Exercise 1.

Consider the following three-address code fragment.

L0	a := 15
	b := 12
L1	if b = 0 goto L6
L2	q := 0
	r := a
L3	if r < b goto L5
L4	r := r - b
	q := q + 1
	goto L3
L5	a := b
	b := r
	goto L1
L6	g := a

1. Give the flow graph of this program.
2. Identify the back edges and the loops.
3. Is there any propagation of a copy statement that can be performed. Justify your answer briefly.
4. Explain how the techniques developed in the course can help discover that two lines of the above program can be eliminated.

Answer 1  

Figure 1: The control flow graph.