1 Programming Languages Implementation of Control Structures Cao Hoaøng Truï Khoa Coâng Ngheä...

1

Programming Languages Implementation of Control

Structures

Cao Hoaøng Truï

Khoa Coâng Ngheä Thoâng TinÑaïi 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) -)

PolishA B + C A -

suitable execution-time representation

10

Execution-Time Representation

• Interpretation: tree structures

+ -

A B C A

prefix or postfix

11


• Compilation: machine code sequences

PUSH APUSH BADDPUSH CPUSH ASUBMUL

A A

B

A + B

A + B

C

A + B

C

A

A + B

C - A

(A+B)(C-A)

12


• Compilation: machine code sequences

PUSH APUSH BADDPUSH CPUSH ASUBMUL

A A

B

A + B

A + B

C

A + B

C

A

A + B

C - A

(A+B)(C-A)

A B + C A -

13

Evaluation

• No simple uniform evaluation rule is satisfactory:

+ -

A B C A

14

Evaluation

• Side effects:

A FOO(X) + A

+

A FOO(X)

A

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

GOTO L JMP L

L

21

Sequencing

begin

S1;

S2;

…

Sn;end

S1 codes

S2 codes

Sn codes

22

Selection

if A = 0 then S1

else S2;

S3;

JEQ0 A L1

S2 codes

JMP L2

S1 codesL1

S3 codesL2

• if:

23

Selection

var E: 0..2;case E of

1: S1;

2: S2;

else: S0

end;

S3;

JMP +v

E v

JMP L0

JMP L1

JMP L2

S1 codesL1

• case:

+1

JMP L3

+2

S2 codes

JMP L3

S0 codes

S3 codes

L2

L0

JUMP table

L3

24

Iteration

for I := E1 to E2 do S;

I := E1;

L0: if I > E2 then GOTO L1;

S; I := I + 1;

GOTO L0;

L1: …

• for:

25

Iteration

while C do S;

L0: if not(C) then GOTO L1;

S;

GOTO L0;

L1: …

• while:

26

Iteration

repeat S until C;

L0: S;

if not(C) then GOTO L0;

• repeat:

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

call A

I0

I1call B

I2

I3

end

return

I4

return

MAIN A B

31

Simple Call-Return

call A

I0

I1

end

MAIN

local data MAIN

I0 CIP (current instruction pointer)

32

Simple Call-Return

call A

I0

I1call B

I2

I3

end

return

MAIN A

local data MAIN

local data A

I1

I2 CIP

33

Simple Call-Return

call A

I0

I1call B

I2

I3

end

return

I4

return

MAIN A B

local data MAIN

local data Alocal data B

I1I3

I4 CIP

34

Recursive Calls

call A

I0

I1

call B

I2

I3end

return

I4

return

MAIN A B

call AI5

local data MAIN

--

--R0

I0 CIP R0 CEP (current environment pointer)

35

Recursive Calls

call A

I0

I1

call B

I2

I3end

return

I4

return

MAIN A B

call AI5

local data MAIN

--

--

local data A

R0

I1R0 R1

I2 CIP R1 CEP

36

Recursive Calls

call A

I0

I1

call B

I2

I3end

return

I4

return

MAIN A B

call AI5

local data MAIN

--

--

local data A

R0

I1

local data B

R1

I3R0 R1 R2

I4 CIP R2 CEP

37

Recursive Calls

call A

I0

I1

call B

I2

I3end

return

I4

return

MAIN A B

call AI5

local data MAIN

--

--

local data A

R0

I1

local data B

R1

I3

local data A

R2

I5R0 R1 R2 R3

I2 CIP R3 CEP

38

Recursive Calls

call A

I0

I1

call B

I2

I3end

return

I4

return

MAIN A B

call AI5

local data MAIN

--

--

local data A

R0

I1

local data B

R1

I3

local data A

R2

I5R0 R1 R2 R3

I2 CIP R3 CEPDynamic chain

39

Central Stack

local data

--

--R0I0CIP

R0CEPcall A

I0

I1end

MAIN

MAIN

40

Central Stack

local data

--

--R0I2CIP

R1CEP

call B

I2

I3return

A

MAIN

local data

R0

I1R1

A

call A

I0

I1end

MAIN

41

Central Stack

local data

--

--R0I4CIP

R2CEP

call A

I4

I5return

B

MAIN

local data

R0

I1R1

A

local data

R1

I3R2

B

call B

I2

I3return

A

42

Central Stack

local data

--

--R0I2CIP

R3CEPcall A

I4

I5return

B

MAIN

local data

R0

I1R1

A

local data

R1

I3R2

Bcall B

I2

I3return

A

local data

R2

I5R2

A

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:

program MAIN;function FAC(N: integer): integer;begin if N <= 1

then FAC := 1else 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


program MAIN;

var X: integer;

…

X := 0;

objectAssociation

51


• Local

• Non-Local

• Global

• Predefined

52

Local Environments

program MAIN;var X: integer;…procedure SUB1;var X: real;…X := 1;

object2

object1

53

Non-Local Environments

program MAIN;var X: integer;…

procedure SUB1;var X: real;…procedure SUB2;…

X := 2;object2

object1

54

Global Environments



X := 3;object2

object1

55

Predefined Environments

program MAIN;

var X: integer;

…

X := MAXINT - 1;

…

write(X)

65535

codes for“write”

56


• Visibility of an association

• Referencing operations

• Local, non-local, global references

57

Visibility


object2

object1

Hidden

Visible

58

Referencing Operations

• Name Environment Data Object

59

Referencing Operations


object2

object1

X (X object2) object2

60


• Visibility of an association

• Referencing operations


61

Local References


object2

object1


62

Non-Local References



X := 2;object2

object1


63

Global References



X := 2;object2

object1


64

Basic Concepts

• Names


• Scope

• Block structure

65

Scope

• The program part (text or execution) within which the binding is effective.

66

Dynamic Scope

• The subprogram activations within which the association is effective.

67

Dynamic Scopeprogram MAIN;var X: integer;…

procedure SUB1;var X: real;…

X := 1;

procedure SUB2;…

X := 2;

object1

MAIN

SUB1

SUB2

68

Static Scope

• The program text within which the declaration is effective.

69

program MAIN;var X: integer; X integer…


X := 1;

procedure SUB2;…

X := 2;

Static Scope

70

program MAIN;var X: integer; X integer…


X := 1;

procedure SUB2;…

X := 2;

Static Scope

Static scopes define dynamic scopes

71

Static Referencing Environments

• Local, non-local, global environments


72

Basic Concepts

• Names


• Scope

• Block structure

73

Block Structureprogram MAIN;…procedure SUB1;…

procedure SUB3;…

procedure SUB4;…

procedure SUB2;…

MAIN

SUB1

SUB3

SUB2

SUB4

74

Static Scope Rules

1. The declarations at the head of each block defines the local referencing environment for that block

MAIN

SUB1

SUB3

SUB2

SUB4

X: real

75

Static Scope Rules

2. If no local declarations exists, then refer to the nearest enclosing block having the declaration in need

MAIN

SUB1

SUB3

SUB2

SUB4

X: real

X: integer

X := 1

76

Static Scope Rules

3. Any local declaration of a block is hidden from its outer blocks

MAIN

SUB1

SUB3

SUB2

SUB4

X: real

77

Static Scope Rules

4. The block name is part of the local referencing environment of the containing block

MAIN

SUB1

SUB3

SUB2

SUB4

78

Data Control

• Basic concepts





79

Local Data and Environments

procedure SUB(X: integer);

var Y: real;Z: array [1..3] of real;procedure SUB1;…

begin…

end;begin

…end;

object2

SUB

object2

object2

X

Y

Z

SUB1

SUB1 code segment

80

Data Control

• Basic concepts





81

Shared Data: Dynamic Scope

program MAIN;procedure SUB1;var X, Y: real;…procedure SUB2;var B, A, U, X: integer; …procedure SUB3;var Z, Y, A, W, V: char; …

MAIN

SUB1

SUB2

SUB3

82



MAIN

XY

SUB1

Return point

83



MAIN

XY

BAUX

SUB1

SUB2

Return point

Return point

Dynamic chain

84



MAIN

XY

BAU

ZYAWV

X

SUB1

SUB2

SUB3

Return point

Return point

Return point

Dynamic chain

85



MAIN

XY

BAU

ZYAWV

X

SUB1

SUB2

SUB3

Return point

Return point

Return point

Dynamic chain

86



MAIN

XY

SUB1

Return point

ABUVWXYZ

Central table

00000110

87



MAIN

XY

BAUX

SUB1

SUB2

Return point

Return point

ABUVWXYZ

Central table

11100110

88



MAIN

XY

BAU

ZYAWV

X

SUB1

SUB2

SUB3

Return point

Return point

Return point

Central table

11111111

ABUVWXYZ

89

Shared Data: Dynamic ScopeMAIN

XY

SUB1

Return pointCentral table

00000110

Hidden stack

ABUVWXYZ

90


XY

BAUX

SUB1

SUB2

Return point

Return point

Central table

11100000

Hidden stackX1111000

ABUVWXYZ

91


XY

BAUX

SUB1

SUB2

Return point

Return point

Central table

11100000

Hidden stackX1111000

ABUVWXYZ

ZYAWV

SUB3

Return point

Y

1A

92

Data Control

• Basic concepts





93

Shared Data: Block Structureprogram MAIN;var X, Y, Z: char;…

procedure SUB2;var X, Y: integer;…


procedure SUB4;…

procedure SUB1;var Y, Z: integer;…

MAIN X, Y, Z: char

SUB2 X, Y: integer

SUB3 X: real

SUB1 Y, Z: integer

SUB4

94

Block Structure: Compile Timeprogram MAIN;var X, Y, Z: char;…



procedure SUB4;…


Symbol tableX1111000

Y

1Z

Scope stack

1

11000

MAIN

95




procedure SUB4;…


Symbol tableX1111

00

Y

1Z

Scope stack

1

11000

SUB2

X1Y

96




procedure SUB4;…


Symbol tableX1111

00

Y

1Z

Scope stack

1

11000

SUB3

X1Y

1X

97




procedure SUB4;…


Symbol tableX1111

00

Y

1Z

Scope stack

1

11000

SUB4

X1Y

1X

98




procedure SUB4;…


Symbol tableX1111

00

Y

1Z

Scope stack

1

11000

SUB4

X1Y

1X

99




procedure SUB4;…


Symbol tableX1111

00

Y

1Z

Scope stack

1

11000

SUB3

X1Y

1X

100




procedure SUB4;…


Symbol tableX111

00

Y

1Z

Scope stack

1

11000

SUB2

X

1X

101




procedure SUB4;…


Symbol tableX111

00

Y

1Z

Scope stack

1

11000

SUB1

X

1X

YZ

102




procedure SUB4;…


Symbol tableX11

00

Y

1Z

Scope stack

1

11000

SUB1

1X

103

Block Structure: Run Timeprogram MAIN;var X, Y, Z: char;…



procedure SUB4;…


MAIN

SUB1

SUB2

SUB3

104




procedure SUB4;…


YZ

MAIN X

--

105




procedure SUB4;…


YZ

SUB1 SCP

Staticchains

YZ

MAIN X

--

RP

106




procedure SUB4;…


YZ

XY

SUB1

SUB2

SCPStaticchains

YZ

MAIN X

--

RP

SCPRP

107




procedure SUB4;…


YZ

XY

X

SUB1

SUB2

SUB3

SCPStaticchains

YZ

MAIN X

--

RP

SCPRP

SCPRP

108

Block Structure: Run Time

A

B

MAIN

A

B

MAIN

C

A

B

C

A

B

MAIN

C

A

B

C

A

B

109

Block Structure: Run Time

YZ

XY

X

SUB1

SUB2

SUB3

SCP Activestaticchain

YZ

MAIN X

--

RP

SCPRP

SCPRP

Display

1

11

0

012

loc Y = Display[1] + offset

110

Data Control

• Basic concepts





111

Parameter Transmission• Parameters:

formal parameters actual parameters

• Transmission: by reference by value

112

Parameter Transmission

procedure SUB2(K: integer; var L: integer);begin

K := K + 10;L := L + 10;write(K, L)

end;procedure SUB1;begin

I := 1;J := 2;SUB2(I, J);write(I, J)

end;

IJ

KL

SUB1

SUB2

SCPRP

SCPRP

12

1

113



K := K + 10;L := L + 10;write(K, L)


I := 1;J := 2;SUB2(I, J);write(I, J)

end;

IJ

KL

SUB1

SUB2

SCPRP

SCPRP

12

11

114



K := K + 10;L := L + 10;write(K, L)


I := 1;J := 2;SUB2(I, J);write(I, J)

end;

IJ

KL

SUB1

SUB2

SCPRP

SCPRP

112

11

115


procedure SUB2(K:integer; var L:integer);

begin K := K + 10;L := L + 10;SUB3(K, L)write(K, L)


I := 1;J := 2;SUB2(I, J);write(I, J)

end;

procedure SUB3(var M, N: integer);

begin M := M + 10;N := N + 10;write(M, N)

end;

Actual parameters are formal parameters of the calling program

116


type VECT = array [1...3] of integer;

procedurre SUB2 (C:VECT; var D:VECT);

var I : integer;begin

C [2] : = C [2] + 10;D [2] = D [2] + 10;for I : = 1 to 3 do write (C [I]);for I : = 1 to 3 do write (D [I])

end;

procedurre SUB1;var A, B : VECT;

J : integer;begin

A [1] : = 7; A [2] = 8; A [3] : = 9;B [1] : = 7; B [2] = 8; B [3] : = 9;SUB2 (A, B);for J : = 1 to 3 do write (A [J]);for J : = 1 to 3 do write (B [J]);

end; Actual parameters are structured data objects

117


type VECT = array [1...3] of integerprocedurre SUB2 (I: integer; var J:

integer);begin

I : = I + 10;J : = J + 10;write (I, J);

end;

procedurre SUB1;var A : VECT;

K : integer;begin

A [1] := 7; A [2] := 8; A [3] := 9;SUB2 (A[1], A[2]);for K := 1 to 3 do write (A[K])

end;

Actual parameters are components of structured data objects

118


type VECT = array [1...3] of integerprocedurre SUB2 (var I, J: integer);begin

I : = I + 1;J : = J + 1;write (I, J);

end;


K : integer;begin

A [1] := 7; A [2] := 8; A [3] := 9;K := 2;SUB2 (K, A[K]);for K := 1 to 3 do write (A[K])

end;

Actual parameters are array components with computed

subscripts

119


type VECT = array [1...3] of integer; VECTPTR = ^ VECT;procedurre SUB2 (R:VECTPTR; var

S:VECTPTR);begin

R^[1] := R^[1] + 10;S^[1] := S^[1] + 10;if . . . then R := S else S := R

end;

procedurre SUB1;var A, B: VECT; P, Q: VECTPTR;begin

A [1] = 7; A [2] := 8; A [3] := 9;B [1] := 7; B [2] := 8; B [3] := 9;P := @A; Q := @B;SUB2 (P, Q);

end; Actual parameters are pointers

120


program MAIN;var X: real;

procedurre SUB2 (X, Y: real; function F(U:real): real);

var Z: real;begin

Z := abs (Y - X);Z := (F(X) + F(Y)) * Z/2;write (Z)

end; begin

X := 3;SUB1

end.

procedurre SUB1;var Y: real;

function FUNC(V:real): real;

beginFUNC := X*V + Y

end;begin

Y := 1;SUB2 (0, 1, FUNC).

end;

Actual parameters are subprograms

121


type VECT = array [1...3] of integerprocedurre SUB2 (name I, J: integer);begin

I : = I + 1;J : = J + 1;write (I, J);

end;


K : integer;begin

A [1] := 7; A [2] := 8; A [3] := 9;K := 2;SUB2 (K, A[K]);for K := 1 to 3 do write (A[K])

end;

Transmissiom by names

122

Formal Parameters and Aliases

• A data object may have more than one name, called aliases.

• Side effects:

I := 1; J := 1;I := J + 10;

J := J*10;

123

Formal Parameters and Aliases

• An actual parameter is a non-local variable and is trasmitted by reference.

• Actual parameters are of the same data object and transmitted by reference.

124

Exercises• Given the following program:

program MAIN;var F: real;

procedure PROC(N: integer; var F: real);var F1, F2: real;begin if (N = 0) or (N = 1) then F := 1 else begin PROC(N-1, F1); PROC(N-2, F2);F := F1 + F2endend

begin PROC(2, F);write(F)

end.

125

Exercises

• Illustrate the code segment and activation records of MAIN and PROC.

• Illustrate the central stack during execution of this program.

1 Programming Languages Implementation of Control Structures Cao Hoaøng Truï Khoa Coâng Ngheä...

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