Programming Languages Implementation of Control Structures

1. Programming LanguagesImplementation of Control Structures Cao Hoaøng Truï Khoa Coâng Ngheä Thoâng Tin Ñaïi Hoïc Baùch Khoa TP. HCM

2. Contents • Sequence control • Data control

3. Sequence Control • Expressions • Statements • Subprograms

4. Expressions • Control mechanism • Syntax • Execution-time representation • Evaluation

5. Control Mechanism • Functional composition: (A + B)*(C - A) * (+ (A, B), - (C, A))

6. Syntax • Infix: A * B + C • binary operations only • computation order ambiguity

7. Syntax • Prefix: • ordinary * (+ (A, B), - (C, A)) • Cambridge Polish (* (+ A B)(- C A)) • Polish * + A B - C A

8. Syntax • Prefix: • different numbers of operands • ordinary/ Cambridge Polish: cumbersome with parentheses • Polish: number of operands known in advance

9. Syntax • Postfix: • ordinary ((A, B) +,(C, A) -) * • Polish A B + C A - * • suitable execution-time representation

10. Execution-Time Representation • Interpretation: • tree structures * + - A B C A • prefix or postfix

11. Execution-Time Representation • Compilation: machine code sequences PUSHA PUSHB ADD PUSHC PUSHA SUB MUL A A A + B B A + B A + B A + B (A+B)*(C-A) C C C - A A

12. Execution-Time Representation • Compilation: machine code sequences PUSHA PUSHB ADD PUSHC PUSHA SUB MUL A A A + B B A B + C A - * A + B A + B A + B (A+B)*(C-A) C C C - A A

13. Evaluation • No simple uniform evaluation rule is satisfactory: * + - A B C A

14. Evaluation • Side effects: A*FOO(X) + A + * A A FOO(X)

15. Evaluation • Side effects: A*B*C = 1020 * 10-20 * 10-20 (A*B)*C = 1* 10-20 = 10-20 A*(B*C) = 1020 * 0 = 0

16. Evaluation • Short-circuit Boolean expressions: if (A = 0)or(B/A > C)then …

17. Evaluation • Short-circuit Boolean expressions: if (A = 0)or else(B/A > C)then …

18. Sequence Control • Expressions • Statements • Subprograms

19. Statements • GOTO • Sequencing • Selection • Iteration

20. GOTO GOTOL JMP L L

21. Sequencing begin S1; S2; … Sn; end S1 codes S2 codes Sn codes

22. Selection • if: if A = 0 then S1 else S2; S3; JEQ0 A L1 S2 codes JMP L2 L1 S1 codes L2 S3 codes

23. Selection E  v • case: var E: 0..2; case E of 1: S1; 2: S2; else:S0 end; S3; JMPa+v a JMP L0 a+1 JUMP table JMP L1 a+2 JMP L2 L1 S1 codes JMP L3 L2 S2 codes JMP L3 L0 S0 codes L3 S3 codes

24. Iteration • for: for I := E1 to E2 doS; I := E1; L0: if I > E2 then GOTOL1; S; I := I + 1; GOTO L0; L1: …

25. Iteration • while: while C do S; L0: if not(C) then GOTOL1; S; GOTO L0; L1: …

26. Iteration • repeat: repeat S until C; L0: S; if not(C) then GOTOL0;

27. Sequence Control • Expressions • Statements • Subprograms

28. Subprograms • Simple call-return • Recursive calls

29. Simple Call-Return • No recursive calls • Explicit calls • Complete execution • Immediate control transfer • Single execution sequence

30. Simple Call-Return MAIN A B I0 I2 I4 call A call B I1 I3 return return end

31. Simple Call-Return MAIN I0 call A I1 end local data MAIN CIP(current instruction pointer) I0

32. Simple Call-Return MAIN A I0 I2 call A call B I1 I3 return end local data A local data MAIN I1 I2 CIP

33. Simple Call-Return MAIN A B I0 I2 I4 call A call B I1 I3 return return end local data B local data A local data MAIN I3 I1 I4 CIP

34. Recursive Calls MAIN A B I0 I2 I4 call A call B call A I1 I3 I5 end return return R0 -- -- local data MAIN CEP(current environment pointer) I0 CIP R0

35. Recursive Calls MAIN A B I0 I2 I4 call A call B call A I1 I3 I5 end return return R0 R1 -- I1 -- R0 local data MAIN local data A I2 CIP R1 CEP

36. Recursive Calls MAIN A B I0 I2 I4 call A call B call A I1 I3 I5 end return return R0 R1 R2 -- I1 I3 -- R0 R1 local data MAIN local data A local data B I4 CIP R2 CEP

37. Recursive Calls MAIN A B I0 I2 I4 call A call B call A I1 I3 I5 end return return R0 R1 R2 R3 -- I1 I3 I5 -- R0 R1 R2 local data MAIN local data A local data B local data A I2 CIP R3 CEP

38. Recursive Calls MAIN A B I0 I2 I4 call A call B call A I1 I3 I5 end return return R0 R1 R2 R3 -- I1 I3 I5 -- R0 R1 R2 local data MAIN local data A local data B local data A Dynamic chain I2 CIP R3 CEP

39. Central Stack MAIN I0 R0 CIP I0 -- MAIN -- call A CEP R0 local data I1 end

40. Central Stack MAIN I0 R0 CIP I2 -- -- MAIN call A CEP R1 local data I1 R1 I1 end R0 A A local data I2 call B I3 return

41. Central Stack A I2 R0 CIP I4 -- -- MAIN call B CEP R2 local data I3 R1 I1 return R0 A B local data R2 I4 I3 R1 B call A local data I5 return

42. Central Stack B R0 -- I4 CIP I2 -- MAIN call A local data CEP R3 I5 R1 I1 return R0 A local data A R2 I3 I2 R1 B call B local data R2 I3 I5 return R2 A local data

43. Exercises • Illustrate the storage representation of A:array[0..1, 1..2, 1..3]of integer using the column-major order. Give the accessing formula for computing the location of A[I, J, K], supposing that the size of an integer is 2 bytes.

44. Exercises • Given the following program: programMAIN; functionFAC(N:integer): integer; beginifN <= 1 thenFAC := 1 else FAC := N * FAC(N - 1) end; begin write(FAC(3)) end. Illustrate the code segment and activation records of MAIN and FAC in the central stack during execution of the program.

45. Contents • Sequence control • Data control

46. Data Control • Basic concepts • Local data and environments • Shared data: dynamic scope • Shared data: block structure • Shared data: parameter transmission

47. Basic Concepts • Names • Referencing environments • Scope • Block structure

48. Names • Variable names • Formal parameter names • Subprogram names • Names for defined types • Names for defined constants

49. Referencing Environments • Association: Name-------->Data Object • Referencing environment = set of associations

50. Referencing Environments programMAIN; varX:integer; … X := 0; object Association