1// align implementation -*- C++ -*-
2
3// Copyright (C) 2014-2024 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 <bit> // std::has_single_bit
34#include <stdint.h> // uintptr_t
35#include <debug/assertions.h> // _GLIBCXX_DEBUG_ASSERT
36#include <bits/version.h>
37
38namespace std _GLIBCXX_VISIBILITY(default)
39{
40_GLIBCXX_BEGIN_NAMESPACE_VERSION
41
42/**
43 * @brief Fit aligned storage in buffer.
44 *
45 * This function tries to fit @a __size bytes of storage with alignment
46 * @a __align into the buffer @a __ptr of size @a __space bytes. If such
47 * a buffer fits then @a __ptr is changed to point to the first byte of the
48 * aligned storage and @a __space is reduced by the bytes used for alignment.
49 *
50 * C++11 20.6.5 [ptr.align]
51 *
52 * @param __align A fundamental or extended alignment value.
53 * @param __size Size of the aligned storage required.
54 * @param __ptr Pointer to a buffer of @a __space bytes.
55 * @param __space Size of the buffer pointed to by @a __ptr.
56 * @return the updated pointer if the aligned storage fits, otherwise nullptr.
57 *
58 * @ingroup memory
59 */
60inline void*
61align(size_t __align, size_t __size, void*& __ptr, size_t& __space) noexcept
62{
63 if (__space < __size)
64 return nullptr;
65 const auto __intptr = reinterpret_cast<uintptr_t>(__ptr);
66 const auto __aligned = (__intptr - 1u + __align) & -__align;
67 const auto __diff = __aligned - __intptr;
68 if (__diff > (__space - __size))
69 return nullptr;
70 else
71 {
72 __space -= __diff;
73 return __ptr = reinterpret_cast<void*>(__aligned);
74 }
75}
76
77#ifdef __glibcxx_assume_aligned // C++ >= 20
78 /** @brief Inform the compiler that a pointer is aligned.
79 *
80 * @tparam _Align An alignment value (i.e. a power of two)
81 * @tparam _Tp An object type
82 * @param __ptr A pointer that is aligned to _Align
83 *
84 * C++20 20.10.6 [ptr.align]
85 *
86 * @ingroup memory
87 */
88 template<size_t _Align, class _Tp>
89 [[nodiscard,__gnu__::__always_inline__]]
90 constexpr _Tp*
91 assume_aligned(_Tp* __ptr) noexcept
92 {
93 static_assert(std::has_single_bit(_Align));
94 if (std::is_constant_evaluated())
95 return __ptr;
96 else
97 {
98 // This function is expected to be used in hot code, where
99 // __glibcxx_assert would add unwanted overhead.
100 _GLIBCXX_DEBUG_ASSERT((uintptr_t)__ptr % _Align == 0);
101 return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
102 }
103 }
104#endif // __glibcxx_assume_aligned
105
106_GLIBCXX_END_NAMESPACE_VERSION
107} // namespace
108
109#endif /* _GLIBCXX_ALIGN_H */
110