SystemVerilog Assertions (SVA) are a powerful mechanism for verifying the correctness of hardware designs. While SVA offers many constructs for expressing complex verification conditions, the goto statement, often associated with repetition, is generally discouraged in assertion writing. This article explores the goto statement's role (or lack thereof) in SVA and highlights better alternatives for achieving repetitive assertion checks.
What is the goto Statement in SystemVerilog?
In SystemVerilog, the goto statement provides an unconditional jump to a labeled statement within the same function or task. It's typically used for transferring control flow outside the normal sequential execution. While it can be employed to create loops, it's considered bad practice in most programming contexts, including SVA, due to its potential to create confusing and hard-to-maintain code.
Why Avoid goto for Repetition in SVA Assertions?
Using goto for repetitive assertion checks in SVA is strongly advised against for several reasons:
-
Readability and Maintainability:
goto-based loops significantly reduce code readability. The control flow becomes convoluted, making it challenging to understand and debug. This is particularly problematic in complex verification environments where assertions are crucial. -
Debugging Complexity: Tracking the execution flow of a
goto-based loop during debugging is far more difficult than tracing a structured loop (e.g.,for,while,repeat). -
Error Proneness: Incorrectly implementing a
goto-based loop can easily lead to infinite loops or unexpected behavior, creating serious verification issues. -
SVA's Richer Constructs: SystemVerilog Assertions provide powerful constructs like
forloops,whileloops, and sequences that offer far more elegant and maintainable ways to express repetitive checks. These built-in constructs are specifically designed for the intricacies of hardware verification, making them inherently safer and easier to manage.
Better Alternatives for Repetitive Assertion Checks
Instead of resorting to goto, leverage SVA's built-in constructs for efficient and readable repetitive assertions:
1. for Loops
The for loop is ideal for iterating a fixed number of times:
for (int i = 0; i < 10; i++)
assert property (@(posedge clk) disable iff (reset) a[i] == b[i]);
This assertion verifies the equality of elements a[i] and b[i] for ten iterations.
2. while Loops
Use while loops for repetitive checks that depend on a condition:
int count = 0;
while (count < 10 && data_valid) begin
assert property (@(posedge clk) disable iff (reset) data_in == expected_data);
count++;
end
This assertion continues to check data_in against expected_data until either ten iterations are complete or data_valid becomes false.
3. Sequences and always blocks (for more complex scenarios)
For complex scenarios involving timing and sequence checking, combine sequences with always blocks or procedural code for cleaner and more structured repetition:
sequence seq_data;
@(posedge clk) data_valid ##1 data_in == expected_data;
endsequence
always @(posedge clk) begin
if (reset) count <= 0;
else if (data_valid) begin
if (count < 10) begin
assert property (seq_data);
count <= count + 1;
end
end
end
This approach allows for sophisticated timing and conditional checks within the repetitive verification.
Conclusion
While technically possible, using the goto statement for creating loops within SystemVerilog Assertions is strongly discouraged. The enhanced readability, maintainability, and reduced error proneness offered by for, while loops, sequences and always blocks make them the preferred methods for implementing repetitive assertion checks. Prioritizing structured programming practices within your SVA code results in more robust and easier-to-manage verification environments. Choose the appropriate method based on the complexity and nature of your repetitive assertion requirements. Remember that clean, well-structured code is crucial for effective verification.
