Fix #6125 (#6432)

Add reference for Bit Twiddling Hacks

THIRD-PARTY-NOTICES

@@ -85,3 +85,19 @@ CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS WITH THE
 SOFTWARE.
+
+License notice for Bit Twiddling Hacks
+--------------------------------------
+
+Bit Twiddling Hacks
+
+By Sean Eron Anderson
+seander@cs.stanford.edu
+
+Individually, the code snippets here are in the public domain (unless otherwise
+noted) — feel free to use them however you please. The aggregate collection and
+descriptions are © 1997-2005 Sean Eron Anderson. The code and descriptions are
+distributed in the hope that they will be useful, but WITHOUT ANY WARRANTY and
+without even the implied warranty of merchantability or fitness for a particular
+purpose. 
+