📘 Semantic Analysis and Intermediate Code Generation

(Compiler Construction — exam-ready, simple explanation with examples, diagrams, and key points)

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🧠 1. Semantic Analysis

✅ Definition

Semantic Analysis is the phase of the compiler that checks whether the program has a meaningful (logically correct) structure according to the language rules.

👉 It is done after syntax analysis and ensures:

“The program is syntactically correct AND makes sense.”

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📌 Position in Compiler

Source Code → Lexical → Syntax → Semantic → Intermediate Code

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🎯 2. Purpose of Semantic Analysis

It checks:

✔ Type checking ✔ Variable declaration ✔ Function correctness ✔ Scope rules ✔ Expression validity

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🔧 3. Main Tasks of Semantic Analysis

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🔹 1. Type Checking (Very Important)

Ensures correct data types are used.

📌 Example:

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

✔ Valid (implicit conversion)

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❌ Invalid Example:

int x = "hello";

👉 Error: type mismatch

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🔹 2. Scope Checking

Ensures variables are used within valid scope.

Example:

{
    int x = 10;
}
print(x);  // ❌ Error

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🔹 3. Function Checking

Checks:

• Correct number of arguments
• Correct return type

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Example:

int add(int a, int b);
add(5);  // ❌ Error

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🔹 4. Declaration Checking

Every variable must be declared before use.

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Example:

x = 10;  // ❌ Error (not declared)

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🌳 4. Symbol Table (Very Important)

✅ Definition

A symbol table stores information about identifiers.

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📊 Contains:

• Variable name
• Data type
• Scope
• Memory location

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📌 Example Table:

Name	Type	Scope
x	int	global
y	float	local

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🧠 5. Semantic Errors

Error Type	Example
Type mismatch	int x = "abc"
Undeclared variable	x = 10
Wrong function call	add(5)
Scope error	using local variable outside block

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📌 6. Difference: Syntax vs Semantic

Feature	Syntax Analysis	Semantic Analysis
Checks	Grammar rules	Meaning rules
Errors	Syntax errors	Logical errors
Example	missing ;	type mismatch
Stage	Before semantic	After syntax

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🧠 7. Intermediate Code Generation (ICG)

✅ Definition

Intermediate Code Generation converts source code into an intermediate representation (IR) that is:

• Independent of machine
• Easier to optimize
• Easier to translate to machine code

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🎯 Purpose

✔ Bridge between source code and machine code ✔ Helps optimization ✔ Makes compiler portable

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📊 8. Forms of Intermediate Code

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🔹 1. Three Address Code (TAC) ⭐ Most Important

Each instruction has at most 3 operands.

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📌 Example:

a = b + c * d

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TAC:

t1 = c * d
t2 = b + t1
a = t2

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🔹 2. Quadruples

Format:

(op, arg1, arg2, result)

Example:

op	arg1	arg2	result
*	c	d	t1
+	b	t1	t2
=	t2		a

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🔹 3. Triples

Format:

(index) op arg1 arg2

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Example:

i	op	arg1	arg2
0	*	c	d
1	+	b	(0)
2	=	(1)	a

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🔹 4. Indirect Triples

• Uses pointer table to refer triples
• Easier optimization

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🧠 9. Expression Translation (Important Exam Part)

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📌 Example:

x = a + b * c

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Step 1: Convert multiplication first

t1 = b * c
t2 = a + t1
x = t2

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🌳 10. Syntax Tree vs Intermediate Code

Feature	Syntax Tree	Intermediate Code
Form	Tree	Linear code
Use	Structure	Execution steps
Level	High	Medium
Machine dependency	No	No

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🔄 11. Flow of Semantic + ICG

Syntax Tree → Semantic Analysis → Annotated Tree → Intermediate Code

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⚠️ 12. Common Exam Points

👉 Frequently asked:

• Define semantic analysis
• Role of symbol table
• Type checking with example
• What is intermediate code?
• Three address code with example
• Quadruples vs triples
• Difference between syntax and semantic analysis
• Translate expression into TAC

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📝 13. Short Notes (Quick Revision)

🔵 Semantic Analysis

• Checks meaning
• Uses symbol table
• Detects logical errors

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🔵 Intermediate Code

• Machine independent
• Easy for optimization
• Common form: TAC

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📊 14. Final Summary Table

Aspect	Semantic Analysis	Intermediate Code
Purpose	Check meaning	Generate IR code
Input	Syntax tree	Annotated tree
Output	Annotated tree	Intermediate code
Focus	Correctness	Translation
Tool	Symbol table	TAC / Quadruples
Errors	Semantic errors	None (after generation)

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✅ Final Conclusion

• Semantic Analysis ensures program meaning is correct
• It uses symbol tables and type checking
• Intermediate Code Generation converts program into machine-independent form
• Both are essential steps between syntax analysis and code optimization

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