1. CS308 Compiler Principles Syntax-Directed Translation Fan Wu Department of Computer Science and Engineering Shanghai Jiao Tong University Fall 2012

2. Compiler Principles Phases of Compilation Lexical Analyzer Syntax Analyzer Semantic Analyzer Intermediate Code Generator Code Optimizer Code Generator Source Language Target Language Intermediate Language Analysis SynthesisSymbol Table

3. Compiler Principles A Model of A Compiler Font End Lexical analyzer reads the source program character by character and returns the tokens of the source program. Parser creates the tree-like syntactic structure of the given program. Intermediate-code generator translates the syntax tree into three- address codes.

4. Compiler Principles Syntax-Directed Translation Associate semantic meanings with the grammar. –generate intermediate codes –put information into the symbol table –perform type checking –issue error messages –perform some other activities –in fact, they may perform almost any activities.

5. Compiler Principles Syntax-Directed Translation Cont’d Syntax-Directed Definitions: –associate a production rule with a set of semantic rules –give high-level specifications for translations –hide many implementation details such as order of evaluation of semantic rules Translation Schemes: –embed program fragments within production bodies –indicate the order of evaluation of semantic actions associated with a production

6. Compiler Principles Syntax-Directed Definition (SDD) A syntax-directed definition is a generalization of a context-free grammar: –Each grammar symbol is associated with a set of attributes. –Each production is associated with a set of semantic rules. Attributes are divided into two kinds: –Synthesized attribute is defined only in terms of attribute values at the node’s children and itself. –Inherited attribute is defined in terms of attribute values the node’s parent, itself, and siblings.

7. Compiler Principles SDD Cont’d In a syntax-directed definition, each production A→α is associated with a set of semantic rules of the form: b=f(c 1,c 2,…,c n )where f is a function,  b is a synthesized attribute of A and c 1,c 2,…,c n are attributes of the grammar symbols in the production ( A→α ).  b is an inherited attribute of one of the grammar symbols in α, and c 1,c 2,…,c n are attributes of the grammar symbols in the production ( A→α ).

8. Compiler Principles Attribute Grammar A semantic rule b=f(c 1,c 2,…,c n ) indicates that the attribute b depends on attributes c 1,c 2,…,c n. In a syntax-directed definition, a semantic rule may not only evaluate the value of an attribute, but also have some side effects such as printing values. An attribute grammar is a syntax-directed definition without side effects.

9. Compiler Principles SDD Example1 Production Semantic Rules L → E returnprint(E.val) E → E 1 + TE.val = E 1.val + T.val E → TE.val = T.val T → T 1 * FT.val = T 1.val * F.val T → FT.val = F.val F → ( E )F.val = E.val F → digitF.val = digit.lexval Symbols E, T, and F are associated with a synthesized attribute val. The token digit has a synthesized attribute lexval (an integer value returned by the lexical analyzer).

10. Compiler Principles SDD Example2 Production Semantic Rules E → E 1 + TE.loc=newtemp(), E.code = E 1.code || T.code || add E 1.loc,T.loc,E.loc E → TE.loc = T.loc, E.code=T.code T → T 1 * FT.loc=newtemp(), T.code = T 1.code || F.code || mult T 1.loc,F.loc,T.loc T → FT.loc = F.loc, T.code=F.code F → ( E )F.loc = E.loc, F.code=E.code F → idF.loc = id.name, F.code=“” Symbols E, T, and F are associated with synthesized attributes loc and code. The token id has a synthesized attribute name. || is the string concatenation operator.

11. Compiler Principles Annotated Parse Tree A parse tree showing the values of attributes at each node is called an annotated parse tree. The process of computing the attributes values at the nodes is called annotating (or decorating) of the parse tree.

12. Compiler Principles Annotated Parse Tree Example Input: 5+3*4 L E.val=17 E.val=5 + T.val=12 T.val=5 T.val=3 * F.val=4 F.val=5 F.val=3 digit.lexval=4 digit.lexval=5 digit.lexval=3

13. Compiler Principles Dependency Graph Semantic rules set up dependencies among attributes. Dependency graph determines the evaluation order of the semantic rules. –An edge from one attribute to another indicates that the value of the former one is needed to compute the later one.

14. Compiler Principles Dependency Graph Example Input: 5+3*4 L E.val=17 E.val=5 T.val=12 T.val=5 T.val=3 F.val=4 F.val=5 F.val=3 digit.lexval=4 digit.lexval=5 digit.lexval=3

15. Compiler Principles Inherited Attributes Example Production Semantic Rules D → T LL.in = T.type T → intT.type = integer T → realT.type = real L → L 1 idL 1.in = L.in, addtype(id.entry,L.in) L → idaddtype(id.entry,L.in) Symbol T is associated with a synthesized attribute type. Symbol L is associated with an inherited attribute in.

16. Compiler Principles A Dependency Graph with Inherited Attributes Input: real p q DL.in=real T LT.type=real L 1.in=real, addtype(q,real) real L idaddtype(p,real)id.entry=q id id.entry=p parse treedependency graph

17. Compiler Principles S & L-Attributed Definitions We will look at two sub-classes of the syntax-directed definitions: –S-Attributed Definitions: only synthesized attributes are used in the syntax-directed definitions. –L-Attributed Definitions: both synthesized and inherited attributes are used in a restricted fashion. dependency-graph edges can go from left to right, but not from right to left

18. Compiler Principles S-Attributed Definitions S-Attributed Definitions: only synthesized attributes are used in the syntax-directed definitions –each rule computes an attribute for the nonterminal at the head of a production from attributes taken from the body of the production –the attributes can be evaluated by performing a postorder traversal of the parse tree –can be implemented naturally with an LR parser –can also be implemented with an LL parser

19. Compiler Principles Bottom-Up Evaluation of S-Attributed Definitions Put the values of the synthesized attributes of the grammar symbols into a parallel stack Evaluate the values of the attributes during reductions Example: A  XYZ A.a=f(X.x,Y.y,Z.z) (all attributes are synthesized) stack parallel-stack top   top  Z Y X. A. A.a. Z.z Y.y X.x.

20. Compiler Principles SDD Example Recall Production Semantic Rules L → E returnprint(E.val) E → E 1 + TE.val = E 1.val + T.val E → TE.val = T.val T → T 1 * FT.val = T 1.val * F.val T → FT.val = F.val F → ( E )F.val = E.val F → digitF.val = digit.lexval Symbols E, T, and F are associated with a synthesized attribute val. The token digit has a synthesized attribute lexval (an integer value returned by the lexical analyzer).

21. Compiler Principles Bottom-Up Eval. of S-Attributed Definitions Production Semantic Rules L → E returnprint(val[top-1]) E → E 1 + Tval[ntop] = val[top-2] + val[top] E → T T → T 1 * Fval[ntop] = val[top-2] * val[top] T → F F → ( E )val[ntop] = val[top-1] F → digitpush digit.lexval At each shift of digit, we also push digit.lexval into val-stack. At all other shifts, we do not put anything into val- stack because other terminals do not have attribute (but we increment the stack pointer for val-stack).

22. Compiler Principles Canonical LR(0) Collection for The Grammar L’→. L L →. Er E →. E+T E →. T T →. T*F T →. F F →. (E) F →. d L’→L. L →E. r E →E. +T E →T. T →T. *F T →F. F → (. E) E →. E+T E →. T T →. T*F T →. F F →. (E) F →. d F →d. L →Er. E →E+. T T →. T*F T →. F F →. (E) F →. d T →T*. F F →. (E) F →. d F →(E. ) E →E. +T E →E+T. T →T. *F T →T*F. F →(E). I0:I0: I1:I1: I2:I2: I4:I4: I3:I3: I5:I5: I6:I6: I7:I7: I 12 : I 11 : I 10 : I9:I9: I 13 : I8:I8: r L E E T T T F F F F ( ( ( ( d d d d ) * + + * 6 3 5 4 6 6 5 5 8 9 4

23. Compiler Principles Bottom-Up Evaluation Example At each shift of digit, we also push digit.lexval into val-stack. stackval-stackinputactionsemantic rule 05+3*4rs6d.lexval(5) into val-stack 0d65+3*4rF→dF.val=d.lexval – do nothing 0F45+3*4rT→FT.val=F.val – do nothing 0T35+3*4rE→TE.val=T.val – do nothing 0E25+3*4rs8push empty slot into val-stack 0E2+85-3*4rs6d.lexval(3) into val-stack 0E2+8d65-3*4rF→dF.val=d.lexval – do nothing 0E2+8F45-3*4rT→FT.val=F.val – do nothing 0E2+8T115-3*4rs9push empty slot into val-stack 0E2+8T11*95-3-4rs6d.lexval(4) into val-stack 0E2+8T11*9d65-3-4rF→d F.val=d.lexval – do nothing 0E2+8T11*9F125-3-4rT→T*FT.val=T 1.val*F.val 0E2+8T115-12rE→E+TE.val=E 1.val+T.val 0E217rs7push empty slot into val-stack 0E2r717-$L→Erprint(17), pop empty slot from val-stack 0L117$acc

24. Compiler Principles Top-Down Eval. of S-Attributed Definitions ProductionsSemantic Rules A → Bprint(B.n0), print(B.n1) B → 0 B 1 B.n0=B 1.n0+1, B.n1=B 1.n1 B → 1 B 1 B.n0=B 1.n0, B.n1=B 1.n1+1 B →  B.n0=0, B.n1=0 B has two synthesized attributes (n0 and n1).

25. Compiler Principles Top-Down Eval. of S-Attributed Definitions In a recursive predictive parser, each non- terminal corresponds to a procedure. procedure A() { call B(); A → B } procedure B() { if (currtoken=0) { consume 0; call B(); } B → 0 B else if (currtoken=1) { consume 1; call B(); } B → 1 B else if (currtoken=$) {} // $ is end-marker B →  else error(“unexpected token”); }

26. Compiler Principles Top-Down Eval. of S-Attributed Definitions procedure A() { int n0,n1;Synthesized attributes of non-terminal B call B(&n0,&n1);are the output parameters of procedure B. print(n0); print(n1); } All the semantic rules can be evaluated procedure B(int *n0, int *n1) { at the end of parsing of production rules if (currtoken=0) { int a,b; consume 0; call B(&a,&b); *n0=a+1; *n1=b; } else if (currtoken=1) { int a,b; consume 1; call B(&a,&b); *n0=a; *n1=b+1; } else if (currtoken=$) {*n0=0; *n1=0; } // $ is end-marker else error(“unexpected token”); }

27. Compiler Principles L-Attributed Definitions L-Attributed Definitions: both synthesized and inherited attributes are used in a restricted fashion. –can always be evaluated by a depth first traversal of the parse tree –can also be evaluated during the parsing

28. Compiler Principles L-Attributed Definitions A syntax-directed definition is L-attributed if each inherited attribute of X j, where 1  j  n, on the right side of A → X 1 X 2...X n depends only on: 1.the inherited attribute of A 2.the attributes of the symbols X 1,...,X j-1 to the left of X j in the production 3.attributes associated with X j itself, under the condition that there is no cycle in the dependency graph involving the attributes of X j Every S-attributed definition is L-attributed, the restrictions only apply to the inherited attributes (not to synthesized attributes).

29. Compiler Principles A L-Attributed SDD ProductionsSemantic Rules T → F T’T’.inh = F.val T’ → * F T’ 1 T’ 1.inh=T’.inh * F.val

30. Compiler Principles A Definition that is NOT L-Attributed ProductionsSemantic Rules A → L ML.in=l(A.i), M.in=m(L.s), A.s=f(M.s) A → Q R R.in=r(A.in), Q.in=q(R.s), A.s=f(Q.s) This syntax-directed definition is not L- attributed because the semantic rule Q.in=q(R.s) violates the restrictions of L- attributed definitions.

31. Compiler Principles Syntax-Directed Translation Schemes (SDT) A syntax-directed translation scheme is a context-free grammar in which: –attributes are associated with the grammar symbols –semantic actions enclosed between braces {} are inserted within the body of productions. Example:A → {... } X {... } Y {... } Semantic Actions

32. Compiler Principles SDT Cont’d In translation schemes, we use semantic action instead of semantic rule used in syntax-directed definitions. Restrictions in designing a translation scheme: –The position of the semantic action on the right side indicates when that semantic action will be evaluated. –These restrictions (motivated by L-attributed definitions) ensure that a semantic action does not refer to an attribute that has not yet computed.

33. Compiler Principles When to evaluate the sematic action? For production B → X {a} Y –If the parse is bottom-up, then we perform action a as soon as this occurrence of X appears on the top of the parsing stack. –If the parse is top-down, we perform a just before we attempt to expand this occurrence of Y (if Y is a nonterminal) or check for Y on the input (if Y is a terminal).

34. Compiler Principles A SDT Example A simple translation scheme that converts infix expressions to the corresponding postfix expressions. E → T R R → + T { print(“+”) } R 1 R →  T → id { print(id.name) } a+b+c  ab+c+ infix expression postfix expression

35. Compiler Principles A SDT Example Cont’d E T R id{print(“a”)} + T {print(“+”)} R id {print(“b”)} + T {print(“+”)} R id {print(“c”)}  A depth first traversal of the parse tree will produce the postfix representation of the infix expression.

36. Compiler Principles SDT for S-Attributed Definition For each associated semantic rule in a S- attributed SDD, append a semantic action to the end of the production body. ProductionSemantic Rule E → E 1 + TE.val = E 1.val + T.val  E → E 1 + T { E.val = E 1.val + T.val }

37. Compiler Principles Bottom-Up Eval. of S-Attributed Definitions Production Actions L → E returnprint(val[top-1]) E → E 1 + Tval[ntop] = val[top-2] + val[top] E → T T → T 1 * Fval[ntop] = val[top-2] * val[top] T → F F → ( E )val[ntop] = val[top-1] F → digitpush digit.lexval At each shift of digit, we also push digit.lexval into val-stack. At all other shifts, we do not put anything into val- stack because other terminals do not have attribute (but we increment the stack pointer for val-stack).

38. Compiler Principles SDT for L-Attributed Definition Conversion rules: 1.An inherited attribute of a symbol on the right side of a production must be computed in a semantic action before that symbol. 2.A semantic action must not refer to a synthesized attribute of a symbol to the right of that semantic action. 3.A synthesized attribute for the non-terminal on the left can only be computed after all attributes it references have been computed (this semantic action is placed at the end of the production body). Any L-attributed definition can always be converted to a corresponding translation scheme satisfying these three rules.

39. Compiler Principles A SDT with Inherited Attributes D → T id { addtype(id.entry,T.type), L.in = T.type } L T → int { T.type = integer } T → real { T.type = real } L → id { addtype(id.entry,L.in), L 1.in = L.in } L 1 L →  This is a translation scheme for an L-attributed definitions.

40. Compiler Principles Implementing SDT Using Recursive-Descent Parsing –Decide the production used to expand A. –Match each terminal appears on the input. –Preserve, in local variables, the values of all attributes needed to compute inherited and synthesized attributes. –Call functions corresponding to nonterminals in the body, and provide them with the proper arguments.

41. Compiler Principles Recursive-Descent Parsing of SDT procedure D() { int Ttype,Lin,identry; call T(&Ttype); consume(id,&identry); addtype(identry,Ttype); Lin=Ttype; call L(Lin);a synthesized attribute (an output parameter) } procedure T(int *Ttype) { if (currtoken is int) { consume(int); *Ttype=TYPEINT; } else if (currtoken is real) { consume(real); *Ttype=TYPEREAL; } else { error(“unexpected type”); } }an inherited attribute (an input parameter) procedure L(int Lin) { if (currtoken is id) { int L1in,identry; consume(id,&identry); addtype(identry,Lin); L1in=Lin; call L(L1in); } else if (currtoken is endmarker) { } else { error(“unexpected token”); } }

42. Compiler Principles Eliminating Left Recursion from SDT A translation scheme with a left recursive grammar. E → E 1 + T { E.val = E 1.val + T.val } E → E 1 - T { E.val = E 1.val - T.val } E → T { E.val = T.val } T → T 1 * F { T.val = T 1.val * F.val } T → F{ T.val = F.val } F → ( E ) { F.val = E.val } F → digit { F.val = digit.lexval } When we eliminate the left recursion from the grammar (to get a suitable grammar for the top- down parsing) we also have to change semantic actions

43. Compiler Principles Eliminating Left Recursion A → A 1 Y { A.a = g(A 1.a,Y.y) }a left recursive grammar with A → X { A.a=f(X.x) }synthesized attributes (a,y,x).  eliminate left recursion inherited attribute of the new non-terminal synthesized attribute of the new non-terminal A → X { R.in=f(X.x) } R { A.a=R.syn } R → Y { R 1.in=g(R.in,Y.y) } R 1 { R.syn = R 1.syn } R →  { R.syn = R.in }

44. Compiler Principles Eliminating Left Recursion Cont’d A parse tree of left recursive grammar AY A.a=g(f(X.x),Y.y) parse tree of non-left-recursive grammar X X.x=f(X.x) A X R.in=f(X.x) R A.a=g(f(X.x,Y.y) Y R 1.in=g(f(X.x),Y.y) R 1 R.syn=g(f(X.x),Y.y)  R 1.syn=R 1.in

45. Compiler Principles Eliminating Left Recursion from SDT A translation scheme with a left recursive grammar. E → E 1 + T { E.val = E 1.val + T.val } E → E 1 - T { E.val = E 1.val - T.val } E → T { E.val = T.val } T → T 1 * F { T.val = T 1.val * F.val } T → F{ T.val = F.val } F → ( E ) { F.val = E.val } F → digit { F.val = digit.lexval } When we eliminate the left recursion from the grammar (to get a suitable grammar for the top- down parsing) we also have to change semantic actions

46. Compiler Principles Eliminating Left Recursion Example inherited attribute synthesized attribute E → T { A.in=T.val } A { E.val=A.syn } A → + T { A 1.in=A.in+T.val } A 1 { A.syn = A 1.syn } A → - T { A 1.in=A.in-T.val } A 1 { A.syn = A 1.syn } A →  { A.syn = A.in } T → F { B.in=F.val } B { T.val=B.syn } B → * F { B 1.in=B.in*F.val } B 1 { B.syn = B 1.syn} B →  { B.syn = B.in } F → ( E ) { F.val = E.val } F → digit { F.val = digit.lexval }

47. Compiler Principles Test Yourself Textbook page 337, Exercise 5.4.3

48. Compiler Principles Intermediate Code Generation with SDT E → T { A.in=T.loc } A { E.loc=A.loc } A → + T { A 1.in=newtemp(); emit(add,A.in,T.loc,A 1.in) } A 1 { A.loc = A 1.loc} A →  { A.loc = A.in } T → F { B.in=F.loc } B { T.loc=B.loc } B → * F { B 1.in=newtemp(); emit(mult,B.in,F.loc,B 1.in) } B 1 { B.loc = B 1.loc} B →  { B.loc = B.in } F → ( E ) { F.loc = E.loc } F → id { F.loc = id.name }

49. Compiler Principles Intermediate Code Generation with Predictive Parsing procedure E(char **Eloc) { char *Ain, *Tloc, *Aloc; call T(&Tloc); Ain=Tloc; call A(Ain,&Aloc); *Eloc=Aloc; } procedure A(char *Ain, char **Aloc) { if (currtok is +) { char *A1in, *Tloc, *A1loc; consume(+); call T(&Tloc); A1in=newtemp(); emit(“add”,Ain,Tloc,A1in); call A(A1in,&A1loc); *Aloc=A1loc; } else { *Aloc = Ain } }

50. Compiler Principles Intermediate Code Generation with Predictive Parsing procedure T(char **Tloc) { char *Bin, *Floc, *Bloc; call F(&Floc); Bin=Floc; call B(Bin,&Bloc); *Tloc=Bloc; } procedure B(char *Bin, char **Bloc) { if (currtok is *) { char *B1in, *Floc, *B1loc; consume(+); call F(&Floc); B1in=newtemp(); emit(“mult”,Bin,Floc,B1in); call B(B1in,&B1loc); Bloc=B1loc; } else { *Bloc = Bin } } procedure F(char **Floc) { if (currtok is “(“) { char *Eloc; consume(“(“); call E(&Eloc); consume(“)”); *Floc=Eloc } else { char *idname; consume(id,&idname); *Floc=idname } }

51. Compiler Principles Bottom-Up Evaluation of L-Attributed SDD In bottom-up evaluation, the semantic actions are evaluated during the reductions. During the bottom-up evaluation of S-attributed definitions, we have a parallel stack to hold synthesized attributes. Problem: Where do we hold inherited attributes? Solution: –Convert the grammar to guarantee the followings: All embedding semantic actions in the translation scheme is moved to the end of the production rules. All inherited attributes is copied into the synthesized attributes.

52. Compiler Principles Moving Embedding Semantic Actions Translation rules: 1.Remove an embedding semantic action S i, and put a new non-terminal M i in the place of the semantic action. 2.Put that semantic action S i to the end of a new production rule M i  for the new non- terminal M i. The semantic action S i will be evaluated when this new production rule is reduced. The evaluation order of the semantic rules are not changed by this transformation.

53. Compiler Principles Removing Embedding Semantic Actions Example1 A  {S 1 } X 1 {S 2 } X 2... {S n } X n  remove embedding semantic actions A  M 1 X 1 M 2 X 2... M n X n M 1  {S 1 } M 2  {S 2 } … M n  {S n }

54. Compiler Principles Removing Embedding Semantic Actions Example2 E → T R R → + T { print(“+”) } R 1 R →  T → id { print(id.name) }  remove embedding semantic actions E → T R R → + T M R 1 R →  T → id { print(id.name) } M →  { print(“+”) }

55. Compiler Principles Copying Inherited Attributes Assume every non-terminal A has an inherited attribute A.i, and every symbol X has a synthesized attribute X.s. For every production rule A  X 1 X 2... X n, –Introduce new marker non-terminals M 1,M 2,...,M n –Replace the production rule with A  M 1 X 1 M 2 X 2... M n X n –Synthesized attributes of X i will be not changed. –Inherited attributes of X i will be copied into the synthesized attribute of M i by the new semantic action added to the end of the new production rule M i. –The inherited attribute of X i can be found in the synthesized attribute of M i (which is immediately available in the stack).

56. Compiler Principles Copying Inherited Attributes Cont’d A  {B.i=f 1 (...)} B {C.i=f 2 (...)} C {A.s= f 3 (...)}  A  {M 1.i=f 1 (...)} M 1 {B.i=M 1.s} B {M 2.i=f 2 (...)} M 2 {C.i=M 2.s} C {A.s= f 3 (...)} M 1  {M 1.s=M 1.i} M 2  {M 2.s=M 2.i}

57. Compiler Principles Translation with Inherited Attributes S  {A.i=1} A {S.s=k(A.i,A.s)} A  {B.i=f(A.i)} B {C.i=g(A.i,B.i,B.s)} C {A.s= h(A.i,B.i,B.s,C.i,C.s)} B  b {B.s=m(B.i,b.s)} C  c {C.s=n(C.i,c.s)} S  {M 1.i=1} M 1 {A.i=M 1.s} A {S.s=k(M 1.s,A.s)} A  {M 2.i=f(A.i)} M 2 {B.i=M 2.s} B {M 3.i=g(A.i,M 2.s,B.s)} M 3 {C.i=M 3.s} C {A.s= h(A.i, M 2.s,B.s, M 3.s,C.s)} B  b {B.s=m(B.i,b.s)} C  c {C.s=n(C.i,c.s)} M 1  {M 1.s=M 1.i} M 2  {M 2.s=M 2.i} M 3  {M 3.s=M 3.i}

58. Compiler Principles Actual Translation Scheme S  {M 1.i=1} M 1 {A.i=M 1.s} A {S.s=k(M 1.s,A.s)} A  {M 2.i=f(A.i)} M 2 {B.i=M 2.s} B {M 3.i=g(A.i,M 2.s,B.s)} M 3 {C.i=M 3.s} C {A.s= h(A.i, M 2.s,B.s, M 3.s,C.s)} B  b {B.s=m(B.i,b.s)} C  c {C.s=n(C.i,c.s)} M 1  {M 1.s= M 1.i} M 2  {M 2.s=M 2.i} M 3  {M 3.s=M 3.i} S  M 1 A { s[ntop]=k(s[top-1],s[top]) } M 1   { s[ntop]=1 } A  M 2 B M 3 C{ s[ntop]=h(s[top-4],s[top-3],s[top-2],s[top-1],s[top]) } M 2   { s[ntop]=f(s[top]) } M 3   { s[ntop]=g(s[top-2],s[top-1],s[top])} B  b { s[ntop]=m(s[top-1],s[top]) } C  c { s[ntop]=n(s[top-1],s[top]) }

59. Compiler Principles Evaluation of Attributes S S.s=k(1,h(..)) A.i=1 A A.s=h(1,f(1),m(..),g(..),n(..)) B.i=f(1) C.i=g(1,f(1),m(..)) BC B.s=m(f(1),b.s) C.s=n(g(..),c.s) bc

60. Compiler Principles Evaluation of Attributes Cont’d stackinput l-attribute stack bc$ M 1 bc$1 M 1 M 2 bc$1 f(1) M 1 M 2 b c$1 f(1) b.s M 1 M 2 B c$1 f(1) m(f(1),b.s) M 1 M 2 B M 3 c$1 f(1) m(f(1),b.s) g(1,f(1),m(f(1),b.s)) M 1 M 2 B M 3 c $1 f(1) m(f(1),b.s) g(1,f(1),m(f(1),b.s)) c.s M 1 M 2 B M 3 C $1 f(1) m(f(1),b.s) g(1,f(1),m(f(1),b.s)) n(g(..),c.s) M 1 A $1 h(f(1),m(..),g(..),n(..)) S $k(1,h(..))

61. Compiler Principles Problems All L-attributed definitions based on LR grammars cannot be evaluated during bottom-up parsing. S  { L.i=0 } L  this translations scheme cannot be implemented L  { L 1.i=L.i+1 } L 1 1 during the bottom-up parsing L   { print(L.i) } S  M 1 L L  M 2 L 1 1  But since L   will be reduced first by the bottom- up L   { print(s[top]) } parser, the translator cannot know the number of 1s. M 1   { s[ntop]=0 } M 2   { s[ntop]=s[top]+1 }

62. Compiler Principles Problems The modified grammar cannot be LR grammar anymore. L  L bL  M L b L  a  L  aNOT LR-grammar M   S’ . L, $ L . M L b, $ L . a, $ M .,a  shift/reduce conflict