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Attributes Migration and Syntax-Directed Translation

Understanding syntax-directed translation process driven by the program's structure and semantic analysis. Learn how semantic actions drive code generation and optimization in the two-part compilation. Explore attribute grammars for formalizing semantic routines.

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Attributes Migration and Syntax-Directed Translation

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  1. Lecture 8 Syntax-Directed Translation Joey Paquet, 2000, 2002

  2. Syntax-Directed Translation • Translation process driven by the syntactic structure of the program, as given by the parser • Semantic actions are integrated in the parsing process • In this view, compilation is divided in two parts: • analysis (syntactic, semantic) • synthesis (code generation and optimization) • The semantic analysis becomes the link between analysis and synthesis Joey Paquet, 2000, 2002

  3. Syntax-Directed Translation • Semantic actions (implemented as semantic routines) finish the analysis phase by performing semantic checking in productions that need such checks • Semantic actions provide a meaning for the productions • They are the starting point for code generation (synthesis) • Thus, the semantic routines are the heart of the compiler Joey Paquet, 2000, 2002

  4. Syntax-Directed Translation • Semantic routines can be formalized using attribute grammars • They augment ordinary context-free grammars with attributes that represent semantic properties such as type, value or correctness used in semantic analysis (checking) and code generation (translation) • It is useful to keep checking and translation facilities distinct in the semantic routines’ implementation • Semantic checking is machine-independent and code generation is not, so separating them gives more flexibility to the compiler Joey Paquet, 2000, 2002

  5. Attribute Migration • Attributes gathered from a child in the abstract syntax tree are called synthetized attributes • Attributes gathered from a sibling in the abstract syntax tree are called inherited attributes Joey Paquet, 2000, 2002

  6. Example: Attribute Migration E1  id = E2 E= : [E2: v: ] [(id:)  defVar] [id.val  v] [E1 .val:  v] E1  E2  E3 E* : [E2 : v2: ] [E3 : v3: ] [E1 .val:  v2  v3 ] E1  E2 + E3 E+ : [E2 : v2: ] [E3 : v3: ] [E1 .val:  v2 + v3 ] E id Eid : [(id:)  defVar] [(E.val: ) id.val] E  const Econst : [const : v : ] [(E.val: )v] Y=3*X+Z E (v3*vx+vz :) id (vY:) = E (v(3*vx)+vz:) E (v3*vx:) + E(vZ:) E (3:) * E(vX:) id (vZ:) const (3:) id (vX:) Joey Paquet, 2000, 2002

  7. E  TE’ E’  +TE’ |  T  FT’ T’  FT’ |  F  id | const T’1 E’2   T’3  Example 2: Attribute Migration • Problems arise when rules are factorized: a+b*c E T1 E’1 F1 + T2 id (va : ) F2 T’2 id (vb : ) F3 * id (vc : ) Joey Paquet, 2000, 2002

  8. T’1 E’2   T’3  Example 2: Attribute Migration • Solution: migrate attributes sideways: a+b*c E T1 E’1 E  TE’ E’  +TE’ |  T  FT’ T’  FT’ |  F  id | const F1 + T2 id (va : ) F2 T’2 id (vb : ) F3 * id (vc : ) Joey Paquet, 2000, 2002

  9. Attr. Migration: Implementation Parse(){ type Es; lookahead = NextToken() if (E(Es);Match('$')) return(true); else return(false); } Joey Paquet, 2000, 2002

  10. Attr. Migration: Implementation E(type &Es){ type Ts,E's if (lookahead is in [0,1,(]) if (T(Ts);E'(Ts,E's);) // E' inherits from T write(E->TE') Es = E's // Synthetised attribute sent up return(true) else return(false) else return(false) } Joey Paquet, 2000, 2002

  11. Attr. Migration: Implementation E'(type &Ti, type &E's){ type Ts,E'2s if (lookahead is in [+]) if (Match('+');T(Ts);E'(Ts,E'2s)) // E' inherits from T write(E'->TE') E's = semcheckop(Ti,E'2s) // Semantic check & synthetized // attribute sent up return(true) else return(false) else if (lookahead is in [$,)] write(E'->epsilon) E's = Ti // Synth. attr. is inhertied // from T (sibling, not child) // and sent up return(true) else return(false) } Joey Paquet, 2000, 2002

  12. Attr. Migration: Implementation T(type &Ts){ type Fs, T's if (lookahead is in [0,1,(]) if (F(Fs);T'(Fs,T's);) // T' inherits from F write(T->FT') Ts = T's // Synthetized attribute sent up return(true) else return(false) else return(false) } Joey Paquet, 2000, 2002

  13. Attr. Migration: Implementation T'(type Fi, type &T's){ type Fs, T'2s if (lookahead is in [*]) if (Match('*');F(Fs);T'(Fs,T'2s)) // T' inherits from F write(T'->*FT') T's = semcheckop(Fi,T'2s) // Sem. check + synth. // attribute sent up return(true) else return(false) else if (lookahead is in [+,$,)] write(T'->epsilon) T's = Fi // Synthetized attr. is // inhertied from F (sibling, // not child) & sent up the tree return(true) else return(false) } Joey Paquet, 2000, 2002

  14. Attr. Migration: Implementation F(type &Fs){ type Es if (lookahead is in [0]) if (Match('id')) write(F->id) Fs = gettype(id.name,table) // Gets the attribute ``type'' // from the symbol table and // sends it up the tree as Fs return(true) else return(false) else if (lookahead is in [(]) if (Match('(');E(Es);Match(')')) write(F->(E)) Fs = Es // Synthetized attribute sent // up the tree return(true) else return(false) else return(false) } Joey Paquet, 2000, 2002

  15. Attr. Migration: Implementation type semcheckop(type ti,type ts){ if (ti == ts) return(ti) else return(typerror) } type gettype(name, table){ if (name is in table) return (type) else return(typerror) } Joey Paquet, 2000, 2002

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