Non-obvious

 from http://graphics.stanford.edu/~seander/bithacks.html
      "Hacker's Delight" by Henry S. Warren, Jr.
      "The GRAPHICS GEMS Series" started by Andrew Glassner
      https://www.geeksforgeeks.org/

 area.c: Compute the area of a polygon
 
 perimeter.c:  Compute the perimeter of a polygon
 
 squareroot.c: Fast squareroot routines for floats and doubles
 
 time.c: Wall clock time

 ticks.c: Tricks mostly using XOR