PCD - Introduction

1. Principles of Compiler
Design
P Sheela Gowr

2. Introduction to Compilers
Purpose of Compilers
• Compilers are essential tools in the software development
process that translate high-level programming languages
into machine code that can be executed by computers.
Function of Compilers
• Compilers perform various tasks, including lexical analysis,
syntax analysis, semantic analysis, code generation, and
optimization.

3. Lexical Analysis
Lexical analysis, also known as scanning, is the process of converting a sequence of characters
into a sequence of tokens. It is the first phase of the compiler design process and plays a
crucial role in the overall compilation process
Techniques
• Deterministic Finite Automaton (DFA): DFA is a type of finite automaton that recognizes regular languages. It is
commonly used in lexical analysis due to its efficiency and simplicity.
• Non-Deterministic Finite Automaton (NFA): NFA is another type of finite automaton that can recognize regular
languages. It is more expressive than DFA but less efficient.
• Regular Expression Matching: Lexical analyzers use regular expressions to match the input characters with the defined
token patterns. Regular expression matching algorithms, such as Thompson's construction or NFA simulation, are
employed to perform this task.
• Tokenization and Token Classification: Once the input characters are tokenized, lexical analyzers classify each token into its
corresponding token type. This classification is based on the defined token patterns and regular expressions.
• Error Handling: Lexical analyzers also handle lexical errors, such as invalid characters or unrecognized tokens. Error
handling techniques, such as error reporting and error recovery, are employed to ensure robustness and user-friendly error
messages.

4. Syntax Analysis
Syntax analysis, also known as parsing, is the process of analyzing the syntactic structure of a
program. It is a crucial phase in the compilation process, where the input source code is
checked for syntactical correctness and transformed into a parse tree or an abstract syntax tree
(AST).
Techniques
• Top-Down Parsing: Top-down parsing starts from the root of the parse tree and applies the grammar rules in a top-down
manner. Recursive descent parsing is a common top-down parsing technique.
• Bottom-Up Parsing: Bottom-up parsing starts from the input tokens and applies the grammar rules in a bottom-up
manner to construct the parse tree. LR parsing and LALR parsing are examples of bottom-up parsing techniques.
• Syntax Error Handling: During syntax analysis, if a syntax error is encountered, the parser generates an error message
and tries to recover from the error. Error recovery techniques include panic mode recovery, error productions, and error
synchronization.
• Ambiguity Resolution: Ambiguity can occur in a grammar when a single input has multiple valid parse trees. Ambiguity
can be resolved by modifying the grammar rules or using disambiguation techniques such as operator precedence and
associativity rules.
• Syntax analysis is a crucial step in the compilation process, as it ensures that the input source code is syntactically correct
and can be further processed by subsequent compiler phases.

5. Semantic Analysis
Semantic analysis is the process of checking the meaning of a program. It ensures that the program is
semantically correct and meaningful according to the rules of the programming language. This stage of
the compiler is responsible for detecting and reporting semantic errors, such as type mismatches,
undeclared variables, and incorrect use of language constructs. Semantic analysis plays a crucial role in
ensuring the reliability and correctness of the compiled program.
Key Concepts and Techniques
Type Checking
• Type checking is a fundamental task in semantic analysis. It involves verifying that the types of expressions and variables are
compatible and consistent throughout the program. Type checking helps prevent runtime errors and ensures that
operations are performed on operands of the correct type.
Symbol Table
• A symbol table is a data structure used by the compiler to store information about variables, functions, and other symbols
in the program. It serves as a lookup table for resolving references to symbols and provides a central repository for storing
and retrieving semantic information.
Scope Analysis
• Scope analysis determines the visibility and accessibility of variables and other symbols in different parts of the program. It
involves identifying the scope of variables, resolving name conflicts, and enforcing scoping rules. Scope analysis helps
ensure that variables are used correctly and consistently within their respective scopes.

6. Semantic Rules
• Semantic rules define the meaning and behavior of language constructs. They specify the
allowed operations, the types of operands, and the results of expressions. Semantic rules are
enforced during semantic analysis to ensure that the program follows the intended semantics
of the programming language.
Error Reporting
• Semantic analysis is responsible for detecting and reporting semantic errors in the program.
When an error is encountered, the compiler should provide informative error messages that
help the programmer identify and fix the issue. Error reporting is an essential aspect of
semantic analysis that aids in program debugging and troubleshooting.

7. Intermediate Code Generation
Intermediate code generation is an important step in the compilation process, where the source
code is converted into an intermediate representation. This intermediate representation serves
as a bridge between the high-level source code and the low-level machine code. It allows for
easier analysis and optimization of the code before generating the final executable.

8. Code Optimization
Code optimization is the process of improving the efficiency and performance of the generated code. It
involves analyzing the code and making modifications to reduce execution time, memory usage, and
overall resource consumption.
• Dead Code Elimination
• Constant Folding
• Loop Optimization
• Inlining
• Register Allocation
• Code Reordering

9. Code Generation
Code generation is the final phase of the compiler design process, where the intermediate representation
(IR) is translated into machine code or assembly code.
Techniques
• Static Single Assignment (SSA): SSA is a popular intermediate representation used in code generation. It
ensures that each variable is assigned only once, simplifying the analysis and optimization phases.
• Peephole Optimization: Peephole optimization is a local optimization technique that involves examining a
small window of instructions and applying transformations to improve code quality and performance.
• Code Templates: Code templates are pre-defined patterns or templates that the compiler uses to generate
code for common programming constructs, such as loops and function calls.
• Code Emission: Code emission is the final step of code generation, where the compiler outputs the
generated machine code or assembly code. This code can then be executed on the target hardware.

10. Conclusion
In this lecture on compiler design, we covered several key points:
• Definition of a compiler: A compiler is a software tool that translates high-level
programming languages into machine code or byte code.
• Phases of compilation: The compilation process consists of several phases, including lexical
analysis, syntax analysis, semantic analysis, code generation, and optimization.
• Importance of compilers: Compilers play a crucial role in software development, as they
enable programmers to write code in high-level languages and have it executed efficiently on
various hardware platforms.
• By understanding the concepts and principles of compiler design, software developers can
create efficient and reliable programs that meet the needs of modern computing systems.