report_qor_suggestions Tcl command.LUT combining reduces logic utilization by combining LUT pairs with shared inputs into single dual-output LUTs that use both O5 and O6 outputs. However, LUT combining can potentially increase congestion because it tends to increase the input/output connectivity for the slices. If LUT combining is high in the congested area (> 40%), you can try using a synthesis strategy that eliminates LUT combining to help alleviate congestion. The Flow_AlternateRoutability synthesis strategy and directive instructs the synthesis tool to not generate any additional LUT combining.
set_option -enable_prepacking 1.You can use the following command to select cells with LUT combining enabled in your design:
select_objects [get_cells -hier -filter {SOFT_HLUTNM != "" || HLUTNM != ""}]The following figure shows the horizontal congestion of a design with and without LUT combining. The cells utilizing LUT combining are highlighted in purple.
To disable LUT combining on a module that overlaps with areas of higher congestion, use the following Tcl command:
reset_property SOFT_HLUTNM [get_cells -hierarchical -filter {NAME =~ <module name> && SOFT_HLUTNM != ""}]