GATE CS 2026 Set 2 — Question 45
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Compiler Design → Code Optimization → Liveness Analysis
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Question
Consider the control flow graph given below.
Which one of the following options is the set of live variables at the exit point of each basic block?
Correct answer
(A) B1:{a, b, c, e, f}, B2:{d, e}, B3:{b, c, e, f}, B4:∅
Solution
We perform backward liveness analysis. . .Topology from graph:
- Uses: , Defs:
- Uses: , Defs:
- Successors: (Note: Graph shows B2 goes to B4. Does B2 go to B3? No line. Does B1 go to B3? Yes.)
- Uses: , Defs:
- Successor:
- Uses: , Defs:
- Successors:
1.Assume initially.
2..
3..
4.Update .
5.Update .
6.Update .
7.Update . (Stable)
Final LiveOut Sets:- B1:
- B2:
- B3:
- B4:
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