Optimizing KObjectHashMap hashIndex()

Motivation:

The IntObjectHashMap benchmarks show the Agrona collections to be faster on put, lookup, and remove. One major difference is that we're using 2 modulus operations each time we increment the position index while iterating.  Agrona uses a mask instead.

Modifications:

Modified the KObjectHashMap to use masking rather than modulus when wrapping the position index. This requires that the capacity be a power of 2.

Result:

Improved performance of IntObjectHashMap.

common/src/main/java/io/netty/util/internal/MathUtil.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2015 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License, version 2.0 (the
+ * "License"); you may not use this file except in compliance with the License. You may obtain a
+ * copy of the License at:
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package io.netty.util.internal;
+
+/**
+ * Math utility methods.
+ */
+public final class MathUtil {
+
+    private MathUtil() {
+    }
+
+    /**
+     * Fast method of finding the next power of 2 greater than or equal to the supplied value.
+     *
+     * If the value is {@code <= 0} then 1 will be returned.
+     *
+     * This method is not suitable for {@link Integer#MIN_VALUE} or numbers greater than 2^30.*
+     * @param value from which to search for next power of 2
+     * @return The next power of 2 or the value itself if it is a power of 2
+     */
+    public static int findNextPositivePowerOfTwo(final int value) {
+        assert value > Integer.MIN_VALUE && value < 0x40000000;
+        return 1 << (32 - Integer.numberOfLeadingZeros(value - 1));
+    }
+}

common/src/main/templates/io/netty/util/collection/KObjectHashMap.template

@@ -15,6 +15,8 @@
 
 package io.netty.util.collection;
 
+import static io.netty.util.internal.MathUtil.findNextPositivePowerOfTwo;
+
 import java.lang.reflect.Array;
 import java.util.AbstractCollection;
 import java.util.Arrays;
@@ -33,7 +35,7 @@ import java.util.NoSuchElementException;
 public class @K@ObjectHashMap<V> implements @K@ObjectMap<V>, Iterable<@K@ObjectMap.Entry<V>> {
 
     /** Default initial capacity. Used if not specified in the constructor */
-    public static final int DEFAULT_CAPACITY = 11;
+    public static final int DEFAULT_CAPACITY = 8;
 
     /** Default load factor. Used if not specified in the constructor */
     public static final float DEFAULT_LOAD_FACTOR = 0.5f;
@@ -52,8 +54,8 @@ public class @K@ObjectHashMap<V> implements @K@ObjectMap<V>, Iterable<@K@ObjectM
 
     private @k@[] keys;
     private V[] values;
-    private Collection<V> valueCollection;
     private int size;
+    private int mask;
 
     public @K@ObjectHashMap() {
         this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR);
@@ -76,7 +78,8 @@ public class @K@ObjectHashMap<V> implements @K@ObjectMap<V>, Iterable<@K@ObjectM
         this.loadFactor = loadFactor;
 
         // Adjust the initial capacity if necessary.
-        int capacity = adjustCapacity(initialCapacity);
+        int capacity = findNextPositivePowerOfTwo(initialCapacity);
+        mask = capacity - 1;
 
         // Allocate the arrays.
         keys = new @k@[capacity];
@@ -238,9 +241,7 @@ public class @K@ObjectHashMap<V> implements @K@ObjectMap<V>, Iterable<@K@ObjectM
 
     @Override
     public Collection<V> values() {
-        Collection<V> valueCollection = this.valueCollection;
-        if (valueCollection == null) {
-            this.valueCollection = valueCollection = new AbstractCollection<V>() {
+        return new AbstractCollection<V>() {
             @Override
             public Iterator<V> iterator() {
                 return new Iterator<V>() {
@@ -269,9 +270,6 @@ public class @K@ObjectHashMap<V> implements @K@ObjectMap<V>, Iterable<@K@ObjectM
         };
     }
 
-        return valueCollection;
-    }
-
     @Override
     public int hashCode() {
         // Hashcode is based on all non-zero, valid keys. We have to scan the whole keys
@@ -351,8 +349,7 @@ public class @K@ObjectHashMap<V> implements @K@ObjectMap<V>, Iterable<@K@ObjectM
      * Returns the hashed index for the given key.
      */
     private int hashIndex(@k@ key) {
-        // Allowing for negative keys by adding the length after the first mod operation.
-        return (hashCode(key) % keys.length + keys.length) % keys.length;
+        return hashCode(key) & mask;
     }
 
     /**
@@ -369,21 +366,13 @@ public class @K@ObjectHashMap<V> implements @K@ObjectMap<V>, Iterable<@K@ObjectM
         size++;
 
         if (size > maxSize) {
-            // Need to grow the arrays. We take care to detect integer overflow,
-            // also limit array size to ArrayList.MAX_ARRAY_SIZE.
-            rehash(adjustCapacity((int) Math.min(keys.length * 2.0, Integer.MAX_VALUE - 8)));
-        } else if (size == keys.length) {
-            // Open addressing requires that we have at least 1 slot available. Need to refresh
-            // the arrays to clear any removed elements.
-            rehash(keys.length);
-        }
+            if(keys.length == Integer.MAX_VALUE) {
+                throw new IllegalStateException("Max capacity reached at size=" + size);
             }
 
-    /**
-     * Adjusts the given capacity value to ensure that it's odd. Even capacities can break probing.
-     */
-    private static int adjustCapacity(int capacity) {
-        return capacity | 1;
+            // Double the capacity.
+            rehash(keys.length << 1);
+        }
     }
 
     /**
@@ -444,6 +433,7 @@ public class @K@ObjectHashMap<V> implements @K@ObjectMap<V>, Iterable<@K@ObjectM
         values = temp;
 
         maxSize = calcMaxSize(newCapacity);
+        mask = newCapacity - 1;
 
         // Insert to the new arrays.
         for (int i = 0; i < oldVals.length; ++i) {