Question

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?

• A. $OUT\left(S_1\right)=USE\left(S_1\right)\cup IN\left(S_2\right)$
• B. $OUT\left(S_1\right)=IN\left(S_1\right)\cup USE\left(S_1\right)$
• C. $OUT\left(S_1\right)=IN\left(S_2\right)$
• D. $OUT\left(S_1\right)=IN\left(S_2\right)\cup OUT\left(S_2\right)$

Answer 1

The correct option is C

$OUT\left(S_1\right)=IN\left(S_2\right)$

In live variable analysis at any node, the set of variables live at just after the block are evaluated using the formula.

$OUT=\cup IN\left(Successornodes\right)$
So, the correct option is

$OUT\left(S_1\right)=IN\left(S_2\right)$
So, the correct option (c).