opt_expr.cc

/*
 *  yosys -- Yosys Open SYnthesis Suite
 *
 *  Copyright (C) 2012  Clifford Wolf <clifford@clifford.at>
 *
 *  Permission to use, copy, modify, and/or distribute this software for any
 *  purpose with or without fee is hereby granted, provided that the above
 *  copyright notice and this permission notice appear in all copies.
 *
 *  THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 *  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 *  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 *  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 *  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 *  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 *  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 *
 */

#include "kernel/register.h"
#include "kernel/sigtools.h"
#include "kernel/celltypes.h"
#include "kernel/utils.h"
#include "kernel/log.h"
#include <stdlib.h>
#include <stdio.h>
#include <algorithm>

USING_YOSYS_NAMESPACE
PRIVATE_NAMESPACE_BEGIN

bool did_something;

void replace_undriven(RTLIL::Module *module, const CellTypes &ct)
{
	SigMap sigmap(module);
	SigPool driven_signals;
	SigPool used_signals;
	SigPool all_signals;

	dict<SigBit, pair<Wire*, State>> initbits;
	pool<Wire*> revisit_initwires;

	for (auto cell : module->cells())
	for (auto &conn : cell->connections()) {
		if (!ct.cell_known(cell->type) || ct.cell_output(cell->type, conn.first))
			driven_signals.add(sigmap(conn.second));
		if (!ct.cell_known(cell->type) || ct.cell_input(cell->type, conn.first))
			used_signals.add(sigmap(conn.second));
	}

	for (auto wire : module->wires()) {
		if (wire->attributes.count(ID::init)) {
			SigSpec sig = sigmap(wire);
			Const initval = wire->attributes.at(ID::init);
			for (int i = 0; i < GetSize(initval) && i < GetSize(wire); i++) {
				if (initval[i] == State::S0 || initval[i] == State::S1)
					initbits[sig[i]] = make_pair(wire, initval[i]);
			}
		}
		if (wire->port_input)
			driven_signals.add(sigmap(wire));
		if (wire->port_output || wire->get_bool_attribute(ID::keep))
			used_signals.add(sigmap(wire));
		all_signals.add(sigmap(wire));
	}

	all_signals.del(driven_signals);
	RTLIL::SigSpec undriven_signals = all_signals.export_all();

	for (auto &c : undriven_signals.chunks())
	{
		RTLIL::SigSpec sig = c;

		if (c.wire->name[0] == '$')
			sig = used_signals.extract(sig);
		if (sig.size() == 0)
			continue;

		Const val(RTLIL::State::Sx, GetSize(sig));
		for (int i = 0; i < GetSize(sig); i++) {
			SigBit bit = sigmap(sig[i]);
			auto cursor = initbits.find(bit);
			if (cursor != initbits.end()) {
				revisit_initwires.insert(cursor->second.first);
				val[i] = cursor->second.second;
			}
		}

		log_debug("Setting undriven signal in %s to constant: %s = %s\n", log_id(module), log_signal(sig), log_signal(val));
		module->connect(sig, val);
		did_something = true;
	}

	if (!revisit_initwires.empty())
	{
		SigMap sm2(module);

		for (auto wire : revisit_initwires) {
			SigSpec sig = sm2(wire);
			Const initval = wire->attributes.at(ID::init);
			for (int i = 0; i < GetSize(initval) && i < GetSize(wire); i++) {
				if (SigBit(initval[i]) == sig[i])
					initval[i] = State::Sx;
			}
			if (initval.is_fully_undef()) {
				log_debug("Removing init attribute from %s/%s.\n", log_id(module), log_id(wire));
				wire->attributes.erase(ID::init);
				did_something = true;
			} else if (initval != wire->attributes.at(ID::init)) {
				log_debug("Updating init attribute on %s/%s: %s\n", log_id(module), log_id(wire), log_signal(initval));
				wire->attributes[ID::init] = initval;
				did_something = true;
			}
		}
	}
}

void replace_cell(SigMap &assign_map, RTLIL::Module *module, RTLIL::Cell *cell,
		const std::string &info YS_ATTRIBUTE(unused), IdString out_port, RTLIL::SigSpec out_val)
{
	RTLIL::SigSpec Y = cell->getPort(out_port);
	out_val.extend_u0(Y.size(), false);

	log_debug("Replacing %s cell `%s' (%s) in module `%s' with constant driver `%s = %s'.\n",
			cell->type.c_str(), cell->name.c_str(), info.c_str(),
			module->name.c_str(), log_signal(Y), log_signal(out_val));
	// log_cell(cell);
	assign_map.add(Y, out_val);
	module->connect(Y, out_val);
	module->remove(cell);
	did_something = true;
}

bool group_cell_inputs(RTLIL::Module *module, RTLIL::Cell *cell, bool commutative, SigMap &sigmap)
{
	IdString b_name = cell->hasPort(ID::B) ? ID::B : ID::A;

	bool a_signed = cell->parameters.at(ID::A_SIGNED).as_bool();
	bool b_signed = cell->parameters.at(b_name.str() + "_SIGNED").as_bool();

	RTLIL::SigSpec sig_a = sigmap(cell->getPort(ID::A));
	RTLIL::SigSpec sig_b = sigmap(cell->getPort(b_name));
	RTLIL::SigSpec sig_y = sigmap(cell->getPort(ID::Y));

	sig_a.extend_u0(sig_y.size(), a_signed);
	sig_b.extend_u0(sig_y.size(), b_signed);

	std::vector<RTLIL::SigBit> bits_a = sig_a, bits_b = sig_b, bits_y = sig_y;

	enum { GRP_DYN, GRP_CONST_A, GRP_CONST_B, GRP_CONST_AB, GRP_N };
	std::map<std::pair<RTLIL::SigBit, RTLIL::SigBit>, std::set<RTLIL::SigBit>> grouped_bits[GRP_N];

	for (int i = 0; i < GetSize(bits_y); i++)
	{
		int group_idx = GRP_DYN;
		RTLIL::SigBit bit_a = bits_a[i], bit_b = bits_b[i];

		if (cell->type == ID($or) && (bit_a == RTLIL::State::S1 || bit_b == RTLIL::State::S1))
			bit_a = bit_b = RTLIL::State::S1;

		if (cell->type == ID($and) && (bit_a == RTLIL::State::S0 || bit_b == RTLIL::State::S0))
			bit_a = bit_b = RTLIL::State::S0;

		if (bit_a.wire == NULL && bit_b.wire == NULL)
			group_idx = GRP_CONST_AB;
		else if (bit_a.wire == NULL)
			group_idx = GRP_CONST_A;
		else if (bit_b.wire == NULL && commutative)
			group_idx = GRP_CONST_A, std::swap(bit_a, bit_b);
		else if (bit_b.wire == NULL)
			group_idx = GRP_CONST_B;

		grouped_bits[group_idx][std::pair<RTLIL::SigBit, RTLIL::SigBit>(bit_a, bit_b)].insert(bits_y[i]);
	}

	for (int i = 0; i < GRP_N; i++)
		if (GetSize(grouped_bits[i]) == GetSize(bits_y))
			return false;

	log_debug("Replacing %s cell `%s' in module `%s' with cells using grouped bits:\n",
			log_id(cell->type), log_id(cell), log_id(module));

	for (int i = 0; i < GRP_N; i++)
	{
		if (grouped_bits[i].empty())
			continue;

		RTLIL::Wire *new_y = module->addWire(NEW_ID, GetSize(grouped_bits[i]));
		RTLIL::SigSpec new_a, new_b;
		RTLIL::SigSig new_conn;

		for (auto &it : grouped_bits[i]) {
			for (auto &bit : it.second) {
				new_conn.first.append(bit);
				new_conn.second.append(RTLIL::SigBit(new_y, new_a.size()));
			}
			new_a.append(it.first.first);
			new_b.append(it.first.second);
		}

		if (cell->type.in(ID($and), ID($or)) && i == GRP_CONST_A) {
			log_debug("  Direct Connection: %s (%s with %s)\n", log_signal(new_b), log_id(cell->type), log_signal(new_a));
			module->connect(new_y, new_b);
			module->connect(new_conn);
			continue;
		}

		RTLIL::Cell *c = module->addCell(NEW_ID, cell->type);

		c->setPort(ID::A, new_a);
		c->parameters[ID::A_WIDTH] = new_a.size();
		c->parameters[ID::A_SIGNED] = false;

		if (b_name == ID::B) {
			c->setPort(ID::B, new_b);
			c->parameters[ID::B_WIDTH] = new_b.size();
			c->parameters[ID::B_SIGNED] = false;
		}

		c->setPort(ID::Y, new_y);
		c->parameters[ID::Y_WIDTH] = new_y->width;
		c->check();

		module->connect(new_conn);

		log_debug("  New cell `%s': A=%s", log_id(c), log_signal(new_a));
		if (b_name == ID::B)
			log_debug(", B=%s", log_signal(new_b));
		log_debug("\n");
	}

	cover_list("opt.opt_expr.fine.group", "$not", "$pos", "$and", "$or", "$xor", "$xnor", cell->type.str());

	module->remove(cell);
	did_something = true;
	return true;
}

void handle_polarity_inv(Cell *cell, IdString port, IdString param, const SigMap &assign_map, const dict<RTLIL::SigSpec, RTLIL::SigSpec> &invert_map)
{
	SigSpec sig = assign_map(cell->getPort(port));
	if (invert_map.count(sig)) {
		log_debug("Inverting %s of %s cell `%s' in module `%s': %s -> %s\n",
				log_id(port), log_id(cell->type), log_id(cell), log_id(cell->module),
				log_signal(sig), log_signal(invert_map.at(sig)));
		cell->setPort(port, (invert_map.at(sig)));
		cell->setParam(param, !cell->getParam(param).as_bool());
	}
}

void handle_clkpol_celltype_swap(Cell *cell, string type1, string type2, IdString port, const SigMap &assign_map, const dict<RTLIL::SigSpec, RTLIL::SigSpec> &invert_map)
{
	log_assert(GetSize(type1) == GetSize(type2));
	string cell_type = cell->type.str();

	if (GetSize(type1) != GetSize(cell_type))
		return;

	for (int i = 0; i < GetSize(type1); i++) {
		log_assert((type1[i] == '?') == (type2[i] == '?'));
		if (type1[i] == '?') {
			if (cell_type[i] != '0' && cell_type[i] != '1' && cell_type[i] != 'N' && cell_type[i] != 'P')
				return;
			type1[i] = cell_type[i];
			type2[i] = cell_type[i];
		}
	}

	if (cell->type.in(type1, type2)) {
		SigSpec sig = assign_map(cell->getPort(port));
		if (invert_map.count(sig)) {
			log_debug("Inverting %s of %s cell `%s' in module `%s': %s -> %s\n",
					log_id(port), log_id(cell->type), log_id(cell), log_id(cell->module),
					log_signal(sig), log_signal(invert_map.at(sig)));
			cell->setPort(port, (invert_map.at(sig)));
			cell->type = cell->type == type1 ? type2 : type1;
		}
	}
}

bool is_one_or_minus_one(const Const &value, bool is_signed, bool &is_negative)
{
	bool all_bits_one = true;
	bool last_bit_one = true;

	if (GetSize(value.bits) < 1)
		return false;

	if (GetSize(value.bits) == 1) {
		if (value.bits[0] != State::S1)
			return false;
		if (is_signed)
			is_negative = true;
		return true;
	}

	for (int i = 0; i < GetSize(value.bits); i++) {
		if (value.bits[i] != State::S1)
			all_bits_one = false;
		if (value.bits[i] != (i ? State::S0 : State::S1))
			last_bit_one = false;
	}

	if (all_bits_one && is_signed) {
		is_negative = true;
		return true;
	}

	return last_bit_one;
}

int get_highest_hot_index(RTLIL::SigSpec signal)
{
	for (int i = GetSize(signal) - 1; i >= 0; i--)
	{
		if (signal[i] == RTLIL::State::S0)
			continue;

		if (signal[i] == RTLIL::State::S1)
			return i;

		break;
	}

	return -1;
}

// if the signal has only one bit set, return the index of that bit.
// otherwise return -1
int get_onehot_bit_index(RTLIL::SigSpec signal)
{
	int bit_index = -1;

	for (int i = 0; i < GetSize(signal); i++)
	{
		if (signal[i] == RTLIL::State::S0)
			continue;

		if (signal[i] != RTLIL::State::S1)
			return -1;

		if (bit_index != -1)
			return -1;

		bit_index = i;
	}

	return bit_index;
}

void replace_const_cells(RTLIL::Design *design, RTLIL::Module *module, bool consume_x, bool mux_undef, bool mux_bool, bool do_fine, bool keepdc, bool clkinv)
{
	if (!design->selected(module))
		return;

	CellTypes ct_combinational;
	ct_combinational.setup_internals();
	ct_combinational.setup_stdcells();

	SigMap assign_map(module);
	dict<RTLIL::SigSpec, RTLIL::SigSpec> invert_map;

	TopoSort<RTLIL::Cell*, RTLIL::IdString::compare_ptr_by_name<RTLIL::Cell>> cells;
	dict<RTLIL::Cell*, std::set<RTLIL::SigBit>> cell_to_inbit;
	dict<RTLIL::SigBit, std::set<RTLIL::Cell*>> outbit_to_cell;

	for (auto cell : module->cells())
		if (design->selected(module, cell) && cell->type[0] == '$') {
			if (cell->type.in(ID($_NOT_), ID($not), ID($logic_not)) &&
					GetSize(cell->getPort(ID::A)) == 1 && GetSize(cell->getPort(ID::Y)) == 1)
				invert_map[assign_map(cell->getPort(ID::Y))] = assign_map(cell->getPort(ID::A));
			if (cell->type.in(ID($mux), ID($_MUX_)) &&
					cell->getPort(ID::A) == SigSpec(State::S1) && cell->getPort(ID::B) == SigSpec(State::S0))
				invert_map[assign_map(cell->getPort(ID::Y))] = assign_map(cell->getPort(ID::S));
			if (ct_combinational.cell_known(cell->type))
				for (auto &conn : cell->connections()) {
					RTLIL::SigSpec sig = assign_map(conn.second);
					sig.remove_const();
					if (ct_combinational.cell_input(cell->type, conn.first))
						cell_to_inbit[cell].insert(sig.begin(), sig.end());
					if (ct_combinational.cell_output(cell->type, conn.first))
						for (auto &bit : sig)
							outbit_to_cell[bit].insert(cell);
				}
			cells.node(cell);
		}

	for (auto &it_right : cell_to_inbit)
	for (auto &it_sigbit : it_right.second)
	for (auto &it_left : outbit_to_cell[it_sigbit])
		cells.edge(it_left, it_right.first);

	cells.sort();

	for (auto cell : cells.sorted)
	{
#define ACTION_DO(_p_, _s_) do { cover("opt.opt_expr.action_" S__LINE__); replace_cell(assign_map, module, cell, input.as_string(), _p_, _s_); goto next_cell; } while (0)
#define ACTION_DO_Y(_v_) ACTION_DO(ID::Y, RTLIL::SigSpec(RTLIL::State::S ## _v_))

		if (clkinv)
		{
			if (cell->type.in(ID($dff), ID($dffe), ID($dffsr), ID($adff), ID($fsm), ID($memrd), ID($memwr)))
				handle_polarity_inv(cell, ID::CLK, ID::CLK_POLARITY, assign_map, invert_map);

			if (cell->type.in(ID($sr), ID($dffsr), ID($dlatchsr))) {
				handle_polarity_inv(cell, ID::SET, ID::SET_POLARITY, assign_map, invert_map);
				handle_polarity_inv(cell, ID::CLR, ID::CLR_POLARITY, assign_map, invert_map);
			}

			if (cell->type.in(ID($dffe), ID($dlatch), ID($dlatchsr)))
				handle_polarity_inv(cell, ID::EN, ID::EN_POLARITY, assign_map, invert_map);

			handle_clkpol_celltype_swap(cell, "$_SR_N?_", "$_SR_P?_", ID::S, assign_map, invert_map);
			handle_clkpol_celltype_swap(cell, "$_SR_?N_", "$_SR_?P_", ID::R, assign_map, invert_map);

			handle_clkpol_celltype_swap(cell, "$_DFF_N_", "$_DFF_P_", ID::C, assign_map, invert_map);

			handle_clkpol_celltype_swap(cell, "$_DFFE_N?_", "$_DFFE_P?_", ID::C, assign_map, invert_map);
			handle_clkpol_celltype_swap(cell, "$_DFFE_?N_", "$_DFFE_?P_", ID::E, assign_map, invert_map);

			handle_clkpol_celltype_swap(cell, "$_DFF_N??_", "$_DFF_P??_", ID::C, assign_map, invert_map);
			handle_clkpol_celltype_swap(cell, "$_DFF_?N?_", "$_DFF_?P?_", ID::R, assign_map, invert_map);

			handle_clkpol_celltype_swap(cell, "$_DFFSR_N??_", "$_DFFSR_P??_", ID::C, assign_map, invert_map);
			handle_clkpol_celltype_swap(cell, "$_DFFSR_?N?_", "$_DFFSR_?P?_", ID::S, assign_map, invert_map);
			handle_clkpol_celltype_swap(cell, "$_DFFSR_??N_", "$_DFFSR_??P_", ID::R, assign_map, invert_map);

			handle_clkpol_celltype_swap(cell, "$_DLATCH_N_", "$_DLATCH_P_", ID::E, assign_map, invert_map);

			handle_clkpol_celltype_swap(cell, "$_DLATCHSR_N??_", "$_DLATCHSR_P??_", ID::E, assign_map, invert_map);
			handle_clkpol_celltype_swap(cell, "$_DLATCHSR_?N?_", "$_DLATCHSR_?P?_", ID::S, assign_map, invert_map);
			handle_clkpol_celltype_swap(cell, "$_DLATCHSR_??N_", "$_DLATCHSR_??P_", ID::R, assign_map, invert_map);
		}

		bool detect_const_and = false;
		bool detect_const_or = false;

		if (cell->type.in(ID($reduce_and), ID($_AND_)))
			detect_const_and = true;

		if (cell->type.in(ID($and), ID($logic_and)) && GetSize(cell->getPort(ID::A)) == 1 && GetSize(cell->getPort(ID::B)) == 1 && !cell->getParam(ID::A_SIGNED).as_bool())
			detect_const_and = true;

		if (cell->type.in(ID($reduce_or), ID($reduce_bool), ID($_OR_)))
			detect_const_or = true;

		if (cell->type.in(ID($or), ID($logic_or)) && GetSize(cell->getPort(ID::A)) == 1 && GetSize(cell->getPort(ID::B)) == 1 && !cell->getParam(ID::A_SIGNED).as_bool())
			detect_const_or = true;

		if (detect_const_and || detect_const_or)
		{
			pool<SigBit> input_bits = assign_map(cell->getPort(ID::A)).to_sigbit_pool();
			bool found_zero = false, found_one = false, found_undef = false, found_inv = false, many_conconst = false;
			SigBit non_const_input = State::Sm;

			if (cell->hasPort(ID::B)) {
				vector<SigBit> more_bits = assign_map(cell->getPort(ID::B)).to_sigbit_vector();
				input_bits.insert(more_bits.begin(), more_bits.end());
			}

			for (auto bit : input_bits) {
				if (bit.wire) {
					if (invert_map.count(bit) && input_bits.count(invert_map.at(bit)))
						found_inv = true;
					if (non_const_input != State::Sm)
						many_conconst = true;
					non_const_input = many_conconst ? State::Sm : bit;
				} else {
					if (bit == State::S0)
						found_zero = true;
					else if (bit == State::S1)
						found_one = true;
					else
						found_undef = true;
				}
			}

			if (detect_const_and && (found_zero || found_inv)) {
				cover("opt.opt_expr.const_and");
				replace_cell(assign_map, module, cell, "const_and", ID::Y, RTLIL::State::S0);
				goto next_cell;
			}

			if (detect_const_or && (found_one || found_inv)) {
				cover("opt.opt_expr.const_or");
				replace_cell(assign_map, module, cell, "const_or", ID::Y, RTLIL::State::S1);
				goto next_cell;
			}

			if (non_const_input != State::Sm && !found_undef) {
				cover("opt.opt_expr.and_or_buffer");
				replace_cell(assign_map, module, cell, "and_or_buffer", ID::Y, non_const_input);
				goto next_cell;
			}
		}

		if (cell->type.in(ID($_XOR_), ID($_XNOR_)) || (cell->type.in(ID($xor), ID($xnor)) && GetSize(cell->getPort(ID::A)) == 1 && GetSize(cell->getPort(ID::B)) == 1 && !cell->getParam(ID::A_SIGNED).as_bool()))
		{
			SigBit sig_a = assign_map(cell->getPort(ID::A));
			SigBit sig_b = assign_map(cell->getPort(ID::B));
			if (!sig_a.wire)
				std::swap(sig_a, sig_b);
			if (sig_b == State::S0 || sig_b == State::S1) {
				if (cell->type.in(ID($xor), ID($_XOR_))) {
					cover("opt.opt_expr.xor_buffer");
					SigSpec sig_y;
					if (cell->type == ID($xor))
						sig_y = (sig_b == State::S1 ? module->Not(NEW_ID, sig_a).as_bit() : sig_a);
					else if (cell->type == ID($_XOR_))
						sig_y = (sig_b == State::S1 ? module->NotGate(NEW_ID, sig_a) : sig_a);
					else log_abort();
					replace_cell(assign_map, module, cell, "xor_buffer", ID::Y, sig_y);
					goto next_cell;
				}
				if (cell->type.in(ID($xnor), ID($_XNOR_))) {
					cover("opt.opt_expr.xnor_buffer");
					SigSpec sig_y;
					if (cell->type == ID($xnor)) {
						sig_y = (sig_b == State::S1 ? sig_a : module->Not(NEW_ID, sig_a).as_bit());
						int width = cell->getParam(ID::Y_WIDTH).as_int();
						sig_y.append(RTLIL::Const(State::S1, width-1));
					}
					else if (cell->type == ID($_XNOR_))
						sig_y = (sig_b == State::S1 ? sig_a : module->NotGate(NEW_ID, sig_a));
					else log_abort();
					replace_cell(assign_map, module, cell, "xnor_buffer", ID::Y, sig_y);
					goto next_cell;
				}
				log_abort();
			}
		}

		if (cell->type.in(ID($reduce_and), ID($reduce_or), ID($reduce_bool), ID($reduce_xor), ID($reduce_xnor), ID($neg)) &&
				GetSize(cell->getPort(ID::A)) == 1 && GetSize(cell->getPort(ID::Y)) == 1)
		{
			if (cell->type == ID($reduce_xnor)) {
				cover("opt.opt_expr.reduce_xnor_not");
				log_debug("Replacing %s cell `%s' in module `%s' with $not cell.\n",
						log_id(cell->type), log_id(cell->name), log_id(module));
				cell->type = ID($not);
				did_something = true;
			} else {
				cover("opt.opt_expr.unary_buffer");
				replace_cell(assign_map, module, cell, "unary_buffer", ID::Y, cell->getPort(ID::A));
			}
			goto next_cell;
		}

		if (do_fine)
		{
			if (cell->type.in(ID($not), ID($pos), ID($and), ID($or), ID($xor), ID($xnor)))
				if (group_cell_inputs(module, cell, true, assign_map))
					goto next_cell;

			if (cell->type.in(ID($logic_not), ID($logic_and), ID($logic_or), ID($reduce_or), ID($reduce_and), ID($reduce_bool)))
			{
				SigBit neutral_bit = cell->type == ID($reduce_and) ? State::S1 : State::S0;

				RTLIL::SigSpec sig_a = assign_map(cell->getPort(ID::A));
				RTLIL::SigSpec new_sig_a;

				for (auto bit : sig_a)
					if (bit != neutral_bit) new_sig_a.append(bit);

				if (GetSize(new_sig_a) == 0)
					new_sig_a.append(neutral_bit);

				if (GetSize(new_sig_a) < GetSize(sig_a)) {
					cover_list("opt.opt_expr.fine.neutral_A", "$logic_not", "$logic_and", "$logic_or", "$reduce_or", "$reduce_and", "$reduce_bool", cell->type.str());
					log_debug("Replacing port A of %s cell `%s' in module `%s' with shorter expression: %s -> %s\n",
							cell->type.c_str(), cell->name.c_str(), module->name.c_str(), log_signal(sig_a), log_signal(new_sig_a));
					cell->setPort(ID::A, new_sig_a);
					cell->parameters.at(ID::A_WIDTH) = GetSize(new_sig_a);
					did_something = true;
				}
			}

			if (cell->type.in(ID($logic_and), ID($logic_or)))
			{
				SigBit neutral_bit = State::S0;

				RTLIL::SigSpec sig_b = assign_map(cell->getPort(ID::B));
				RTLIL::SigSpec new_sig_b;

				for (auto bit : sig_b)
					if (bit != neutral_bit) new_sig_b.append(bit);

				if (GetSize(new_sig_b) == 0)
					new_sig_b.append(neutral_bit);

				if (GetSize(new_sig_b) < GetSize(sig_b)) {
					cover_list("opt.opt_expr.fine.neutral_B", "$logic_and", "$logic_or", cell->type.str());
					log_debug("Replacing port B of %s cell `%s' in module `%s' with shorter expression: %s -> %s\n",
							cell->type.c_str(), cell->name.c_str(), module->name.c_str(), log_signal(sig_b), log_signal(new_sig_b));
					cell->setPort(ID::B, new_sig_b);
					cell->parameters.at(ID::B_WIDTH) = GetSize(new_sig_b);
					did_something = true;
				}
			}

			if (cell->type == ID($reduce_and))
			{
				RTLIL::SigSpec sig_a = assign_map(cell->getPort(ID::A));

				RTLIL::State new_a = RTLIL::State::S1;
				for (auto &bit : sig_a.to_sigbit_vector())
					if (bit == RTLIL::State::Sx) {
						if (new_a == RTLIL::State::S1)
							new_a = RTLIL::State::Sx;
					} else if (bit == RTLIL::State::S0) {
						new_a = RTLIL::State::S0;
						break;
					} else if (bit.wire != NULL) {
						new_a = RTLIL::State::Sm;
					}

				if (new_a != RTLIL::State::Sm && RTLIL::SigSpec(new_a) != sig_a) {
					cover("opt.opt_expr.fine.$reduce_and");
					log_debug("Replacing port A of %s cell `%s' in module `%s' with constant driver: %s -> %s\n",
							cell->type.c_str(), cell->name.c_str(), module->name.c_str(), log_signal(sig_a), log_signal(new_a));
					cell->setPort(ID::A, sig_a = new_a);
					cell->parameters.at(ID::A_WIDTH) = 1;
					did_something = true;
				}
			}

			if (cell->type.in(ID($logic_not), ID($logic_and), ID($logic_or), ID($reduce_or), ID($reduce_bool)))
			{
				RTLIL::SigSpec sig_a = assign_map(cell->getPort(ID::A));

				RTLIL::State new_a = RTLIL::State::S0;
				for (auto &bit : sig_a.to_sigbit_vector())
					if (bit == RTLIL::State::Sx) {
						if (new_a == RTLIL::State::S0)
							new_a = RTLIL::State::Sx;
					} else if (bit == RTLIL::State::S1) {
						new_a = RTLIL::State::S1;
						break;
					} else if (bit.wire != NULL) {
						new_a = RTLIL::State::Sm;
					}

				if (new_a != RTLIL::State::Sm && RTLIL::SigSpec(new_a) != sig_a) {
					cover_list("opt.opt_expr.fine.A", "$logic_not", "$logic_and", "$logic_or", "$reduce_or", "$reduce_bool", cell->type.str());
					log_debug("Replacing port A of %s cell `%s' in module `%s' with constant driver: %s -> %s\n",
							cell->type.c_str(), cell->name.c_str(), module->name.c_str(), log_signal(sig_a), log_signal(new_a));
					cell->setPort(ID::A, sig_a = new_a);
					cell->parameters.at(ID::A_WIDTH) = 1;
					did_something = true;
				}
			}

			if (cell->type.in(ID($logic_and), ID($logic_or)))
			{
				RTLIL::SigSpec sig_b = assign_map(cell->getPort(ID::B));

				RTLIL::State new_b = RTLIL::State::S0;
				for (auto &bit : sig_b.to_sigbit_vector())
					if (bit == RTLIL::State::Sx) {
						if (new_b == RTLIL::State::S0)
							new_b = RTLIL::State::Sx;
					} else if (bit == RTLIL::State::S1) {
						new_b = RTLIL::State::S1;
						break;
					} else if (bit.wire != NULL) {
						new_b = RTLIL::State::Sm;
					}

				if (new_b != RTLIL::State::Sm && RTLIL::SigSpec(new_b) != sig_b) {
					cover_list("opt.opt_expr.fine.B", "$logic_and", "$logic_or", cell->type.str());
					log_debug("Replacing port B of %s cell `%s' in module `%s' with constant driver: %s -> %s\n",
							cell->type.c_str(), cell->name.c_str(), module->name.c_str(), log_signal(sig_b), log_signal(new_b));
					cell->setPort(ID::B, sig_b = new_b);
					cell->parameters.at(ID::B_WIDTH) = 1;
					did_something = true;
				}
			}

			if (cell->type.in(ID($add), ID($sub)))
			{
				RTLIL::SigSpec sig_a = assign_map(cell->getPort(ID::A));
				RTLIL::SigSpec sig_b = assign_map(cell->getPort(ID::B));
				RTLIL::SigSpec sig_y = cell->getPort(ID::Y);
				bool sub = cell->type == ID($sub);

				int i;
				for (i = 0; i < GetSize(sig_y); i++) {
					RTLIL::SigBit b = sig_b.at(i, State::Sx);
					RTLIL::SigBit a = sig_a.at(i, State::Sx);
					if (b == State::S0 && a != State::Sx)
						module->connect(sig_y[i], a);
					else if (sub && b == State::S1 && a == State::S1)
						module->connect(sig_y[i], State::S0);
					else if (!sub && a == State::S0 && b != State::Sx)
						module->connect(sig_y[i], b);
					else
						break;
				}
				if (i > 0) {
					cover_list("opt.opt_expr.fine", "$add", "$sub", cell->type.str());
					cell->setPort(ID::A, sig_a.extract_end(i));
					cell->setPort(ID::B, sig_b.extract_end(i));
					cell->setPort(ID::Y, sig_y.extract_end(i));
					cell->fixup_parameters();
					did_something = true;
				}
			}

			if (cell->type == ID($alu))
			{
				RTLIL::SigSpec sig_a = assign_map(cell->getPort(ID::A));
				RTLIL::SigSpec sig_b = assign_map(cell->getPort(ID::B));
				RTLIL::SigBit sig_ci = assign_map(cell->getPort(ID::CI));
				RTLIL::SigBit sig_bi = assign_map(cell->getPort(ID::BI));
				RTLIL::SigSpec sig_x = cell->getPort(ID::X);
				RTLIL::SigSpec sig_y = cell->getPort(ID::Y);
				RTLIL::SigSpec sig_co = cell->getPort(ID::CO);

				bool sub = (sig_ci == State::S1 && sig_bi == State::S1);

				// If not a subtraction, yet there is a carry or B is inverted
				//   then no optimisation is possible as carry will not be constant
				if (!sub && (sig_ci != State::S0 || sig_bi != State::S0))
					goto next_cell;

				int i;
				for (i = 0; i < GetSize(sig_y); i++) {
					RTLIL::SigBit b = sig_b.at(i, State::Sx);
					RTLIL::SigBit a = sig_a.at(i, State::Sx);
					if (b == State::S0 && a != State::Sx) {
						module->connect(sig_y[i], sig_a[i]);
						module->connect(sig_x[i], sub ? module->Not(NEW_ID, a).as_bit() : a);
						module->connect(sig_co[i], sub ? State::S1 : State::S0);
					}
					else if (sub && b == State::S1 && a == State::S1) {
						module->connect(sig_y[i], State::S0);
						module->connect(sig_x[i], module->Not(NEW_ID, a));
						module->connect(sig_co[i], State::S0);
					}
					else if (!sub && a == State::S0 && b != State::Sx) {
						module->connect(sig_y[i], b);
						module->connect(sig_x[i], b);
						module->connect(sig_co[i], State::S0);
					}
					else
						break;
				}
				if (i > 0) {
					cover("opt.opt_expr.fine.$alu");
					cell->setPort(ID::A, sig_a.extract_end(i));
					cell->setPort(ID::B, sig_b.extract_end(i));
					cell->setPort(ID::X, sig_x.extract_end(i));
					cell->setPort(ID::Y, sig_y.extract_end(i));
					cell->setPort(ID::CO, sig_co.extract_end(i));
					cell->fixup_parameters();
					did_something = true;
				}
			}
		}

		if (cell->type.in(ID($reduce_xor), ID($reduce_xnor), ID($shift), ID($shiftx), ID($shl), ID($shr), ID($sshl), ID($sshr),
					ID($lt), ID($le), ID($ge), ID($gt), ID($neg), ID($add), ID($sub), ID($mul), ID($div), ID($mod), ID($pow)))
		{
			RTLIL::SigSpec sig_a = assign_map(cell->getPort(ID::A));
			RTLIL::SigSpec sig_b = cell->hasPort(ID::B) ? assign_map(cell->getPort(ID::B)) : RTLIL::SigSpec();

			if (cell->type.in(ID($shl), ID($shr), ID($sshl), ID($sshr), ID($shift), ID($shiftx)))
				sig_a = RTLIL::SigSpec();

			for (auto &bit : sig_a.to_sigbit_vector())
				if (bit == RTLIL::State::Sx)
					goto found_the_x_bit;

			for (auto &bit : sig_b.to_sigbit_vector())
				if (bit == RTLIL::State::Sx)
					goto found_the_x_bit;

			if (0) {
		found_the_x_bit:
				cover_list("opt.opt_expr.xbit", "$reduce_xor", "$reduce_xnor", "$shl", "$shr", "$sshl", "$sshr", "$shift", "$shiftx",
						"$lt", "$le", "$ge", "$gt", "$neg", "$add", "$sub", "$mul", "$div", "$mod", "$pow", cell->type.str());
				if (cell->type.in(ID($reduce_xor), ID($reduce_xnor), ID($lt), ID($le), ID($ge), ID($gt)))
					replace_cell(assign_map, module, cell, "x-bit in input", ID::Y, RTLIL::State::Sx);
				else
					replace_cell(assign_map, module, cell, "x-bit in input", ID::Y, RTLIL::SigSpec(RTLIL::State::Sx, GetSize(cell->getPort(ID::Y))));
				goto next_cell;
			}
		}

		if (cell->type.in(ID($shiftx), ID($shift))) {
			SigSpec sig_a = assign_map(cell->getPort(ID::A));
			int width;
			bool trim_x = cell->type == ID($shiftx) || !keepdc;
			bool trim_0 = cell->type == ID($shift);
			for (width = GetSize(sig_a); width > 1; width--) {
				if ((trim_x && sig_a[width-1] == State::Sx) ||
					(trim_0 && sig_a[width-1] == State::S0))
					continue;
				break;
			}

			if (width < GetSize(sig_a)) {
				cover_list("opt.opt_expr.trim", "$shiftx", "$shift", cell->type.str());
				sig_a.remove(width, GetSize(sig_a)-width);
				cell->setPort(ID::A, sig_a);
				cell->setParam(ID::A_WIDTH, width);
				did_something = true;
				goto next_cell;
			}
		}

		if (cell->type.in(ID($_NOT_), ID($not), ID($logic_not)) && GetSize(cell->getPort(ID::Y)) == 1 &&
				invert_map.count(assign_map(cell->getPort(ID::A))) != 0) {
			cover_list("opt.opt_expr.invert.double", "$_NOT_", "$not", "$logic_not", cell->type.str());
			replace_cell(assign_map, module, cell, "double_invert", ID::Y, invert_map.at(assign_map(cell->getPort(ID::A))));
			goto next_cell;
		}

		if (cell->type.in(ID($_MUX_), ID($mux)) && invert_map.count(assign_map(cell->getPort(ID::S))) != 0) {
			cover_list("opt.opt_expr.invert.muxsel", "$_MUX_", "$mux", cell->type.str());
			log_debug("Optimizing away select inverter for %s cell `%s' in module `%s'.\n", log_id(cell->type), log_id(cell), log_id(module));
			RTLIL::SigSpec tmp = cell->getPort(ID::A);
			cell->setPort(ID::A, cell->getPort(ID::B));
			cell->setPort(ID::B, tmp);
			cell->setPort(ID::S, invert_map.at(assign_map(cell->getPort(ID::S))));
			did_something = true;
			goto next_cell;
		}

		if (cell->type == ID($_NOT_)) {
			RTLIL::SigSpec input = cell->getPort(ID::A);
			assign_map.apply(input);
			if (input.match("1")) ACTION_DO_Y(0);
			if (input.match("0")) ACTION_DO_Y(1);
			if (input.match("*")) ACTION_DO_Y(x);
		}

		if (cell->type == ID($_AND_)) {
			RTLIL::SigSpec input;
			input.append(cell->getPort(ID::B));
			input.append(cell->getPort(ID::A));
			assign_map.apply(input);
			if (input.match(" 0")) ACTION_DO_Y(0);
			if (input.match("0 ")) ACTION_DO_Y(0);
			if (input.match("11")) ACTION_DO_Y(1);
			if (input.match("**")) ACTION_DO_Y(x);
			if (input.match("1*")) ACTION_DO_Y(x);
			if (input.match("*1")) ACTION_DO_Y(x);
			if (consume_x) {
				if (input.match(" *")) ACTION_DO_Y(0);
				if (input.match("* ")) ACTION_DO_Y(0);
			}
			if (input.match(" 1")) ACTION_DO(ID::Y, input.extract(1, 1));
			if (input.match("1 ")) ACTION_DO(ID::Y, input.extract(0, 1));
		}

		if (cell->type == ID($_OR_)) {
			RTLIL::SigSpec input;
			input.append(cell->getPort(ID::B));
			input.append(cell->getPort(ID::A));
			assign_map.apply(input);
			if (input.match(" 1")) ACTION_DO_Y(1);
			if (input.match("1 ")) ACTION_DO_Y(1);
			if (input.match("00")) ACTION_DO_Y(0);
			if (input.match("**")) ACTION_DO_Y(x);
			if (input.match("0*")) ACTION_DO_Y(x);
			if (input.match("*0")) ACTION_DO_Y(x);
			if (consume_x) {
				if (input.match(" *")) ACTION_DO_Y(1);
				if (input.match("* ")) ACTION_DO_Y(1);
			}
			if (input.match(" 0")) ACTION_DO(ID::Y, input.extract(1, 1));
			if (input.match("0 ")) ACTION_DO(ID::Y, input.extract(0, 1));
		}

		if (cell->type == ID($_XOR_)) {
			RTLIL::SigSpec input;
			input.append(cell->getPort(ID::B));
			input.append(cell->getPort(ID::A));
			assign_map.apply(input);
			if (input.match("00")) ACTION_DO_Y(0);
			if (input.match("01")) ACTION_DO_Y(1);
			if (input.match("10")) ACTION_DO_Y(1);
			if (input.match("11")) ACTION_DO_Y(0);
			if (input.match(" *")) ACTION_DO_Y(x);
			if (input.match("* ")) ACTION_DO_Y(x);
		}

		if (cell->type == ID($_MUX_)) {
			RTLIL::SigSpec input;
			input.append(cell->getPort(ID::S));
			input.append(cell->getPort(ID::B));
			input.append(cell->getPort(ID::A));
			assign_map.apply(input);
			if (input.extract(2, 1) == input.extract(1, 1))
				ACTION_DO(ID::Y, input.extract(2, 1));
			if (input.match("  0")) ACTION_DO(ID::Y, input.extract(2, 1));
			if (input.match("  1")) ACTION_DO(ID::Y, input.extract(1, 1));
			if (input.match("01 ")) ACTION_DO(ID::Y, input.extract(0, 1));
			if (input.match("10 ")) {
				cover("opt.opt_expr.mux_to_inv");
				cell->type = ID($_NOT_);
				cell->setPort(ID::A, input.extract(0, 1));
				cell->unsetPort(ID::B);
				cell->unsetPort(ID::S);
				goto next_cell;
			}
			if (input.match("11 ")) ACTION_DO_Y(1);
			if (input.match("00 ")) ACTION_DO_Y(0);
			if (input.match("** ")) ACTION_DO_Y(x);
			if (input.match("01*")) ACTION_DO_Y(x);
			if (input.match("10*")) ACTION_DO_Y(x);
			if (mux_undef) {
				if (input.match("*  ")) ACTION_DO(ID::Y, input.extract(1, 1));
				if (input.match(" * ")) ACTION_DO(ID::Y, input.extract(2, 1));
				if (input.match("  *")) ACTION_DO(ID::Y, input.extract(2, 1));
			}
		}

		if (cell->type.in(ID($_TBUF_), ID($tribuf))) {
			RTLIL::SigSpec input = cell->getPort(cell->type == ID($_TBUF_) ? ID::E : ID::EN);
			RTLIL::SigSpec a = cell->getPort(ID::A);
			assign_map.apply(input);
			assign_map.apply(a);
			if (input == State::S1)
				ACTION_DO(ID::Y, cell->getPort(ID::A));
			if (input == State::S0 && !a.is_fully_undef()) {
				cover("opt.opt_expr.action_" S__LINE__);
				log_debug("Replacing data input of %s cell `%s' in module `%s' with constant undef.\n",
					cell->type.c_str(), cell->name.c_str(), module->name.c_str());
				cell->setPort(ID::A, SigSpec(State::Sx, GetSize(a)));
				did_something = true;
				goto next_cell;
			}
		}

		if (cell->type.in(ID($eq), ID($ne), ID($eqx), ID($nex)))
		{
			RTLIL::SigSpec a = cell->getPort(ID::A);
			RTLIL::SigSpec b = cell->getPort(ID::B);

			if (cell->parameters[ID::A_WIDTH].as_int() != cell->parameters[ID::B_WIDTH].as_int()) {
				int width = max(cell->parameters[ID::A_WIDTH].as_int(), cell->parameters[ID::B_WIDTH].as_int());
				a.extend_u0(width, cell->parameters[ID::A_SIGNED].as_bool() && cell->parameters[ID::B_SIGNED].as_bool());
				b.extend_u0(width, cell->parameters[ID::A_SIGNED].as_bool() && cell->parameters[ID::B_SIGNED].as_bool());
			}

			RTLIL::SigSpec new_a, new_b;

			log_assert(GetSize(a) == GetSize(b));
			for (int i = 0; i < GetSize(a); i++) {
				if (a[i].wire == NULL && b[i].wire == NULL && a[i] != b[i] && a[i].data <= RTLIL::State::S1 && b[i].data <= RTLIL::State::S1) {
					cover_list("opt.opt_expr.eqneq.isneq", "$eq", "$ne", "$eqx", "$nex", cell->type.str());
					RTLIL::SigSpec new_y = RTLIL::SigSpec(cell->type.in(ID($eq), ID($eqx)) ?  RTLIL::State::S0 : RTLIL::State::S1);
					new_y.extend_u0(cell->parameters[ID::Y_WIDTH].as_int(), false);
					replace_cell(assign_map, module, cell, "isneq", ID::Y, new_y);
					goto next_cell;
				}
				if (a[i] == b[i])
					continue;
				new_a.append(a[i]);
				new_b.append(b[i]);
			}

			if (new_a.size() == 0) {
				cover_list("opt.opt_expr.eqneq.empty", "$eq", "$ne", "$eqx", "$nex", cell->type.str());
				RTLIL::SigSpec new_y = RTLIL::SigSpec(cell->type.in(ID($eq), ID($eqx)) ?  RTLIL::State::S1 : RTLIL::State::S0);
				new_y.extend_u0(cell->parameters[ID::Y_WIDTH].as_int(), false);
				replace_cell(assign_map, module, cell, "empty", ID::Y, new_y);
				goto next_cell;
			}

			if (new_a.size() < a.size() || new_b.size() < b.size()) {
				cover_list("opt.opt_expr.eqneq.resize", "$eq", "$ne", "$eqx", "$nex", cell->type.str());
				cell->setPort(ID::A, new_a);
				cell->setPort(ID::B, new_b);
				cell->parameters[ID::A_WIDTH] = new_a.size();
				cell->parameters[ID::B_WIDTH] = new_b.size();
			}
		}

		if (cell->type.in(ID($eq), ID($ne)) && cell->parameters[ID::Y_WIDTH].as_int() == 1 &&
				cell->parameters[ID::A_WIDTH].as_int() == 1 && cell->parameters[ID::B_WIDTH].as_int() == 1)
		{
			RTLIL::SigSpec a = assign_map(cell->getPort(ID::A));
			RTLIL::SigSpec b = assign_map(cell->getPort(ID::B));

			if (a.is_fully_const() && !b.is_fully_const()) {
				cover_list("opt.opt_expr.eqneq.swapconst", "$eq", "$ne", cell->type.str());
				cell->setPort(ID::A, b);
				cell->setPort(ID::B, a);
				std::swap(a, b);
			}

			if (b.is_fully_const()) {
				if (b.is_fully_undef()) {
					RTLIL::SigSpec input = b;
					ACTION_DO(ID::Y, Const(State::Sx, GetSize(cell->getPort(ID::Y))));
				} else
				if (b.as_bool() == (cell->type == ID($eq))) {
					RTLIL::SigSpec input = b;
					ACTION_DO(ID::Y, cell->getPort(ID::A));
				} else {
					cover_list("opt.opt_expr.eqneq.isnot", "$eq", "$ne", cell->type.str());
					log_debug("Replacing %s cell `%s' in module `%s' with inverter.\n", log_id(cell->type), log_id(cell), log_id(module));
					cell->type = ID($not);
					cell->parameters.erase(ID::B_WIDTH);
					cell->parameters.erase(ID::B_SIGNED);
					cell->unsetPort(ID::B);
					did_something = true;
				}
				goto next_cell;
			}
		}

		if (cell->type.in(ID($eq), ID($ne)) &&
				(assign_map(cell->getPort(ID::A)).is_fully_zero() || assign_map(cell->getPort(ID::B)).is_fully_zero()))
		{
			cover_list("opt.opt_expr.eqneq.cmpzero", "$eq", "$ne", cell->type.str());
			log_debug("Replacing %s cell `%s' in module `%s' with %s.\n", log_id(cell->type), log_id(cell),
					log_id(module), cell->type == ID($eq) ? "$logic_not" : "$reduce_bool");
			cell->type = cell->type == ID($eq) ? ID($logic_not) : ID($reduce_bool);
			if (assign_map(cell->getPort(ID::A)).is_fully_zero()) {
				cell->setPort(ID::A, cell->getPort(ID::B));
				cell->setParam(ID::A_SIGNED, cell->getParam(ID::B_SIGNED));
				cell->setParam(ID::A_WIDTH, cell->getParam(ID::B_WIDTH));
			}
			cell->unsetPort(ID::B);
			cell->unsetParam(ID::B_SIGNED);
			cell->unsetParam(ID::B_WIDTH);
			did_something = true;
			goto next_cell;
		}

		if (cell->type.in(ID($shl), ID($shr), ID($sshl), ID($sshr), ID($shift), ID($shiftx)) && assign_map(cell->getPort(ID::B)).is_fully_const())
		{
			bool sign_ext = cell->type == ID($sshr) && cell->getParam(ID::A_SIGNED).as_bool();
			int shift_bits = assign_map(cell->getPort(ID::B)).as_int(cell->type.in(ID($shift), ID($shiftx)) && cell->getParam(ID::B_SIGNED).as_bool());

			if (cell->type.in(ID($shl), ID($sshl)))
				shift_bits *= -1;

			RTLIL::SigSpec sig_a = assign_map(cell->getPort(ID::A));
			RTLIL::SigSpec sig_y(cell->type == ID($shiftx) ? RTLIL::State::Sx : RTLIL::State::S0, cell->getParam(ID::Y_WIDTH).as_int());

			if (GetSize(sig_a) < GetSize(sig_y))
				sig_a.extend_u0(GetSize(sig_y), cell->getParam(ID::A_SIGNED).as_bool());

			for (int i = 0; i < GetSize(sig_y); i++) {
				int idx = i + shift_bits;
				if (0 <= idx && idx < GetSize(sig_a))
					sig_y[i] = sig_a[idx];
				else if (GetSize(sig_a) <= idx && sign_ext)
					sig_y[i] = sig_a[GetSize(sig_a)-1];
			}

			cover_list("opt.opt_expr.constshift", "$shl", "$shr", "$sshl", "$sshr", "$shift", "$shiftx", cell->type.str());

			log_debug("Replacing %s cell `%s' (B=%s, SHR=%d) in module `%s' with fixed wiring: %s\n",
					log_id(cell->type), log_id(cell), log_signal(assign_map(cell->getPort(ID::B))), shift_bits, log_id(module), log_signal(sig_y));

			module->connect(cell->getPort(ID::Y), sig_y);
			module->remove(cell);

			did_something = true;
			goto next_cell;
		}

		if (!keepdc)
		{
			bool identity_wrt_a = false;
			bool identity_wrt_b = false;
			bool arith_inverse = false;

			if (cell->type.in(ID($add), ID($sub), ID($alu), ID($or), ID($xor)))
			{
				RTLIL::SigSpec a = assign_map(cell->getPort(ID::A));
				RTLIL::SigSpec b = assign_map(cell->getPort(ID::B));

				bool sub = cell->type == ID($sub);

				if (cell->type == ID($alu)) {
					RTLIL::SigBit sig_ci = assign_map(cell->getPort(ID::CI));
					RTLIL::SigBit sig_bi = assign_map(cell->getPort(ID::BI));

					sub = (sig_ci == State::S1 && sig_bi == State::S1);

					// If not a subtraction, yet there is a carry or B is inverted
					//   then no optimisation is possible as carry will not be constant
					if (!sub && (sig_ci != State::S0 || sig_bi != State::S0))
						goto next_cell;
				}

				if (!sub && a.is_fully_const() && a.as_bool() == false)
					identity_wrt_b = true;

				if (b.is_fully_const() && b.as_bool() == false)
					identity_wrt_a = true;
			}

			if (cell->type.in(ID($shl), ID($shr), ID($sshl), ID($sshr), ID($shift), ID($shiftx)))
			{
				RTLIL::SigSpec b = assign_map(cell->getPort(ID::B));

				if (b.is_fully_const() && b.as_bool() == false)
					identity_wrt_a = true;
			}

			if (cell->type == ID($mul))
			{
				RTLIL::SigSpec a = assign_map(cell->getPort(ID::A));
				RTLIL::SigSpec b = assign_map(cell->getPort(ID::B));

				if (a.is_fully_const() && is_one_or_minus_one(a.as_const(), cell->getParam(ID::A_SIGNED).as_bool(), arith_inverse))
					identity_wrt_b = true;
				else
				if (b.is_fully_const() && is_one_or_minus_one(b.as_const(), cell->getParam(ID::B_SIGNED).as_bool(), arith_inverse))
					identity_wrt_a = true;
			}

			if (cell->type == ID($div))
			{
				RTLIL::SigSpec b = assign_map(cell->getPort(ID::B));

				if (b.is_fully_const() && b.size() <= 32 && b.as_int() == 1)
					identity_wrt_a = true;
			}

			if (identity_wrt_a || identity_wrt_b)
			{
				if (identity_wrt_a)
					cover_list("opt.opt_expr.identwrt.a", "$add", "$sub", "$alu", "$or", "$xor", "$shl", "$shr", "$sshl", "$sshr", "$shift", "$shiftx", "$mul", "$div", cell->type.str());
				if (identity_wrt_b)
					cover_list("opt.opt_expr.identwrt.b", "$add", "$sub", "$alu", "$or", "$xor", "$shl", "$shr", "$sshl", "$sshr", "$shift", "$shiftx", "$mul", "$div", cell->type.str());

				log_debug("Replacing %s cell `%s' in module `%s' with identity for port %c.\n",
					cell->type.c_str(), cell->name.c_str(), module->name.c_str(), identity_wrt_a ? 'A' : 'B');

				if (cell->type == ID($alu)) {
					int y_width = GetSize(cell->getPort(ID::Y));
					module->connect(cell->getPort(ID::X), RTLIL::Const(State::S0, y_width));
					module->connect(cell->getPort(ID::CO), RTLIL::Const(State::S0, y_width));
					cell->unsetPort(ID::BI);
					cell->unsetPort(ID::CI);
					cell->unsetPort(ID::X);
					cell->unsetPort(ID::CO);
				}

				if (!identity_wrt_a) {
					cell->setPort(ID::A, cell->getPort(ID::B));
					cell->setParam(ID::A_WIDTH, cell->getParam(ID::B_WIDTH));
					cell->setParam(ID::A_SIGNED, cell->getParam(ID::B_SIGNED));
				}

				cell->type = arith_inverse ? ID($neg) : ID($pos);
				cell->unsetPort(ID::B);
				cell->parameters.erase(ID::B_WIDTH);
				cell->parameters.erase(ID::B_SIGNED);
				cell->check();

				did_something = true;
				goto next_cell;
			}
		}

		if (mux_bool && cell->type.in(ID($mux), ID($_MUX_)) &&
				cell->getPort(ID::A) == State::S0 && cell->getPort(ID::B) == State::S1) {
			cover_list("opt.opt_expr.mux_bool", "$mux", "$_MUX_", cell->type.str());
			replace_cell(assign_map, module, cell, "mux_bool", ID::Y, cell->getPort(ID::S));
			goto next_cell;
		}

		if (mux_bool && cell->type.in(ID($mux), ID($_MUX_)) &&
				cell->getPort(ID::A) == State::S1 && cell->getPort(ID::B) == State::S0) {
			cover_list("opt.opt_expr.mux_invert", "$mux", "$_MUX_", cell->type.str());
			log_debug("Replacing %s cell `%s' in module `%s' with inverter.\n", log_id(cell->type), log_id(cell), log_id(module));
			cell->setPort(ID::A, cell->getPort(ID::S));
			cell->unsetPort(ID::B);
			cell->unsetPort(ID::S);
			if (cell->type == ID($mux)) {
				Const width = cell->parameters[ID::WIDTH];
				cell->parameters[ID::A_WIDTH] = width;
				cell->parameters[ID::Y_WIDTH] = width;
				cell->parameters[ID::A_SIGNED] = 0;
				cell->parameters.erase(ID::WIDTH);
				cell->type = ID($not);
			} else
				cell->type = ID($_NOT_);
			did_something = true;
			goto next_cell;
		}

		if (consume_x && mux_bool && cell->type.in(ID($mux), ID($_MUX_)) && cell->getPort(ID::A) == State::S0) {
			cover_list("opt.opt_expr.mux_and", "$mux", "$_MUX_", cell->type.str());
			log_debug("Replacing %s cell `%s' in module `%s' with and-gate.\n", log_id(cell->type), log_id(cell), log_id(module));
			cell->setPort(ID::A, cell->getPort(ID::S));
			cell->unsetPort(ID::S);
			if (cell->type == ID($mux)) {
				Const width = cell->parameters[ID::WIDTH];
				cell->parameters[ID::A_WIDTH] = width;
				cell->parameters[ID::B_WIDTH] = width;
				cell->parameters[ID::Y_WIDTH] = width;
				cell->parameters[ID::A_SIGNED] = 0;
				cell->parameters[ID::B_SIGNED] = 0;
				cell->parameters.erase(ID::WIDTH);
				cell->type = ID($and);
			} else
				cell->type = ID($_AND_);
			did_something = true;
			goto next_cell;
		}

		if (consume_x && mux_bool && cell->type.in(ID($mux), ID($_MUX_)) && cell->getPort(ID::B) == State::S1) {
			cover_list("opt.opt_expr.mux_or", "$mux", "$_MUX_", cell->type.str());
			log_debug("Replacing %s cell `%s' in module `%s' with or-gate.\n", log_id(cell->type), log_id(cell), log_id(module));
			cell->setPort(ID::B, cell->getPort(ID::S));
			cell->unsetPort(ID::S);
			if (cell->type == ID($mux)) {
				Const width = cell->parameters[ID::WIDTH];
				cell->parameters[ID::A_WIDTH] = width;
				cell->parameters[ID::B_WIDTH] = width;
				cell->parameters[ID::Y_WIDTH] = width;
				cell->parameters[ID::A_SIGNED] = 0;
				cell->parameters[ID::B_SIGNED] = 0;
				cell->parameters.erase(ID::WIDTH);
				cell->type = ID($or);
			} else
				cell->type = ID($_OR_);
			did_something = true;
			goto next_cell;
		}

		if (mux_undef && cell->type.in(ID($mux), ID($pmux))) {
			RTLIL::SigSpec new_a, new_b, new_s;
			int width = GetSize(cell->getPort(ID::A));
			if ((cell->getPort(ID::A).is_fully_undef() && cell->getPort(ID::B).is_fully_undef()) ||
					cell->getPort(ID::S).is_fully_undef()) {
				cover_list("opt.opt_expr.mux_undef", "$mux", "$pmux", cell->type.str());
				replace_cell(assign_map, module, cell, "mux_undef", ID::Y, cell->getPort(ID::A));
				goto next_cell;
			}
			for (int i = 0; i < cell->getPort(ID::S).size(); i++) {
				RTLIL::SigSpec old_b = cell->getPort(ID::B).extract(i*width, width);
				RTLIL::SigSpec old_s = cell->getPort(ID::S).extract(i, 1);
				if (old_b.is_fully_undef() || old_s.is_fully_undef())
					continue;
				new_b.append(old_b);
				new_s.append(old_s);
			}
			new_a = cell->getPort(ID::A);
			if (new_a.is_fully_undef() && new_s.size() > 0) {
				new_a = new_b.extract((new_s.size()-1)*width, width);
				new_b = new_b.extract(0, (new_s.size()-1)*width);
				new_s = new_s.extract(0, new_s.size()-1);
			}
			if (new_s.size() == 0) {
				cover_list("opt.opt_expr.mux_empty", "$mux", "$pmux", cell->type.str());
				replace_cell(assign_map, module, cell, "mux_empty", ID::Y, new_a);
				goto next_cell;
			}
			if (new_a == RTLIL::SigSpec(RTLIL::State::S0) && new_b == RTLIL::SigSpec(RTLIL::State::S1)) {
				cover_list("opt.opt_expr.mux_sel01", "$mux", "$pmux", cell->type.str());
				replace_cell(assign_map, module, cell, "mux_sel01", ID::Y, new_s);
				goto next_cell;
			}
			if (cell->getPort(ID::S).size() != new_s.size()) {
				cover_list("opt.opt_expr.mux_reduce", "$mux", "$pmux", cell->type.str());
				log_debug("Optimized away %d select inputs of %s cell `%s' in module `%s'.\n",
						GetSize(cell->getPort(ID::S)) - GetSize(new_s), log_id(cell->type), log_id(cell), log_id(module));
				cell->setPort(ID::A, new_a);
				cell->setPort(ID::B, new_b);
				cell->setPort(ID::S, new_s);
				if (new_s.size() > 1) {
					cell->type = ID($pmux);
					cell->parameters[ID::S_WIDTH] = new_s.size();
				} else {
					cell->type = ID($mux);
					cell->parameters.erase(ID::S_WIDTH);
				}
				did_something = true;
			}
		}

#define FOLD_1ARG_CELL(_t) \
		if (cell->type == ID($##_t)) { \
			RTLIL::SigSpec a = cell->getPort(ID::A); \
			assign_map.apply(a); \
			if (a.is_fully_const()) { \
				RTLIL::Const dummy_arg(RTLIL::State::S0, 1); \
				RTLIL::SigSpec y(RTLIL::const_ ## _t(a.as_const(), dummy_arg, \
						cell->parameters[ID::A_SIGNED].as_bool(), false, \
						cell->parameters[ID::Y_WIDTH].as_int())); \
				cover("opt.opt_expr.const.$" #_t); \
				replace_cell(assign_map, module, cell, stringf("%s", log_signal(a)), ID::Y, y); \
				goto next_cell; \
			} \
		}
#define FOLD_2ARG_CELL(_t) \
		if (cell->type == ID($##_t)) { \
			RTLIL::SigSpec a = cell->getPort(ID::A); \
			RTLIL::SigSpec b = cell->getPort(ID::B); \
			assign_map.apply(a), assign_map.apply(b); \
			if (a.is_fully_const() && b.is_fully_const()) { \
				RTLIL::SigSpec y(RTLIL::const_ ## _t(a.as_const(), b.as_const(), \
						cell->parameters[ID::A_SIGNED].as_bool(), \
						cell->parameters[ID::B_SIGNED].as_bool(), \
						cell->parameters[ID::Y_WIDTH].as_int())); \
				cover("opt.opt_expr.const.$" #_t); \
				replace_cell(assign_map, module, cell, stringf("%s, %s", log_signal(a), log_signal(b)), ID::Y, y); \
				goto next_cell; \
			} \
		}

		FOLD_1ARG_CELL(not)
		FOLD_2ARG_CELL(and)
		FOLD_2ARG_CELL(or)
		FOLD_2ARG_CELL(xor)
		FOLD_2ARG_CELL(xnor)

		FOLD_1ARG_CELL(reduce_and)
		FOLD_1ARG_CELL(reduce_or)
		FOLD_1ARG_CELL(reduce_xor)
		FOLD_1ARG_CELL(reduce_xnor)
		FOLD_1ARG_CELL(reduce_bool)

		FOLD_1ARG_CELL(logic_not)
		FOLD_2ARG_CELL(logic_and)
		FOLD_2ARG_CELL(logic_or)

		FOLD_2ARG_CELL(shl)
		FOLD_2ARG_CELL(shr)
		FOLD_2ARG_CELL(sshl)
		FOLD_2ARG_CELL(sshr)
		FOLD_2ARG_CELL(shift)
		FOLD_2ARG_CELL(shiftx)

		FOLD_2ARG_CELL(lt)
		FOLD_2ARG_CELL(le)
		FOLD_2ARG_CELL(eq)
		FOLD_2ARG_CELL(ne)
		FOLD_2ARG_CELL(gt)
		FOLD_2ARG_CELL(ge)

		FOLD_2ARG_CELL(add)
		FOLD_2ARG_CELL(sub)
		FOLD_2ARG_CELL(mul)
		FOLD_2ARG_CELL(div)
		FOLD_2ARG_CELL(mod)
		FOLD_2ARG_CELL(pow)

		FOLD_1ARG_CELL(pos)
		FOLD_1ARG_CELL(neg)

		// be very conservative with optimizing $mux cells as we do not want to break mux trees
		if (cell->type == ID($mux)) {
			RTLIL::SigSpec input = assign_map(cell->getPort(ID::S));
			RTLIL::SigSpec inA = assign_map(cell->getPort(ID::A));
			RTLIL::SigSpec inB = assign_map(cell->getPort(ID::B));
			if (input.is_fully_const())
				ACTION_DO(ID::Y, input.as_bool() ? cell->getPort(ID::B) : cell->getPort(ID::A));
			else if (inA == inB)
				ACTION_DO(ID::Y, cell->getPort(ID::A));
		}

		if (!keepdc && cell->type == ID($mul))
		{
			bool a_signed = cell->parameters[ID::A_SIGNED].as_bool();
			bool b_signed = cell->parameters[ID::B_SIGNED].as_bool();
			bool swapped_ab = false;

			RTLIL::SigSpec sig_a = assign_map(cell->getPort(ID::A));
			RTLIL::SigSpec sig_b = assign_map(cell->getPort(ID::B));
			RTLIL::SigSpec sig_y = assign_map(cell->getPort(ID::Y));

			if (sig_b.is_fully_const() && sig_b.size() <= 32)
				std::swap(sig_a, sig_b), std::swap(a_signed, b_signed), swapped_ab = true;

			if (sig_a.is_fully_def() && sig_a.size() <= 32)
			{
				int a_val = sig_a.as_int();

				if (a_val == 0)
				{
					cover("opt.opt_expr.mul_shift.zero");

					log_debug("Replacing multiply-by-zero cell `%s' in module `%s' with zero-driver.\n",
							cell->name.c_str(), module->name.c_str());

					module->connect(RTLIL::SigSig(sig_y, RTLIL::SigSpec(0, sig_y.size())));
					module->remove(cell);

					did_something = true;
					goto next_cell;
				}

				for (int i = 1; i < (a_signed ? sig_a.size()-1 : sig_a.size()); i++)
					if (a_val == (1 << i))
					{
						if (swapped_ab)
							cover("opt.opt_expr.mul_shift.swapped");
						else
							cover("opt.opt_expr.mul_shift.unswapped");

						log_debug("Replacing multiply-by-%d cell `%s' in module `%s' with shift-by-%d.\n",
								a_val, cell->name.c_str(), module->name.c_str(), i);

						if (!swapped_ab) {
							cell->setPort(ID::A, cell->getPort(ID::B));
							cell->parameters.at(ID::A_WIDTH) = cell->parameters.at(ID::B_WIDTH);
							cell->parameters.at(ID::A_SIGNED) = cell->parameters.at(ID::B_SIGNED);
						}

						std::vector<RTLIL::SigBit> new_b = RTLIL::SigSpec(i, 6);

						while (GetSize(new_b) > 1 && new_b.back() == RTLIL::State::S0)
							new_b.pop_back();

						cell->type = ID($shl);
						cell->parameters[ID::B_WIDTH] = GetSize(new_b);
						cell->parameters[ID::B_SIGNED] = false;
						cell->setPort(ID::B, new_b);
						cell->check();

						did_something = true;
						goto next_cell;
					}
			}
		}

		if (!keepdc && cell->type.in(ID($div), ID($mod)))
		{
			bool b_signed = cell->parameters[ID::B_SIGNED].as_bool();
			SigSpec sig_b = assign_map(cell->getPort(ID::B));
			SigSpec sig_y = assign_map(cell->getPort(ID::Y));

			if (sig_b.is_fully_def() && sig_b.size() <= 32)
			{
				int b_val = sig_b.as_int();

				if (b_val == 0)
				{
					cover("opt.opt_expr.divmod_zero");

					log_debug("Replacing divide-by-zero cell `%s' in module `%s' with undef-driver.\n",
							cell->name.c_str(), module->name.c_str());

					module->connect(RTLIL::SigSig(sig_y, RTLIL::SigSpec(State::Sx, sig_y.size())));
					module->remove(cell);

					did_something = true;
					goto next_cell;
				}

				for (int i = 1; i < (b_signed ? sig_b.size()-1 : sig_b.size()); i++)
					if (b_val == (1 << i))
					{
						if (cell->type == ID($div))
						{
							cover("opt.opt_expr.div_shift");

							log_debug("Replacing divide-by-%d cell `%s' in module `%s' with shift-by-%d.\n",
									b_val, cell->name.c_str(), module->name.c_str(), i);

							std::vector<RTLIL::SigBit> new_b = RTLIL::SigSpec(i, 6);

							while (GetSize(new_b) > 1 && new_b.back() == RTLIL::State::S0)
								new_b.pop_back();

							cell->type = ID($shr);
							cell->parameters[ID::B_WIDTH] = GetSize(new_b);
							cell->parameters[ID::B_SIGNED] = false;
							cell->setPort(ID::B, new_b);
							cell->check();
						}
						else
						{
							cover("opt.opt_expr.mod_mask");

							log_debug("Replacing modulo-by-%d cell `%s' in module `%s' with bitmask.\n",
									b_val, cell->name.c_str(), module->name.c_str());

							std::vector<RTLIL::SigBit> new_b = RTLIL::SigSpec(State::S1, i);

							if (b_signed)
								new_b.push_back(State::S0);

							cell->type = ID($and);
							cell->parameters[ID::B_WIDTH] = GetSize(new_b);
							cell->setPort(ID::B, new_b);
							cell->check();
						}

						did_something = true;
						goto next_cell;
					}
			}
		}

		// remove redundant pairs of bits in ==, ===, !=, and !==
		// replace cell with const driver if inputs can't be equal
		if (do_fine && cell->type.in(ID($eq), ID($ne), ID($eqx), ID($nex)))
		{
			pool<pair<SigBit, SigBit>> redundant_cache;
			mfp<SigBit> contradiction_cache;

			contradiction_cache.promote(State::S0);
			contradiction_cache.promote(State::S1);

			int a_width = cell->getParam(ID::A_WIDTH).as_int();
			int b_width = cell->getParam(ID::B_WIDTH).as_int();

			bool is_signed = cell->getParam(ID::A_SIGNED).as_bool();
			int width = is_signed ? std::min(a_width, b_width) : std::max(a_width, b_width);

			SigSpec sig_a = cell->getPort(ID::A);
			SigSpec sig_b = cell->getPort(ID::B);

			int redundant_bits = 0;

			for (int i = width-1; i >= 0; i--)
			{
				SigBit bit_a = i < a_width ? assign_map(sig_a[i]) : State::S0;
				SigBit bit_b = i < b_width ? assign_map(sig_b[i]) : State::S0;

				if (bit_a != State::Sx && bit_a != State::Sz &&
						bit_b != State::Sx && bit_b != State::Sz)
					contradiction_cache.merge(bit_a, bit_b);

				if (bit_b < bit_a)
					std::swap(bit_a, bit_b);

				pair<SigBit, SigBit> key(bit_a, bit_b);

				if (redundant_cache.count(key)) {
					if (i < a_width) sig_a.remove(i);
					if (i < b_width) sig_b.remove(i);
					redundant_bits++;
					continue;
				}

				redundant_cache.insert(key);
			}

			if (contradiction_cache.find(State::S0) == contradiction_cache.find(State::S1))
			{
				SigSpec y_sig = cell->getPort(ID::Y);
				Const y_value(cell->type.in(ID($eq), ID($eqx)) ? 0 : 1, GetSize(y_sig));

				log_debug("Replacing cell `%s' in module `%s' with constant driver %s.\n",
					log_id(cell), log_id(module), log_signal(y_value));

				module->connect(y_sig, y_value);
				module->remove(cell);

				did_something = true;
				goto next_cell;
			}

			if (redundant_bits)
			{
				log_debug("Removed %d redundant input bits from %s cell `%s' in module `%s'.\n",
						redundant_bits, log_id(cell->type), log_id(cell), log_id(module));

				cell->setPort(ID::A, sig_a);
				cell->setPort(ID::B, sig_b);
				cell->setParam(ID::A_WIDTH, GetSize(sig_a));
				cell->setParam(ID::B_WIDTH, GetSize(sig_b));

				did_something = true;
				goto next_cell;
			}
		}

		// simplify comparisons
		if (do_fine && cell->type.in(ID($lt), ID($ge), ID($gt), ID($le)))
		{
			IdString cmp_type = cell->type;
			SigSpec var_sig = cell->getPort(ID::A);
			SigSpec const_sig = cell->getPort(ID::B);
			int var_width = cell->parameters[ID::A_WIDTH].as_int();
			int const_width = cell->parameters[ID::B_WIDTH].as_int();
			bool is_signed = cell->getParam(ID::A_SIGNED).as_bool();

			if (!const_sig.is_fully_const())
			{
				std::swap(var_sig, const_sig);
				std::swap(var_width, const_width);
				if (cmp_type == ID($gt))
					cmp_type = ID($lt);
				else if (cmp_type == ID($lt))
					cmp_type = ID($gt);
				else if (cmp_type == ID($ge))
					cmp_type = ID($le);
				else if (cmp_type == ID($le))
					cmp_type = ID($ge);
			}

			if (const_sig.is_fully_def() && const_sig.is_fully_const())
			{
				std::string condition, replacement;
				SigSpec replace_sig(State::S0, GetSize(cell->getPort(ID::Y)));
				bool replace = false;
				bool remove = false;

				if (!is_signed)
				{ /* unsigned */
					if (const_sig.is_fully_zero() && cmp_type == ID($lt)) {
						condition   = "unsigned X<0";
						replacement = "constant 0";
						replace_sig[0] = State::S0;
						replace = true;
					}
					if (const_sig.is_fully_zero() && cmp_type == ID($ge)) {
						condition   = "unsigned X>=0";
						replacement = "constant 1";
						replace_sig[0] = State::S1;
						replace = true;
					}
					if (const_width == var_width && const_sig.is_fully_ones() && cmp_type == ID($gt)) {
						condition   = "unsigned X>~0";
						replacement = "constant 0";
						replace_sig[0] = State::S0;
						replace = true;
					}
					if (const_width == var_width && const_sig.is_fully_ones() && cmp_type == ID($le)) {
						condition   = "unsigned X<=~0";
						replacement = "constant 1";
						replace_sig[0] = State::S1;
						replace = true;
					}

					int const_bit_hot = get_onehot_bit_index(const_sig);
					if (const_bit_hot >= 0 && const_bit_hot < var_width)
					{
						RTLIL::SigSpec var_high_sig(RTLIL::State::S0, var_width - const_bit_hot);
						for (int i = const_bit_hot; i < var_width; i++) {
							var_high_sig[i - const_bit_hot] = var_sig[i];
						}

						if (cmp_type == ID($lt))
						{
							condition   = stringf("unsigned X<%s", log_signal(const_sig));
							replacement = stringf("!X[%d:%d]", var_width - 1, const_bit_hot);
							module->addLogicNot(NEW_ID, var_high_sig, cell->getPort(ID::Y));
							remove = true;
						}
						if (cmp_type == ID($ge))
						{
							condition   = stringf("unsigned X>=%s", log_signal(const_sig));
							replacement = stringf("|X[%d:%d]", var_width - 1, const_bit_hot);
							module->addReduceOr(NEW_ID, var_high_sig, cell->getPort(ID::Y));
							remove = true;
						}
					}

					int const_bit_set = get_highest_hot_index(const_sig);
					if (const_bit_set >= var_width)
					{
						string cmp_name;
						if (cmp_type == ID($lt) || cmp_type == ID($le))
						{
							if (cmp_type == ID($lt)) cmp_name = "<";
							if (cmp_type == ID($le)) cmp_name = "<=";
							condition   = stringf("unsigned X[%d:0]%s%s", var_width - 1, cmp_name.c_str(), log_signal(const_sig));
							replacement = "constant 1";
							replace_sig[0] = State::S1;
							replace = true;
						}
						if (cmp_type == ID($gt) || cmp_type == ID($ge))
						{
							if (cmp_type == ID($gt)) cmp_name = ">";
							if (cmp_type == ID($ge)) cmp_name = ">=";
							condition   = stringf("unsigned X[%d:0]%s%s", var_width - 1, cmp_name.c_str(), log_signal(const_sig));
							replacement = "constant 0";
							replace_sig[0] = State::S0;
							replace = true;
						}
					}
				}
				else
				{ /* signed */
					if (const_sig.is_fully_zero() && cmp_type == ID($lt))
					{
						condition   = "signed X<0";
						replacement = stringf("X[%d]", var_width - 1);
						replace_sig[0] = var_sig[var_width - 1];
						replace = true;
					}
					if (const_sig.is_fully_zero() && cmp_type == ID($ge))
					{
						condition   = "signed X>=0";
						replacement = stringf("X[%d]", var_width - 1);
						module->addNot(NEW_ID, var_sig[var_width - 1], cell->getPort(ID::Y));
						remove = true;
					}
				}

				if (replace || remove)
				{
					log_debug("Replacing %s cell `%s' (implementing %s) with %s.\n",
							log_id(cell->type), log_id(cell), condition.c_str(), replacement.c_str());
					if (replace)
						module->connect(cell->getPort(ID::Y), replace_sig);
					module->remove(cell);
					did_something = true;
					goto next_cell;
				}
			}
		}

	next_cell:;
#undef ACTION_DO
#undef ACTION_DO_Y
#undef FOLD_1ARG_CELL
#undef FOLD_2ARG_CELL
	}
}

struct OptExprPass : public Pass {
	OptExprPass() : Pass("opt_expr", "perform const folding and simple expression rewriting") { }
	void help() YS_OVERRIDE
	{
		//   |---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|
		log("\n");
		log("    opt_expr [options] [selection]\n");
		log("\n");
		log("This pass performs const folding on internal cell types with constant inputs.\n");
		log("It also performs some simple expression rewriting.\n");
		log("\n");
		log("    -mux_undef\n");
		log("        remove 'undef' inputs from $mux, $pmux and $_MUX_ cells\n");
		log("\n");
		log("    -mux_bool\n");
		log("        replace $mux cells with inverters or buffers when possible\n");
		log("\n");
		log("    -undriven\n");
		log("        replace undriven nets with undef (x) constants\n");
		log("\n");
		log("    -clkinv\n");
		log("        optimize clock inverters by changing FF types\n");
		log("\n");
		log("    -fine\n");
		log("        perform fine-grain optimizations\n");
		log("\n");
		log("    -full\n");
		log("        alias for -mux_undef -mux_bool -undriven -fine\n");
		log("\n");
		log("    -keepdc\n");
		log("        some optimizations change the behavior of the circuit with respect to\n");
		log("        don't-care bits. for example in 'a+0' a single x-bit in 'a' will cause\n");
		log("        all result bits to be set to x. this behavior changes when 'a+0' is\n");
		log("        replaced by 'a'. the -keepdc option disables all such optimizations.\n");
		log("\n");
	}
	void execute(std::vector<std::string> args, RTLIL::Design *design) YS_OVERRIDE
	{
		bool mux_undef = false;
		bool mux_bool = false;
		bool undriven = false;
		bool clkinv = false;
		bool do_fine = false;
		bool keepdc = false;

		log_header(design, "Executing OPT_EXPR pass (perform const folding).\n");
		log_push();

		size_t argidx;
		for (argidx = 1; argidx < args.size(); argidx++) {
			if (args[argidx] == "-mux_undef") {
				mux_undef = true;
				continue;
			}
			if (args[argidx] == "-mux_bool") {
				mux_bool = true;
				continue;
			}
			if (args[argidx] == "-undriven") {
				undriven = true;
				continue;
			}
			if (args[argidx] == "-clkinv") {
				clkinv = true;
				continue;
			}
			if (args[argidx] == "-fine") {
				do_fine = true;
				continue;
			}
			if (args[argidx] == "-full") {
				mux_undef = true;
				mux_bool = true;
				undriven = true;
				do_fine = true;
				continue;
			}
			if (args[argidx] == "-keepdc") {
				keepdc = true;
				continue;
			}
			break;
		}
		extra_args(args, argidx, design);

		CellTypes ct(design);
		for (auto module : design->selected_modules())
		{
			log("Optimizing module %s.\n", log_id(module));

			if (undriven) {
				did_something = false;
				replace_undriven(module, ct);
				if (did_something)
					design->scratchpad_set_bool("opt.did_something", true);
			}

			do {
				do {
					did_something = false;
					replace_const_cells(design, module, false, mux_undef, mux_bool, do_fine, keepdc, clkinv);
					if (did_something)
						design->scratchpad_set_bool("opt.did_something", true);
				} while (did_something);
				replace_const_cells(design, module, true, mux_undef, mux_bool, do_fine, keepdc, clkinv);
				if (did_something)
					design->scratchpad_set_bool("opt.did_something", true);
			} while (did_something);

			log_suppressed();
		}

		log_pop();
	}
} OptExprPass;

PRIVATE_NAMESPACE_END