1. Modern Compiler Implementation in Java Semantic Analysis Chapter 5 CMSC 431 Spring, 2003

2. UMBC 2 Symbol Tables Symbol Tables  Environments –Mapping IDs to Types and Locations –Definitions  Insert in the table –Use  Lookup ID Scope –Where the IDs are “visible” Ex: formal parameters, local variables in MiniJava -> inside the method where defined

3. UMBC 3 Environments A set of bindings ( -> ) Initial Env  0 Class C { int a; int b; int c; Env   =  0 + {a -> int, b -> int, c -> int} public void m() { System.out.println(a+c); int j = a+b; Env   =  1 + {j -> int} String a = “hello”; Env   =  2 + {a -> String} System.out.println(a);

4. UMBC 4 Environments (Cont’d) Env   =  2 + {a -> String} System.out.println(a); } Env   } Env  

5. UMBC 5 Implementing Environments Functional Style –Keep previous env and create new one –When restored, discard new one and back to old Imperative Style –Destructive update the env(symbol tables) –Undo : need “undo stack”

6. UMBC 6 Multiple Symbol Tables : ML-style structure M = sturct structure E = struct val a = 5 end   +   structure N = struct val b = 10 val a = E.a + b end   +     structure D = struct val d = E.a + N.a end End   Initial Env  0   = {a -> int}   = {E ->   }   = {b -> int,a -> int}   = {N ->   }   = {d -> int}   = {D ->   }   =        

7. UMBC 7 Multiple Symbol Tables : Java-style Package M;   class E { static int a = 5; }   class N { static int b = 10 static int a = E.a + b }   class D { static int d = E.a+ N.a }   End   Initial Env  0   = {a -> int}   = {E ->   }   = {b -> int,a -> int}   = {N ->   }   = {d -> int}   = {D ->   }   =        

8. UMBC 8 Implementation – Imperative Symbol Table a  b  d  c  See Appel Program 5.2 (p106) Update Undo   ’  d    Using a Hash Table

9. UMBC 9 Implementation – Functional Symbol Table Efficient Functional Approach  ’  a   would return [  a  If implemented with a Hashtable would have to create O(n) buckets for each scope Is this a good idea?

10. UMBC 10 Implementation - Tree dog 3 bat 1 dog 3 camel 2 emu 42 m1m1 m2m2 m 1 = { bat | -> 1, camel | -> 2, dog | -> 3 } m 2 = { m 1 + emu | -> 42 } How could this be implemented? Want m 2 from m 1 in O(n)

11. UMBC 11 Symbols See Program 5.5 on pg109 Symbol Representation –Comparing symbols for equality is fast. –Extracting an integer hash key is fast. –Comparing two symbols for “greater-than” is fast. See Program 5.6 on P110 for implementation.

12. UMBC 12 Some sample program(I) /** * The Table class is similar to java.util.Dictionary, * except that each key must be a Symbol and there is * a scope mechanism. */ public class Table { private java.util.Dictionary dict = new java.util.Hashtable(); private Symbol top; private Binder marks; public Table(){}

13. UMBC 13 Some sample program(II) /** * Gets the object associated with the specified * symbol in the Table. */ public Object get(Symbol key) { Binder e = (Binder)dict.get(key); if (e==null) return null; else return e.value; } /** * Puts the specified value into the Table, * bound to the specified Symbol. */ public void put(Symbol key, Object value) { dict.put(key, new Binder(value, top, (Binder)dict.get(key))); top = key; }

14. UMBC 14 Some sample program(III) /** * Remembers the current state of the Table. */ public void beginScope() {marks = new Binder(null,top,marks); top=null;} /** * Restores the table to what it was at the most recent * beginScope that has not already been ended. */ public void endScope() { while (top!=null) { Binder e = (Binder)dict.get(top); if (e.tail!=null) dict.put(top,e.tail); else dict.remove(top); top = e.prevtop; } top=marks.prevtop; marks=marks.tail; }

15. UMBC 15 Some sample program(IV) package Symbol; class Binder { Object value; Symbol prevtop; Binder tail; Binder(Object v, Symbol p, Binder t) { value=v; prevtop=p; tail=t; }

16. UMBC 16 Type-Checking in MiniJava Binding for type-checking in MiniJava –Variable and formal parameter Var name type of variable –Method Method name result type, parameters( including position information), local variables –Class Class name variables, method declaration, parent class

17. UMBC 17 Symbol Table: example See Figure 5.7 on page 111 Primitive types –int -> IntegerType() –Boolean -> BooleanType() Other types –Int [] -> IntArrayType() –Class -> IdentifierType(String s)

18. UMBC 18 SymbolTable : Real Story class SymbolTable { public SymbolTable(); public boolean addClass(String id, String parent); public Class getClass(String id); public boolean containsClass(String id); public Type getVarType(Method m, Class c, String id); public Method getMethod(String id, String classScope); public Type getMethodType(String id, String classScope); public boolean compareTypes(Type t1, Type t2); }

19. UMBC 19 Be careful! getVarType(Method m, Class c, String id) –In c.m, find variable id –Local variable in method –Parameter in parameter list –Variable in the class –Variable in the parent class getMethod(), getMethodType() –May be defined in the parent Classes compareTypes() –Primitive types : int, boolean, IntArrayType –Subtype : IdentifierType

20. UMBC 20 SymbolTalbe : Class class Class { public Class(String id, String parent); public String getId(); public Type type(); public boolean addMethod(String id, Type type); public Method getMethod(String id); public boolean containsMethod(String id); public boolean addVar(String id, Type type); public Variable getVar(String id); public boolean containsVar(String id); public String parent(); }

21. UMBC 21 SymbolTable : Variable class Variable{ public Variable(String id, Type type); public String id(); public Type type() }

22. UMBC 22 SymbolTable : Method class Method { public Method(String id, Type type); public String getId(); public Type type(); public boolean addParam(String id, Type type); public Variable getParamAt(int i); public boolean getParam(String id); public boolean containsParam(String id); public boolean addVar(String id, Type type); public Variable getVar(String id); public boolean containsVar(String id);

23. UMBC 23 Type-Checking : Two Phase Build Symbol Table Type-check statements and expressions public class Main { public static void main(String [] args) { try { Program root = new MiniJavaParser(System.in).Goal(); BuildSymbolTableVisitor v1 = new BuildSymbolTableVisitor(); root.accept(v1); root.accept(new TypeCheckVisitor(v1.getSymTab())); } catch (ParseException e) { System.out.println(e.toString()); }

24. UMBC 24 BuildSymbolTableVisitor(); See Program 5.8 on Page 112 public class BuildSymbolTableVisitor extends TypeDepthFirstVisitor { …. private Class currClass; private Method currMethod; …… // Type t; // Identifier i; public Type visit(VarDecl n) { Type t = n.t.accept(this); String id = n.i.toString();

25. UMBC 25 BuildSymbolTableVisitor(); - Cont’d if (currMethod == null){ if (!currClass.addVar(id,t)){ System.out.println(id + "is already defined in " + currClass.getId()); System.exit(-1); } } else { if (!currMethod.addVar(id,t)){ System.out.println(id + "is already defined in " + currClass.getId() + "." + currMethod.getId()); System.exit(-1); } return null; }

26. UMBC 26 BuildSymbolTableVisitor() :TypeVisitor() public Type visit(MainClass n); public Type visit(ClassDeclSimple n); public Type visit(ClassDeclExtends n); public Type visit(VarDecl n); public Type visit(MethodDecl n); public Type visit(Formal n); public Type visit(IntArrayType n); public Type visit(BooleanType n); public Type visit(IntegerType n); public Type visit(IdentifierType n);

27. UMBC 27 TypeCheckVisitor(SymbolTable); See Program 5.9 on page 113 package visitor; import syntaxtree.*; public class TypeCheckVisitor extends DepthFirstVisitor { static Class currClass; static Method currMethod; static SymbolTable symbolTable; public TypeCheckVisitor(SymbolTable s){ symbolTable = s; }

28. UMBC 28 TypeCheckVisitor(SymbolTable); - Cont’d // Identifier i; // Exp e; public void visit(Assign n) { Type t1 = symbolTable.getVarType(currMethod,currClass, n.i.toString()); Type t2 = n.e.accept(new TypeCheckExpVisitor() ); if (symbolTable.compareTypes(t1,t2)==false){ System.out.println("Type error in assignment to " +n.i.toString()); System.exit(0); }

29. UMBC 29 TypeCheckExpVisitor() package visitor; import syntaxtree.*; public class TypeCheckExpVisitor extends TypeDepthFirstVisitor { // Exp e1,e2; public Type visit(Plus n) { if (! (n.e1.accept(this) instanceof IntegerType) ) { System.out.println("Left side of Plus must be of type integer"); System.exit(-1); } if (! (n.e2.accept(this) instanceof IntegerType) ) { System.out.println("Right side of Plus must be of type integer"); System.exit(-1); } return new IntegerType(); }

30. UMBC 30 TypeCheckVisitor : Visitor() public void visit(MainClass n); public void visit(ClassDeclSimple n); public void visit(ClassDeclExtends n); public void visit(MethodDecl n); public void visit(If n); public void visit(While n); public void visit(Print n); public void visit(Assign n); public void visit(ArrayAssign n);

31. UMBC 31 TypeCheckExpVisitor() : TypeVisitor() public Type visit(And n); public Type visit(LessThan n); public Type visit(Plus n); public Type visit(Minus n); public Type visit(Times n); public Type visit(ArrayLookup n); public Type visit(ArrayLength n); public Type visit(Call n); public Type visit(IntegerLiteral n); public Type visit(True n); public Type visit(False n); public Type visit(IdentifierExp n); public Type visit(This n); public Type visit(NewArray n); public Type visit(NewObject n); public Type visit(Not n);

32. UMBC 32 Overloading of Operators, …. When operators are overloaded, the compiler must explicitly generate the code for the type conversion. For an assignment statement, both sides have the same type. When we allow extension of classes, the right hand side is a subtype of lhs.

33. UMBC 33 Method Calls e.m(…) Lookup method in the SymbolTable to get parameter list and result type Find m in class e The parameter types must be matched against the actual arguments. Result type becomes the type of the method call as a whole. Etc, etc, …….

34. UMBC 34 Error Handling For a type error or an undeclared identifier, it should print an error message. And must go on….. Recovery from type errors? –Do as if it were correct. –Not a big deal in the examples we will examine but the reality is bit more complicated. What are some approaches the book mentions?

35. UMBC 35 References Modern Compiler Implementation in Java. 2 nd Ed. A. Appel, Cambridge University Press, Chapter 5