Dynamic addition/submission in Stratix-V DSP block

there is a template in Quartus > Edit > Insert Template > Verilog > Full Designs > Arithmetic > DSP Features > Dynamic Add/Sub

// Quartus II Verilog Template
// Dynamic add/sub control
// For use with the Stratix V, Arria-V, Cyclone-V and later device families
module dynamic_addsub(a1, b1, a2, b2, a3, b3, a4, b4, a5, b5, a6, b6,
	addnsub12, addnsub34, addnsub56, subnadd_chain34, subnadd_chain56,
	clock, ena, reset, s);
parameter a_width = 18;
parameter b_width = 18;
parameter o_width = 64;
input  a1;
input  b1;
input  a2;
input  b2;
input  a3;
input  b3;
input  a4;
input  b4;
input  a5;
input  b5;
input  a6;
input  b6;
input addnsub12;
input addnsub34;
input addnsub56;
input subnadd_chain34;
input subnadd_chain56;
input clock;
input ena;
input reset;
output  s;
reg signed  a1_reg, a2_reg, a3_reg, a4_reg, a5_reg, a6_reg;
reg signed  b1_reg, b2_reg, b3_reg, b4_reg, b5_reg, b6_reg;
reg signed  s12, s1234, s;
//Dynamic add/sub can be used in basic modes: sum-of-2 18x18, 36+18x18, two level sum-of-4 18x18, sum-of-2 27x27 and sum-of-2 36x18.
//Input and output signals of the dynamic add/sub operation must be explicitly defined. 
wire signed  p1, p2, p3, p4, p5, p6;
wire signed  p12, p34, p56;
assign p1 = a1_reg*b1_reg;
assign p2 = a2_reg*b2_reg;
assign p3 = a3_reg*b3_reg;
assign p4 = a4_reg*b4_reg;
assign p5 = a5_reg*b5_reg;
assign p6 = a6_reg*b6_reg;
assign p12 = addnsub12 ? (p1 + p2) : (p1 - p2);
assign p34 = addnsub34 ? (p3 + p4) : (p3 - p4);
assign p56 = addnsub56 ? (p5 + p6) : (p5 - p6);
//Dynamic add/sub is also applicable to chainout adder or accumulator (not both). 
//Dynamic add/sub is not applicable to 18x18 systolic mode.
always @(posedge clock or posedge reset)
if (reset) begin
	a1_reg <= 0;
	a2_reg <= 0;
	a3_reg <= 0;
	a4_reg <= 0;
	a5_reg <= 0;
	a6_reg <= 0;
	b1_reg <= 0;
	b2_reg <= 0;
	b3_reg <= 0;
	b4_reg <= 0;
	b5_reg <= 0;
	b6_reg <= 0;
	s12 <= 0;
	s1234 <= 0;
	s <= 0;
end else begin
	if (ena) begin
		a1_reg <= a1;
		a2_reg <= a2;
		a3_reg <= a3;
		a4_reg <= a4;
		a5_reg <= a5;
		a6_reg <= a6;
		b1_reg <= b1;
		b2_reg <= b2;
		b3_reg <= b3;
		b4_reg <= b4;
		b5_reg <= b5;
		b6_reg <= b6;
		s12 <= p12;
		s1234 <= subnadd_chain34 ? (s12 - p34) : (s12 + p34);
		s <= subnadd_chain56 ? (s1234 - p56) : (s1234 + p56);
	end
end
endmodule