ShenandoahOptimizeFinals is an illegal optimization

[Roland Westrelin]

This is a follow up to:

http://mail.openjdk.java.net/pipermail/shenandoah-dev/2017-October/003960.html

If we have something like:

public class TestFinal {
    final int field;
    static TestFinal tf;
    TestFinal() {
        tf = this;
        field = 0x42;
    }

    public static void main(String[] args) {
        new TestFinal();
        // read tf.field
    }
}

the object can be evacuated while it is being constructed between the 2
assignments. In that case tf points to from space with a value of zero
for field "field". If tf.field is used later own, because field is
final, with +ShenandoahOptimizeFinals, there will be no read barrier (in
compiled code), so it will read 0 instead of 42.

I observe something similar in practice with
sun.security.jca.ProviderList:

    private final List<Provider> userList = new AbstractList<Provider>() {
        public int size() {
            return configs.length;
        }
        public Provider get(int index) {
            return getProvider(index);
        }
    };

initializes an instance of an inner class which contains a reference to
ProviderList in a final field. That happens before the instance itself
is initialized so fields of ProviderList may be set after the object is
evacuated but userList may have a reference to the object before it is
evacuated. The following code in java.security.SecureRandom:

    private static String getPrngAlgorithm() {
        for (Provider p : Providers.getProviderList().providers()) {
            for (Service s : p.getServices()) {
                if (s.getType().equals("SecureRandom")) {
                    return s.getAlgorithm();
                }
            }
        }
        return null;
    }

goes through method size() above which reads configs in ProviderList
through the final field and may get a null value for configs eventhough
it was initialized in the evacuated copy.

I'm working on a patch that disables ShenandoahOptimizeFinals for
instance fields.

Roland.