Modern Compiler Implementation in Java

Semantic Analysis

Chapter 5CMSC 431

Spring, 2003

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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

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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);

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Environments (Cont’d)

Env = 2 + {a -> String}

System.out.println(a);

System.out.println(a);

System.out.println(a);

}

Env

}

Env

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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”

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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 -> }

=

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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 -> }

=

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a

b

d

c

See Appel Program 5.2 (p106)

Update

Undo

’d

Using a Hash Table

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• 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?

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Implementation - Tree

dog 3

bat 1

dog 3

camel 2

emu 42

m1

m2

m1 = { bat |-> 1 , camel |-> 2, dog |-> 3 }

m2 = {m1 + emu |-> 42 }

How could this be implemented?

Want m2 from m1 in O(n)

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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.

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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(){}

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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; }

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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;

}

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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;

}

}

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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

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Symbol Table: example

See Figure 5.7 on page 111

• Primitive types– int -> IntegerType()– Boolean -> BooleanType()

• Other types– Int [] -> IntArrayType()– Class -> IdentifierType(String s)

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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);}

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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

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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();}

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SymbolTable : Variable

class Variable{ public Variable(String id, Type type); public String id(); public Type type()}

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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);

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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()); } }}

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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();

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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;}

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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);

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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; }

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// 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); }}

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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(); }

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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);

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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);

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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.

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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, …….

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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?

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References

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