8306075: Micro-optimize Enum.hashCode

Co-authored-by: Aleksey Shipilev <shade@openjdk.org>
Reviewed-by: redestad, shade, rriggs, liach, apangin, jvernee

src/java.base/share/classes/java/lang/Enum.java

@@ -37,6 +37,8 @@ import java.lang.constant.DynamicConstantDesc;
 import java.lang.invoke.MethodHandles;
 import java.util.Optional;
 
+import jdk.internal.vm.annotation.Stable;
+
 import static java.util.Objects.requireNonNull;
 
 /**
@@ -166,13 +168,28 @@ public abstract class Enum<E extends Enum<E>>
         return this==other;
     }
 
+    /**
+     * The hash code of this enumeration constant.
+     */
+    @Stable
+    private int hash;
+
     /**
      * Returns a hash code for this enum constant.
      *
      * @return a hash code for this enum constant.
      */
     public final int hashCode() {
-        return super.hashCode();
+        // Once initialized, the hash field value does not change.
+        // HotSpot's identity hash code generation also never returns zero
+        // as the identity hash code. This makes zero a convenient marker
+        // for the un-initialized value for both @Stable and the lazy
+        // initialization code below.
+        int hc = hash;
+        if (hc == 0) {
+            hc = hash = System.identityHashCode(this);
+        }
+        return hc;
     }
 
     /**