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The C and C++ Include Header Files

/usr/include/c++/11/bits/align.h


$ cat -n /usr/include/c++/11/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	#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 */