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AR# 63514

Vivado - LUT utilization mis-match between report_utilization and Find results

Description

In some cases, report_utilization reports a smaller number of LUTs than the Find results based on LUT cell type.
 
In the below example, report_utilization reports 5 LUTs but Find Results returns 9.

Is this expected behavior?

report_utilization:
 

Find Results:





Solution

In examples like the above, the mis-match is expected.

The difference is due to LUT-combining.
 
A count of primitive LUTs (Find Results) does not account for LUT-combining, whereas report_utilization does account for LUT-combining and as a result reports less.
 
The report_utilization result is the final actual LUT utilization.

In this example, there are 4 pairs of LUT4 which are combined into LUT6, so report_utilization reports 4+1=5 LUTs.
 
The LUTs that are optimized with LUT-combining have a "SOFT_HLUTNM" property.

get_property SOFT_HLUTNM [get_cells Componentx/DSP48_V6.DSP48X/U0/i_synth/i_synth_option.i_synth_model/i_op2/g0_b18]
 
soft_lutpair3

AR# 63514
Date Created 02/09/2015
Last Updated 03/26/2015
Status Active
Type General Article
Tools
  • Vivado Design Suite