Operation with different size operands in Verilog/SystemVerilog

When an operation such as an addtion or a substraction is done using different size operands than final variable, it is necessary to extend sign to ensure the operation is done properly.

Example:

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logic signed [21:0] acc;
logic signed [5:0] data_in;
logic [3:0] offset;

Wrong:

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acc = data_in + offset;

Sign on data_in will not be respected. data_in will be filled with 0 before doing the operation and won’t be taken as negative (if applies).

Correct:

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acc = {{16{data_in[5]}},data_in} + offset;

Extend sign to match number of acc_add bits before doing operation