Compiler explanation


Compiler – Based on a similar lab work in Caen University
Antlr, Calculator and MVaP language
The objectives of the lab work (on machine) :

  1. to be familiar with a lexical and syntactic analyser tool. We have choosen the ANTLRtool,
    which uses Java language.
    Your friend is the ANTLRdocumentation : https ://
  2. In a second part, you will have to design a programming language dealing with variables,
    conditional structures and loops. Apart from conditional structures, assignements to
    variables and loops, the language is intented to manipulate aritmetical and boolean
    expressions. You will use antlr for the lexical, syntactic and code generation. The target
    language for the code generation will be the MVaP language, the language of the MVaP
    stack machine. The descriptions of the instructions are given in the MVaP-Cheat-Sheet
  3. Quick start of Antlr
    Question. Download the install file install.txt and follow the instructions to install
    ANTLR. The installation instructions are for a Gnu/Linux or similar operating systems (Unix,
    Mac OS X). I recommend then to read https ://
    The following is an attribute grammar for arithmetical expressions using sum and product.
    The first axiom is always start, et every non-terminal should start with a lower case letter.
    Tokens always are in upper-case. We end the start axiom with the system token EOF, which
    is automatically produced by the lexical analyser after reading the input.
    grammar Calculator;
    // grammar rules
    : expr EOF;
    : expr ’*’ expr
    | expr ’+’ expr
    | INT
    //lexical rules. When something is unmatched, we skip it.
    NEWLINE : ’\r’? ’\n’ -> skip;
    WS : (’ ’|’\t’)+ -> skip;
    INT : (’0’…’9’)+;
    UNMATCH : . -> skip;
    Question. Explain why this grammar is ambigous.
    Question. Put this grammar in a file (say Calculator.g4) and compile (follow the instructions
    in the install.txt file).
    Question. Use the grun tool to visualise the syntactic tree (see install.txt file for the corresponding
    command line). A use case with grun :
    grun Calculator ’start’ -gui
    Analyse the results with the following expressions :
    24+ 24- 6
    58+2 1
    5+2 * 8+ 3
  4. *4 / 5+38
    Question. We use attribute grammars to evaluate such arithmetical expressions, by associating
    each rule with an attribute. We can do the following for the addition rule for instance.
    expr returns [int val]
    : a=expr ’+’ b=expr {$val=$a.val +$b.val;}
    Complete so that the result of the evaluation of the arithmetical expression is printed.
    To obtain the text of a token, use the token’s text attribute, and the token’s attribute int to
    get the integer value held by the token if the text is a valid numeric string. See https ://
    for the set of token’s and rules attributes,
    given by ANTLR and also the syntax for adding your own attributes and retreiving their values.
    Question. Analyse the output of the syntactic tree (with grun) and of the result of the
    evaluation of the expression 5 + 2 ∗ 8 + 3. Permute now in expr the order in which the rules for
    the sum and product are given, and do the same as above with the expression 5 + 2 ∗ 8 + 3.
    Do we have same outputs ? Why ?
  5. Calculator
    The objective is to enrich our calculator language to manipulate complex arithmetical
    expressions with variables and types, loops, printing and reading, and conditional structures.
    We recommend to use standard techniques to deal with ambiguity. For instance, for
    arithmetical expressions, division and product have high priorities on sum and substractions,
    with same priorities we evaluate from left to right. We recommend to read again
    https :// to know which rules are
    used by default by ANTLR to resolve ambiguity.
    We will add more and more rules to our grammar Calculator.g4.
    Question. Enrich the grammar to include divisions, substractions and parenthesized expressions.
    Test with the following expressions (they all evaluate to the same value). We suppose
    from now that every expression is ended by a line break, but you can enrich by adding for
    instance the possibility to end them also with semi-colons (you encapsulate them in a token
    (56711 + 2)/115-1008
    (56711 + 2)/(115)
    (567*11 + 2)/11/5
    (567*11 + 2)/(11/5)-1114
    Question. We have seen in exercises’ lecture rules for manipulating boolean expressions
    with comparisons between arithmetical expressions. Complete these rules to add comparisons
    between boolean expressions and add the rules to the Calculator grammar. We recall that
    the comparisons operations are == (equality), <> (difference), < (less than), > (greater
    than), <= (less or equal than) et >= (greater or equal than), and the operators for boolean
    expressions are and, or and not. The rule should also accept parenthesized boolean expressions.
    Test with the following expressions.
    true and false or true and not true
    true or false and false
    true or (false or false)
    false or (true and not false)
    (not true and false) or (true and not false)
    false or not true or true and not 42 == 42
    false or 42 <= 40
    false or (56711 + 2)/115-1008 <= 58+2-1/-1
  6. <> 51
    42+1+2+-3 <> 58+2-1/-1
    (56711 + 2)/11/5 < (56711 + 2)/(11/5)-1114
    (56711 + 2)/11/5 <= (56711 + 2)/(11/5)-1114
    58+21 > 6*4/5+38
    58+21 >= 6*4/5+38
  7. == (567*11 + 2)/11/5
  8. Code Generation
    The goal is to use the MVaP machine to execute the programs written in our language,
    and then we need to generate a machine code instead of evaluating directly the expressions.
    The target language for the code generation will be the MVaP language.
    Reminder. If you did not yet install MVaP, please follow the instructions in install.txt.
    Read also the MVaP-Cheat-Sheet.pdf file for the set of instructions of the MVaP machine.
    Until now, we have associated an integer attribute with each rule to store the result of the
    evaluation. For the code generation with target language MVaP, we will instead use a string
    accumulator to store the MVaP code.
    Question. Modify the grammar so that it generates, for each rule, the associated MVaP
    code, instead of evaluating its value.
    Tests. Test your parser to check that you really generate the right MVaP, you can use the
    expressions above. Run the MVaP generated code to check that the results are correct (see
    install.txt on how to run MVaP machine). The tool grun can be quickly too heavy for
    testing. Instead, you can use the following to automate the tests :
    import org.antlr.v4.runtime.*;
    public class MainCalculator {
    public static void main(String args[]) throws Exception {
    CalculatorLexer lex;
    if (args.length == 0)
    lex = new CalculatorLexer(new ANTLRInputStream(;
    lex = new CalculatorLexer(new ANTLRFileStream(args[0]));
    CommonTokenStream tokens = new CommonTokenStream(lex);
    CalculatorParser parser = new CalculatorParser(tokens);
    try {
    parser.start(); // start the first axiom of the grammar
    } catch (RecognitionException e) {
    } catch (Exception e) {
  9. Global Variables
    We would like to be able to manipulate global variables and types. We will manipulate
    only int, float and bool types. Values by default for variables is 0. In our language, any variable
    should be declared before first use. One should be able to assign to variables constants, e.g.,,
    x = 4, as well expressions, e.g.,, x = 6 ∗ y (in this latter we are expecting that the value of y
    is computed and multiplied with 6 before assigning it to x).
    We will follow the C language structure as follows : a program in our language starts with
    declarations of variables first and then can do computations. See below, a proposition for the
    structure of the file Calculator and a beginning of the rules.
    grammar Calculator;
    @parser::members {
    private int _label = 0;
    /* generator of label names /
    private int nextLabel() { return _label++; }
    start : calcul EOF;
    calcul returns [ String code ]
    @init{ $code = new String(); } // we initialise $code, to use it as accumulator
    @after{ System.out.println($code); } // we print the MVaP code produced by the parser
    : (decl { $code += $decl.code; })*
    (instruction { $code += $instruction.code; })*
    { $code += ” HALT\n”; }
    : (NEWLINE | ’;’)+
    decl returns [ String code ] // declarations of variables
    : TYPE IDENTIFIER EndInstruction
    // to be completed
    | TYPE IDENTIFIER ’=’ expression EndInstruction
    // to be completed
    instruction returns [ String code ] // an instruction other than declaration
    : expression EndInstruction // boolean or arithmetical expressions
    //to be completed
    | assignment EndInstruction // assigning values to variables
    // to be completed
    | EndInstruction
    assignment returns [ String code ] // Assign an expression
    : IDENTIFIER ’=’ expression
    //to be completed
    // lexer
    TYPE : ’int’ | ’float’ | ’bool’; // three types
    IDENTIFIER : ; //to be completed, any sequence of [a-zA-Z0-9_], starting always with [a-zA-Z] or _
    Question. To deal with variables, you will probably need symbol tables to be able to decide
    whether an identifier has been already used, and also to be able to recover the value of a variable
    whenever it is used, in particular know its address in the stack (recall that the declarations are
    done at the beginning, and then if you declare variables first in your MVaP code, you are able
    to compute their addresses in the stack). To keep track of the tuples (id-variable,type-variable,
    adress-in-stack), you can use for instance an HashMap, declared and initialised in the header
    @parser : :members.
    Modify now the Calculator grammar to generate MVaP code for all the rules we have
    written so far (arithmetical and boolean expressions), and for declarations and assignments of
    global variables.
    Use the test collection to test your grammar. Each of the wanted behaviour has a test file
    with a collection of expressions to be tested. From now on, it is better to use the test set to
    check whether your proposal corresponds to the wanted behaviour.
    Adding Input and Output operations. To ease the tests and the use of our language,
    it might interesting to include a mechanism for reading from the system input and writing
    on the system output. We recall that the MVaP machine has two dedicated instructions READ
    and WRITE. The following rules allow to deal with input and output operations. Add the io
    rule to the grammar file and modify also the instruction rule so that one can readln and
    println in our language.
    io returns [ String code ]
    : READ ’(’ IDENTIFIER ’)’
    int at = // to be completed, adress of the variable $IDENTIFIER.text;
    $code=” READ” ;
    $code+=”\n”; // new line after READ
    $code+=” STOREG” +” ” + at + “\n”;
    | WRITE ’(’ expression ’)’
    $code = $expression.code
  10. ” WRITE” + “\n”
  11. ” POP” + “\n”;
    WRITE : ’println’;
    READ : ’readln’;
  12. Blocks
    In order to deal with conditional structures and loops with several instructions, we need
    to be able to encapsulate a set of instructions in a block, so that we make a difference between
    the instructions to execute inside the conditional structure, and the others outside.
    Question. Add to the grammar the possibility to manipulate a block of instructions, encapsulated
    between { and }. The beginning of the rule can be the following :
    block returns [ String code ]
    @init{ $code = new String ()}
    : // to be completed
  13. While Loops
    Let’s enrich our language with while loops. The wanted syntax is the following, with cond
    a boolean expression :
    while (cond)
    Question. Add the necessary rules to our grammar file. We are expecting that you allow
    the two following forms.
    while (cond)
    // 1 single instruction.
    // or a block of instructions
    while (cond) {
    //several instructions
  14. Conditional Structures
    Let’s now add jumps to our language with the if . . . then else . . . construction. The intended
    syntax is the following (cond still a boolean expression) :
    if (cond)
    else instructionelse
    The else is not mandatory. Also, as with the while instruction, the set of instructions
    instructionthen and instructionelse might be a single or a block of instructions.
    Question. Add the necessary rules for manipulating the if . . . then else . . . instruction.