From 42c356c49c04a735fe27e672ff31452170f84b40 Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Mon, 31 Dec 2018 23:53:23 +0000
Subject: [PATCH] opt_lut: eliminate LUTs evaluating to constants or inputs.

---
 passes/opt/opt_lut.cc     | 81 +++++++++++++++++++++++++++++++++++++++
 tests/opt/opt_lut_elim.il | 19 +++++++++
 tests/opt/opt_lut_elim.ys |  3 ++
 tests/opt/opt_lut_port.ys |  1 +
 4 files changed, 104 insertions(+)
 create mode 100644 tests/opt/opt_lut_elim.il
 create mode 100644 tests/opt/opt_lut_elim.ys

diff --git a/passes/opt/opt_lut.cc b/passes/opt/opt_lut.cc
index be050c713..261af538f 100644
--- a/passes/opt/opt_lut.cc
+++ b/passes/opt/opt_lut.cc
@@ -187,6 +187,87 @@ struct OptLutWorker
 		}
 		show_stats_by_arity();
 
+		log("\n");
+		log("Eliminating LUTs.\n");
+		for (auto lut : luts)
+		{
+			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");
+					module->connect(lut_output, value);
+
+					module->remove(lut);
+					luts.erase(lut);
+					luts_arity.erase(lut);
+					luts_dlogics.erase(lut);
+					luts_dlogic_inputs.erase(lut);
+				}
+			}
+		}
+		show_stats_by_arity();
+
 		log("\n");
 		log("Combining LUTs.\n");
 		pool<RTLIL::Cell*> worklist = luts;
diff --git a/tests/opt/opt_lut_elim.il b/tests/opt/opt_lut_elim.il
new file mode 100644
index 000000000..75675d983
--- /dev/null
+++ b/tests/opt/opt_lut_elim.il
@@ -0,0 +1,19 @@
+module \test
+  wire input 1 \i
+
+  wire output 2 \o1
+  cell $lut $1
+    parameter \LUT 16'0110100110010110
+    parameter \WIDTH 4
+    connect \A { \i 3'000 }
+    connect \Y \o1
+  end
+
+  wire output 2 \o2
+  cell $lut $2
+    parameter \LUT 16'0110100010010110
+    parameter \WIDTH 4
+    connect \A { \i 3'000 }
+    connect \Y \o2
+  end
+end
diff --git a/tests/opt/opt_lut_elim.ys b/tests/opt/opt_lut_elim.ys
new file mode 100644
index 000000000..8e5e23aea
--- /dev/null
+++ b/tests/opt/opt_lut_elim.ys
@@ -0,0 +1,3 @@
+read_ilang opt_lut_elim.il
+opt_lut
+select -assert-count 0 t:$lut
diff --git a/tests/opt/opt_lut_port.ys b/tests/opt/opt_lut_port.ys
index 51dfd988b..3cb4ecb23 100644
--- a/tests/opt/opt_lut_port.ys
+++ b/tests/opt/opt_lut_port.ys
@@ -1,2 +1,3 @@
 read_ilang opt_lut_port.il
+opt_lut
 select -assert-count 2 t:$lut
-- 
2.30.2