prefix_or.v

`default_nettype none

/*
 * Prefix-Or operation.
 * b = { ..., a[2] | a[1] | a[0], a[1] | a[0], a[0] }
 */
module prefix_or #(
	parameter WIDTH = 0
) (
	input wire clk,
	input wire [WIDTH - 1:0] a,
	output wire [WIDTH - 1:0] b,
);

	wire [WIDTH:0] c;

	assign c[0] = 1'b0;

	genvar i;
	for (i = 0; i < WIDTH; i = i + 1) begin
		/*
		 * Equivalent naive implementation is (starting at i = 1)
		 * assign b[i] = a[i] | b[i - 1];
		 * There are of course fancy ways to arrange your luts to
		 * reduce the tree efficiently. But why bother when we have
		 * carry chains? :)
		 */
		SB_CARRY carry (
			.I0(1'b1),
			.I1(a[i]),
			.CI(c[i]),
			.CO(c[i + 1])
		);

		assign b[i] = a[i] | c[i];
	end
endmodule