The loops in testFill and testCopy are not vectorized if MyValue contains more than one field:

public class Test {
    static inline class MyValue {
        int x = 42;
        int y = 42;
    }

    public static void testFill(MyValue[] array) {
        for (int i = 0; i < array.length; ++i) {
            array[i] = new MyValue();
        }
    }
    
    public static void testCopy(MyValue[] array1, MyValue[] array2) {
        for (int i = 0; i < array1.length; ++i) {
            array1[i] = array2[i];
        }
    }

    public static void main(String[] args) {
        MyValue[] array = new MyValue[100];
        for (int i = 0; i < 100_000; ++i) {
            testFill(array);
            testCopy(array, array);
        }
    }
}

Update: For JEP 401, flat arrays can only be created via an internal API. For example, via ValueClass.newNullRestrictedNonAtomicArray. Above test would need to be updated accordingly.