Syntax-directed translation (SDT) schemes are a powerful technique in compiler design that combines syntax analysis (parsing) with semantic analysis (meaning extraction) within a single framework. Here's a breakdown of key concepts:
What are SDT Schemes?
- A notation that associates semantic actions (code) with productions in a context-free grammar (CFG).
- These actions are triggered during the parsing process, allowing for the generation of intermediate code, symbol table updates, or other semantic tasks based on the structure of the input program.
Components:
- Context-Free Grammar (CFG): Defines the valid syntactic structure of the input program.
- Productions: Rules in the CFG that specify how non-terminal symbols can be rewritten as terminal symbols (keywords, identifiers, etc.) or other non-terminal symbols.
- Semantic Actions: Code snippets embedded within productions, written in a suitable programming language (often a high-level language).
- Attributes: Associated with grammar symbols (terminals and non-terminals) to hold values or information during parsing.
Types of SDT Schemes:
- Attribute Evaluation Schemes: Focus on how attribute values are computed and propagated during parsing. Common types include:
- S-attributed: Attributes only depend on the symbols on the right-hand side (RHS) of a production.
- L-attributed: Attributes can depend on both LHS and RHS symbols.
- LR Parsing-based Schemes: Leverage the parsing power of LR parsers (like LR(0), SLR, LALR) to handle bottom-up parsing with semantic actions.
Benefits of SDT Schemes:
- Improved Readability: Semantic actions integrated with grammar rules enhance code maintainability.
- Modular Design: Separates syntax analysis from semantic analysis, promoting modularity.
- Error Handling: Can be integrated with semantic actions to perform type checking and other validations during parsing.
Example (Simple Arithmetic Expression):
E -> T + E | T T -> F * T | F F -> (E) | id {type of E; // attribute associated with E}E -> T {type of E = type of T; } + E {}| T {type of E = type of T; }T -> F {type of T = type of F; } * T {}| F {type of T = type of F; }{type of F; // attribute associated with F}F -> ( E ) {type of F = type of E; }| id {} // assume identifier type is looked up elsewhere
In this example, semantic actions calculate the type of expressions during parsing by propagating type information from sub-expressions to higher-level ones.
Summary:
Syntax-directed translation schemes provide a powerful approach for combining syntax and semantic analysis in compiler design. By embedding semantic actions within grammar productions, you can perform meaning extraction and other tasks while parsing the input program, leading to more readable, modular, and robust compiler implementations.