The LUT routethrough inserter checks that a CARRY4 is driven by a LUT cell and inserts a routethrough if it isn't. LUT routethroughs may also need to be inserted if a CARRY4 is driven by a LUT cell that drives more than one pin - like several CARRY4 cells.
The LUT routethrough inserter checks that a CARRY4 is driven by a LUT cell and inserts a routethrough if it isn't. LUT routethroughs may also need to be inserted if a CARRY4 is driven by a LUT cell that drives more than one pin - like several CARRY4 cells.