🧠 Code Generation: Design of a Code Generator (Compiler Construction)

Code generation is the final major phase of a compiler where intermediate code is converted into target machine code or assembly code.

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📌 1. Code Generation

📖 Definition

Code generation is the process of converting intermediate representation (IR) of a program into target machine code.

👉 It produces executable code for the CPU.

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🧠 Input and Output

Input	Output
Intermediate Code (TAC, DAG, etc.)	Machine / Assembly Code

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📊 Basic Code Generator Structure

Intermediate Code
        ↓
Code Generator
        ↓
Target Machine Code

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🧠 2. Design of a Code Generator

📖 Definition

The design of a code generator refers to how the compiler component converts IR into efficient machine code while considering correctness, speed, and memory usage.

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⚙️ Main Goals of Code Generator

✔ Correct output ✔ Fast execution ✔ Efficient memory usage ✔ Minimal instructions ✔ Proper register usage

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🧠 3. Major Tasks of Code Generator

🔹 1. Instruction Selection

Choosing correct machine instructions for IR.

Example:

t1 = a + b

Machine code:

MOV R1, a
ADD R1, b

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🔹 2. Register Allocation

Assign variables to CPU registers.

👉 Registers are faster than memory.

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🔹 3. Instruction Ordering

Arrange instructions for better performance.

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🔹 4. Storage Management

Decide where variables will be stored:

• Stack
• Heap
• Registers

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🔹 5. Code Optimization (Local level)

Improve generated code without changing meaning.

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🧠 4. Input to Code Generator

The code generator takes Intermediate Representation (IR) like:

🔹 Three Address Code (TAC)

t1 = a + b
t2 = t1 * c

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🧠 5. Target Program

Output may be:

• Assembly code
• Machine code

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Example:

LOAD R1, a
ADD R1, b
STORE t1, R1

LOAD R2, t1
MUL R2, c
STORE t2, R2

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🧠 6. Issues in Code Generation (Very Important Exam Topic)

🔴 1. Input Form

• TAC, DAG, or syntax tree affects output quality

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🔴 2. Target Machine

• Number of registers
• Instruction set

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🔴 3. Memory Management

• Stack / heap usage

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🔴 4. Instruction Cost

• Some instructions are faster than others

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🔴 5. Register Availability

• Limited registers → need spilling

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🧠 7. Register Allocation

📖 Definition

Assigning variables to CPU registers to reduce memory access.

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🔹 Types:

1. Static Allocation

• Done at compile time

2. Dynamic Allocation

• Done during execution

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📊 Example:

a → R1
b → R2
c → memory (if registers full)

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🧠 8. Simple Code Generation Example

High-level code:

x = a + b * c;

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Step 1: Intermediate Code

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

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Step 2: Target Code

MOV R1, b
MUL R1, c
MOV R2, a
ADD R2, R1
MOV x, R2

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🧠 9. Code Generation Techniques

🔹 1. Simple Code Generation

• Direct translation
• Not optimized

🔹 2. Optimal Code Generation

• Minimizes instruction cost
• Uses registers efficiently

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🧠 10. Peephole Optimization (Important)

📖 Definition

A technique that improves small segments of code.

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🔍 Example:

Before:

MOV R1, a
MOV R1, a

After:

MOV R1, a

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🧠 11. Code Generation Strategy (Important Points)

✔ Prefer registers over memory ✔ Minimize number of instructions ✔ Avoid redundant operations ✔ Reuse computed values ✔ Reduce memory access

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🧠 12. Code Generator Design Diagram

Intermediate Code
        ↓
Instruction Selection
        ↓
Register Allocation
        ↓
Instruction Ordering
        ↓
Target Code Generation

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📌 13. Key Exam Points

✔ Code generation is final compiler phase ✔ Input = Intermediate Code ✔ Output = Machine/Assembly Code ✔ Main tasks:

• Instruction selection
• Register allocation
• Storage management ✔ Goal = efficient and correct code ✔ Uses TAC as common input

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🎯 Final Exam Definition

Code generation is the compiler phase that converts intermediate representation of a program into target machine code. The design of a code generator focuses on instruction selection, register allocation, and efficient use of memory and CPU resources to produce optimized executable code.

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📊 FINAL REVISION TABLE

Concept	Description
Code generation	IR → machine code
Input	Intermediate code (TAC)
Output	Assembly / machine code
Instruction selection	Choose machine instructions
Register allocation	Assign variables to registers
Optimization	Improve performance
Peephole optimization	Local code improvement

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If you want next, I can explain:

• 🔥 Code optimization techniques (global + local)
• 🔥 DAG-based code generation (very important exam question)
• 🔥 Register allocation graph coloring method
• 🔥 OR full 10-mark solved questions

Just tell 👍