Make UnsignedBytes.lexicographicalComparator() use Arrays.compareUnsigned when it's available.

This provides another [alternative to using `Unsafe`](#6806).

And port the benchmark to JMH, which for now means making it Google-internal. (Not that we have our _Caliper_ benchmarks actually _running_ externally, either, IIRC.)

The benchmarks suggest that the old, `Unsafe`-based implementation is faster up to 64 elements or so but that it's a matter of only about a nanosecond. The new implementation pulls ahead for larger arrays, including an advantage of about 5-10 ns for 1024 elements.

For now, I've included this implementation only in the JRE flavor of Guava. We could include it in the Android flavor, too, to see if it helps [under API Level 33+](https://developer.android.com/reference/java/util/Arrays#compareUnsigned(byte[],%20byte[])). But we really would want to do yet more benchmarking for that.

RELNOTES=n/a
PiperOrigin-RevId: 714130759

Author: cpovirk

guava-tests/benchmark/com/google/common/primitives/UnsignedBytesBenchmark.java

@@ -16,6 +16,7 @@
 
 package com.google.common.primitives;
 
+import static com.google.common.base.StandardSystemProperty.JAVA_SPECIFICATION_VERSION;
 import static com.google.common.primitives.UnsignedBytes.max;
 import static com.google.common.primitives.UnsignedBytes.min;
 import static com.google.common.truth.Truth.assertThat;
@@ -242,12 +243,14 @@ public void testLexicographicalComparatorChoice() throws Exception {
     Comparator<byte[]> defaultComparator = UnsignedBytes.lexicographicalComparator();
     assertThat(defaultComparator).isNotNull();
     assertThat(UnsignedBytes.lexicographicalComparator()).isSameInstanceAs(defaultComparator);
-    if (unsafeComparatorAvailable()) {
-      assertThat(Class.forName(unsafeComparatorClassName()))
-          .isSameInstanceAs(defaultComparator.getClass());
+    if (!isJava8()) {
+      assertThat(defaultComparator.getClass())
+          .isEqualTo(UnsignedBytes.ArraysCompareUnsignedComparator.class);
+    } else if (unsafeComparatorAvailable()) {
+      assertThat(defaultComparator.getClass())
+          .isEqualTo(Class.forName(unsafeComparatorClassName()));
     } else {
-      assertThat(UnsignedBytes.lexicographicalComparatorJavaImpl())
-          .isSameInstanceAs(defaultComparator);
+      assertThat(defaultComparator).isEqualTo(UnsignedBytes.lexicographicalComparatorJavaImpl());
     }
   }
 
@@ -360,4 +363,8 @@ public void testSortDescendingIndexed() {
   public void testNulls() {
     new NullPointerTester().testAllPublicStaticMethods(UnsignedBytes.class);
   }
+
+  private static boolean isJava8() {
+    return JAVA_SPECIFICATION_VERSION.value().equals("1.8");
+  }
 }

guava-tests/test/com/google/common/primitives/UnsignedBytesTest.java

@@ -23,13 +23,15 @@
 import com.google.common.annotations.J2ktIncompatible;
 import com.google.common.annotations.VisibleForTesting;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
+import com.google.j2objc.annotations.J2ObjCIncompatible;
 import java.lang.reflect.Field;
 import java.nio.ByteOrder;
 import java.security.AccessController;
 import java.security.PrivilegedActionException;
 import java.security.PrivilegedExceptionAction;
 import java.util.Arrays;
 import java.util.Comparator;
+import org.jspecify.annotations.Nullable;
 import sun.misc.Unsafe;
 
 /**
@@ -439,6 +441,12 @@ public String toString() {
      * to do so.
      */
     static Comparator<byte[]> getBestComparator() {
+      Comparator<byte[]> arraysCompareUnsignedComparator =
+          ArraysCompareUnsignedComparatorMaker.INSTANCE.tryMakeArraysCompareUnsignedComparator();
+      if (arraysCompareUnsignedComparator != null) {
+        return arraysCompareUnsignedComparator;
+      }
+
       try {
         Class<?> theClass = Class.forName(UNSAFE_COMPARATOR_NAME);
 
@@ -455,6 +463,44 @@ static Comparator<byte[]> getBestComparator() {
     }
   }
 
+  private enum ArraysCompareUnsignedComparatorMaker {
+    INSTANCE {
+      /** Implementation used by non-J2ObjC environments. */
+      // We use Arrays.compareUnsigned only after confirming that it's available at runtime.
+      @SuppressWarnings("Java8ApiChecker")
+      @IgnoreJRERequirement
+      @Override
+      @J2ObjCIncompatible
+      @Nullable Comparator<byte[]> tryMakeArraysCompareUnsignedComparator() {
+        try {
+          // Compare AbstractFuture.VarHandleAtomicHelperMaker.
+          Arrays.class.getMethod("compareUnsigned", byte[].class, byte[].class);
+        } catch (NoSuchMethodException beforeJava9) {
+          return null;
+        }
+        return ArraysCompareUnsignedComparator.INSTANCE;
+      }
+    };
+
+    /** Implementation used by J2ObjC environments, overridden for other environments. */
+    @Nullable Comparator<byte[]> tryMakeArraysCompareUnsignedComparator() {
+      return null;
+    }
+  }
+
+  @J2ObjCIncompatible
+  enum ArraysCompareUnsignedComparator implements Comparator<byte[]> {
+    INSTANCE;
+
+    @Override
+    // We use the class only after confirming that Arrays.compareUnsigned is available at runtime.
+    @SuppressWarnings("Java8ApiChecker")
+    @IgnoreJRERequirement
+    public int compare(byte[] left, byte[] right) {
+      return Arrays.compareUnsigned(left, right);
+    }
+  }
+
   private static byte flip(byte b) {
     return (byte) (b ^ 0x80);
   }