Exercise 5.

We recall the syntax-directed definition for the translation of flow of control statements into three-address code, using the jump method with symbolic labels. In each of the above productions, E is a boolean expression to be translated. This boolean expression E is associated with two labels (which are inherited attributes)

• E.true the label to which control flows if E is true,
• E.false the label to which control flows if E is false.

Similarly, S is a flow of control statement associated with 2 or 3 attributes among

• S.begin which is a label attached to the first 3-address code in the translation of S, S.begin is a synthesized attribute,
• S.next which is a label attached to the first 3-address code statement to be executed after the code for S , S.next is an inherited attribute,
• S.code is the translation code for S, as usual it is a synthesized attribute.

Production	Semantic Rule
S  $\longmapsto$  $\bf if$ E $\bf then$ S1	E.true := newlabel
	E.false := S.next
	S1.next := S.next
	S.code := E.code | | generate(E.true ':') | | S1.code
S  $\longmapsto$  $\bf if$ E $\bf then$ S1 $\bf else$ S2	E.true := newlabel
	E.false := newlabel
	S1.next := S.next
	S2.next := S.next
	code1 := E.code | | generate(E.true ':') | | S1.code
	code2 := generate('goto' S.next) | |
	code3 := generate(E.false ':') | | S2.code
	S.code := code1 | | code2 | | code3
S  $\longmapsto$  $\bf while$ E $\bf repeat$ S1	S.begin := newlabel
	E.true := newlabel
	E.false := S.next
	S1.next := S.begin
	code1 := generate(S.begin ':') | | E.code
	code2 := generate(E.true ':') | | S1.code
	code3 := generate('goto' S.begin)
	S.code := code1 | | code2 | | code3

Some languages allow repeat-until loop statements of the form

    repeat {
         s1;
    } 
    until (e);

where s1 is a statement and e is a boolean expression. (Iterations of the loop body are executed until e becomes true. Construct a syntax-directed definition for the translation of these repeat-until loop statements into three-address code.

Answer 5