c++ atomic: would function call act as memory barrier?

A compiler barrier is not the same thing as a memory barrier. A compiler barrier prevents the compiler from moving code across the barrier. A memory barrier (loosely speaking) prevents the hardware from moving reads and writes across the barrier. For atomics you need both, and you also need to ensure that values don't get torn when read or written.

Formally, no, if only because Link-Time Code Generation is a valid implementation choice and need not be optional.

There's also a second oversight, and that's escape analysis. The claim is that "the compiler has no idea what the function’s side effects will be.", but if no pointers to my local variables escape from my function, then the compiler does know for sure that no other function changes them.