Semantic Analysis

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Arjit Sharma
02 Mins

What Is Semantic Analysis ?

semantic-analyzer-intro

After syntax analysis confirms that the structure of your code is valid, semantic analysis checks that the code also has meaning.

Example:

int x = "hello";  // Syntax is fine, but semantically invalid

Here, the parser is okay with the structure, but the semantic analyzer will raise an error because you’re assigning a string to an integer.

What Does Semantic Analyzer Check ?

what-semantic-analyzer-checks

  1. Type Checking
    • Are values used with the correct data types?
    • Example: int x = 5 + true; → invalid in most languages
  2. Scope Resolution
    • Are variables declared before use?
    • Is a variable accessed within its valid scope?
  3. Function Signatures and Calls
    • Are functions called with the right number and type of arguments?
  4. Variable Declarations and Usage
    • Prevents using undeclared variables
    • Checks for duplicate declarations

Internal Working of Semantic Analysis

Semantic analysis takes the Abstract Syntax Tree (AST) generated during syntax analysis and verifies whether the program is logically meaningful.

working-of-semantic-analyzer

Semantic analysis walks through this AST and attaches additional information such as variable types, scopes, function signatures, expression types. To perform these checks efficiently, the compiler maintains a symbol table alongside the AST.

AST with Semantic Information

ast-with-semantic-info

During semantic analysis, the AST becomes enriched with semantic details.

For example:

  • variables get associated with their data types
  • expressions are checked for type compatibility
  • function calls are validated against parameter definitions

In the diagram above:

  • x : int means variable x has integer type
  • sum(x : int, y : int) verifies argument types
  • BinaryExpr (a + b : int) confirms the expression result type

This annotated AST helps later compiler phases like optimization and code generation.

Symbol Table

symbol-tree-with-semantic-info

The symbol table is a compiler data structure that stores information about identifiers used in the program.

It typically keeps track of:

  • variable names
  • data types
  • scopes
  • function definitions
  • parameters
  • memory-related metadata

For example:

  • x belongs to the main scope
  • a and b are parameters of sum
  • sum is a function returning int

Semantic analysis continuously consults the symbol table while traversing the AST to validate declarations and resolve identifiers correctly.

Conclusion

  • Semantic analysis ensures code is meaningful, using the AST.
  • It uses a symbol table to track declarations, scopes, and types
  • Errors at this stage are not structural but meaning-related
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