Pascal JVM compiler (2019)

The project is implemented for educational purposes. Its main goal is to get experience in a compiler implementation.

The compiler implements an analysis of a subset of Pascal language grammar. A Pascal program is translated to Java byte code.

Features

The compiler supports

• Type declaration block
• Variable declaration block (Boolean, Integer, Real, Char)
• Operators block
• Number constants
• Arithmetic and logical expressions (constants, variables, +, -, *, /, =, <>, <, <=, >, >=, in, NOT, AND, OR, (, ))
• Assignment operator (:=)
• Compound statement (begin .. end)
• Conditional statement (if .. then ..)
• while, repeat, for loops
• Functions declaration and invocation

Example

In this README, the compiler’s work is illustrated by the example of a Pascal program that implements a ternary search algorithm for a square equation solution. In each section, one of the modules of the compiler is discussed and illustrated by a screenshot of the program parts containing the messages provided by the discussed module (other modules’ messages are blurred). You can see a complete program listing below.

Lexical analysis

Lexer is a compiler module that transforms a program text into a sequence of language symbols. It also reveals lexical errors:

• Constant range violation (i.e., for an integer value, the range is -32768..32767)
• Detection of the unsupported characters (i.e. %, !, $)

Syntax analysis

Syntax Analyzer checks the sequence of symbols for language syntax rules and generates syntax errors. For example, having a set of high-level BNF syntax rules and an if expression a < 0,0, Syntax Analyzer detects unexpected , and generates an error message.

<if statement> ::= if <expression> then <statement> | if <expression> then <statement> else <statement>
<expression> ::= <simple expression> | <simple expression> <relational operator> <simple expression>
<simple expression> ::= <term> | <sign> <term>| <simple expression> <adding operator> <term>
<term> ::= <factor> | <term> <multiplying operator> <factor>
<factor> ::= <variable> | <unsigned constant> | ( <expression> ) | <function designator> | <set> | not <factor>
<variable> ::= <identifier>
<unsigned constant> ::= <unsigned number> | <string> | < constant identifier> < nil>
<relational operator> ::= = | <> | < | <= | >= | > | in

Also, the Syntax Analyzer implements basic error neutralization technics. Neutralization is a process of recovery after error detection. For example, we can skip some sequence of symbols until we meet an appropriate symbol like in the expression above we skip , and 0 until we meet then which is a legal part of the if statement.

Semantic analysis

Semantic Analyzer checks the program for semantic errors. Supported types of errors:

• Occurrence of an undeclared identifier (type, variable, function)
• Multiple identifier declaration
• Incompatible operands types of an operator
• Wrong number of arguments in function invocation

Code generation

The module generates Java bytecode for provided Pascal program using the library ASM. The compiler uses a core API of the library. While parsing a program, the Code Generator builds a sequence of Java bytecode instructions that operate on the list of local variables and operand stack.

An example of bytecode for a more minor program.

// class version 52.0 (52)
// access flags 0x1
public class PascalProgram {

  // access flags 0x9
  public static main([Ljava/lang/String;)V
   L0
    LOCALVARIABLE hours1 I L0 L1 1
    LOCALVARIABLE minutes1 I L0 L1 2
    LOCALVARIABLE hours2 I L0 L1 3
    LOCALVARIABLE minutes2 I L0 L1 4
    LOCALVARIABLE allMinutes1 I L0 L1 5
    LOCALVARIABLE allMinutes2 I L0 L1 6
    LOCALVARIABLE result I L0 L1 7
    LDC 15
    ISTORE 1
    LDC 25
    ISTORE 2
    LDC 15
    ISTORE 3
    LDC 21
    ISTORE 4
    ILOAD 1
    LDC 60
    IMUL
    ILOAD 2
    IADD
    ISTORE 5
    ILOAD 3
    LDC 60
    IMUL
    ILOAD 4
    IADD
    ISTORE 6
    ILOAD 6
    ILOAD 5
    ISUB
    ISTORE 7
   L1
    RETURN
    MAXSTACK = 0
    MAXLOCALS = 0
}