8273000: Remove WeakReference-based class initialisation barrier implementation

Reviewed-by: psandoz, mchung

src/java.base/share/classes/java/lang/invoke/DirectMethodHandle.java

@@ -33,7 +33,6 @@ import sun.invoke.util.VerifyAccess;
 import sun.invoke.util.VerifyType;
 import sun.invoke.util.Wrapper;
 
-import java.lang.ref.WeakReference;
 import java.util.Arrays;
 import java.util.Objects;
 import java.util.function.Function;
@@ -363,27 +362,12 @@ class DirectMethodHandle extends MethodHandle {
             VerifyAccess.isSamePackage(ValueConversions.class, cls)) {
             // It is a system class.  It is probably in the process of
             // being initialized, but we will help it along just to be safe.
-            if (UNSAFE.shouldBeInitialized(cls)) {
             UNSAFE.ensureClassInitialized(cls);
-            }
             return false;
         }
         return UNSAFE.shouldBeInitialized(cls);
     }
 
-    private static class EnsureInitialized extends ClassValue<WeakReference<Thread>> {
-        @Override
-        protected WeakReference<Thread> computeValue(Class<?> type) {
-            UNSAFE.ensureClassInitialized(type);
-            if (UNSAFE.shouldBeInitialized(type))
-                // If the previous call didn't block, this can happen.
-                // We are executing inside <clinit>.
-                return new WeakReference<>(Thread.currentThread());
-            return null;
-        }
-        static final EnsureInitialized INSTANCE = new EnsureInitialized();
-    }
-
     private void ensureInitialized() {
         if (checkInitialized(member)) {
             // The coast is clear.  Delete the <clinit> barrier.
@@ -397,24 +381,12 @@ class DirectMethodHandle extends MethodHandle {
     }
     private static boolean checkInitialized(MemberName member) {
         Class<?> defc = member.getDeclaringClass();
-        WeakReference<Thread> ref = EnsureInitialized.INSTANCE.get(defc);
-        if (ref == null) {
-            return true;  // the final state
-        }
-        // Somebody may still be running defc.<clinit>.
-        if (ref.refersTo(Thread.currentThread())) {
-            // If anybody is running defc.<clinit>, it is this thread.
-            if (UNSAFE.shouldBeInitialized(defc))
-                // Yes, we are running it; keep the barrier for now.
-                return false;
-        } else {
-            // We are in a random thread.  Block.
         UNSAFE.ensureClassInitialized(defc);
-        }
+        // Once we get here either defc was fully initialized by another thread, or
-        assert(!UNSAFE.shouldBeInitialized(defc));
+        // defc was already being initialized by the current thread. In the latter case
-        // put it into the final state
+        // the barrier must remain. We can detect this simply by checking if initialization
-        EnsureInitialized.INSTANCE.remove(defc);
+        // is still needed.
-        return true;
+        return !UNSAFE.shouldBeInitialized(defc);
     }
 
     /*non-public*/

src/java.base/share/classes/jdk/internal/misc/Unsafe.java

@@ -1143,9 +1143,14 @@ public final class Unsafe {
     }
 
     /**
-     * Ensures the given class has been initialized. This is often
+     * Ensures the given class has been initialized (see JVMS-5.5 for details).
-     * needed in conjunction with obtaining the static field base of a
+     * This is often needed in conjunction with obtaining the static field base
-     * class.
+     * of a class.
+     *
+     * The call returns when either class {@code c} is fully initialized or
+     * class {@code c} is being initialized and the call is performed from
+     * the initializing thread. In the latter case a subsequent call to
+     * {@link #shouldBeInitialized} will return {@code true}.
      */
     public void ensureClassInitialized(Class<?> c) {
         if (c == null) {