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

   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
  38 namespace 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  */
  60 inline void*
  61 align(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 #if __cplusplus > 201703L
  78 #define __cpp_lib_assume_aligned 201811L
  79   /** @brief Inform the compiler that a pointer is aligned.
  80    *
  81    *  @tparam _Align An alignment value (i.e. a power of two)
  82    *  @tparam _Tp    An object type
  83    *  @param  __ptr  A pointer that is aligned to _Align
  84    *
  85    *  C++20 20.10.6 [ptr.align]
  86    *
  87    *  @ingroup memory
  88    */
  89   template<size_t _Align, class _Tp>
  90     [[nodiscard,__gnu__::__always_inline__]]
  91     constexpr _Tp*
  92     assume_aligned(_Tp* __ptr) noexcept
  93     {
  94       static_assert(std::has_single_bit(_Align));
  95       if (std::is_constant_evaluated())
  96         return __ptr;
  97       else
  98         {
  99           // This function is expected to be used in hot code, where
 100           // __glibcxx_assert would add unwanted overhead.
 101           _GLIBCXX_DEBUG_ASSERT((uintptr_t)__ptr % _Align == 0);
 102           return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
 103         }
 104     }
 105 #endif // C++2a
 106
 107 _GLIBCXX_END_NAMESPACE_VERSION
 108 } // namespace
 109
 110 #endif /* _GLIBCXX_ALIGN_H */