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XTRAN Demonstration:  Analyzing Constant/Expression PL/I PROCEDURE Arguments

PL/I normally passes arguments to PROCEDUREs "by reference"; that is, what is passed is the address of the argument.  However, if a calling argument is a constant or an expression, PL/I passes it "by copy":  The PL/I compiler generates code to create a local copy of the constant or expression value and passes the copy's address as the argument.

When XTRAN translates PL/I to C/C++, it reflects this behavior:  It normally generates code to pass an argument's address, but if the argument is a constant or expression, it generates code to declare a local copy and pass its address.

However, the chances are that if PL/I code passes a constant or expression as an argument, then the called PROCEDURE doesn't modify the corresponding parameter.  Even if it does, it modifies only the copy whose address was passed to it.  Therefore, in the translated C/C++ we can safely pass the parameter by value.  In fact, XTRAN extends the PL/I language by adding a new declaration attribute, XTR_BYVAL, which tells XTRAN to translate the PROCEDURE parameter and all corresponding calling arguments as "pass by value".

The knowledge of which PROCEDURE parameters actually have matching arguments that are constants or expressions is scattered throughout all of the PL/I modules to be translated.

XTRAN's powerful PL/I analysis capability comes to the rescue.  The following example uses a set of XTRAN rules ("meta-language") that examines all arguments of all PROCEDURE calls in the code it is given and creates a database of constant and expression argument occurrences, capturing the following information for each such occurrence:

We run this analysis on each module to be translated; the meta-code appends the occurrence information to the database file we're creating.  When we have all the data, we use another set of meta-code that creates a report of the data, eliminating duplicates and sorting occurrences appropriately.

We can then modify the appropriate PROCEDURE parameter declarations in the PL/I code to be translated, adding XTR_BYVAL where appropriate.  Click here to see an actual translation example using this technique for the code below.

The following input to, and output from, XTRAN is untouched except for line numbers added to the input for reference.


* Input to XTRAN (demcpa-w.pli):

 1      DCL extprc1 ENTRY (FIXED (15) BIN, FLOAT (5) BIN,
 2        FIXED (31) BIN) EXT;
 3      DCL extprc2 ENTRY (CHAR (*), FIXED (31) DEC, FIXED (15) BIN) EXT;
 4
 5 %    DCL (ppint1, ppint2) FIXED;
 6 %    ppint1 = 1;
 7 %    ppint2 = 2;
 8 %    DCL (ppchr1, ppchr2) CHAR;
 9 %    ppchr1 = '''abc''';
10 %    ppchr2 = '''xy''';
11 %    DCL ppexpr FIXED;
12 %    ppexpr = ppint1 + 1;
13
14 prc: PROCEDURE;
15      DCL (int1, int2) FIXED (31) BIN;
16      DCL (flt1, flt2) FLOAT (5) DEC;
17      DCL (chr1, chr2) CHAR (5);
18
19 nstprc: PROCEDURE (arg1, arg2);              /*nstprc: PROCEDURE...;*/
20          DCL arg1 FIXED (31) BIN XTR_BYVAL;
21          DCL (arg2, i) FIXED (31) BIN;
22          i = extprc1(1, 2.0, i);             /*const args 1 & 2*/
23          END nstprc;
24      extprc1(int1, flt1, i);                 /*no const args*/
25      extprc1(1, flt1, int1);                 /*const arg 1*/
26      extprc1(int1 + 1, flt1, int1);          /*expr arg 1*/
27      extprc1(ppexpr, flt1, int1);            /*expr arg 1*/
28      extprc1(int1, 1.0, int1);               /*const arg 2*/
29      extprc1(ppint1, flt2, int1);            /*const arg 1*/
30      extprc2(chr1, int2, int1);              /*no const args*/
31      extprc2('abc', int2, int1);             /*const arg 1*/
32      extprc2(chr1, 2, int1);                 /*const arg 2*/
33      extprc2('a' | 'bc', 2, int1);           /*expr arg 1, const arg 2*/
34      extprc2(ppchr1, chr2, int1);            /*const arg 1*/
35      extprc2('abc', ppchr2, int1);           /*const arg numbers 1,2*/
36      nstprc(int1, int2);                     /*no const args*/
37      nstprc(1, int2);                        /*const arg 1*/
38      end prc;

* Output from XTRAN:

                 Constant/Expression PROCEDURE Argument Numbers

Legend:

  [X] means declared "external"
  [S] means declared "static"
  [N] means nested declaration
  [I] means inferred declaration

extprc1[X]
    Argument 1
        demcpa-w.pli:  22,25,26,27,29
    Argument 2
        demcpa-w.pli:  22,28
extprc2[X]
    Argument 1
        demcpa-w.pli:  31,33,34,35
    Argument 2
        demcpa-w.pli:  32,33,35
nstprc[N]
    Argument 1
        demcpa-w.pli:  37

COPYRIGHT 2008; reproduction prohibited without permission.  Revised 2006-11-27

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