Dead code elimination in Compiler Design - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is dead code in programming?

Dead code refers to parts of a program that are never executed or whose results are never used. These code sections do not affect the program's output.

beginner

What is the purpose of dead code elimination in compilers?

Dead code elimination removes unnecessary code that does not affect the program's behavior, making the program smaller and faster.

intermediate

How does dead code elimination improve program performance?

By removing unused code, the program uses less memory and runs faster because the processor executes fewer instructions.

beginner

Give an example of dead code.

If a variable is assigned a value but never used later, the assignment is dead code. For example:
int x = 5; // x is never used

intermediate

Can dead code elimination affect program correctness?

No, dead code elimination only removes code that does not change the program's output or behavior, so it keeps the program correct.

What does dead code elimination do?

A Changes program logic to improve results

B Adds extra code for debugging

C Removes code that never runs or affects output

D Encrypts the program code

Dead code elimination removes code that does not run or affect the program's output.

Which of these is an example of dead code?

A A function called multiple times

B A print statement showing output

C A loop that runs 10 times

D A variable assigned but never used

Assigning a variable that is never used is dead code.

Why do compilers perform dead code elimination?

A To make programs smaller and faster

B To add comments automatically

C To change program results

D To increase program size

Removing dead code reduces program size and improves speed.

Can dead code elimination remove code that changes program output?

A No, it adds extra code instead

B No, it only removes code that does not affect output

C Yes, to fix bugs

D Yes, it changes program logic

Dead code elimination preserves program correctness by only removing unused code.

Which of the following is NOT a benefit of dead code elimination?

A Adds new features

B Reduces program size

C Improves program speed

D Simplifies code

Dead code elimination does not add features; it removes unnecessary code.

Explain what dead code elimination is and why it is important in compilers.

Think about code that never runs or affects the program.

Describe an example of dead code and how a compiler might remove it.

Consider code that assigns a value but never uses it.

Practice

1. What is the main purpose of dead code elimination in compiler design?

easy

A. To remove code that never affects the program's output

B. To add extra debugging information

C. To increase the size of the compiled program

D. To change the program's logic for optimization

Solution

1. Step 1: Understand dead code

   Dead code is code that does not affect the program's output or behavior.

2. Step 2: Purpose of elimination

   Removing such code makes the program faster and smaller without changing its meaning.

3. Final Answer:

   To remove code that never affects the program's output -> Option A

4. Quick Check:

   Dead code elimination = Remove unused code [OK]

Hint: Dead code does not run or affect results, so remove it [OK]

Common Mistakes:

• Thinking dead code changes program logic
• Confusing dead code elimination with debugging
• Believing it increases program size

2. Which of the following code snippets is an example of dead code that can be eliminated?

easy

A. int z = 2; z = z + 1; return z;

B. int x = 5; x = 10; return x;

C. int y = 3; if (false) { y = 7; } return y;

D. int a = 4; a = a * 2; return a;

Solution

1. Step 1: Identify dead code

   The code inside if (false) never runs, so y = 7; is dead code.

2. Step 2: Check other options

   Other assignments affect the returned value, so they are not dead code.

3. Final Answer:

   int y = 3; if (false) { y = 7; } return y; -> Option C

4. Quick Check:

   Code inside false condition = Dead code [OK]

Hint: Code inside 'if(false)' or unreachable blocks is dead code [OK]

Common Mistakes:

• Thinking all variable reassignments are dead code
• Ignoring conditions that never execute
• Confusing dead code with useful code

3. Consider the following code snippet:

int a = 10;
int b = 20;
if (a > b) {
  b = a + 5;
}
return b;

After dead code elimination, what will be the returned value?

medium

A. 10

B. 20

C. 25

D. Compiler error

Solution

1. Step 1: Analyze the condition

   The condition a > b is 10 > 20, which is false, so the if block never runs.

2. Step 2: Identify dead code and final value

   The assignment inside the if block is dead code and removed. The value of b remains 20 and is returned.

3. Final Answer:

   20 -> Option B

4. Quick Check:

   False condition skips code, return original b = 20 [OK]

Hint: If condition is false, code inside is dead and removed [OK]

Common Mistakes:

• Assuming if block runs and changes b
• Returning a instead of b
• Confusing dead code with runtime error

4. Identify the dead code in the following snippet and suggest the correct fix:

int x = 5;
return x;
x = 10;

medium

A. No dead code; code is correct as is

B. The return statement is dead code and should be removed

C. The variable declaration int x = 5; is dead code

D. The assignment x = 10; is dead code and should be removed

Solution

1. Step 1: Understand code flow

   After return x;, the function exits, so x = 10; never runs.

2. Step 2: Identify dead code and fix

   The assignment x = 10; is dead code and should be removed to clean the code.

3. Final Answer:

   The assignment x = 10; is dead code and should be removed -> Option D

4. Quick Check:

   Code after return is dead and removable [OK]

Hint: Code after return is dead and can be removed [OK]

Common Mistakes:

• Thinking code after return runs
• Removing return statement mistakenly
• Confusing variable declaration as dead code

5. You have this code:

int sum = 0;
for (int i = 0; i < 5; i++) {
  if (false) {
    sum += i;
  }
}
return sum;

How does dead code elimination optimize this loop?

hard

A. Removes the entire loop since it does nothing

B. Removes only the if block but keeps the loop

C. Keeps all code as is because loop runs 5 times

D. Changes the loop to run 0 times

Solution

1. Step 1: Analyze the loop body

   The if condition is always false, so sum += i; never runs, making the loop body dead code.

2. Step 2: Optimize the loop

   Since the loop does not change sum, the entire loop can be removed, returning 0 directly.

3. Final Answer:

   Removes the entire loop since it does nothing -> Option A

4. Quick Check:

   Loop with dead body = Remove whole loop [OK]

Hint: If loop body never runs, remove whole loop [OK]

Common Mistakes:

• Removing only if block but keeping loop
• Assuming loop changes sum despite false condition
• Changing loop count incorrectly