Behavior of `t.atomic.rmw.cmpxchg` instructions

Hi,

As the 17. and 18. steps in the spec:

17. If c1 equals c2, then:
    Let c be c2.
18. Else:
    Let c be c3.

And the 21. step will write the c back to the memory.
Where c1 is the origin value on the memory, c2 is the expected value, c3 is the value to replace.

Maybe I can realize this as following simply:

if (memory[addr] != expected)
    memory[addr] = replacement;

But the closest function in C++ std::atomic<T>::compare_exchange_strong has the different behavior.

if (memory[addr] == expected)
   memory[addr] = replacement;

The atomic_rmw instructions are similar to the member functions of std::atomic<T> in C++ STL, such as fetch_add(), fetch_xor, etc.

Is it necessary to have the same behavior between cmpxchg instructions and std::atomic<T>::compare_exchange_strong in C++?

Thanks.

Start with steps 17–21 of the linked WebAssembly threads specification and compare their t.atomic.rmw.cmpxchg semantics with C++ std::atomic<T>::compare_exchange_strong. No repository files or tests are identified; done would require resolving whether the specified behavior is intentional and determining any resulting specification change.