Dead code elimination in Compiler Design - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary goal of dead code elimination in compilers?

Choose the best description of what dead code elimination aims to achieve during program compilation.

A Convert high-level code into machine code

B Remove code that does not affect the program's output or behavior

C Optimize code to run faster by parallelizing instructions

D Add debugging information to the compiled program

💡 Hint

Think about code that never runs or whose results are never used.

Dead code elimination removes instructions that do not impact the final program result, improving efficiency without changing behavior.

📋 Factual

intermediate

Which of the following is an example of dead code?

Identify the code snippet that represents dead code.

Compiler Design

int x = 5;
int y = 10;
x = 7;
return y;

A The assignment <code>int y = 10;</code> because <code>y</code> is returned

B The assignment <code>x = 7;</code> because it changes <code>x</code>

C The assignment <code>int x = 5;</code> because <code>x</code> is reassigned before use

D The <code>return y;</code> statement because it ends the function

💡 Hint

Dead code is code that never affects the program's output.

The initial assignment int x = 5; is dead because x is immediately overwritten by x = 7; before any use.

🔍 Analysis

advanced

What is the effect of dead code elimination on program size and performance?

Analyze how removing dead code impacts the compiled program.

A It reduces program size and can improve runtime performance by removing unnecessary instructions

B It increases program size by adding extra checks for dead code

C It has no effect on program size but slows down execution

D It only affects debugging symbols without changing size or speed

💡 Hint

Think about what happens when unnecessary code is removed.

Removing dead code reduces the number of instructions, which lowers program size and can speed up execution by avoiding useless operations.

❓ Comparison

advanced

How does dead code elimination differ from unreachable code elimination?

Choose the statement that best distinguishes dead code elimination from unreachable code elimination.

A Dead code elimination removes code whose results are never used; unreachable code elimination removes code that can never be executed

B Dead code elimination removes unreachable code; unreachable code elimination removes code with side effects

C Dead code elimination only removes comments; unreachable code elimination removes loops

D Dead code elimination and unreachable code elimination are the same process

💡 Hint

Consider the difference between code that runs but has no effect and code that never runs at all.

Dead code elimination targets code that runs but whose results are unused, while unreachable code elimination removes code that cannot be reached during execution.

❓ Reasoning

expert

Given the following code snippet, what is the value of variable z after dead code elimination?

Consider this code:

int x = 3;
int y = 4;
int z = x + y;
x = 5;
// no further use of x or y

After dead code elimination, what is the value of z?

A 5

B 8

C 3

D 7

💡 Hint

Focus on which assignments affect z and which are dead.

The value of z is 3 + 4 = 7. The assignment x = 5; is dead because x is not used after that. So z remains 7.