MSGPACK_ENABLE_GCC_CXX_ATOMIC - Failed

[wfjm]

When running cmake on the cpp_master branch on Ubuntu 18.04 I get a

    -- Performing Test MSGPACK_ENABLE_GCC_CXX_ATOMIC
    -- Performing Test MSGPACK_ENABLE_GCC_CXX_ATOMIC - Failed

The test code in CMakeLists.txt reads

#include <bits/atomicity.h>
int atomic_sub(int i) { return __gnu_cxx::__exchange_and_add(&i, -1) - 1; }
int atomic_add(int i) { return __gnu_cxx::__exchange_and_add(&i, 1) + 1; }
int main(int argc, char * argv[])
{
    atomic_sub(1);
    atomic_add(1);
}

The culprit is that since quite some time the header is under

#include <ext/atomicity.h>

Looking through some old systems is see that

• up to gcc 4.1.1 I find bits/atomicity.h
• after gcc 4.6.3 I find ext/atomicity.h

so the transition was somewhere between gcc 4.1.1 and 4.6.3. Ubuntu 18.04 has gcc version 7.5.0

The code compiles and tests without atomicity.h, hard to judge whether there are performance impacts.
In case atomic actions are important, it seems to me that std::atomic is the natural choice these days.

In closing the steps to reproduce (on an Ubuntu 18.04 system, likely on all current Ubuntu/Debian systems):

  git clone https://github.com/msgpack/msgpack-c.git
  cd msgpack-c
  git checkout cpp_master 
  git checkout 70912ff

  mkdir build
  cd build
  cmake -DMSGPACK_CXX17=ON ..

[redboltz]

In case atomic actions are important, it seems to me that std::atomic is the natural choice these days.

Yes. since C++11, msgpack-c uses std::atomic.
See

msgpack-c/include/msgpack/v1/unpack.hpp

Lines 257 to 264 in 6598c62

	inline void incr_count(void* buffer)
	{
	#if defined(MSGPACK_USE_CPP03)
	_msgpack_sync_incr_and_fetch(reinterpret_cast<volatile _msgpack_atomic_counter_t*>(buffer));
	#else // defined(MSGPACK_USE_CPP03)
	++*reinterpret_cast<std::atomic<unsigned int>*>(buffer);
	#endif // defined(MSGPACK_USE_CPP03)
	}

For C++03, msgpack-c provides _msgpack_sync_incr_and_fetch. It is not a standard way so there are various compiler support.

msgpack-c/include/msgpack/sysdep.hpp

Lines 44 to 69 in 6598c62

	#ifdef _WIN32
	# if defined(_KERNEL_MODE)
	# define _msgpack_atomic_counter_header <ntddk.h>
	# else
	# define _msgpack_atomic_counter_header <windows.h>
	# if !defined(WIN32_LEAN_AND_MEAN)
	# define WIN32_LEAN_AND_MEAN
	# endif /* WIN32_LEAN_AND_MEAN */
	# endif
	typedef long _msgpack_atomic_counter_t;
	#if defined(_AMD64_) || defined(_M_X64) || defined(_M_ARM64)
	# define _msgpack_sync_decr_and_fetch(ptr) _InterlockedDecrement(ptr)
	# define _msgpack_sync_incr_and_fetch(ptr) _InterlockedIncrement(ptr)
	#else
	# define _msgpack_sync_decr_and_fetch(ptr) InterlockedDecrement(ptr)
	# define _msgpack_sync_incr_and_fetch(ptr) InterlockedIncrement(ptr)
	#endif
	#elif defined(__GNUC__) && ((__GNUC__*10 + __GNUC_MINOR__) < 41)
	# define _msgpack_atomic_counter_header "msgpack/gcc_atomic.hpp"
	#else
	typedef unsigned int _msgpack_atomic_counter_t;
	# define _msgpack_sync_decr_and_fetch(ptr) __sync_sub_and_fetch(ptr, 1)
	# define _msgpack_sync_incr_and_fetch(ptr) __sync_add_and_fetch(ptr, 1)
	#endif

https://github.com/msgpack/msgpack-c/blob/6598c6228fb9ce798a7bc1e6300cc8af514af0a7/include/msgpack/gcc_atomic.hpp
is only for older compilers.

However the top level CMakeLists.txt always check it. And outputs Failed message.
I think that cmake process itself is successfully finished even if Failed is output.

Is that right ?

If it is, the problem is not so critical but msgpack-c shouldn't output misleading message at the cmake process.
I will investigate it.

[redboltz]

#906 should fix the issue.

[redboltz]

#906 merged.