Code Optimization

A variable $x$ is said to be live at a statement $S_i$ in a program if the following three conditions hold simultaneously: i. There exists a statement $S_j$ that uses $x$ ii. There is a path from $S_i$ to $S_j$ in the flow graph corresponding to the program iii. The path has no intervening assignment to $x$ including at $S_i$ and $S_j$ 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

Consider the intermediate code given below.

(1)  i = 1
(2)  j = 1
(3)  t1 = 5 * i
(4)  t2 = t1 + j
(5)  t3 = 4 * t2
(6)  t4 = t3
(7)  a[t4] = -1
(8)  j = j + 1
(9)  if j<=5 goto (3)
(10) i=i+1
(11) if i<5 goto (2)

The number of nodes and edges in the control-flow-graph constructed for the above code, respectively, are