README.md

🔨 PL/0 Compiler

This is a compiler for the programming language PL/0 introduced by Niklaus Wirth in the book, Algorithms + Data Structures = Programs, as an example of how to construct a compiler. This language is a minimal Pascal, has a really simple grammar and supports all basic features of a programming language, like: constants, variables, procedures, control flow and so on. Because of its simplicity, I consider it a good starting point to learn how build compilers with LLVM as backend. For more details about this language check out the Wikipedia article.

Important

The focus of this project is not to design a good and fast compiler, but just to learn how use the LLVM to develop compilers. So all the components of this compiler (lexer, parser, error reporting, etc.) are really basic.

👩‍💻 Installation

📜 Prerequisites

• g++ or clang++ compiler
• LLVM
• make

⚒️ How to build the compiler?

In your terminal run the following command to build the compiler:

make

🧪 Example

Let's take the following PL/0 program:

const ADD = 0, SUB = 1, MULT = 2, DIV = 3;
var firstOperand, secondOperand, operator, done, zeroDivisionError; 

procedure printErrorCode;
const ERROR = 2147483647;
begin
   !(-ERROR) - 1
end;

procedure setZeroDivisionError;
begin
   zeroDivisionError := 1
end;

procedure resetZeroDivisionError;
begin
   zeroDivisionError := 0
end;

procedure add;
begin
   !(firstOperand + secondOperand)
end;

procedure sub;
begin
   !(firstOperand - secondOperand)
end;

procedure mult;
begin
   !(firstOperand * secondOperand)
end;

procedure div;
begin
   if secondOperand # 0 then
      !(firstOperand / secondOperand);

   if secondOperand = 0 then
      call setZeroDivisionError
end;

begin
   while done = 0 do
   begin
      ?operator;

      if operator < 0 then
         operator := -operator;

      if operator > DIV then
         done := 1;

      if operator <= DIV then
      begin

         ?firstOperand;
         ?secondOperand;

         if operator = ADD then
            call add;

         if operator = SUB then
            call sub;

         if operator = MULT then
            call mult;

         if operator = DIV then
         begin
            call div;
            if zeroDivisionError = 1 then
            begin
               call printErrorCode;
               call resetZeroDivisionError
            end
         end

      end
   end
end.

To generate the executable we run:

./pl0 myprogram.pl0

And when the compilation is finished we execute the executable produced:

./myprogram

🔭 Resources

• LLVM Kaleidoscope
• Clang Source Code
• LLVM Language Reference