GLIBCXX_ALIGN_H
31 #define _GLIBCXX_ALIGN_H 1
32 
33 #include <bits/c++config.h>
34 
35 #include <bit> // std::has_single_bit
36 #include <stdint.h> // uintptr_t
37 #include <debug/assertions.h> // _GLIBCXX_DEBUG_ASSERT
38 
39 namespace std _GLIBCXX_VISIBILITY(default)
40 {
41 _GLIBCXX_BEGIN_NAMESPACE_VERSION
42 
43 /**
44  * @brief Fit aligned storage in buffer.
45  *
46  * This function tries to fit @a __size bytes of storage with alignment
47  * @a __align into the buffer @a __ptr of size @a __space bytes. If such
48  * a buffer fits then @a __ptr is changed to point to the first byte of the
49  * aligned storage and @a __space is reduced by the bytes used for alignment.
50  *
51  * C++11 20.6.5 [ptr.align]
52  *
53  * @param __align A fundamental or extended alignment value.
54  * @param __size Size of the aligned storage required.
55  * @param __ptr Pointer to a buffer of @a __space bytes.
56  * @param __space Size of the buffer pointed to by @a __ptr.
57  * @return the updated pointer if the aligned storage fits, otherwise nullptr.
58  *
59  * @ingroup memory
60  */
61 inline void*
62 align(size_t __align, size_t __size, void*& __ptr, size_t& __space) noexcept
63 {
64  if (__space < __size)
65  return nullptr;
66  const auto __intptr = reinterpret_cast<uintptr_t>(__ptr);
67  const auto __aligned = (__intptr - 1u + __align) & -__align;
68  const auto __diff = __aligned - __intptr;
69  if (__diff > (__space - __size))
70  return nullptr;
71  else
72  {
73  __space -= __diff;
74  return __ptr = reinterpret_cast<void*>(__aligned);
75  }
76 }
77 
78 #if __cplusplus > 201703L
79 #define __cpp_lib_assume_aligned 201811L
80  /** @brief Inform the compiler that a pointer is aligned.
81  *
82  * @tparam _Align An alignment value (i.e. a power of two)
83  * @tparam _Tp An object type
84  * @param __ptr A pointer that is aligned to _Align
85  *
86  * C++20 20.10.6 [ptr.align]
87  *
88  * @ingroup memory
89  */
90  template<size_t _Align, class _Tp>
91  [[nodiscard,__gnu__::__always_inline__]]
92  constexpr _Tp*
93  assume_aligned(_Tp* __ptr) noexcept
94  {
95  static_assert(std::has_single_bit(_Align));
96  if (std::is_constant_evaluated())
97  return __ptr;
98  else
99  {
100  // This function is expected to be used in hot code, where
101  // __glibcxx_assert would add unwanted overhead.
102  _GLIBCXX_DEBUG_ASSERT((uintptr_t)__ptr % _Align == 0);
103  return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
104  }
105  }
106 #endif // C++2a
107 
108 _GLIBCXX_END_NAMESPACE_VERSION
109 } // namespace
110 
111 #endif /* _GLIBCXX_ALIGN_H */
void * align(size_t __align, size_t __size, void *&__ptr, size_t &__space) noexcept
Fit aligned storage in buffer.
Definition: align.h:62
ISO C++ entities toplevel namespace is std.