Merge pull request #4789 from YosysHQ/emil/sklansky-adder

Add a Sklansky option for `$lcu` mapping

Author: widlarizer

techlibs/common/Makefile.inc

@@ -37,3 +37,4 @@ $(eval $(call add_share_file,share,techlibs/common/cmp2lcu.v))
 $(eval $(call add_share_file,share,techlibs/common/cmp2softlogic.v))
 $(eval $(call add_share_file,share/choices,techlibs/common/choices/kogge-stone.v))
 $(eval $(call add_share_file,share/choices,techlibs/common/choices/han-carlson.v))
+$(eval $(call add_share_file,share/choices,techlibs/common/choices/sklansky.v))

techlibs/common/choices/sklansky.v

@@ -0,0 +1,37 @@
+(* techmap_celltype = "$lcu" *)
+module _80_lcu_sklansky (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
+			// iterate in reverse so we don't confuse a result from this stage and the previous
+			for (j = WIDTH - 1; j >= 0; j = j - 1) begin
+				if (j & 2**i) begin
+					g[j] = g[j] | p[j] & g[(j & ~(2**i - 1)) - 1];
+					p[j] = p[j] & p[(j & ~(2**i - 1)) - 1];
+				end
+			end
+		end
+	end
+
+	assign CO = g;
+endmodule

tests/techmap/sklansky.tcl

@@ -0,0 +1,15 @@
+yosys -import
+
+read_verilog +/choices/sklansky.v
+read_verilog -icells lcu_refined.v
+design -save init
+
+for {set i 1} {$i <= 16} {incr i} {
+    design -load init
+    chparam -set WIDTH $i
+    yosys proc
+    opt_clean
+    equiv_make lcu _80_lcu_sklansky equiv
+    equiv_simple equiv
+    equiv_status -assert equiv
+}