Cadence SKILL

SKILL
Paradigm functional, OOP
Developer Cadence Design Systems
First appeared 1990
Stable release
? / ?
Typing discipline dynamic
Major implementations
Cadence Allegro, Cadence APD, Cadence Concept HDL and Cadence Virtuoso. Major LISP implementation — Cadence UniCAD.
Dialects
SKILL, SKILL++
Influenced by
Scheme, Common Lisp, CLOS

SKILL is a Lisp dialect used as a scripting language and PCell (parameterized cells) description language used in many EDA software suites by Cadence Design Systems. It was originally put forth in an IEEE paper[1] in 1990.

History

SKILL was originally based on a flavor of Lisp called “Franz Lisp” created at UC Berkeley by the students of Professor Richard J. Fateman. SKILL is not an acronym; it is a name. For trademark reasons Cadence prefers it be capitalized.[2]

Franz Lisp and all other flavors of LISP were eventually superseded by an ANSI standard for Lisp called "Common Lisp." Historically, SKILL was known as IL. SKILL was a library of IL functions. The name was originally an initialism for Silicon Compiler Interface Language (SCIL), pronounced "SKIL", which then morphed into "SKILL", a plain English word that was easier for everyone to remember.

"IL" was just Interface Language. Whilst SKILL was used initially to describe the API rather than the language, the snappier name stuck. The name "IL" remains a common file extension used for SKILL code ".il" designating that the code contained in the file has lisp-2 semantics. Another possible file extension is ".ils" designating that the content has lisp-1 semantics.

Syntax

Comments

Comments are delimited by either the traditional Lisp semicolon

(car mylist) ; Comment from semicolon to end of the line

or C-style comments

/* Comment */   car(mylist)   /* Another comment */

Function call

SKILL programmers have a choice of expression syntaxes. Traditional Lisp syntax such as 

(car mylist)

can be mixed with C-like syntax such as 

car(mylist)

White space between the function name and the opening parenthesis, as in 

car (mylist)

is also allowed but has a different meaning. For example, 

(list car (mylist))

or equivalently 

list(car mylist())

has the traditional lisp meaning of a call to the list function with two operands car and (mylist), the first of which is a variable reference, and the second a call to the mylist function. Usually this is not what the beginner programmer intended. Thus, SKILL novices usually incorrectly think of car (mylist) as a syntax error.

Certain arithmetic operations can be also called using an infix notation. Thus each of the following is recognized and in fact are represented the same internally

(plus 1 2 3 4 5 6 7)
plus(1 2 3 4 5 6 7)
1+2+3+4+5+6+7

Functions may be called using several different indirect second order functions.

(inSkill  (apply 'plus '(1 2 3 4 5 6 7)))
(inScheme (apply plus '(1 2 3 4 5 6 7)))
(inSkill  (apply 'plus 1 2 3 '(4 5 6 7)))
(inScheme (apply plus 1 2 3 '(4 5 6 7)))
(inSkill  (funcall 'plus 1 2 3 4 5 6 7)))
(inScheme (funcall plus 1 2 3 4 5 6 7)))

Function definition

This infix notation was introduced to make it easier for programmers familiar with C and other procedural languages to understand and write SKILL code.

SKILL indeed internally represents the language as traditional Lisp S-expression, the surface syntax loosens this restriction compared to other lisps. More elaborate examples look quite different from their Lisp equivalents. For example, the factorial function in SKILL may be represented several different ways which are all compiled to the identical internal representation.

;; C style
procedure (factorial(n)
    if ( n <= 1 then
        1
    else
        n * factorial(n-1)
    )
)

;; LISP style
(procedure (factorial n)
    (if (leqp n 1)
        then 1
        else (times n (factorial (difference n 1)))))

;; Hybrid style
(procedure (factorial n)
    (if n <= 1
        then 1
        else n * (factorial n-1)))

Objects and types

The SKILL language supports several built-in object types: symbol, list, fixnum, flonum, string, port, assocTable, array, funobj, envobj, and stdobj.

Functions

The SKILL language supports both dynamic (sometimes referred to as special) and lexical variables. However, a single function is limited to interpret its variables homogeneously. There are several ways of syntactically indicating whether a function be interpreted lexically or dynamically. One of which are to use (inSkill ..) and (inScheme ...) forms indicating dynamic and lexical scoping respectively.

(inSkill (lambda (A B) (A B)))

In the inSkill example A and B are dynamic variables in the parameter list of the lambda (the lambda list), and (A B) within the body of the function is interpreted as a call to the globally defined function A passing the value of the local dynamic variable B.

(inScheme (lambda (A B) (A B)))

In the inScheme example A and B are lexical variables in both the lambda list, and also (A B) within the body of the function which is interpreted as a call to the function A which has been passed and a parameter the value of the local lexical variable B.

inSkill and inScheme forms may be mixed within a single function with some degree of flexibility.

(inScheme (lambda (A B) (A (inSkill B) B)))

In this example a lexical 2-ary function is created which calls A, a function passed as its first argument, passing two parameters: the value of the dynamic variable B and also the value of the local lexical variable B.


SKILL supports several kinds of functions. In addition to the functions and special forms built into the language, users can create functions in their own applications of several varieties.

Anonymous functions including lexical closures.

(lambda (A) 
  (lambda (B)
    (plus A B)))

Lambda functions which evaluate their arguments using normal left-to-right evaluation rules.

(defun (A B) 
  (list A A B B))

Nlambda functions which do not evaluate their arguments, but pass their operands unevaluated at run time.

(nprocedure (A) 
  (when (listp A) 
    (eval A)))

Macros which are evaluated by expanding at load/compile time. The expressions a macro returns (evaluates to) become input for the compiler, and is thus evaluated at run time.

(defmacro nif (num if_plus if_zero if_minus)
  (let ((var (gensym)))
    `(let ((,var ,num))
       (cond ((plusp ,var) ,if_plus) 
             ((zerop ,var) ,if_zero)
             (t ,if_minus)))))

SKILL supports CLOS-like generic functions which are allowed to have an optional default implementation.

(defgeneric generic_add (a b c))

SKILL supports CLOS-like methods which specialize on all their required arguments. (Older versions of SKILL only support specialization of the first argument.)

(defmethod generic_add ((a fixnum) (b number) (c list))
  (apply plus a b c))

Local functions of two sorts are supported with flet and labels. If local functions are defined with flet such as inner1 and inner2 below, neither one can see the other's definition.

(defun outer ()
  (flet ((inner1 ()
           (printf "hello"))
         (inner2 ()
           (printf "world")))
    (inner1)
    (printf " ")
    (inner2)
    (newline)))

If local functions are defined with labels such as inner1 and inner2 below, all of them see the other's definition. Local function are only supported in lexical mode.

(defun outer ()
  (labels ((inner1 ()
            (printf "hello"))
          (inner2 ()
            (inner1)
            (printf " world\n")))
    (inner2)))

Additional Commands

Additional commands are added to the language for functions specific to a certain tool. For example, functions specific to the PCB Editor have the prefix “axl” (e.g. axlDBGetDesign), and commands specific to the Design Entry tool have the prefix “cn” (e.g. cnGetDwgInfo).

Examples

Here are some examples of SKILL.

First, a basic “Hello world”:

println("Hello, world!")

SKILL supports tail call optimization, if it is explicitly enabled. Here is a tail recursive version of factorial which requires no stack space for the recursion if optimizeTailCall is enabled.

(sstatus optimizeTailCall t)

(defun factorial (n)
  (flet ((repeat (n factorial)
           (if (plusp n)
               (repeat (sub1 n) 
                       n * factorial))
               factorial)))
    (repeat n 1)))

The following definition is a more idiomatic iterative definition.

(defun factorial (n)
  (let ((answer 1))
    (for i 2 n
      answer = answer * i)
    answer)))

This example shows how variables are assigned and scoped, using = and let, respectively:

procedure( swap()
  let( ((a 1) (b 2))
    c = a
    a = b
    b = c
    printf("%d %d %d\n" a b c)
  )
)

The variables a and b are scoped within the let function, but variable c is not. As a result, c becomes a global variable that can be accessed without the scope of the swap function. Here is what happens when swap is executed and a, b and c are then printed:

> swap
2 1 1
t
> a
*Error* toplevel: undefined variable - a
> b
*Error* toplevel: undefined variable - b
> c
1

References

  1. ↑ Barnes, T. J.: “SKILL: a CAD system extension language”, 27th ACM/IEEE Design Automation Conference, 1990. Proceedings; pp. 266-271
  2. ↑ "Re: SKILL stands for ...?".

Academic:

User Groups

More SKILL Programming Examples: http://www.cadence.com/community/forums/T/10274.aspx

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