VLSI DV Interview Puzzles · All levels

Queue of Handle Cells Still Aliases

A sequence item stores a queue of class-handle cells. Why does `dst.cells[0].v` change when only `src` is modified after copy?

Puzzle

Difficulty: Medium · Puzzle 2 of 6 · Topic: Handle and Copy Puzzles

A sequence item stores a queue of class-handle cells. Why does `dst.cells[0].v` change when only `src` is modified after copy?

Code

systemverilog

class cell extends uvm_object;
  `uvm_object_utils(cell)
  int v;
  function new(string name="cell"); super.new(name); endfunction
endclass

class tr extends uvm_sequence_item;
  `uvm_object_utils(tr)
  cell cells[$];
  function new(string name="tr");
    super.new(name);
    cells.push_back(cell::type_id::create("c0"));
    cells.push_back(cell::type_id::create("c1"));
  endfunction
  function void do_copy(uvm_object rhs);
    tr r;
    if (!$cast(r, rhs)) return;
    cells = r.cells;
  endfunction
endclass

initial begin
  tr src = tr::type_id::create("src");
  tr dst = tr::type_id::create("dst");
  src.cells[0].v = 5;
  dst.copy(src);
  src.cells[0].v = 99;
  $display("src=%0d dst=%0d", src.cells[0].v, dst.cells[0].v);
end

Hint

Queue assignment copies queue structure, but inspect the element type.

Step-by-step solution

diagram

1) `cells` is a queue of class handles, not packed values.
2) `cells = r.cells` duplicates queue indices but each element still points to the same `cell` objects.
3) Mutating `src.cells[0].v` mutates the shared object referenced by both queues.
4) Deep-copy by resizing `cells` and cloning each element in a loop.

Answer

Answer: Because queue assignment is shallow for class-handle elements; both queues share identical `cell` objects.

Why candidates get it wrong

People treat class-handle queues like queues of ints and assume assignment implies value-copy.

Interviewer follow-up

How would you deep-copy while preserving element dynamic type if `cell` has subclasses?

Navigate