Semantic Analysis


In compiler design, semantic analysis is a crucial phase following syntax analysis (parsing). It ensures that a program's source code adheres to the semantic rules defined by the programming language, guaranteeing its meaningfulness and consistency.

Key Functions

  • Type Checking: Verifies that variables, expressions, and operations use data types in a compatible and meaningful way. For example, it detects errors where a numeric operation is attempted on a string.
  • Scope Resolution: Determines the visibility and accessibility of variables, functions, and other identifiers based on their declaration and usage context. This prevents conflicts caused by duplicate names or out-of-scope references.
  • Definite Assignment Checking: Ensures that variables are assigned a value before they are used, preventing unpredictable behavior due to uninitialized variables.
  • Control Flow Analysis: Examines the logical flow of the program, detecting issues like unreachable code, infinite loops, or missing return statements in functions.
  • Symbol Table Management: Maintains a symbol table, which stores information about declared entities (variables, functions, classes, etc.) and their attributes (data types, scope, etc.). This organized representation is crucial for semantic checks and subsequent code generation.


  • Attribute Grammars: A formal way to associate attributes (e.g., data type, scope) with nodes in a syntax tree. Semantic rules define how attributes are calculated and propagated through the tree.
  • Abstract Syntax Trees (ASTs): Serve as the primary data structure for semantic analysis. An AST captures the essential structure of the code while abstracting away from concrete syntax details.
  • Visitor Pattern: Traverses the AST, applying type checking, scope resolution, and other semantic checks to each node.

Benefits of Semantic Analysis

  • Early Error Detection: Catches a wide range of programming errors during compilation, preventing unexpected program behavior or crashes at runtime.
  • Improved Code Reliability: Semantic analysis helps ensure that code is internally consistent and adheres to the language's intended meaning.
  • Enhanced Code Understanding: The process of semantic analysis forces a deeper understanding of the program's logic and the relationships between its elements.
  • Optimization Opportunities: Information gathered during semantic analysis can guide the compiler in making code optimizations.


Consider the following code snippet:

int x = 10;
string y = "hello";
z = x + y; // Type error!

A semantic analyzer would detect a type mismatch error in the last line, as you cannot directly add a number and a string.

Relationship to Other Compiler Phases

  • Syntax Analysis: Precedes semantic analysis. It confirms that the code is structurally valid according to the language's grammar.
  • Code Generation: Follows semantic analysis. It translates the semantically valid code into machine-executable instructions or an intermediate representation.