From 1908897b93a90ce0a44fe43de481b687817963f0 Mon Sep 17 00:00:00 2001
From: Claes Redestad <redestad@openjdk.org>
Date: Tue, 2 Apr 2019 11:24:40 +0200
Subject: [PATCH] 8221723: Avoid storing zero to String.hash

Reviewed-by: shade, prappo, jiangli
---
 src/java.base/share/classes/java/lang/String.java   | 10 ++++++++--
 .../org/openjdk/bench/java/lang/StringHashCode.java | 13 ++++++++++++-
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/src/java.base/share/classes/java/lang/String.java b/src/java.base/share/classes/java/lang/String.java
index 1f2d8c5fa92..f4f11fc6a54 100644
--- a/src/java.base/share/classes/java/lang/String.java
+++ b/src/java.base/share/classes/java/lang/String.java
@@ -1510,8 +1510,14 @@ public final class String
     public int hashCode() {
         int h = hash;
         if (h == 0 && value.length > 0) {
-            hash = h = isLatin1() ? StringLatin1.hashCode(value)
-                                  : StringUTF16.hashCode(value);
+            h = isLatin1() ? StringLatin1.hashCode(value)
+                           : StringUTF16.hashCode(value);
+            // Avoid issuing a store if the calculated value is also zero:
+            // in addition to a minor performance benefit, this allows storing
+            // Strings with zero hash code in read-only memory.
+            if (h != 0) {
+                hash = h;
+            }
         }
         return h;
     }
diff --git a/test/micro/org/openjdk/bench/java/lang/StringHashCode.java b/test/micro/org/openjdk/bench/java/lang/StringHashCode.java
index e00cdb4b050..96dbb0c5bda 100644
--- a/test/micro/org/openjdk/bench/java/lang/StringHashCode.java
+++ b/test/micro/org/openjdk/bench/java/lang/StringHashCode.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014 Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2019, Oracle and/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
@@ -42,11 +42,13 @@ public class StringHashCode {
 
     private String hashcode;
     private String hashcode0;
+    private String empty;
 
     @Setup
     public void setup() {
         hashcode = "abcdefghijkl";
         hashcode0 = new String(new char[]{72, 90, 100, 89, 105, 2, 72, 90, 100, 89, 105, 2});
+        empty = new String();
     }
 
     /**
@@ -66,4 +68,13 @@ public class StringHashCode {
     public int notCached() {
         return hashcode0.hashCode();
     }
+
+    /**
+     * Benchmark testing String.hashCode() with the empty string. Since the
+     * empty String has hashCode = 0, this value is always recalculated.
+     */
+    @Benchmark
+    public int empty() {
+        return empty.hashCode();
+    }
 }