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align.h revision 1.1 
      1 // align implementation -*- C++ -*-
      2 
      3 // Copyright (C) 2014-2022 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