HDLbits Popcount3的一点疑问

题目：
A “population count” circuit counts the number of '1’s in an input vector. Build a population count circuit for a 3-bit input vector.

module top_module( 
    input [2:0] in,
    output [1:0] out );
    
    reg [1:0] out1;
    integer i;
    always@(*) begin
        out1 = 2'd0;
        for