Add JMH benchmark for the square-and-add kernel

jmh/src/main/java/com/ankurm/vectorapi/VectorBench.java

@@ -0,0 +1,63 @@
+package com.ankurm.vectorapi;
+
+import jdk.incubator.vector.*;
+import org.openjdk.jmh.annotations.*;
+import org.openjdk.jmh.infra.Blackhole;
+import java.util.Random;
+import java.util.concurrent.TimeUnit;
+
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+@State(Scope.Thread)
+@Warmup(iterations = 5, time = 1)
+@Measurement(iterations = 5, time = 1)
+@Fork(1)
+public class VectorBench {
+    static final VectorSpecies<Float> SP = FloatVector.SPECIES_PREFERRED;
+    float[] a, b, c;
+
+    @Setup
+    public void setup() {
+        int n = 8192;
+        a = new float[n]; b = new float[n]; c = new float[n];
+        Random r = new Random(42);
+        for (int i = 0; i < n; i++) { a[i] = r.nextFloat(); b[i] = r.nextFloat(); }
+    }
+
+    // The result array is written, then handed to JMH's Blackhole so neither the
+    // stores nor the arithmetic can be eliminated as dead code.
+    @Benchmark
+    public void scalar(Blackhole bh) {
+        for (int i = 0; i < a.length; i++) c[i] = -(a[i]*a[i] + b[i]*b[i]);
+        bh.consume(c);
+    }
+
+    @Benchmark
+    public void vector(Blackhole bh) {
+        int i = 0, bound = SP.loopBound(a.length);
+        for (; i < bound; i += SP.length()) {
+            var va = FloatVector.fromArray(SP, a, i);
+            var vb = FloatVector.fromArray(SP, b, i);
+            va.mul(va).add(vb.mul(vb)).neg().intoArray(c, i);
+        }
+        for (; i < a.length; i++) c[i] = -(a[i]*a[i] + b[i]*b[i]);
+        bh.consume(c);
+    }
+
+    // Same kernel, but returning a value derived from the output. Returned values
+    // are consumed by JMH automatically. Note what this costs: the checksum loop is
+    // itself measured, which is why this variant reports ~9x the time of vector().
+    @Benchmark
+    public float vectorChecksum() {
+        int i = 0, bound = SP.loopBound(a.length);
+        for (; i < bound; i += SP.length()) {
+            var va = FloatVector.fromArray(SP, a, i);
+            var vb = FloatVector.fromArray(SP, b, i);
+            va.mul(va).add(vb.mul(vb)).neg().intoArray(c, i);
+        }
+        for (; i < a.length; i++) c[i] = -(a[i]*a[i] + b[i]*b[i]);
+        float sum = 0;
+        for (float v : c) sum += v;
+        return sum;
+    }
+}