Randomizing arrays in SystemVerilog without the unique keyword presents a unique challenge. The unique keyword ensures that all elements within an array are distinct. Omitting it allows for duplicate values, requiring different randomization strategies. This article explores effective techniques for achieving this, addressing common questions and best practices.
Why Avoid the unique Keyword?
While the unique keyword simplifies array randomization by guaranteeing uniqueness, it's not always desirable. Sometimes, allowing duplicate values is crucial for modeling specific scenarios, like packet bursts with repeated data or error conditions involving redundant data transmission. Forcing uniqueness in such cases would be inaccurate and hinder realistic simulation.
Randomizing Arrays Without unique: Methods and Considerations
Here are several approaches to randomize arrays without the unique constraint, each with its own advantages and disadvantages:
1. Using randc for Constrained Randomization
The randc data type, combined with appropriate constraints, provides a powerful method for controlled randomization. While it doesn't explicitly prevent duplicates, careful constraint definition can influence the probability of duplicates.
class packet;
randc bit [7:0] data[10]; // Array of 8-bit data, allowing duplicates
constraint data_constraint {
foreach(data[i]) data[i] inside {[0:255]}; //Example constraint; adjust as needed.
}
endclass
module test;
packet p;
initial begin
p = new();
repeat (100) begin
p.randomize();
$display("Packet Data: ", p.data);
end
end
endmodule
This example uses randc to allow for duplicates within the data array. The data_constraint limits each byte to a value between 0 and 255. The frequency of duplicates depends on the range and the number of array elements. More tightly constrained ranges reduce the likelihood of duplicates.
2. Post-Randomization Check and Re-randomization
This method involves randomizing the array without any constraints, then checking for duplicates. If duplicates are found, the array is re-randomized. This iterative approach is simple but can be computationally expensive for large arrays or tight constraints.
function bit check_duplicates(randc bit [7:0] arr[]);
for (int i = 0; i < arr.size(); i++)
for (int j = i + 1; j < arr.size(); j++)
if (arr[i] == arr[j]) return 1; // Duplicate found
return 0; // No duplicates
endfunction
class packet;
rand bit [7:0] data[10]; //Allow duplicates
...
function void randomize();
do begin
super.randomize();
end while (check_duplicates(data));
endfunction
endclass
This code defines a function check_duplicates to detect duplicate values. The randomize function uses a do-while loop to repeatedly randomize the array until no duplicates exist.
3. Using a Helper Function for Random Value Generation
This approach involves creating a helper function that generates a random value and ensures it isn't already in the array. This offers more control over the randomization process compared to simple re-randomization.
function bit [7:0] get_unique_random_byte(ref bit [7:0] arr[]);
bit [7:0] random_byte;
do begin
random_byte = $random() % 256;
end while (arr.find(random_byte) != -1); //Check if the value exists in the array already.
return random_byte;
endfunction
class packet;
bit [7:0] data[10];
...
function void randomize();
for (int i = 0; i < data.size(); i++)
data[i] = get_unique_random_byte(data);
endfunction
endclass
This uses a helper function get_unique_random_byte that keeps generating random values until a unique one is found. This provides a more efficient solution compared to the simple re-randomization method.
Choosing the Right Approach
The best approach depends on your specific needs and the characteristics of your simulation. For simple cases, re-randomization might suffice. For large arrays or complex scenarios with performance constraints, a helper function or careful constraint definition is recommended. Always consider the trade-off between simulation speed and the accuracy of the modeling. Using randc with constraints gives you a middle ground. It allows duplicates but can control their probability, offering efficiency while maintaining a degree of control.
This detailed explanation and code examples provide a comprehensive guide to randomizing arrays in SystemVerilog without relying on the unique keyword. Remember to choose the method that best suits your simulation requirements and performance goals.
