On sparc/gcc environment, boost/detail/sp_counted_base.hpp will use sp_counted_base_pt.hpp, which of course implements counter protection via pthread_mutex operations. Other gcc-environments (x86, x64, ia64, ppc) have the concept of using asm directives for atomic increment & decrement, which obviously is more efficient than the mutex operations. The boost/detail/atomic_count_gcc.hpp implementation seems like a more efficient mechanism for incrementing/decrementing the shared_ptr reference counters. In fact, the shared_ptr_nmt.hpp implementation does use boost::detail::atomic_count for the reference count (albeit directly, rather than through the shared_count template). I am using atomic_count as the basis for my internal reference counting for my intrusive_ptr-based objects and am happy with this. So I'm curious as to why sp_counted_base.hpp does not use the atomic increment/decrement model for sparc/gcc that it does for other gcc environments?