Chapter 5. Syntax-Directed Translation

2 Fig Syntax-directed definition of a simple desk calculator ProductionSemantic Rules L  E n print ( E.val ) E  E 1 + T E.val := E 1.val + T.val E  TE  T E.val := T.val T  T 1 * F T.val := T 1.val + F.val T  FT  F T.val := F.val F  ( E ) F.val := E.val F  digit F.val := digit.lexval

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

4 Fig Syntax-directed definition with inherited attribute L.in. ProductionSemantic Rules D  T L L.in := T.type T  int T.type := integer T  real T.type := real L  L 1, id L 1.in := L.in addtype ( id.entry, L.in ) L  id addtype ( id.entry, L.in )

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

6 Fig E.val is synthesized from E 1.val and E 2.val

7 Fig Dependency graph for parse tree of Fig. 5.5

8 Fig Syntax-directed definition for constructing a syntax tree for an expression ProductionSemantic Rules E  E 1 + T E.nptr := mknode(‘+’, E 1.nptr, T.nptr) E  E 1 - T E.nptr := mknode(‘-’, E 1.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 Construction of a syntax-tree for a-4+c

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

11 Fig Instructions for constructing the dag of Fig (1) p 1 := mkleaf ( id, a ) (2) p 2 := mkleaf ( id, a ) (3) p 3 := mkleaf ( id, b ) (4) p 4 := mkleaf ( id, c ) (5) p 5 := mknode ( ‘-’, p 3, p 4 ) (6) p 6 := mknode ( ‘*’, p 2, p 5 ) (7) p 7 := mknode ( ‘+’, p 1, p 6 ) (8) p 8 := mkleaf ( id, b ) (9) p 9 := mkleaf ( id, c ) (10) p 10 := mknode ( ‘-’, p 8, p 9 ) (11) p 11 := mkleaf ( id, d ) (12) p 12 := mknode ( ‘*’, p 10, p 11 ) (13) p 13 := mknode ( ‘+’, p 7, p 12 )

12 Fig Parser stack with a field for synthesized attributes

13 Fig Implementation of a desk calculator with an LR parser. ProductionCode Fragment L  E n print ( val [top] ) E  E 1 + T val [ntop] := val [top – 2]+val [top] E  TE  T T  T 1 * F val [ntop] := val [top – 2]×val [top] T  FT  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 X j, 1≤ j≤n, on the right side of A → X 1 X 2 · · · X n, depends only on 1. the attributes of the symbols X 1, X 2, · · ·, X j-1 to the left of X j in the production and 2.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 The type of an identifier can be passed by copy rules using inherited attributes as shown in Fig (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  { L 1.in := L.in } L 1, 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 is as in Fig For clarity, we show the corresponding grammar symbol instead of a stack state and the actual identifier instead of the token id. Fig At each node for L, L.in = T.type.

17 Fig Whenever a right side for L is reduced, T is just below the right side. InputstateProduction 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 The value of T.type is used in place of L.in ProductionCode Fragment D  T L ; T  int val [ntop] := integer T  real val [ntop] := real L  L 1, id addtype (val [top], val [top−3] ) L  id addtype (val [top], val [top−1] )

19 Example 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. ProductionSemantic Rules S → aAC C.i := A.s S → bABC C.i := A.s C → c C.s := g ( C.i )

20 Fig Copying an attribute value through a marker M. ProductionSemantic 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 → єM → є M.s := M.i є (a) original production(b) modified dependencies

21 Fig Semantic rules generating code for a while statement ProductionSemantic Rules S -> while E do S 1 S.begin := newlabel ; S.after := newlabel ; S.code := gen( S.begin ‘ :’) || E.code || gen( ‘ if ’ E.place ‘ = ‘ ‘ 0 ’ ‘ goto’ S.after ) || S 1.code || gen(‘ goto’ S.begin ) || gen( S.after ‘ : ‘ )