Variable assignment are not order guaranteed

Inspired by this document, CPU can sometimes change your execution order and compiler would optimize your code. For example

This is a reordering problem^[1]

int v;
bool v_ready = false;

void threadA() {
	v = 42;
	v_ready = true;
}

void threadB() {
	while (!v_ready) { /* wait */}
	const int my_v = v;
	// do something with my_v
}

In this example, we explicitly assign v_ready after v in threadA, however, due to memory locality by the cache mechanism, these two variable are cached in the same cache line, so this makes these variable almost value changed at the same time.

As a result, we can not guarantee v_ready is assigned after v

This brings the importance of atomicity... !

int v = 0;
std::atomic_bool v_ready(false);

void threadA() {
	v = 42;
	v_ready = true;
}

void threadB() {
	while (!v_ready) { /* wait */}
	const int my_v = v;
	// do something with my_v
}

This will make sure v_ready is assigned after v ! So that threadB would get the right value of 4.