Add Kogge-Stone LCU module implementation

Author: shifengzhicheng

flow/platforms/common/lcu_kogge_stone.v

@@ -0,0 +1,34 @@
+(* techmap_celltype = "$lcu" *)
+module _80_lcu_kogge_stone (P, G, CI, CO);
+	parameter WIDTH = 2;
+
+	(* force_downto *)
+	input [WIDTH-1:0] P, G;
+	input CI;
+
+	(* force_downto *)
+	output [WIDTH-1:0] CO;
+
+	integer i, j;
+	(* force_downto *)
+	reg [WIDTH-1:0] p, g;
+
+	wire [1023:0] _TECHMAP_DO_ = "proc; opt -fast";
+
+	always @* begin
+		p = P;
+		g = G;
+
+		// in almost all cases CI will be constant zero
+		g[0] = g[0] | (p[0] & CI);
+
+		for (i = 0; i < $clog2(WIDTH); i = i + 1) begin
+			for (j = WIDTH - 1; j >= 2**i; j = j - 1) begin
+				g[j] = g[j] | p[j] & g[j - 2**i];
+				p[j] = p[j] & p[j - 2**i];
+			end
+		end
+	end
+
+	assign CO = g;
+endmodule