8302323: Add repeat methods to StringBuilder/StringBuffer

Reviewed-by: tvaleev, redestad

src/java.base/share/classes/java/lang/String.java

@@ -4563,14 +4563,36 @@ public final class String
         final int limit = len * count;
         final byte[] multiple = new byte[limit];
         System.arraycopy(value, 0, multiple, 0, len);
-        int copied = len;
-        for (; copied < limit - copied; copied <<= 1) {
-            System.arraycopy(multiple, 0, multiple, copied, copied);
-        }
-        System.arraycopy(multiple, 0, multiple, copied, limit - copied);
+        repeatCopyRest(multiple, 0, limit, len);
         return new String(multiple, coder);
     }
 
+    /**
+     * Used to perform copying after the initial insertion. Copying is optimized
+     * by using power of two duplication. First pass duplicates original copy,
+     * second pass then duplicates the original and the copy yielding four copies,
+     * third pass duplicates four copies yielding eight copies, and so on.
+     * Finally, the remainder is filled in with prior copies.
+     *
+     * @implNote The technique used here is significantly faster than hand-rolled
+     * loops or special casing small numbers due to the intensive optimization
+     * done by intrinsic {@code System.arraycopy}.
+     *
+     * @param buffer    destination buffer
+     * @param offset    offset in the destination buffer
+     * @param limit     total replicated including what is already in the buffer
+     * @param copied    number of bytes that have already in the buffer
+     */
+    static void repeatCopyRest(byte[] buffer, int offset, int limit, int copied) {
+        // Initial copy is in the buffer.
+        for (; copied < limit - copied; copied <<= 1) {
+            // Power of two duplicate.
+            System.arraycopy(buffer, offset, buffer, offset + copied, copied);
+        }
+        // Duplicate remainder.
+        System.arraycopy(buffer, offset, buffer, offset + copied, limit - copied);
+    }
+
     ////////////////////////////////////////////////////////////////
 
     /**