Your code makes use of fetch_add() on the atomic, which gives the following guarantee:
Atomically replaces the current value with the result of arithmetic addition of the value and arg. The operation is read-modify-write operation. Memory is affected according to the value of order.
The semantics are crystal clear: before the operation it’s m, after the operation it’s m+2, and no thread accesses to what’s between these two states because the operation is atomic.
Edit: additional elements regarding your alternate question
Whatever Boehm and Adve may say, the C++ compilers obey to the following standard clause:
1.9/5: A conforming implementation executing a well-formed program shall produce the same observable behavior as one of the possible
executions of the corresponding instance of the abstract machine with
the same program and the same input.
If a C++ compiler would generate code that could allow speculative updates to interfere with the observable behavior of the program (aka getting something else than 5 or 7), it would not be standard compliant, because it would fail to ensure the guarantee mentioned in my initial answer.