LECTURE 14 IN COMPUTER SCIENCE ENGINEERING

Lecture 14: Compiler Design

In this lecture, we study Compiler Design, which explains how high-level programming code (e.g., C, Java, Python) is translated into machine code that can be executed by a computer. Compilers are essential for bridging the gap between human-readable programming languages and low-level hardware instructions.

1. Introduction to Compilers

A compiler is a system software that converts source code written in a high-level language into executable machine code or intermediate code.

Example: A C program is compiled into assembly or machine instructions, which are then executed by the CPU.

2. Phases of Compiler Design

Compilation happens in a series of steps, commonly called the phases of a compiler:

Lexical Analysis: Converts source code into tokens (keywords, identifiers, symbols). Example: int x = 5; → Tokens: int, x, =, 5, ;
Syntax Analysis (Parsing): Checks whether tokens follow grammar rules of the language. Creates a parse tree.
Semantic Analysis: Ensures meaningful correctness (type checking, variable declarations). Example: Prevents adding a number to a string unless explicitly allowed.
Intermediate Code Generation: Produces machine-independent representation of code, often in 3-address format.
Optimization: Improves efficiency without changing meaning. Example: Eliminating redundant calculations.
Code Generation: Translates optimized intermediate code into machine instructions.
Code Linking and Loading: Resolves external libraries and prepares executable code for the OS.

3. Lexical Analysis in Detail

The lexical analyzer, or lexer, scans input characters and groups them into tokens. It also removes whitespace and comments. This is the first phase of compilation and relies on regular expressions and finite automata.

4. Parsing Techniques

Parsers analyze the grammatical structure of the code. Two major approaches are:

Top-Down Parsing: Begins at the start symbol and expands using production rules (e.g., LL parser).
Bottom-Up Parsing: Begins with input symbols and reduces them to the start symbol (e.g., LR parser).

5. Semantic Analysis

The semantic analyzer uses a symbol table to check:

Type consistency (e.g., cannot assign a string to an integer variable).
Variable/function declarations before use.
Scope rules (local vs. global variables).

6. Intermediate Code Representation

Compilers often use intermediate code to simplify translation into multiple machine languages. Common forms include:

Three-Address Code (TAC): Simple instructions like t1 = a + b.
Abstract Syntax Trees (AST): Tree representation of program structure.
Bytecode: Portable code used by virtual machines (e.g., Java JVM).

7. Code Optimization

Optimization improves execution speed and reduces resource usage. Types include:

Peephole Optimization: Small local improvements (e.g., replacing x * 2 with x << 1).
Loop Optimization: Reducing repeated calculations inside loops.
Dead Code Elimination: Removing code that will never execute.

8. Code Generation

Converts intermediate code into target machine code. Example: t1 = a + b might become assembly: ADD R1, A, B.

9. Compiler vs. Interpreter

Compilers translate entire code into machine code before execution (e.g., C, C++). Interpreters translate and execute line by line (e.g., Python, JavaScript).

10. Applications of Compiler Design

Development of new programming languages.
Portability of applications across platforms (via intermediate code like bytecode).
Performance improvements in software execution.
Static analysis tools for debugging and security.

11. Summary

A compiler translates high-level code into machine code.
Compilation occurs in multiple phases: lexical, syntax, semantic, intermediate, optimization, code generation.
Optimization improves performance without changing correctness.
Intermediate code ensures portability across systems.

Next Lecture (15): Human-Computer Interaction (HCI) and User Interface Design

LECTURE 15 IN COMPUTER SCIENCE ENGINEERING