Can't compile a module. VVP stucks.

module alu(i_a, i_b , i_oper, o_y, o_status, i_rsn, i_clk);
parameter BITS = 4;
parameter OPER = 3;
input logic signed [BITS - 1:0] i_a, i_b;
input logic [OPER-2:0] i_oper;
input logic i_clk;
input logic i_rsn;
output logic signed [BITS - 1:0] o_y;
output logic [3:0] o_status;
logic [BITS - 1:0] o_y_mux;
logic [BITS - 1:0] o_y_1;
logic [BITS - 1:0] o_y_2;
logic [BITS - 1:0] o_y_3;
logic [BITS - 1:0] o_y_4;
logic [3:0] o_status_mux;
logic [3:0] o_status_1;
logic [3:0] o_status_2;
logic [3:0] o_status_3;
logic [3:0] o_status_4;

ashift_example1 #(.BITS(BITS)) case1 (.i_a(i_a), .i_b(i_b), .o_arith_left(o_y_1), .o_error(o_status_1));
U2_to_ZM #(.BITS(BITS)) case2 (.i_a(i_a), .o_zm(o_y_2), .o_status(o_status_2));
porownanie #(.BITS(BITS)) case3 (.i_a(i_a), .i_b(i_b), .o_smaller(o_y_3), .o_error(o_status_3));
replace #(.BITS(BITS)) case4 (.i_a(i_a), .i_b(i_b), .o_replaced(o_y_4), .o_error(o_status_4));

always_comb
begin

{o_y_mux, o_status_mux} = '0;
case (i_oper)

2'b00 : o_y_mux = o_y_1;
2'b01 : o_y_mux = o_y_2;
2'b10 : o_y_mux = o_y_3;
2'b11 : o_y_mux = o_y_4;
default : o_y_mux = '0;

endcase
case (i_oper)
2'b00 : o_status_mux = o_status_1;
2'b01 : o_status_mux = o_status_2;
2'b10 : o_status_mux = o_status_3;
2'b11 : o_status_mux = o_status_4;
default : o_status_mux = '0;
endcase
end

always_ff @(posedge i_clk)
begin
if(i_rsn == 1)
begin
o_y <= o_y_mux;
o_status <= o_status_mux;
end
else
begin
o_y <= '0;
o_status <= '0;
end
end
endmodule

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module ashift_example1(i_a, i_b, o_arith_left, o_error);
parameter BITS = 4;
input logic signed [BITS-1:0] i_a, i_b;
output logic signed [BITS-1:0] o_arith_left;
output logic [3:0] o_error;

integer x;
always_comb
begin
x = 0;
o_arith_left = '0;
o_error = '0;
if(i_b >= 0)
begin
o_arith_left = i_a <<< i_b;
if(i_a < 0)
begin
o_arith_left[BITS-1] = 'd1;
end
else
begin
o_arith_left[BITS-1] = 'd0;
end
end
else
begin
o_error[0] = 1;
o_arith_left = 'x;
end
for (int i = 0; i<BITS;i++)
begin
if(o_arith_left == 1)
begin
x = x+1;
end
end
if (x % 2 == 0 && o_arith_left != '0)
begin
o_error[1] = 1;
end
if(o_arith_left == '1)
begin
o_error[2] = 1;
end
end
endmodule

module U2_to_ZM (i_a , o_zm, o_status);

parameter BITS = 4;
input logic signed [BITS-1:0] i_a;
output logic signed [BITS-1:0] o_zm;
output logic [3:0] o_status;
logic signed [BITS-1:0] one;

integer x;
always_comb
begin
x = 0;
one = i_a;
o_status = '0;
if(i_a < 0)
begin
for (int i = 0; i<BITS-1; i++)
begin
one = ~i_a;
end
if(one == '1)
begin
o_status[3] = 1;
o_status[0] = 1;
o_zm = 'x;
end
else
begin
o_zm = one + 1;
end
end
else
begin
o_zm = one;
end
for (int i = 0; i<BITS;i++)
begin
if(o_zm == 1)
begin
x = x + 1;
end
end
if (x % 2 == 0 && o_zm != '0)
begin
o_status[1] = 1;
end
if (o_zm == '1)
begin
o_status[2] = 1;
end
end
endmodule

module porownanie(i_a, i_b, o_smaller, o_error);
parameter BITS = 4;
input logic signed [BITS-1:0] i_a, i_b;
output logic signed [BITS-1:0] o_smaller;
output logic [3:0] o_error;

always_comb
begin
o_error = '0;
o_smaller = '0;
if(i_a<=i_b)
begin
o_smaller = 'd1;
end
else
begin
o_smaller = '0;
end
if(o_smaller == '1)
begin
o_error[2] = 1;
end
end
endmodule

module replace(i_a, i_b, o_replaced, o_error);
parameter BITS = 4;
input logic signed [BITS-1:0] i_a, i_b;
output logic signed [BITS-1:0] o_replaced;
output logic [3:0] o_error;

integer x;
integer i;
always_comb
begin
x = 0;
i = 0;
o_replaced = '0;
o_error = '0;
if(i_b < 0 || i_b > BITS)
begin
o_error[0] = 1;
o_replaced = 'x;
end
else
begin
i = i_b;
o_replaced = i_a;
o_replaced = 1;
end
for (int i = 0; i<BITS;i++)
begin
if(o_replaced == 1)
begin
x = x+1;
end
end
if (x % 2 == 0 && o_replaced != '0)
begin
o_error[1] = 1;
end
if (o_replaced == '1)
begin
o_error[2] = 1;
end
end
endmodule

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