[SPARK-21967][CORE] org.apache.spark.unsafe.types.UTF8String#compareTo Should Compare 8 Bytes at a Time for Better Performance

## What changes were proposed in this pull request?

* Using 64 bit unsigned long comparison instead of unsigned int comparison in `org.apache.spark.unsafe.types.UTF8String#compareTo` for better performance.
* Making `IS_LITTLE_ENDIAN` a constant for correctness reasons (shouldn't use a non-constant in `compareTo` implementations and it def. is a constant per JVM)

## How was this patch tested?

Build passes and the functionality is widely covered by existing tests as far as I can see.

Author: Armin <[email protected]>

Closes apache#19180 from original-brownbear/SPARK-21967.

Author: original-brownbear

common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java

@@ -64,7 +64,8 @@ public final class UTF8String implements Comparable<UTF8String>, Externalizable,
     5, 5, 5, 5,
     6, 6};
 
-  private static boolean isLittleEndian = ByteOrder.nativeOrder() == ByteOrder.LITTLE_ENDIAN;
+  private static final boolean IS_LITTLE_ENDIAN =
+      ByteOrder.nativeOrder() == ByteOrder.LITTLE_ENDIAN;
 
   private static final UTF8String COMMA_UTF8 = UTF8String.fromString(",");
   public static final UTF8String EMPTY_UTF8 = UTF8String.fromString("");
@@ -220,7 +221,7 @@ public long getPrefix() {
     // After getting the data, we use a mask to mask out data that is not part of the string.
     long p;
     long mask = 0;
-    if (isLittleEndian) {
+    if (IS_LITTLE_ENDIAN) {
       if (numBytes >= 8) {
         p = Platform.getLong(base, offset);
       } else if (numBytes > 4) {
@@ -1097,10 +1098,23 @@ public UTF8String copy() {
   @Override
   public int compareTo(@Nonnull final UTF8String other) {
     int len = Math.min(numBytes, other.numBytes);
-    // TODO: compare 8 bytes as unsigned long
-    for (int i = 0; i < len; i ++) {
+    int wordMax = (len / 8) * 8;
+    long roffset = other.offset;
+    Object rbase = other.base;
+    for (int i = 0; i < wordMax; i += 8) {
+      long left = getLong(base, offset + i);
+      long right = getLong(rbase, roffset + i);
+      if (left != right) {
+        if (IS_LITTLE_ENDIAN) {
+          return Long.compareUnsigned(Long.reverseBytes(left), Long.reverseBytes(right));
+        } else {
+          return Long.compareUnsigned(left, right);
+        }
+      }
+    }
+    for (int i = wordMax; i < len; i++) {
       // In UTF-8, the byte should be unsigned, so we should compare them as unsigned int.
-      int res = (getByte(i) & 0xFF) - (other.getByte(i) & 0xFF);
+      int res = (getByte(i) & 0xFF) - (Platform.getByte(rbase, roffset + i) & 0xFF);
       if (res != 0) {
         return res;
       }