Semantic Analysis

Back to Compiler Design
Arjit Sharma
01 Min

What Is Semantic Analysis?

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 It Check?

  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

Using the AST

Semantic analysis works with the AST, adding details like variable types or scopes. The AST is a simplified tree from syntax analysis, acting as an intermediate representation (IR) for this phase.

Symbol Table

Semantic analysis heavily uses a symbol table, which keeps track of:

  • Variable names
  • Types
  • Scopes
  • Function definitions

It’s like the compiler’s cheatsheet for the code.

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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