SystemVerilog Assertions (SVA) are a powerful mechanism for verifying the correctness of hardware designs. While SVA offers a rich set of constructs for expressing complex verification conditions, the goto statement, often associated with repetition, is generally discouraged in SVA. This article will explore why, while providing a clear understanding of how it could be used (for educational purposes only). We'll also delve into safer and more readable alternatives for achieving repetitive assertion checks.
Understanding the goto Statement in SystemVerilog
In procedural SystemVerilog code, the goto statement allows unconditional branching to a labeled statement. This can be used to create loops, although it's generally considered bad practice due to its potential to produce spaghetti code, making verification logic difficult to understand and maintain.
The same principle applies within SVA. While technically possible, using goto for repetitive assertions is highly problematic. Let's illustrate with a contrived example:
module test;
bit [7:0] data;
initial begin
repeat (10) begin
data = $random;
// Instead of using a loop, consider the following (incorrect) goto example:
// label_start:
// assert property (@(posedge clk) data > 50) else $error("Data too low!");
// if (data < 100) goto label_start;
end
end
endmodule
The commented-out code shows a flawed attempt at using goto to repeatedly check the assertion. This approach is incredibly messy and makes the code practically unreadable and unmaintainable. Furthermore, it makes debugging significantly harder.
Why Avoid goto in SVA Assertions?
-
Readability and Maintainability:
gotomakes the code flow extremely difficult to follow. This severely impacts the readability and maintainability of your assertions, particularly in large and complex verification environments. Other developers (and even your future self) will struggle to understand the logic. -
Debugging Complexity: Tracing the execution path of code containing
gotostatements is significantly more challenging. Debugging becomes a nightmare, increasing development time and frustration. -
Verilog's inherent looping constructs: SystemVerilog provides superior constructs like
repeat,for, andwhileloops, which are designed specifically for iterative operations and offer better control and readability. These should always be preferred overgoto. -
Verification Tool Support: Some verification tools might not handle
gotostatements efficiently, impacting simulation performance or causing unexpected behavior.
Better Alternatives for Repetitive Assertion Checks
Instead of resorting to the goto statement, SystemVerilog provides elegant and efficient ways to implement repetitive assertion checks:
1. repeat Loops within always blocks (for procedural code)
For procedural code checking conditions, the repeat loop within an always block is a suitable alternative. This creates a clean and understandable loop for repeated assertion checks.
2. always blocks with a counter
This enables the creation of iterative processes. The counter determines when to terminate the loop. Assertions can be included within the always block.
3. Using sequence operators and property combinators
SVA's powerful sequence operators (like ##, |, &) and property combinators (like implies, until) provide a far more concise and readable way to express complex verification conditions, often eliminating the need for explicit looping constructs altogether. This is the recommended approach for most scenarios.
Conclusion
While the goto statement might technically be possible within SVA, it's strongly advised to avoid it. Its use leads to unreadable, unmaintainable, and difficult-to-debug code. SystemVerilog provides superior alternatives, such as repeat loops, always blocks, sequences and property combinators, for handling repetitive assertion checks. Prioritize clarity, maintainability, and the use of built-in constructs to create robust and efficient verification environments. This improves both the quality of your code and the overall efficiency of your verification process.
