ADDITIONAL SYSTEM INFORMATION :
Ubuntu 18.04, Java versions 14 and 15.0.1

A DESCRIPTION OF THE PROBLEM :
A program with multiple threads modifying the same object is not successfully synchronizing access to that object. The program works correctly under Java 8 and Java 11, but not under Java 14 or Java 15.0.1

REGRESSION : Last worked in version 11

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
The attached program creates 10 threads, each of which increments an Integer counter 100,000 times. The counter is declared as volatile and the increment operation is synchronized.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Final result 1000000
ACTUAL -
Result less than 1000000

---------- BEGIN SOURCE ----------
public class CounterThread2 extends Thread {

public static void main(String[] args) {
Thread[] threads = new Thread[10];
for (int i = 0; i < 10; ++i) {
threads[i] = new CounterThread2("Counter-" + i);
}
for (Thread t : threads) {
t.start();
}

// Wait for all threads to end
boolean done = false;
while (!done) {
boolean alive = false;
for (int i = 0; i < 10; ++i) {
if (threads[i].isAlive()) alive = true;
}
done = !alive;
}
System.out.println("End - end - end");
}

// Counter variable
private static volatile Integer counter = 0;

// Object attributes
private String threadName;

public CounterThread2(String name) {
super(name);
this.threadName = name;
}

public void run() {
for (int i = 0; i < 100000; i++) {
synchronized(counter) {
counter++;
}
System.out.println("Count " + counter + " (" + threadName + ")");
}
}
}
---------- END SOURCE ----------

FREQUENCY : often