diff --git a/test/sim/tb_cxxrtl/Makefile b/test/sim/tb_cxxrtl/Makefile
index 00beff0..24ca1e8 100644
--- a/test/sim/tb_cxxrtl/Makefile
+++ b/test/sim/tb_cxxrtl/Makefile
@@ -22,7 +22,7 @@ SYNTH_CMD += chparam -set RESET_VECTOR $(CPU_RESET_VECTOR) $(TOP);
 SYNTH_CMD += chparam -set REDUCED_BYPASS $(REDUCED_BYPASS) $(TOP);
 SYNTH_CMD += chparam -set MULDIV_UNROLL $(MULDIV_UNROLL) $(TOP);
 SYNTH_CMD += chparam -set MUL_FAST $(MUL_FAST) $(TOP);
-SYNTH_CMD += prep -flatten -top $(TOP); async2sync;
+SYNTH_CMD += hierarchy -top $(TOP); proc; opt_clean;
 SYNTH_CMD += splitnets -driver;
 SYNTH_CMD += write_cxxrtl dut.cpp