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SKILL for the Skilled: Making Programs Clear and Concise

Comments(6)Filed under: Custom IC Design, Virtuoso, SKILL, analog, Team SKILL, Allegro, programming

The SKILL programming language augments Cadence core tool functionality for Virtuoso and Allegro customers. It is also an important development tool for internal Cadence services organizations as well as Cadence product development groups. We see the value, power, flexibility, and elegance of the language as an enabling tool for customizing and enhancing design environments. These capabilities are made possible by the tight integration of the SKILL programming language into the Cadence platform.

This post introduces an upcoming series of articles, SKILL for the Skilled, which will attempt to better enable users to exploit the power and elegance of this fun and interesting programming language.

WARNING: What you are about to read is HIGHLY OPINIONATED!

SKILL Functions: Short and Clear

SKILL programs are usually made up of functions. A function should be short, clear, and express intent. Some programming languages force the programmer to transform the software problem into what the language can express, rather than allowing the programmer to transform the language to fit the problem at hand. The unfortunate result is often that the SKILL program looks like a C program, and is twice to ten times as long as it needs to be. We'll look at this ability to transform the language in upcoming articles.

Example Program

To illustrate the idea that functions should be short and clear, here are two different implementations of the same conceptual function. The confusing one (#1) uses an imperative style. Avoid this style. The second one (#2) uses a functional style. Without sacrificing clarity, it expresses in two lines what the imperative style expresses in nine lines. It is clearer, probably easier to debug, most certainly more efficient computation-wise, and scales better to larger programs.

Implementation #1 -- needlessly confusing

procedure( abs_less_than_100(x)
prog( (value)
value = abs(x)
if( value < 100
then return(t)
else return(nil)
) ; if
) ; prog
) ; procedure
Implementation #2 -- clear and simple
(procedure (abs_less_than_100 x)
(abs x) < 100)

What were the programmers thinking?

What are the problems of Implementation #1? What was programmer #1's thought process? He was probably trying to think like a Von Neumann machine -- in terms of an arithmetic unit and moving values around between registers until the goal is achieved. Programmer #2 was probably trying to express a mathematical expression.

A more natural way to think

I claim that the mathematical evaluation model is an easier, more natural way for humans to think than trying awkwardly to think like a machine that's moving data between registers. Don't try to make the problem harder than it is.

Some mistakes to avoid

Aside from the two very different ways of thinking, there are several other issues I have with implementation #1.

  • It declares the useless variable: value. There is normally no need to declare a value which is used only once.
  • It unnecessarily uses  prog/return. There is no need to use prog/return if you want to return the value that is already in the tail position.
  • It unnecessarily uses if/then/else. There is no need to test an expression for TRUE/FALSE, and thereafter produce the same TRUE/FALSE using if/then/else.
  • It fills up half the screen space with lines containing only close parentheses. There is no need to put parentheses on separate lines. Rather use an editor which enforces indentation.
  • Its only comments are redundant; and worse, they will probably be out of date as soon as someone edits the function and forgets to check all the comments.
SKILL is easy, flexible, powerful, and elegant

The SKILL language is on the one hand easy to learn and easy to use for simple use-once-and-discard scripting tasks. On the other hand the power and flexibility of the language is evident when experienced programmers develop well designed, high quality SKILL based software applications. The elegance of the SKILL language is often underestimated. A well written SKILL program is easy to understand, takes fewer lines of code to implement, and incurs shorter development times than with most other language alternatives, assuming the developers understand the tools they are using.

Until now, it has been somewhat difficult to find insightful articles written about SKILL and how to apply it to day to day problems. I hope this series of articles enables you to get more benefit and enjoyment out of the SKILL programming language.

Jim Newton 

See Also:

Comments(6)

By Andrew Beckett on November 8, 2010
A couple of thoughts on Jim's post.
I don't entirely agree with the statement about  lines with just close parentheses. I think there can be some benefit in having them on separate lines as this can encourage readability of the code, provided the indentation is consistent. Similarly commenting the close parenthesis when it is a long way from the open parenthesis can help a reader (particularly one who is less familiar with SKILL) understand the code. But commenting every close is unnecessary, of course - the indentation should allow you to follow the code.
Conciseness is a laudable quality, but should not be the only aim. It's always important to ensure that code can be easily comprehended, and so if that means using an unnecessary variable in order to self-document the code, that can be a good thing. Of course in this very simple example, there is absolutely no need to have the prog, the if, or the local variable; the simplified function is just as easy to understand and is more concise.
Just in case anyone is confused by the semi-LISP syntax of the improved example, you could equally well writ e this as:
procedure(abs_less_than_100(x)
 abs(x)<100
)
Both are equivalent. Similarly true LISPers would write this as:
(defun abs_less_than_100 (x)
 (lessp (abs x) 100)
)
which is also equivalent!
The key points that I see from Jim's post is that SKILL is a functional programming language - everything you do is a function - and so it makes sense to think in terms of functions which return things. That's quite different from languages like C - so in SKILL, if(), case(), while(), foreach() etc all have return values - so you  might as well take advantage of that!
Andrew

By Team SKILL on November 8, 2010
Thanks for the response Andrew. Different people have different experience of course, but the problem which I've seen more often is that people write functions which contain too much clutter, not that people write functions which are too terse.  This is the problem I was addressing here.
About the close paren:  Yes, if it is too far away from the open paren it can indeed be a problem.  But the advise I'd give in that case is "Don't put the close paren very far away from the opening paren."   If your function is too long to fit on a screen, don't get a bigger screen, just split the function.  This also means you have to think about how to refactor it and find appropriate names for the components.  If you don't want to make the component functions global, you can make local functions for which there are very nice new syntaxes for in 615:  flet and labels.
-jim

By Ole Ejlersen on November 8, 2010
Hi

If you use a text editor that understand parathesis it does not really matter where the paran is placed. An example of that is UltraEdit that also have a skill plugin, read the skill post at www.cadence.com/.../17154.aspx

Best regards

Ole


By Damien Diederen on November 9, 2010
Jim: Flet/labels in 6.1.5?  That's great news, indeed!

I believe one of the reasons a lot of the SKILL code found in the wild is so cluttered is that people are trying to avoid external dependencies in pcells... and the lack of support for local functions (lambda is very unwieldy in pure SKILL) pushes them to write loooooooong and repetitive bodies of code.

It also makes your article very to-the-point; a nice followup (when 6.1.5 is out) would be to show how to write a clean (but still standalone) pcell using these constructs to define short local functions!

-Damien

P.-S. -- Are flet/labels going to be available in both language modes?  Is tracing local functions going to be possible?


By Team SKILL on November 9, 2010
Hi Damien, thanks for the positive feedback and the great questions.
Yes, destructuringBind, labels, and flet will be standard in 615.  The in the first release flet and labels will refuse to run (runtime error) if they are encountered in non-scheme code.  If the functions are needed in dynamically scoped SKILL code, an enhancement CCR will need to be filed.  However, destructuringBind will work in both dialects.
For anyone who wants to find out more of flet, labels, and destrucuringBind, the will be discussed in more detail in upcoming articles on this blog.  In the meantime, you can take a look at the following web pages to see how they work in Common Lisp.
www.lispworks.com/.../m_destru.htm
www.lispworks.com/.../s_flet_.htm
Or you can do a simple google search for: lisp flet labels destructuring-bind
kind regards
-jim

By Team SKILL on November 9, 2010
Hi Damien,
as to your comment about PCell code which is unmanageably long because of dependency management, I could not agree more.  The OODI (object oriented device infrastructure) which can be standard in 616, but is available to early customer who request it along with 615, will address many of these problems.
Using OODI, devices are implemented as SKILL++ classes, not in terms of pcDefinePCell. This means that (1) by default PCell code will be lexically scoped, (2) code reuse can be enforced to a limited extent  by the SKILL++ object system in terms of the class hierarchy, and (3) the base classes such as pcoStandardDevice will provide lots of powerful methods for easily creating complex PCells with very little customer code necessary.
I think this will be a good step toward decreasing code size of customer PCells.
-jim

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