XTRAN Demonstration: Analyzing Knots in PL/I Code

The following analysis used an XTRAN rules file comprising 134 non-comment lines of "meta-code" (XTRAN's rules language) to analyze PL/I code and show any knots it finds.

A knot is the crossing of two GOTOs. It is an important measure of code quality for two reasons:

It decreases the readability and maintainability of the code.
It defeats automatic restructuring of the code, which can eliminate GOTOs by imposing IF, ELSE, and DO statements with equivalent functionality. (Click here for an example using XTRAN to restructure PL/I code.)

Note that the analysis output shows not only the number of knots, but also the actual crossings from which that number was determined.

The XTRAN rules themselves are not specific to PL/I; they are language-independent, and therefore usable without change to analyze any 3GL language that has GOTOs.

The input to and output from XTRAN are untouched, except that line numbers have been added to the PL/I source code for reference.

Input to XTRAN: demknt-a.pli

19 prc:    PROCEDURE;
20         DCL i FIXED (31) BIN;
21                                /* Nesting           Range      */
22                                /* ============================ */
23
24 lbl0:   GOTO lbl14;            /* >--------------\  lbl0-lbl14 */
25 lbl1:   i = 1;                 /* <--------\     |             */
26 lbl2:   GOTO lbl10;            /* >-----\  |     |  lbl2-lbl10 */
27 lbl3:   i = 3;                 /*       |  |     |             */
28 lbl4:   GOTO lbl1;             /* >-----x--/     |  lbl1-lbl4  */
29 lbl5:   i = 5;                 /* <-----x-----\  |             */
30 lbl6:   GOTO lbl13;            /* >-----x--\  |  |  lbl6-lbl13 */
31 lbl7:   GOTO lbl9;             /* >--\  |  |  |  |  lbl7-lbl9  */
32 lbl8:   i = 8;                 /*    |  |  |  |  |             */
33 lbl9:   i = 9;                 /* <--/  |  |  |  |             */
34 lbl10:  i = 10;                /* <-----/  |  |  |             */
35 lbl11:  GOTO lbl5;             /* >--------x--/  |  lbl5-lbl11 */
36 lbl12:  i = 12;                /*          |     |             */
37 lbl13:  i = 13;                /* <--------/     |             */
38 lbl14:  i = 14;                /* <--------------/             */
39         END prc;

Excerpt from XTRAN run log:

Knots found in source code:

demknt-a.pli:  GOTO at line 26 crosses GOTO at line 28
demknt-a.pli:  GOTO at line 26 crosses GOTO at line 30
demknt-a.pli:  GOTO at line 26 crosses GOTO at line 35
demknt-a.pli:  GOTO at line 30 crosses GOTO at line 35

demknt-a.pli has 4 knots.

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