System verilog: define empty vector

Question

In VHDL I can write (0 downto 1) and will generate a Vector with 0 elements.

I want to do this to "rightalign" a field in a union, if needed. Samplecode:

module example #(int ITEMS=3)(output logic[4:0] result);
union {
  logic [4:0] result;
  struct {
    logic systemstatus;
    logic [4-1-ITEMS-1:0] padding;
    logic [ITEMS-1:0] itemStatus;
  } bitfield;
} myStruct;
endmodule;

I want to generate a struct that looks roughly like this:

systemstatus | 0padder | item3 | item2 | item1 //ITEMS==3
systemstatus | item4   | item3 | item2 | item1 //ITEMS==4

As you can see the padding field disappears for 4 items. However the code above will not do what I want in system verilog since instead of creating a vector with 0 elements, I will always create a vector with at least one element ([0:0]), and when the expression becomes negative ([-1:0]]) I will create a vector with two elements.

Is there a way to let padding "decay/degenerate" to an empty vector? I know I could make myStruct.bitfield.itemStatus simply always the right size and always access a subvector. But the point is to do the magic once in the struct and not worry about it anymore in the rest of the code.

Answer 1

You want to use the let statement instead of a union. The let statement is like a macro, except it is limited to a scope, and limited by substituting expressions, not any arbitrary text.

module example #(int ITEMS=3)(output logic[4:0] result);
  let systemstatus = result[4];
  let itemStatus = result[ITEMS-1:0];
  ...
endmodule

If you still want do this using a union, you can get what you want by creating your union with two structs. Also note that you need to use a packed union and struct to guarantee bit alignment.

union packed {
  logic [4:0] result;
  struct packed {
    logic status;
    logic [3:0] padding;
  } system;
  struct packed {
    logic [4: 1+4-ITEMS] padding;
    logic [ITEMS-1:0] status;
  } item;
} myStruct; 

Now you can refer to mystruct.system.status and mystruct.item.status.