wreduce: Optimize signedness when possible

passes/opt/wreduce.cc

@@ -263,6 +263,19 @@ struct WreduceWorker
 		}
 	}
 
+	int reduced_opsize(const SigSpec &inp, bool signed_)
+	{
+		int size = GetSize(inp);
+		if (signed_) {
+			while (size >= 2 && inp[size - 1] == inp[size - 2])
+				size--;
+		} else {
+			while (size >= 1 && inp[size - 1] == State::S0)
+				size--;
+		}
+		return size;
+	}
+
 	void run_cell(Cell *cell)
 	{
 		bool did_something = false;
@@ -295,6 +308,45 @@ struct WreduceWorker
 		bool port_a_signed = false;
 		bool port_b_signed = false;
 
+		// For some operations if the output is no wider than either of the inputs
+		// we are free to choose the signedness of the operands
+		if (cell->type.in(ID($mul), ID($add), ID($sub)) &&
+				max_port_a_size == GetSize(sig) &&
+				max_port_b_size == GetSize(sig)) {
+			SigSpec sig_a = mi.sigmap(cell->getPort(ID::A)), sig_b = mi.sigmap(cell->getPort(ID::B));
+
+			// Remove top bits from sig_a and sig_b which are not visible on the output
+			sig_a.extend_u0(max_port_a_size);
+			sig_b.extend_u0(max_port_b_size);
+
+			int signed_cost, unsigned_cost;
+			if (cell->type == ID($mul)) {
+				signed_cost = reduced_opsize(sig_a, true) * reduced_opsize(sig_b, true);
+				unsigned_cost = reduced_opsize(sig_a, false) * reduced_opsize(sig_b, false);
+			} else {
+				signed_cost = max(reduced_opsize(sig_a, true), reduced_opsize(sig_b, true));
+				unsigned_cost = max(reduced_opsize(sig_a, false), reduced_opsize(sig_b, false));
+			}
+
+			if (!port_a_signed && !port_b_signed && signed_cost < unsigned_cost) {
+				log("Converting cell %s.%s (%s) from unsigned to signed.\n",
+						log_id(module), log_id(cell), log_id(cell->type));
+				cell->setParam(ID::A_SIGNED, 1);
+				cell->setParam(ID::B_SIGNED, 1);
+				port_a_signed = true;
+				port_b_signed = true;
+				did_something = true;
+			} else if (port_a_signed && port_b_signed && unsigned_cost < signed_cost) {
+				log("Converting cell %s.%s (%s) from signed to unsigned.\n",
+						log_id(module), log_id(cell), log_id(cell->type));
+				cell->setParam(ID::A_SIGNED, 0);
+				cell->setParam(ID::B_SIGNED, 0);
+				port_a_signed = false;
+				port_b_signed = false;
+				did_something = true;
+			}
+		}
+
 		if (max_port_a_size >= 0 && cell->type != ID($shiftx))
 			run_reduce_inport(cell, 'A', max_port_a_size, port_a_signed, did_something);
 

tests/various/wreduce2.ys

@@ -0,0 +1,22 @@
+read_verilog <<EOF
+module top(a, b, y);
+	parameter awidth = 6;
+	parameter bwidth = 8;
+	parameter ywidth = 14;
+
+	input [awidth-1:0] a;
+	input [bwidth-1:0] b;
+	output [ywidth-1:0] y;
+
+	wire [ywidth-1:0] aext = {{(ywidth-awidth){a[awidth-1]}}, a};
+	wire [ywidth-1:0] bext = {{(ywidth-bwidth){b[bwidth-1]}}, b};
+
+	assign y = aext*bext;
+endmodule
+EOF
+
+opt_clean
+wreduce
+select -assert-count 1 t:$mul
+select -assert-count 1 t:$mul r:A_SIGNED=1 r:B_SIGNED=1 %i %i
+select -assert-count 1 t:$mul r:A_WIDTH=6 r:B_WIDTH=8 %i %i