sta: very crude static timing analysis pass

Co-authored-by: Eddie Hung <eddie@fpgeh.com>

9 files changed, 502 insertions, 62 deletions

diff --git a/backends/aiger/xaiger.cc b/backends/aiger/xaiger.cc
index 66955d88e..e223f185e 100644
--- a/backends/aiger/xaiger.cc
+++ b/backends/aiger/xaiger.cc
@@ -261,26 +261,27 @@ struct XAigerWriter
 
 				if (!timing.count(inst_module->name))
 					timing.setup_module(inst_module);
-				auto &t = timing.at(inst_module->name).arrival;
-				for (const auto &conn : cell->connections()) {
-					auto port_wire = inst_module->wire(conn.first);
-					if (!port_wire->port_output)
+
+				for (auto &i : timing.at(inst_module->name).arrival) {
+					if (!cell->hasPort(i.first.name))
 						continue;
 
-					for (int i = 0; i < GetSize(conn.second); i++) {
-						auto d = t.at(TimingInfo::NameBit(conn.first,i), 0);
-						if (d == 0)
-							continue;
+					auto port_wire = inst_module->wire(i.first.name);
+					log_assert(port_wire->port_output);
+
+					auto d = i.second.first;
+					if (d == 0)
+						continue;
+					auto offset = i.first.offset;
 
 #ifndef NDEBUG
-						if (ys_debug(1)) {
-							static std::set<std::tuple<IdString,IdString,int>> seen;
-							if (seen.emplace(inst_module->name, conn.first, i).second) log("%s.%s[%d] abc9_arrival = %d\n",
-									log_id(cell->type), log_id(conn.first), i, d);
-						}
-#endif
-						arrival_times[conn.second[i]] = d;
+					if (ys_debug(1)) {
+						static pool<std::pair<IdString,TimingInfo::NameBit>> seen;
+						if (seen.emplace(inst_module->name, i.first).second) log("%s.%s[%d] abc9_arrival = %d\n",
+								log_id(cell->type), log_id(i.first.name), offset, d);
 					}
+#endif
+					arrival_times[cell->getPort(i.first.name)[offset]] = d;
 				}
 
 				if (abc9_flop)
diff --git a/kernel/constids.inc b/kernel/constids.inc
index 8ccb60089..566b76217 100644
--- a/kernel/constids.inc
+++ b/kernel/constids.inc
@@ -179,6 +179,7 @@ X(SRC_WIDTH)
 X(SRST)
 X(SRST_POLARITY)
 X(SRST_VALUE)
+X(sta_arrival)
 X(STATE_BITS)
 X(STATE_NUM)
 X(STATE_NUM_LOG2)
diff --git a/kernel/rtlil.cc b/kernel/rtlil.cc
index 88153a380..cd0f5ab12 100644
--- a/kernel/rtlil.cc
+++ b/kernel/rtlil.cc
@@ -480,6 +480,35 @@ vector<string> RTLIL::AttrObject::get_hdlname_attribute() const
 	return split_tokens(get_string_attribute(ID::hdlname), " ");
 }
 
+void RTLIL::AttrObject::set_intvec_attribute(RTLIL::IdString id, const vector<int> &data)
+{
+	std::stringstream attrval;
+	for (auto &i : data) {
+		if (attrval.tellp() > 0)
+			attrval << " ";
+		attrval << i;
+	}
+	attributes[id] = RTLIL::Const(attrval.str());
+}
+
+vector<int> RTLIL::AttrObject::get_intvec_attribute(RTLIL::IdString id) const
+{
+	vector<int> data;
+	auto it = attributes.find(id);
+	if (it != attributes.end())
+		for (const auto &s : split_tokens(attributes.at(id).decode_string())) {
+			char *end = nullptr;
+			errno = 0;
+			long value = strtol(s.c_str(), &end, 10);
+			if (end != s.c_str() + s.size())
+				log_cmd_error("Literal for intvec attribute has invalid format");
+			if (errno == ERANGE || value < INT_MIN || value > INT_MAX)
+				log_cmd_error("Literal for intvec attribute is out of range");
+			data.push_back(value);
+		}
+	return data;
+}
+
 bool RTLIL::Selection::selected_module(RTLIL::IdString mod_name) const
 {
 	if (full_selection)
diff --git a/kernel/rtlil.h b/kernel/rtlil.h
index 68481b81c..073110f16 100644
--- a/kernel/rtlil.h
+++ b/kernel/rtlil.h
@@ -718,6 +718,9 @@ struct RTLIL::AttrObject
 
 	void set_hdlname_attribute(const vector<string> &hierarchy);
 	vector<string> get_hdlname_attribute() const;
+
+	void set_intvec_attribute(RTLIL::IdString id, const vector<int> &data);
+	vector<int> get_intvec_attribute(RTLIL::IdString id) const;
 };
 
 struct RTLIL::SigChunk
diff --git a/kernel/timinginfo.h b/kernel/timinginfo.h
index 9d88ac027..e7e4eab6e 100644
--- a/kernel/timinginfo.h
+++ b/kernel/timinginfo.h
@@ -49,9 +49,9 @@ struct TimingInfo
 
 	struct ModuleTiming
 	{
-		RTLIL::IdString type;
 		dict<BitBit, int> comb;
-		dict<NameBit, int> arrival, required;
+		dict<NameBit, std::pair<int,NameBit>> arrival, required;
+		bool has_inputs;
 	};
 
 	dict<RTLIL::IdString, ModuleTiming> data;
@@ -120,11 +120,10 @@ struct TimingInfo
 				}
 			}
 			else if (cell->type == ID($specify3)) {
-				auto src = cell->getPort(ID::SRC);
+				auto src = cell->getPort(ID::SRC).as_bit();
 				auto dst = cell->getPort(ID::DST);
-				for (const auto &c : src.chunks())
-					if (!c.wire->port_input)
-						log_error("Module '%s' contains specify cell '%s' where SRC '%s' is not a module input.\n", log_id(module), log_id(cell), log_signal(src));
+				if (!src.wire || !src.wire->port_input)
+					log_error("Module '%s' contains specify cell '%s' where SRC '%s' is not a module input.\n", log_id(module), log_id(cell), log_signal(src));
 				for (const auto &c : dst.chunks())
 					if (!c.wire->port_output)
 						log_error("Module '%s' contains specify cell '%s' where DST '%s' is not a module output.\n", log_id(module), log_id(cell), log_signal(dst));
@@ -136,34 +135,49 @@ struct TimingInfo
 					max = 0;
 				}
 				for (const auto &d : dst) {
-					auto &v = t.arrival[NameBit(d)];
-					v = std::max(v, max);
+					auto r = t.arrival.insert(NameBit(d));
+					auto &v = r.first->second;
+					if (r.second || v.first < max) {
+						v.first = max;
+						v.second = NameBit(src);
+					}
 				}
 			}
 			else if (cell->type == ID($specrule)) {
-				auto type = cell->getParam(ID::TYPE).decode_string();
-				if (type != "$setup" && type != "$setuphold")
+				IdString type = cell->getParam(ID::TYPE).decode_string();
+				if (type != ID($setup) && type != ID($setuphold))
 					continue;
 				auto src = cell->getPort(ID::SRC);
-				auto dst = cell->getPort(ID::DST);
+				auto dst = cell->getPort(ID::DST).as_bit();
 				for (const auto &c : src.chunks())
-					if (!c.wire->port_input)
+					if (!c.wire || !c.wire->port_input)
 						log_error("Module '%s' contains specify cell '%s' where SRC '%s' is not a module input.\n", log_id(module), log_id(cell), log_signal(src));
-				for (const auto &c : dst.chunks())
-					if (!c.wire->port_input)
-						log_error("Module '%s' contains specify cell '%s' where DST '%s' is not a module input.\n", log_id(module), log_id(cell), log_signal(dst));
+				if (!dst.wire || !dst.wire->port_input)
+					log_error("Module '%s' contains specify cell '%s' where DST '%s' is not a module input.\n", log_id(module), log_id(cell), log_signal(dst));
 				int max = cell->getParam(ID::T_LIMIT_MAX).as_int();
 				if (max < 0) {
 					log_warning("Module '%s' contains specify cell '%s' with T_LIMIT_MAX < 0 which is currently unsupported. Clamping to 0.\n", log_id(module), log_id(cell));
 					max = 0;
 				}
 				for (const auto &s : src) {
-					auto &v = t.required[NameBit(s)];
-					v = std::max(v, max);
+					auto r = t.required.insert(NameBit(s));
+					auto &v = r.first->second;
+					if (r.second || v.first < max) {
+						v.first = max;
+						v.second = NameBit(dst);
+					}
 				}
 			}
 		}
 
+		for (auto port_name : module->ports) {
+			auto wire = module->wire(port_name);
+			if (wire->port_input) {
+				t.has_inputs = true;
+				break;
+			}
+		}
+
 		return t;
 	}
 
diff --git a/passes/cmds/Makefile.inc b/passes/cmds/Makefile.inc
index 53bfd40c6..01e052fdf 100644
--- a/passes/cmds/Makefile.inc
+++ b/passes/cmds/Makefile.inc
@@ -40,3 +40,4 @@ endif
 OBJS += passes/cmds/scratchpad.o
 OBJS += passes/cmds/logger.o
 OBJS += passes/cmds/printattrs.o
+OBJS += passes/cmds/sta.o
\ No newline at end of file
diff --git a/passes/cmds/sta.cc b/passes/cmds/sta.cc
new file mode 100644
index 000000000..13e1ee13c
--- /dev/null
+++ b/passes/cmds/sta.cc
@@ -0,0 +1,312 @@
+/*
+ *  yosys -- Yosys Open SYnthesis Suite
+ *
+ *  Copyright (C) 2012  Clifford Wolf <clifford@clifford.at>
+ *            (C) 2019  Eddie Hung <eddie@fpgeh.com>
+ *
+ *  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/yosys.h"
+#include "kernel/sigtools.h"
+#include "kernel/timinginfo.h"
+#include <deque>
+
+USING_YOSYS_NAMESPACE
+PRIVATE_NAMESPACE_BEGIN
+
+struct StaWorker
+{
+	Design *design;
+	Module *module;
+	SigMap sigmap;
+
+	struct t_data {
+		Cell* driver;
+		IdString dst_port, src_port;
+		vector<tuple<SigBit,int,IdString>> fanouts;
+		SigBit backtrack;
+		t_data() : driver(nullptr) {}
+	};
+	dict<SigBit, t_data> data;
+	std::deque<SigBit> queue;
+	struct t_endpoint {
+		Cell *sink;
+		IdString port;
+		int required;
+		t_endpoint() : sink(nullptr), required(0) {}
+	};
+	dict<SigBit, t_endpoint> endpoints;
+
+	int maxarrival;
+	SigBit maxbit;
+
+	pool<SigBit> driven;
+
+	StaWorker(RTLIL::Module *module) : design(module->design), module(module), sigmap(module), maxarrival(0)
+	{
+		TimingInfo timing;
+
+		for (auto cell : module->cells())
+		{
+			Module *inst_module = design->module(cell->type);
+			if (!inst_module) {
+				log_warning("Cell type '%s' not recognised! Ignoring.\n", log_id(cell->type));
+				continue;
+			}
+
+			if (!inst_module->get_blackbox_attribute()) {
+				log_warning("Cell type '%s' is not a black- nor white-box! Ignoring.\n", log_id(cell->type));
+				continue;
+			}
+
+			IdString derived_type = inst_module->derive(design, cell->parameters);
+			inst_module = design->module(derived_type);
+			log_assert(inst_module);
+
+			if (!timing.count(derived_type)) {
+				auto &t = timing.setup_module(inst_module);
+				if (t.has_inputs && t.comb.empty() && t.arrival.empty() && t.required.empty())
+					log_warning("Module '%s' has no timing arcs!\n", log_id(cell->type));
+			}
+
+			auto &t = timing.at(derived_type);
+			if (t.comb.empty() && t.arrival.empty() && t.required.empty())
+				continue;
+
+			pool<std::pair<SigBit,TimingInfo::NameBit>> src_bits, dst_bits;
+
+			for (auto &conn : cell->connections()) {
+				auto rhs = sigmap(conn.second);
+				for (auto i = 0; i < GetSize(rhs); i++) {
+					const auto &bit = rhs[i];
+					if (!bit.wire)
+						continue;
+					TimingInfo::NameBit namebit(conn.first,i);
+					if (cell->input(conn.first)) {
+						src_bits.insert(std::make_pair(bit,namebit));
+
+						auto it = t.required.find(namebit);
+						if (it == t.required.end())
+							continue;
+						auto r = endpoints.insert(bit);
+						if (r.second || r.first->second.required < it->second.first) {
+							r.first->second.sink = cell;
+							r.first->second.port = conn.first;
+							r.first->second.required = it->second.first;
+						}
+					}
+					if (cell->output(conn.first)) {
+						dst_bits.insert(std::make_pair(bit,namebit));
+						auto &d = data[bit];
+						d.driver = cell;
+						d.dst_port = conn.first;
+						driven.insert(bit);
+
+						auto it = t.arrival.find(namebit);
+						if (it == t.arrival.end())
+							continue;
+						const auto &s = it->second.second;
+						if (cell->hasPort(s.name)) {
+							auto s_bit = sigmap(cell->getPort(s.name)[s.offset]);
+							if (s_bit.wire)
+								data[s_bit].fanouts.emplace_back(bit,it->second.first,s.name);
+						}
+					}
+				}
+			}
+
+			for (const auto &s : src_bits)
+				for (const auto &d : dst_bits) {
+					auto it = t.comb.find(TimingInfo::BitBit(s.second,d.second));
+					if (it == t.comb.end())
+						continue;
+					data[s.first].fanouts.emplace_back(d.first,it->second,s.second.name);
+				}
+		}
+
+		for (auto port_name : module->ports) {
+			auto wire = module->wire(port_name);
+			if (wire->port_input) {
+				for (const auto &b : sigmap(wire)) {
+					queue.emplace_back(b);
+					driven.insert(b);
+				}
+				// All primary inputs to arrive at time zero
+				wire->set_intvec_attribute(ID::sta_arrival, std::vector<int>(GetSize(wire), 0));
+			}
+			if (wire->port_output)
+				for (const auto &b : sigmap(wire))
+					if (b.wire)
+						endpoints.insert(b);
+		}
+	}
+
+	void run()
+	{
+		while (!queue.empty()) {
+			auto b = queue.front();
+			queue.pop_front();
+			auto it = data.find(b);
+			if (it == data.end())
+				continue;
+			const auto& src_arrivals = b.wire->get_intvec_attribute(ID::sta_arrival);
+			log_assert(GetSize(src_arrivals) == GetSize(b.wire));
+			auto src_arrival = src_arrivals[b.offset];
+			for (const auto &d : it->second.fanouts) {
+				const auto &dst_bit = std::get<0>(d);
+				auto dst_arrivals = dst_bit.wire->get_intvec_attribute(ID::sta_arrival);
+				if (dst_arrivals.empty())
+					dst_arrivals = std::vector<int>(GetSize(dst_bit.wire), -1);
+				else
+					log_assert(GetSize(dst_arrivals) == GetSize(dst_bit.wire));
+				auto &dst_arrival = dst_arrivals[dst_bit.offset];
+				auto new_arrival = src_arrival + std::get<1>(d);
+				if (dst_arrival < new_arrival) {
+					auto dst_wire = dst_bit.wire;
+					dst_arrival = std::max(dst_arrival, new_arrival);
+					dst_wire->set_intvec_attribute(ID::sta_arrival, dst_arrivals);
+					queue.emplace_back(dst_bit);
+
+					data[dst_bit].backtrack = b;
+					data[dst_bit].src_port = std::get<2>(d);
+
+					auto it = endpoints.find(dst_bit);
+					if (it != endpoints.end())
+						new_arrival += it->second.required;
+					if (new_arrival > maxarrival && driven.count(b)) {
+						maxarrival = new_arrival;
+						maxbit = dst_bit;
+					}
+				}
+			}
+		}
+
+		auto b = maxbit;
+		if (b == SigBit()) {
+			log("No timing paths found.\n");
+			return;
+		}
+
+		log("Latest arrival time in '%s' is %d:\n", log_id(module), maxarrival);
+		auto it = endpoints.find(maxbit);
+		if (it != endpoints.end() && it->second.sink)
+			log("  %6d %s (%s.%s)\n", maxarrival, log_id(it->second.sink), log_id(it->second.sink->type), log_id(it->second.port));
+		else {
+			log("  %6d (%s)\n", maxarrival, b.wire->port_output ? "<primary output>" : "<unknown>");
+			if (!b.wire->port_output)
+				log_warning("Critical-path does not terminate in a recognised endpoint.\n");
+		}
+		auto jt = data.find(b);
+		while (jt != data.end()) {
+			int arrival = b.wire->get_intvec_attribute(ID::sta_arrival)[b.offset];
+			if (jt->second.driver) {
+				log("           %s\n", log_signal(b));
+				log("  %6d %s (%s.%s->%s)\n", arrival, log_id(jt->second.driver), log_id(jt->second.driver->type), log_id(jt->second.src_port), log_id(jt->second.dst_port));
+			}
+			else if (b.wire->port_input)
+				log("  %6d   %s (%s)\n", arrival, log_signal(b), "<primary input>");
+			else
+				log_abort();
+			b = jt->second.backtrack;
+			jt = data.find(b);
+		}
+
+		std::map<int, unsigned> arrival_histogram;
+		for (const auto &i : endpoints) {
+			const auto &b = i.first;
+			if (!driven.count(b))
+				continue;
+
+			if (!b.wire->attributes.count(ID::sta_arrival)) {
+				log_warning("Endpoint %s.%s has no (* sta_arrival *) value.\n", log_id(module), log_signal(b));
+				continue;
+			}
+
+			auto arrival = b.wire->get_intvec_attribute(ID::sta_arrival)[b.offset];
+			if (arrival < 0) {
+				log_warning("Endpoint %s.%s has no (* sta_arrival *) value.\n", log_id(module), log_signal(b));
+				continue;
+			}
+			arrival += i.second.required;
+			arrival_histogram[arrival]++;
+		}
+		// Adapted from https://github.com/YosysHQ/nextpnr/blob/affb12cc27ebf409eade062c4c59bb98569d8147/common/timing.cc#L946-L969
+		if (arrival_histogram.size() > 0) {
+			unsigned num_bins = 20;
+			unsigned bar_width = 60;
+			auto min_arrival = arrival_histogram.begin()->first;
+			auto max_arrival = arrival_histogram.rbegin()->first;
+			auto bin_size = std::max<unsigned>(1, ceil((max_arrival - min_arrival + 1) / float(num_bins)));
+			std::vector<unsigned> bins(num_bins);
+			unsigned max_freq = 0;
+			for (const auto &i : arrival_histogram) {
+				auto &bin = bins[(i.first - min_arrival) / bin_size];
+				bin += i.second;
+				max_freq = std::max(max_freq, bin);
+			}
+			bar_width = std::min(bar_width, max_freq);
+
+			log("\n");
+			log("Arrival histogram:\n");
+			log(" legend: * represents %d endpoint(s)\n", max_freq / bar_width);
+			log("         + represents [1,%d) endpoint(s)\n", max_freq / bar_width);
+			for (int i = num_bins-1; i >= 0; --i)
+				log("(%6d, %6d] |%s%c\n", min_arrival + bin_size * (i + 1), min_arrival + bin_size * i,
+						std::string(bins[i] * bar_width / max_freq, '*').c_str(),
+						(bins[i] * bar_width) % max_freq > 0 ? '+' : ' ');
+		}
+	}
+};
+
+struct StaPass : public Pass {
+	StaPass() : Pass("sta", "perform static timing analysis") { }
+	void help() override
+	{
+		//   |---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|
+		log("\n");
+		log("    sta [options] [selection]\n");
+		log("\n");
+		log("This command performs static timing analysis on the design. (Only considers\n");
+		log("paths within a single module, so the design must be flattened.)\n");
+		log("\n");
+	}
+	void execute(std::vector<std::string> args, RTLIL::Design *design) override
+	{
+		log_header(design, "Executing STA pass (static timing analysis).\n");
+
+		/*
+		size_t argidx;
+		for (argidx = 1; argidx < args.size(); argidx++) {
+			if (args[argidx] == "-TODO") {
+				continue;
+			}
+			break;
+		}
+		*/
+
+		extra_args(args, 1, design);
+
+		for (Module *module : design->selected_modules())
+		{
+			if (module->has_processes_warn())
+				continue;
+
+			StaWorker worker(module);
+			worker.run();
+		}
+	}
+} StaPass;
+
+PRIVATE_NAMESPACE_END
diff --git a/passes/techmap/abc9_ops.cc b/passes/techmap/abc9_ops.cc
index 7a6959971..29fe74ec7 100644
--- a/passes/techmap/abc9_ops.cc
+++ b/passes/techmap/abc9_ops.cc
@@ -648,40 +648,38 @@ void prep_delays(RTLIL::Design *design, bool dff_mode)
 		auto inst_module = design->module(cell->type);
 		log_assert(inst_module);
 
-		auto &t = timing.at(cell->type).required;
-		for (auto &conn : cell->connections_) {
-			auto port_wire = inst_module->wire(conn.first);
+		for (auto &i : timing.at(cell->type).required) {
+			auto port_wire = inst_module->wire(i.first.name);
 			if (!port_wire)
 				log_error("Port %s in cell %s (type %s) from module %s does not actually exist",
-						log_id(conn.first), log_id(cell), log_id(cell->type), log_id(module));
-			if (!port_wire->port_input)
-				continue;
-			if (conn.second.is_fully_const())
+						log_id(i.first.name), log_id(cell), log_id(cell->type), log_id(module));
+			log_assert(port_wire->port_input);
+
+			auto d = i.second.first;
+			if (d == 0)
 				continue;
 
-			SigSpec O = module->addWire(NEW_ID, GetSize(conn.second));
-			for (int i = 0; i < GetSize(conn.second); i++) {
-				auto d = t.at(TimingInfo::NameBit(conn.first,i), 0);
-				if (d == 0)
-					continue;
+			auto offset = i.first.offset;
+			auto O = module->addWire(NEW_ID);
+			auto rhs = cell->getPort(i.first.name);
 
 #ifndef NDEBUG
-				if (ys_debug(1)) {
-					static std::set<std::tuple<IdString,IdString,int>> seen;
-					if (seen.emplace(cell->type, conn.first, i).second) log("%s.%s[%d] abc9_required = %d\n",
-							log_id(cell->type), log_id(conn.first), i, d);
-				}
+			if (ys_debug(1)) {
+				static pool<std::pair<IdString,TimingInfo::NameBit>> seen;
+				if (seen.emplace(cell->type, i.first).second) log("%s.%s[%d] abc9_required = %d\n",
+						log_id(cell->type), log_id(i.first.name), offset, d);
+			}
 #endif
-				auto r = box_cache.insert(d);
-				if (r.second) {
-					r.first->second = delay_module->derive(design, {{ID::DELAY, d}});
-					log_assert(r.first->second.begins_with("$paramod$__ABC9_DELAY\\DELAY="));
-				}
-				auto box = module->addCell(NEW_ID, r.first->second);
-				box->setPort(ID::I, conn.second[i]);
-				box->setPort(ID::O, O[i]);
-				conn.second[i] = O[i];
+			auto r = box_cache.insert(d);
+			if (r.second) {
+				r.first->second = delay_module->derive(design, {{ID::DELAY, d}});
+				log_assert(r.first->second.begins_with("$paramod$__ABC9_DELAY\\DELAY="));
 			}
+			auto box = module->addCell(NEW_ID, r.first->second);
+			box->setPort(ID::I, rhs[offset]);
+			box->setPort(ID::O, O);
+			rhs[offset] = O;
+			cell->setPort(i.first.name, rhs);
 		}
 	}
 }
@@ -1006,16 +1004,16 @@ void prep_box(RTLIL::Design *design)
 				log_assert(GetSize(wire) == 1);
 				auto it = t.find(TimingInfo::NameBit(port_name,0));
 				if (it == t.end())
-					// Assume no connectivity if no setup time
-					ss << "-";
+					// Assume that no setup time means zero
+					ss << 0;
 				else {
-					ss << it->second;
+					ss << it->second.first;
 
 #ifndef NDEBUG
 					if (ys_debug(1)) {
 						static std::set<std::pair<IdString,IdString>> seen;
 						if (seen.emplace(module->name, port_name).second) log("%s.%s abc9_required = %d\n", log_id(module),
-								log_id(port_name), it->second);
+								log_id(port_name), it->second.first);
 					}
 #endif
 				}
diff --git a/tests/various/sta.ys b/tests/various/sta.ys
new file mode 100644
index 000000000..156c31c47
--- /dev/null
+++ b/tests/various/sta.ys
@@ -0,0 +1,81 @@
+read_verilog -specify <<EOT
+module buffer(input i, output o);
+specify
+(i => o) = 10;
+endspecify
+endmodule
+
+module top(input i);
+wire w;
+buffer b(.i(i), .o(w));
+endmodule
+EOT
+
+logger -expect warning "Critical-path does not terminate in a recognised endpoint\." 1
+sta
+
+
+design -reset
+read_verilog -specify <<EOT
+module top(input i, output o, p);
+assign o = i;
+endmodule
+EOT
+
+logger -expect log "No timing paths found\." 1
+sta
+
+
+design -reset
+read_verilog -specify <<EOT
+module buffer(input i, output o);
+specify
+(i => o) = 10;
+endspecify
+endmodule
+
+module top(input i, output o, p);
+buffer b(.i(i), .o(o));
+endmodule
+EOT
+
+sta
+
+
+design -reset
+read_verilog -specify <<EOT
+module buffer(input i, output o);
+specify
+(i => o) = 10;
+endspecify
+endmodule
+
+module top(input i, output o, p);
+buffer b(.i(i), .o(o));
+const0 c(.o(p));
+endmodule
+EOT
+
+logger -expect warning "Cell type 'const0' not recognised! Ignoring\." 1
+sta
+
+
+design -reset
+read_verilog -specify <<EOT
+module buffer(input i, output o);
+specify
+(i => o) = 10;
+endspecify
+endmodule
+module const0(output o);
+endmodule
+
+module top(input i, output o, p);
+buffer b(.i(i), .o(o));
+const0 c(.o(p));
+endmodule
+EOT
+
+sta
+
+logger -expect-no-warnings