1 files changed, 112 insertions, 8 deletions

diff --git a/passes/opt/opt_lut.cc b/passes/opt/opt_lut.cc
index be050c713..26855fd70 100644
--- a/passes/opt/opt_lut.cc
+++ b/passes/opt/opt_lut.cc
@@ -36,7 +36,7 @@ struct OptLutWorker
 	dict<RTLIL::Cell*, pool<RTLIL::Cell*>> luts_dlogics;
 	dict<RTLIL::Cell*, pool<int>> luts_dlogic_inputs;
 
-	int combined_count = 0;
+	int eliminated_count = 0, combined_count = 0;
 
 	bool evaluate_lut(RTLIL::Cell *lut, dict<SigBit, bool> inputs)
 	{
@@ -133,7 +133,7 @@ struct OptLutWorker
 				// Second, make sure that the connection to dedicated logic is legal. If it is not legal,
 				// it means one of the two things:
 				//   * The connection is spurious. I.e. this is dedicated logic that will be packed
-				//     with some other LUT, and it just happens to be conected to this LUT as well.
+				//     with some other LUT, and it just happens to be connected to this LUT as well.
 				//   * The connection is illegal.
 				// In either of these cases, we don't need to concern ourselves with preserving the connection
 				// between this LUT and this dedicated logic cell.
@@ -188,7 +188,7 @@ struct OptLutWorker
 		show_stats_by_arity();
 
 		log("\n");
-		log("Combining LUTs.\n");
+		log("Eliminating LUTs.\n");
 		pool<RTLIL::Cell*> worklist = luts;
 		while (worklist.size())
 		{
@@ -198,6 +198,106 @@ struct OptLutWorker
 				break;
 			}
 
+			auto lut = worklist.pop();
+			SigSpec lut_input = sigmap(lut->getPort("\\A"));
+			pool<int> &lut_dlogic_inputs = luts_dlogic_inputs[lut];
+
+			vector<SigBit> lut_inputs;
+			for (auto &bit : lut_input)
+			{
+				if (bit.wire)
+					lut_inputs.push_back(sigmap(bit));
+			}
+
+			bool const0_match = true;
+			bool const1_match = true;
+			vector<bool> input_matches;
+			for (size_t i = 0; i < lut_inputs.size(); i++)
+				input_matches.push_back(true);
+
+			for (int eval = 0; eval < 1 << lut_inputs.size(); eval++)
+			{
+				dict<SigBit, bool> eval_inputs;
+				for (size_t i = 0; i < lut_inputs.size(); i++)
+					eval_inputs[lut_inputs[i]] = (eval >> i) & 1;
+				bool value = evaluate_lut(lut, eval_inputs);
+				if (value != 0)
+					const0_match = false;
+				if (value != 1)
+					const1_match = false;
+				for (size_t i = 0; i < lut_inputs.size(); i++)
+				{
+					if (value != eval_inputs[lut_inputs[i]])
+						input_matches[i] = false;
+				}
+			}
+
+			int input_match = -1;
+			for (size_t i = 0; i < lut_inputs.size(); i++)
+				if (input_matches[i])
+					input_match = i;
+
+			if (const0_match || const1_match || input_match != -1)
+			{
+				log("Found redundant cell %s.%s.\n", log_id(module), log_id(lut));
+
+				SigBit value;
+				if (const0_match)
+				{
+					log("  Cell evaluates constant 0.\n");
+					value = State::S0;
+				}
+				if (const1_match)
+				{
+					log("  Cell evaluates constant 1.\n");
+					value = State::S1;
+				}
+				if (input_match != -1) {
+					log("  Cell evaluates signal %s.\n", log_signal(lut_inputs[input_match]));
+					value = lut_inputs[input_match];
+				}
+
+				if (lut_dlogic_inputs.size())
+				{
+					log("  Not eliminating cell (connected to dedicated logic).\n");
+				}
+				else
+				{
+					SigSpec lut_output = lut->getPort("\\Y");
+					for (auto &port : index.query_ports(lut_output))
+					{
+						if (port.cell != lut && luts.count(port.cell))
+							worklist.insert(port.cell);
+					}
+
+					module->connect(lut_output, value);
+					sigmap.add(lut_output, value);
+
+					module->remove(lut);
+					luts.erase(lut);
+					luts_arity.erase(lut);
+					luts_dlogics.erase(lut);
+					luts_dlogic_inputs.erase(lut);
+
+					eliminated_count++;
+					if (limit > 0)
+						limit--;
+				}
+			}
+		}
+		show_stats_by_arity();
+
+		log("\n");
+		log("Combining LUTs.\n");
+		worklist = luts;
+		while (worklist.size())
+		{
+			if (limit == 0)
+			{
+				log("Limit reached.\n");
+				break;
+			}
+
 			auto lutA = worklist.pop();
 			SigSpec lutA_input = sigmap(lutA->getPort("\\A"));
 			SigSpec lutA_output = sigmap(lutA->getPort("\\Y")[0]);
@@ -487,16 +587,20 @@ struct OptLutPass : public Pass {
 		}
 		extra_args(args, argidx, design);
 
-		int total_count = 0;
+		int eliminated_count = 0, combined_count = 0;
 		for (auto module : design->selected_modules())
 		{
-			OptLutWorker worker(dlogic, module, limit - total_count);
-			total_count += worker.combined_count;
+			OptLutWorker worker(dlogic, module, limit - eliminated_count - combined_count);
+			eliminated_count += worker.eliminated_count;
+			combined_count   += worker.combined_count;
 		}
-		if (total_count)
+		if (eliminated_count)
+			design->scratchpad_set_bool("opt.did_something", true);
+		if (combined_count)
 			design->scratchpad_set_bool("opt.did_something", true);
 		log("\n");
-		log("Combined %d LUTs.\n", total_count);
+		log("Eliminated %d LUTs.\n", eliminated_count);
+		log("Combined %d LUTs.\n", combined_count);
 	}
 } OptLutPass;