GATE CS 2023 Set 1 — Question 37
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Compiler Design → Code Optimization → Liveness Analysis
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Question
Consider the control flow graph shown.Which one of the following choices correctly lists the set of live variables at the exit point of each basic block?
Correct answer
(D) B1: \a, i, j\, B2: \a, j\, B3: \a, j\, B4: \a, i, j\
Solution
Live variable analysis is performed backwards. A variable is live at a point if there is a path to a use of that variable that does not redefine it.Let and be the set of live variables at the entry and exit of block , respectively.
From the graph:
From the graph:
- (Note: the loop back to B2 entry means is a successor of and )
- B1:
- B2:
- B3:
- B4:
1.
2.
3.
4.
5.
6.
Testing option (D):- . (Consistent with )
- . (Consistent with )
- .
- . (Consistent with )
- . (Consistent with )
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