1. Compiler Construction Sohail Aslam Lecture 40

2. 2 Boolean Expressions E → E 1 and M E 2 {backpatch(E 1.truelist, M.quad); E.truelist = E 2.truelist; E.falselist = merge(E 1.falselist, E 2.falselist);} So the target for E 1.truelist must be the beginning of code generated for E 2

3. 3 Boolean Expressions E → E 1 and M E 2 {backpatch(E 1.truelist, M.quad); E.truelist = E 2.truelist; E.falselist = merge(E 1.falselist, E 2.falselist);} This target is obtained using the marker nonterminal M.

4. 4 Boolean Expressions E → E 1 and M E 2 {backpatch(E 1.truelist, M.quad); E.truelist = E 2.truelist; E.falselist = merge(E 1.falselist, E 2.falselist);} M.quad records the number of the first statement of E 2.code.

5. 5 Boolean Expressions: OR E → E 1 or M E 2 { backpatch(E 1.falselist, M.quad); E.truelist = merge(E 1.truelist, E 2.truelist); E.falselist = E 2.falselist; } If E 1 is false, need to test E 2

6. 6 Boolean Experssions E → not E 1 { E.truelist = E 1.falselist; E.falselist = E 1.truelist; }

7. 7 Boolean Experssions E → ( E 1 ) { E.truelist = E 1.truelist; E.falselist = E 1.falselist; }

8. 8 Boolean Experssions E → id 1 relop id 2 { E.truelist = makelist(nextquad()); E.falselist = makelist(nextquad()+1); emit(‘if’ id 1 relop id 2 ‘goto _’); emit(‘goto _’ ); }

9. 9 Boolean Experssions E → true { E.truelist = makelist(nextquad()); emit(‘goto _’ ); }

10. 10 Boolean Experssions E → false { E.falselist = makelist(nextquad()); emit(‘goto _’ ); }

11. 11 Boolean Expressions M →  { M.quad = nextquad(); }

12. 12 BackpatchingBackpatching  consider again, the boolean expression a < b or c < d and e < f  We carry out a bottom-up parse

13. 13 BackpatchingBackpatching  In response to reduction of a < b to E, the two quadruples 100: if a < b goto _ 101: goto _ are generated

14. 14 RecallRecall E → id 1 relop id 2 { E.truelist = makelist(nextquad()); E.falselist = makelist(nextquad()+1); emit(‘if’ id 1 relop id 2 ‘goto _’); emit(‘goto _’ ); } View this in a parse tree

15. 15 E.t = {100} E.f = {101} c < da < be < forand

16. 16 BackpatchingBackpatching  The marker non-terminal M in the production E → E 1 or M E 2 records the value of nextquad which at this time is 102.

17. 17 E.t = {100} E.f = {101} c < d a < b e < f M.q = 102 orand 

18. 18 BackpatchingBackpatching  The reduction of c < d to E, the two quadruples 102: if c < d goto _ 103: goto _ are generated

19. 19 E.t = {100} E.f = {101} E.t = {102} E.f = {103} c < d a < b e < f M.q = 102 or and 

20. 20 BackpatchingBackpatching  The marker non-terminal M in the production E → E 1 and M E 2 records the value of nextquad which at this time is 104.

21. 21 E.t = {100} E.f = {101} E.t = {102} E.f = {103} c < d a < b e < f M.q = 104 M.q = 102 or and  

22. 22 BackpatchingBackpatching  Reducing e < f to E, generates 104: if e < f goto _ 105: goto _

23. 23 E.t = {100} E.f = {101} E.t = {102} E.f = {103} E.t = {104} E.f = {105} c < d a < b e < f M.q = 104 M.q = 102 or and  

24. 24 BackpatchingBackpatching  We now reduce by the production E → E 1 and M E 2

25. 25 E.t = {100} E.f = {101} E.t = {104} E.f = {103, 105} E.t = {102} E.f = {103} E.t = {104} E.f = {105} c < d a < b e < f M.q = 104 M.q = 102 orand  

26. 26 Semantic Actions E → E 1 and M E 2 {backpatch(E 1.truelist, M.quad); E.truelist = E 2.truelist; E.falselist = merge(E 1.falselist, E 2.falselist);}

27. 27 BackpatchingBackpatching The semantic action calls backpatch({102},104) where {102} denotes E 1.truelist containing only 102

28. 28 BackpatchingBackpatching The six statements generated so far are thus 100: if a < b goto _ 101: goto _ 102: if c < d goto _ 103: goto _ 104: if e < f goto _ 105: goto _

29. 29 BackpatchingBackpatching The semantic action calls backpatch({102},104) which fills in 104 in statement 102.

30. 30 BackpatchingBackpatching The call fills in 104 in statement 102 100: if a < b goto _ 101: goto _ 102: if c < d goto 103: goto _ 104: if e < f goto _ 105: goto _ 104

31. 31 Semantic Actions E → E 1 and M E 2 {backpatch(E 1.truelist, M.quad); E.truelist = E 2.truelist; E.falselist = merge(E 1.falselist, E 2.falselist);}

32. 32 E.t = {100} E.f = {101} E.t = {104} E.f = {103, 105} E.t = {102} E.f = {103} E.t = {104} E.f = {105} c < d a < b e < f M.q = 104 M.q = 102 or and   104 103105

33. 33 BackpatchingBackpatching  We now reduce by the production E → E 1 or M E 2

34. 34 E.t = {100, 104} E.f = {103, 105} E.t = {100} E.f = {101} E.t = {104} E.f = {103, 105} E.t = {102} E.f = {103} E.t = {104} E.f = {105} c < d a < b e < f M.q = 104 M.q = 102 or and  

35. 35 Semantic Actions E → E 1 or M E 2 { backpatch(E 1.falselist, M.quad); E.truelist = merge(E 1.truelist, E 2.truelist); E.falselist = E 2.falselist; }

36. 36 BackpatchingBackpatching The semantic action calls backpatch({101},102) which fills in 102 in statement 101.

37. 37 BackpatchingBackpatching 100: if a < b goto _ 101: goto 102: if c < d goto 104 103: goto _ 104: if e < f goto _ 105: goto _ 102

38. 38 BackpatchingBackpatching 100: if a < b goto _ 101: goto 102 102: if c < d goto 104 103: goto _ 104: if e < f goto _ 105: goto _ These instructions will have their targets filled later in the compilation

39. 39 Semantic Actions E → E 1 or M E 2 { backpatch(E 1.falselist, M.quad); E.truelist = merge(E 1.truelist, E 2.truelist); E.falselist = E 2.falselist; }

40. 40 E.t = {100, 104} E.f = {103, 105} E.t = {100} E.f = {101} E.t = {104} E.f = {103, 105} E.t = {102} E.f = {103} E.t = {104} E.f = {105} c < d a < b e < f M.q = 104 M.q = 102 or and   {100} {104} {103, 105}

41. 41 Flow-of-Control Statements  We now use backpatching to translate flow-of-control statements in one pass  We will use the same list- handling procedures as before

42. 42 Flow-of-Control Statements  We now use backpatching to translate flow-of-control statements in one pass  We will use the same list- handling procedures as before

43. 43 Flow-of-Control Statements S → if E then S | if E then S else S | while E do S | begin L end | A L → L ; S | S