libstdc++-v3/include/bits/align.h

   1 // align implementation -*- C++ -*-
   2
   3 // Copyright (C) 2014-2023 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 #define __glibcxx_want_assume_aligned
  36 #include <bits/version.h>
  37
  38 #include <bit>          // std::has_single_bit
  39 #include <stdint.h>     // uintptr_t
  40 #include <debug/assertions.h> // _GLIBCXX_DEBUG_ASSERT
  41
  42 namespace std _GLIBCXX_VISIBILITY(default)
  43 {
  44 _GLIBCXX_BEGIN_NAMESPACE_VERSION
  45
  46 /**
  47  *  @brief Fit aligned storage in buffer.
  48  *
  49  *  This function tries to fit @a __size bytes of storage with alignment
  50  *  @a __align into the buffer @a __ptr of size @a __space bytes.  If such
  51  *  a buffer fits then @a __ptr is changed to point to the first byte of the
  52  *  aligned storage and @a __space is reduced by the bytes used for alignment.
  53  *
  54  *  C++11 20.6.5 [ptr.align]
  55  *
  56  *  @param __align   A fundamental or extended alignment value.
  57  *  @param __size    Size of the aligned storage required.
  58  *  @param __ptr     Pointer to a buffer of @a __space bytes.
  59  *  @param __space   Size of the buffer pointed to by @a __ptr.
  60  *  @return the updated pointer if the aligned storage fits, otherwise nullptr.
  61  *
  62  *  @ingroup memory
  63  */
  64 inline void*
  65 align(size_t __align, size_t __size, void*& __ptr, size_t& __space) noexcept
  66 {
  67   if (__space < __size)
  68     return nullptr;
  69   const auto __intptr = reinterpret_cast<uintptr_t>(__ptr);
  70   const auto __aligned = (__intptr - 1u + __align) & -__align;
  71   const auto __diff = __aligned - __intptr;
  72   if (__diff > (__space - __size))
  73     return nullptr;
  74   else
  75     {
  76       __space -= __diff;
  77       return __ptr = reinterpret_cast<void*>(__aligned);
  78     }
  79 }
  80
  81 #ifdef __cpp_lib_assume_aligned // C++ >= 20
  82   /** @brief Inform the compiler that a pointer is aligned.
  83    *
  84    *  @tparam _Align An alignment value (i.e. a power of two)
  85    *  @tparam _Tp    An object type
  86    *  @param  __ptr  A pointer that is aligned to _Align
  87    *
  88    *  C++20 20.10.6 [ptr.align]
  89    *
  90    *  @ingroup memory
  91    */
  92   template<size_t _Align, class _Tp>
  93     [[nodiscard,__gnu__::__always_inline__]]
  94     constexpr _Tp*
  95     assume_aligned(_Tp* __ptr) noexcept
  96     {
  97       static_assert(std::has_single_bit(_Align));
  98       if (std::is_constant_evaluated())
  99         return __ptr;
 100       else
 101         {
 102           // This function is expected to be used in hot code, where
 103           // __glibcxx_assert would add unwanted overhead.
 104           _GLIBCXX_DEBUG_ASSERT((uintptr_t)__ptr % _Align == 0);
 105           return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
 106         }
 107     }
 108 #endif // __cpp_lib_assume_aligned
 109
 110 _GLIBCXX_END_NAMESPACE_VERSION
 111 } // namespace
 112
 113 #endif /* _GLIBCXX_ALIGN_H */