diff --git a/hadoop-common-project/hadoop-common/CHANGES.txt b/hadoop-common-project/hadoop-common/CHANGES.txt
index 36d208f09a3..e5bd5ec3fc4 100644
--- a/hadoop-common-project/hadoop-common/CHANGES.txt
+++ b/hadoop-common-project/hadoop-common/CHANGES.txt
@@ -658,6 +658,9 @@ Release 2.6.0 - UNRELEASED
     HADOOP-11250. fix endmacro of set_find_shared_library_without_version in
     CMakeLists (Yi Liu via Colin P. McCabe)
 
+    HADOOP-11221. IdentityHashStore assumes System.identityHashCode() is
+    non-negative. (Jinghui Wang via szetszwo)
+
 Release 2.5.1 - 2014-09-05
 
   INCOMPATIBLE CHANGES
diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/IdentityHashStore.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/IdentityHashStore.java
index 4209488d39a..3ae4bbac659 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/IdentityHashStore.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/IdentityHashStore.java
@@ -92,9 +92,10 @@ public final class IdentityHashStore<K, V> {
   }
 
   private void putInternal(Object k, Object v) {
-    int hash = System.identityHashCode(k);
-    final int numEntries = buffer.length / 2;
-    int index = hash % numEntries;
+    final int hash = System.identityHashCode(k);
+    final int numEntries = buffer.length >>  1;
+    //computing modulo with the assumption buffer.length is power of 2
+    int index = hash & (numEntries-1);
     while (true) {
       if (buffer[2 * index] == null) {
         buffer[2 * index] = k;
@@ -127,9 +128,10 @@ public final class IdentityHashStore<K, V> {
     if (buffer == null) {
       return -1;
     }
-    final int numEntries = buffer.length / 2;
-    int hash = System.identityHashCode(k);
-    int index = hash % numEntries;
+    final int numEntries = buffer.length >> 1;
+    final int hash = System.identityHashCode(k);
+    //computing modulo with the assumption buffer.length is power of 2
+    int index = hash & (numEntries -1);
     int firstIndex = index;
     do {
       if (buffer[2 * index] == k) {