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 */ |
112 | |