leading zero counter
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A design for high speed leading-zero counter
Leading Zero Counter：一个比较简明的设计
// This code is under development and not yet released to the public.
// Until it is released, the code is under the copyright of ETH Zurich and
// the University of Bologna, and may contain confidential and/or unpublished
// work. Any reuse/redistribution is strictly forbidden without written
// permission from ETH Zurich.
//
// Bug fixes and contributions will eventually be released under the
// SolderPad open hardware license in the context of the PULP platform
// (http://www.pulp-platform.org), under the copyright of ETH Zurich and the
// University of Bologna.
/// A trailing zero counter / leading zero counter.
/// Set MODE to 0 for trailing zero counter => cnt_o is the number of trailing zeros (from the LSB)
/// Set MODE to 1 for leading zero counter => cnt_o is the number of leading zeros (from the MSB)
/// If the input does not contain a zero, `empty_o` is asserted. Additionally `cnt_o` contains
/// the maximum number of zeros - 1. For example:
/// in_i = 000_0000, empty_o = 1, cnt_o = 6 (mode = 0)
/// in_i = 000_0001, empty_o = 0, cnt_o = 0 (mode = 0)
/// in_i = 000_1000, empty_o = 0, cnt_o = 3 (mode = 0)
/// Furthermore, this unit contains a more efficient implementation for Verilator (simulation only).
/// This speeds up simulation significantly.
module lzc #(
/// The width of the input vector.
parameter int unsigned WIDTH = 2,
parameter bit MODE = 1'b0, // 0 -> trailing zero, 1 -> leading zero
// Dependent parameters. Do not change!
parameter int unsigned CNT_WIDTH = WIDTH == 1 ? 1 : $clog2(WIDTH)
) (
input logic [WIDTH-1:0] in_i,
output logic [CNT_WIDTH-1:0] cnt_o,
output logic empty_o // asserted if all bits in in_i are zero
);
if (WIDTH == 1) begin: gen_degenerate_lzc
assign cnt_o[0] = !in_i[0];
assign empty_o = !in_i[0];
end else begin: gen_lzc
localparam int unsigned NUM_LEVELS = $clog2(WIDTH);
// pragma translate_off
initial begin
assert(WIDTH > 0) else $fatal(1, "input must be at least one bit wide");
end
// pragma translate_on
logic [WIDTH-1:0][NUM_LEVELS-1:0] index_lut;
logic [2**NUM_LEVELS-1:0] sel_nodes;
logic [2**NUM_LEVELS-1:0][NUM_LEVELS-1:0] index_nodes;
logic [WIDTH-1:0] in_tmp;
// reverse vector if required
always_comb begin : flip_vector
for (int unsigned i = 0; i < WIDTH; i++) begin
in_tmp[i] = (MODE) ? in_i[WIDTH-1-i] : in_i[i];
end
end
for (genvar j = 0; unsigned'(j) < WIDTH; j++) begin : g_index_lut
assign index_lut[j] = (NUM_LEVELS)'(unsigned'(j));
end
for (genvar level = 0; unsigned'(level) < NUM_LEVELS; level++) begin : g_levels
if (unsigned'(level) == NUM_LEVELS-1) begin : g_last_level
for (genvar k = 0; k < 2**level; k++) begin : g_level
// if two successive indices are still in the vector...
if (unsigned'(k) * 2 < WIDTH-1) begin
assign sel_nodes[2**level-1+k] = in_tmp[k*2] | in_tmp[k*2+1];
assign index_nodes[2**level-1+k] = (in_tmp[k*2] == 1'b1) ? index_lut[k*2] :
index_lut[k*2+1];
end
// if only the first index is still in the vector...
if (unsigned'(k) * 2 == WIDTH-1) begin
assign sel_nodes[2**level-1+k] = in_tmp[k*2];
assign index_nodes[2**level-1+k] = index_lut[k*2];
end
// if index is out of range
if (unsigned'(k) * 2 > WIDTH-1) begin
assign sel_nodes[2**level-1+k] = 1'b0;
assign index_nodes[2**level-1+k] = '0;
end
end
end else begin
for (genvar l = 0; l < 2**level; l++) begin : g_level
assign sel_nodes[2**level-1+l] = sel_nodes[2**(level+1)-1+l*2] | sel_nodes[2**(level+1)-1+l*2+1];
assign index_nodes[2**level-1+l] = (sel_nodes[2**(level+1)-1+l*2] == 1'b1) ? index_nodes[2**(level+1)-1+l*2] :
index_nodes[2**(level+1)-1+l*2+1];
end
end
end
assign cnt_o = NUM_LEVELS > unsigned'(0) ? index_nodes[0] : {($clog2(WIDTH)){1'b0}};
assign empty_o = NUM_LEVELS > unsigned'(0) ? ~sel_nodes[0] : ~(|in_i);
end : gen_lzc
endmodule : lzc
rr_arb_tree
popcount
