The code to insert FF routethroughs checks if both the CO and O pin of a carry4 drive a non-FF cell. If they both do, a FF routethrough is inserted. However, a FF routethrough isn't needed if the O3 pin drives a non-FF cell and the CO3 drives the CIN pin of another CARRY4 in the same chain since the COUT site pin can be used.
This PR just takes care of this case so extra FF routethroughs aren't inserted.
The code to insert FF routethroughs checks if both the CO and O pin of a carry4 drive a non-FF cell. If they both do, a FF routethrough is inserted. However, a FF routethrough isn't needed if the O3 pin drives a non-FF cell and the CO3 drives the CIN pin of another CARRY4 in the same chain since the COUT site pin can be used.
This PR just takes care of this case so extra FF routethroughs aren't inserted.