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Lecture 8

Lecture 8. Syntax-Directed Translation. 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:

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Lecture 8

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  1. Lecture 8 Syntax-Directed Translation Joey Paquet, 2000, 2002, 2008, 2012

  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: code generation (synthesis) is conditional to positive analysis Joey Paquet, 2000, 2002, 2008, 2012

  3. Syntax-Directed Translation • Semantic actions (implemented as semantic routines) finish the analysis phase by performing semantic checking in productions that need such checks, e.g. type checking • Semantic actions assemble information in order to check and generate a meaning for the program elements generated by 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, 2008, 2012

  4. Syntax-Directed Translation • Semantic routines can be formalized using attribute grammars • Attribute grammars 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 (front/back end) Joey Paquet, 2000, 2002, 2008, 2012

  5. Attributes • An attribute is a property of a programming language construct, including data type, value, memory location/size, translated code, etc. • Implementation-wise, they are also called semantic records. • The process of computing the value of an attribute is called binding. Static binding concerns binding that can be done at compile-time, and dynamic binding happens at run-time, e.g. for polymorphism. • Here we are concerned solely on static compile-time binding. Joey Paquet, 2000, 2002, 2008, 2012

  6. Attributes Migration • Static attribute binding is done by gathering, propagating, and aggregating attributes while traversing the parse tree. • Attributes are gathered at tree leaves, propagated across tree nodes, and aggregated at some parent nodes when all necessary information is available. • This can be done as the program is being parsed using syntax-directed translation. Joey Paquet, 2000, 2002, 2008, 2012

  7. 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, 2008, 2012

  8. 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, 2008, 2012

  9. 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, 2008, 2012

  10. 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, 2008, 2012

  11. Attr. Migration: Implementation Parse(){ type Es; //semantic record created //before the call lookahead = NextToken() if (E(Es);Match('$')) //passed as a reference //to parsing functions //that will compute its value return(true); else return(false); } Joey Paquet, 2000, 2002, 2008, 2012

  12. 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, 2008, 2012

  13. 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, 2008, 2012

  14. 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 Fs from F write(T->FT') Ts = T's // Synthetized attribute sent up return(true) else return(false) else return(false) } Joey Paquet, 2000, 2002, 2008, 2012

  15. 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) // Semantic check and // synthetized attribute sent up return(true) else return(false) else if (lookahead is in [+,$,)] write(T'->epsilon) T's = Fi // Synthetized attribute is // inhertied from F sibling // and sent up the tree return(true) else return(false) } Joey Paquet, 2000, 2002, 2008, 2012

  16. 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 from E // sent up the tree as attribute // of F return(true) else return(false) else return(false) } Joey Paquet, 2000, 2002, 2008, 2012

  17. 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, 2008, 2012

  18. T’1 E’2   T’3  Attribute Migration E Es T1 E’1 E’s E’s E’s E’s Ts Ts Ti Ti F1 + T2 T’s T’s T’s T’s T’s Fi Fi Fi Fs Fs Fs Fs Fs Fs id (va : ) F2 T’2 id (vb : ) F3 * Joey Paquet, 2000, 2002, 2008, 2012 18 id (vc : )

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