Tuesday, 8 January 2008

Checker implementation dilema

Ever encountered the checker implementation dilemma? How to implement it? Using temporal expressions or using methods? Once in a lifetime, every verification engineer comes to a stage where they have to take decision about the implementation of a checker. Once, I made a wrong decision and implemented a complex event-based checker in temporal expressions using "expect" statement. After that I was introduced to the hell of debugging temporal expressions in Specman. Event chart is a junk feature in the Specman. Filtering of the events, having full path of the events are not so easy to see in the events chart. Even, when-subtypes' events can not be viewed separately.
After having a lot of trouble debugging temporal expressions, I re-wrote the entire checker in the methods format using "check that" and "if else" construct. Debugging of method based checker is such an easy task in Specman, as Specman has a beautiful and reliable debugger. Lot of features, though might be basic, are quiet useful when it comes to debug the method based checkers. In Specman debugger, one can put conditional and unconditional breakpoints, can see the hierarchy of the threads, can add watch points on e fields and a lot more. I will discuss about debugging in Specman for the beginners in the separate post.
After that experience, I concluded following things for the checker implementation.
  1. Write down the checker in the simple english to understand its complexity.
  2. If checker is simple, I mean, just contains 2 to 3 basic events to check, then go for temporal expressions for the checker implementation.
  3. If checker is even a bit complex having more than 3 events, just go for the method based checker implementation. You will save a hell lot of time which might be lost in the debugging temporal expressions.
Method based checker implementation looks more readable, even though it requires extra codes to implement and it's a bit slower than temporal expressions based checkers. But at the end, you are going to save lot of time having clean checker.

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