Line data    Source code

       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         611 : align(size_t __align, size_t __size, void*& __ptr, size_t& __space) noexcept
      63             : {
      64         611 :   if (__space < __size)
      65           0 :     return nullptr;
      66         611 :   const auto __intptr = reinterpret_cast<uintptr_t>(__ptr);
      67         611 :   const auto __aligned = (__intptr - 1u + __align) & -__align;
      68         611 :   const auto __diff = __aligned - __intptr;
      69         611 :   if (__diff > (__space - __size))
      70           0 :     return nullptr;
      71             :   else
      72             :     {
      73         611 :       __space -= __diff;
      74         611 :       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 */