Understanding Compiler Passes: Multi-Pass vs. One-Pass Compilation

Learn about compiler passes and the difference between multi-pass and one-pass compilation. This guide explains how compilers process source code, detailing the stages involved in translating human-readable code into machine instructions, and the trade-offs between these two common compiler approaches.

Understanding Compiler Passes

What is a Compiler Pass?

A compiler pass is a single complete scan of the source code during compilation. Compilers often use multiple passes to transform source code (like C or C++) into machine-readable instructions. There are two main approaches: multi-pass and one-pass compilers.

Multi-Pass Compilers

Multi-pass compilers process the source code multiple times, with each pass performing a specific task. Here's a typical example:

  1. First Pass (Lexical Analysis): The compiler reads the source code, breaks it into individual tokens (keywords, identifiers, operators, etc.), and stores the results in an intermediate representation (often a file).
  2. Second Pass (Syntax Analysis/Parsing): The compiler reads the tokens, builds a syntax tree (abstract syntax tree or AST), and checks for grammatical correctness (syntax).
  3. Third Pass (Semantic Analysis): The compiler examines the syntax tree to verify that the code makes sense according to the programming language's rules. It checks for type errors, undeclared variables, and other semantic issues.
  4. Further Passes (Optimization, Code Generation): Additional passes might perform code optimization or generate the final machine code.

One-Pass Compilers

One-pass compilers process the source code just once. This simplifies the compiler but might impose restrictions on the language's design. Here's how a one-pass compiler works:

  1. Lexical Analysis and Tokenization: The compiler reads and breaks down each line of code into tokens.
  2. Syntax Analysis: The syntax of each line is checked, and a corresponding part of the syntax tree is created.
  3. Semantic Analysis and Code Generation: The semantic meaning of the code is checked and machine code for that line is generated.
  4. Repetition: Steps 1-3 are repeated for each line.