gdbsupport/packed.h

/* Copyright (C) 2022 Free Software Foundation, Inc.

   This file is part of GDB.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#ifndef PACKED_H
#define PACKED_H

#include "traits.h"

/* Each instantiation and full specialization of the packed template
   defines a type that behaves like a given scalar type, but that has
   byte alignment, and, may optionally have a smaller size than the
   given scalar type.  This is typically used as alternative to
   bit-fields (and ENUM_BITFIELD), when the fields must have separate
   memory locations to avoid data races.  */

template<typename T, size_t Bytes = sizeof (T)>
struct packed
{
public:
  packed () noexcept = default;

  packed (T val)
  {
    m_val = val;

    /* Ensure size and aligment are what we expect.  */
    gdb_static_assert (sizeof (packed) == Bytes);
    gdb_static_assert (alignof (packed) == 1);

    /* Make sure packed can be wrapped with std::atomic.  */
#if HAVE_IS_TRIVIALLY_COPYABLE
    gdb_static_assert (std::is_trivially_copyable<packed>::value);
#endif
    gdb_static_assert (std::is_copy_constructible<packed>::value);
    gdb_static_assert (std::is_move_constructible<packed>::value);
    gdb_static_assert (std::is_copy_assignable<packed>::value);
    gdb_static_assert (std::is_move_assignable<packed>::value);
  }

  operator T () const noexcept
  {
    return m_val;
  }

private:
  T m_val : (Bytes * HOST_CHAR_BIT) ATTRIBUTE_PACKED;
};

/* Add some comparisons between std::atomic<packed<T>> and T.  We need
   this because the regular comparisons would require two implicit
   conversions to go from T to std::atomic<packed<T>>:

     T         -> packed<T>
     packed<T> -> std::atomic<packed<T>>

   and C++ only does one.  */

template<typename T, size_t Bytes>
bool operator== (T lhs, const std::atomic<packed<T, Bytes>> &rhs)
{
  return lhs == rhs.load ();
}

template<typename T, size_t Bytes>
bool operator== (const std::atomic<packed<T, Bytes>> &lhs, T rhs)
{
  return lhs.load () == rhs;
}

template<typename T, size_t Bytes>
bool operator!= (T lhs, const std::atomic<packed<T, Bytes>> &rhs)
{
  return !(lhs == rhs);
}

template<typename T, size_t Bytes>
bool operator!= (const std::atomic<packed<T, Bytes>> &lhs, T rhs)
{
  return !(lhs == rhs);
}

#endif