InstConstraintVisitor.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License. 
 *
 */ 
package org.apache.bcel.verifier.structurals;


import org.apache.bcel.Constants;
import org.apache.bcel.Repository;
import org.apache.bcel.classfile.Constant;
import org.apache.bcel.classfile.ConstantClass;
import org.apache.bcel.classfile.ConstantDouble;
import org.apache.bcel.classfile.ConstantFieldref;
import org.apache.bcel.classfile.ConstantFloat;
import org.apache.bcel.classfile.ConstantInteger;
import org.apache.bcel.classfile.ConstantLong;
import org.apache.bcel.classfile.ConstantString;
import org.apache.bcel.classfile.Field;
import org.apache.bcel.classfile.JavaClass;
import org.apache.bcel.generic.*;
import org.apache.bcel.verifier.VerificationResult;
import org.apache.bcel.verifier.Verifier;
import org.apache.bcel.verifier.VerifierFactory;
import org.apache.bcel.verifier.exc.AssertionViolatedException;
import org.apache.bcel.verifier.exc.StructuralCodeConstraintException;


/**
 * A Visitor class testing for valid preconditions of JVM instructions.
 * The instance of this class will throw a StructuralCodeConstraintException
 * instance if an instruction is visitXXX()ed which has preconditions that are
 * not satisfied.
 * TODO: Currently, the JVM's behaviour concerning monitors (MONITORENTER,
 * MONITOREXIT) is not modeled in JustIce.
 *
 * @version $Id: InstConstraintVisitor.java 1606478 2014-06-29 12:07:40Z ggregory $
 * @author Enver Haase
 * @see org.apache.bcel.verifier.exc.StructuralCodeConstraintException
 * @see org.apache.bcel.verifier.exc.LinkingConstraintException
 */
public class InstConstraintVisitor extends EmptyVisitor{

    private static final ObjectType GENERIC_ARRAY = ObjectType.getInstance("org.apache.bcel.verifier.structurals.GenericArray");

    /**
     * The constructor. Constructs a new instance of this class.
     */
    public InstConstraintVisitor(){}

    /**
     * The Execution Frame we're working on.
     *
     * @see #setFrame(Frame f)
     * @see #locals()
     * @see #stack()
     */
    private Frame frame = null;

    /**
     * The ConstantPoolGen we're working on.
     * 
     * @see #setConstantPoolGen(ConstantPoolGen cpg)
     */
    private ConstantPoolGen cpg = null;

    /**
     * The MethodGen we're working on.
     * 
     * @see #setMethodGen(MethodGen mg)
     */
    private MethodGen mg = null;

    /**
     * The OperandStack we're working on.
     *
     * @see #setFrame(Frame f)
     */
    private OperandStack stack(){
        return frame.getStack();
    }

    /**
     * The LocalVariables we're working on.
     *
     * @see #setFrame(Frame f)
     */
    private LocalVariables locals(){
        return frame.getLocals();
    }

    /**
   * This method is called by the visitXXX() to notify the acceptor of this InstConstraintVisitor
   * that a constraint violation has occured. This is done by throwing an instance of a
   * StructuralCodeConstraintException.
   * @throws org.apache.bcel.verifier.exc.StructuralCodeConstraintException always.
   */
    private void constraintViolated(Instruction violator, String description){
        String fq_classname = violator.getClass().getName();
        throw new StructuralCodeConstraintException("Instruction "+ fq_classname.substring(fq_classname.lastIndexOf('.')+1) +" constraint violated: " + description);
    }

    /**
     * This returns the single instance of the InstConstraintVisitor class.
     * To operate correctly, other values must have been set before actually
     * using the instance.
     * Use this method for performance reasons.
     *
     * @see #setConstantPoolGen(ConstantPoolGen cpg)
     * @see #setMethodGen(MethodGen mg)
     */
    public void setFrame(Frame f){
        this.frame = f;
        //if (singleInstance.mg == null || singleInstance.cpg == null) throw new AssertionViolatedException("Forgot to set important values first.");
    }

    /**
     * Sets the ConstantPoolGen instance needed for constraint
     * checking prior to execution.
     */    
    public void setConstantPoolGen(ConstantPoolGen cpg){
        this.cpg = cpg;
    }

    /**
     * Sets the MethodGen instance needed for constraint
     * checking prior to execution.
     */
    public void setMethodGen(MethodGen mg){
        this.mg = mg;
    }

    /**
     * Assures index is of type INT.
     * @throws org.apache.bcel.verifier.exc.StructuralCodeConstraintException if the above constraint is not satisfied.
     */
    private void indexOfInt(Instruction o, Type index){
        if (! index.equals(Type.INT)) {
            constraintViolated(o, "The 'index' is not of type int but of type "+index+".");
        }
    }

    /**
     * Assures the ReferenceType r is initialized (or Type.NULL).
     * Formally, this means (!(r instanceof UninitializedObjectType)), because
     * there are no uninitialized array types.
     * @throws org.apache.bcel.verifier.exc.StructuralCodeConstraintException if the above constraint is not satisfied.
     */
    private void referenceTypeIsInitialized(Instruction o, ReferenceType r){
        if (r instanceof UninitializedObjectType){
            constraintViolated(o, "Working on an uninitialized object '"+r+"'.");
        }
    }

    /** Assures value is of type INT. */
    private void valueOfInt(Instruction o, Type value){
        if (! value.equals(Type.INT)) {
            constraintViolated(o, "The 'value' is not of type int but of type "+value+".");
        }
    }

    /**
     * Assures arrayref is of ArrayType or NULL;
     * returns true if and only if arrayref is non-NULL.
     * @throws org.apache.bcel.verifier.exc.StructuralCodeConstraintException if the above constraint is violated.
      */
    private boolean arrayrefOfArrayType(Instruction o, Type arrayref){
        if (! ((arrayref instanceof ArrayType) || arrayref.equals(Type.NULL)) ) {
            constraintViolated(o, "The 'arrayref' does not refer to an array but is of type "+arrayref+".");
        }
        return (arrayref instanceof ArrayType);
    }

    /***************************************************************/
    /* MISC                                                        */
    /***************************************************************/
    /**
     * Ensures the general preconditions of an instruction that accesses the stack.
     * This method is here because BCEL has no such superinterface for the stack
     * accessing instructions; and there are funny unexpected exceptions in the
     * semantices of the superinterfaces and superclasses provided.
     * E.g. SWAP is a StackConsumer, but DUP_X1 is not a StackProducer.
     * Therefore, this method is called by all StackProducer, StackConsumer,
     * and StackInstruction instances via their visitXXX() method.
     * Unfortunately, as the superclasses and superinterfaces overlap, some instructions
     * cause this method to be called two or three times. [TODO: Fix this.]
     *
     * @see #visitStackConsumer(StackConsumer o)
     * @see #visitStackProducer(StackProducer o)
     * @see #visitStackInstruction(StackInstruction o)
     */
    private void _visitStackAccessor(Instruction o){
        int consume = o.consumeStack(cpg); // Stack values are always consumed first; then produced.
        if (consume > stack().slotsUsed()){
            constraintViolated(o, "Cannot consume "+consume+" stack slots: only "+stack().slotsUsed()+" slot(s) left on stack!\nStack:\n"+stack());
        }

        int produce = o.produceStack(cpg) - o.consumeStack(cpg); // Stack values are always consumed first; then produced.
        if ( produce + stack().slotsUsed() > stack().maxStack() ){
            constraintViolated(o, "Cannot produce "+produce+" stack slots: only "+(stack().maxStack()-stack().slotsUsed())+" free stack slot(s) left.\nStack:\n"+stack());
        }
    }

    /***************************************************************/
    /* "generic"visitXXXX methods where XXXX is an interface       */
    /* therefore, we don't know the order of visiting; but we know */
    /* these methods are called before the visitYYYY methods below */
    /***************************************************************/

    /**
     * Assures the generic preconditions of a LoadClass instance.
     * The referenced class is loaded and pass2-verified.
     */
    @Override
    public void visitLoadClass(LoadClass o){
        ObjectType t = o.getLoadClassType(cpg);
        if (t != null){// null means "no class is loaded"
            Verifier v = VerifierFactory.getVerifier(t.getClassName());
            VerificationResult vr = v.doPass2();
            if (vr.getStatus() != VerificationResult.VERIFIED_OK){
                constraintViolated((Instruction) o, "Class '"+o.getLoadClassType(cpg).getClassName()+"' is referenced, but cannot be loaded and resolved: '"+vr+"'.");
            }
        }
    }

    /**
     * Ensures the general preconditions of a StackConsumer instance.
     */
    @Override
    public void visitStackConsumer(StackConsumer o){
        _visitStackAccessor((Instruction) o);
    }

    /**
     * Ensures the general preconditions of a StackProducer instance.
     */
    @Override
    public void visitStackProducer(StackProducer o){
        _visitStackAccessor((Instruction) o);
    }


    /***************************************************************/
    /* "generic" visitYYYY methods where YYYY is a superclass.     */
    /* therefore, we know the order of visiting; we know           */
    /* these methods are called after the visitXXXX methods above. */
    /***************************************************************/
    /**
     * Ensures the general preconditions of a CPInstruction instance.
     */
    @Override
    public void visitCPInstruction(CPInstruction o){
        int idx = o.getIndex();
        if ((idx < 0) || (idx >= cpg.getSize())){
            throw new AssertionViolatedException("Huh?! Constant pool index of instruction '"+o+"' illegal? Pass 3a should have checked this!");
        }
    }

    /**
     * Ensures the general preconditions of a FieldInstruction instance.
     */
     @Override
    public void visitFieldInstruction(FieldInstruction o){
         // visitLoadClass(o) has been called before: Every FieldOrMethod
         // implements LoadClass.
         // visitCPInstruction(o) has been called before.
        // A FieldInstruction may be: GETFIELD, GETSTATIC, PUTFIELD, PUTSTATIC 
            Constant c = cpg.getConstant(o.getIndex());
            if (!(c instanceof ConstantFieldref)){
                constraintViolated(o, "Index '"+o.getIndex()+"' should refer to a CONSTANT_Fieldref_info structure, but refers to '"+c+"'.");
            }
            // the o.getClassType(cpg) type has passed pass 2; see visitLoadClass(o).
            Type t = o.getType(cpg);
            if (t instanceof ObjectType){
                String name = ((ObjectType)t).getClassName();
                Verifier v = VerifierFactory.getVerifier( name );
                VerificationResult vr = v.doPass2();
                if (vr.getStatus() != VerificationResult.VERIFIED_OK){
                    constraintViolated(o, "Class '"+name+"' is referenced, but cannot be loaded and resolved: '"+vr+"'.");
                }
            }
     }

    /**
     * Ensures the general preconditions of an InvokeInstruction instance.
     */
     @Override
    public void visitInvokeInstruction(InvokeInstruction o){
         // visitLoadClass(o) has been called before: Every FieldOrMethod
         // implements LoadClass.
         // visitCPInstruction(o) has been called before.
        //TODO
     }

    /**
     * Ensures the general preconditions of a StackInstruction instance.
     */
    @Override
    public void visitStackInstruction(StackInstruction o){
        _visitStackAccessor(o);
    }

    /**
     * Assures the generic preconditions of a LocalVariableInstruction instance.
     * That is, the index of the local variable must be valid.
     */
    @Override
    public void visitLocalVariableInstruction(LocalVariableInstruction o){
        if (locals().maxLocals() <= (o.getType(cpg).getSize()==1? o.getIndex() : o.getIndex()+1) ){
            constraintViolated(o, "The 'index' is not a valid index into the local variable array.");
        }
    }

    /**
     * Assures the generic preconditions of a LoadInstruction instance.
     */
    @Override
    public void visitLoadInstruction(LoadInstruction o){
        //visitLocalVariableInstruction(o) is called before, because it is more generic.

        // LOAD instructions must not read Type.UNKNOWN
        if (locals().get(o.getIndex()) == Type.UNKNOWN){
            constraintViolated(o, "Read-Access on local variable "+o.getIndex()+" with unknown content.");
        }

        // LOAD instructions, two-slot-values at index N must have Type.UNKNOWN
        // as a symbol for the higher halve at index N+1
        // [suppose some instruction put an int at N+1--- our double at N is defective]
        if (o.getType(cpg).getSize() == 2){
            if (locals().get(o.getIndex()+1) != Type.UNKNOWN){
                constraintViolated(o, "Reading a two-locals value from local variables "+o.getIndex()+" and "+(o.getIndex()+1)+" where the latter one is destroyed.");
            }
        }

        // LOAD instructions must read the correct type.
        if (!(o instanceof ALOAD)){
            if (locals().get(o.getIndex()) != o.getType(cpg) ){
                constraintViolated(o, "Local Variable type and LOADing Instruction type mismatch: Local Variable: '"+locals().get(o.getIndex())+"'; Instruction type: '"+o.getType(cpg)+"'.");
            }
        }
        else{ // we deal with an ALOAD
            if (!(locals().get(o.getIndex()) instanceof ReferenceType)){
                constraintViolated(o, "Local Variable type and LOADing Instruction type mismatch: Local Variable: '"+locals().get(o.getIndex())+"'; Instruction expects a ReferenceType.");
            }
            // ALOAD __IS ALLOWED__ to put uninitialized objects onto the stack!
            //referenceTypeIsInitialized(o, (ReferenceType) (locals().get(o.getIndex())));
        }

        // LOAD instructions must have enough free stack slots.
        if ((stack().maxStack() - stack().slotsUsed()) < o.getType(cpg).getSize()){
            constraintViolated(o, "Not enough free stack slots to load a '"+o.getType(cpg)+"' onto the OperandStack.");
        }
    }

    /**
     * Assures the generic preconditions of a StoreInstruction instance.
     */
    @Override
    public void visitStoreInstruction(StoreInstruction o){
        //visitLocalVariableInstruction(o) is called before, because it is more generic.

        if (stack().isEmpty()){ // Don't bother about 1 or 2 stack slots used. This check is implicitely done below while type checking.
            constraintViolated(o, "Cannot STORE: Stack to read from is empty.");
        }

        if ( (!(o instanceof ASTORE)) ){
            if (! (stack().peek() == o.getType(cpg)) ){// the other xSTORE types are singletons in BCEL.
                constraintViolated(o, "Stack top type and STOREing Instruction type mismatch: Stack top: '"+stack().peek()+"'; Instruction type: '"+o.getType(cpg)+"'.");
            }
        }
        else{ // we deal with ASTORE
            Type stacktop = stack().peek();
            if ( (!(stacktop instanceof ReferenceType)) && (!(stacktop instanceof ReturnaddressType)) ){
                constraintViolated(o, "Stack top type and STOREing Instruction type mismatch: Stack top: '"+stack().peek()+"'; Instruction expects a ReferenceType or a ReturnadressType.");
            }
            //if (stacktop instanceof ReferenceType){
            //    referenceTypeIsInitialized(o, (ReferenceType) stacktop);
            //}
        }
    }

    /**
     * Assures the generic preconditions of a ReturnInstruction instance.
     */
    @Override
    public void visitReturnInstruction(ReturnInstruction o){
        Type method_type = mg.getType();
        if (method_type == Type.BOOLEAN ||
            method_type == Type.BYTE ||
            method_type == Type.SHORT ||
            method_type == Type.CHAR){
                method_type = Type.INT;
            }

        if (o instanceof RETURN){
            if (method_type != Type.VOID){
                constraintViolated(o, "RETURN instruction in non-void method.");
            }
            else{
                return;
            }
        }
        if (o instanceof ARETURN){
            if (stack().peek() == Type.NULL){
                return;
            }
            if (! (stack().peek() instanceof ReferenceType)){
                constraintViolated(o, "Reference type expected on top of stack, but is: '"+stack().peek()+"'.");
            }
            referenceTypeIsInitialized(o, (ReferenceType) (stack().peek()));
            //ReferenceType objectref = (ReferenceType) (stack().peek());
            // TODO: This can only be checked if using Staerk-et-al's "set of object types" instead of a
            // "wider cast object type" created during verification.
            //if (! (objectref.isAssignmentCompatibleWith(mg.getType())) ){
            //    constraintViolated(o, "Type on stack top which should be returned is a '"+stack().peek()+"' which is not assignment compatible with the return type of this method, '"+mg.getType()+"'.");
            //}
        }
        else{
            if (! ( method_type.equals( stack().peek() ))){
                constraintViolated(o, "Current method has return type of '"+mg.getType()+"' expecting a '"+method_type+"' on top of the stack. But stack top is a '"+stack().peek()+"'.");
            }
        }
    }

    /***************************************************************/
    /* "special"visitXXXX methods for one type of instruction each */
    /***************************************************************/

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitAALOAD(AALOAD o){
        Type arrayref = stack().peek(1);
        Type index    = stack().peek(0);

        indexOfInt(o, index);
        if (arrayrefOfArrayType(o, arrayref)){
            if (! (((ArrayType) arrayref).getElementType() instanceof ReferenceType)){
                constraintViolated(o, "The 'arrayref' does not refer to an array with elements of a ReferenceType but to an array of "+((ArrayType) arrayref).getElementType()+".");
            }    
            //referenceTypeIsInitialized(o, (ReferenceType) (((ArrayType) arrayref).getElementType()));
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitAASTORE(AASTORE o){
        try {
        Type arrayref = stack().peek(2);
        Type index    = stack().peek(1);
        Type value    = stack().peek(0);

        indexOfInt(o, index);
        if (!(value instanceof ReferenceType)){
            constraintViolated(o, "The 'value' is not of a ReferenceType but of type "+value+".");
        }else{
            //referenceTypeIsInitialized(o, (ReferenceType) value);
        }
        // Don't bother further with "referenceTypeIsInitialized()", there are no arrays
        // of an uninitialized object type. 
        if (arrayrefOfArrayType(o, arrayref)){
            if (! (((ArrayType) arrayref).getElementType() instanceof ReferenceType)){
                constraintViolated(o, "The 'arrayref' does not refer to an array with elements of a ReferenceType but to an array of "+((ArrayType) arrayref).getElementType()+".");
            }
            if (! ((ReferenceType)value).isAssignmentCompatibleWith(((ArrayType) arrayref).getElementType())){
                constraintViolated(o, "The type of 'value' ('"+value+"') is not assignment compatible to the components of the array 'arrayref' refers to. ('"+((ArrayType) arrayref).getElementType()+"')");
            }
        }
        } catch (ClassNotFoundException e) {
        // FIXME: maybe not the best way to handle this
        throw new AssertionViolatedException("Missing class: " + e, e);
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitACONST_NULL(ACONST_NULL o){
        // Nothing needs to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitALOAD(ALOAD o){
        //visitLoadInstruction(LoadInstruction) is called before.

        // Nothing else needs to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitANEWARRAY(ANEWARRAY o){
        if (!stack().peek().equals(Type.INT)) {
            constraintViolated(o, "The 'count' at the stack top is not of type '"+Type.INT+"' but of type '"+stack().peek()+"'.");
        // The runtime constant pool item at that index must be a symbolic reference to a class,
        // array, or interface type. See Pass 3a.
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitARETURN(ARETURN o){
        if (! (stack().peek() instanceof ReferenceType) ){
            constraintViolated(o, "The 'objectref' at the stack top is not of a ReferenceType but of type '"+stack().peek()+"'.");
        }
        ReferenceType objectref = (ReferenceType) (stack().peek());
        referenceTypeIsInitialized(o, objectref);

        // The check below should already done via visitReturnInstruction(ReturnInstruction), see there.
        // It cannot be done using Staerk-et-al's "set of object types" instead of a
        // "wider cast object type", anyway.
        //if (! objectref.isAssignmentCompatibleWith(mg.getReturnType() )){
        //    constraintViolated(o, "The 'objectref' type "+objectref+" at the stack top is not assignment compatible with the return type '"+mg.getReturnType()+"' of the method.");
        //}
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitARRAYLENGTH(ARRAYLENGTH o){
        Type arrayref = stack().peek(0);
        arrayrefOfArrayType(o, arrayref);
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitASTORE(ASTORE o){
        if (! ( (stack().peek() instanceof ReferenceType) || (stack().peek() instanceof ReturnaddressType) ) ){
            constraintViolated(o, "The 'objectref' is not of a ReferenceType or of ReturnaddressType but of "+stack().peek()+".");
        }
        //if (stack().peek() instanceof ReferenceType){
        //    referenceTypeIsInitialized(o, (ReferenceType) (stack().peek()) );
        //}
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitATHROW(ATHROW o){
        try {
        // It's stated that 'objectref' must be of a ReferenceType --- but since Throwable is
        // not derived from an ArrayType, it follows that 'objectref' must be of an ObjectType or Type.NULL.
        if (! ((stack().peek() instanceof ObjectType) || (stack().peek().equals(Type.NULL))) ){
            constraintViolated(o, "The 'objectref' is not of an (initialized) ObjectType but of type "+stack().peek()+".");
        }

        // NULL is a subclass of every class, so to speak.
        if (stack().peek().equals(Type.NULL)) {
            return;
        }

        ObjectType exc = (ObjectType) (stack().peek());
        ObjectType throwable = (ObjectType) (Type.getType("Ljava/lang/Throwable;"));
        if ( (! (exc.subclassOf(throwable)) ) && (! (exc.equals(throwable))) ){
            constraintViolated(o, "The 'objectref' is not of class Throwable or of a subclass of Throwable, but of '"+stack().peek()+"'.");
        }
        } catch (ClassNotFoundException e) {
        // FIXME: maybe not the best way to handle this
        throw new AssertionViolatedException("Missing class: " + e, e);
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitBALOAD(BALOAD o){
        Type arrayref = stack().peek(1);
        Type index    = stack().peek(0);
        indexOfInt(o, index);
        if (arrayrefOfArrayType(o, arrayref)){
            if (! ( (((ArrayType) arrayref).getElementType().equals(Type.BOOLEAN)) ||
                    (((ArrayType) arrayref).getElementType().equals(Type.BYTE)) ) ){
                constraintViolated(o, "The 'arrayref' does not refer to an array with elements of a Type.BYTE or Type.BOOLEAN but to an array of '"+((ArrayType) arrayref).getElementType()+"'.");
            }
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitBASTORE(BASTORE o){
        Type arrayref = stack().peek(2);
        Type index    = stack().peek(1);
        Type value    = stack().peek(0);

        indexOfInt(o, index);
        valueOfInt(o, value);
        if (arrayrefOfArrayType(o, arrayref)){
            if (! ( (((ArrayType) arrayref).getElementType().equals(Type.BOOLEAN)) ||
                    (((ArrayType) arrayref).getElementType().equals(Type.BYTE)) ) ) {
                constraintViolated(o, "The 'arrayref' does not refer to an array with elements of a Type.BYTE or Type.BOOLEAN but to an array of '"+((ArrayType) arrayref).getElementType()+"'.");
            }
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitBIPUSH(BIPUSH o){
        // Nothing to do...
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitBREAKPOINT(BREAKPOINT o){
        throw new AssertionViolatedException("In this JustIce verification pass there should not occur an illegal instruction such as BREAKPOINT.");
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitCALOAD(CALOAD o){
        Type arrayref = stack().peek(1);
        Type index = stack().peek(0);

        indexOfInt(o, index);
        arrayrefOfArrayType(o, arrayref);
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitCASTORE(CASTORE o){
        Type arrayref = stack().peek(2);
        Type index = stack().peek(1);
        Type value = stack().peek(0);

        indexOfInt(o, index);
        valueOfInt(o, value);
        if (arrayrefOfArrayType(o, arrayref)){
            if (! ((ArrayType) arrayref).getElementType().equals(Type.CHAR) ){
                constraintViolated(o, "The 'arrayref' does not refer to an array with elements of type char but to an array of type "+((ArrayType) arrayref).getElementType()+".");
            }
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitCHECKCAST(CHECKCAST o){
        // The objectref must be of type reference.
        Type objectref = stack().peek(0);
        if (!(objectref instanceof ReferenceType)){
            constraintViolated(o, "The 'objectref' is not of a ReferenceType but of type "+objectref+".");
        }
        //else{
        //    referenceTypeIsInitialized(o, (ReferenceType) objectref);
        //}
        // The unsigned indexbyte1 and indexbyte2 are used to construct an index into the runtime constant pool of the
        // current class (�3.6), where the value of the index is (indexbyte1 << 8) | indexbyte2. The runtime constant
        // pool item at the index must be a symbolic reference to a class, array, or interface type.
        Constant c = cpg.getConstant(o.getIndex());
        if (! (c instanceof ConstantClass)){
            constraintViolated(o, "The Constant at 'index' is not a ConstantClass, but '"+c+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitD2F(D2F o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitD2I(D2I o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitD2L(D2L o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDADD(DADD o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.DOUBLE){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'double', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDALOAD(DALOAD o){
        indexOfInt(o, stack().peek());
        if (stack().peek(1) == Type.NULL){
            return;
        } 
        if (! (stack().peek(1) instanceof ArrayType)){
            constraintViolated(o, "Stack next-to-top must be of type double[] but is '"+stack().peek(1)+"'.");
        }
        Type t = ((ArrayType) (stack().peek(1))).getBasicType();
        if (t != Type.DOUBLE){
            constraintViolated(o, "Stack next-to-top must be of type double[] but is '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDASTORE(DASTORE o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
        indexOfInt(o, stack().peek(1));
        if (stack().peek(2) == Type.NULL){
            return;
        } 
        if (! (stack().peek(2) instanceof ArrayType)){
            constraintViolated(o, "Stack next-to-next-to-top must be of type double[] but is '"+stack().peek(2)+"'.");
        }
        Type t = ((ArrayType) (stack().peek(2))).getBasicType();
        if (t != Type.DOUBLE){
            constraintViolated(o, "Stack next-to-next-to-top must be of type double[] but is '"+stack().peek(2)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDCMPG(DCMPG o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.DOUBLE){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'double', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDCMPL(DCMPL o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.DOUBLE){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'double', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDCONST(DCONST o){
        // There's nothing to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDDIV(DDIV o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.DOUBLE){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'double', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDLOAD(DLOAD o){
        //visitLoadInstruction(LoadInstruction) is called before.

        // Nothing else needs to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDMUL(DMUL o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.DOUBLE){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'double', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDNEG(DNEG o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDREM(DREM o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.DOUBLE){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'double', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDRETURN(DRETURN o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDSTORE(DSTORE o){
        //visitStoreInstruction(StoreInstruction) is called before.

        // Nothing else needs to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDSUB(DSUB o){
        if (stack().peek() != Type.DOUBLE){
            constraintViolated(o, "The value at the stack top is not of type 'double', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.DOUBLE){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'double', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDUP(DUP o){
        if (stack().peek().getSize() != 1){
            constraintViolated(o, "Won't DUP type on stack top '"+stack().peek()+"' because it must occupy exactly one slot, not '"+stack().peek().getSize()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDUP_X1(DUP_X1 o){
        if (stack().peek().getSize() != 1){
            constraintViolated(o, "Type on stack top '"+stack().peek()+"' should occupy exactly one slot, not '"+stack().peek().getSize()+"'.");
        }
        if (stack().peek(1).getSize() != 1){
            constraintViolated(o, "Type on stack next-to-top '"+stack().peek(1)+"' should occupy exactly one slot, not '"+stack().peek(1).getSize()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDUP_X2(DUP_X2 o){
        if (stack().peek().getSize() != 1){
            constraintViolated(o, "Stack top type must be of size 1, but is '"+stack().peek()+"' of size '"+stack().peek().getSize()+"'.");
        }
        if (stack().peek(1).getSize() == 2){
            return; // Form 2, okay.
        }
        //stack().peek(1).getSize == 1.
        if (stack().peek(2).getSize() != 1){
            constraintViolated(o, "If stack top's size is 1 and stack next-to-top's size is 1, stack next-to-next-to-top's size must also be 1, but is: '"+stack().peek(2)+"' of size '"+stack().peek(2).getSize()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDUP2(DUP2 o){
        if (stack().peek().getSize() == 2){
            return; // Form 2, okay.
        }
        //stack().peek().getSize() == 1.
        if (stack().peek(1).getSize() != 1){
            constraintViolated(o, "If stack top's size is 1, then stack next-to-top's size must also be 1. But it is '"+stack().peek(1)+"' of size '"+stack().peek(1).getSize()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDUP2_X1(DUP2_X1 o){
        if (stack().peek().getSize() == 2){
            if (stack().peek(1).getSize() != 1){
                constraintViolated(o, "If stack top's size is 2, then stack next-to-top's size must be 1. But it is '"+stack().peek(1)+"' of size '"+stack().peek(1).getSize()+"'.");
            }
            else{
                return; // Form 2
            }
        }
        else{ // stack top is of size 1
            if ( stack().peek(1).getSize() != 1 ){
                constraintViolated(o, "If stack top's size is 1, then stack next-to-top's size must also be 1. But it is '"+stack().peek(1)+"' of size '"+stack().peek(1).getSize()+"'.");
            }
            if ( stack().peek(2).getSize() != 1 ){
                constraintViolated(o, "If stack top's size is 1, then stack next-to-next-to-top's size must also be 1. But it is '"+stack().peek(2)+"' of size '"+stack().peek(2).getSize()+"'.");
            }
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitDUP2_X2(DUP2_X2 o){

        if (stack().peek(0).getSize() == 2){
             if (stack().peek(1).getSize() == 2){
                return; // Form 4
            }
            // stack top size is 2, next-to-top's size is 1
            if ( stack().peek(2).getSize() != 1 ){
                constraintViolated(o, "If stack top's size is 2 and stack-next-to-top's size is 1, then stack next-to-next-to-top's size must also be 1. But it is '"+stack().peek(2)+"' of size '"+stack().peek(2).getSize()+"'.");
            }
            else{
                return; // Form 2
            }
        }
        else{// stack top is of size 1
            if (stack().peek(1).getSize() == 1){
                if ( stack().peek(2).getSize() == 2 ){
                    return; // Form 3
                }
                if ( stack().peek(3).getSize() == 1){
                    return; // Form 1
                }
            }
        }
        constraintViolated(o, "The operand sizes on the stack do not match any of the four forms of usage of this instruction.");
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitF2D(F2D o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitF2I(F2I o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitF2L(F2L o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFADD(FADD o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.FLOAT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'float', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFALOAD(FALOAD o){
        indexOfInt(o, stack().peek());
        if (stack().peek(1) == Type.NULL){
            return;
        } 
        if (! (stack().peek(1) instanceof ArrayType)){
            constraintViolated(o, "Stack next-to-top must be of type float[] but is '"+stack().peek(1)+"'.");
        }
        Type t = ((ArrayType) (stack().peek(1))).getBasicType();
        if (t != Type.FLOAT){
            constraintViolated(o, "Stack next-to-top must be of type float[] but is '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFASTORE(FASTORE o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
        indexOfInt(o, stack().peek(1));
        if (stack().peek(2) == Type.NULL){
            return;
        } 
        if (! (stack().peek(2) instanceof ArrayType)){
            constraintViolated(o, "Stack next-to-next-to-top must be of type float[] but is '"+stack().peek(2)+"'.");
        }
        Type t = ((ArrayType) (stack().peek(2))).getBasicType();
        if (t != Type.FLOAT){
            constraintViolated(o, "Stack next-to-next-to-top must be of type float[] but is '"+stack().peek(2)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFCMPG(FCMPG o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.FLOAT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'float', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFCMPL(FCMPL o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.FLOAT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'float', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFCONST(FCONST o){
        // nothing to do here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFDIV(FDIV o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.FLOAT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'float', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFLOAD(FLOAD o){
        //visitLoadInstruction(LoadInstruction) is called before.

        // Nothing else needs to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFMUL(FMUL o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.FLOAT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'float', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFNEG(FNEG o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFREM(FREM o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.FLOAT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'float', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFRETURN(FRETURN o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFSTORE(FSTORE o){
        //visitStoreInstruction(StoreInstruction) is called before.

        // Nothing else needs to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitFSUB(FSUB o){
        if (stack().peek() != Type.FLOAT){
            constraintViolated(o, "The value at the stack top is not of type 'float', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.FLOAT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'float', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitGETFIELD(GETFIELD o){
        try {
        Type objectref = stack().peek();
        if (! ( (objectref instanceof ObjectType) || (objectref == Type.NULL) ) ){
            constraintViolated(o, "Stack top should be an object reference that's not an array reference, but is '"+objectref+"'.");
        }

        String field_name = o.getFieldName(cpg);

        JavaClass jc = Repository.lookupClass(o.getClassType(cpg).getClassName());
        Field[] fields = jc.getFields();
        Field f = null;
        for (Field field : fields) {
            if (field.getName().equals(field_name)){
                  Type f_type = Type.getType(field.getSignature());
                  Type o_type = o.getType(cpg);
                    /* TODO: Check if assignment compatibility is sufficient.
                   * What does Sun do?
                   */
                  if (f_type.equals(o_type)){
                        f = field;
                        break;
                    }
            }
        }

        if (f == null){
            JavaClass[] superclasses = jc.getSuperClasses();
            outer:
            for (JavaClass superclass : superclasses) {
                fields = superclass.getFields();
                for (Field field : fields) {
                    if (field.getName().equals(field_name)) {
                        Type f_type = Type.getType(field.getSignature());
                        Type o_type = o.getType(cpg);
                        if (f_type.equals(o_type)) {
                            f = field;
                            if ((f.getAccessFlags() & (Constants.ACC_PUBLIC | Constants.ACC_PROTECTED)) == 0) {
                                f = null;
                            }
                            break outer;
                        }
                    }
                }
            }
            if (f == null) {
                throw new AssertionViolatedException("Field '" + field_name + "' not found in " + jc.getClassName());
            }
        }

        if (f.isProtected()){
            ObjectType classtype = o.getClassType(cpg);
            ObjectType curr = ObjectType.getInstance(mg.getClassName());

            if (    classtype.equals(curr) ||
                        curr.subclassOf(classtype)    ){
                Type t = stack().peek();
                if (t == Type.NULL){
                    return;
                }
                if (! (t instanceof ObjectType) ){
                    constraintViolated(o, "The 'objectref' must refer to an object that's not an array. Found instead: '"+t+"'.");
                }
                ObjectType objreftype = (ObjectType) t;
                if (! ( objreftype.equals(curr) ||
                            objreftype.subclassOf(curr) ) ){
                    //TODO: One day move to Staerk-et-al's "Set of object types" instead of "wider" object types
                    //      created during the verification.
                    //      "Wider" object types don't allow us to check for things like that below.
                    //constraintViolated(o, "The referenced field has the ACC_PROTECTED modifier, and it's a member of the current class or a superclass of the current class. However, the referenced object type '"+stack().peek()+"' is not the current class or a subclass of the current class.");
                }
            } 
        }

        // TODO: Could go into Pass 3a.
        if (f.isStatic()){
            constraintViolated(o, "Referenced field '"+f+"' is static which it shouldn't be.");
        }

        } catch (ClassNotFoundException e) {
        // FIXME: maybe not the best way to handle this
        throw new AssertionViolatedException("Missing class: " + e, e);
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitGETSTATIC(GETSTATIC o){
        // Field must be static: see Pass 3a.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitGOTO(GOTO o){
        // nothing to do here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitGOTO_W(GOTO_W o){
        // nothing to do here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitI2B(I2B o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitI2C(I2C o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitI2D(I2D o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitI2F(I2F o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitI2L(I2L o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitI2S(I2S o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIADD(IADD o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIALOAD(IALOAD o){
        indexOfInt(o, stack().peek());
        if (stack().peek(1) == Type.NULL){
            return;
        } 
        if (! (stack().peek(1) instanceof ArrayType)){
            constraintViolated(o, "Stack next-to-top must be of type int[] but is '"+stack().peek(1)+"'.");
        }
        Type t = ((ArrayType) (stack().peek(1))).getBasicType();
        if (t != Type.INT){
            constraintViolated(o, "Stack next-to-top must be of type int[] but is '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIAND(IAND o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIASTORE(IASTORE o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        indexOfInt(o, stack().peek(1));
        if (stack().peek(2) == Type.NULL){
            return;
        } 
        if (! (stack().peek(2) instanceof ArrayType)){
            constraintViolated(o, "Stack next-to-next-to-top must be of type int[] but is '"+stack().peek(2)+"'.");
        }
        Type t = ((ArrayType) (stack().peek(2))).getBasicType();
        if (t != Type.INT){
            constraintViolated(o, "Stack next-to-next-to-top must be of type int[] but is '"+stack().peek(2)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitICONST(ICONST o){
        //nothing to do here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIDIV(IDIV o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIF_ACMPEQ(IF_ACMPEQ o){
        if (!(stack().peek() instanceof ReferenceType)){
            constraintViolated(o, "The value at the stack top is not of a ReferenceType, but of type '"+stack().peek()+"'.");
        }
        //referenceTypeIsInitialized(o, (ReferenceType) (stack().peek()) );

        if (!(stack().peek(1) instanceof ReferenceType)){
            constraintViolated(o, "The value at the stack next-to-top is not of a ReferenceType, but of type '"+stack().peek(1)+"'.");
        }
        //referenceTypeIsInitialized(o, (ReferenceType) (stack().peek(1)) );

    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIF_ACMPNE(IF_ACMPNE o){
        if (!(stack().peek() instanceof ReferenceType)){
            constraintViolated(o, "The value at the stack top is not of a ReferenceType, but of type '"+stack().peek()+"'.");
            //referenceTypeIsInitialized(o, (ReferenceType) (stack().peek()) );
        }
        if (!(stack().peek(1) instanceof ReferenceType)){
            constraintViolated(o, "The value at the stack next-to-top is not of a ReferenceType, but of type '"+stack().peek(1)+"'.");
            //referenceTypeIsInitialized(o, (ReferenceType) (stack().peek(1)) );
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIF_ICMPEQ(IF_ICMPEQ o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIF_ICMPGE(IF_ICMPGE o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIF_ICMPGT(IF_ICMPGT o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIF_ICMPLE(IF_ICMPLE o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIF_ICMPLT(IF_ICMPLT o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIF_ICMPNE(IF_ICMPNE o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIFEQ(IFEQ o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIFGE(IFGE o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIFGT(IFGT o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIFLE(IFLE o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIFLT(IFLT o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIFNE(IFNE o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIFNONNULL(IFNONNULL o){
        if (!(stack().peek() instanceof ReferenceType)){
            constraintViolated(o, "The value at the stack top is not of a ReferenceType, but of type '"+stack().peek()+"'.");
        }
        referenceTypeIsInitialized(o, (ReferenceType) (stack().peek()) );    
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIFNULL(IFNULL o){
        if (!(stack().peek() instanceof ReferenceType)){
            constraintViolated(o, "The value at the stack top is not of a ReferenceType, but of type '"+stack().peek()+"'.");
        }
        referenceTypeIsInitialized(o, (ReferenceType) (stack().peek()) );    
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIINC(IINC o){
        // Mhhh. In BCEL, at this time "IINC" is not a LocalVariableInstruction.
        if (locals().maxLocals() <= (o.getType(cpg).getSize()==1? o.getIndex() : o.getIndex()+1) ){
            constraintViolated(o, "The 'index' is not a valid index into the local variable array.");
        }

        indexOfInt(o, locals().get(o.getIndex()));
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitILOAD(ILOAD o){
        // All done by visitLocalVariableInstruction(), visitLoadInstruction()
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIMPDEP1(IMPDEP1 o){
        throw new AssertionViolatedException("In this JustIce verification pass there should not occur an illegal instruction such as IMPDEP1.");
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIMPDEP2(IMPDEP2 o){
        throw new AssertionViolatedException("In this JustIce verification pass there should not occur an illegal instruction such as IMPDEP2.");
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIMUL(IMUL o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitINEG(INEG o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitINSTANCEOF(INSTANCEOF o){
        // The objectref must be of type reference.
        Type objectref = stack().peek(0);
        if (!(objectref instanceof ReferenceType)){
            constraintViolated(o, "The 'objectref' is not of a ReferenceType but of type "+objectref+".");
        }
        //else{
        //    referenceTypeIsInitialized(o, (ReferenceType) objectref);
        //}
        // The unsigned indexbyte1 and indexbyte2 are used to construct an index into the runtime constant pool of the
        // current class (�3.6), where the value of the index is (indexbyte1 << 8) | indexbyte2. The runtime constant
        // pool item at the index must be a symbolic reference to a class, array, or interface type.
        Constant c = cpg.getConstant(o.getIndex());
        if (! (c instanceof ConstantClass)){
            constraintViolated(o, "The Constant at 'index' is not a ConstantClass, but '"+c+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitINVOKEINTERFACE(INVOKEINTERFACE o){
        // Method is not native, otherwise pass 3 would not happen.

        int count = o.getCount();
        if (count == 0){
            constraintViolated(o, "The 'count' argument must not be 0.");
        }
        // It is a ConstantInterfaceMethodref, Pass 3a made it sure.
        // TODO: Do we want to do anything with it?
        //ConstantInterfaceMethodref cimr = (ConstantInterfaceMethodref) (cpg.getConstant(o.getIndex()));

        // the o.getClassType(cpg) type has passed pass 2; see visitLoadClass(o).

        Type t = o.getType(cpg);
        if (t instanceof ObjectType){
            String name = ((ObjectType)t).getClassName();
            Verifier v = VerifierFactory.getVerifier( name );
            VerificationResult vr = v.doPass2();
            if (vr.getStatus() != VerificationResult.VERIFIED_OK){
                constraintViolated(o, "Class '"+name+"' is referenced, but cannot be loaded and resolved: '"+vr+"'.");
            }
        }


        Type[] argtypes = o.getArgumentTypes(cpg);
        int nargs = argtypes.length;

        for (int i=nargs-1; i>=0; i--){
            Type fromStack = stack().peek( (nargs-1) - i );    // 0 to nargs-1
            Type fromDesc = argtypes[i];
            if (fromDesc == Type.BOOLEAN ||
                    fromDesc == Type.BYTE ||
                    fromDesc == Type.CHAR ||
                    fromDesc == Type.SHORT){
                fromDesc = Type.INT;
            }
            if (! fromStack.equals(fromDesc)){
                if (fromStack instanceof ReferenceType && fromDesc instanceof ReferenceType){
                    ReferenceType rFromStack = (ReferenceType) fromStack;
                    //ReferenceType rFromDesc = (ReferenceType) fromDesc;
                    // TODO: This can only be checked when using Staerk-et-al's "set of object types"
                    // instead of a "wider cast object type" created during verification.
                    //if ( ! rFromStack.isAssignmentCompatibleWith(rFromDesc) ){
                    //    constraintViolated(o, "Expecting a '"+fromDesc+"' but found a '"+fromStack+"' on the stack (which is not assignment compatible).");
                    //}
                    referenceTypeIsInitialized(o, rFromStack);
                }
                else{
                    constraintViolated(o, "Expecting a '"+fromDesc+"' but found a '"+fromStack+"' on the stack.");
                }
            }
        }

        Type objref = stack().peek(nargs);
        if (objref == Type.NULL){
            return;
        }
        if (! (objref instanceof ReferenceType) ){
            constraintViolated(o, "Expecting a reference type as 'objectref' on the stack, not a '"+objref+"'.");
        }
        referenceTypeIsInitialized(o, (ReferenceType) objref);
        if (!(objref instanceof ObjectType)){
            if (!(objref instanceof ArrayType)){
                constraintViolated(o, "Expecting an ObjectType as 'objectref' on the stack, not a '"+objref+"'."); // could be a ReturnaddressType
            }
            else{
                objref = GENERIC_ARRAY;
            }
        }

        // String objref_classname = ((ObjectType) objref).getClassName();
        // String theInterface = o.getClassName(cpg);
        // TODO: This can only be checked if we're using Staerk-et-al's "set of object types"
        //       instead of "wider cast object types" generated during verification.
        //if ( ! Repository.implementationOf(objref_classname, theInterface) ){
        //    constraintViolated(o, "The 'objref' item '"+objref+"' does not implement '"+theInterface+"' as expected.");
        //}    

        int counted_count = 1; // 1 for the objectref
        for (int i=0; i<nargs; i++){
            counted_count += argtypes[i].getSize();
        }
        if (count != counted_count){
            constraintViolated(o, "The 'count' argument should probably read '"+counted_count+"' but is '"+count+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitINVOKESPECIAL(INVOKESPECIAL o){
        try {
        // Don't init an object twice.
        if ( (o.getMethodName(cpg).equals(Constants.CONSTRUCTOR_NAME)) && (!(stack().peek(o.getArgumentTypes(cpg).length) instanceof UninitializedObjectType)) ){
            constraintViolated(o, "Possibly initializing object twice. A valid instruction sequence must not have an uninitialized object on the operand stack or in a local variable during a backwards branch, or in a local variable in code protected by an exception handler. Please see The Java Virtual Machine Specification, Second Edition, 4.9.4 (pages 147 and 148) for details.");
        }

        // the o.getClassType(cpg) type has passed pass 2; see visitLoadClass(o).

        Type t = o.getType(cpg);
        if (t instanceof ObjectType){
            String name = ((ObjectType)t).getClassName();
            Verifier v = VerifierFactory.getVerifier( name );
            VerificationResult vr = v.doPass2();
            if (vr.getStatus() != VerificationResult.VERIFIED_OK){
                constraintViolated(o, "Class '"+name+"' is referenced, but cannot be loaded and resolved: '"+vr+"'.");
            }
        }


        Type[] argtypes = o.getArgumentTypes(cpg);
        int nargs = argtypes.length;

        for (int i=nargs-1; i>=0; i--){
            Type fromStack = stack().peek( (nargs-1) - i );    // 0 to nargs-1
            Type fromDesc = argtypes[i];
            if (fromDesc == Type.BOOLEAN ||
                    fromDesc == Type.BYTE ||
                    fromDesc == Type.CHAR ||
                    fromDesc == Type.SHORT){
                fromDesc = Type.INT;
            }
            if (! fromStack.equals(fromDesc)){
                if (fromStack instanceof ReferenceType && fromDesc instanceof ReferenceType){
                    ReferenceType rFromStack = (ReferenceType) fromStack;
                    ReferenceType rFromDesc = (ReferenceType) fromDesc;
                    // TODO: This can only be checked using Staerk-et-al's "set of object types", not
                    // using a "wider cast object type".
                    if ( ! rFromStack.isAssignmentCompatibleWith(rFromDesc) ){
                        constraintViolated(o, "Expecting a '"+fromDesc+"' but found a '"+fromStack+"' on the stack (which is not assignment compatible).");
                    }
                    referenceTypeIsInitialized(o, rFromStack);
                }
                else{
                    constraintViolated(o, "Expecting a '"+fromDesc+"' but found a '"+fromStack+"' on the stack.");
                }
            }
        }

        Type objref = stack().peek(nargs);
        if (objref == Type.NULL){
            return;
        }
        if (! (objref instanceof ReferenceType) ){
            constraintViolated(o, "Expecting a reference type as 'objectref' on the stack, not a '"+objref+"'.");
        }
        String objref_classname = null;
        if ( !(o.getMethodName(cpg).equals(Constants.CONSTRUCTOR_NAME))){
            referenceTypeIsInitialized(o, (ReferenceType) objref);
            if (!(objref instanceof ObjectType)){
                if (!(objref instanceof ArrayType)){
                    constraintViolated(o, "Expecting an ObjectType as 'objectref' on the stack, not a '"+objref+"'."); // could be a ReturnaddressType
                }
                else{
                    objref = GENERIC_ARRAY;
                }
            }

            objref_classname = ((ObjectType) objref).getClassName();        
        }
        else{
            if (!(objref instanceof UninitializedObjectType)){
                constraintViolated(o, "Expecting an UninitializedObjectType as 'objectref' on the stack, not a '"+objref+"'. Otherwise, you couldn't invoke a method since an array has no methods (not to speak of a return address).");
            }
            objref_classname = ((UninitializedObjectType) objref).getInitialized().getClassName();
        }


        String theClass = o.getClassName(cpg);
        if ( ! Repository.instanceOf(objref_classname, theClass) ){
            constraintViolated(o, "The 'objref' item '"+objref+"' does not implement '"+theClass+"' as expected.");
        }    

        } catch (ClassNotFoundException e) {
        // FIXME: maybe not the best way to handle this
        throw new AssertionViolatedException("Missing class: " + e, e);
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitINVOKESTATIC(INVOKESTATIC o){
        try {
        // Method is not native, otherwise pass 3 would not happen.

        Type t = o.getType(cpg);
        if (t instanceof ObjectType){
            String name = ((ObjectType)t).getClassName();
            Verifier v = VerifierFactory.getVerifier( name );
            VerificationResult vr = v.doPass2();
            if (vr.getStatus() != VerificationResult.VERIFIED_OK){
                constraintViolated(o, "Class '"+name+"' is referenced, but cannot be loaded and resolved: '"+vr+"'.");
            }
        }

        Type[] argtypes = o.getArgumentTypes(cpg);
        int nargs = argtypes.length;

        for (int i=nargs-1; i>=0; i--){
            Type fromStack = stack().peek( (nargs-1) - i );    // 0 to nargs-1
            Type fromDesc = argtypes[i];
            if (fromDesc == Type.BOOLEAN ||
                    fromDesc == Type.BYTE ||
                    fromDesc == Type.CHAR ||
                    fromDesc == Type.SHORT){
                fromDesc = Type.INT;
            }
            if (! fromStack.equals(fromDesc)){
                if (fromStack instanceof ReferenceType && fromDesc instanceof ReferenceType){
                    ReferenceType rFromStack = (ReferenceType) fromStack;
                    ReferenceType rFromDesc = (ReferenceType) fromDesc;
                    // TODO: This check can possibly only be done using Staerk-et-al's "set of object types"
                    // instead of a "wider cast object type" created during verification.
                    if ( ! rFromStack.isAssignmentCompatibleWith(rFromDesc) ){
                        constraintViolated(o, "Expecting a '"+fromDesc+"' but found a '"+fromStack+"' on the stack (which is not assignment compatible).");
                    }
                    referenceTypeIsInitialized(o, rFromStack);
                }
                else{
                    constraintViolated(o, "Expecting a '"+fromDesc+"' but found a '"+fromStack+"' on the stack.");
                }
            }
        }
        } catch (ClassNotFoundException e) {
        // FIXME: maybe not the best way to handle this
        throw new AssertionViolatedException("Missing class: " + e, e);
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitINVOKEVIRTUAL(INVOKEVIRTUAL o){
        try {
        // the o.getClassType(cpg) type has passed pass 2; see visitLoadClass(o).

        Type t = o.getType(cpg);
        if (t instanceof ObjectType){
            String name = ((ObjectType)t).getClassName();
            Verifier v = VerifierFactory.getVerifier( name );
            VerificationResult vr = v.doPass2();
            if (vr.getStatus() != VerificationResult.VERIFIED_OK){
                constraintViolated(o, "Class '"+name+"' is referenced, but cannot be loaded and resolved: '"+vr+"'.");
            }
        }


        Type[] argtypes = o.getArgumentTypes(cpg);
        int nargs = argtypes.length;

        for (int i=nargs-1; i>=0; i--){
            Type fromStack = stack().peek( (nargs-1) - i );    // 0 to nargs-1
            Type fromDesc = argtypes[i];
            if (fromDesc == Type.BOOLEAN ||
                    fromDesc == Type.BYTE ||
                    fromDesc == Type.CHAR ||
                    fromDesc == Type.SHORT){
                fromDesc = Type.INT;
            }
            if (! fromStack.equals(fromDesc)){
                if (fromStack instanceof ReferenceType && fromDesc instanceof ReferenceType){
                    ReferenceType rFromStack = (ReferenceType) fromStack;
                    ReferenceType rFromDesc = (ReferenceType) fromDesc;
                    // TODO: This can possibly only be checked when using Staerk-et-al's "set of object types" instead
                    // of a single "wider cast object type" created during verification.
                    if ( ! rFromStack.isAssignmentCompatibleWith(rFromDesc) ){
                        constraintViolated(o, "Expecting a '"+fromDesc+"' but found a '"+fromStack+"' on the stack (which is not assignment compatible).");
                    }
                    referenceTypeIsInitialized(o, rFromStack);
                }
                else{
                    constraintViolated(o, "Expecting a '"+fromDesc+"' but found a '"+fromStack+"' on the stack.");
                }
            }
        }

        Type objref = stack().peek(nargs);
        if (objref == Type.NULL){
            return;
        }
        if (! (objref instanceof ReferenceType) ){
            constraintViolated(o, "Expecting a reference type as 'objectref' on the stack, not a '"+objref+"'.");
        }
        referenceTypeIsInitialized(o, (ReferenceType) objref);
        if (!(objref instanceof ObjectType)){
            if (!(objref instanceof ArrayType)){
                constraintViolated(o, "Expecting an ObjectType as 'objectref' on the stack, not a '"+objref+"'."); // could be a ReturnaddressType
            }
            else{
                objref = GENERIC_ARRAY;
            }
        }

        String objref_classname = ((ObjectType) objref).getClassName();

        String theClass = o.getClassName(cpg);

        if ( ! Repository.instanceOf(objref_classname, theClass) ){
            constraintViolated(o, "The 'objref' item '"+objref+"' does not implement '"+theClass+"' as expected.");
        }    
        } catch (ClassNotFoundException e) {
        // FIXME: maybe not the best way to handle this
        throw new AssertionViolatedException("Missing class: " + e, e);
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIOR(IOR o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIREM(IREM o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIRETURN(IRETURN o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitISHL(ISHL o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitISHR(ISHR o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitISTORE(ISTORE o){
        //visitStoreInstruction(StoreInstruction) is called before.

        // Nothing else needs to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitISUB(ISUB o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIUSHR(IUSHR o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitIXOR(IXOR o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.INT){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'int', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitJSR(JSR o){
        // nothing to do here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitJSR_W(JSR_W o){
        // nothing to do here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitL2D(L2D o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitL2F(L2F o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitL2I(L2I o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLADD(LADD o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLALOAD(LALOAD o){
        indexOfInt(o, stack().peek());
        if (stack().peek(1) == Type.NULL){
            return;
        } 
        if (! (stack().peek(1) instanceof ArrayType)){
            constraintViolated(o, "Stack next-to-top must be of type long[] but is '"+stack().peek(1)+"'.");
        }
        Type t = ((ArrayType) (stack().peek(1))).getBasicType();
        if (t != Type.LONG){
            constraintViolated(o, "Stack next-to-top must be of type long[] but is '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLAND(LAND o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLASTORE(LASTORE o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
        indexOfInt(o, stack().peek(1));
        if (stack().peek(2) == Type.NULL){
            return;
        } 
        if (! (stack().peek(2) instanceof ArrayType)){
            constraintViolated(o, "Stack next-to-next-to-top must be of type long[] but is '"+stack().peek(2)+"'.");
        }
        Type t = ((ArrayType) (stack().peek(2))).getBasicType();
        if (t != Type.LONG){
            constraintViolated(o, "Stack next-to-next-to-top must be of type long[] but is '"+stack().peek(2)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLCMP(LCMP o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLCONST(LCONST o){
        // Nothing to do here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLDC(LDC o){
        // visitCPInstruction is called first.

        Constant c = cpg.getConstant(o.getIndex());
        if     (!    (    ( c instanceof ConstantInteger) ||
                    ( c instanceof ConstantFloat    )    ||
                    ( c instanceof ConstantString    )    ||
                    ( c instanceof ConstantClass    ) )    ){
            constraintViolated(o, "Referenced constant should be a CONSTANT_Integer, a CONSTANT_Float, a CONSTANT_String or a CONSTANT_Class, but is '"+c+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    public void visitLDC_W(LDC_W o){
        // visitCPInstruction is called first.

        Constant c = cpg.getConstant(o.getIndex());
        if     (!    (    ( c instanceof ConstantInteger) ||
                    ( c instanceof ConstantFloat    )    ||
                    ( c instanceof ConstantString    )    ||
                    ( c instanceof ConstantClass    ) )    ){
            constraintViolated(o, "Referenced constant should be a CONSTANT_Integer, a CONSTANT_Float, a CONSTANT_String or a CONSTANT_Class, but is '"+c+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLDC2_W(LDC2_W o){
        // visitCPInstruction is called first.

        Constant c = cpg.getConstant(o.getIndex());
        if     (!    (    ( c instanceof ConstantLong) ||
                            ( c instanceof ConstantDouble )    )    ){
            constraintViolated(o, "Referenced constant should be a CONSTANT_Integer, a CONSTANT_Float or a CONSTANT_String, but is '"+c+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLDIV(LDIV o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLLOAD(LLOAD o){
        //visitLoadInstruction(LoadInstruction) is called before.

        // Nothing else needs to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLMUL(LMUL o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLNEG(LNEG o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLOOKUPSWITCH(LOOKUPSWITCH o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        // See also pass 3a.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLOR(LOR o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLREM(LREM o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLRETURN(LRETURN o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLSHL(LSHL o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLSHR(LSHR o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLSTORE(LSTORE o){
        //visitStoreInstruction(StoreInstruction) is called before.

        // Nothing else needs to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLSUB(LSUB o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLUSHR(LUSHR o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitLXOR(LXOR o){
        if (stack().peek() != Type.LONG){
            constraintViolated(o, "The value at the stack top is not of type 'long', but of type '"+stack().peek()+"'.");
        }
        if (stack().peek(1) != Type.LONG){
            constraintViolated(o, "The value at the stack next-to-top is not of type 'long', but of type '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitMONITORENTER(MONITORENTER o){
        if (! ((stack().peek()) instanceof ReferenceType)){
            constraintViolated(o, "The stack top should be of a ReferenceType, but is '"+stack().peek()+"'.");
        }
        //referenceTypeIsInitialized(o, (ReferenceType) (stack().peek()) );
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitMONITOREXIT(MONITOREXIT o){
        if (! ((stack().peek()) instanceof ReferenceType)){
            constraintViolated(o, "The stack top should be of a ReferenceType, but is '"+stack().peek()+"'.");
        }
        //referenceTypeIsInitialized(o, (ReferenceType) (stack().peek()) );
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitMULTIANEWARRAY(MULTIANEWARRAY o){
        int dimensions = o.getDimensions();
        // Dimensions argument is okay: see Pass 3a.
        for (int i=0; i<dimensions; i++){
            if (stack().peek(i) != Type.INT){
                constraintViolated(o, "The '"+dimensions+"' upper stack types should be 'int' but aren't.");
            }
        }
        // The runtime constant pool item at that index must be a symbolic reference to a class,
        // array, or interface type. See Pass 3a.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitNEW(NEW o){
        //visitCPInstruction(CPInstruction) has been called before.
        //visitLoadClass(LoadClass) has been called before.

        Type t = o.getType(cpg);
        if (! (t instanceof ReferenceType)){
            throw new AssertionViolatedException("NEW.getType() returning a non-reference type?!");
        }
        if (! (t instanceof ObjectType)){
            constraintViolated(o, "Expecting a class type (ObjectType) to work on. Found: '"+t+"'.");
        }
        ObjectType obj = (ObjectType) t;

        //e.g.: Don't instantiate interfaces
        if (! obj.referencesClass()){
            constraintViolated(o, "Expecting a class type (ObjectType) to work on. Found: '"+obj+"'.");
        }        
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitNEWARRAY(NEWARRAY o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitNOP(NOP o){
        // nothing is to be done here.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitPOP(POP o){
        if (stack().peek().getSize() != 1){
            constraintViolated(o, "Stack top size should be 1 but stack top is '"+stack().peek()+"' of size '"+stack().peek().getSize()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitPOP2(POP2 o){
        if (stack().peek().getSize() != 2){
            constraintViolated(o, "Stack top size should be 2 but stack top is '"+stack().peek()+"' of size '"+stack().peek().getSize()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitPUTFIELD(PUTFIELD o){
        try {

        Type objectref = stack().peek(1);
        if (! ( (objectref instanceof ObjectType) || (objectref == Type.NULL) ) ){
            constraintViolated(o, "Stack next-to-top should be an object reference that's not an array reference, but is '"+objectref+"'.");
        }

        String field_name = o.getFieldName(cpg);

        JavaClass jc = Repository.lookupClass(o.getClassType(cpg).getClassName());
        Field[] fields = jc.getFields();
        Field f = null;
        for (Field field : fields) {
            if (field.getName().equals(field_name)){
                  Type f_type = Type.getType(field.getSignature());
                  Type o_type = o.getType(cpg);
                    /* TODO: Check if assignment compatibility is sufficient.
                   * What does Sun do?
                   */
                  if (f_type.equals(o_type)){
                        f = field;
                        break;
                    }
            }
        }
        if (f == null){
            throw new AssertionViolatedException("Field '" + field_name + "' not found in " + jc.getClassName());
        }

        Type value = stack().peek();
        Type t = Type.getType(f.getSignature());
        Type shouldbe = t;
        if (shouldbe == Type.BOOLEAN ||
                shouldbe == Type.BYTE ||
                shouldbe == Type.CHAR ||
                shouldbe == Type.SHORT){
            shouldbe = Type.INT;
        }
        if (t instanceof ReferenceType){
            ReferenceType rvalue = null;
            if (value instanceof ReferenceType){
                rvalue = (ReferenceType) value;
                referenceTypeIsInitialized(o, rvalue);
            }
            else{
                constraintViolated(o, "The stack top type '"+value+"' is not of a reference type as expected.");
            }
            // TODO: This can possibly only be checked using Staerk-et-al's "set-of-object types", not
            // using "wider cast object types" created during verification.
            // Comment it out if you encounter problems. See also the analogon at visitPUTSTATIC.
            if (!(rvalue.isAssignmentCompatibleWith(shouldbe))){
                constraintViolated(o, "The stack top type '"+value+"' is not assignment compatible with '"+shouldbe+"'.");
            }
        }
        else{
            if (shouldbe != value){
                constraintViolated(o, "The stack top type '"+value+"' is not of type '"+shouldbe+"' as expected.");
            }
        }

        if (f.isProtected()){
            ObjectType classtype = o.getClassType(cpg);
            ObjectType curr = ObjectType.getInstance(mg.getClassName());

            if (    classtype.equals(curr) ||
                        curr.subclassOf(classtype)    ){
                Type tp = stack().peek(1);
                if (tp == Type.NULL){
                    return;
                }
                if (! (tp instanceof ObjectType) ){
                    constraintViolated(o, "The 'objectref' must refer to an object that's not an array. Found instead: '"+tp+"'.");
                }
                ObjectType objreftype = (ObjectType) tp;
                if (! ( objreftype.equals(curr) ||
                            objreftype.subclassOf(curr) ) ){
                    constraintViolated(o, "The referenced field has the ACC_PROTECTED modifier, and it's a member of the current class or a superclass of the current class. However, the referenced object type '"+stack().peek()+"' is not the current class or a subclass of the current class.");
                }
            } 
        }

        // TODO: Could go into Pass 3a.
        if (f.isStatic()){
            constraintViolated(o, "Referenced field '"+f+"' is static which it shouldn't be.");
        }

        } catch (ClassNotFoundException e) {
        // FIXME: maybe not the best way to handle this
        throw new AssertionViolatedException("Missing class: " + e, e);
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitPUTSTATIC(PUTSTATIC o){
        try {
        String field_name = o.getFieldName(cpg);
        JavaClass jc = Repository.lookupClass(o.getClassType(cpg).getClassName());
        Field[] fields = jc.getFields();
        Field f = null;
        for (Field field : fields) {
            if (field.getName().equals(field_name)){
                    Type f_type = Type.getType(field.getSignature());
                  Type o_type = o.getType(cpg);
                    /* TODO: Check if assignment compatibility is sufficient.
                   * What does Sun do?
                   */
                  if (f_type.equals(o_type)){
                        f = field;
                        break;
                    }
            }
        }
        if (f == null){
            throw new AssertionViolatedException("Field '" + field_name + "' not found in " + jc.getClassName());
        }
        Type value = stack().peek();
        Type t = Type.getType(f.getSignature());
        Type shouldbe = t;
        if (shouldbe == Type.BOOLEAN ||
                shouldbe == Type.BYTE ||
                shouldbe == Type.CHAR ||
                shouldbe == Type.SHORT){
            shouldbe = Type.INT;
        }
        if (t instanceof ReferenceType){
            ReferenceType rvalue = null;
            if (value instanceof ReferenceType){
                rvalue = (ReferenceType) value;
                referenceTypeIsInitialized(o, rvalue);
            }
            else{
                constraintViolated(o, "The stack top type '"+value+"' is not of a reference type as expected.");
            }
            // TODO: This can possibly only be checked using Staerk-et-al's "set-of-object types", not
            // using "wider cast object types" created during verification.
            // Comment it out if you encounter problems. See also the analogon at visitPUTFIELD.
            if (!(rvalue.isAssignmentCompatibleWith(shouldbe))){
                constraintViolated(o, "The stack top type '"+value+"' is not assignment compatible with '"+shouldbe+"'.");
            }
        }
        else{
            if (shouldbe != value){
                constraintViolated(o, "The stack top type '"+value+"' is not of type '"+shouldbe+"' as expected.");
            }
        }
        // TODO: Interface fields may be assigned to only once. (Hard to implement in
        //       JustIce's execution model). This may only happen in <clinit>, see Pass 3a.

        } catch (ClassNotFoundException e) {
        // FIXME: maybe not the best way to handle this
        throw new AssertionViolatedException("Missing class: " + e, e);
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitRET(RET o){
        if (! (locals().get(o.getIndex()) instanceof ReturnaddressType)){
            constraintViolated(o, "Expecting a ReturnaddressType in local variable "+o.getIndex()+".");
        }
        if (locals().get(o.getIndex()) == ReturnaddressType.NO_TARGET){
            throw new AssertionViolatedException("Oops: RET expecting a target!");
        }
        // Other constraints such as non-allowed overlapping subroutines are enforced
        // while building the Subroutines data structure.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitRETURN(RETURN o){
        if (mg.getName().equals(Constants.CONSTRUCTOR_NAME)){// If we leave an <init> method
            if ((Frame._this != null) && (!(mg.getClassName().equals(Type.OBJECT.getClassName()))) ) {
                constraintViolated(o, "Leaving a constructor that itself did not call a constructor.");
            }
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitSALOAD(SALOAD o){
        indexOfInt(o, stack().peek());
        if (stack().peek(1) == Type.NULL){
            return;
        } 
        if (! (stack().peek(1) instanceof ArrayType)){
            constraintViolated(o, "Stack next-to-top must be of type short[] but is '"+stack().peek(1)+"'.");
        }
        Type t = ((ArrayType) (stack().peek(1))).getBasicType();
        if (t != Type.SHORT){
            constraintViolated(o, "Stack next-to-top must be of type short[] but is '"+stack().peek(1)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitSASTORE(SASTORE o){
        if (stack().peek() != Type.INT){
            constraintViolated(o, "The value at the stack top is not of type 'int', but of type '"+stack().peek()+"'.");
        }
        indexOfInt(o, stack().peek(1));
        if (stack().peek(2) == Type.NULL){
            return;
        } 
        if (! (stack().peek(2) instanceof ArrayType)){
            constraintViolated(o, "Stack next-to-next-to-top must be of type short[] but is '"+stack().peek(2)+"'.");
        }
        Type t = ((ArrayType) (stack().peek(2))).getBasicType();
        if (t != Type.SHORT){
            constraintViolated(o, "Stack next-to-next-to-top must be of type short[] but is '"+stack().peek(2)+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitSIPUSH(SIPUSH o){
        // nothing to do here. Generic visitXXX() methods did the trick before.
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitSWAP(SWAP o){
        if (stack().peek().getSize() != 1){
            constraintViolated(o, "The value at the stack top is not of size '1', but of size '"+stack().peek().getSize()+"'.");
        }
        if (stack().peek(1).getSize() != 1){
            constraintViolated(o, "The value at the stack next-to-top is not of size '1', but of size '"+stack().peek(1).getSize()+"'.");
        }
    }

    /**
     * Ensures the specific preconditions of the said instruction.
     */
    @Override
    public void visitTABLESWITCH(TABLESWITCH o){
        indexOfInt(o, stack().peek());
        // See Pass 3a.
    }

}