I think you missed my point. memset_ptr is a const (not changed by this program... theoretically) volatile (allowed to be changed by the system, theoretically)

I think you missed my point. memset_ptr is a const (not changed by this program... theoretically) volatile (allowed to be changed by the system, theoretically) pointer to memset. In THIS PARTICULAR CASE, memset_ptr points to memset. The compiler however doesn't know that it won't change due to another processes, but we do. So the compiler shouldn't be able to optimize out the call directly to the function pointer because it introduces a possible race condition: the program reads that memset_ptr points to memset, then the pointer changes (due to some other process changing it), but the program still calls memset, and not memset_ptr. The optimization allows for a possible race condition to occur.