diff --git a/backends/firrtl/firrtl.cc b/backends/firrtl/firrtl.cc
index 32410a651..0917ecba6 100644
--- a/backends/firrtl/firrtl.cc
+++ b/backends/firrtl/firrtl.cc
@@ -23,7 +23,11 @@
 #include "kernel/celltypes.h"
 #include "kernel/cellaigs.h"
 #include "kernel/log.h"
+#include <algorithm>
 #include <string>
+#include <regex>
+#include <vector>
+#include <cmath>
 
 USING_YOSYS_NAMESPACE
 PRIVATE_NAMESPACE_BEGIN
@@ -37,6 +41,7 @@ static const FDirection FD_NODIRECTION = 0x0;
 static const FDirection FD_IN = 0x1;
 static const FDirection FD_OUT = 0x2;
 static const FDirection FD_INOUT = 0x3;
+static const int FIRRTL_MAX_DSH_WIDTH_ERROR = 20; // For historic reasons, this is actually one greater than the maximum allowed shift width
 
 // Get a port direction with respect to a specific module.
 FDirection getPortFDirection(IdString id, Module *module)
@@ -173,6 +178,26 @@ struct FirrtlWorker
 	void process_instance(RTLIL::Cell *cell, vector<string> &wire_exprs)
 	{
 		std::string cell_type = fid(cell->type);
+		std::string instanceOf;
+		// If this is a parameterized module, its parent module is encoded in the cell type
+		if (cell->type.substr(0, 8) == "$paramod")
+		{
+			std::string::iterator it;
+			for (it = cell_type.begin(); it < cell_type.end(); it++)
+			{
+				switch (*it) {
+					case '\\': /* FALL_THROUGH */
+					case '=': /* FALL_THROUGH */
+					case '\'': /* FALL_THROUGH */
+					case '$': instanceOf.append("_"); break;
+					default: instanceOf.append(1, *it); break;
+				}
+			}
+		}
+		else
+		{
+			instanceOf = cell_type;
+		}
 
 		std::string cell_name = cellname(cell);
 		std::string cell_name_comment;
@@ -182,7 +207,13 @@ struct FirrtlWorker
 			cell_name_comment = "";
 		// Find the module corresponding to this instance.
 		auto instModule = design->module(cell->type);
-		wire_exprs.push_back(stringf("%s" "inst %s%s of %s", indent.c_str(), cell_name.c_str(), cell_name_comment.c_str(), cell_type.c_str()));
+		// If there is no instance for this, just return.
+		if (instModule == NULL)
+		{
+			log_warning("No instance for %s.%s\n", cell_type.c_str(), cell_name.c_str());
+			return;
+		}
+		wire_exprs.push_back(stringf("%s" "inst %s%s of %s", indent.c_str(), cell_name.c_str(), cell_name_comment.c_str(), instanceOf.c_str()));
 
 		for (auto it = cell->connections().begin(); it != cell->connections().end(); ++it) {
 			if (it->second.size() > 0) {
@@ -194,20 +225,20 @@ struct FirrtlWorker
 				std::string source, sink;
 				switch (dir) {
 					case FD_INOUT:
-						log_warning("Instance port connection %s.%s is INOUT; treating as OUT\n", log_id(cell_type), log_signal(it->second));
+						log_warning("Instance port connection %s.%s is INOUT; treating as OUT\n", cell_type.c_str(), log_signal(it->second));
 					case FD_OUT:
 						source = firstName;
 						sink = secondName;
 						break;
 					case FD_NODIRECTION:
-						log_warning("Instance port connection %s.%s is NODIRECTION; treating as IN\n", log_id(cell_type), log_signal(it->second));
+						log_warning("Instance port connection %s.%s is NODIRECTION; treating as IN\n", cell_type.c_str(), log_signal(it->second));
 						/* FALL_THROUGH */
 					case FD_IN:
 						source = secondName;
 						sink = firstName;
 						break;
 					default:
-						log_error("Instance port %s.%s unrecognized connection direction 0x%x !\n", log_id(cell_type), log_signal(it->second), dir);
+						log_error("Instance port %s.%s unrecognized connection direction 0x%x !\n", cell_type.c_str(), log_signal(it->second), dir);
 						break;
 				}
 				wire_exprs.push_back(stringf("\n%s%s <= %s", indent.c_str(), sink.c_str(), source.c_str()));
@@ -217,6 +248,20 @@ struct FirrtlWorker
 
 	}
 
+	// Given an expression for a shift amount, and a maximum width,
+	//  generate the FIRRTL expression for equivalent dynamic shift taking into account FIRRTL shift semantics.
+	std::string gen_dshl(const string b_expr, const int b_padded_width)
+	{
+		string result = b_expr;
+		if (b_padded_width >= FIRRTL_MAX_DSH_WIDTH_ERROR) {
+			int max_shift_width_bits = FIRRTL_MAX_DSH_WIDTH_ERROR - 1;
+			string max_shift_string = stringf("UInt<%d>(%d)", max_shift_width_bits, (1<<max_shift_width_bits) - 1);
+			// Deal with the difference in semantics between FIRRTL and verilog
+			result = stringf("mux(gt(%s, %s), %s, bits(%s, %d, 0))", b_expr.c_str(), max_shift_string.c_str(), max_shift_string.c_str(), b_expr.c_str(), max_shift_width_bits - 1);
+		}
+		return result;
+	}
+
 	void run()
 	{
 		f << stringf("  module %s:\n", make_id(module->name));
@@ -225,6 +270,12 @@ struct FirrtlWorker
 		for (auto wire : module->wires())
 		{
 			const auto wireName = make_id(wire->name);
+			// If a wire has initial data, issue a warning since FIRRTL doesn't currently support it.
+			if (wire->attributes.count("\\init")) {
+				log_warning("Initial value (%s) for (%s.%s) not supported\n",
+							wire->attributes.at("\\init").as_string().c_str(),
+							log_id(module), log_id(wire));
+			}
 			if (wire->port_id)
 			{
 				if (wire->port_input && wire->port_output)
@@ -240,7 +291,8 @@ struct FirrtlWorker
 
 		for (auto cell : module->cells())
 		{
-		  // Is this cell is a module instance?
+			bool extract_y_bits = false;		// Assume no extraction of final bits will be required.
+		    // Is this cell is a module instance?
 			if (cell->type[0] != '$')
 			{
 				process_instance(cell, wire_exprs);
@@ -264,21 +316,21 @@ struct FirrtlWorker
 				}
 
 				string primop;
-                                bool always_uint = false;
+				bool always_uint = false;
 				if (cell->type == "$not") primop = "not";
-				if (cell->type == "$neg") primop = "neg";
-				if (cell->type == "$logic_not") {
+				else if (cell->type == "$neg") primop = "neg";
+				else if (cell->type == "$logic_not") {
                                         primop = "eq";
                                         a_expr = stringf("%s, UInt(0)", a_expr.c_str());
                                 }
-				if (cell->type == "$reduce_and") primop = "andr";
-				if (cell->type == "$reduce_or") primop = "orr";
-				if (cell->type == "$reduce_xor") primop = "xorr";
-				if (cell->type == "$reduce_xnor") {
+				else if (cell->type == "$reduce_and") primop = "andr";
+				else if (cell->type == "$reduce_or") primop = "orr";
+				else if (cell->type == "$reduce_xor") primop = "xorr";
+				else if (cell->type == "$reduce_xnor") {
                                         primop = "not";
                                         a_expr = stringf("xorr(%s)", a_expr.c_str());
                                 }
-				if (cell->type == "$reduce_bool") {
+				else if (cell->type == "$reduce_bool") {
 					primop = "neq";
 					// Use the sign of the a_expr and its width as the type (UInt/SInt) and width of the comparand.
 					bool a_signed = cell->parameters.at("\\A_SIGNED").as_bool();
@@ -297,14 +349,15 @@ struct FirrtlWorker
 				continue;
 			}
 			if (cell->type.in("$add", "$sub", "$mul", "$div", "$mod", "$xor", "$and", "$or", "$eq", "$eqx",
-                                        "$gt", "$ge", "$lt", "$le", "$ne", "$nex", "$shr", "$sshr", "$sshl", "$shl",
-                                        "$logic_and", "$logic_or"))
+							  "$gt", "$ge", "$lt", "$le", "$ne", "$nex", "$shr", "$sshr", "$sshl", "$shl",
+							  "$logic_and", "$logic_or"))
 			{
 				string y_id = make_id(cell->name);
 				bool is_signed = cell->parameters.at("\\A_SIGNED").as_bool();
 				int y_width =  cell->parameters.at("\\Y_WIDTH").as_int();
 				string a_expr = make_expr(cell->getPort("\\A"));
 				string b_expr = make_expr(cell->getPort("\\B"));
+				int b_padded_width = cell->parameters.at("\\B_WIDTH").as_int();
 				wire_decls.push_back(stringf("    wire %s: UInt<%d>\n", y_id.c_str(), y_width));
 
 				if (cell->parameters.at("\\A_SIGNED").as_bool()) {
@@ -315,67 +368,93 @@ struct FirrtlWorker
 					if (cell->parameters.at("\\B_SIGNED").as_bool()) {
 						b_expr = "asSInt(" + b_expr + ")";
 					}
-					b_expr = stringf("pad(%s, %d)", b_expr.c_str(), y_width);
+					if (b_padded_width < y_width) {
+						auto b_sig = cell->getPort("\\B");
+						b_padded_width = y_width;
+					}
 				}
 
-				a_expr = stringf("pad(%s, %d)", a_expr.c_str(), y_width);
+				auto a_sig = cell->getPort("\\A");
 
 				if (cell->parameters.at("\\A_SIGNED").as_bool()  & (cell->type == "$shr")) {
 					a_expr = "asUInt(" + a_expr + ")";
 				}
 
 				string primop;
-                                bool always_uint = false;
+				bool always_uint = false;
 				if (cell->type == "$add") primop = "add";
-				if (cell->type == "$sub") primop = "sub";
-				if (cell->type == "$mul") primop = "mul";
-				if (cell->type == "$div") primop = "div";
-				if (cell->type == "$mod") primop = "rem";
-				if (cell->type == "$and") {
+				else if (cell->type == "$sub") primop = "sub";
+				else if (cell->type == "$mul") primop = "mul";
+				else if (cell->type == "$div") primop = "div";
+				else if (cell->type == "$mod") primop = "rem";
+				else if (cell->type == "$and") {
                                         primop = "and";
                                         always_uint = true;
                                 }
-				if (cell->type == "$or" ) {
+				else if (cell->type == "$or" ) {
                                         primop =  "or";
                                         always_uint = true;
                                 }
-				if (cell->type == "$xor") {
+				else if (cell->type == "$xor") {
                                         primop = "xor";
                                         always_uint = true;
                                 }
-				if ((cell->type == "$eq") | (cell->type == "$eqx")) {
+				else if ((cell->type == "$eq") | (cell->type == "$eqx")) {
                                         primop = "eq";
                                         always_uint = true;
                                 }
-				if ((cell->type == "$ne") | (cell->type == "$nex")) {
+				else if ((cell->type == "$ne") | (cell->type == "$nex")) {
                                         primop = "neq";
                                         always_uint = true;
                                 }
-				if (cell->type == "$gt") {
+				else if (cell->type == "$gt") {
                                         primop = "gt";
                                         always_uint = true;
                                 }
-				if (cell->type == "$ge") {
+				else if (cell->type == "$ge") {
                                         primop = "geq";
                                         always_uint = true;
                                 }
-				if (cell->type == "$lt") {
+				else if (cell->type == "$lt") {
                                         primop = "lt";
                                         always_uint = true;
                                 }
-				if (cell->type == "$le") {
+				else if (cell->type == "$le") {
                                         primop = "leq";
                                         always_uint = true;
                                 }
-				if ((cell->type == "$shl") | (cell->type == "$sshl")) primop = "dshl";
-				if ((cell->type == "$shr") | (cell->type == "$sshr")) primop = "dshr";
-				if ((cell->type == "$logic_and")) {
+				else if ((cell->type == "$shl") | (cell->type == "$sshl")) {
+					// FIRRTL will widen the result (y) by the amount of the shift.
+					// We'll need to offset this by extracting the un-widened portion as Verilog would do.
+					extract_y_bits = true;
+					// Is the shift amount constant?
+					auto b_sig = cell->getPort("\\B");
+					if (b_sig.is_fully_const()) {
+						primop = "shl";
+					} else {
+						primop = "dshl";
+						// Convert from FIRRTL left shift semantics.
+						b_expr = gen_dshl(b_expr, b_padded_width);
+					}
+				}
+				else if ((cell->type == "$shr") | (cell->type == "$sshr")) {
+					// We don't need to extract a specific range of bits.
+					extract_y_bits = false;
+					// Is the shift amount constant?
+					auto b_sig = cell->getPort("\\B");
+					if (b_sig.is_fully_const()) {
+						primop = "shr";
+					} else {
+						primop = "dshr";
+					}
+				}
+				else if ((cell->type == "$logic_and")) {
                                         primop = "and";
                                         a_expr = "neq(" + a_expr + ", UInt(0))";
                                         b_expr = "neq(" + b_expr + ", UInt(0))";
                                         always_uint = true;
                                 }
-				if ((cell->type == "$logic_or")) {
+				else if ((cell->type == "$logic_or")) {
                                         primop = "or";
                                         a_expr = "neq(" + a_expr + ", UInt(0))";
                                         b_expr = "neq(" + b_expr + ", UInt(0))";
@@ -388,6 +467,11 @@ struct FirrtlWorker
 
 				string expr = stringf("%s(%s, %s)", primop.c_str(), a_expr.c_str(), b_expr.c_str());
 
+				// Deal with FIRRTL's "shift widens" semantics
+				if (extract_y_bits) {
+					expr = stringf("bits(%s, %d, 0)", expr.c_str(), y_width - 1);
+				}
+
 				if ((is_signed && !always_uint) || cell->type.in("$sub"))
 					expr = stringf("asUInt(%s)", expr.c_str());
 
@@ -513,7 +597,65 @@ struct FirrtlWorker
 				continue;
 			}
 
-			log_error("Cell type not supported: %s (%s.%s)\n", log_id(cell->type), log_id(module), log_id(cell));
+			// This may be a parameterized module - paramod.
+			if (cell->type.substr(0, 8) == "$paramod")
+			{
+				process_instance(cell, wire_exprs);
+				continue;
+			}
+			if (cell->type == "$shiftx") {
+				// assign y = a[b +: y_width];
+				// We'll extract the correct bits as part of the primop.
+
+				string y_id = make_id(cell->name);
+				int y_width =  cell->parameters.at("\\Y_WIDTH").as_int();
+				string a_expr = make_expr(cell->getPort("\\A"));
+				// Get the initial bit selector
+				string b_expr = make_expr(cell->getPort("\\B"));
+				wire_decls.push_back(stringf("    wire %s: UInt<%d>\n", y_id.c_str(), y_width));
+
+				if (cell->getParam("\\B_SIGNED").as_bool()) {
+					// Use validif to constrain the selection (test the sign bit)
+					auto b_string = b_expr.c_str();
+					int b_sign = cell->parameters.at("\\B_WIDTH").as_int() - 1;
+					b_expr = stringf("validif(not(bits(%s, %d, %d)), %s)", b_string, b_sign, b_sign, b_string);
+				}
+				string expr = stringf("dshr(%s, %s)", a_expr.c_str(), b_expr.c_str());
+
+				cell_exprs.push_back(stringf("    %s <= %s\n", y_id.c_str(), expr.c_str()));
+				register_reverse_wire_map(y_id, cell->getPort("\\Y"));
+				continue;
+			}
+			if (cell->type == "$shift") {
+				// assign y = a >> b;
+				//  where b may be negative
+
+				string y_id = make_id(cell->name);
+				int y_width =  cell->parameters.at("\\Y_WIDTH").as_int();
+				string a_expr = make_expr(cell->getPort("\\A"));
+				string b_expr = make_expr(cell->getPort("\\B"));
+				auto b_string = b_expr.c_str();
+				int b_padded_width = cell->parameters.at("\\B_WIDTH").as_int();
+				string expr;
+				wire_decls.push_back(stringf("    wire %s: UInt<%d>\n", y_id.c_str(), y_width));
+
+				if (cell->getParam("\\B_SIGNED").as_bool()) {
+					// We generate a left or right shift based on the sign of b.
+					std::string dshl = stringf("bits(dshl(%s, %s), 0, %d)", a_expr.c_str(), gen_dshl(b_expr, b_padded_width).c_str(), y_width);
+					std::string dshr = stringf("dshr(%s, %s)", a_expr.c_str(), b_string);
+					expr = stringf("mux(%s < 0, %s, %s)",
+									 b_string,
+									 dshl.c_str(),
+									 dshr.c_str()
+									 );
+				} else {
+					expr = stringf("dshr(%s, %s)", a_expr.c_str(), b_string);
+				}
+				cell_exprs.push_back(stringf("    %s <= %s\n", y_id.c_str(), expr.c_str()));
+				register_reverse_wire_map(y_id, cell->getPort("\\Y"));
+				continue;
+			}
+			log_warning("Cell type not supported: %s (%s.%s)\n", log_id(cell->type), log_id(module), log_id(cell));
 		}
 
 		for (auto conn : module->connections())
@@ -629,38 +771,53 @@ struct FirrtlBackend : public Backend {
 		log("    write_firrtl [options] [filename]\n");
 		log("\n");
 		log("Write a FIRRTL netlist of the current design.\n");
+		log("The following commands are executed by this command:\n");
+		log("        pmuxtree\n");
 		log("\n");
 	}
 	void execute(std::ostream *&f, std::string filename, std::vector<std::string> args, RTLIL::Design *design) YS_OVERRIDE
 	{
-		size_t argidx;
-		for (argidx = 1; argidx < args.size(); argidx++)
-		{
-			// if (args[argidx] == "-aig") {
-			// 	aig_mode = true;
-			// 	continue;
-			// }
-			break;
+		size_t argidx = args.size();	// We aren't expecting any arguments.
+
+		// If we weren't explicitly passed a filename, use the last argument (if it isn't a flag).
+		if (filename == "") {
+			if (argidx > 0 && args[argidx - 1][0] != '-') {
+				// extra_args and friends need to see this argument.
+				argidx -= 1;
+				filename = args[argidx];
+			}
 		}
 		extra_args(f, filename, args, argidx);
 
+		if (!design->full_selection())
+			log_cmd_error("This command only operates on fully selected designs!\n");
+
 		log_header(design, "Executing FIRRTL backend.\n");
+		log_push();
 
-		Module *top = design->top_module();
-
-		if (top == nullptr)
-			log_error("No top module found!\n");
+		Pass::call(design, stringf("pmuxtree"));
 
 		namecache.clear();
 		autoid_counter = 0;
 
+		// Get the top module, or a reasonable facsimile - we need something for the circuit name.
+		Module *top = design->top_module();
+		Module *last = nullptr;
+		// Generate module and wire names.
 		for (auto module : design->modules()) {
 			make_id(module->name);
+			last = module;
+			if (top == nullptr && module->get_bool_attribute("\\top")) {
+				top = module;
+			}
 			for (auto wire : module->wires())
 				if (wire->port_id)
 					make_id(wire->name);
 		}
 
+		if (top == nullptr)
+			top = last;
+
 		*f << stringf("circuit %s:\n", make_id(top->name));
 
 		for (auto module : design->modules())
diff --git a/backends/verilog/verilog_backend.cc b/backends/verilog/verilog_backend.cc
index 4b5a13941..d351a6266 100644
--- a/backends/verilog/verilog_backend.cc
+++ b/backends/verilog/verilog_backend.cc
@@ -293,7 +293,7 @@ void dump_const(std::ostream &f, const RTLIL::Const &data, int width = -1, int o
 	}
 }
 
-void dump_reg_init(std::ostream &f, SigSpec sig, bool write_equals = true)
+void dump_reg_init(std::ostream &f, SigSpec sig)
 {
 	Const initval;
 	bool gotinit = false;
@@ -308,7 +308,7 @@ void dump_reg_init(std::ostream &f, SigSpec sig, bool write_equals = true)
 	}
 
 	if (gotinit) {
-		if (write_equals) f << " = ";
+		f << " = ";
 		dump_const(f, initval);
 	}
 }
@@ -1250,14 +1250,7 @@ void dump_cell(std::ostream &f, std::string indent, RTLIL::Cell *cell)
 	dump_attributes(f, indent, cell->attributes);
 	f << stringf("%s" "%s", indent.c_str(), id(cell->type, false).c_str());
 
-	std::string init;
-	if (reg_ct.count(cell->type) && cell->hasPort("\\Q")) {
-		std::stringstream ss;
-		dump_reg_init(ss, cell->getPort("\\Q"), false /* write_equals */);
-		init = ss.str();
-	}
-
-	if (!defparam && (cell->parameters.size() > 0 || !init.empty())) {
+	if (!defparam && cell->parameters.size() > 0) {
 		f << stringf(" #(");
 		for (auto it = cell->parameters.begin(); it != cell->parameters.end(); ++it) {
 			if (it != cell->parameters.begin())
@@ -1267,11 +1260,6 @@ void dump_cell(std::ostream &f, std::string indent, RTLIL::Cell *cell)
 			dump_const(f, it->second, -1, 0, false, is_signed);
 			f << stringf(")");
 		}
-		if (!init.empty()) {
-			if (!cell->parameters.empty())
-				f << stringf(",");
-			f << stringf("\n%s  .INIT(%s)", indent.c_str(), init.c_str());
-		}
 		f << stringf("\n%s" ")", indent.c_str());
 	}
 
@@ -1313,17 +1301,24 @@ void dump_cell(std::ostream &f, std::string indent, RTLIL::Cell *cell)
 	}
 	f << stringf("\n%s" ");\n", indent.c_str());
 
-	if (defparam && (cell->parameters.size() > 0 || !init.empty())) {
+	if (defparam && cell->parameters.size() > 0) {
 		for (auto it = cell->parameters.begin(); it != cell->parameters.end(); ++it) {
 			f << stringf("%sdefparam %s.%s = ", indent.c_str(), cell_name.c_str(), id(it->first).c_str());
 			bool is_signed = (it->second.flags & RTLIL::CONST_FLAG_SIGNED) != 0;
 			dump_const(f, it->second, -1, 0, false, is_signed);
 			f << stringf(";\n");
 		}
-		if (!init.empty())
-			f << stringf("%sdefparam %s.INIT = %s;\n", indent.c_str(), cell_name.c_str(), init.c_str());
 	}
 
+	if (reg_ct.count(cell->type) && cell->hasPort("\\Q")) {
+		std::stringstream ss;
+		dump_reg_init(ss, cell->getPort("\\Q"));
+		if (!ss.str().empty()) {
+			f << stringf("%sinitial %s.Q", indent.c_str(), cell_name.c_str());
+			f << ss.str();
+			f << ";\n";
+		}
+	}
 }
 
 void dump_conn(std::ostream &f, std::string indent, const RTLIL::SigSpec &left, const RTLIL::SigSpec &right)
diff --git a/techlibs/common/simcells.v b/techlibs/common/simcells.v
index b9957bd5e..289673e82 100644
--- a/techlibs/common/simcells.v
+++ b/techlibs/common/simcells.v
@@ -451,9 +451,8 @@ endmodule
 //-                 1 1 | y
 //-
 module \$_SR_NN_ (S, R, Q);
-parameter INIT = 1'bx;
 input S, R;
-output reg Q = INIT;
+output reg Q;
 always @(negedge S, negedge R) begin
 	if (R == 0)
 		Q <= 0;
@@ -476,9 +475,8 @@ endmodule
 //-                 1 0 | y
 //-
 module \$_SR_NP_ (S, R, Q);
-parameter INIT = 1'bx;
 input S, R;
-output reg Q = INIT;
+output reg Q;
 always @(negedge S, posedge R) begin
 	if (R == 1)
 		Q <= 0;
@@ -501,9 +499,8 @@ endmodule
 //-                 0 1 | y
 //-
 module \$_SR_PN_ (S, R, Q);
-parameter INIT = 1'bx;
 input S, R;
-output reg Q = INIT;
+output reg Q;
 always @(posedge S, negedge R) begin
 	if (R == 0)
 		Q <= 0;
@@ -526,9 +523,8 @@ endmodule
 //-                 0 0 | y
 //-
 module \$_SR_PP_ (S, R, Q);
-parameter INIT = 1'bx;
 input S, R;
-output reg Q = INIT;
+output reg Q;
 always @(posedge S, posedge R) begin
 	if (R == 1)
 		Q <= 0;
@@ -546,9 +542,8 @@ endmodule
 //- type is usually only used in netlists for formal verification.)
 //-
 module \$_FF_ (D, Q);
-parameter INIT = 1'bx;
 input D;
-output reg Q = INIT;
+output reg Q;
 always @($global_clock) begin
 	Q <= D;
 end
@@ -567,9 +562,8 @@ endmodule
 //-                 - - | q
 //-
 module \$_DFF_N_ (D, C, Q);
-parameter INIT = 1'bx;
 input D, C;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C) begin
 	Q <= D;
 end
@@ -587,9 +581,8 @@ endmodule
 //-                 - - | q
 //-
 module \$_DFF_P_ (D, C, Q);
-parameter INIT = 1'bx;
 input D, C;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C) begin
 	Q <= D;
 end
@@ -607,9 +600,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFFE_NN_ (D, C, E, Q);
-parameter INIT = 1'bx;
 input D, C, E;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C) begin
 	if (!E) Q <= D;
 end
@@ -627,9 +619,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFFE_NP_ (D, C, E, Q);
-parameter INIT = 1'bx;
 input D, C, E;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C) begin
 	if (E) Q <= D;
 end
@@ -647,9 +638,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFFE_PN_ (D, C, E, Q);
-parameter INIT = 1'bx;
 input D, C, E;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C) begin
 	if (!E) Q <= D;
 end
@@ -667,9 +657,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFFE_PP_ (D, C, E, Q);
-parameter INIT = 1'bx;
 input D, C, E;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C) begin
 	if (E) Q <= D;
 end
@@ -688,9 +677,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFF_NN0_ (D, C, R, Q);
-parameter INIT = 1'bx;
 input D, C, R;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C or negedge R) begin
 	if (R == 0)
 		Q <= 0;
@@ -712,9 +700,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFF_NN1_ (D, C, R, Q);
-parameter INIT = 1'bx;
 input D, C, R;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C or negedge R) begin
 	if (R == 0)
 		Q <= 1;
@@ -736,9 +723,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFF_NP0_ (D, C, R, Q);
-parameter INIT = 1'bx;
 input D, C, R;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C or posedge R) begin
 	if (R == 1)
 		Q <= 0;
@@ -760,9 +746,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFF_NP1_ (D, C, R, Q);
-parameter INIT = 1'bx;
 input D, C, R;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C or posedge R) begin
 	if (R == 1)
 		Q <= 1;
@@ -784,9 +769,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFF_PN0_ (D, C, R, Q);
-parameter INIT = 1'bx;
 input D, C, R;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C or negedge R) begin
 	if (R == 0)
 		Q <= 0;
@@ -808,9 +792,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFF_PN1_ (D, C, R, Q);
-parameter INIT = 1'bx;
 input D, C, R;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C or negedge R) begin
 	if (R == 0)
 		Q <= 1;
@@ -832,9 +815,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFF_PP0_ (D, C, R, Q);
-parameter INIT = 1'bx;
 input D, C, R;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C or posedge R) begin
 	if (R == 1)
 		Q <= 0;
@@ -856,9 +838,8 @@ endmodule
 //-                 - - - | q
 //-
 module \$_DFF_PP1_ (D, C, R, Q);
-parameter INIT = 1'bx;
 input D, C, R;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C or posedge R) begin
 	if (R == 1)
 		Q <= 1;
@@ -881,9 +862,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DFFSR_NNN_ (C, S, R, D, Q);
-parameter INIT = 1'bx;
 input C, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C, negedge S, negedge R) begin
 	if (R == 0)
 		Q <= 0;
@@ -909,9 +889,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DFFSR_NNP_ (C, S, R, D, Q);
-parameter INIT = 1'bx;
 input C, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C, negedge S, posedge R) begin
 	if (R == 1)
 		Q <= 0;
@@ -937,9 +916,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DFFSR_NPN_ (C, S, R, D, Q);
-parameter INIT = 1'bx;
 input C, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C, posedge S, negedge R) begin
 	if (R == 0)
 		Q <= 0;
@@ -964,9 +942,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DFFSR_NPP_ (C, S, R, D, Q);
-parameter INIT = 1'bx;
 input C, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @(negedge C, posedge S, posedge R) begin
 	if (R == 1)
 		Q <= 0;
@@ -991,9 +968,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DFFSR_PNN_ (C, S, R, D, Q);
-parameter INIT = 1'bx;
 input C, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C, negedge S, negedge R) begin
 	if (R == 0)
 		Q <= 0;
@@ -1019,9 +995,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DFFSR_PNP_ (C, S, R, D, Q);
-parameter INIT = 1'bx;
 input C, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C, negedge S, posedge R) begin
 	if (R == 1)
 		Q <= 0;
@@ -1047,9 +1022,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DFFSR_PPN_ (C, S, R, D, Q);
-parameter INIT = 1'bx;
 input C, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C, posedge S, negedge R) begin
 	if (R == 0)
 		Q <= 0;
@@ -1074,9 +1048,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DFFSR_PPP_ (C, S, R, D, Q);
-parameter INIT = 1'bx;
 input C, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @(posedge C, posedge S, posedge R) begin
 	if (R == 1)
 		Q <= 0;
@@ -1099,9 +1072,8 @@ endmodule
 //-                 - - | q
 //-
 module \$_DLATCH_N_ (E, D, Q);
-parameter INIT = 1'bx;
 input E, D;
-output reg Q = INIT;
+output reg Q;
 always @* begin
 	if (E == 0)
 		Q <= D;
@@ -1120,9 +1092,8 @@ endmodule
 //-                 - - | q
 //-
 module \$_DLATCH_P_ (E, D, Q);
-parameter INIT = 1'bx;
 input E, D;
-output reg Q = INIT;
+output reg Q;
 always @* begin
 	if (E == 1)
 		Q <= D;
@@ -1143,9 +1114,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DLATCHSR_NNN_ (E, S, R, D, Q);
-parameter INIT = 1'bx;
 input E, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @* begin
 	if (R == 0)
 		Q <= 0;
@@ -1171,9 +1141,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DLATCHSR_NNP_ (E, S, R, D, Q);
-parameter INIT = 1'bx;
 input E, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @* begin
 	if (R == 1)
 		Q <= 0;
@@ -1199,9 +1168,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DLATCHSR_NPN_ (E, S, R, D, Q);
-parameter INIT = 1'bx;
 input E, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @* begin
 	if (R == 0)
 		Q <= 0;
@@ -1226,9 +1194,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DLATCHSR_NPP_ (E, S, R, D, Q);
-parameter INIT = 1'bx;
 input E, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @* begin
 	if (R == 1)
 		Q <= 0;
@@ -1253,9 +1220,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DLATCHSR_PNN_ (E, S, R, D, Q);
-parameter INIT = 1'bx;
 input E, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @* begin
 	if (R == 0)
 		Q <= 0;
@@ -1281,9 +1247,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DLATCHSR_PNP_ (E, S, R, D, Q);
-parameter INIT = 1'bx;
 input E, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @* begin
 	if (R == 1)
 		Q <= 0;
@@ -1309,9 +1274,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DLATCHSR_PPN_ (E, S, R, D, Q);
-parameter INIT = 1'bx;
 input E, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @* begin
 	if (R == 0)
 		Q <= 0;
@@ -1336,9 +1300,8 @@ endmodule
 //-                 - - - - | q
 //-
 module \$_DLATCHSR_PPP_ (E, S, R, D, Q);
-parameter INIT = 1'bx;
 input E, S, R, D;
-output reg Q = INIT;
+output reg Q;
 always @* begin
 	if (R == 1)
 		Q <= 0;
diff --git a/techlibs/common/simlib.v b/techlibs/common/simlib.v
index a1e0c1575..8e43fe058 100644
--- a/techlibs/common/simlib.v
+++ b/techlibs/common/simlib.v
@@ -1464,11 +1464,10 @@ module \$dff (CLK, D, Q);
 
 parameter WIDTH = 0;
 parameter CLK_POLARITY = 1'b1;
-parameter INIT = {WIDTH > 0 ? WIDTH : 1{1'bx}};
 
 input CLK;
 input [WIDTH-1:0] D;
-output reg [WIDTH-1:0] Q = INIT;
+output reg [WIDTH-1:0] Q;
 wire pos_clk = CLK == CLK_POLARITY;
 
 always @(posedge pos_clk) begin
@@ -1484,11 +1483,10 @@ module \$dffe (CLK, EN, D, Q);
 parameter WIDTH = 0;
 parameter CLK_POLARITY = 1'b1;
 parameter EN_POLARITY = 1'b1;
-parameter INIT = {WIDTH > 0 ? WIDTH : 1{1'bx}};
 
 input CLK, EN;
 input [WIDTH-1:0] D;
-output reg [WIDTH-1:0] Q = INIT;
+output reg [WIDTH-1:0] Q;
 wire pos_clk = CLK == CLK_POLARITY;
 
 always @(posedge pos_clk) begin
@@ -1506,11 +1504,10 @@ parameter WIDTH = 0;
 parameter CLK_POLARITY = 1'b1;
 parameter SET_POLARITY = 1'b1;
 parameter CLR_POLARITY = 1'b1;
-parameter INIT = {WIDTH > 0 ? WIDTH : 1{1'bx}};
 
 input CLK;
 input [WIDTH-1:0] SET, CLR, D;
-output reg [WIDTH-1:0] Q = INIT;
+output reg [WIDTH-1:0] Q;
 
 wire pos_clk = CLK == CLK_POLARITY;
 wire [WIDTH-1:0] pos_set = SET_POLARITY ? SET : ~SET;
@@ -1540,11 +1537,10 @@ parameter WIDTH = 0;
 parameter CLK_POLARITY = 1'b1;
 parameter ARST_POLARITY = 1'b1;
 parameter ARST_VALUE = 0;
-parameter INIT = {WIDTH > 0 ? WIDTH : 1{1'bx}};
 
 input CLK, ARST;
 input [WIDTH-1:0] D;
-output reg [WIDTH-1:0] Q = INIT;
+output reg [WIDTH-1:0] Q;
 wire pos_clk = CLK == CLK_POLARITY;
 wire pos_arst = ARST == ARST_POLARITY;
 
@@ -1563,11 +1559,10 @@ module \$dlatch (EN, D, Q);
 
 parameter WIDTH = 0;
 parameter EN_POLARITY = 1'b1;
-parameter INIT = {WIDTH > 0 ? WIDTH : 1{1'bx}};
 
 input EN;
 input [WIDTH-1:0] D;
-output reg [WIDTH-1:0] Q = INIT;
+output reg [WIDTH-1:0] Q;
 
 always @* begin
 	if (EN == EN_POLARITY)
@@ -1585,11 +1580,10 @@ parameter WIDTH = 0;
 parameter EN_POLARITY = 1'b1;
 parameter SET_POLARITY = 1'b1;
 parameter CLR_POLARITY = 1'b1;
-parameter INIT = {WIDTH > 0 ? WIDTH : 1{1'bx}};
 
 input EN;
 input [WIDTH-1:0] SET, CLR, D;
-output reg [WIDTH-1:0] Q = INIT;
+output reg [WIDTH-1:0] Q;
 
 wire pos_en = EN == EN_POLARITY;
 wire [WIDTH-1:0] pos_set = SET_POLARITY ? SET : ~SET;
diff --git a/tests/asicworld/run-test.sh b/tests/asicworld/run-test.sh
index d5708c456..c22ab6928 100755
--- a/tests/asicworld/run-test.sh
+++ b/tests/asicworld/run-test.sh
@@ -11,4 +11,4 @@ do
 done
 shift "$((OPTIND-1))"
 
-exec ${MAKE:-make} -f ../tools/autotest.mk $seed EXTRA_FLAGS="-e" *.v
+exec ${MAKE:-make} -f ../tools/autotest.mk $seed EXTRA_FLAGS+="-e" *.v
diff --git a/tests/asicworld/xfirrtl b/tests/asicworld/xfirrtl
new file mode 100644
index 000000000..c782a2bd6
--- /dev/null
+++ b/tests/asicworld/xfirrtl
@@ -0,0 +1,24 @@
+# This file contains the names of verilog files to exclude from verilog to FIRRTL regression tests due to known failures.
+code_hdl_models_arbiter.v	error: reg rst; cannot be driven by primitives or continuous assignment.
+code_hdl_models_clk_div_45.v	yosys issue: 2nd PMUXTREE pass yields: ERROR: Negative edge clock on FF clk_div_45.$procdff$49.
+code_hdl_models_d_ff_gates.v	combinational loop
+code_hdl_models_d_latch_gates.v	combinational loop
+code_hdl_models_dff_async_reset.v	$adff
+code_hdl_models_tff_async_reset.v	$adff
+code_hdl_models_uart.v	$adff
+code_specman_switch_fabric.v	subfield assignment (bits() <= ...)
+code_tidbits_asyn_reset.v	$adff
+code_tidbits_reg_seq_example.v	$adff
+code_verilog_tutorial_always_example.v	empty module
+code_verilog_tutorial_escape_id.v	make_id issues (name begins with a digit)
+code_verilog_tutorial_explicit.v	firrtl backend bug (empty module)
+code_verilog_tutorial_first_counter.v	error: reg rst; cannot be driven by primitives or continuous assignment.
+code_verilog_tutorial_fsm_full.v	error: reg reset; cannot be driven by primitives or continuous assignment.
+code_verilog_tutorial_if_else.v	empty module (everything is under 'always @ (posedge clk)')
+[code_verilog_tutorial_n_out_primitive.v	empty module
+code_verilog_tutorial_parallel_if.v	empty module (everything is under 'always @ (posedge clk)')
+code_verilog_tutorial_simple_function.v	empty module (no hardware)
+code_verilog_tutorial_simple_if.v	empty module (everything is under 'always @ (posedge clk)')
+code_verilog_tutorial_task_global.v	empty module (everything is under 'always @ (posedge clk)')
+code_verilog_tutorial_v2k_reg.v		empty module
+code_verilog_tutorial_which_clock.v	 $adff
diff --git a/tests/simple/xfirrtl b/tests/simple/xfirrtl
new file mode 100644
index 000000000..00e89b389
--- /dev/null
+++ b/tests/simple/xfirrtl
@@ -0,0 +1,26 @@
+# This file contains the names of verilog files to exclude from verilog to FIRRTL regression tests due to known failures.
+arraycells.v	inst id[0] of
+dff_different_styles.v
+generate.v	combinational loop
+hierdefparam.v	inst id[0] of
+i2c_master_tests.v   $adff
+macros.v	drops modules
+mem2reg.v	drops modules
+mem_arst.v	$adff
+memory.v	$adff
+multiplier.v	inst id[0] of
+muxtree.v	drops modules
+omsp_dbg_uart.v	$adff
+operators.v	$pow
+paramods.v	subfield assignment (bits() <= ...)
+partsel.v	drops modules
+process.v	drops modules
+realexpr.v	drops modules
+scopes.v	original verilog issues ( -x where x isn't declared signed)
+sincos.v	$adff
+specify.v	no code (empty module generates error
+subbytes.v	$adff
+task_func.v	drops modules
+values.v	combinational loop
+vloghammer.v	combinational loop
+wreduce.v	original verilog issues ( -x where x isn't declared signed)
diff --git a/tests/tools/autotest.mk b/tests/tools/autotest.mk
index c68678929..e0f2bcdc1 100644
--- a/tests/tools/autotest.mk
+++ b/tests/tools/autotest.mk
@@ -1,7 +1,7 @@
 
-EXTRA_FLAGS=
-SEED=
-
+# Don't bother defining default values for SEED and EXTRA_FLAGS.
+# Their "natural" default values should be sufficient,
+#   and they may be overridden in the environment.
 ifneq ($(strip $(SEED)),)
 SEEDOPT=-S$(SEED)
 endif
diff --git a/tests/tools/autotest.sh b/tests/tools/autotest.sh
index 0ffa062e3..65fd4cb1f 100755
--- a/tests/tools/autotest.sh
+++ b/tests/tools/autotest.sh
@@ -17,12 +17,18 @@ scriptfiles=""
 scriptopt=""
 toolsdir="$(cd $(dirname $0); pwd)"
 warn_iverilog_git=false
+# The following are used in verilog to firrtl regression tests.
+# Typically these will be passed as environment variables:
+#EXTRA_FLAGS="--firrtl2verilog 'java -cp /.../firrtl/utils/bin/firrtl.jar firrtl.Driver'"
+# The tests are skipped if firrtl2verilog is the empty string (the default).
+firrtl2verilog=""
+xfirrtl="../xfirrtl"
 
 if [ ! -f $toolsdir/cmp_tbdata -o $toolsdir/cmp_tbdata.c -nt $toolsdir/cmp_tbdata ]; then
 	( set -ex; ${CC:-gcc} -Wall -o $toolsdir/cmp_tbdata $toolsdir/cmp_tbdata.c; ) || exit 1
 fi
 
-while getopts xmGl:wkjvref:s:p:n:S:I:B: opt; do
+while getopts xmGl:wkjvref:s:p:n:S:I:B:-: opt; do
 	case "$opt" in
 		x)
 			use_xsim=true ;;
@@ -61,8 +67,24 @@ while getopts xmGl:wkjvref:s:p:n:S:I:B: opt; do
 			minclude_opts="$minclude_opts +incdir+$OPTARG" ;;
         B)
 			backend_opts="$backend_opts $OPTARG" ;;
+		-)
+			case "${OPTARG}" in
+			    xfirrtl)
+			    	xfirrtl="${!OPTIND}"
+				OPTIND=$(( $OPTIND + 1 ))
+				;;
+			    firrtl2verilog)
+			    	firrtl2verilog="${!OPTIND}"
+				OPTIND=$(( $OPTIND + 1 ))
+				;;
+			    *)
+			    	if [ "$OPTERR" == 1 ] && [ "${optspec:0:1}" != ":" ]; then
+				    echo "Unknown option --${OPTARG}" >&2
+				fi
+				;;
+			esac;;
 		*)
-			echo "Usage: $0 [-x|-m] [-G] [-w] [-k] [-j] [-v] [-r] [-e] [-l libs] [-f frontend] [-s script] [-p cmdstring] [-n iters] [-S seed] [-I incdir] [-B backend_opt] verilog-files\n" >&2
+			echo "Usage: $0 [-x|-m] [-G] [-w] [-k] [-j] [-v] [-r] [-e] [-l libs] [-f frontend] [-s script] [-p cmdstring] [-n iters] [-S seed] [-I incdir] [-B backend_opt] [--xfirrtl FIRRTL test exclude file] [--firrtl2verilog command to generate verilog from firrtl] verilog-files\n" >&2
 			exit 1
 	esac
 done
@@ -118,6 +140,7 @@ do
 			"$toolsdir"/../../yosys -f "$frontend $include_opts" -b "verilog" -o ${bn}_ref.v ../${fn}
 			frontend="verilog"
 		fi
+		rm -f ${bn}_ref.fir
 
 		if [ ! -f ../${bn}_tb.v ]; then
 			"$toolsdir"/../../yosys -f "$frontend $include_opts" -b "test_autotb $autotb_opts" -o ${bn}_tb.v ${bn}_ref.v
@@ -157,6 +180,13 @@ do
 		else
 			test_passes -f "$frontend $include_opts" -p "hierarchy; proc; opt; memory; opt; fsm; opt -full -fine" ${bn}_ref.v
 			test_passes -f "$frontend $include_opts" -p "hierarchy; synth -run coarse; techmap; opt; abc -dff" ${bn}_ref.v
+			if [ -n "$firrtl2verilog" ]; then
+			    if test -z "$xfirrtl" || ! grep "$fn" "$xfirrtl" ; then
+				"$toolsdir"/../../yosys -b "firrtl" -o ${bn}_ref.fir -f "$frontend $include_opts" -p "prep -nordff; proc; opt; memory; opt; fsm; opt -full -fine; pmuxtree" ${bn}_ref.v
+				$firrtl2verilog -i ${bn}_ref.fir -o ${bn}_ref.fir.v  -X verilog
+				test_passes -f "$frontend $include_opts" -p "hierarchy; proc; opt; memory; opt; fsm; opt -full -fine" ${bn}_ref.fir.v
+			    fi
+			fi
 		fi
 		touch ../${bn}.log
 	}
@@ -169,14 +199,18 @@ do
 		( set -ex; body; ) > ${bn}.err 2>&1
 	fi
 
+	did_firrtl=""
+	if [ -f ${bn}.out/${bn}_ref.fir ]; then
+	    did_firrtl="+FIRRTL "
+	fi
 	if [ -f ${bn}.log ]; then
 		mv ${bn}.err ${bn}.log
-		echo "${status_prefix}-> ok"
+		echo "${status_prefix}${did_firrtl}-> ok"
 	elif [ -f ${bn}.skip ]; then
 		mv ${bn}.err ${bn}.skip
 		echo "${status_prefix}-> skip"
 	else
-		echo "${status_prefix}-> ERROR!"
+		echo "${status_prefix}${did_firrtl}-> ERROR!"
 		if $warn_iverilog_git; then
 			echo "Note: Make sure that 'iverilog' is an up-to-date git checkout of Icarus Verilog."
 		fi