Are these allowed optimizations in C++?

Let std::atomic<std::int64_t> num{0}; be defined somewhere accessible/visible in the code. Is the C++ compiler allowed to replace each of the following two codes with an empty code (something that does nothing)? Similarly, are these optimizations allowed to happen at runtime? I am just trying to get a better understanding of how things work.

num.fetch_add(1,std::memory_order_relaxed);
num.fetch_sub(1,std::memory_order_relaxed);

and

num.fetch_add(1,std::memory_order_relaxed);
std::this_thread::yield();
num.fetch_sub(1,std::memory_order_relaxed);