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  #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 */