Spring Semester 2021
line decor
line decor


Course Project

Design and implementation of a compiler for the MiniJava language (a small subset of Java)

To implement the compiler you will use the tools JavaCC and JTB

The implementation for phases 2 and 3 of the project will be done in Java utilizing the visitor pattern

Homework Description Deadline
1 Implementation of a LL(1) parser for a simple calculator and a translator to Java for a simple language 18/4/2021
2 Semantic Check (MiniJava) 16/05/2021
3 Generating intermediate code (MiniJava -> LLVM) 13/06/2021

Homework 1 - LL(1) Calculator Parser - Translator to Java

Part 1

For the first part of this homework you should implement a simple calculator. The calculator should accept expressions with the addition, subtraction, and exponentiation operators, as well as parentheses. The grammar (for multi-digit numbers) is summarized in:

exp -> num | exp op exp | (exp)

op -> + | - | **

num -> digit | digit num

digit -> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

You need to change this grammar to support priority between the operators, to remove the left recursion for LL parsing, etc.

This part of the homework is divided in two tasks:

  1. For practice, you can write the FIRST+ & FOLLOW sets for the LL(1) version of the above grammar. In the end you will summarize them in a single lookahead table (include a row for every derivation in your final grammar). This part will not be graded.

  2. You have to write a recursive descent parser in Java that reads expressions and computes the values or prints "parse error" if there is a syntax error. You don't need to identify blank spaces. You can read the symbols one-by-one (as in the C getchar() function). The expression must end with a newline or EOF.

Your parser should read its input from the standard input (e.g., via an InputStream on System.in) and write the computed values of expressions to the standard output (System.out). Parse errors should be reported on standard error (System.err).

Part 2

In the second part of this homework you will implement a parser and translator for a language supporting string operations. The language supports the concatenation (+) operator over strings, function definitions and calls, conditionals (if-else i.e, every "if" must be followed by an "else"), and the following logical expressions:

  • is-prefix-of (string1 prefix string2): Whether string1 is a prefix of string2.
  • is-suffix-of (string1 suffix string2): Whether string1 is a suffix of string2.

All values in the language are strings.

The precedence of the operator expressions is defined as: precedence(if) < precedence(concat).

Your parser, based on a context-free grammar, will translate the input language into Java. You will use JavaCUP for the generation of the parser combined either with a hand-written lexer or a generated-one (e.g., using JFlex, which is encouraged).

You will infer the desired syntax of the input and output languages from the examples below. The output language is a subset of Java so it can be compiled using javac and executed using Java or online Java compilers like this, if you want to test your output.

There is no need to perform type checking for the argument types or a check for the number of function arguments. You can assume that the program input will always be semantically correct.

Note that each file of Java source code you produce must have the same name as the public Java class in it. For your own convenience you can name the public class "Main" and the generated files "Main.java". In order to compile a file named Main.java you need to execute the command: javac Main.java. In order to execute the produced Main.class file you need to execute: java Main.

To execute the program successfully, the "Main" class of your Java program must have a method with the following signature: public static void main(String[] args), which will be the main method of your program, containing all the translated statements of the input program. Moreover, for each function declaration of the input program, the translated Java program must contain an equivalent static method of the same name. Finally, keep in mind that in the input language the function declarations must precede all statements.

As with the first part of this assignment, you should accept input programs from stdin and print output Java programs to stdout.

Example #1


name()  {

surname() {

fullname(first_name, sep, last_name) {
    first_name + sep + last_name

fullname(name(), " ", surname())

Output (Java):

public class Main {
    public static void main(String[] args) {
        System.out.println(fullname(name(), " ", surname()));
    public static String name() {
        return "John";
    public static String surname() {
        return "Doe";
    public static String fullname(String first_name, String sep, String last_name) {
        return first_name + sep + last_name;

Example #2


name() {

repeat(x) {
    x + x

cond_repeat(c, x) {
    if (c prefix "yes")
        if("yes" prefix c)

cond_repeat("yes", name())
cond_repeat("no", "Jane")

Example #3


findLangType(langName) {
    if ("Java" prefix langName)
        if(langName prefix "Java")
            if("script" suffix langName)
        if ("script" suffix langName)
            "Probably Dynamic"


Homework 2 – MiniJava Static Checking (Semantic Analysis)

This homework introduces your semester project, which consists of building a compiler for MiniJava, a subset of Java. MiniJava is designed so that its programs can be compiled by a full Java compiler like javac.

Here is a partial, textual description of the language. Much of it can be safely ignored (most things are well defined in the grammar or derived from the requirement that each MiniJava program is also a Java program):

  • MiniJava is fully object-oriented, like Java. It does not allow global functions, only classes, fields and methods. The basic types are int, boolean, and int [] which is an array of int. You can build classes that contain fields of these basic types or of other classes. Classes contain methods with arguments of basic or class types, etc.

  • MiniJava supports single inheritance but not interfaces. It does not support function overloading, which means that each method name must be unique. In addition, all methods are inherently polymorphic (i.e., “virtual” in C++ terminology). This means that foo can be defined in a subclass if it has the same return type and argument types (ordered) as in the parent, but it is an error if it exists with other argument types or return type in the parent. Also all methods must have a return type–there are no void methods. Fields in the base and derived class are allowed to have the same names, and are essentially different fields.

  • All MiniJava methods are “public” and all fields “protected”. A class method cannot access fields of another class, with the exception of its superclasses. Methods are visible, however. A class’s own methods can be called via “this”. E.g., this.foo(5) calls the object’s own foo method, a.foo(5) calls the foo method of object a. Local variables are defined only at the beginning of a method. A name cannot be repeated in local variables (of the same method) and cannot be repeated in fields (of the same class). A local variable x shadows a field x of the surrounding class.
  • In MiniJava, constructors and destructors are not defined. The new operator calls a default void constructor. In addition, there are no inner classes and there are no static methods or fields. By exception, the pseudo-static method “main” is handled specially in the grammar. A MiniJava program is a file that begins with a special class that contains the main method and specific arguments that are not used. The special class has no fields. After it, other classes are defined that can have fields and methods.
    Notably, an A class can contain a field of type B, where B is defined later in the file. But when we have “class B extends A”, A must be defined before B. As you’ll notice in the grammar, MiniJava offers very simple ways to construct expressions and only allows < comparisons. There are no lists of operations, e.g., 1 + 2 + 3, but a method call on one object may be used as an argument for another method call. In terms of logical operators, MiniJava allows the logical and (“&&”) and the logical not (“!”). For int arrays, the assignment and [] operators are allowed, as well as the a.length expression, which returns the size of array a. We have “while” and “if” code blocks. The latter are always followed by an “else”. Finally, the assignment “A a = new B();” when B extends A is correct, and the same applies when a method expects a parameter of type A and a B instance is given instead.

  • You should only accept expressions of type int as the argument of the PrintStatement.

The MiniJava grammar in BNF can be downloaded here. You can make small changes to grammar, but you must accept everything that MiniJava accepts and reject anything that is rejected by the full Java language. Making changes is not recommended because it will make your job harder in subsequent homework assignments. Normally you won’t need to touch the grammar.

The MiniJava grammar in JavaCC form is here. You will use the JTB tool to convert it into a grammar that produces class hierarchies. Then you will write one or more visitors who will take control over the MiniJava input file and will tell whether it is semantically correct, or will print an error message. It isn’t necessary for the compiler to report precisely what error it encountered and compilation can end at the first error. But you should not miss errors or report errors in correct programs.

The visitors you will build should be subclasses of the visitors generated by JTB, but they may also contain methods and fields to hold information during static checking, to transfer information from one visitor to the next, etc. In the end, you will have a Main class that runs the semantic analysis initiating the parser that was produced by JavaCC and executing the visitors you wrote. You will turn in your grammar file, if you have made changes, otherwise just the code produced by JavaCC and JTB alongside your own classes that implement the visitors, etc. and a Main. The Main should parse and statically check all the MiniJava files that are given as arguments.

Also, for every MiniJava file, your program should store and print some useful data for every class such as the names and the offsets of every field and method this class contains. For MiniJava we have only three types of fields (int, boolean and pointers). Ints are stored in 4 bytes, booleans in 1 byte and pointers in 8 bytes (we consider functions and int arrays as pointers). Corresponding offsets are shown in the example below:



  A.i : 0
  A.flag : 4
  A.j : 5
  A.foo : 0
  A.fa: 8
  B.type : 9
  B.k : 17
  B.bla : 16

There will be a tutorial for JavaCC and JTB. You can use these files as MiniJava examples and to test your program. Obviously you are free to make up your own files, however the homework will be graded purely on how your compiler performs on all the files we will test it against (both the above sample files and others). You can share ideas and test files, but you are not allowed to share code.

Your program should run as follows:

java [MainClassName] [file1] [file2] ... [fileN]

That is, your program must perform semantic analysis on all files given as arguments. May the Force be with you!

Homework 3 - Generating intermediate code (MiniJava -> LLVM)

In this part of the project you have to write visitors that convert MiniJava code into the intermediate representation used by the LLVM compiler project. The MiniJava language is the same as in the previous exercise. The LLVM language is documented in the LLVM Language Reference Manual, although you will use only a subset of the instructions.


Some of the available types that might be useful are:

  • i1 - a single bit, used for booleans (practically takes up one byte)
  • i8 - a single byte
  • i8* - similar to a char* pointer
  • i32 - a single integer
  • i32* - a pointer to an integer, can be used to point to an integer array
  • static arrays, e.g., [20 x i8] - a constant array of 20 characters

Instructions to be used

  • declare is used for the declaration of external methods. Only a few specific methods (e.g., calloc, printf) need to be declared.
    Example: declare i32 @puts(i8*)

  • define is used for defining our own methods. The return and argument types need to be specified, and the method needs to end with a ret instruction of the same type.
    Example: define i32 @main(i32 %argc, i8** argv) {...}

  • ret is the return instruction. It is used to return the control flow and a value to the caller of the current function. Example: ret i32 %rv

  • alloca is used to allocate space on the stack of the current function for local variables. It returns a pointer to the given type. This space is freed when the method returns.
    Example: %ptr = alloca i32

  • store is used to store a value to a memory location. The parameters are the value to be stored and a pointer to the memory.
    Example: store i32 %val, i32* %ptr

  • load is used to load a value from a memory location. The parameters are the type of the value and a pointer to the memory.
    Example: %val = load i32, i32* %ptr

  • call is used to call a method. The result can be assigned to a register. (LLVM bitcode temporary variables are called "registers".) The return type and parameters (with their types) need to be specified.
    Example: %result = call i8* @calloc(i32 1, i32 %val)

  • add, and, sub, mul, xor are used for mathematical operations. The result is the same type as the operands.
    Example: %sum = add i32 %a, %b

  • icmp is used for comparing two operands. icmp slt for instance does a signed comparison of the operands and will return i1 1 if the first operand is less than the second, otherwise i1 0.
    Example: %case = icmp slt i32 %a, %b

  • br with a i1 operand and two labels will jump to the first label if the i1 is one, and to the second label otherwise.
    Example: br i1 %case, label %if, label %else

  • br with only a single label will jump to that label.
    Example: br label %goto

  • label: declares a label with the given name. The instruction before declaring a label needs to be a br operation, even if that br is simply a jump to the label.
    Example: label123:

  • bitcast is used to cast between different pointer types. It takes the value and type to be cast, and the type that it will be cast to.
    Example: %ptr = bitcast i32* %ptr2 to i8**

  • getelementptr is used to get the pointer to an element of an array from a pointer to that array and the index of the element. The result is also a pointer to the type that is passed as the first parameter (in the case below it's an i8*). This example is like doing ptr_idx = &ptr[idx] in C (you still need to do a load to get the actual value at that position).
    Example: %ptr_idx = getelementptr i8, i8* %ptr, i32 %idx

  • constant is used to define a constant, such as a string. The size of the constant needs to be declared too. In the example below, the string is 12 bytes ([12 x i8]). The result is a pointer to the given type (in the example below, @.str is a [12 x i8]*).
    Example: @.str = constant [12 x i8] c"Hello world\00"

  • global is used for declaring global variables - something you will need to do for creating v-tables. Just like constant, the result is a pointer to the given type.
    @.vtable = global [2 x i8*] [i8* bitcast (i32 ()* @func1 to i8*), i8* bitcast (i8* (i32, i32*)* @func2 to i8*)]

  • phi is used for selecting a value from previous basic blocks, depending on which one was executed before the current block. Phi instructions must be the first in a basic block. It takes as arguments a list of pairs. Each pair contains the value to be selected and the predecessor block for that value. This is necessary in single-assignment languages, in places where multiple control-flow paths join, such as if-else statements, if one wants to select a value from the different paths. In the context of the exercise, you will need this for short-circuiting and (&&) expressions.
    br i1 1, label %lb1, label %lb2
        %a = add i32 0, 100
        br label %lb3
        %b = add i32 0, 200
        br label %lb3
        %c = phi i32 [%a, %lb1], [%b, %lb2]


If you do not remember or haven't seen how a virtual table (v-table) is constructed, essentially it is a table of function pointers, pointed at by the first 8 bytes of an object. The v-table defines an address for each dynamic function the object supports. Consider a function foo in position 0 and bar in position 1 of the table (with actual offset 8). If a method is overridden, the overriding version is inserted in the same location of the virtual table as the overridden version. Virtual calls are implemented by finding the address of the function to call through the virtual table. If we wanted to depict this in C, imagine that object obj is located at location x and we are calling foo which is in the 3rd position (offset 16) of the v-table. The address of the function that is going to be called is in memory location (*x) + 16.


You will need to execute the produced LLVM IR files in order to see that their output is the same as compiling the input java file with javac and executing it with java. To do that, you will need Clang with version >=4.0.0. You may download it on your Linux machine, or use it via SSH on the linuxvm machines.

In Ubuntu Trusty

  1. sudo apt update && sudo apt install clang-4.0
  2. Save the code to a file (e.g. ex.ll)
  3. clang-4.0 -o out1 ex.ll
  4. ./out1

In linuxvm machines

  1. /home/users/thp06/clang/clang -o out1 ex.ll
  2. ./out1


Your program should run as follows:
java [MainClassName] [file1.java] [file2.java] ... [fileN.java]
That is, your program must compile to LLVM IR all .java files given as arguments. Moreover, the outputs must be stored in files named file1.ll, file2.ll, ... fileN.ll respectively.


  • You will need to use a lot of registers in order to 'glue' expressions together. This means that each visitor will produce the code for storing the value of an expression to a register, and then return the name of that register so that other expressions may use it, if necessary.
  • Registers are single-assignment. This means you can only write to them once (but read any number of times). This also implies that registers cannot be used for local variables of the source program. Instead, you will allocate space on the stack using alloca and keep the address in a register. You will use the load and store instructions to read and write to that local variable.
  • Because registers are single-assignment, you will probably need to keep a counter to produce new ones. For example, you may produce registers of the form %_1, %_2, etc.
  • You will only support compilation to a 64-bit architecture: pointers are 8-bytes long.
  • Everything new in Java is initialized to zeroes.
  • Memory allocated with @calloc will leak since you're not implementing a Garbage Collector, but that's fine for this homework.
  • You will need to check each array access in order not to write or read beyond the limits of an array. If an illegal read/write is attempted, you will print the message "Out of bounds" and the program will exit (you may call the @throw_oob defined below for that). Of course, you need to know the length of an array for that.
  • You will also need to check if an array is allocated with a negative length, and do the same process as above in that case.
  • You may see some examples of LLVM code produced for different Java input files here (corresponding to the earlier MiniJava examples from HW2).
  • You may define the following helper methods once in your output files, in order to be able to call @calloc, @print_int and @throw_oob.
declare i8* @calloc(i32, i32)
declare i32 @printf(i8*, ...)
declare void @exit(i32)

@_cint = constant [4 x i8] c"%d\0a\00"
@_cOOB = constant [15 x i8] c"Out of bounds\0a\00"
define void @print_int(i32 %i) {
    %_str = bitcast [4 x i8]* @_cint to i8*
    call i32 (i8*, ...) @printf(i8* %_str, i32 %i)
    ret void

define void @throw_oob() {
    %_str = bitcast [15 x i8]* @_cOOB to i8*
    call i32 (i8*, ...) @printf(i8* %_str)
    call void @exit(i32 1)
    ret void

Example program

The program below demonstrates all of the above instructions. It creates an array of 3 methods (add, sub and mul), calls all of them with the same arguments and prints the results.

@.funcs = global [3 x i8*] [i8* bitcast (i32 (i32*, i32*)* @add to i8*),
                            i8* bitcast (i32 (i32*, i32*)* @sub to i8*),
                            i8* bitcast (i32 (i32*, i32*)* @mul to i8*)]

declare i32 @printf(i8*, ...)
@.comp_str = constant [15 x i8] c"%d %c %d = %d\0A\00"
@.ret_val = constant [20 x i8] c"Returned value: %d\0A\00"

define i32 @main() {
    ; allocate local variables
    %ptr_a = alloca i32
    %ptr_b = alloca i32
    %count = alloca i32

    ; initialize var values
    store i32 100, i32* %ptr_a
    store i32 50, i32* %ptr_b
    store i32 0, i32* %count
    br label %loopstart

    ;load %i from %count
    %i = load i32, i32* %count
    ; while %i < 3
    %fin = icmp slt i32 %i, 3
    br i1 %fin, label %next, label %end

    ; get pointer to %i'th element of the @.funcs array
    %func_ptr = getelementptr [3 x i8*], [3 x i8*]* @.funcs, i32 0, i32 %i
    ; load %i'th element that contains an i8* to the method
    %func_addr = load i8*, i8** %func_ptr
    ; cast i8* to actual method type in order to call it
    %func = bitcast i8* %func_addr to i32 (i32*, i32*)*
    ; call casted method
    %result = call i32 %func(i32* %ptr_a, i32* %ptr_b)

    ; print result
    %str = bitcast [20 x i8]* @.ret_val to i8*
    call i32 (i8*, ...) @printf(i8* %str, i32 %result)

    ; increase %i and store to %count
    %next_i = add i32 %i, 1
    store i32 %next_i, i32* %count
    ; go to loopstart
    br label %loopstart

    ret i32 0

define i32 @add(i32* %a, i32* %b) {
    %str = bitcast [15 x i8]* @.comp_str to i8*

    ; load values from addresses
    %val_a = load i32, i32* %a
    %val_b = load i32, i32* %b

    ; add them and print the result
    %res = add i32 %val_a, %val_b
    call i32 (i8*, ...) @printf(i8* %str, i32 %val_a, [1 x i8] c"+", i32 %val_b, i32 %res)

    ; return the result
    ret i32 %res

define i32 @sub(i32* %a, i32* %b) {
    ; similar as above
    %str = bitcast [15 x i8]* @.comp_str to i8*
    %val_a = load i32, i32* %a
    %val_b = load i32, i32* %b
    %res = sub i32 %val_a, %val_b
    call i32 (i8*, ...) @printf(i8* %str, i32 %val_a, [1 x i8] c"-", i32 %val_b, i32 %res)
    ret i32 %res

define i32 @mul(i32* %a, i32* %b) {
    ; similar as above
    %str = bitcast [15 x i8]* @.comp_str to i8*
    %val_a = load i32, i32* %a
    %val_b = load i32, i32* %b
    %res = mul i32 %val_a, %val_b
    call i32 (i8*, ...) @printf(i8* %str, i32 %val_a, [1 x i8] c"*", i32 %val_b, i32 %res)
    ret i32 %res