zeal

Question

Consider the following control flow graph

The live-variable analysis determines for each point $p$ in a program which variables are live at that point. A live variable v at point p is one where there exists a path from point p to another point q where v is used without v being redefined anywhere along that path.

The sets for the livevariable dataflow problem are:
use[b] = variables used in block b before any definition
def[b] = variables defined in block $b$ and not later killed in $b$
in $[\mathrm{b}]=$ variables live on entry to block $b$
out [b] = variables live on exit from block b
The dataflow equations for live variables are
in [b] = use[b] $\cup$ (out[b] $-\operatorname{def}[b])$
$\operatorname{out}[b]=U_{s \in \operatorname{Succ}[b]} \operatorname{in}[s]$

What will be the in and out set for basic block B2?

• A. in- $\{x, y\}$, out- $\{x, y\}$
  
• B. in- $\{x, y\}$, out- $\{y, z\}$
  
• C. in- $\{\mathrm{x}, \mathrm{y}\}$, out- $\{\mathrm{x}, \mathrm{y}, \mathrm{z}\}$
  
• D. in- $\{x, y, a, b\}$, out- $\{y, z\}$

Comments

$in(B_2)=(x,y),out(B_2)=(x,y)$??

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answer?

 

Answer 1

Ans is B