Do not call opt -mux_undef (part of -full) before muxcover

techlibs/xilinx/synth_xilinx.cc

@@ -270,6 +270,10 @@ struct SynthXilinxPass : public ScriptPass
 		}
 
 		if (check_label("fine")) {
+		        if (widemux > 0)
+				run("opt -fast -mux_bool -undriven -fine"); // Necessary to omit -mux_undef otherwise muxcover
+									    // performs less efficiently
+			else
 				run("opt -fast -full");
 			run("memory_map");
 			run("dffsr2dff");