[boost] Lock free shared_ptr on gcc, help needed

So, shared_ptr is now lock-free on Windows, but not on the other platforms. We need a portable atomic primitive for that. The only alternative are the internal libstdc++ helpers __atomic_add and __exchange_and_add. But in order to use them correctly, we need to know what their exact semantics are with respect to memory synchronization. To the best of my knowledge, this is not documented anywhere. What do we do now? -- Peter Dimov http://www.pdimov.com