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;
     }
 
     /**

test/micro/org/openjdk/bench/java/lang/EnumHashCode.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package org.openjdk.bench.java.lang;
+
+import org.openjdk.jmh.annotations.*;
+
+import java.util.concurrent.TimeUnit;
+
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Thread)
+@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
+@Measurement(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
+@Fork(3)
+public class EnumHashCode {
+
+    enum E {
+        A, B, C;
+    }
+
+    Enum e = E.A;
+
+    @Benchmark
+    public int constant() {
+        // Tests that hash code is constant-foldable
+        return E.A.hashCode();
+    }
+
+    @Benchmark
+    public int field() {
+        // Tests that hash code is efficient
+        return e.hashCode();
+    }
+
+}