Modern Compiler Implementation in Java Semantic Analysis

1
Modern Compiler Implementation in JavaSemantic
Analysis

• Chapter 5
• CMSC 431
• Spring, 2003

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
• -gt inside the method where defined

3
Environments

• A set of bindings ( -gt )
• Initial Env s0
• Class C
• int a int b int c
• Env s1 s0 a -gt int, b -gt int, c -gt int
• public void m()
• System.out.println(ac)
• int j ab
• Env s2 s1 j -gt int
• String a hello
• Env s3 s2 a -gt String
• System.out.println(a)

4
Environments (Contd)

• Env s3 s2 a -gt String
• System.out.println(a)
• System.out.println(a)
• System.out.println(a)
• Env s1
• Env s0

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
Multiple Symbol Tables ML-style
Initial Env s0 s1 a -gt int s2 E -gt s1
s3 b -gt int,a -gt int s4 N -gt s3 s5
d -gt int s6 D -gt s5 s7 s2 s4 s6

• structure M sturct
• structure E struct
• val a 5
• end s0 s2
• structure N struct
• val b 10
• val a E.a b
• end s0 s2 s4
• structure D struct
• val d E.a N.a
• end
• End s7

7
Multiple Symbol Tables Java-style
Initial Env s0 s1 a -gt int s2 E -gt s1
s3 b -gt int,a -gt int s4 N -gt s3 s5
d -gt int s6 D -gt s5 s7 s2 s4 s6
Package M s7 class E static int a 5
s7 class N static int b 10 static
int a E.a b s7 class D static int
d E.a N.a s7 End s7
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Implementation Imperative Symbol Table
Using a Hash Table
Update
s
s s d -gt t4
Undo
a
t1
d
t4
b
t3
c
t2
See Appel Program 5.2 (p106)
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Implementation Functional Symbol Table

• Efficient Functional Approach
• s s a -gt t
• would return s a -gt t
• 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
m1
m2
How could this be implemented?
m2 m1 emu -gt 42
Want m2 from m1 in O(n)
m1 bat -gt 1 , camel -gt 2, dog -gt 3
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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 (enull) 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(ke
  y)))
• 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)
  topnull
• /
• 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
• topmarks.prevtop
• marksmarks.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)
• valuev prevtopp tailt

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Type-Checking in MiniJava

• Binding for type-checking in MiniJava
• Variable and formal parameter
• Var name lt-gt type of variable
• Method
• Method name lt-gt result type, parameters(
  including position information), local variables
• Class
• Class name lt-gt variables, method declaration,
  parent class

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Symbol Table example

• See Figure 5.7 on page 111
• Primitive types
• int -gt IntegerType()
• Boolean -gt BooleanType()
• Other types
• Int -gt IntArrayType()
• Class -gt 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() - Contd

• 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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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)

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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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TypeCheckVisitor(SymbolTable) - Contd

• // Identifier i
• // Exp e
• public void visit(Assign n)
• Type t1 symbolTable.getVarType(currMethod,curr
  Class,
• 
  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?

35
References

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