Such as the code displayed below, how can I dump the internal signal just like _x into waveform when I use verilator with the version==4.210? Did I need to switch to upper version?

    integer i;
    reg [7:0] _x; // how to dump the internal signal in waveform?
    always @(*) begin
        if(!en) begin y=3'bzzz; ptr=0; _x=x; end
        else begin
            ptr=0;
            y = 0;
            _x=~_x;
            for(i = 0; i < 8; i = i+1)
                if(x[i] == 1) begin y = {7-i}[2:0]; ptr=1; end
        end
    end

By the way, the variable i typed integer is dumped automatically in the waveform so that it may be surprise for me.

Furthermore, this is the cmd that I used: verilator --cc --exe --build --trace -Wno-fatal -j `nproc` sim_main.cpp top.v.

I just read the mannual of verilator simply and roughly but nothing interesting found.