Chapter 5.

Syntax-Directed

Translation

2

Fig. 5.2. Syntax-directed definition of a simple desk calculator

Production Semantic Rules

L E n print ( E.val )

E E1 + T E.val := E1.val + T.val

E T E.val := T.val

T T1 * F T.val := T1.val + F.val

T F T.val := F.val

F ( E ) F.val := E.val

F digit F.val := digit.lexval

3

Fig. 5.3. Annotated parse tree for 3*5+4n.

L

nE.val = 19

T.val = 4

F.val = 4

digit.lexval = 4

T.val = 15

E.val = 15

T.val = 3

F.val = 3

digit.lexval = 3

* F.val = 5

+

digit.lexval = 5

4

Fig. 5.4. Syntax-directed definition with inherited attribute L.in.

Production Semantic Rules

D T L L.in := T.type

T int T.type := integer

T real T.type := real

L L1 , id L1.in := L.in

addtype ( id.entry , L.in )

L id addtype ( id.entry , L.in )

5

Fig. 5.5. Parse tree with inherited attribute in at each node labeled L.

D

L.in = realT.type = real

real

,

id1

id2

id3L.in = real

L.in = real

,

6

Fig. 5.6. E.val is synthesized from E1.val and E2.val

val val

valE

+E1 E2

7

Fig. 5.7. Dependency graph for parse tree of Fig. 5.5

D

LT

real

,

id1

id2

id3L

L

,

4type

in 5

3 entry

2 entry

1 entry

in 7

in 9

6

8

10

8

Fig. 5.9. Syntax-directed definition for constructing a syntax tree for an expression

Production Semantic Rules

E E1 + T E.nptr := mknode(‘+’, E1.nptr, T.nptr)

E E1 - T E.nptr := mknode(‘-’, E1.nptr, T.nptr)

E T E.nptr := T.nptr

T ( E )

T id

T.nptr := E.nptr

T.nptr := mkleaf (id, id.entry)

T num T.nptr := mkleaf (num, num.val)

9

Fig. 5.10. Construction of a syntax-tree for a-4+c

10

Fig. 5.11. Dag for the expression a+a*(b-c)+(b-c)*d.

11

Fig. 5.12. Instructions for constructing the dag of Fig. 5.11

(1) p1 := mkleaf ( id, a )

(2) p2 := mkleaf ( id, a )

(3) p3 := mkleaf ( id, b )

(4) p4 := mkleaf ( id, c )

(5) p5 := mknode ( ‘-’, p3, p4 )

(6) p6 := mknode ( ‘*’, p2, p5 )

(7) p7 := mknode ( ‘+’, p1, p6 )

(8) p8 := mkleaf ( id, b )

(9) p9 := mkleaf ( id, c )

(10) p10 := mknode ( ‘-’, p8, p9 )

(11) p11 := mkleaf ( id, d )

(12) p12 := mknode ( ‘*’, p10, p11 )

(13) p13 := mknode ( ‘+’, p7, p12 )

12

Fig. 5.15. Parser stack with a field for synthesized attributes

13

Fig. 5.16. Implementation of a desk calculator with an LR parser.

Production Code Fragment

L E n print ( val [top] )

E E1 + T val [ntop] := val [top – 2]+val [top]

E T

T T1 * F val [ntop] := val [top – 2]×val [top]

T F

F ( E ) val [ntop] := val [top – 1]

F digit

14

L-Attributed Definitions

A syntax-directed definition is L-attribute if each inherited attribute of Xj,

1≤ j≤n, on the right side of A → X1X2 · · · Xn , depends only on

1. the attributes of the symbols X1 , X2 , · · · , Xj-1 to the left of Xj in the pr

oduction and

the inherited attributes of A.

Note that every S-attributes definition is L-attributed, because the

restrictions (1) and (2) apply only to inherited attributes

15

Example 5.17. The type of an identifier can be passed by copy rules using inherited attributes as shown in Fig. 5.32 (adapted from Fig. 5.7). We shall first examine the moves made by a bottom-up parser on the input

real p, q, r

then we show how the value of attirbute T.type can be accessed when the productions for L are applied. The translation scheme we wish to implement is

D T L { L.in := T.type }T int { T.type := integer }T real { T.type := real }L { L1.in := L.in } L1 , id { addtype ( id.entry , L.in ) }L id { addtype ( id.entry , L.in ) }

16

If we ignore the actions in the above translation scheme, the sequence of moves made by the parser on the input of Fig. 5.32. is as in Fig. 5.33. For clarity, we show the corresponding grammar symbol instead of a stack state and the actual identifier instead of the token id.

D

LT

real

,

p

q

rL

L

,

typein

in

in

Fig.5.32. At each node for L, L.in = T.type.

17

Fig. 5.33.Whenever a right side for L is reduced, T is just below the right side.

Input state Production Used

real p,q,r −

p,q,r real

p,q,r T T → real

,q,r T p

,q,r T L L → id

q,r T L ,

,r T L , q

,r T L L → L , id

r T L ,

T L , r

T L L → L , id

D D → T L

18

Fig. 5.34. The value of T.type is used in place of L.in

Production Code Fragment

D T L ;

T int val [ntop] := integer

T real val [ntop] := real

L L1 , id addtype (val [top] , val [top−3] )

L id addtype (val [top] , val [top−1] )

19

Example 5.18. As an instance where we cannot predict the position, consider the

following translation scheme:

(5.6)

C inherits the synthesized attribute A.s by a copy rule. Note that there may or

may not be a B between A and C in the stack. When reduction by C → c is

performed, the value of C.i is either in val [top−1]or in val [top−2], but it is

not clear which case applies.

Production Semantic Rules

S → aAC C.i := A.s

S → bABC C.i := A.s

C → c C.s := g ( C.i )

20

Fig. 5.35. Copying an attribute value through a marker M.

Production Semantic Rules

S → aAC C.i := A.s

S → bABMC M.i := A.s ; C.i := M.s

C → c C.s := g ( C.i )

M → є M.s := M.i

S

Cb A Bs

i

S

Cb A Bs i

Mi s

є

(a) original production (b) modified dependencies

21Fig. 8.7. Semantic rules generating code for a while statement

Production Semantic Rules

S -> while E do S1 S.begin := newlabel ;

S.after := newlabel ;

S.code := gen( S.begin ‘ :’) ||

E.code ||

gen( ‘ if ’ E.place ‘ = ‘ ‘ 0 ’ ‘ goto’ S.after ) ||

S1.code ||

gen(‘ goto’ S.begin ) ||

gen( S.after ‘ : ‘ )