8271862: C2 intrinsic for Reference.refersTo() is often not used

Reviewed-by: kbarrett, mchung

src/java.base/share/classes/java/lang/ref/PhantomReference.java

@@ -69,8 +69,12 @@ public T get() {
      * do reference processing concurrently.
      */
     @Override
+    boolean refersToImpl(T obj) {
+        return refersTo0(obj);
+    }
+
     @IntrinsicCandidate
-    native final boolean refersTo0(Object o);
+    private native boolean refersTo0(Object o);
 
     /**
      * Creates a new phantom reference that refers to the given object and

src/java.base/share/classes/java/lang/ref/Reference.java

@@ -363,13 +363,20 @@ public T get() {
      * @since 16
      */
     public final boolean refersTo(T obj) {
-        return refersTo0(obj);
+        return refersToImpl(obj);
     }
 
     /* Implementation of refersTo(), overridden for phantom references.
+     * This method exists only to avoid making refersTo0() virtual. Making
+     * refersTo0() virtual has the undesirable effect of C2 often preferring
+     * to call the native implementation over the intrinsic.
      */
+    boolean refersToImpl(T obj) {
+        return refersTo0(obj);
+    }
+
     @IntrinsicCandidate
-    native boolean refersTo0(Object o);
+    private native boolean refersTo0(Object o);
 
     /**
      * Clears this reference object.  Invoking this method will not cause this