Semantic Analysis in Compiler Design

Semantic Analysis is the third phase of a compiler, performed after syntax analysis. It ensures that a program is semantically correct, meaning it follows the logical rules of the programming language.

While syntax analysis checks the structure of the program, semantic analysis checks its meaning, such as correct use of variables, types, and expressions.

It works on the parse tree (or AST) and symbol table to:

• Verify type correctness
• Ensure proper declaration and scope of identifiers
• Validate expressions and function calls

During this phase, the syntax tree is enriched with type and scope information, forming an Annotated Syntax Tree, which is used in later stages like intermediate code generation.

Need

Even if a program is syntactically correct, it may still be incorrect logically.

Example:

int a;
a = "hello"; // syntactically correct but semantically wrong

Syntax is valid, but type rules are violated.

Thus, semantic analysis ensures:

• Correct interpretation of meaning
• Consistency in program behavior
• Error detection before code generation

Role of Semantic Analyzer

The Semantic Analyzer performs the following major tasks:

• Type Checking
• Scope Resolution
• Symbol Table Management
• Function Call Validation
• Intermediate Representation Support
• Syntax Tree Annotation

Functions

1. Type Checking

Ensures that operands of operators are of compatible types.

Example:

int x = 10;
float y = x + 2.5; // valid

Invalid case:

int x = "abc"; // type mismatch

2. Scope Resolution

Determines the region in which a variable is accessible.

Example:

{
 int x = 10;
}
x = 20; // error: x out of scope

3. Type Conversion (Coercion)

Automatically converts data types when required.

Example:

float x = 10.5;
float y = x * 2; // 2 → 2.0 (implicit conversion)

4. Function Checking

Ensures:

• Correct number of arguments
• Correct argument types
• Correct return type

Example:

int add(int a, int b);
add(5); // error: missing argument

5. Label Checking

Ensures that all labels used in the program are properly declared and referenced.

6. Flow Control Check

Ensures proper use of control flow statements.

Examples:

• break only inside loops or switch
• return matches function type

7. Symbol Table Handling

Semantic analysis uses the symbol table to:

• Store variable names, types, scope
• Track function definitions
• Ensure correct usage of identifiers

Semantic Errors

Semantic errors are detected after syntax analysis. Common errors include:

1. Type Mismatch

int x = "hello";

2. Undeclared Variables

x = 10; // not declared

3. Multiple Declarations

int x;
int x; // redeclaration

4. Reserved Keyword Misuse

int while = 5;

5. Incorrect Function Usage

int add(int a, int b);
add(1); // wrong number of arguments

Static vs Dynamic Semantics

Static Semantics

• Checked at compile time
• Includes type checking, scope rules, declarations
• Helps detect errors early

Dynamic Semantics

• Checked at runtime
• Defines actual behavior of program execution

Examples:

• Division by zero
• Null pointer access

Annotated Syntax Tree

During semantic analysis, additional information is added to the syntax tree.

Example:

x = a + b;

After annotation:

• a → int
• b → int
• a + b → int
• x → int

This helps in intermediate code generation

Example of Semantic Analysis

float x = 10.1;
float y = x * 30;

Steps:

• x is float
• 30 is integer
• Convert 30 → 30.0
• Perform multiplication