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GATE | GATE-CS-2015 (Set 1) | Question 65
  • Last Updated : 20 Feb, 2015

A variable x is said to be live at a statement Si in a program if the following three conditions hold simultaneously:

1. There exists a statement Sj that uses x
2. There is a path from Si to Sj in the flow
   graph corresponding to the program
3. The path has no intervening assignment to x 
   including at Si and Sj


The variables which are live both at the statement in basic block 2 and at the statement in basic block 3 of the above control flow graph are
(A) p, s, u
(B) r, s, u
(C) r, u
(D) q, v

Answer: (C)

Explanation: Live variable analysis is useful in compilers to find variables in each program that may be needed in future.

As per the definition given in question, a variable is live if it holds a value that may be needed in the future. In other words,  it is used in future before any new assignment.

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