Compiler Construction

Compiler Compiler ConstructionConstruction


Sohail Aslam

Lecture 39

2

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean ExperssionsIn programming languages, boolean expressions have two primary purposes:• compute logical values

x = a < b && d > e• conditional expressions in flow-

of-control statements

3

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean ExperssionsIn programming languages, boolean expressions have two primary purposes:• compute logical values

x = a < b && d > e• conditional expressions in flow-

of-control statements

4

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean ExperssionsConsider the grammar

E → E or E | E and E | not E | ( E ) | id relop id | true | false

5

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean Experssions We will implement the

boolean expression by flow of control method

I.e., representing the value of a boolean expression by a position reached in the program

6

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean Experssions We will implement the

boolean expression by flow of control method

I.e., representing the value of a boolean expression by a position reached in the program

7

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean Experssions

E → id1 relop id2

E.code = gen(‘if’ id1 relop id2 ‘goto’ E.true)

|| gen(‘goto’ E.false)

8

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean ExperssionsE → true

E.code = gen(‘goto’ E.true)

E → falseE.code = gen(‘goto’ E.false)

9

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean ExperssionsE → E1 or E2

E1.true = E.true E1.false = newlabel()E2.true = E.true E2.false = E.falseE.code = E1.code ||

gen(E1.false ‘:’) || E2.code

10

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean ExperssionsE → E1 and E2

E1.true = newlabel() E1.false = E.false E2.true = E.true E2.false = E.falseE.code = E1.code ||

gen(E1.true ‘:’) || E2.code

11


E → not E1

E1.true = E.false E1.false = E.true E.code = E1.code

12


E → ( E1 )

E1.true = E.true E1.false = E.false E.code = E1.code

13

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean Experssions Consider the expression

a < b or c < d and e < f

Suppose the true and false exits for the entire expression are Ltrue and Lfalse

14

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean Experssions Consider the expression


Suppose the true and false exits for the entire expression are Ltrue and Lfalse

15

if a < b goto Ltruegoto L1

L1: if c < d goto L2goto Lfalse

L2: if e < f goto Ltruegoto Lfalse

if a < b or c < d and e < f

16

while a < b if c < d then x = y + z else x = y – z

Consider the while statement

17

L1: if a < b goto L2

goto Lnext

L2: if c < d goto L3

goto L4

L3: t1 = y + z

x = t1

goto L1

L4: t2 = y – zx = t2goto L1

Lnext: nop

while a < b if c < d then x = y + z else x = y – z

18

ImplementationImplementationImplementationImplementation The easiest way to implement

syntax-directed definitions is to use two passes

• construct a syntax tree for the input

• walk the tree in depth-first order

19

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean Experssions The problem in generating

three-address code in one pass is that we may not know the labels that the control must go to when we generate jump statements

20


However, by using a technique called back-patching, we can generate code in one pass.

21


we will generate the jumps with targets temporarily left unspecified

22

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean Experssions Each such statement will be

put on a list of goto statements

We will fill the labels when the proper label can be determined (backpatch)

23

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean Experssions Each such statement will be

put on a list of goto statements

We will fill the labels when the proper label can be determined (backpatch)

24

BackpatchingBackpatchingBackpatchingBackpatching Assume that the quadruples

are put into an array

Labels will be indicies into this array

To manipulate list of labels, we will use three functions:

25

BackpatchingBackpatchingBackpatchingBackpatching

1. makelist(i) creates and returns a new list containing only i, the index of quadruple

26


2. merge(p1, p2) concatenates lists pointed to by p1 and p2 and returns the concatenated list

27


3. backpatch(p, i) inserts i as the target label for each of the goto statements on list pointed to by p

28

Boolean ExpressionsBoolean ExpressionsBoolean ExpressionsBoolean Expressions We now construct a

translation scheme suitable for producing quads (IR) for boolean expressions during bottom-up parsing

29

The grammar we use is

E → E1 or M E2

| E1 and M E2

| not E1 | ( E1 )| id1 relop id2| true| false

M → {M.quad = nextquad()}

M is the marker non-terminal

M.quad records the number of first statement of E2

30

Boolean ExpressionsBoolean ExpressionsBoolean ExpressionsBoolean Expressions We will associate synthesized

attributes truelist and falselist with the nonterminal E

Incomplete jumps will be placed on these list

31

Boolean ExpressionsBoolean ExpressionsBoolean ExpressionsBoolean Expressions We associate the semantic

action

{ M.quad = nextquad() }

with the production M →

32

Boolean ExpressionsBoolean ExpressionsBoolean ExpressionsBoolean Expressions

The function nextquad() returns the index of the next quadruple to follow

33

Boolean Expressions: ANDBoolean Expressions: ANDBoolean Expressions: ANDBoolean Expressions: ANDE → E1 and M E2{

backpatch(E1.truelist, M.quad); E.truelist = E2.truelist;E.falselist = merge(E1.falselist,

E2.falselist);}

Let us look at the mechanics

34


E → E1 and M E2

{ backpatch(E1.truelist, M.quad); E.truelist = E2.truelist;E.falselist = merge(E1.falselist, E2.falselist);}

If E1 is false, E is also false

35

Boolean ExpressionsBoolean ExpressionsBoolean ExpressionsBoolean ExpressionsE → E1 and M E2


so the statements on E1.falselist become part of E.falselist

36


E → E1 and M E2


If E1 is true, we must test E2



Sohail Aslam

Lecture 40

38



So the target for E1.truelist must be the beginning of code generated for E2

39



This target is obtained using the markernonterminal M.

40


E → E1 and M E2


M.quad records the number of the firststatement of E2.code.

41

Boolean Expressions: ORBoolean Expressions: ORBoolean Expressions: ORBoolean Expressions: ORE → E1 or M E2

{backpatch(E1.falselist, M.quad); E.truelist = merge(E1.truelist, E2.truelist);E.falselist = E2.falselist;

}If E1 is false, need to test E2

42


E → not E1

{E.truelist = E1.falselist;E.falselist = E1.truelist;

}

43


E → ( E1 ){

E.truelist = E1.truelist;E.falselist = E1.falselist;

}

44

Boolean ExperssionsBoolean ExperssionsBoolean ExperssionsBoolean ExperssionsE → id1 relop id2

{E.truelist = makelist(nextquad()); E.falselist = makelist(nextquad()

+1); emit(‘if’ id1 relop id2 ‘goto _’) ;emit(‘goto _’ );

}

45


E → true{

E.truelist = makelist(nextquad());

emit(‘goto _’ );}

46


E → false{

E.falselist = makelist(nextquad());

emit(‘goto _’ );}

47


M → {

M.quad = nextquad();

}

48


consider again, the boolean expression


We carry out a bottom-up parse

49

BackpatchingBackpatchingBackpatchingBackpatching In response to reduction of

a < b to E, the two quadruples

100: if a < b goto _101: goto _

are generated

50

RecallRecallRecallRecallE → id1 relop id2

{E.truelist = makelist(nextquad()); E.falselist = makelist(nextquad()

+1); emit(‘if’ id1 relop id2 ‘goto _’) ;emit(‘goto _’ );

}View this in a parse tree

51

E.t = {100}E.f = {101}

c < da < b e < for and

52

BackpatchingBackpatchingBackpatchingBackpatching The marker non-terminal M in

the production

E → E1 or M E2

records the value of nextquad which at this time is 102.

53

E.t = {100}E.f = {101}

c < d

a < b

e < f

M.q = 102

or and

54

BackpatchingBackpatchingBackpatchingBackpatching The reduction of c < d to E,

the two quadruples

102: if c < d goto _103: goto _

are generated

55

E.t = {100}E.f = {101}

E.t = {102}E.f = {103}

c < d

a < b

e < f

M.q = 102

or and

56

BackpatchingBackpatchingBackpatchingBackpatching The marker non-terminal M in

the production

E → E1 and M E2

records the value of nextquad which at this time is 104.

57

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

58

BackpatchingBackpatchingBackpatchingBackpatching Reducing e < f to E,

generates

104: if e < f goto _105: goto _

59

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

60

BackpatchingBackpatchingBackpatchingBackpatching We now reduce by the

production

E → E1 and M E2

61

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

62

Semantic ActionsSemantic ActionsSemantic ActionsSemantic Actions

E → E1 and M E2


63


The semantic action calls

backpatch({102},104)

where {102} denotes E1.truelist containing only 102

64


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 _

65




which fills in 104 in statement 102.

66


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

67

Semantic ActionsSemantic ActionsSemantic ActionsSemantic Actions

E → E1 and M E2


68

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

104103 105

69

BackpatchingBackpatchingBackpatchingBackpatching We now reduce by the

production

E → E1 or M E2

70

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

71

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsE → E1 or M E2


}

72




which fills in 102 in statement 101.

73


100: if a < b goto _101: goto102: if c < d goto 104

103: goto _104: if e < f goto _105: goto _

102

74

BackpatchingBackpatchingBackpatchingBackpatching100: if a < b goto _101: goto 102102: 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

75

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsE → E1 or M E2


}

76

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 = 102or

and

{100} {104}{103, 105}

77

Flow-of-Control StatementsFlow-of-Control StatementsFlow-of-Control StatementsFlow-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

78

Flow-of-Control StatementsFlow-of-Control StatementsFlow-of-Control StatementsFlow-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

79

Flow-of-Control StatementsFlow-of-Control StatementsFlow-of-Control StatementsFlow-of-Control StatementsS →if E then S

| if E then S else S| while E do S| begin L end| A

L → L ; S| S

S denotes a statement

L is a statementlistA is assignment

80

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsS → if E then M1 S1 N else M2 S2

{backpatch(E.truelist, M1.quad); backpatch(E.falselist, M2.quad); S.nextlist = merge(S1.nextlist,

merge( N.nextlist,S2.nextlist));}

If E is true, jump to M1.quad which is start of code for S1

N is marker non-terminal to introduce jump over code for S2

81

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsN → { N.nextlist = makelist(nextQuad()); emit(‘goto_’); }The attribute N.nextlist records the quad number of the goto it generates

82

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsM → { M.quad = nextQuad(); }

83

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsS → if E then M S1 {

backpatch(E.truelist, M.quad); S.nextlist = merge(

E.falselist,S1.nextlist);}

If E is true, jump to M.quad which is start of code for S1

84

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsS → while M1 E do M2 S1 {

backpatch(S1.nextlist, M1.quad); backpatch(E.truelist, M2.quad); S.nextlist = E.falselist;emit( ‘goto’ M1.quad);

}

85

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsS → begin L end { S.nextlist = L.nextlist; }

86

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsS → A{ S.nextlist = nil; }

initializes S.nextlist to an empty list

87

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsS → L1 ; M S {

backpatch(L1.nextlist, M.quad); L.nextlist = S.nextlist;

}

Statement following L1 in order of execution is beginning of S

88

Semantic ActionsSemantic ActionsSemantic ActionsSemantic ActionsL → S{ L.nextlist = S.nextlist; }

89

ExampleExampleExampleExample

if a<b or c<d and e<f then x=y+z else x=y-z

if E1 or M E2 then x=y+z else x=y-z

90

if E1 or M E2 then x=y+z else x=y-z

if E then x=y+z else x=y-z{ E.truelist=[100,104]

E.falselist=[103,105] }

91

100 if a < b goto _

101 goto 102

102 if c < d goto 104

103 goto _

104 if e < f goto _

105 goto_

106

107

108

109

92

if E then M1 x=y+z else x=y-z M1 → { M1.quad = 106 }

if E then M1 A else x=y-z A → x=y+z { emit(‘x=y+z’) }

if E then M1 S1 else A S1 → A{ S1.nextlist = nil}

if E then M1 S1 N else x=y-z N → { N.nextlist = [107]emit(‘goto _’ }

93

100 if a < b goto _

101 goto 102

102 if c < d goto 104

103 goto _

104 if e < f goto _

105 goto _

106 x=y+z

107 goto _

108

109

94

if E then M1 S1 N else M2 x=y-zM2 → { M2.quad = 108 }

if E then M1 S1 N else M2 A A → x=y-z { emit(‘x=y-z’) }

if E then M1 S1 N else M2 S2 S2 → A{ S2.nextlist = nil }

S{ backpatch([100,104],106) backpatch([103,105],108)S.nextlist=[107]}

95

100 if a < b goto _

101 goto 102

102 if c < d goto 104

103 goto _

104 if e < f goto _

105 goto _

106 x=y+z

107 goto _

108 x=y-z

109

106

106

108

108

96

100 if a < b goto 106

101 goto 102

102 if c < d goto 104

103 goto 108

104 if e < f goto 106

105 goto 108

106 x=y+z

107 goto _

108 x=y-z

109

97

Semantic Actions in YACCSemantic Actions in YACCSemantic Actions in YACCSemantic Actions in YACC

The syntax-directed translation statements can be conveniently specified in YACC

The %union will require more fields because the attributes vary

98

Semantic Actions in YACCSemantic Actions in YACCSemantic Actions in YACCSemantic Actions in YACC

The actual mechanics will be covered in the handout for the syntax-directed translation phase of the course project

Documents

Compiler Construction