Problem with while loop in an array

A for loop will work in your case, as you're only going through every entry in the table. With the while loop, imagine what would happen if no entries are 0? it will just loop forever, in 0 time. The synthesisor has no way of knowing that some entries will always be zero.

process(clk)
  variable num_packets : integer;
begin
  if rising_edge(clk) begin
    num_packets := 0;
    for i in packets_in'range loop
      if packets_in(i) /= ZERO_64 then
        num_packets := num_packets + 1;
      end if;
    end loop;
    output <= num_packets;
  end if;
end process;