Relaxed ordering and visibility

@tag(programming)

I'm really curious that whether the relaxed memory ordering can ensure to see the last recent value in the total modification order. It appears that the C++ memory model alone is not enough.

The C++ memory model only states that

Implementations should make atomic stores visible to atomic loads within a reasonable amount of time.

which means in the code

std::atomic<int> g{0};

// thread 1
g.store(42);

// thread 2
int a = g.load();
// do stuff with a
int b = g.load();

if thread 1 has executed the storing, thread 2 is not guaranteed to load 42 immediately.

However, C++ standard does ensure the visibility of RMW(Read-Modify-Write) operations as the standard says:

Atomic read-modify-write operations shall always read the last value (in the modification order) written before the write associated with the read-modify-write operation.

which means you don't need to worry about operations like fetch_xx and compare_exchange.