GATE CSE 2026 | Set 2 | Compiler Design| Memory Based | Question 34

Answer 1

In  this question we have to check   a path from exit  point where read of that variable happen without any write before it .

Block B1 Live variable = { a , b , c, e,f}

Block B2 Live variable = {d,e}

Block B3  Live variable = {}

Bolck B4 Live variable = {b,c,e,f}

So option A

Answer 2

Source: Ullman

Available variables = {a,b,c,d,e,f,g}

At the exit of B1

• a is first used before defined => LIVE {B1 -> B2 OR B1 -> B4}

• b is first used before defined => LIVE {B1 -> B4 -> B1}

• c is first used before defined => LIVE {B1 -> B4 -> B1}

• d is first defined before used => DEAD {B1 -> B2}

• e is first used before defined => LIVE {B1 -> B2}

• f is first used before defined => LIVE {B1 -> B4}

• g is first defined before used => DEAD {B1 -> B2 -> B3}

Live at exit of B1 = {a,b,c,e,f}

At the exit of B2

• d is first used before defined => LIVE {B2 -> B3}

• e is first used before defined => LIVE {B2 -> B3}

• g is first defined before used => DEAD {B2 -> B3}

Live at exit of B2 = {d,e}

At the exit of B3

• NO variable is LIVE

Live at exit of B3 = { }

At the exit of B4

• a is first defined before used => DEAD {B4 -> B1}

• b is first used before defined => LIVE {B4 -> B1}

• c is first used before defined => LIVE {B4 -> B1}

• d is first defined before use => DEAD {B4 -> B1 -> B2}

• e is first used before defined => LIVE {B4 -> B1 -> B2}

• f is first used before defined => LIVE {B4 -> B1 -> B4}

• g is first defined before used => DEAD {B4 -> B1 -> B2 -> B3}

Live at exit of B4 = {b,c,e,f}

Answer: Option A

For better understanding of Method 1: Liveness Analysis - GATE PYQs & Practice Questions | CFG with Loop | Live Variable | Compiler Design