SystemVerilog Assertions (SVA) are a powerful mechanism for verifying the behavior of digital designs. They allow you to specify properties that your design should satisfy, and the simulator checks these properties during simulation. While SVA offers a rich set of constructs for expressing complex assertions, the use of goto statements within assertions is generally discouraged and considered bad practice. This article explores why, and offers better alternatives.
Why Avoid goto in SystemVerilog Assertions?
The goto statement, while available in SystemVerilog, is rarely used—and even less so within assertions. This is because:
-
Readability and Maintainability:
gotostatements make code significantly harder to read and understand. They disrupt the natural flow of execution, making it difficult to trace the program's logic and debug potential issues. In the context of assertions, this is particularly problematic, as assertions need to be easily understandable to ensure their correctness and effectiveness. -
Complexity and Debugging: The use of
gotocan introduce unexpected behavior and make debugging a nightmare. Complex jumps can create intricate control flow paths, making it difficult to isolate errors. The intricate nature of SVA itself makes this even worse. -
Verification Difficulty: Complex control flow introduced by
gotomakes it significantly harder for verification tools to analyze and optimize the assertions. This might lead to longer simulation times or even incorrect results. -
Alternative Constructs: SystemVerilog provides superior constructs to handle iterative and conditional behavior within assertions, rendering
gotounnecessary and undesirable. These includerepeat,while,for, and conditional statements (if,case,else).
How to Achieve Repetition in SystemVerilog Assertions (Without goto)
Let's examine how to achieve repetitive behavior in SVA without resorting to goto. Consider a scenario where you want to check a signal's value for a sequence of clock cycles. Instead of using goto, use the repeat construct:
property my_property;
@(posedge clk)
repeat (5)
a == 1;
endproperty
assert property (my_property);
This assertion checks if signal a is equal to 1 for five consecutive clock cycles. This is far clearer and more maintainable than a goto-based implementation.
Other Common Scenarios and Their Solutions
Here are a few frequently encountered scenarios and how to handle repetition without goto:
Scenario 1: Checking a condition until a specific event occurs.
Instead of using goto to jump back to a check, use a while loop within an always block combined with a disable iff clause. This limits the execution to a controlled duration.
always @(posedge clk) begin
while (condition && !event_occurred) begin
// Check conditions
if (some_condition) begin
event_occurred = 1'b1;
end
end
end
Scenario 2: Iterating over an array.
Use a for loop to iterate over the elements of an array:
for (int i = 0; i < array_size; i++) begin
assert (array[i] == expected_value[i]);
end
Conclusion: Embrace the Elegance of SVA's Built-in Constructs
SystemVerilog Assertions provide a powerful and expressive language for formal verification. While goto is technically available, it should be avoided in SVA due to its negative impact on readability, maintainability, and verification efficiency. SystemVerilog offers a suite of elegant and efficient constructs—repeat, while, for, and conditional statements—that provide far better alternatives for handling repetition and control flow within assertions. Embrace these constructs to write clear, concise, and easily maintainable assertions. Using these best practices will ensure your verification process is robust, reliable, and significantly easier to manage.
