On Thu, Mar 24, 2005 at 05:40:07PM +0200, Peter Dimov wrote:
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?
From my brief survey of the primitives, the memory synchronisation isn't well defined ... it depends on the particular implementation on a
I can find out if you like - I am using the primitives for the libstdc++ version of shared_ptr so was going to dig into their semantics this weekend and ask on the libstdc++ list. particular platform (the i386 implementations, for example, just use a mutex, so aren't lock-free at all). If the implementation for platform X uses memory barriers then it will be well-defined on platform X. N.B. libstdc++ had a compare_and_swap primitive until a couple of years ago, when it was removed because nothing was using it and it was a headache when porting libstdc++ to new architectures. I'm going to ask about the possibility of resurrecting that compare_and_swap(), otherwise shared_count::add_ref_lock() needs to use a lock. jon