read_verilog rom.v
proc
flatten
equiv_opt -assert -map +/ice40/cells_sim.v synth_ice40 # equivalency check
design -load postopt # load the post-opt design (otherwise equiv_opt loads the pre-opt design)
cd top # Constrain all select calls below inside the top module
select -assert-max 6 t:SB_LUT4
select -assert-none t:SB_LUT4 %% t:* %D