LAB Optimization

LUTs are not a good metric. A 6-input LUT uses the whole ALM, while a 4-input LUT uses half of it, i.e. you can get twice as many 4LUTs, 3LUTs, 2LUTs as 6LUTs. (5LUTs can usually be packed with 3LUTs, or share an input. I believe the handbook shows all the combinations). The Logic Utilization is the number you should be looking at to determine how full it is. In 12.1 the numbers were redone to use ALMs instead of LUTs, and is much easier to understand. The Logic Utilization is the same in 12.1, just the breakdown is done differently. (Before 12.1 it reported LUTs as a number that might be too low, but later on accounted for "Unpairable LUTs" because of 6,7 input LUTs, etc. If you ignore the Logic Utilization and just looked at LUTs without looking at the rest, you were not getting the full story...)