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/* Copyright (C) 2022-2024 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"
#include <atomic>
/* 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. */
/* There are two implementations here -- one standard compliant, using
a byte array for internal representation, and another that relies
on bitfields and attribute packed (and attribute gcc_struct on
Windows). The latter is preferable, as it is more convenient when
debugging GDB -- printing a struct packed variable prints its field
using its natural type, which is particularly useful if the type is
an enum -- but may not work on all compilers. */
/* Clang targeting Windows does not support attribute gcc_struct, so
we use the alternative byte array implementation there. */
#if defined _WIN32 && defined __clang__
# define PACKED_USE_ARRAY 1
#else
# define PACKED_USE_ARRAY 0
#endif
/* For the preferred implementation, we need gcc_struct on Windows, as
otherwise the size of e.g., "packed<int, 1>" will be larger than
what we want. Clang targeting Windows does not support attribute
gcc_struct. */
#if !PACKED_USE_ARRAY && defined _WIN32 && !defined __clang__
# define ATTRIBUTE_GCC_STRUCT __attribute__((__gcc_struct__))
#else
# define ATTRIBUTE_GCC_STRUCT
#endif
template<typename T, size_t Bytes = sizeof (T)>
struct ATTRIBUTE_GCC_STRUCT packed
{
public:
packed () noexcept = default;
packed (T val)
{
static_assert (sizeof (ULONGEST) >= sizeof (T));
#if PACKED_USE_ARRAY
ULONGEST tmp = val;
for (int i = (Bytes - 1); i >= 0; --i)
{
m_bytes[i] = (gdb_byte) tmp;
tmp >>= HOST_CHAR_BIT;
}
#else
m_val = val;
#endif
/* Ensure size and aligment are what we expect. */
static_assert (sizeof (packed) == Bytes);
static_assert (alignof (packed) == 1);
/* Make sure packed can be wrapped with std::atomic. */
#if HAVE_IS_TRIVIALLY_COPYABLE
static_assert (std::is_trivially_copyable<packed>::value);
#endif
static_assert (std::is_copy_constructible<packed>::value);
static_assert (std::is_move_constructible<packed>::value);
static_assert (std::is_copy_assignable<packed>::value);
static_assert (std::is_move_assignable<packed>::value);
}
operator T () const noexcept
{
#if PACKED_USE_ARRAY
ULONGEST tmp = 0;
for (int i = 0;;)
{
tmp |= m_bytes[i];
if (++i == Bytes)
break;
tmp <<= HOST_CHAR_BIT;
}
return (T) tmp;
#else
return m_val;
#endif
}
private:
#if PACKED_USE_ARRAY
gdb_byte m_bytes[Bytes];
#else
T m_val : (Bytes * HOST_CHAR_BIT) ATTRIBUTE_PACKED;
#endif
};
/* Add some comparisons between std::atomic<packed<T>> and packed<T>
and T. We need this because even though std::atomic<T> doesn't
define these operators, the relational expressions still work via
implicit conversions. Those wouldn't work when wrapped in packed
without these operators, because they'd require two implicit
conversions to go from T to packed<T> to std::atomic<packed<T>>
(and back), and C++ only does one. */
#define PACKED_ATOMIC_OP(OP) \
template<typename T, size_t Bytes> \
bool operator OP (const std::atomic<packed<T, Bytes>> &lhs, \
const std::atomic<packed<T, Bytes>> &rhs) \
{ \
return lhs.load () OP rhs.load (); \
} \
\
template<typename T, size_t Bytes> \
bool operator OP (T lhs, const std::atomic<packed<T, Bytes>> &rhs) \
{ \
return lhs OP rhs.load (); \
} \
\
template<typename T, size_t Bytes> \
bool operator OP (const std::atomic<packed<T, Bytes>> &lhs, T rhs) \
{ \
return lhs.load () OP rhs; \
} \
\
template<typename T, size_t Bytes> \
bool operator OP (const std::atomic<packed<T, Bytes>> &lhs, \
packed<T, Bytes> rhs) \
{ \
return lhs.load () OP rhs; \
} \
\
template<typename T, size_t Bytes> \
bool operator OP (packed<T, Bytes> lhs, \
const std::atomic<packed<T, Bytes>> &rhs) \
{ \
return lhs OP rhs.load (); \
}
PACKED_ATOMIC_OP (==)
PACKED_ATOMIC_OP (!=)
PACKED_ATOMIC_OP (>)
PACKED_ATOMIC_OP (<)
PACKED_ATOMIC_OP (>=)
PACKED_ATOMIC_OP (<=)
#undef PACKED_ATOMIC_OP
#endif
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