Strange atomic_flag example for c++20 at cppreference.com - c++

Question

cppreference.com std::atomic_flag listed two examples of spinlock in prior c++20 and c++20. The last modified date is 21 July 2020, at 12:58.
Prior c++20:
#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
std::atomic_flag lock = ATOMIC_FLAG_INIT;
void f(int n)
{
for (int cnt = 0; cnt < 100; ++cnt) {
while (lock.test_and_set(std::memory_order_acquire)) // acquire lock
; // spin
std::cout << "Output from thread " << n << '\n';
lock.clear(std::memory_order_release); // release lock
}
}
int main()
{
std::vector<std::thread> v;
for (int n = 0; n < 10; ++n) {
v.emplace_back(f, n);
}
for (auto& t : v) {
t.join();
}
}
c++ 20:
#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
std::atomic_flag lock = ATOMIC_FLAG_INIT;
void f(int n)
{
for (int cnt = 0; cnt < 100; ++cnt) {
while (lock.test_and_set(std::memory_order_acquire)) // acquire lock
while (lock.test(std::memory_order_relaxed)) // test lock
; // spin
std::cout << "Output from thread " << n << '\n';
lock.clear(std::memory_order_release); // release lock
}
}
int main()
{
std::vector<std::thread> v;
for (int n = 0; n < 10; ++n) {
v.emplace_back(f, n);
}
for (auto& t : v) {
t.join();
}
}
Two things that bother me in the c++20 example are:
(1) ATOMIC_FLAG_INIT is deprecated in c++20 and the default constructor should store value to false for us.
(2) The "optimization" by introducing while (lock.test(std::memory_order_relaxed)) after the flag has been set to true does not make sense to me. Shouldn't while (lock.test(std::memory_order_relaxed)) always immediately return in that case? Why is it an optimization to the prior c++20 example then?
Edit:
c++20 has introduced test() for atomic which simply checks if the flag is true by doing an atomic load. It is placed at the inner loop when test_and_set() has failed so that the computer first spins inside the test() while loop before going back to test_and_set() the second time.

Answer 1

See the comment:
that edit came from stackoverflow.com/q/62318642/2945027 – Cubbi