Compiler Design: Phases and Structure

• This document discusses compiler design, focusing on the phases, structure, and key components of a compiler.
• It also covers assemblers, linkers, and loaders as part of the compilation process.

Compiler Overview

• A compiler translates source code (high-level language) into target code (low-level language, e.g., assembly or machine code).
• The compilation process can be broken down into distinct phases.

Phases of Compilation

• Lexical Analysis (Scanning): Converts source code into a stream of tokens.
  • Tokens are basic building blocks like keywords, identifiers, operators, and constants.
  • It identifies and categorizes these tokens.
• Syntax Analysis (Parsing): Constructs a syntax tree from the tokens.
  • This tree represents the grammatical structure of the source code.
  • Checks if the input program follows the grammar rules of the programming language.
• Semantic Analysis: Checks the syntax tree for semantic errors.
  • Includes type checking, ensuring that variables are used consistently with their declared types.
  • Gathers type information for subsequent code generation.
• Intermediate Code Generation: Translates the syntax tree into an intermediate representation (IR).
  • IR is a machine-independent representation of the program, simplifying optimization and code generation.
  • Examples of IR include three-address code and abstract syntax trees.
• Code Optimization: Improves the intermediate code to enhance performance.
  • Aims to reduce execution time and memory usage.
  • Optimizations can include dead code elimination, constant folding, and loop unrolling.
• Code Generation: Transforms the optimized intermediate code into target code.
  • Target code can be assembly language or machine code.
  • Involves instruction selection, register allocation, and code scheduling.

Compiler Structure

• Front End: Includes lexical analysis, syntax analysis, semantic analysis, and intermediate code generation.
  • Handles the language-specific aspects of compilation.
• Back End: Includes code optimization and code generation.
  • Deals with the target machine-specific aspects of compilation.
• Symbol Table: A data structure that stores information about identifiers used in the source code.
  • Stores names, types, scopes, and other relevant details.
  • Used in various phases of the compilation process.

Assembler

• Translates assembly language code into machine code.
• Assembly code uses mnemonics to represent machine instructions.
• Assemblers perform tasks such as:
  • Converting mnemonics to opcodes.
  • Resolving symbolic addresses to numerical addresses.
  • Creating object files containing machine code and relocation information.

Linker

• Combines multiple object files into a single executable file.
• Resolves external references between object files.
• Performs relocation, adjusting addresses in the code to reflect their final positions in memory.
• Libraries are often linked to the program by the linker to include pre-compiled functions.

Loader

• Loads the executable file into memory and prepares it for execution.
• Allocates memory space for the program.
• Resolves symbolic links.
• Initializes registers and sets the program counter to the starting address of the program.
• Dynamic Linking: Loading and linking of shared libraries at runtime.
  • Reduces the size of executable files.
  • Allows multiple programs to share the same library code in memory.

More Details On Compilation Phases

• Lexical Analysis breaks source code into a stream of tokens where each token represents a basic element of the language, like identifiers, keywords, operators and constants.
  • Removes whitespace and comments.
  • Identifies lexical errors, such as invalid characters.
• Syntax Analysis constructs a syntax tree from the tokens, reflecting the grammatical structure.
  • Checks if the input program conforms to the language's grammar.
  • Employs techniques like top-down (LL parsing) or bottom-up (LR parsing).
• Semantic Analysis checks the syntax tree for semantic consistency.
  • Type checking ensures operations are performed on compatible types.
  • Scope resolution determines the visibility of variables.
  • Error detection includes undeclared variables and type mismatches.
• Intermediate Code Generation translates the syntax tree into an intermediate representation (IR).
  • IR is machine-independent, facilitating portability and optimization.
  • Common IR forms include three-address code and abstract syntax trees.
• Code Optimization improves the intermediate code to minimize resource usage.
  • Optimizations like constant folding, dead code elimination, and loop unrolling are applied.
  • Can be machine-dependent or machine-independent.
• Code Generation transforms the optimized IR into target code.
  • Involves instruction selection, register allocation, and code scheduling.
  • Target code can be assembly language, machine code, or another high-level language.

Symbol Table Details

• The symbol table is a crucial data structure that maintains information about identifiers.
• Stores names, types, scopes, and attributes of variables, functions, and other program constructs.
• Used in multiple compiler phases for information retrieval and validation.

Assembler Tasks

• The assembler translates assembly language into machine code by:
  • Converting assembly mnemonics into their corresponding opcodes.
  • Resolving symbolic addresses like variable names into numerical memory addresses.
  • Generating object files that contain machine code and necessary relocation information.

Linker Functions

• The linker combines multiple object files to produce a final executable.
• Resolves external references by connecting calls to functions or variables defined in other files.
• Performs relocation by adjusting addresses to the executable's memory layout.
• Libraries, both static and shared, are integrated to include external functions and resources.

Loader Operations

• The loader's role is to prepare the executable for running.
• Allocates memory space required by the program.
• Resolves symbolic links to shared libraries, especially in dynamic linking.
• Initializes registers and sets the program counter to the program's entry point.

Types of Linking

• Static Linking: Libraries are copied into the executable during the linking phase.
  • Results in larger executable files.
  • No external dependencies after compilation.
• Dynamic Linking: Linking of shared libraries occurs at runtime.
  • Reduces executable size and saves disk space.
  • Requires the presence of shared libraries on the target system.