1 // align implementation -*- C++ -*-
3 // Copyright (C) 2014-2024 Free Software Foundation, Inc.
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)
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.
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.
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/>.
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}
30 #ifndef _GLIBCXX_ALIGN_H
31 #define _GLIBCXX_ALIGN_H 1
33 #include <bit> // std::has_single_bit
34 #include <debug/assertions.h> // _GLIBCXX_DEBUG_ASSERT
35 #include <bits/version.h>
37 namespace std
_GLIBCXX_VISIBILITY(default)
39 _GLIBCXX_BEGIN_NAMESPACE_VERSION
42 * @brief Fit aligned storage in buffer.
44 * This function tries to fit @a __size bytes of storage with alignment
45 * @a __align into the buffer @a __ptr of size @a __space bytes. If such
46 * a buffer fits then @a __ptr is changed to point to the first byte of the
47 * aligned storage and @a __space is reduced by the bytes used for alignment.
49 * C++11 20.6.5 [ptr.align]
51 * @param __align A fundamental or extended alignment value.
52 * @param __size Size of the aligned storage required.
53 * @param __ptr Pointer to a buffer of @a __space bytes.
54 * @param __space Size of the buffer pointed to by @a __ptr.
55 * @return the updated pointer if the aligned storage fits, otherwise nullptr.
60 align(size_t __align
, size_t __size
, void*& __ptr
, size_t& __space
) noexcept
64 const auto __intptr
= reinterpret_cast<__UINTPTR_TYPE__
>(__ptr
);
65 const auto __aligned
= (__intptr
- 1u + __align
) & -__align
;
66 const auto __diff
= __aligned
- __intptr
;
67 if (__diff
> (__space
- __size
))
72 return __ptr
= reinterpret_cast<void*>(__aligned
);
76 #ifdef __glibcxx_assume_aligned // C++ >= 20
77 /** @brief Inform the compiler that a pointer is aligned.
79 * @tparam _Align An alignment value (i.e. a power of two)
80 * @tparam _Tp An object type
81 * @param __ptr A pointer that is aligned to _Align
83 * C++20 20.10.6 [ptr.align]
87 template<size_t _Align
, class _Tp
>
88 [[nodiscard
,__gnu__::__always_inline__
]]
90 assume_aligned(_Tp
* __ptr
) noexcept
92 static_assert(std::has_single_bit(_Align
));
93 if (std::is_constant_evaluated())
97 // This function is expected to be used in hot code, where
98 // __glibcxx_assert would add unwanted overhead.
99 _GLIBCXX_DEBUG_ASSERT((__UINTPTR_TYPE__
)__ptr
% _Align
== 0);
100 return static_cast<_Tp
*>(__builtin_assume_aligned(__ptr
, _Align
));
103 #endif // __glibcxx_assume_aligned
105 _GLIBCXX_END_NAMESPACE_VERSION
108 #endif /* _GLIBCXX_ALIGN_H */