GATE CSE 2021 Set 2 Q48: Liveness: relation between OUT(S1) and IN(S2)

For a statement $S$ in a program, in the context of liveness analysis, the following sets are defined:

$USE(S)$ : the set of variables used in $S$

$IN(S)$ : the set of variables that are live at the entry of $S$

$OUT(S)$ : the set of variables that are live at the exit of $S$

Consider a basic block that consists of two statements, $S_1$ followed by $S_2$ . Which one of the following statements is correct?

Try it: multiple choice: exactly one correct option

Wrong answer costs 2/3 mark in GATE.

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Answer: A (official GATE 2021 final answer key)

Concept used: OUT of a statement is the union of IN over its successors.

First step: Inside a basic block, $S_1$ has exactly one successor, $S_2$ .

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Source: official GATE 2021 Set 2 CS question paper and final answer key, organised by IIT Bombay. Question paper · Answer key