I'm getting this error for my verilog code, "Illegal operation for constant expression"

Question

when ever I compile this code, I get the following errors.

module mv2_generate
(
    input [127:0] c_array [1:0],
    input [127:0] p_array [1:0],
    input [127:0] p1_array [1:0],
    output reg [15:0] min_mv
);

   //genvar index;
   integer a, b, index, m;
  //genvar m;
  // a= (m*7)+m+7;
  // b= (m*7)+m;

   reg [7:0] read_dataC;  //registers for C,P,P'
   reg [7:0] read_dataP;
   reg [7:0] read_dataP1;
   reg [15:0] out_pe0;
   reg pe0_en;
   pe PE0(.a(read_dataC),.b(read_dataP),.en(pe0_en),.pe_out(out_pe0));

always @*
begin
//generate
   for (index=0; index<2; index=index+1)
    begin
    // assign  n=n+1;
    //  a=7;
    //  b=0;


      for (m=0; m<16; m=m+1)
      begin
       if(index<2)
        begin
         if (m>=0)
          begin
           read_dataC = c_array [index] [(m*7)+m+7:(m*7)+m];
           read_dataP = p_array [index] [(m*7)+m+7:(m*7)+m];
           // read_dataC = c_array [index] [a:b];
          // read_dataP = p_array [index] [a:b];
          #50;
          $display("pe out: %d",out_pe0);
          //pe PE0(read_dataC, read_dataP, out_pe0);
         end

       // a= a+8;
       // b= b+8;
       end
      end
    end
  end
//endgenerate

     //assign min_mv= out_pe0;

endmodule 

 // 
 module pe(input [7:0] a, input [7:0] b, input en, output reg [7:0] pe_out);
       //reg [15:0] acc_temp = acc;
        always @* begin
        //$display("End of Sim: %d", en);
         if(en) begin
               if (a<b) begin
                        assign pe_out = b - a;
                 end
                 else if (a==b) begin
                       assign pe_out = 8'd0;
                 end
                 else begin
                       assign pe_out = a - b;
                 end
                 //acc_temp = acc_temp + pe_out;
                 //acc = acc_temp;
                  //$display("End of Sim: %d", acc);
             end
             else begin
              pe_out = 8'd0;
             end
             end
        endmodule

The errors

ncverilog(64): 15.20-s029: (c) Copyright 1995-2017 Cadence Design Systems, Inc.
file: mv2test.v
           read_dataC = c_array [index] [(m*7)+m+7:(m*7)+m];
                                          |
ncvlog: *E,NOTPAR (mv2test.v,38|42): Illegal operand for constant expression [4(IEEE)].
           read_dataC = c_array [index] [(m*7)+m+7:(m*7)+m];
                                               |
ncvlog: *E,NOTPAR (mv2test.v,38|47): Illegal operand for constant expression [4(IEEE)].
           read_dataC = c_array [index] [(m*7)+m+7:(m*7)+m];
                                                    |
ncvlog: *E,NOTPAR (mv2test.v,38|52): Illegal operand for constant expression [4(IEEE)].
           read_dataC = c_array [index] [(m*7)+m+7:(m*7)+m];
                                                         |
ncvlog: *E,NOTPAR (mv2test.v,38|57): Illegal operand for constant expression [4(IEEE)].
           read_dataP = p_array [index] [(m*7)+m+7:(m*7)+m];
                                          |
ncvlog: *E,NOTPAR (mv2test.v,39|42): Illegal operand for constant expression [4(IEEE)].
           read_dataP = p_array [index] [(m*7)+m+7:(m*7)+m];
                                               |
ncvlog: *E,NOTPAR (mv2test.v,39|47): Illegal operand for constant expression [4(IEEE)].
           read_dataP = p_array [index] [(m*7)+m+7:(m*7)+m];
                                                    |
ncvlog: *E,NOTPAR (mv2test.v,39|52): Illegal operand for constant expression [4(IEEE)].
           read_dataP = p_array [index] [(m*7)+m+7:(m*7)+m];
                                                         |
ncvlog: *E,NOTPAR (mv2test.v,39|57): Illegal operand for constant expression [4(IEEE)].
    module worklib.mv2_generate:v
        errors: 8, warnings: 0
module pe(input [7:0] a, input [7:0] b, input en, output reg [7:0] pe_out);
        |
ncvlog: *W,RECOME (mv2test.v,83|8): recompiling design unit worklib.pe:v.
    First compiled from line 59 of mv2test.v.
ncverilog: *E,VLGERR: An error occurred during parsing.  Review the log file for errors with the code *E and fix those identified problems to proceed.  Exiting with code (status 1).

Answer 1

This is illegal in Verilog (and SystemVerilog):

c_array [index] [(m*7)+m+7:(m*7)+m];

Specifically, you cannot have a variable on the right hand side of a : in a part select. Instead you need to write either this:

c_array [index] [(m*7)+m+7 -: 8];

or this:

c_array [index] [(m*7)+m +: 8];

The value on the left hand side of the -: and +: operators is the starting index. The number on the right hand side is the width. This must be constant. Hence, your error. The -: operator counts down from the starting index; the +: operator counts up. The direction of the original array declaration does not matter: you can use either operator irrespective of the direction of the array.