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
_GLIBCXX_VISIBILITY(default)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 */
112