call_once uses incorrect barrier intrinsic on Visual Studio

[ybungalobill@…]

boost\thread\win32\interlocked_read.hpp uses _ReadWriteBarrier() as a barrier. However, ​according to MSDN and the machine code generated, it doesn't emit any memory barrier instructions at all:

The _ReadBarrier, _WriteBarrier, and _ReadWriteBarrier compiler intrinsics prevent only compiler re-ordering. To prevent the CPU from re-ordering read and write operations, use the MemoryBarrier macro.

So one should use the MemoryBarrier() macro instead. In fact according ​this page _ReadWriteBarrier is not necessary for volatile variables access since VS2005.

Thanks,

25. Galka.

[viboes]

Could you provide a test that proves the issue. And a patch to solve it?

[viboes]

wouldn't be enough to remove these lines and let the implementation using boost/detail/interlocked.hpp?

#ifdef BOOST_MSVC

extern "C" void _ReadWriteBarrier(void);
#pragma intrinsic(_ReadWriteBarrier)

namespace boost
{
    namespace detail
    {
        inline long interlocked_read_acquire(long volatile* x) BOOST_NOEXCEPT
        {
            long const res=*x;
            _ReadWriteBarrier();
            return res;
        }
        inline void* interlocked_read_acquire(void* volatile* x) BOOST_NOEXCEPT
        {
            void* const res=*x;
            _ReadWriteBarrier();
            return res;
        }

        inline void interlocked_write_release(long volatile* x,long value) BOOST_NOEXCEPT
        {
            _ReadWriteBarrier();
            *x=value;
        }
        inline void interlocked_write_release(void* volatile* x,void* value) BOOST_NOEXCEPT
        {
            _ReadWriteBarrier();
            *x=value;
        }
    }
}

#else
#endif

[Niall Douglas]

I believe Boost requires VS2005 minimum now, so I have removed the ReadWriteBarrier completely as volatile reads always acquire and volatile writes always release. MSVC generates the appropriate acquire-release opcodes on ARM, and on x86 they are unnecessary as all loads acquire and all stores release. This ticket can now be closed. Niall

[viboes]

​https://github.com/boostorg/thread/commit/6fe7f44ea047b5f4c0182fa73fb1755af0bcf11a

[viboes]

​https://github.com/boostorg/thread/commit/6fe7f44ea047b5f4c0182fa73fb1755af0bcf11a