Add opt_dff pass.

12 files changed, 1790 insertions, 3 deletions

diff --git a/passes/opt/Makefile.inc b/passes/opt/Makefile.inc
index 3133927bb..64fee76b3 100644
--- a/passes/opt/Makefile.inc
+++ b/passes/opt/Makefile.inc
@@ -5,6 +5,7 @@ OBJS += passes/opt/opt_mem.o
 OBJS += passes/opt/opt_muxtree.o
 OBJS += passes/opt/opt_reduce.o
 OBJS += passes/opt/opt_rmdff.o
+OBJS += passes/opt/opt_dff.o
 OBJS += passes/opt/opt_share.o
 OBJS += passes/opt/opt_clean.o
 OBJS += passes/opt/opt_expr.o
diff --git a/passes/opt/opt_dff.cc b/passes/opt/opt_dff.cc
new file mode 100644
index 000000000..a47071a30
--- /dev/null
+++ b/passes/opt/opt_dff.cc
@@ -0,0 +1,875 @@
+/*
+ *  yosys -- Yosys Open SYnthesis Suite
+ *
+ *  Copyright (C) 2012  Clifford Wolf <clifford@clifford.at>
+ *  Copyright (C) 2020  Marcelina Kościelnicka <mwk@0x04.net>
+ *
+ *  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/log.h"
+#include "kernel/register.h"
+#include "kernel/rtlil.h"
+#include "kernel/satgen.h"
+#include "kernel/sigtools.h"
+#include "kernel/ffinit.h"
+#include "kernel/ff.h"
+#include "passes/techmap/simplemap.h"
+#include <stdio.h>
+#include <stdlib.h>
+
+USING_YOSYS_NAMESPACE
+PRIVATE_NAMESPACE_BEGIN
+
+struct OptDffOptions
+{
+	bool nosdff;
+	bool nodffe;
+	bool simple_dffe;
+	bool sat;
+	bool keepdc;
+};
+
+struct OptDffWorker
+{
+	const OptDffOptions &opt;
+
+	Module *module;
+	typedef std::pair<RTLIL::Cell*, int> cell_int_t;
+	SigMap sigmap;
+	FfInitVals initvals;
+	dict<SigBit, int> bitusers;
+	dict<SigBit, cell_int_t> bit2mux;
+	dict<SigBit, RTLIL::Cell*> bit2driver;
+
+	typedef std::map<RTLIL::SigBit, bool> pattern_t;
+	typedef std::set<pattern_t> patterns_t;
+	typedef std::pair<RTLIL::SigBit, bool> ctrl_t;
+	typedef std::set<ctrl_t> ctrls_t;
+
+	ezSatPtr ez;
+	SatGen satgen;
+	pool<Cell*> sat_cells;
+
+	// Used as a queue.
+	std::vector<Cell *> dff_cells;
+
+	OptDffWorker(const OptDffOptions &opt, Module *mod) : opt(opt), module(mod), sigmap(mod), initvals(&sigmap, mod), ez(), satgen(ez.get(), &sigmap) {
+		// Gathering three kinds of information here for every sigmapped SigBit:
+		//
+		// - bitusers: how many users it has (muxes will only be merged into FFs if this is 1, making the FF the only user)
+		// - bit2mux: the mux cell and bit index that drives it, if any
+		// - bit2driver: the cell driving it, if any
+
+		for (auto wire : module->wires())
+		{
+			if (wire->port_output)
+				for (auto bit : sigmap(wire))
+					bitusers[bit]++;
+		}
+
+		for (auto cell : module->cells()) {
+			if (cell->type.in(ID($mux), ID($pmux), ID($_MUX_))) {
+				RTLIL::SigSpec sig_y = sigmap(cell->getPort(ID::Y));
+				for (int i = 0; i < GetSize(sig_y); i++)
+					bit2mux[sig_y[i]] = cell_int_t(cell, i);
+			}
+
+			for (auto conn : cell->connections()) {
+				bool is_output = cell->output(conn.first);
+				if (is_output) {
+					for (auto bit : sigmap(conn.second))
+						bit2driver[bit] = cell;
+				}
+				if (!is_output || !cell->known()) {
+					for (auto bit : sigmap(conn.second))
+						bitusers[bit]++;
+				}
+			}
+
+			if (module->design->selected(module, cell) && RTLIL::builtin_ff_cell_types().count(cell->type))
+				dff_cells.push_back(cell);
+		}
+
+	}
+
+	std::function<void(Cell*)> sat_import_cell = [&](Cell *c) {
+		if (!sat_cells.insert(c).second)
+			return;
+		if (!satgen.importCell(c))
+			return;
+		for (auto &conn : c->connections()) {
+			if (!c->input(conn.first))
+				continue;
+			for (auto bit : sigmap(conn.second))
+				if (bit2driver.count(bit))
+					sat_import_cell(bit2driver.at(bit));
+		}
+	};
+
+	State combine_const(State a, State b) {
+		if (a == State::Sx && !opt.keepdc)
+			return b;
+		if (b == State::Sx && !opt.keepdc)
+			return a;
+		if (a == b)
+			return a;
+		return State::Sm;
+	}
+
+	patterns_t find_muxtree_feedback_patterns(RTLIL::SigBit d, RTLIL::SigBit q, pattern_t path)
+	{
+		patterns_t ret;
+
+		if (d == q) {
+			ret.insert(path);
+			return ret;
+		}
+
+		if (bit2mux.count(d) == 0 || bitusers[d] > 1)
+			return ret;
+
+		cell_int_t mbit = bit2mux.at(d);
+		RTLIL::SigSpec sig_a = sigmap(mbit.first->getPort(ID::A));
+		RTLIL::SigSpec sig_b = sigmap(mbit.first->getPort(ID::B));
+		RTLIL::SigSpec sig_s = sigmap(mbit.first->getPort(ID::S));
+		int width = GetSize(sig_a), index = mbit.second;
+
+		for (int i = 0; i < GetSize(sig_s); i++)
+			if (path.count(sig_s[i]) && path.at(sig_s[i]))
+			{
+				ret = find_muxtree_feedback_patterns(sig_b[i*width + index], q, path);
+
+				if (sig_b[i*width + index] == q) {
+					RTLIL::SigSpec s = mbit.first->getPort(ID::B);
+					s[i*width + index] = RTLIL::Sx;
+					mbit.first->setPort(ID::B, s);
+				}
+
+				return ret;
+			}
+
+		pattern_t path_else = path;
+
+		for (int i = 0; i < GetSize(sig_s); i++)
+		{
+			if (path.count(sig_s[i]))
+				continue;
+
+			pattern_t path_this = path;
+			path_else[sig_s[i]] = false;
+			path_this[sig_s[i]] = true;
+
+			for (auto &pat : find_muxtree_feedback_patterns(sig_b[i*width + index], q, path_this))
+				ret.insert(pat);
+
+			if (sig_b[i*width + index] == q) {
+				RTLIL::SigSpec s = mbit.first->getPort(ID::B);
+				s[i*width + index] = RTLIL::Sx;
+				mbit.first->setPort(ID::B, s);
+			}
+		}
+
+		for (auto &pat : find_muxtree_feedback_patterns(sig_a[index], q, path_else))
+			ret.insert(pat);
+
+		if (sig_a[index] == q) {
+			RTLIL::SigSpec s = mbit.first->getPort(ID::A);
+			s[index] = RTLIL::Sx;
+			mbit.first->setPort(ID::A, s);
+		}
+
+		return ret;
+	}
+
+	void simplify_patterns(patterns_t&)
+	{
+		// TBD
+	}
+
+	ctrl_t make_patterns_logic(const patterns_t &patterns, const ctrls_t &ctrls, bool make_gates)
+	{
+		if (patterns.empty() && GetSize(ctrls) == 1) {
+			return *ctrls.begin();
+		}
+
+		RTLIL::SigSpec or_input;
+
+		for (auto pat : patterns)
+		{
+			RTLIL::SigSpec s1, s2;
+			for (auto it : pat) {
+				s1.append(it.first);
+				s2.append(it.second);
+			}
+
+			RTLIL::SigSpec y = module->addWire(NEW_ID);
+			RTLIL::Cell *c = module->addNe(NEW_ID, s1, s2, y);
+
+			if (make_gates) {
+				simplemap(module, c);
+				module->remove(c);
+			}
+
+			or_input.append(y);
+		}
+		for (auto item : ctrls) {
+			if (item.second)
+				or_input.append(item.first);
+			else if (make_gates)
+				or_input.append(module->NotGate(NEW_ID, item.first));
+			else
+				or_input.append(module->Not(NEW_ID, item.first));
+		}
+
+		if (GetSize(or_input) == 0)
+			return ctrl_t(State::S1, true);
+
+		if (GetSize(or_input) == 1)
+			return ctrl_t(or_input, true);
+
+		RTLIL::SigSpec y = module->addWire(NEW_ID);
+		RTLIL::Cell *c = module->addReduceAnd(NEW_ID, or_input, y);
+
+		if (make_gates) {
+			simplemap(module, c);
+			module->remove(c);
+		}
+
+		return ctrl_t(y, true);
+	}
+
+	ctrl_t combine_resets(const ctrls_t &ctrls, bool make_gates)
+	{
+		if (GetSize(ctrls) == 1) {
+			return *ctrls.begin();
+		}
+
+		RTLIL::SigSpec or_input;
+
+		bool final_pol = false;
+		for (auto item : ctrls) {
+			if (item.second)
+				final_pol = true;
+		}
+
+		for (auto item : ctrls) {
+			if (item.second == final_pol)
+				or_input.append(item.first);
+			else if (make_gates)
+				or_input.append(module->NotGate(NEW_ID, item.first));
+			else
+				or_input.append(module->Not(NEW_ID, item.first));
+		}
+
+		RTLIL::SigSpec y = module->addWire(NEW_ID);
+		RTLIL::Cell *c = final_pol ? module->addReduceOr(NEW_ID, or_input, y) : module->addReduceAnd(NEW_ID, or_input, y);
+
+		if (make_gates) {
+			simplemap(module, c);
+			module->remove(c);
+		}
+
+		return ctrl_t(y, final_pol);
+	}
+
+	bool run() {
+		// We have all the information we need, and the list of FFs to process as well.  Do it.
+		bool did_something = false;
+		while (!dff_cells.empty()) {
+			Cell *cell = dff_cells.back();
+			dff_cells.pop_back();
+			// Break down the FF into pieces.
+			FfData ff(&initvals, cell);
+			bool changed = false;
+
+			if (!ff.width) {
+				module->remove(cell);
+				did_something = true;
+				continue;
+			}
+
+			if (ff.has_sr) {
+				bool sr_removed = false;
+				std::vector<int> keep_bits;
+				// Check for always-active S/R bits.
+				for (int i = 0; i < ff.width; i++) {
+					if (ff.sig_clr[i] == (ff.pol_clr ? State::S1 : State::S0) || (!opt.keepdc && ff.sig_clr[i] == State::Sx)) {
+						// Always-active clear — connect Q bit to 0.
+						initvals.remove_init(ff.sig_q[i]);
+						module->connect(ff.sig_q[i], State::S0);
+						log("Handling always-active CLR at position %d on %s (%s) from module %s (changing to const driver).\n",
+								i, log_id(cell), log_id(cell->type), log_id(module));
+						sr_removed = true;
+					} else if (ff.sig_set[i] == (ff.pol_set ? State::S1 : State::S0) || (!opt.keepdc && ff.sig_set[i] == State::Sx)) {
+						// Always-active set — connect Q bit to 1 if clear inactive, 0 if reset active.
+						initvals.remove_init(ff.sig_q[i]);
+						if (!ff.pol_clr) {
+							module->connect(ff.sig_q[i], ff.sig_clr[i]);
+						} else if (ff.is_fine) {
+							module->addNotGate(NEW_ID, ff.sig_q[i], ff.sig_clr[i]);
+						} else {
+							module->addNot(NEW_ID, ff.sig_q[i], ff.sig_clr[i]);
+						}
+						log("Handling always-active SET at position %d on %s (%s) from module %s (changing to combinatorial circuit).\n",
+								i, log_id(cell), log_id(cell->type), log_id(module));
+						sr_removed = true;
+					} else {
+						keep_bits.push_back(i);
+					}
+				}
+				if (sr_removed) {
+					if (keep_bits.empty()) {
+						module->remove(cell);
+						did_something = true;
+						continue;
+					}
+					ff = ff.slice(keep_bits);
+					changed = true;
+				}
+
+				if (ff.pol_clr ? ff.sig_clr.is_fully_zero() : ff.sig_clr.is_fully_ones()) {
+					// CLR is useless, try to kill it.
+					bool failed = false;
+					for (int i = 0; i < ff.width; i++)
+						if (ff.sig_set[i] != ff.sig_set[0])
+							failed = true;
+					if (!failed) {
+						log("Removing never-active CLR on %s (%s) from module %s.\n",
+								log_id(cell), log_id(cell->type), log_id(module));
+						ff.has_sr = false;
+						ff.has_arst = true;
+						ff.pol_arst = ff.pol_set;
+						ff.sig_arst = ff.sig_set[0];
+						ff.val_arst = Const(State::S1, ff.width);
+						changed = true;
+					}
+				} else if (ff.pol_set ? ff.sig_set.is_fully_zero() : ff.sig_set.is_fully_ones()) {
+					// SET is useless, try to kill it.
+					bool failed = false;
+					for (int i = 0; i < ff.width; i++)
+						if (ff.sig_clr[i] != ff.sig_clr[0])
+							failed = true;
+					if (!failed) {
+						log("Removing never-active SET on %s (%s) from module %s.\n",
+								log_id(cell), log_id(cell->type), log_id(module));
+						ff.has_sr = false;
+						ff.has_arst = true;
+						ff.pol_arst = ff.pol_clr;
+						ff.sig_arst = ff.sig_clr[0];
+						ff.val_arst = Const(State::S0, ff.width);
+						changed = true;
+					}
+				} else if (ff.pol_clr == ff.pol_set) {
+					// Try a more complex conversion to plain async reset.
+					State val_neutral = ff.pol_set ? State::S0 : State::S1;
+					Const val_arst;
+					SigSpec sig_arst;
+					if (ff.sig_clr[0] == val_neutral)
+						sig_arst = ff.sig_set[0];
+					else
+						sig_arst = ff.sig_clr[0];
+					bool failed = false;
+					for (int i = 0; i < ff.width; i++) {
+						if (ff.sig_clr[i] == sig_arst && ff.sig_set[i] == val_neutral)
+							val_arst.bits.push_back(State::S0);
+						else if (ff.sig_set[i] == sig_arst && ff.sig_clr[i] == val_neutral)
+							val_arst.bits.push_back(State::S1);
+						else
+							failed = true;
+					}
+					if (!failed) {
+						log("Converting CLR/SET to ARST on %s (%s) from module %s.\n",
+								log_id(cell), log_id(cell->type), log_id(module));
+						ff.has_sr = false;
+						ff.has_arst = true;
+						ff.val_arst = val_arst;
+						ff.sig_arst = sig_arst;
+						ff.pol_arst = ff.pol_clr;
+						changed = true;
+					}
+				}
+			}
+
+			if (ff.has_arst) {
+				if (ff.sig_arst == (ff.pol_arst ? State::S0 : State::S1)) {
+					// Always-inactive reset — remove.
+					log("Removing never-active ARST on %s (%s) from module %s.\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					ff.has_arst = false;
+					changed = true;
+				} else if (ff.sig_arst == (ff.pol_arst ? State::S1 : State::S0) || (!opt.keepdc && ff.sig_arst == State::Sx)) {
+					// Always-active async reset — change to const driver.
+					log("Handling always-active ARST on %s (%s) from module %s (changing to const driver).\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					initvals.remove_init(ff.sig_q);
+					module->remove(cell);
+					module->connect(ff.sig_q, ff.val_arst);
+					did_something = true;
+					continue;
+				}
+			}
+
+			if (ff.has_srst) {
+				if (ff.sig_srst == (ff.pol_srst ? State::S0 : State::S1)) {
+					// Always-inactive reset — remove.
+					log("Removing never-active SRST on %s (%s) from module %s.\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					ff.has_srst = false;
+					changed = true;
+				} else if (ff.sig_srst == (ff.pol_srst ? State::S1 : State::S0) || (!opt.keepdc && ff.sig_srst == State::Sx)) {
+					// Always-active sync reset — connect to D instead.
+					log("Handling always-active SRST on %s (%s) from module %s (changing to const D).\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					ff.has_srst = false;
+					if (!ff.ce_over_srst)
+						ff.has_en = false;
+					ff.sig_d = ff.val_d = ff.val_srst;
+					ff.d_is_const = true;
+					changed = true;
+				}
+			}
+
+			if (ff.has_en) {
+				if (ff.sig_en == (ff.pol_en ? State::S0 : State::S1) || (!opt.keepdc && ff.sig_en == State::Sx)) {
+					// Always-inactive enable — remove.
+					if (ff.has_clk && ff.has_srst && !ff.ce_over_srst) {
+						log("Handling never-active EN on %s (%s) from module %s (connecting SRST instead).\n",
+								log_id(cell), log_id(cell->type), log_id(module));
+						// FF with sync reset — connect the sync reset to D instead.
+						ff.pol_en = ff.pol_srst;
+						ff.sig_en = ff.sig_srst;
+						ff.has_srst = false;
+						ff.sig_d = ff.val_d = ff.val_srst;
+						ff.d_is_const = true;
+						changed = true;
+					} else {
+						log("Handling never-active EN on %s (%s) from module %s (removing D path).\n",
+								log_id(cell), log_id(cell->type), log_id(module));
+						// The D input path is effectively useless, so remove it (this will be a const-input D latch, SR latch, or a const driver).
+						ff.has_d = ff.has_en = ff.has_clk = false;
+						changed = true;
+					}
+				} else if (ff.sig_en == (ff.pol_en ? State::S1 : State::S0)) {
+					// Always-active enable.
+					if (ff.has_clk) {
+						// For FF, just remove the useless enable.
+						log("Removing always-active EN on %s (%s) from module %s.\n",
+								log_id(cell), log_id(cell->type), log_id(module));
+						ff.has_en = false;
+						changed = true;
+					} else {
+						// For latches, make a comb circuit, nuke the latch.
+						log("Handling always-active EN on %s (%s) from module %s (changing to combinatorial circuit).\n",
+								log_id(cell), log_id(cell->type), log_id(module));
+						initvals.remove_init(ff.sig_q);
+						module->remove(cell);
+						if (ff.has_sr) {
+							SigSpec tmp;
+							if (ff.is_fine) {
+								if (ff.pol_set)
+									tmp = module->MuxGate(NEW_ID, ff.sig_d, State::S1, ff.sig_set);
+								else
+									tmp = module->MuxGate(NEW_ID, State::S1, ff.sig_d, ff.sig_set);
+								if (ff.pol_clr)
+									module->addMuxGate(NEW_ID, tmp, State::S0, ff.sig_clr, ff.sig_q);
+								else
+									module->addMuxGate(NEW_ID, State::S0, tmp, ff.sig_clr, ff.sig_q);
+							} else {
+								if (ff.pol_set)
+									tmp = module->Or(NEW_ID, ff.sig_d, ff.sig_set);
+								else
+									tmp = module->Or(NEW_ID, ff.sig_d, module->Not(NEW_ID, ff.sig_set));
+								if (ff.pol_clr)
+									module->addAnd(NEW_ID, tmp, module->Not(NEW_ID, ff.sig_clr), ff.sig_q);
+								else
+									module->addAnd(NEW_ID, tmp, ff.sig_clr, ff.sig_q);
+							}
+						} else if (ff.has_arst) {
+							if (ff.is_fine) {
+								if (ff.pol_arst)
+									module->addMuxGate(NEW_ID, ff.sig_d, ff.val_arst[0], ff.sig_arst, ff.sig_q);
+								else
+									module->addMuxGate(NEW_ID, ff.val_arst[0], ff.sig_d, ff.sig_arst, ff.sig_q);
+							} else {
+								if (ff.pol_arst)
+									module->addMux(NEW_ID, ff.sig_d, ff.val_arst, ff.sig_arst, ff.sig_q);
+								else
+									module->addMux(NEW_ID, ff.val_arst, ff.sig_d, ff.sig_arst, ff.sig_q);
+							}
+						} else {
+							module->connect(ff.sig_q, ff.sig_d);
+						}
+						did_something = true;
+						continue;
+					}
+				}
+			}
+
+			if (ff.has_clk) {
+				if (ff.sig_clk.is_fully_const()) {
+					// Const clock — the D input path is effectively useless, so remove it (this will be a const-input D latch, SR latch, or a const driver).
+					log("Handling const CLK on %s (%s) from module %s (removing D path).\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					ff.has_d = ff.has_en = ff.has_clk = ff.has_srst = false;
+					changed = true;
+				}
+			}
+
+			if (ff.has_d && ff.sig_d == ff.sig_q) {
+				// Q wrapped back to D, can be removed.
+				if (ff.has_clk && ff.has_srst) {
+					// FF with sync reset — connect the sync reset to D instead.
+					log("Handling D = Q on %s (%s) from module %s (conecting SRST instead).\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					if (ff.has_en && ff.ce_over_srst) {
+						if (!ff.pol_en) {
+							if (ff.is_fine)
+								ff.sig_en = module->NotGate(NEW_ID, ff.sig_en);
+							else
+								ff.sig_en = module->Not(NEW_ID, ff.sig_en);
+						}
+						if (!ff.pol_srst) {
+							if (ff.is_fine)
+								ff.sig_srst = module->NotGate(NEW_ID, ff.sig_srst);
+							else
+								ff.sig_srst = module->Not(NEW_ID, ff.sig_srst);
+						}
+						if (ff.is_fine)
+							ff.sig_en = module->AndGate(NEW_ID, ff.sig_en, ff.sig_srst);
+						else
+							ff.sig_en = module->And(NEW_ID, ff.sig_en, ff.sig_srst);
+						ff.pol_en = true;
+					} else {
+						ff.pol_en = ff.pol_srst;
+						ff.sig_en = ff.sig_srst;
+					}
+					ff.has_en = true;
+					ff.has_srst = false;
+					ff.sig_d = ff.val_d = ff.val_srst;
+					ff.d_is_const = true;
+					changed = true;
+				} else {
+					// The D input path is effectively useless, so remove it (this will be a const-input D latch, SR latch, or a const driver).
+					log("Handling D = Q on %s (%s) from module %s (removing D path).\n",
+							log_id(cell), log_id(cell->type), log_id(module));
+					ff.has_d = ff.has_en = ff.has_clk = false;
+					changed = true;
+				}
+			}
+
+			// Now check if any bit can be replaced by a constant.
+			pool<int> removed_sigbits;
+			for (int i = 0; i < ff.width; i++) {
+				State val = ff.val_init[i];
+				if (ff.has_arst)
+					val = combine_const(val, ff.val_arst[i]);
+				if (ff.has_srst)
+					val = combine_const(val, ff.val_srst[i]);
+				if (ff.has_sr) {
+					if (ff.sig_clr[i] != (ff.pol_clr ? State::S0 : State::S1))
+						val = combine_const(val, State::S0);
+					if (ff.sig_set[i] != (ff.pol_set ? State::S0 : State::S1))
+						val = combine_const(val, State::S1);
+				}
+				if (val == State::Sm)
+					continue;
+				if (ff.has_d) {
+					if (!ff.sig_d[i].wire) {
+						val = combine_const(val, ff.sig_d[i].data);
+						if (val == State::Sm)
+							continue;
+					} else {
+						if (!opt.sat)
+							continue;
+						// For each register bit, try to prove that it cannot change from the initial value. If so, remove it
+						if (!bit2driver.count(ff.sig_d[i]))
+							continue;
+						if (val != State::S0 && val != State::S1)
+							continue;
+
+						sat_import_cell(bit2driver.at(ff.sig_d[i]));
+
+						int init_sat_pi = satgen.importSigSpec(val).front();
+						int q_sat_pi = satgen.importSigBit(ff.sig_q[i]);
+						int d_sat_pi = satgen.importSigBit(ff.sig_d[i]);
+
+						// Try to find out whether the register bit can change under some circumstances
+						bool counter_example_found = ez->solve(ez->IFF(q_sat_pi, init_sat_pi), ez->NOT(ez->IFF(d_sat_pi, init_sat_pi)));
+
+						// If the register bit cannot change, we can replace it with a constant
+						if (counter_example_found)
+							continue;
+					}
+				}
+				log("Setting constant %d-bit at position %d on %s (%s) from module %s.\n", val ? 1 : 0,
+						i, log_id(cell), log_id(cell->type), log_id(module));
+
+				initvals.remove_init(ff.sig_q[i]);
+				module->connect(ff.sig_q[i], val);
+				removed_sigbits.insert(i);
+			}
+			if (!removed_sigbits.empty()) {
+				std::vector<int> keep_bits;
+				for (int i = 0; i < ff.width; i++)
+					if (!removed_sigbits.count(i))
+						keep_bits.push_back(i);
+				if (keep_bits.empty()) {
+					module->remove(cell);
+					did_something = true;
+					continue;
+				}
+				ff = ff.slice(keep_bits);
+				changed = true;
+			}
+
+			// The cell has been simplified as much as possible already.  Now try to spice it up with enables / sync resets.
+			if (ff.has_clk) {
+				if (!ff.has_arst && !ff.has_sr && (!ff.has_srst || !ff.has_en || ff.ce_over_srst) && !opt.nosdff) {
+					// Try to merge sync resets.
+					std::map<ctrls_t, std::vector<int>> groups;
+					std::vector<int> remaining_indices;
+					Const val_srst;
+
+					for (int i = 0 ; i < ff.width; i++) {
+						ctrls_t resets;
+						State reset_val = State::Sx;
+						if (ff.has_srst)
+							reset_val = ff.val_srst[i];
+						while (bit2mux.count(ff.sig_d[i]) && bitusers[ff.sig_d[i]] == 1) {
+							cell_int_t mbit = bit2mux.at(ff.sig_d[i]);
+							if (GetSize(mbit.first->getPort(ID::S)) != 1)
+								break;
+							SigBit s = mbit.first->getPort(ID::S);
+							SigBit a = mbit.first->getPort(ID::A)[mbit.second];
+							SigBit b = mbit.first->getPort(ID::B)[mbit.second];
+							// Workaround for funny memory WE pattern.
+							if ((a == State::S0 || a == State::S1) && (b == State::S0 || b == State::S1))
+								break;
+							if ((b == State::S0 || b == State::S1) && (b == reset_val || reset_val == State::Sx)) {
+								// This is better handled by CE pattern.
+								if (a == ff.sig_q[i])
+									break;
+								reset_val = b.data;
+								resets.insert(ctrl_t(s, true));
+								ff.sig_d[i] = a;
+							} else if ((a == State::S0 || a == State::S1) && (a == reset_val || reset_val == State::Sx)) {
+								// This is better handled by CE pattern.
+								if (b == ff.sig_q[i])
+									break;
+								reset_val = a.data;
+								resets.insert(ctrl_t(s, false));
+								ff.sig_d[i] = b;
+							} else {
+								break;
+							}
+						}
+
+						if (!resets.empty()) {
+							if (ff.has_srst)
+								resets.insert(ctrl_t(ff.sig_srst, ff.pol_srst));
+							groups[resets].push_back(i);
+						} else
+							remaining_indices.push_back(i);
+						val_srst.bits.push_back(reset_val);
+					}
+
+					for (auto &it : groups) {
+						FfData new_ff = ff.slice(it.second);
+						new_ff.val_srst = Const();
+						for (int i = 0; i < new_ff.width; i++) {
+							int j = it.second[i];
+							new_ff.val_srst.bits.push_back(val_srst[j]);
+						}
+						ctrl_t srst = combine_resets(it.first, ff.is_fine);
+
+						new_ff.has_srst = true;
+						new_ff.sig_srst = srst.first;
+						new_ff.pol_srst = srst.second;
+						if (new_ff.has_en)
+							new_ff.ce_over_srst = true;
+						Cell *new_cell = new_ff.emit(module, NEW_ID);
+						if (new_cell)
+							dff_cells.push_back(new_cell);
+						log("Adding SRST signal on %s (%s) from module %s (D = %s, Q = %s, rval = %s).\n",
+								log_id(cell), log_id(cell->type), log_id(module), log_signal(new_ff.sig_d), log_signal(new_ff.sig_q), log_signal(new_ff.val_srst));
+					}
+
+					if (remaining_indices.empty()) {
+						module->remove(cell);
+						did_something = true;
+						continue;
+					} else if (GetSize(remaining_indices) != ff.width) {
+						ff = ff.slice(remaining_indices);
+						changed = true;
+					}
+				}
+				if ((!ff.has_srst || !ff.has_en || !ff.ce_over_srst) && !opt.nodffe) {
+					// Try to merge enables.
+					std::map<std::pair<patterns_t, ctrls_t>, std::vector<int>> groups;
+					std::vector<int> remaining_indices;
+
+					for (int i = 0 ; i < ff.width; i++) {
+						// First, eat up as many simple muxes as possible.
+						ctrls_t enables;
+						while (bit2mux.count(ff.sig_d[i]) && bitusers[ff.sig_d[i]] == 1) {
+							cell_int_t mbit = bit2mux.at(ff.sig_d[i]);
+							if (GetSize(mbit.first->getPort(ID::S)) != 1)
+								break;
+							SigBit s = mbit.first->getPort(ID::S);
+							SigBit a = mbit.first->getPort(ID::A)[mbit.second];
+							SigBit b = mbit.first->getPort(ID::B)[mbit.second];
+							if (a == ff.sig_q[i]) {
+								enables.insert(ctrl_t(s, true));
+								ff.sig_d[i] = b;
+							} else if (b == ff.sig_q[i]) {
+								enables.insert(ctrl_t(s, false));
+								ff.sig_d[i] = a;
+							} else {
+								break;
+							}
+						}
+
+						patterns_t patterns;
+						if (!opt.simple_dffe)
+							patterns = find_muxtree_feedback_patterns(ff.sig_d[i], ff.sig_q[i], pattern_t());
+						if (!patterns.empty() || !enables.empty()) {
+							if (ff.has_en)
+								enables.insert(ctrl_t(ff.sig_en, ff.pol_en));
+							simplify_patterns(patterns);
+							groups[std::make_pair(patterns, enables)].push_back(i);
+						} else
+							remaining_indices.push_back(i);
+					}
+
+					for (auto &it : groups) {
+						FfData new_ff = ff.slice(it.second);
+						ctrl_t en = make_patterns_logic(it.first.first, it.first.second, ff.is_fine);
+
+						new_ff.has_en = true;
+						new_ff.sig_en = en.first;
+						new_ff.pol_en = en.second;
+						new_ff.ce_over_srst = false;
+						Cell *new_cell = new_ff.emit(module, NEW_ID);
+						if (new_cell)
+							dff_cells.push_back(new_cell);
+						log("Adding EN signal on %s (%s) from module %s (D = %s, Q = %s).\n",
+								log_id(cell), log_id(cell->type), log_id(module), log_signal(new_ff.sig_d), log_signal(new_ff.sig_q));
+					}
+
+					if (remaining_indices.empty()) {
+						module->remove(cell);
+						did_something = true;
+						continue;
+					} else if (GetSize(remaining_indices) != ff.width) {
+						ff = ff.slice(remaining_indices);
+						changed = true;
+					}
+				}
+			}
+
+			if (changed) {
+				// Rebuild the FF.
+				IdString name = cell->name;
+				module->remove(cell);
+				ff.emit(module, name);
+				did_something = true;
+			}
+		}
+		return did_something;
+	}
+};
+
+struct OptDffPass : public Pass {
+	OptDffPass() : Pass("opt_dff", "perform DFF optimizations") { }
+	void help() override
+	{
+		//   |---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|
+		log("\n");
+		log("    opt_dff [-nodffe] [-nosdff] [-keepdc] [-sat] [selection]\n");
+		log("\n");
+		log("This pass converts flip-flops to a more suitable type by merging clock enables\n");
+		log("and synchronous reset multiplexers, removing unused control inputs, or potentially\n");
+		log("removes the flip-flop altogether, converting it to a constant driver.\n");
+		log("\n");
+		log("    -nodffe\n");
+		log("        disables dff -> dffe conversion, and other transforms recognizing clock enable\n");
+		log("\n");
+		log("    -nosdff\n");
+		log("        disables dff -> sdff conversion, and other transforms recognizing sync resets\n");
+		log("\n");
+		log("    -simple-dffe\n");
+		log("        only enables clock enable recognition transform for obvious cases\n");
+		log("\n");
+		log("    -sat\n");
+		log("        additionally invoke SAT solver to detect and remove flip-flops (with\n");
+		log("        non-constant inputs) that can also be replaced with a constant driver\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) override
+	{
+		log_header(design, "Executing OPT_DFF pass (perform DFF optimizations).\n");
+		OptDffOptions opt;
+		opt.nodffe = false;
+		opt.nosdff = false;
+		opt.simple_dffe = false;
+		opt.keepdc = false;
+		opt.sat = false;
+
+		size_t argidx;
+		for (argidx = 1; argidx < args.size(); argidx++) {
+			if (args[argidx] == "-nodffe") {
+				opt.nodffe = true;
+				continue;
+			}
+			if (args[argidx] == "-nosdff") {
+				opt.nosdff = true;
+				continue;
+			}
+			if (args[argidx] == "-simple-dffe") {
+				opt.simple_dffe = true;
+				continue;
+			}
+			if (args[argidx] == "-keepdc") {
+				opt.keepdc = true;
+				continue;
+			}
+			if (args[argidx] == "-sat") {
+				opt.sat = true;
+				continue;
+			}
+			break;
+		}
+		extra_args(args, argidx, design);
+
+		bool did_something = false;
+		for (auto mod : design->selected_modules()) {
+			OptDffWorker worker(opt, mod);
+			if (worker.run())
+				did_something = true;
+		}
+
+		if (did_something)
+			design->scratchpad_set_bool("opt.did_something", true);
+	}
+} OptDffPass;
+
+PRIVATE_NAMESPACE_END
diff --git a/tests/opt/opt_dff_arst.ys b/tests/opt/opt_dff_arst.ys
new file mode 100644
index 000000000..2aa3b7a26
--- /dev/null
+++ b/tests/opt/opt_dff_arst.ys
@@ -0,0 +1,101 @@
+### Always-active ARST removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+output [11:0] Q;
+input ARST;
+input EN;
+
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff0 (.CLK(CLK), .ARST(1'b1), .D(D), .Q(Q[1:0]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .ARST_POLARITY(1'b0), .ARST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .ARST(1'b0), .EN(EN), .D(D), .Q(Q[3:2]));
+$adlatch #(.EN_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff2 (.EN(EN), .ARST(1'b1), .D(D), .Q(Q[5:4]));
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .ARST(1'bx), .D(D), .Q(Q[7:6]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .ARST_POLARITY(1'b0), .ARST_VALUE(2'h2), .WIDTH(2)) ff4 (.CLK(CLK), .ARST(1'bx), .EN(EN), .D(D), .Q(Q[9:8]));
+$adlatch #(.EN_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff5 (.EN(EN), .ARST(1'bx), .D(D), .Q(Q[11:10]));
+
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:*
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$adlatch
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:*
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 2 t:$_DFF_???_
+select -assert-count 2 t:$_DFFE_????_
+select -assert-count 2 t:$_DLATCH_???_
+
+design -reset
+
+
+### Never-active ARST removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+output [5:0] Q;
+input ARST;
+input EN;
+
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff0 (.CLK(CLK), .ARST(1'b0), .D(D), .Q(Q[1:0]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .ARST_POLARITY(1'b0), .ARST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .ARST(1'b1), .EN(EN), .D(D), .Q(Q[3:2]));
+$adlatch #(.EN_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff2 (.EN(EN), .ARST(1'b0), .D(D), .Q(Q[5:4]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:$adff
+select -assert-none t:$adffe
+select -assert-none t:$adlatch
+select -assert-count 1 t:$dff
+select -assert-count 1 t:$dffe
+select -assert-count 1 t:$dlatch
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:$_DFF_???_
+select -assert-none t:$_DFFE_????_
+select -assert-none t:$_DLATCH_???_
+select -assert-count 2 t:$_DFF_P_
+select -assert-count 2 t:$_DFFE_PP_
+select -assert-count 2 t:$_DLATCH_P_
+
+design -reset
diff --git a/tests/opt/opt_dff_clk.ys b/tests/opt/opt_dff_clk.ys
new file mode 100644
index 000000000..f3aefa406
--- /dev/null
+++ b/tests/opt/opt_dff_clk.ys
@@ -0,0 +1,45 @@
+### Never-toggling CLK removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input EN;
+input [1:0] D;
+(* init = 18'h15555 *)
+output [17:0] Q;
+input SRST;
+input ARST;
+input [1:0] CLR;
+input [1:0] SET;
+
+$dff #(.CLK_POLARITY(1'b1), .WIDTH(2)) ff0 (.CLK(1'b0), .D(D), .Q(Q[1:0]));
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff1 (.CLK(1'b1), .EN(EN), .D(D), .Q(Q[3:2]));
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(1'bx), .ARST(ARST), .D(D), .Q(Q[5:4]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(1'b0), .EN(EN), .ARST(ARST), .D(D), .Q(Q[7:6]));
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff4 (.CLK(1'b1), .SRST(SRST), .D(D), .Q(Q[9:8]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff5 (.CLK(1'bx), .EN(EN), .SRST(SRST), .D(D), .Q(Q[11:10]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff6 (.CLK(1'bx), .EN(EN), .SRST(SRST), .D(D), .Q(Q[13:12]));
+$dffsr #(.CLK_POLARITY(1'b1), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff7 (.CLK(1'b1), .SET(SET), .CLR(CLR), .D(D), .Q(Q[15:14]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff8 (.CLK(1'bx), .EN(EN), .SET(SET), .CLR(CLR), .D(D), .Q(Q[17:16]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 2 t:$dlatch
+select -assert-count 2 t:$sr
+select -assert-none t:$dlatch t:$sr %% %n t:* %i
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 4 t:$_DLATCH_?_
+select -assert-count 4 t:$_SR_??_
+select -assert-none t:$_DLATCH_?_ t:$_SR_??_ %% %n t:* %i
diff --git a/tests/opt/opt_dff_const.ys b/tests/opt/opt_dff_const.ys
new file mode 100644
index 000000000..6a7dec7fa
--- /dev/null
+++ b/tests/opt/opt_dff_const.ys
@@ -0,0 +1,49 @@
+### Replace FFs with a const.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input EN;
+(* init=84'haaaaaaaaaaaaaaaaaaaaa *)
+output [83:0] Q;
+input SRST;
+input ARST;
+input [3:0] CLR;
+input [3:0] SET;
+
+$dff #(.CLK_POLARITY(1'b1), .WIDTH(4)) ff0 (.CLK(CLK), .D(4'hc), .Q(Q[3:0]));
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(4)) ff1 (.CLK(CLK), .EN(EN), .D(4'hc), .Q(Q[7:4]));
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(8'hf0), .WIDTH(8)) ff2 (.CLK(CLK), .ARST(ARST), .D(8'hcc), .Q(Q[15:8]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(8'hf0), .WIDTH(8)) ff3 (.CLK(CLK), .EN(EN), .ARST(ARST), .D(8'hcc), .Q(Q[23:16]));
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(8'hf0), .WIDTH(8)) ff4 (.CLK(CLK), .SRST(SRST), .D(8'hcc), .Q(Q[31:24]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(8'hf0), .WIDTH(8)) ff5 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(8'hcc), .Q(Q[39:32]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(8'hf0), .WIDTH(8)) ff6 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(8'hcc), .Q(Q[47:40]));
+$dffsr #(.CLK_POLARITY(1'b1), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(8)) ff7 (.CLK(CLK), .SET({SET, 4'hf}), .CLR({4'h0, CLR}), .D(8'hcc), .Q(Q[55:48]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b0), .SET_POLARITY(1'b1), .WIDTH(8)) ff8 (.CLK(CLK), .EN(EN), .SET({SET, 4'h0}), .CLR({4'hf, CLR}), .D(8'hcc), .Q(Q[63:56]));
+
+$dlatch #(.EN_POLARITY(1'b1), .WIDTH(4)) ff9 (.EN(EN), .D(4'hc), .Q(Q[67:64]));
+$adlatch #(.EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(8'hf0), .WIDTH(8)) ff10 (.EN(EN), .ARST(ARST), .D(8'hcc), .Q(Q[75:68]));
+$dlatchsr #(.EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b1), .WIDTH(8)) ff11 (.EN(EN), .SET({SET, 4'h0}), .CLR({4'h0, CLR}), .D(8'hcc), .Q(Q[83:76]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 1 t:$dff r:WIDTH=2 %i
+select -assert-count 1 t:$dffe r:WIDTH=2 %i
+select -assert-count 1 t:$adff r:WIDTH=6 %i
+select -assert-count 1 t:$adffe r:WIDTH=6 %i
+select -assert-count 1 t:$sdff r:WIDTH=6 %i
+select -assert-count 1 t:$sdffe r:WIDTH=6 %i
+select -assert-count 1 t:$sdffce r:WIDTH=6 %i
+select -assert-count 1 t:$dffsr r:WIDTH=6 %i
+select -assert-count 1 t:$dffsre r:WIDTH=6 %i
+select -assert-count 1 t:$dlatch r:WIDTH=2 %i
+select -assert-count 1 t:$adlatch r:WIDTH=6 %i
+select -assert-count 1 t:$dlatchsr r:WIDTH=6 %i
diff --git a/tests/opt/opt_dff_en.ys b/tests/opt/opt_dff_en.ys
new file mode 100644
index 000000000..06ee6c63d
--- /dev/null
+++ b/tests/opt/opt_dff_en.ys
@@ -0,0 +1,157 @@
+### Always-active EN removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+output [15:0] Q;
+input SRST;
+input ARST;
+input [1:0] CLR;
+input [1:0] SET;
+
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff0 (.CLK(CLK), .EN(1'b1), .D(D), .Q(Q[1:0]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .EN(1'b0), .ARST(ARST), .D(D), .Q(Q[3:2]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .EN(1'b1), .SRST(SRST), .D(D), .Q(Q[5:4]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .EN(1'b1), .SRST(SRST), .D(D), .Q(Q[7:6]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff4 (.CLK(CLK), .EN(1'b0), .SET(SET), .CLR(CLR), .D(D), .Q(Q[9:8]));
+
+$dlatch #(.EN_POLARITY(1'b1), .WIDTH(2)) ff5 (.EN(1'b1), .D(D), .Q(Q[11:10]));
+$adlatch #(.EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff6 (.EN(1'b0), .ARST(ARST), .D(D), .Q(Q[13:12]));
+$dlatchsr #(.EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff7 (.EN(1'b0), .SET(SET), .CLR(CLR), .D(D), .Q(Q[15:14]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+# Equivalence check will fail for unmapped adlatch and dlatchsr due to negative hold hack.
+delete top/ff6 top/ff7
+equiv_opt -undef -assert -multiclock opt_dff
+
+design -load orig
+delete top/ff6 top/ff7
+simplemap
+equiv_opt -undef -assert -multiclock opt_dff
+
+design -load orig
+opt_dff
+select -assert-count 0 t:$dffe
+select -assert-count 0 t:$adffe
+select -assert-count 0 t:$sdffe
+select -assert-count 0 t:$sdffce
+select -assert-count 0 t:$dffsre
+select -assert-count 0 t:$dlatch
+select -assert-count 0 t:$adlatch
+select -assert-count 0 t:$dlatchsr
+select -assert-count 1 t:$dff
+select -assert-count 2 t:$sdff
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$dffsr
+
+design -load orig
+simplemap
+opt_dff
+select -assert-count 0 t:$_DFFE_*
+select -assert-count 0 t:$_SDFFE_*
+select -assert-count 0 t:$_SDFFCE_*
+select -assert-count 0 t:$_DFFSRE_*
+select -assert-count 0 t:$_DLATCH*
+select -assert-count 2 t:$_DFF_P_
+select -assert-count 4 t:$_SDFF_PP?_
+select -assert-count 2 t:$_DFF_PP?_
+select -assert-count 2 t:$_DFFSR_PNP_
+
+design -reset
+
+
+
+### Never-active EN removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+(* init = 32'h55555555 *)
+output [31:0] Q;
+input SRST;
+input ARST;
+input [1:0] CLR;
+input [1:0] SET;
+
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff0 (.CLK(CLK), .EN(1'b0), .D(D), .Q(Q[1:0]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .EN(1'b1), .ARST(ARST), .D(D), .Q(Q[3:2]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .EN(1'b0), .SRST(SRST), .D(D), .Q(Q[5:4]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .EN(1'b0), .SRST(SRST), .D(D), .Q(Q[7:6]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff4 (.CLK(CLK), .EN(1'b1), .SET(SET), .CLR(CLR), .D(D), .Q(Q[9:8]));
+
+$dlatch #(.EN_POLARITY(1'b1), .WIDTH(2)) ff5 (.EN(1'b0), .D(D), .Q(Q[11:10]));
+$adlatch #(.EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff6 (.EN(1'b1), .ARST(ARST), .D(D), .Q(Q[13:12]));
+$dlatchsr #(.EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff7 (.EN(1'b1), .SET(SET), .CLR(CLR), .D(D), .Q(Q[15:14]));
+
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff8 (.CLK(CLK), .EN(1'bx), .D(D), .Q(Q[17:16]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff9 (.CLK(CLK), .EN(1'bx), .ARST(ARST), .D(D), .Q(Q[19:18]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff10 (.CLK(CLK), .EN(1'bx), .SRST(SRST), .D(D), .Q(Q[21:20]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff11 (.CLK(CLK), .EN(1'bx), .SRST(SRST), .D(D), .Q(Q[23:22]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff12 (.CLK(CLK), .EN(1'bx), .SET(SET), .CLR(CLR), .D(D), .Q(Q[25:24]));
+
+$dlatch #(.EN_POLARITY(1'b1), .WIDTH(2)) ff13 (.EN(1'bx), .D(D), .Q(Q[27:26]));
+$adlatch #(.EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff14 (.EN(1'bx), .ARST(ARST), .D(D), .Q(Q[29:28]));
+$dlatchsr #(.EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff15 (.EN(1'bx), .SET(SET), .CLR(CLR), .D(D), .Q(Q[31:30]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 2 t:$dffe
+select -assert-count 4 t:$dlatch
+select -assert-count 4 t:$sr
+select -assert-none t:$dffe t:$dlatch t:$sr %% %n t:* %i
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 2 t:$dffe
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$sdffe
+select -assert-count 1 t:$sdffce
+select -assert-count 1 t:$dffsre
+select -assert-count 3 t:$dlatch
+select -assert-count 1 t:$adlatch
+select -assert-count 1 t:$dlatchsr
+select -assert-count 2 t:$sr
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 4 t:$_DFFE_??_
+select -assert-count 8 t:$_DLATCH_?_
+select -assert-count 8 t:$_SR_??_
+select -assert-none t:$_DFFE_??_ t:$_DLATCH_?_ t:$_SR_??_ %% %n t:* %i
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 4 t:$_DFFE_??_
+select -assert-count 2 t:$_DFFE_????_
+select -assert-count 2 t:$_SDFFE_????_
+select -assert-count 2 t:$_SDFFCE_????_
+select -assert-count 2 t:$_DFFSRE_????_
+select -assert-count 6 t:$_DLATCH_?_
+select -assert-count 2 t:$_DLATCH_???_
+select -assert-count 2 t:$_DLATCHSR_???_
+select -assert-count 4 t:$_SR_??_
diff --git a/tests/opt/opt_dff_mux.ys b/tests/opt/opt_dff_mux.ys
new file mode 100644
index 000000000..ed01bed59
--- /dev/null
+++ b/tests/opt/opt_dff_mux.ys
@@ -0,0 +1,86 @@
+### CE and SRST matching.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input NE, NS;
+input EN;
+output [23:0] Q;
+input [23:0] D;
+input SRST;
+input ARST;
+input [1:0] CLR;
+input [1:0] SET;
+
+$dff #(.CLK_POLARITY(1'b1), .WIDTH(2)) ff0 (.CLK(CLK), .D(NS ? 2'h2 : NE ? D[1:0] : Q[1:0]), .Q(Q[1:0]));
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff1 (.CLK(CLK), .EN(EN), .D(NS ? 2'h2 : NE ? D[3:2] : Q[3:2]), .Q(Q[3:2]));
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .ARST(ARST), .D(NS ? 2'h2 : NE ? D[5:4] : Q[5:4]), .Q(Q[5:4]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .EN(EN), .ARST(ARST), .D(NS ? 2'h2 : NE ? D[7:6] : Q[7:6]), .Q(Q[7:6]));
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff4 (.CLK(CLK), .SRST(SRST), .D(NS ? 2'h2 : NE ? D[9:8] : Q[9:8]), .Q(Q[9:8]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff5 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(NS ? 2'h2 : NE ? D[11:10] : Q[11:10]), .Q(Q[11:10]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff6 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(NS ? 2'h2 : NE ? D[13:12] : Q[13:12]), .Q(Q[13:12]));
+$dffsr #(.CLK_POLARITY(1'b1), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff7 (.CLK(CLK), .SET(SET), .CLR(CLR), .D(NS ? 2'h2 : NE ? D[15:14] : Q[15:14]), .Q(Q[15:14]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff8 (.CLK(CLK), .EN(EN), .SET(SET), .CLR(CLR), .D(NS ? 2'h2 : NE ? D[17:16] : Q[17:16]), .Q(Q[17:16]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+clean
+select -assert-count 0 t:$dff
+select -assert-count 0 t:$dffe
+select -assert-count 0 t:$adff
+select -assert-count 2 t:$adffe
+select -assert-count 0 t:$dffsr
+select -assert-count 2 t:$dffsre
+select -assert-count 0 t:$sdff
+select -assert-count 3 t:$sdffe
+select -assert-count 2 t:$sdffce
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -nodffe -nosdff
+design -load postopt
+clean
+select -assert-count 1 t:$dff
+select -assert-count 1 t:$dffe
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsre
+select -assert-count 1 t:$sdff
+select -assert-count 1 t:$sdffe
+select -assert-count 1 t:$sdffce
+equiv_opt -undef -assert -multiclock opt_dff -nodffe
+design -load postopt
+clean
+select -assert-count 0 t:$dff
+select -assert-count 0 t:$dffe
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsre
+select -assert-count 2 t:$sdff
+select -assert-count 1 t:$sdffe
+select -assert-count 2 t:$sdffce
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -nosdff
+design -load postopt
+clean
+select -assert-count 0 t:$dff
+select -assert-count 2 t:$dffe
+select -assert-count 0 t:$adff
+select -assert-count 2 t:$adffe
+select -assert-count 0 t:$dffsr
+select -assert-count 2 t:$dffsre
+select -assert-count 0 t:$sdff
+select -assert-count 2 t:$sdffe
+select -assert-count 1 t:$sdffce
diff --git a/tests/opt/opt_dff_qd.ys b/tests/opt/opt_dff_qd.ys
new file mode 100644
index 000000000..afc96c42f
--- /dev/null
+++ b/tests/opt/opt_dff_qd.ys
@@ -0,0 +1,56 @@
+### Q = D case.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input EN;
+(* init = 24'h555555 *)
+output [23:0] Q;
+input SRST;
+input ARST;
+input [1:0] CLR;
+input [1:0] SET;
+
+$dff #(.CLK_POLARITY(1'b1), .WIDTH(2)) ff0 (.CLK(CLK), .D(Q[1:0]), .Q(Q[1:0]));
+$dffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(2)) ff1 (.CLK(CLK), .EN(EN), .D(Q[3:2]), .Q(Q[3:2]));
+$adff #(.CLK_POLARITY(1'b1), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .ARST(ARST), .D(Q[5:4]), .Q(Q[5:4]));
+$adffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .EN(EN), .ARST(ARST), .D(Q[7:6]), .Q(Q[7:6]));
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff4 (.CLK(CLK), .SRST(SRST), .D(Q[9:8]), .Q(Q[9:8]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff5 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(Q[11:10]), .Q(Q[11:10]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff6 (.CLK(CLK), .EN(EN), .SRST(SRST), .D(Q[13:12]), .Q(Q[13:12]));
+$dffsr #(.CLK_POLARITY(1'b1), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff7 (.CLK(CLK), .SET(SET), .CLR(CLR), .D(Q[15:14]), .Q(Q[15:14]));
+$dffsre #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff8 (.CLK(CLK), .EN(EN), .SET(SET), .CLR(CLR), .D(Q[17:16]), .Q(Q[17:16]));
+
+$dlatch #(.EN_POLARITY(1'b1), .WIDTH(2)) ff9 (.EN(EN), .D(Q[19:18]), .Q(Q[19:18]));
+$adlatch #(.EN_POLARITY(1'b0), .ARST_POLARITY(1'b1), .ARST_VALUE(2'h2), .WIDTH(2)) ff10 (.EN(EN), .ARST(ARST), .D(Q[21:20]), .Q(Q[21:20]));
+$dlatchsr #(.EN_POLARITY(1'b0), .CLR_POLARITY(1'b1), .SET_POLARITY(1'b0), .WIDTH(2)) ff11 (.EN(EN), .SET(SET), .CLR(CLR), .D(Q[23:22]), .Q(Q[23:22]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+# Equivalence check will fail for unmapped adlatch and dlatchsr due to negative hold hack.
+delete top/ff10 top/ff11
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+
+design -load orig
+opt_dff -keepdc
+select -assert-count 1 t:$and
+select -assert-count 3 t:$dffe
+select -assert-count 3 t:$dlatch
+select -assert-count 3 t:$sr
+select -assert-none t:$and t:$dffe t:$dlatch t:$sr %% %n t:* %i
+
+design -load orig
+simplemap
+opt_dff -keepdc
+select -assert-count 2 t:$_AND_
+select -assert-count 6 t:$_DFFE_??_
+select -assert-count 6 t:$_DLATCH_?_
+select -assert-count 6 t:$_SR_??_
+select -assert-none t:$_AND_ t:$_DFFE_??_ t:$_DLATCH_?_ t:$_SR_??_ %% %n t:* %i
+
diff --git a/tests/opt/opt_dff_sr.ys b/tests/opt/opt_dff_sr.ys
new file mode 100644
index 000000000..daedb115c
--- /dev/null
+++ b/tests/opt/opt_dff_sr.ys
@@ -0,0 +1,304 @@
+### Always-active SET/CLR removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input CLR;
+input SET;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR({CLR, CLR, CLR, 1'b1, 1'b0, 1'bx}), .SET({1'b1, 1'b0, 1'bx, SET, SET, SET}), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .CLR({CLR, CLR, CLR, 1'b1, 1'b0, 1'bx}), .SET({1'b1, 1'b0, 1'bx, SET, SET, SET}), .D(D), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .CLR({CLR, CLR, CLR, 1'b1, 1'b0, 1'bx}), .SET({1'b1, 1'b0, 1'bx, SET, SET, SET}), .D(D), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR({CLR, CLR, CLR, 1'b1, 1'b0, 1'bx}), .SET({1'b1, 1'b0, 1'bx, SET, SET, SET}), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsr r:WIDTH=2 %i
+select -assert-count 1 t:$dffsre
+select -assert-count 1 t:$dffsre r:WIDTH=2 %i
+select -assert-count 1 t:$dlatchsr
+select -assert-count 1 t:$dlatchsr r:WIDTH=2 %i
+select -assert-none t:$sr
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsr r:WIDTH=4 %i
+select -assert-count 1 t:$dffsre
+select -assert-count 1 t:$dffsre r:WIDTH=4 %i
+select -assert-count 1 t:$dlatchsr
+select -assert-count 1 t:$dlatchsr r:WIDTH=4 %i
+select -assert-count 1 t:$sr
+select -assert-count 1 t:$sr r:WIDTH=4 %i
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 1 t:$_DFF_PP0_
+select -assert-count 1 t:$_DFF_PP1_
+select -assert-count 1 t:$_DFFE_PN0P_
+select -assert-count 1 t:$_DFFE_PN1P_
+select -assert-count 1 t:$_DLATCH_PP0_
+select -assert-count 1 t:$_DLATCH_PN1_
+select -assert-none t:$_DFF_PP0_ t:$_DFF_PP1_ t:$_DFFE_PN0P_ t:$_DFFE_PN1P_ t:$_DLATCH_PP0_ t:$_DLATCH_PN1_ t:$_NOT_ %% %n t:* %i
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$_DFF_PP0_
+select -assert-count 1 t:$_DFF_PP1_
+select -assert-count 2 t:$_DFFSR_PPP_
+select -assert-count 1 t:$_DFFE_PN0P_
+select -assert-count 1 t:$_DFFE_PN1P_
+select -assert-count 2 t:$_DFFSRE_PNNP_
+select -assert-count 1 t:$_DLATCH_PP0_
+select -assert-count 1 t:$_DLATCH_PN1_
+select -assert-count 2 t:$_DLATCHSR_PNP_
+select -assert-count 1 t:$_DLATCH_P_
+select -assert-count 1 t:$_DLATCH_N_
+select -assert-count 2 t:$_SR_PN_
+select -assert-none t:$_DFF_PP0_ t:$_DFF_PP1_ t:$_DFFSR_PPP_ t:$_DFFE_PN0P_ t:$_DFFE_PN1P_ t:$_DFFSRE_PNNP_ t:$_DLATCH_PP0_ t:$_DLATCH_PN1_ t:$_DLATCHSR_PNP_ t:$_NOT_ t:$_DLATCH_N_ t:$_DLATCH_P_ t:$_SR_PN_ %% %n t:* %i
+
+design -reset
+
+
+
+### Never-active CLR removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input CLR;
+input SET;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR(6'h00), .SET({6{SET}}), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .D(D), .CLR(6'h3f), .SET({6{SET}}), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .D(D), .CLR(6'h00), .SET({6{SET}}), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR(6'h3f), .SET({6{SET}}), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 0 t:$dffsr
+select -assert-count 0 t:$dffsre
+select -assert-count 0 t:$dlatchsr
+select -assert-count 0 t:$sr
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$adlatch
+select -assert-count 1 t:$dlatch
+
+design -reset
+
+
+
+### Never-active CLR removal (not applicable).
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input CLR;
+input SET;
+input ALT;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR(6'h00), .SET({{5{SET}}, ALT}), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .D(D), .CLR(6'h3f), .SET({{5{SET}}, ALT}), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .D(D), .CLR(6'h00), .SET({{5{SET}}, ALT}), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR(6'h3f), .SET({{5{SET}}, ALT}), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsre
+select -assert-count 1 t:$dlatchsr
+select -assert-count 1 t:$sr
+select -assert-count 0 t:$adff
+select -assert-count 0 t:$adffe
+select -assert-count 0 t:$adlatch
+select -assert-count 0 t:$dlatch
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 0 t:$_DFFSR_*
+select -assert-count 0 t:$_DFFSRE_*
+select -assert-count 0 t:$_DLATCHSR_*
+select -assert-count 0 t:$_SR_*
+select -assert-count 6 t:$_DFF_PP1_
+select -assert-count 6 t:$_DFFE_PN1P_
+select -assert-count 6 t:$_DLATCH_PN1_
+select -assert-count 6 t:$_DLATCH_P_
+
+design -reset
+
+
+
+### Never-active SET removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input CLR;
+input SET;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR({6{CLR}}), .SET(6'h00), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .D(D), .CLR({6{CLR}}), .SET(6'h3f), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .D(D), .CLR({6{CLR}}), .SET(6'h3f), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR({6{CLR}}), .SET(6'h00), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 0 t:$dffsr
+select -assert-count 0 t:$dffsre
+select -assert-count 0 t:$dlatchsr
+select -assert-count 0 t:$sr
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$adlatch
+select -assert-count 1 t:$dlatch
+
+design -reset
+
+
+
+### Never-active CLR removal (not applicable).
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input CLR;
+input SET;
+input ALT;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR({{5{CLR}}, ALT}), .SET(6'h00), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .D(D), .CLR({{5{CLR}}, ALT}), .SET(6'h3f), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .D(D), .CLR({{5{CLR}}, ALT}), .SET(6'h3f), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR({{5{CLR}}, ALT}), .SET(6'h00), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$dffsr
+select -assert-count 1 t:$dffsre
+select -assert-count 1 t:$dlatchsr
+select -assert-count 1 t:$sr
+select -assert-count 0 t:$adff
+select -assert-count 0 t:$adffe
+select -assert-count 0 t:$adlatch
+select -assert-count 0 t:$dlatch
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 0 t:$_DFFSR_*
+select -assert-count 0 t:$_DFFSRE_*
+select -assert-count 0 t:$_DLATCHSR_*
+select -assert-count 0 t:$_SR_*
+select -assert-count 6 t:$_DFF_PP0_
+select -assert-count 6 t:$_DFFE_PN0P_
+select -assert-count 6 t:$_DLATCH_PP0_
+select -assert-count 6 t:$_DLATCH_N_
+
+design -reset
+
+
+
+### SET/CLR merge into ARST.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [5:0] D;
+output [23:0] Q;
+input ARST;
+input EN;
+
+$dffsr #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b1), .CLR_POLARITY(1'b1), .WIDTH(6)) ff0 (.CLK(CLK), .CLR({ARST, 5'h00}), .SET({1'b0, {5{ARST}}}), .D(D), .Q(Q[5:0]));
+$dffsre #(.CLK_POLARITY(1'b1), .SET_POLARITY(1'b0), .CLR_POLARITY(1'b0), .EN_POLARITY(1'b1), .WIDTH(6)) ff1 (.CLK(CLK), .EN(EN), .D(D), .CLR({ARST, 5'h1f}), .SET({1'b1, {5{ARST}}}), .Q(Q[11:6]));
+$dlatchsr #(.SET_POLARITY(1'b0), .CLR_POLARITY(1'b1), .EN_POLARITY(1'b1), .WIDTH(6)) ff2 (.EN(EN), .D(D), .CLR({ARST, 5'h00}), .SET({1'b1, {5{ARST}}}), .Q(Q[17:12]));
+$sr #(.SET_POLARITY(1'b1), .CLR_POLARITY(1'b0), .WIDTH(6)) ff3 (.CLR({ARST, 5'h1f}), .SET({1'b0, {5{ARST}}}), .Q(Q[23:18]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 0 t:$dffsr
+select -assert-count 0 t:$dffsre
+select -assert-count 1 t:$dlatchsr
+select -assert-count 1 t:$sr
+select -assert-count 1 t:$adff
+select -assert-count 1 t:$adff r:ARST_VALUE=6'h1f %i
+select -assert-count 1 t:$adffe
+select -assert-count 1 t:$adffe r:ARST_VALUE=6'h1f %i
+select -assert-count 0 t:$adlatch
+select -assert-count 0 t:$dlatch
diff --git a/tests/opt/opt_dff_srst.ys b/tests/opt/opt_dff_srst.ys
new file mode 100644
index 000000000..4a77de0b8
--- /dev/null
+++ b/tests/opt/opt_dff_srst.ys
@@ -0,0 +1,113 @@
+### Always-active SRST removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+(* init=12'h555 *)
+output [11:0] Q;
+input SRST;
+input EN;
+
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff0 (.CLK(CLK), .SRST(1'b1), .D(D), .Q(Q[1:0]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .SRST(1'b0), .EN(EN), .D(D), .Q(Q[3:2]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .SRST(1'b0), .EN(EN), .D(D), .Q(Q[5:4]));
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff3 (.CLK(CLK), .SRST(1'bx), .D(D), .Q(Q[7:6]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff4 (.CLK(CLK), .SRST(1'bx), .EN(EN), .D(D), .Q(Q[9:8]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff5 (.CLK(CLK), .SRST(1'bx), .EN(EN), .D(D), .Q(Q[11:10]));
+
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-count 0 t:$sdff
+select -assert-count 0 t:$sdffe
+select -assert-count 0 t:$sdffce
+select -assert-count 4 t:$dff
+select -assert-count 2 t:$dffe
+
+design -load orig
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 1 t:$sdff
+select -assert-count 1 t:$sdffe
+select -assert-count 1 t:$sdffce
+select -assert-count 2 t:$dff
+select -assert-count 1 t:$dffe
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:$_SDFF_???_
+select -assert-none t:$_SDFFE_????_
+select -assert-none t:$_SDFFCE_????_
+select -assert-count 8 t:$_DFF_?_
+select -assert-count 4 t:$_DFFE_??_
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff -keepdc
+design -load postopt
+select -assert-count 2 t:$_SDFF_???_
+select -assert-count 2 t:$_SDFFE_????_
+select -assert-count 2 t:$_SDFFCE_????_
+select -assert-count 4 t:$_DFF_?_
+select -assert-count 2 t:$_DFFE_??_
+
+design -reset
+
+
+### Never-active SRST removal.
+
+read_verilog -icells <<EOT
+
+module top(...);
+
+input CLK;
+input [1:0] D;
+output [5:0] Q;
+input SRST;
+input EN;
+
+$sdff #(.CLK_POLARITY(1'b1), .SRST_POLARITY(1'b1), .SRST_VALUE(2'h2), .WIDTH(2)) ff0 (.CLK(CLK), .SRST(1'b0), .D(D), .Q(Q[1:0]));
+$sdffe #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff1 (.CLK(CLK), .SRST(1'b1), .EN(EN), .D(D), .Q(Q[3:2]));
+$sdffce #(.CLK_POLARITY(1'b1), .EN_POLARITY(1'b1), .SRST_POLARITY(1'b0), .SRST_VALUE(2'h2), .WIDTH(2)) ff2 (.CLK(CLK), .SRST(1'b1), .EN(EN), .D(D), .Q(Q[5:4]));
+
+endmodule
+
+EOT
+
+design -save orig
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:$sdff
+select -assert-none t:$sdffe
+select -assert-none t:$sdffce
+select -assert-count 1 t:$dff
+select -assert-count 2 t:$dffe
+
+design -load orig
+simplemap
+
+equiv_opt -undef -assert -multiclock opt_dff
+design -load postopt
+select -assert-none t:$_SDFF_???_
+select -assert-none t:$_SDFFE_????_
+select -assert-none t:$_SDFFCE_????_
+select -assert-count 2 t:$_DFF_P_
+select -assert-count 4 t:$_DFFE_PP_
+
+design -reset
+
diff --git a/tests/opt/opt_rmdff.ys b/tests/opt/opt_rmdff.ys
index 6fb24fd66..998414597 100644
--- a/tests/opt/opt_rmdff.ys
+++ b/tests/opt/opt_rmdff.ys
@@ -4,7 +4,7 @@ design -stash gold
 
 read_verilog -icells opt_rmdff.v
 proc
-opt_rmdff
+opt_dff
 
 select -assert-count 0 c:remove*
 select -assert-min 7 c:keep*
diff --git a/tests/opt/opt_rmdff_sat.ys b/tests/opt/opt_rmdff_sat.ys
index 1c3dd9c05..231c43ecb 100644
--- a/tests/opt/opt_rmdff_sat.ys
+++ b/tests/opt/opt_rmdff_sat.ys
@@ -1,5 +1,5 @@
 read_verilog opt_rmdff_sat.v
 prep -flatten
-opt_rmdff -sat
-synth
+opt_dff -sat -nosdff
+simplemap
 select -assert-count 5 t:$_DFF_P_