Heuristics are used. For example, memory accesses relative to the stack pointer will be assumed to be thread-local, as the stack isn’t shared between threads. A

Heuristics are used. For example, memory accesses relative to the stack pointer will be assumed to be thread-local, as the stack isn’t shared between threads. And that’s just one of the tricks in the toolbox. :-) The result of those is that the expensive atomics aren’t applied to all accesses at all on hardware that doesn’t expose a TSO memory model.