This extended prac is designed to take you the best part of two weeks. Hand in your solutions before lunch time on Monday 30 October, correctly packaged in a transparent folder with your cover sheet and individual assessment sheets. Since the practical will have been done on a group basis, please hand in one copy of the cover sheet for each member of the group. These will be returned to you in due course, signed by the marker. Please make it clear whose folder you have used for the electronic submission, for example g03A1234. Please resist the temptation to carve up the practical, with each group member only doing one task. The group experience is best when you work on tasks together.
The reason for requiring all submissions by 30th October is to free you up during swot week to prepare for the final examinations. I shall try to get the marking done as soon as possible after that.
In this practical you are to
You will need this prac sheet and your text book. As usual, copies of the prac sheet are also available at http://www.cs.ru.ac.za/CSc301/Translators/trans.htm.
When you have completed this practical you should understand
Hopefully after doing these exercises (and studying the attributed grammar and the various other support modules carefully) you will find you have learned a lot more about compilers and programming languages than you ever did before (and, I suspect, a lot more than undergraduates at any other university in this country). I also hope that you will have begun to appreciate how useful it is to be able to base a really large and successful project on a clear formalism - namely the use of attributed context-free grammars - and will have learned to appreciate the use of sophisticated tools like Coco/R.
By the hand-in date you are required to hand in, besides the cover sheets (one per group member):
I do NOT require listings of any Java or C# code produced by Coco/R.
Keep the prac sheet and your solutions until the end of the semester. Check carefully that your mark has been entered into the Departmental Records.
You are referred to the rules for practical submission which are clearly stated in our Departmental Handbook. However, for this course pracs must be posted in the "hand-in" box outside the laboratory and not given to demonstrators.
A rule not stated there, but which should be obvious, is that you are not allowed to hand in another group's or student's work as your own. Attempts to do this will result in (at best) a mark of zero and (at worst) severe disciplinary action and the loss of your DP. You are allowed - even encouraged - to work and study with other students, but if you do this you are asked to acknowledge that you have done so. You are expected to be familiar with the University Policy on Plagiarism, which you can consult at:
http://www.scifac.ru.ac.za/plag.htm
The tasks are presented below in an order which, if followed, should make the practical an enjoyable and enriching experience. Please do not try to leave everything to the last few hours, or you will come horribly short. You must work consistently, and with a view to getting an overview of the entire project, as the various components and tasks all interact in ways that will probably not at first be apparent. Please take the opportunity of coming to consult with me at any stage if you are in doubt as how best to continue. By all means experiment in other ways and with other extensions if you feel so inclined.
This version of Parva has been restricted so as not to include functions. This means that there will be no practical work set on chapter 14 of the text. Because of the timing of our courses this is unavoidable, if highly regrettable. You should be warned that some of the material of that chapter may be examinable.
You are advised that it is in your best interests to take this opportunity of really studying the code in the Parva grammar and its support files. The exercises have been designed to try to force you to do that, but it is always tempting just to guess and to hack. With a program of this size that often leads to wasting more time than it saves. Finally, remember the advice given in an earlier lecture:
Keep it as simple as you can, but no simpler.
Throughout this project you will need to ensure that the features you explore are correctly implemented. This means that you will have to get a feel for understanding code sequences produced by the compiler. The best way to do this is usually to write some very minimal programs, that do not necessarily do anything useful, but simply have one, or maybe two or three statements, the object code for which is easily predicted and understood.
When the Parva compiler has finished compiling a program, say SILLY.PAV, you will find that it creates a file SILLY.COD in which the stack machine assembler code appears. Studying this is often very enlightening.
Useful test programs are small ones like the following. There are some specimen test programs in the kit, but these are deliberately incomplete, wrong in places, too large in some cases and so on. Get a feel for developing some of your own.
$D+ // Turn on debugging mode
void main (void) {
int i;
int[] List = new int[10];
while (true) { // infinite loop, can generate an index error
read(i);
List[i] = 100;
}
}
The debugging pragma
It is useful when writing a compiler to be able to produce debugging output - but sometimes this just clutters up a production quality compiler. The PARVA.ATG grammar makes use of the PRAGMAS option of Coco/R (see text, page 250) to allow pragmas like those shown to have the desired effect.
$D+ /* Turn debugging mode on */
$D- /* Turn debugging mode off */
There are several files that you need, zipped up in the file PRAC25.ZIP (Java) or PRAC25C.ZIP (C#).
J:
md prac25
cd prac25
copy i:\csc301\trans\prac25.zip
unzip prac25.zip
This will create several other directories "below" the prac25 directory:
J:\prac25
J:\prac25\Library
J:\prac25\Parva
containing the Java classes for the I/O Library and the code generator and symbol table handler.
You will also find the executable version of Coco/R and batch files for running it, frame files, and various sample data, code and the Parva grammar, contained in files with extensions like
*.ATG, *.PAV
We have already commented on the $D+ pragma. At present it is only used to request the printout of a symbol table. How would you change the system so that one would have to use a similar pragma or command line option if one wanted to obtain the assembler code file - so that the ".COD" file with the assembler code listing would only be produced if it were really needed?
$C+ /* Request that the .COD file be produced */
$C- /* Request that the .COD file not be produced */
Another useful (run-time) debugging aid is the undocumented stackdump statement. Compilation of this is also controlled by the $D pragma (in other words, the stack dumping code is only generated when in debug mode - much of the time you are testing your compiler you will probably be working in "debugging" mode, I expect).
Hint: This addition is almost trivially easy. You will also need to look at (and probably modify) the Parva.frame file, which is used as the basis for constructing the compiler proper (see page 268).
If, like me, you first learned programming in languages other than Java, you may persist in making silly mistakes when writing Parva programs. For example, Pascal programmers easily confuse the roles played by ==, != and = with the similar operators in Pascal denoted by =, <> and :=, and are prone to introduce words like then into IfStatements, giving rise to code like
if (a = b) then c := d;
if (x <> y) then p := q;
instead of
if (a == b) c = d;
if (x != y) p = q;
Can you think of (and implement) ways to handle these errors sympathetically - that is to say, to report them but then to "recover" from them without further ado?
(Confusing = with == in Boolean expressions is something C, C++ and Java programs can easily do as well. It is particularly nasty in C/C++, where a perfectly legal statement like
if (a = b) x = y;
does not compare a and b, but assigns b to a, as you probably know).
Identifiers that are undeclared by virtue of mistyped declarations tend to be annoying, for they result in many subsequent errors being reported. Implement a strategy where each undeclared identifier is flagged as undeclared at the point of first reference, and then quietly entered as a variable of the noType pseudo-type in the symbol table. Can you extend the idea to recognize an undeclared array reference variable and thereby reduce the plethora of "unexpected subscript" messages that this non-declaration might otherwise generate?
The remaining tasks all involve coming to terms with the code generation process.
A very easy one: add the Pascal-like Repeat loop to Parva, as exemplified by
repeat a = b; c = c + 10; until (c > 100);
Add an else option to the IfStatement. Oh, yes, it is trivial to add it to the grammar. But be careful. Some IfStatements will have else parts, others may not, and the code generator has to be able to produce the correct code for whatever form is actually to be compiled. The following silly examples are all valid.
if (a == 1) { c = d; }
if (a == 1) {}
if (a == 1) {} else {}
if (a == 1) {} else { b = 1; }
In an earlier practical we suggested that an IfStatement might also incorporate optional elsif clauses - the grammar for this simply being on the lines of
IfStatement = "if" "(" Condition ")" Statement
{ "elsif" "(" Condition ")" Statement }
[ "else" Statement ] .
Implement this extension (make sure all the branches are correctly set up).
The BreakStatement is syntactically simple, but takes a bit of thought. Give it some! Be careful - breaks can only appear within loops, there might be several break statements inside a single loop, and loops can be nested inside one another.
At last! Let's really make Parva useful and turn it into Parva++ by adding the increment and decrement statement forms exemplified by
int parva;
int [] list = new int[10];
...
parva++;
list[10]--;
--parva;
Suggestions for doing this - specifically by introducing new operations into the PVM - are made in section 13.5.1 of the text. Be careful - only integer variables (and array elements) can be handled in this way.
The operator precedences in Parva as supplied resemble those in Pascal and Modula, where only three basic levels are supported. Modify Parva so that it uses a precedence structure based on that in C++ or Java (the basic grammar modifications were discussed in earlier practicals). Take special care to deal with "short circuit" semantics correctly (see section 13.5.2) and with type compatibility issues (see section 12.6.8)
Parva is looking closer to C/C++/Java with each successive long hour spent in the Hamilton Labs. Seems a pity to stop now, so go right on and extend the system to allow statements like
int parva;
int [] list = new int[10];
...
parva *= 2;
list[10] += parva;
Once again, section 13.5.1 should give you some ideas of how to proceed. And, once again, remember that variables have types that require compatibilty with the operators to be checked.
Many languages provide for a ForStatement in one or other form. Although most people are familiar with these, their semantics and implementation can actually be quite tricky.
Suppose we were to add a simple Pascal-style for loop to Parva, to allow statements with concrete syntax defined by
ForStatement = "for" Ident "=" Expression ("to" | "downto") Expression Statement .
for example
for i = 1 to 10 write(i); // forwards 1 2 3 4 5 6 7 8 9 10
for i = 10 downto 1 write(i); // backwards 10 9 8 7 6 5 4 3 2 1
for i = 10 to 5 write(i); // no effect
for b = false to true write(b); // writes "false true"
for i = i - 1 to i + 6 write(i); // what does this do?
for i = 1 to 5 read(i); // should we allow this?
for i = 1 to 5 i = i + 1; // should we allow this?
What would you suggest in the way of constraints, how would you enforce the constraints, and how would you generate code for these sorts of statements?
Hint: you may like to add some more oprations to the PVM to assist in this regard.
In Pascal the word do is also required, as below. Would it be a good idea here too?
ForStatement = "for" Ident "=" Expression ("to" | "downto") Expression "do" Statement .