I wish people would not say "undefined behavior" to mean something that looks vaguely iffy or dangerous. It has a pretty specific meaning. [1] x84/64 has a pre

I wish people would not say "undefined behavior" to mean something that looks vaguely iffy or dangerous. It has a pretty specific meaning. [1] x84/64 has a pretty strong memory model. Bareback sharing of 64-bit variables has pretty much acquire-release semantics. Of course you should still say atomic and acq_rel when you actually mean that, but it's a question of portable code, not UB. Volatile stops the compiler from shuffling accesses around so that has can affect inter-thread observation order in some cases. [1]: http://blog.llvm.org/2011/05/what-every-c-programmer-should-know.html?m=1 http://blog.llvm.org/2011/05/what-every-c-programmer-should-...