Abstract Syntax Trees. AST Data Structure. Visitor Interface. Accept methods. Visitor Methodology for AST Traversal CS412/CS413

Transcription

1 Abstract Syta Trees CS412/CS413 Itroductio to Copilers Ti Teitelbau Lecture 12: Visitors; Sybol Tables February 18, 2005 Separate AST costructio fro seatic checkig phase Traverse the AST ad perfor seatic checks (or other actios) oly after the tree has bee built ad its structure is stable This approach is less error-proe It is better whe efficiecy is ot a critical issue CS 412/413 Sprig 2005 Itroductio to Copilers 1 CS 412/413 Sprig 2005 Itroductio to Copilers 2 Visitor Methodology for AST Traversal Visitor patter: useful OO prograig patter that separates data structure defiitio (e.g., the AST) fro code that traverses the structure (e.g., the ae resolutio code ad the type checkig code). Defie a Visitor erface for all traversals of the AST Eted each AST class with a ethod that accepts ay Visitor Code each traversal as a separate class that ipleets the Visitor erface AST Data Structure abstract class Epr Epr { class Add Addeteds Epr Epr { Epr Epr e1, e1, e2 e2 class Nu Nueteds Epr Epr { value class Id Ideteds Epr Epr { Strig ae CS 412/413 Sprig 2005 Itroductio to Copilers 3 CS 412/413 Sprig 2005 Itroductio to Copilers 4 Visitor Iterface Accept ethods erface Visitor { void visit(add e); e); void visit(nu e); e); void visit(id e); e); abstract class classepr {{ abstract public publicvoid voidaccept(visitor v); v); class classadd Addeteds Epr Epr { { public publicvoid voidaccept(visitor v) v) {{ v.visit(this); class classnu Nueteds Epr Epr { { public publicvoid voidaccept(visitor v) v) {{ v.visit(this); class classid Ideteds Epr Epr {{ public publicvoid voidaccept(visitor v) v) {{ v.visit(this); The declared type of this is the subclass it which it occurs. Overload resolutio of v.visit(this); ivokes appropriate visit fuctio i the Visitor. CS 412/413 Sprig 2005 Itroductio to Copilers 5 CS 412/413 Sprig 2005 Itroductio to Copilers 6

2 Visitor Methods For each kid of traversal, ipleet the Visitor erface, e.g., class PostfiOutputVisitor ipleets Visitor { void visit(add e) { e.e1.accept(this); e.e2.accept(this); Syste.out.pr( + ); void visit(nu e) { Syste.out.pr(value); void visit(id e) { Syste.out.pr(id); Dispatch i the visit ethods eliiates case aalysis o AST subclasses To traverse epressio e: PostfiOutputVisitor v = ew PostfiOutputVisitor(); e.accept(v); Iherited ad Sythesized Iforatio So far, OK for traversal ad actio w/o couicatio of values But we eed a way to pass iforatio Dow the AST (iherited) Up the AST (sythesized) To pass iforatio dow the AST add paraeter to visit fuctios To pass iforatio up the AST add retur value to visit fuctios CS 412/413 Sprig 2005 Itroductio to Copilers 7 CS 412/413 Sprig 2005 Itroductio to Copilers 8 Visitor Iterface (2) Accept ethods (2) erface Visitor { Object visit(add e, e, Object ih); Object visit(nu e, e, Object ih); ih); Object visit(id e, e, Object ih); ih); abstract class classepr {{ abstract public publicobject Objectaccept(Visitor v, v, Object Object ih); ih); class classadd Addeteds Epr Epr { { public publicobject Objectaccept(Visitor v, v, Object Object ih) ih) {{ retur returv.visit(this, ih); ih); class classnu Nueteds Epr Epr { { public publicobject Objectaccept(Visitor v, v, Object Object ih) ih) {{ retur returv.visit(this, ih); ih); class classid Ideteds Epr Epr {{ public publicobject Objectaccept(Visitor v, v, Object Object ih) ih) {{ retur returv.visit(this, ih); ih); CS 412/413 Sprig 2005 Itroductio to Copilers 9 CS 412/413 Sprig 2005 Itroductio to Copilers 10 Visitor Methods (2) For kid of traversal, ipleet the Visitor erface, e.g., class EvaluatioVisitor ipleets Visitor { Object visit(add e, Object ih) { left = () e.e1.accept(this, ih); right = () e.e2.accept(this, ih); retur left+right; Object visit(nu e, Object ih) { retur value; Object visit(id e, Object ih) { retur Lookup(id, (SybolTable)ih); To traverse epressio e: EvaluatioVisitor v = ew EvaluatioVisitor (); e.accept(v); CS 412/413 Sprig 2005 Itroductio to Copilers 11 Icorrect Progras Leically ad sytactically correct progras ay still cotai other errors! Leical ad syta aalysis are ot powerful eough to esure the correct usage of iables, objects, fuctios, stateets, etc. CS 412/413 Sprig 2005 Itroductio to Copilers 12

3 Icorrect Progras Eaple 1: leical aalysis does ot distiguish betwee differet iable or fuctio idetifiers (it returs the sae toke for all idetifiers) a; a; a = 1; b = 1; Eaple 2: syta aalysis does ot correlate the declaratios with the uses of iables i the progra: a; a = 1; a= 1; Eaple 3: syta aalysis does ot correlate the types fro the declaratios with the uses of iables: a; a; a = 1; a = 1.0; CS 412/413 Sprig 2005 Itroductio to Copilers 13 Goals of Seatic Aalysis Seatic aalysis = esure that the progra satisfies a set of rules regardig the usage of prograig costructs (iables, objects, epressios, stateets) Eaples of seatic rules: Variables ust be declared before beig used A iable should ot be declared ultiple ties i the sae scope I a assiget stateet, the iable ad the assiged epressio ust have the sae type The coditio of a if-stateet ust have type boolea Soe categories of rules: Seatic rules regardig types Seatic rules regardig scopes CS 412/413 Sprig 2005 Itroductio to Copilers 14 Type Iforatio Type iforatio = describes what kid of values correspod to differet costructs: iables, stateets, epressios, fuctios iables: a; eger epressios: (a+1) == 2 boolea stateets: a = 1.0 ig-po fuctios: pow(, ) Type Checkig Type checkig = set of rules that esure the type cosistecy of differet costructs i the progra Eaples: The type of a iable ust atch the type fro its declaratio The operads of arithetic epressios (+, *, -, /) ust have eger types; the result has eger type The operads of copariso epressios (==,!=) ust have eger or strig types; the result has boolea type CS 412/413 Sprig 2005 Itroductio to Copilers 15 CS 412/413 Sprig 2005 Itroductio to Copilers 16 Type Checkig More eaples: For each assiget stateet, the type of the updated iable ust atch the type of the epressio beig assiged For each call stateet foo(v 1,, v ), the type of each actual uet v i ust atch the type of the correspodig foral uet f i fro the declaratio of fuctio foo The type of the retur value ust atch the retur type fro the declaratio of the fuctio Type checkig: et two lectures. Scope Iforatio Scope iforatio = characterizes the declaratio of idetifiers ad the portios of the progra where it is allowed to use each idetifier Eaple idetifiers: iables, fuctios, objects, labels Leical scope = tetual regio i the progra Stateet block Foral uet list Object body Fuctio or ethod body Module body Whole progra (ultiple odules) Scope of a idetifier: the leical scope i which it is valid CS 412/413 Sprig 2005 Itroductio to Copilers 17 CS 412/413 Sprig 2005 Itroductio to Copilers 18

4 Scope Iforatio Scope of iables i stateet blocks: { a; scope of iable a { b; scope of iable b I C: Scope of file static iables: curret file Scope of eteral iables: whole progra Scope of autoatic iables, foral paraeters, ad fuctio static iables: the fuctio Scope of foral uets of fuctios/ethods: factorial( ) { Scope of labels: void f() { goto l; l: a =1; goto l; Scope Iforatio scope of foral paraeter scope of label l CS 412/413 Sprig 2005 Itroductio to Copilers 19 CS 412/413 Sprig 2005 Itroductio to Copilers 20 Scope Iforatio Scope of object fields ad ethods: class A { private ; public void g() { =1; class B eteds A { public h() { g(); scope of field scope of ethod g CS 412/413 Sprig 2005 Itroductio to Copilers 21 Seatic Rules for Scopes Mai rules regardig scopes: Rule 1: Use a idetifier oly if defied i eclosig scope Rule 2: Do ot declare idetifiers of the sae kid with idetical aes ore tha oce i the sae leical scope Ca declare idetifiers with the sae ae with idetical or overlappig leical scopes if they are of differet kids class X { X; void X( X) { X: for(;;) break X; X( X) { X; goto X; { X; X: X = 1; Not Recoeded! CS 412/413 Sprig 2005 Itroductio to Copilers 22 Sybol Tables Seatic checks refer to properties of idetifiers i the progra -- their scope or type Need a eviroet to store the iforatio about idetifiers = sybol table Each etry i the sybol table cotais the ae of a idetifier additioal iforatio: its kid, its type, if it is costat, NAME foo tp KIND fu TYPE bool bool ATTRIBUTES eter cost cost Scope Iforatio How to capture the scope iforatio i the sybol table? Idea: There is a hierarchy of scopes i the progra Use a siilar hierarchy of sybol tables Oe sybol table for each scope Each sybol table cotais the sybols declared i that leical scope CS 412/413 Sprig 2005 Itroductio to Copilers 23 CS 412/413 Sprig 2005 Itroductio to Copilers 24

5 Eaple Idetifiers With Sae Nae ; void f( ) {, y; { i, j; ; { ; l: ; g( ) { bool t; ; i j fuc f sytab y f g l Global sytab fu void fu lab t fuc g sytab bool The hierarchical structure of sybol tables autoatically solves the proble of resolvig ae collisios (idetifiers with the sae ae ad overlappig scopes) To fid which is the declaratio of a idetifier that is active at a progra po: Start fro the curret scope Go up i the hierarchy util you fid a idetifier with the sae ae CS 412/413 Sprig 2005 Itroductio to Copilers 25 CS 412/413 Sprig 2005 Itroductio to Copilers 26 ; void f( ) {, y; { i, j; = 1; { ; l: = 2; g( ) { bool t; = 3; i j Eaple y Global sytab f fu void g fu l lab = 1 = 2 t bool = 3 ; Catchig Seatic Errors void f( ) {, y; { i, j; = 1; { ; l: i = 2; g( ) { bool t; = 3; i j y f g l fu void fu lab = 1 i = 2 t Error! bool = 3 CS 412/413 Sprig 2005 Itroductio to Copilers 27 CS 412/413 Sprig 2005 Itroductio to Copilers 28 Sybol Table Operatios Two operatios: To build sybol tables, we eed to isert ew idetifiers i the table I the subsequet stages of the copiler we eed to access the iforatio fro the table: use a lookup fuctio Caot build sybol tables durig leical aalysis hierarchy of scopes ecoded i the syta Build the sybol tables: while parsig, usig the seatic actios After the AST is costructed CS 412/413 Sprig 2005 Itroductio to Copilers 29 Array Ipleetatio Siple ipleetatio = array Oe etry per sybol Sca the array for lookup, copare ae at each etry foo tp fu bool bool Disadvatage: table has fied size eed to kow i advace the uber of etries CS 412/413 Sprig 2005 Itroductio to Copilers 30

6 List Ipleetatio Dyaic structure = list Oe cell per etry i the table Ca grow dyaically durig copilatio foo fuc bool Disadvatage: iefficiet for le sybol tables eed to sca half the list o average tp Var bool Hash Table Ipleetatio Efficiet ipleetatio = hash table It is a array of lists (buckets) Uses a hashig fuctio to ap the sybol ae to the correspodig bucket: hashfuc : strig Good hash fuctio = eve distributio i the buckets foo fuc hashfuc( ) = 0, hashfuc( foo ) = 3 tp bool CS 412/413 Sprig 2005 Itroductio to Copilers 31 CS 412/413 Sprig 2005 Itroductio to Copilers 32 Forward Refereces Forward refereces = use a idetifier withi the scope of its declaratio, but before it is declared Ay copiler phase that uses the iforatio fro the sybol table ust be perfored after the table is costructed Caot type-check ad build sybol table at the sae tie Eaple: class A { () { retur (); () { retur 1; CS 412/413 Sprig 2005 Itroductio to Copilers 33 Suary Seatic checks esure the correct usage of iables, objects, epressios, stateets, fuctios, ad labels i the progra Scope seatic checks esure that idetifiers are correctly used withi the scope of their declaratio Type seatic checks esures the type cosistecy of ious costructs i the progra Sybol tables: a data structure for storig iforatio about sybols i the progra Used i seatic aalysis ad subsequet copiler stages Net tie: type-checkig CS 412/413 Sprig 2005 Itroductio to Copilers 34