Understanding Semantic Analysis in Compiler Design

Ordered, rooted tree that represents the syntactic structure of a string according to some context-free grammar.

Semantic analysis is a vital phase in the compiler design process. It follows the syntax analysis phase and uses the parse tree and the symbol table to check the source program for semantic errors. It also gathers type information for the subsequent code generation phase. This article will delve into the role of the semantic analyzer, attribute grammars, type checking, and the design of a semantic analyzer.

Role of the Semantic Analyzer

The semantic analyzer is responsible for ensuring that the parse tree generated by the syntax analyzer is semantically correct. This means that the operations are meaningful. For example, it checks that operators are used with compatible types, variables are declared before use, function calls match declarations, etc. If the semantic analyzer detects an error, it generates an error message.

Attribute Grammars and Semantic Analysis

Attribute grammars are a powerful tool used in semantic analysis. They extend context-free grammars by attaching attributes to the grammar symbols and by associating computations with the grammar rules. The computations, also known as semantic rules, can access the attributes of the grammar symbols in the rule's scope.

There are two types of attributes: synthesized attributes, which are computed from the attribute values of the children nodes, and inherited attributes, which are computed from the attribute values of the parent and/or sibling nodes. Attribute grammars allow the semantic analyzer to carry out complex semantic checks and computations.

Type Checking

Type checking is a crucial part of semantic analysis. It ensures that the types of an operation's operands match the operation's type expectations. For example, it checks that you're not trying to add a number and a string, or call a function with the wrong number or types of arguments.

There are two types of type checking: static type checking, which is performed at compile time, and dynamic type checking, which is performed at runtime. Static type checking can catch many type errors before the program is run, while dynamic type checking allows for more flexibility in the types of expressions and statements a program can execute.

Designing a Semantic Analyzer

Designing a semantic analyzer involves defining the attribute grammar and the semantic rules for the source language, and implementing the attribute computations in the compiler. The semantic analyzer typically works in conjunction with the syntax analyzer and the symbol table manager.

The syntax analyzer calls the semantic analyzer whenever it reduces a grammar rule, passing it the grammar symbols in the rule and their attribute values. The semantic analyzer then computes the attribute values for the left-hand side symbol of the rule, updates the symbol table as needed, and checks for semantic errors.

In conclusion, semantic analysis is a critical phase in compiler design, ensuring that the source program is semantically correct and gathering type information for code generation. Understanding the role of the semantic analyzer, attribute grammars, type checking, and the design of a semantic analyzer is crucial for anyone interested in compiler design.