[uecker]

The can of worms is not so big actually. In general, observable behavior is only I/O and volatile accesses. This is not about side effects in general (which can be optimized according to the "as if" rule). So many things can still be reordered vs potentially trapping operations. Also potentially trapping operations can be reordered. For multi-threading we have "happens before" relationship, so a natural interpretation is that everything which happens before the UB is safe.

The reordering of a volatile access after a potentially trapping operation is not conforming. I think it is an important property of volatile that it prevents this optimization, so I hope that GCC will be fixed eventually. A potentially trapping operation can also not be hoisted above a function call, and compilers that did this all got fixed in the mean time.

https://developercommunity.visualstudio.com/t/Invalid-optimi...