[ARROW-5639][Java] Remove floating point computation from BaseVariabl…

…eWidthVector#getOffsetBufferValueCapacity

java/performance/src/test/java/org/apache/arrow/vector/VariableWidthVectorBenchmarks.java

@@ -0,0 +1,93 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF 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 org.apache.arrow.vector;
+
+import java.util.concurrent.TimeUnit;
+
+import org.apache.arrow.memory.BufferAllocator;
+import org.apache.arrow.memory.RootAllocator;
+import org.junit.Test;
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.BenchmarkMode;
+import org.openjdk.jmh.annotations.Mode;
+import org.openjdk.jmh.annotations.OutputTimeUnit;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.Setup;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.annotations.TearDown;
+import org.openjdk.jmh.runner.Runner;
+import org.openjdk.jmh.runner.RunnerException;
+import org.openjdk.jmh.runner.options.Options;
+import org.openjdk.jmh.runner.options.OptionsBuilder;
+
+/**
+ * Benchmarks for {@link BaseVariableWidthVector}.
+ */
+@State(Scope.Benchmark)
+public class VariableWidthVectorBenchmarks {
+
+  private static final int VECTOR_CAPACITY = 16 * 1024;
+
+  private static final int VECTOR_LENGTH = 1024;
+
+  private static final int ALLOCATOR_CAPACITY = 1024 * 1024;
+
+  private BufferAllocator allocator;
+
+  private VarCharVector vector;
+
+  /**
+   * Setup benchmarks.
+   */
+  @Setup
+  public void prepare() {
+    allocator = new RootAllocator(ALLOCATOR_CAPACITY);
+    vector = new VarCharVector("vector", allocator);
+    vector.allocateNew(VECTOR_CAPACITY, VECTOR_LENGTH);
+  }
+
+  /**
+   * Tear down benchmarks.
+   */
+  @TearDown
+  public void tearDown() {
+    vector.close();
+    allocator.close();
+  }
+
+  /**
+   * Test {@link BaseVariableWidthVector#getValueCapacity()}.
+   * @return useless. To avoid DCE by JIT.
+   */
+  @Benchmark
+  @BenchmarkMode(Mode.AverageTime)
+  @OutputTimeUnit(TimeUnit.NANOSECONDS)
+  public int getValueCapacity() {
+    return vector.getValueCapacity();
+  }
+
+  @Test
+  public void evaluate() throws RunnerException {
+    Options opt = new OptionsBuilder()
+            .include(VariableWidthVectorBenchmarks.class.getSimpleName())
+            .forks(1)
+            .build();
+
+    new Runner(opt).run();
+  }
+}

java/vector/src/main/java/org/apache/arrow/vector/BaseVariableWidthVector.java

@@ -211,7 +211,7 @@ private int getValidityBufferValueCapacity() {
   }
 
   private int getOffsetBufferValueCapacity() {
-    return (int) ((offsetBuffer.capacity() * 1.0) / OFFSET_WIDTH);
+    return offsetBuffer.capacity() / OFFSET_WIDTH;
   }
 
   /**