[mono.git] / mcs / mcs / flowanalysis.cs

//
// flowanalyis.cs: The control flow analysis code
//
// Author:
//   Martin Baulig (martin@ximian.com)
//
// (C) 2001, 2002, 2003 Ximian, Inc.
//

using System;
using System.Text;
using System.Collections;
using System.Reflection;
using System.Reflection.Emit;
using System.Diagnostics;

namespace Mono.CSharp
{
        // <summary>
        //   A new instance of this class is created every time a new block is resolved
        //   and if there's branching in the block's control flow.
        // </summary>
        public abstract class FlowBranching
        {
                // <summary>
                //   The type of a FlowBranching.
                // </summary>
                public enum BranchingType {
                        // Normal (conditional or toplevel) block.
                        Block,

                        // A loop block.
                        LoopBlock,

                        // Try/Catch block.
                        Exception,

                        // Switch block.
                        Switch,

                        // Switch section.
                        SwitchSection
                }

                // <summary>
                //   The type of one sibling of a branching.
                // </summary>
                public enum SiblingType {
                        Conditional,
                        SwitchSection,
                        Try,
                        Catch,
                        Finally
                }

                // <summary>
                //   This is used in the control flow analysis code to specify whether the
                //   current code block may return to its enclosing block before reaching
                //   its end.
                // </summary>
                public enum FlowReturns {
                        Undefined = 0,

                        // It can never return.
                        Never,

                        // This means that the block contains a conditional return statement
                        // somewhere.
                        Sometimes,

                        // The code always returns, ie. there's an unconditional return / break
                        // statement in it.
                        Always,

                        // The code always throws an exception.
                        Exception,

                        // The current code block is unreachable.  This happens if it's immediately
                        // following a FlowReturns.Always block.
                        Unreachable
                }

                public static FlowBranching CreateBranching (FlowBranching parent, BranchingType type, Block block, Location loc)
                {
                        switch (type) {
                        case BranchingType.Exception:
                                return new FlowBranchingException (parent, type, block, loc);

                        case BranchingType.Switch:
                                return new FlowBranchingBlock (parent, type, SiblingType.SwitchSection, block, loc);

                        default:
                                return new FlowBranchingBlock (parent, type, SiblingType.Conditional, block, loc);
                        }
                }

                // <summary>
                //   The type of this flow branching.
                // </summary>
                public readonly BranchingType Type;

                // <summary>
                //   The block this branching is contained in.  This may be null if it's not
                //   a top-level block and it doesn't declare any local variables.
                // </summary>
                public readonly Block Block;

                // <summary>
                //   The parent of this branching or null if this is the top-block.
                // </summary>
                public readonly FlowBranching Parent;

                // <summary>
                //   Start-Location of this flow branching.
                // </summary>
                public readonly Location Location;

                // <summary>
                //   If this is an infinite loop.
                // </summary>
                public bool Infinite;

                // <summary>
                //   If we may leave the current loop.
                // </summary>
                public bool MayLeaveLoop;

                //
                // Private
                //
                VariableMap param_map, local_map;

                static int next_id = 0;
                int id;

                // <summary>
                //   Performs an `And' operation on the FlowReturns status
                //   (for instance, a block only returns Always if all its siblings
                //   always return).
                // </summary>
                public static FlowReturns AndFlowReturns (FlowReturns a, FlowReturns b)
                {
                        if (a == FlowReturns.Undefined)
                                return b;
                        if (b == FlowReturns.Unreachable)
                                return a;

                        switch (a) {
                        case FlowReturns.Never:
                                if (b == FlowReturns.Never)
                                        return FlowReturns.Never;
                                else
                                        return FlowReturns.Sometimes;

                        case FlowReturns.Sometimes:
                                return FlowReturns.Sometimes;

                        case FlowReturns.Always:
                                if ((b == FlowReturns.Always) || (b == FlowReturns.Exception))
                                        return FlowReturns.Always;
                                else
                                        return FlowReturns.Sometimes;

                        case FlowReturns.Exception:
                                if (b == FlowReturns.Exception)
                                        return FlowReturns.Exception;
                                else if (b == FlowReturns.Always)
                                        return FlowReturns.Always;
                                else
                                        return FlowReturns.Sometimes;
                        }

                        return b;
                }

                // <summary>
                //   The vector contains a BitArray with information about which local variables
                //   and parameters are already initialized at the current code position.
                // </summary>
                public class UsageVector {
                        // <summary>
                        //   The type of this branching.
                        // </summary>
                        public readonly SiblingType Type;

                        // <summary>
                        //   Start location of this branching.
                        // </summary>
                        public readonly Location Location;

                        // <summary>
                        //   If this is true, then the usage vector has been modified and must be
                        //   merged when we're done with this branching.
                        // </summary>
                        public bool IsDirty;

                        // <summary>
                        //   The number of parameters in this block.
                        // </summary>
                        public readonly int CountParameters;

                        // <summary>
                        //   The number of locals in this block.
                        // </summary>
                        public readonly int CountLocals;

                        // <summary>
                        //   If not null, then we inherit our state from this vector and do a
                        //   copy-on-write.  If null, then we're the first sibling in a top-level
                        //   block and inherit from the empty vector.
                        // </summary>
                        public readonly UsageVector InheritsFrom;

                        //
                        // Private.
                        //
                        MyBitVector locals, parameters;
                        FlowReturns RealReturns, RealBreaks, RealReachable;
                        bool is_finally;

                        static int next_id = 0;
                        int id;

                        //
                        // Normally, you should not use any of these constructors.
                        //
                        public UsageVector (SiblingType type, UsageVector parent, Location loc, int num_params, int num_locals)
                        {
                                this.Type = type;
                                this.Location = loc;
                                this.InheritsFrom = parent;
                                this.CountParameters = num_params;
                                this.CountLocals = num_locals;
                                this.RealReturns = FlowReturns.Never;
                                this.RealBreaks = FlowReturns.Never;
                                this.RealReachable = FlowReturns.Always;

                                if (parent != null) {
                                        locals = new MyBitVector (parent.locals, CountLocals);
                                        if (num_params > 0)
                                                parameters = new MyBitVector (parent.parameters, num_params);
                                        RealReturns = parent.Returns;
                                        RealBreaks = parent.Breaks;
                                } else {
                                        locals = new MyBitVector (null, CountLocals);
                                        if (num_params > 0)
                                                parameters = new MyBitVector (null, num_params);
                                }

                                id = ++next_id;
                        }

                        public UsageVector (SiblingType type, UsageVector parent, Location loc)
                                : this (type, parent, loc, parent.CountParameters, parent.CountLocals)
                        { }

                        // <summary>
                        //   This does a deep copy of the usage vector.
                        // </summary>
                        public UsageVector Clone ()
                        {
                                UsageVector retval = new UsageVector (Type, null, Location, CountParameters, CountLocals);

                                retval.locals = locals.Clone ();
                                if (parameters != null)
                                        retval.parameters = parameters.Clone ();
                                retval.RealReturns = RealReturns;
                                retval.RealBreaks = RealBreaks;
                                retval.RealReachable = RealReachable;

                                return retval;
                        }

                        public bool IsAssigned (VariableInfo var)
                        {
                                if (!var.IsParameter && AlwaysBreaks)
                                        return true;

                                return var.IsAssigned (var.IsParameter ? parameters : locals);
                        }

                        public void SetAssigned (VariableInfo var)
                        {
                                if (!var.IsParameter && AlwaysBreaks)
                                        return;

                                var.SetAssigned (var.IsParameter ? parameters : locals);
                        }

                        public bool IsFieldAssigned (VariableInfo var, string name)
                        {
                                if (!var.IsParameter && AlwaysBreaks)
                                        return true;

                                return var.IsFieldAssigned (var.IsParameter ? parameters : locals, name);
                        }

                        public void SetFieldAssigned (VariableInfo var, string name)
                        {
                                if (!var.IsParameter && AlwaysBreaks)
                                        return;

                                var.SetFieldAssigned (var.IsParameter ? parameters : locals, name);
                        }

                        // <summary>
                        //   Specifies when the current block returns.
                        //   If this is FlowReturns.Unreachable, then control can never reach the
                        //   end of the method (so that we don't need to emit a return statement).
                        //   The same applies for FlowReturns.Exception, but in this case the return
                        //   value will never be used.
                        // </summary>
                        public FlowReturns Returns {
                                get {
                                        return RealReturns;
                                }
                        }

                        // <summary>
                        //   Specifies whether control may return to our containing block
                        //   before reaching the end of this block.  This happens if there
                        //   is a break/continue/goto/return in it.
                        //   This can also be used to find out whether the statement immediately
                        //   following the current block may be reached or not.
                        // </summary>
                        public FlowReturns Breaks {
                                get {
                                        return RealBreaks;
                                }
                        }

                        public FlowReturns Reachable {
                                get {
                                        return RealReachable;
                                }
                        }

                        public bool AlwaysBreaks {
                                get {
                                        return (Breaks == FlowReturns.Always) ||
                                                (Breaks == FlowReturns.Exception) ||
                                                (Breaks == FlowReturns.Unreachable);
                                }
                        }

                        public bool MayBreak {
                                get {
                                        return Breaks != FlowReturns.Never;
                                }
                        }

                        public bool AlwaysReturns {
                                get {
                                        return (Returns == FlowReturns.Always) ||
                                                (Returns == FlowReturns.Exception);
                                }
                        }

                        public bool MayReturn {
                                get {
                                        return (Returns == FlowReturns.Sometimes) ||
                                                (Returns == FlowReturns.Always);
                                }
                        }

                        public void Break ()
                        {
                                RealBreaks = FlowReturns.Always;
                        }

                        public void Return ()
                        {
                                RealReturns = FlowReturns.Always;
                        }

                        public bool IsUnreachable {
                                get {
                                        return (Reachable == FlowReturns.Exception) ||
                                                (Reachable == FlowReturns.Unreachable);
                                }
                        }

                        public void Unreachable ()
                        {
                                // If we're already unreachable, don't modify the reason why.
                                if (!IsUnreachable)
                                        RealReachable = FlowReturns.Unreachable;
                        }

                        public void NeverReachable ()
                        {
                                // If we're already unreachable, don't modify the reason why.
                                if (!IsUnreachable)
                                        RealReachable = FlowReturns.Never;
                        }

                        public void Throw ()
                        {
                                // If we're already unreachable, don't modify the reason why.
                                if (!IsUnreachable)
                                        RealReachable = FlowReturns.Exception;
                        }

                        // <summary>
                        //   Merges a child branching.
                        // </summary>
                        public FlowReturns MergeChild (MyBitVector new_params, MyBitVector new_locals,
                                                       FlowReturns new_returns, FlowReturns new_breaks,
                                                       FlowReturns new_reachable)
                        {
                                Report.Debug (2, "MERGING CHILD", this, new_params, new_locals, new_returns, new_breaks,
                                              new_reachable);

                                RealReturns = new_returns;
                                RealBreaks = new_breaks;
                                RealReachable = new_reachable;

                                //
                                // We've now either reached the point after the branching or we will
                                // never get there since we always return or always throw an exception.
                                //
                                // If we can reach the point after the branching, mark all locals and
                                // parameters as initialized which have been initialized in all branches
                                // we need to look at (see above).
                                //

                                Report.Debug (2, "MERGING CHILD #1", this, Returns, Breaks, Reachable, new_locals, new_params);

                                if ((Reachable == FlowReturns.Always) || (Reachable == FlowReturns.Sometimes) ||
                                    (Reachable == FlowReturns.Never)) {
                                        if ((Returns == FlowReturns.Always) || (Breaks == FlowReturns.Always))
                                                RealReachable = FlowReturns.Never;
                                        if ((Type == SiblingType.SwitchSection) && (Reachable != FlowReturns.Never)) {
                                                Report.Error (163, Location, "Control cannot fall through from one " +
                                                              "case label to another");
                                        }

                                        if (new_locals != null)
                                                locals.Or (new_locals);

                                        if (new_params != null)
                                                parameters.Or (new_params);
                                }

                                Report.Debug (2, "MERGING CHILD DONE", this);

                                return Returns;
                        }

                        // <summary>
                        //   Tells control flow analysis that the current code position may be reached with
                        //   a forward jump from any of the origins listed in `origin_vectors' which is a
                        //   list of UsageVectors.
                        //
                        //   This is used when resolving forward gotos - in the following example, the
                        //   variable `a' is uninitialized in line 8 becase this line may be reached via
                        //   the goto in line 4:
                        //
                        //      1     int a;
                        //
                        //      3     if (something)
                        //      4        goto World;
                        //
                        //      6     a = 5;
                        //
                        //      7  World:
                        //      8     Console.WriteLine (a);
                        //
                        // </summary>
                        public void MergeJumpOrigins (ICollection origin_vectors)
                        {
                                Report.Debug (1, "MERGING JUMP ORIGIN", this);

                                RealBreaks = FlowReturns.Never;
                                RealReturns = FlowReturns.Never;
                                if (Reachable != FlowReturns.Always)
                                        RealReachable = FlowReturns.Always;

                                if (origin_vectors == null)
                                        return;

                                foreach (UsageVector vector in origin_vectors) {
                                        Report.Debug (1, "  MERGING JUMP ORIGIN", vector);

                                        locals.And (vector.locals);
                                        if (parameters != null)
                                                parameters.And (vector.parameters);
                                        RealBreaks = AndFlowReturns (RealBreaks, vector.Breaks);
                                        RealReturns = AndFlowReturns (RealReturns, vector.Returns);
                                        RealReachable = AndFlowReturns (RealReachable, vector.Reachable);
                                }

                                Report.Debug (1, "MERGING JUMP ORIGIN DONE", this);
                        }

                        // <summary>
                        //   This is used at the beginning of a finally block if there were
                        //   any return statements in the try block or one of the catch blocks.
                        // </summary>
                        public void MergeFinallyOrigins (ICollection finally_vectors)
                        {
                                Report.Debug (1, "MERGING FINALLY ORIGIN", this);

                                RealBreaks = FlowReturns.Never;

                                foreach (UsageVector vector in finally_vectors) {
                                        Report.Debug (1, "  MERGING FINALLY ORIGIN", vector);

                                        if (parameters != null)
                                                parameters.And (vector.parameters);
                                        RealBreaks = AndFlowReturns (Breaks, vector.Breaks);
                                }

                                is_finally = true;

                                Report.Debug (1, "MERGING FINALLY ORIGIN DONE", this);
                        }

                        public void CheckOutParameters (FlowBranching branching)
                        {
                                if (parameters != null)
                                        branching.CheckOutParameters (parameters, branching.Location);
                        }

                        // <summary>
                        //   Performs an `or' operation on the locals and the parameters.
                        // </summary>
                        public void Or (UsageVector new_vector)
                        {
                                locals.Or (new_vector.locals);
                                if (parameters != null)
                                        parameters.Or (new_vector.parameters);
                        }

                        // <summary>
                        //   Performs an `and' operation on the locals.
                        // </summary>
                        public void AndLocals (UsageVector new_vector)
                        {
                                locals.And (new_vector.locals);
                        }

                        public bool HasParameters {
                                get {
                                        return parameters != null;
                                }
                        }

                        public bool HasLocals {
                                get {
                                        return locals != null;
                                }
                        }

                        // <summary>
                        //   Returns a deep copy of the parameters.
                        // </summary>
                        public MyBitVector Parameters {
                                get {
                                        if (parameters != null)
                                                return parameters.Clone ();
                                        else
                                                return null;
                                }
                        }

                        // <summary>
                        //   Returns a deep copy of the locals.
                        // </summary>
                        public MyBitVector Locals {
                                get {
                                        return locals.Clone ();
                                }
                        }

                        public MyBitVector ParameterVector {
                                get {
                                        return parameters;
                                }
                        }

                        public MyBitVector LocalVector {
                                get {
                                        return locals;
                                }
                        }

                        //
                        // Debugging stuff.
                        //

                        public override string ToString ()
                        {
                                StringBuilder sb = new StringBuilder ();

                                sb.Append ("Vector (");
                                sb.Append (id);
                                sb.Append (",");
                                sb.Append (Returns);
                                sb.Append (",");
                                sb.Append (Breaks);
                                sb.Append (",");
                                sb.Append (Reachable);
                                if (parameters != null) {
                                        sb.Append (" - ");
                                        sb.Append (parameters);
                                }
                                sb.Append (" - ");
                                sb.Append (locals);
                                sb.Append (")");

                                return sb.ToString ();
                        }
                }

                // <summary>
                //   Creates a new flow branching which is contained in `parent'.
                //   You should only pass non-null for the `block' argument if this block
                //   introduces any new variables - in this case, we need to create a new
                //   usage vector with a different size than our parent's one.
                // </summary>
                protected FlowBranching (FlowBranching parent, BranchingType type, SiblingType stype,
                                         Block block, Location loc)
                {
                        Parent = parent;
                        Block = block;
                        Location = loc;
                        Type = type;
                        id = ++next_id;

                        UsageVector vector;
                        if (Block != null) {
                                param_map = Block.ParameterMap;
                                local_map = Block.LocalMap;

                                UsageVector parent_vector = parent != null ? parent.CurrentUsageVector : null;
                                vector = new UsageVector (stype, parent_vector, loc, param_map.Length, local_map.Length);
                        } else {
                                param_map = Parent.param_map;
                                local_map = Parent.local_map;
                                vector = new UsageVector (stype, Parent.CurrentUsageVector, loc);
                        }

                        AddSibling (vector);
                }

                public abstract UsageVector CurrentUsageVector {
                        get;
                }                               

                // <summary>
                //   Creates a sibling of the current usage vector.
                // </summary>
                public virtual void CreateSibling (SiblingType type)
                {
                        AddSibling (new UsageVector (type, Parent.CurrentUsageVector, Location));

                        Report.Debug (1, "CREATED SIBLING", CurrentUsageVector);
                }

                protected abstract void AddSibling (UsageVector uv);

                public abstract void Break ();
                public abstract void Return ();
                public abstract void Goto ();
                public abstract void Throw ();
                public abstract void Label (ArrayList origin_vectors);

                // <summary>
                //   Check whether all `out' parameters have been assigned.
                // </summary>
                public void CheckOutParameters (MyBitVector parameters, Location loc)
                {
                        for (int i = 0; i < param_map.Count; i++) {
                                VariableInfo var = param_map [i];

                                if (var == null)
                                        continue;

                                if (var.IsAssigned (parameters))
                                        continue;

                                Report.Error (177, loc, "The out parameter `" +
                                              param_map.VariableNames [i] + "' must be " +
                                              "assigned before control leave the current method.");
                        }
                }

                protected class MergeResult
                {
                        public MyBitVector Parameters;
                        public MyBitVector Locals;
                        public FlowReturns Returns;
                        public FlowReturns Breaks;
                        public FlowReturns Reachable;
                        public bool MayLeaveLoop;

                        public MergeResult (MyBitVector parameters, MyBitVector locals, FlowReturns returns, FlowReturns breaks,
                                            FlowReturns reachable, bool may_leave_loop)
                        {
                                this.Parameters = parameters;
                                this.Locals = locals;
                                this.Returns = returns;
                                this.Breaks = breaks;
                                this.Reachable = reachable;
                                this.MayLeaveLoop = may_leave_loop;
                        }
                }

                protected MergeResult Merge (ArrayList children)
                {
                        MyBitVector locals = null;
                        MyBitVector parameters = null;

                        FlowReturns returns = FlowReturns.Undefined;
                        FlowReturns breaks = FlowReturns.Undefined;
                        FlowReturns reachable = FlowReturns.Undefined;

                        Report.Debug (2, "MERGING CHILDREN", this, Type, children.Count);

                        int children_count = children.Count;
                        for (int ix = 0; ix < children_count; ix++){
                                UsageVector child = (UsageVector) children [ix];
                                
                                Report.Debug (2, "  MERGING CHILD", child, child.AlwaysBreaks, child.AlwaysReturns,
                                              child.IsUnreachable, child.Locals, child.Parameters,
                                              child.Returns, child.Breaks, child.Reachable);

                                reachable = AndFlowReturns (reachable, child.Reachable);

                                // Ignore unreachable children.
                                if (child.IsUnreachable)
                                        continue;

                                returns = AndFlowReturns (returns, child.Returns);
                                breaks = AndFlowReturns (breaks, child.Breaks);
                                        
                                // A local variable is initialized after a flow branching if it
                                // has been initialized in all its branches which do neither
                                // always return or always throw an exception.
                                //
                                // If a branch may return, but does not always return, then we
                                // can treat it like a never-returning branch here: control will
                                // only reach the code position after the branching if we did not
                                // return here.
                                //
                                // It's important to distinguish between always and sometimes
                                // returning branches here:
                                //
                                //    1   int a;
                                //    2   if (something) {
                                //    3      return;
                                //    4      a = 5;
                                //    5   }
                                //    6   Console.WriteLine (a);
                                //
                                // The if block in lines 3-4 always returns, so we must not look
                                // at the initialization of `a' in line 4 - thus it'll still be
                                // uninitialized in line 6.
                                //
                                // On the other hand, the following is allowed:
                                //
                                //    1   int a;
                                //    2   if (something)
                                //    3      a = 5;
                                //    4   else
                                //    5      return;
                                //    6   Console.WriteLine (a);
                                //
                                // Here, `a' is initialized in line 3 and we must not look at
                                // line 5 since it always returns.
                                // 
                                if (!child.AlwaysReturns && !child.AlwaysBreaks)
                                        MyBitVector.And (ref locals, child.LocalVector);

                                // An `out' parameter must be assigned in all branches which do
                                // not always throw an exception.
                                if ((child.Type != SiblingType.Catch) &&
                                    (child.ParameterVector != null) && (child.Breaks != FlowReturns.Exception))
                                        MyBitVector.And (ref parameters, child.ParameterVector);
                        }

                        Report.Debug (2, "MERGING CHILDREN DONE", Type, parameters, locals, returns, breaks, reachable,
                                      Infinite, MayLeaveLoop, this);

                        if (Infinite && !MayLeaveLoop) {
                                Report.Debug (1, "INFINITE", returns, breaks, this);

                                if (returns == FlowReturns.Never) {
                                        // We're actually infinite.
                                        breaks = FlowReturns.Unreachable;
                                        returns = FlowReturns.Unreachable;
                                } else if ((returns == FlowReturns.Sometimes) || (returns == FlowReturns.Always)) {
                                        // If we're an infinite loop and do not break, the code after
                                        // the loop can never be reached.  However, if we may return
                                        // from the loop, then we do always return (or stay in the loop
                                        // forever).
                                        returns = FlowReturns.Always;
                                }
                        }

                        if (returns == FlowReturns.Undefined)
                                returns = FlowReturns.Never;
                        if (breaks == FlowReturns.Undefined)
                                breaks = FlowReturns.Never;

                        return new MergeResult (parameters, locals, returns, breaks, reachable, MayLeaveLoop);
                }

                protected abstract MergeResult Merge ();

                // <summary>
                //   Merge a child branching.
                // </summary>
                public FlowReturns MergeChild (FlowBranching child)
                {
                        MergeResult result = child.Merge ();

                        CurrentUsageVector.MergeChild (
                                result.Parameters, result.Locals, result.Returns, result.Breaks, result.Reachable);

                        if ((child.Type != BranchingType.LoopBlock) && (child.Type != BranchingType.SwitchSection))
                                MayLeaveLoop |= child.MayLeaveLoop;

                        if (result.Reachable == FlowReturns.Exception)
                                return FlowReturns.Exception;
                        else
                                return result.Returns;
                }

                // <summary>
                //   Does the toplevel merging.
                // </summary>
                public FlowReturns MergeTopBlock ()
                {
                        if ((Type != BranchingType.Block) || (Block == null))
                                throw new NotSupportedException ();

                        UsageVector vector = new UsageVector (
                                SiblingType.Conditional, null, Location, param_map.Length, local_map.Length);

                        MergeResult result = Merge ();
                        vector.MergeChild (result.Parameters, result.Locals, result.Returns, result.Breaks, result.Reachable);

                        if (vector.Reachable != FlowReturns.Exception)
                                CheckOutParameters (vector.Parameters, Location);
                        else
                                return FlowReturns.Exception;

                        return result.Returns;
                }

                public virtual bool InTryBlock ()
                {
                        if (Parent != null)
                                return Parent.InTryBlock ();
                        else
                                return false;
                }

                public virtual void AddFinallyVector (UsageVector vector)
                {
                        if (Parent != null)
                                Parent.AddFinallyVector (vector);
                        else
                                throw new NotSupportedException ();
                }

                public bool IsAssigned (VariableInfo vi)
                {
                        return CurrentUsageVector.IsAssigned (vi);
                }

                public bool IsFieldAssigned (VariableInfo vi, string field_name)
                {
                        if (CurrentUsageVector.IsAssigned (vi))
                                return true;

                        return CurrentUsageVector.IsFieldAssigned (vi, field_name);
                }

                public void SetAssigned (VariableInfo vi)
                {
                        CurrentUsageVector.SetAssigned (vi);
                }

                public void SetFieldAssigned (VariableInfo vi, string name)
                {
                        CurrentUsageVector.SetFieldAssigned (vi, name);
                }

                public override string ToString ()
                {
                        StringBuilder sb = new StringBuilder ();
                        sb.Append (GetType ());
                        sb.Append (" (");

                        sb.Append (id);
                        sb.Append (",");
                        sb.Append (Type);
                        if (Block != null) {
                                sb.Append (" - ");
                                sb.Append (Block.ID);
                                sb.Append (" - ");
                                sb.Append (Block.StartLocation);
                        }
                        sb.Append (" - ");
                        // sb.Append (Siblings.Length);
                        // sb.Append (" - ");
                        sb.Append (CurrentUsageVector);
                        sb.Append (")");
                        return sb.ToString ();
                }
        }

        public class FlowBranchingBlock : FlowBranching
        {
                UsageVector current_vector;
                ArrayList siblings = new ArrayList ();

                public FlowBranchingBlock (FlowBranching parent, BranchingType type, SiblingType stype,
                                           Block block, Location loc)
                        : base (parent, type, stype, block, loc)
                { }

                public override UsageVector CurrentUsageVector {
                        get { return current_vector; }
                }

                protected override void AddSibling (UsageVector sibling)
                {
                        siblings.Add (sibling);
                        current_vector = sibling;
                }

                public override void Break ()
                {
                        if (Type == BranchingType.SwitchSection)
                                CurrentUsageVector.NeverReachable ();
                        else {
                                if (Type == BranchingType.LoopBlock)
                                        MayLeaveLoop = true;
                                CurrentUsageVector.Break ();
                        }
                }

                public override void Return ()
                {
                        CurrentUsageVector.Return ();
                }

                public override void Goto ()
                {
                        CurrentUsageVector.Unreachable ();
                }

                public override void Throw ()
                {
                        CurrentUsageVector.Throw ();
                }

                public override void Label (ArrayList origin_vectors)
                {
                        CurrentUsageVector.MergeJumpOrigins (origin_vectors);
                }

                protected override MergeResult Merge ()
                {
                        MergeResult result = Merge (siblings);
                        if (Type == BranchingType.LoopBlock)
                                result.MayLeaveLoop = false;
                        return result;
                }
        }

        public class FlowBranchingException : FlowBranching
        {
                ArrayList finally_vectors;

                bool has_params;
                UsageVector current_vector;
                UsageVector try_vector;
                ArrayList catch_vectors = new ArrayList ();
                UsageVector finally_vector;

                public FlowBranchingException (FlowBranching parent, BranchingType type, Block block, Location loc)
                        : base (parent, type, SiblingType.Try, block, loc)
                {
                        finally_vectors = new ArrayList ();
                        has_params = current_vector.HasParameters;
                }

                protected override void AddSibling (UsageVector sibling)
                {
                        if (sibling.Type == SiblingType.Try) {
                                try_vector = sibling;
                                catch_vectors.Add (sibling);
                        } else if (sibling.Type == SiblingType.Catch)
                                catch_vectors.Add (sibling);
                        else if (sibling.Type == SiblingType.Finally) {
                                // sibling.MergeFinallyOrigins (finally_vectors);
                                finally_vector = sibling;
                        } else
                                throw new InvalidOperationException ();

                        current_vector = sibling;
                }

                public override UsageVector CurrentUsageVector {
                        get { return current_vector; }
                }

                public override bool InTryBlock ()
                {
                        return true;
                }

                public override void AddFinallyVector (UsageVector vector)
                {
                        finally_vectors.Add (vector.Clone ());
                }

                public override void Break ()
                {
                        CurrentUsageVector.Break ();
                }

                public override void Return ()
                {
                        CurrentUsageVector.Return ();
                }

                public override void Goto ()
                {
                        CurrentUsageVector.Unreachable ();
                }

                public override void Throw ()
                {
                        CurrentUsageVector.Throw ();
                }

                public override void Label (ArrayList origin_vectors)
                {
                        CurrentUsageVector.MergeJumpOrigins (origin_vectors);
                }

                protected void MergeFinally (MyBitVector f_params, ref MergeResult result)
                {
                        foreach (UsageVector vector in finally_vectors) {
                                MyBitVector temp_params = f_params.Clone ();
                                temp_params.Or (vector.Parameters);

                                CheckOutParameters (temp_params, Location);
                        }
                }

                protected override MergeResult Merge ()
                {
                        MergeResult result = Merge (catch_vectors);

                        if (has_params) {
                                if (finally_vector != null) {
                                        MergeFinally (finally_vector.Parameters, ref result);
                                        MyBitVector.Or (ref result.Parameters, finally_vector.ParameterVector);
                                } else
                                        MergeFinally (result.Parameters, ref result);
                        }

                        if (finally_vector != null)
                                MyBitVector.Or (ref result.Locals, finally_vector.LocalVector);

                        return result;
                }
        }

        // <summary>
        //   This is used by the flow analysis code to keep track of the type of local variables
        //   and variables.
        //
        //   The flow code uses a BitVector to keep track of whether a variable has been assigned
        //   or not.  This is easy for fundamental types (int, char etc.) or reference types since
        //   you can only assign the whole variable as such.
        //
        //   For structs, we also need to keep track of all its fields.  To do this, we allocate one
        //   bit for the struct itself (it's used if you assign/access the whole struct) followed by
        //   one bit for each of its fields.
        //
        //   This class computes this `layout' for each type.
        // </summary>
        public class TypeInfo
        {
                public readonly Type Type;

                // <summary>
                //   Total number of bits a variable of this type consumes in the flow vector.
                // </summary>
                public readonly int TotalLength;

                // <summary>
                //   Number of bits the simple fields of a variable of this type consume
                //   in the flow vector.
                // </summary>
                public readonly int Length;

                // <summary>
                //   This is only used by sub-structs.
                // </summary>
                public readonly int Offset;

                // <summary>
                //   If this is a struct.
                // </summary>
                public readonly bool IsStruct;       

                // <summary>
                //   If this is a struct, all fields which are structs theirselves.
                // </summary>
                public TypeInfo[] SubStructInfo;

                protected readonly StructInfo struct_info;
                private static Hashtable type_hash = new Hashtable ();

                public static TypeInfo GetTypeInfo (Type type)
                {
                        TypeInfo info = (TypeInfo) type_hash [type];
                        if (info != null)
                                return info;

                        info = new TypeInfo (type);
                        type_hash.Add (type, info);
                        return info;
                }

                public static TypeInfo GetTypeInfo (TypeContainer tc)
                {
                        TypeInfo info = (TypeInfo) type_hash [tc.TypeBuilder];
                        if (info != null)
                                return info;

                        info = new TypeInfo (tc);
                        type_hash.Add (tc.TypeBuilder, info);
                        return info;
                }

                private TypeInfo (Type type)
                {
                        this.Type = type;

                        struct_info = StructInfo.GetStructInfo (type);
                        if (struct_info != null) {
                                Length = struct_info.Length;
                                TotalLength = struct_info.TotalLength;
                                SubStructInfo = struct_info.StructFields;
                                IsStruct = true;
                        } else {
                                Length = 0;
                                TotalLength = 1;
                                IsStruct = false;
                        }
                }

                private TypeInfo (TypeContainer tc)
                {
                        this.Type = tc.TypeBuilder;

                        struct_info = StructInfo.GetStructInfo (tc);
                        if (struct_info != null) {
                                Length = struct_info.Length;
                                TotalLength = struct_info.TotalLength;
                                SubStructInfo = struct_info.StructFields;
                                IsStruct = true;
                        } else {
                                Length = 0;
                                TotalLength = 1;
                                IsStruct = false;
                        }
                }

                protected TypeInfo (StructInfo struct_info, int offset)
                {
                        this.struct_info = struct_info;
                        this.Offset = offset;
                        this.Length = struct_info.Length;
                        this.TotalLength = struct_info.TotalLength;
                        this.SubStructInfo = struct_info.StructFields;
                        this.Type = struct_info.Type;
                        this.IsStruct = true;
                }

                public int GetFieldIndex (string name)
                {
                        if (struct_info == null)
                                return 0;

                        return struct_info [name];
                }

                public TypeInfo GetSubStruct (string name)
                {
                        if (struct_info == null)
                                return null;

                        return struct_info.GetStructField (name);
                }

                // <summary>
                //   A struct's constructor must always assign all fields.
                //   This method checks whether it actually does so.
                // </summary>
                public bool IsFullyInitialized (FlowBranching branching, VariableInfo vi, Location loc)
                {
                        if (struct_info == null)
                                return true;

                        bool ok = true;
                        for (int i = 0; i < struct_info.Count; i++) {
                                FieldInfo field = struct_info.Fields [i];

                                if (!branching.IsFieldAssigned (vi, field.Name)) {
                                        Report.Error (171, loc,
                                                      "Field `" + TypeManager.CSharpName (Type) +
                                                      "." + field.Name + "' must be fully initialized " +
                                                      "before control leaves the constructor");
                                        ok = false;
                                }
                        }

                        return ok;
                }

                public override string ToString ()
                {
                        return String.Format ("TypeInfo ({0}:{1}:{2}:{3})",
                                              Type, Offset, Length, TotalLength);
                }

                protected class StructInfo {
                        public readonly Type Type;
                        public readonly FieldInfo[] Fields;
                        public readonly TypeInfo[] StructFields;
                        public readonly int Count;
                        public readonly int CountPublic;
                        public readonly int CountNonPublic;
                        public readonly int Length;
                        public readonly int TotalLength;
                        public readonly bool HasStructFields;

                        private static Hashtable field_type_hash = new Hashtable ();
                        private Hashtable struct_field_hash;
                        private Hashtable field_hash;

                        protected bool InTransit = false;

                        // Private constructor.  To save memory usage, we only need to create one instance
                        // of this class per struct type.
                        private StructInfo (Type type)
                        {
                                this.Type = type;

                                field_type_hash.Add (type, this);

                                if (type is TypeBuilder) {
                                        TypeContainer tc = TypeManager.LookupTypeContainer (type);

                                        ArrayList fields = tc.Fields;

                                        ArrayList public_fields = new ArrayList ();
                                        ArrayList non_public_fields = new ArrayList ();

                                        if (fields != null) {
                                                foreach (Field field in fields) {
                                                        if ((field.ModFlags & Modifiers.STATIC) != 0)
                                                                continue;
                                                        if ((field.ModFlags & Modifiers.PUBLIC) != 0)
                                                                public_fields.Add (field.FieldBuilder);
                                                        else
                                                                non_public_fields.Add (field.FieldBuilder);
                                                }
                                        }

                                        CountPublic = public_fields.Count;
                                        CountNonPublic = non_public_fields.Count;
                                        Count = CountPublic + CountNonPublic;

                                        Fields = new FieldInfo [Count];
                                        public_fields.CopyTo (Fields, 0);
                                        non_public_fields.CopyTo (Fields, CountPublic);
                                } else {
                                        FieldInfo[] public_fields = type.GetFields (
                                                BindingFlags.Instance|BindingFlags.Public);
                                        FieldInfo[] non_public_fields = type.GetFields (
                                                BindingFlags.Instance|BindingFlags.NonPublic);

                                        CountPublic = public_fields.Length;
                                        CountNonPublic = non_public_fields.Length;
                                        Count = CountPublic + CountNonPublic;

                                        Fields = new FieldInfo [Count];
                                        public_fields.CopyTo (Fields, 0);
                                        non_public_fields.CopyTo (Fields, CountPublic);
                                }

                                struct_field_hash = new Hashtable ();
                                field_hash = new Hashtable ();

                                Length = 0;
                                StructFields = new TypeInfo [Count];
                                StructInfo[] sinfo = new StructInfo [Count];

                                InTransit = true;

                                for (int i = 0; i < Count; i++) {
                                        FieldInfo field = (FieldInfo) Fields [i];

                                        sinfo [i] = GetStructInfo (field.FieldType);
                                        if (sinfo [i] == null)
                                                field_hash.Add (field.Name, ++Length);
                                        else if (sinfo [i].InTransit) {
                                                Report.Error (523, String.Format (
                                                                      "Struct member '{0}.{1}' of type '{2}' causes " +
                                                                      "a cycle in the structure layout",
                                                                      type, field.Name, sinfo [i].Type));
                                                sinfo [i] = null;
                                                return;
                                        }
                                }

                                InTransit = false;

                                TotalLength = Length + 1;
                                for (int i = 0; i < Count; i++) {
                                        FieldInfo field = (FieldInfo) Fields [i];

                                        if (sinfo [i] == null)
                                                continue;

                                        field_hash.Add (field.Name, TotalLength);

                                        HasStructFields = true;
                                        StructFields [i] = new TypeInfo (sinfo [i], TotalLength);
                                        struct_field_hash.Add (field.Name, StructFields [i]);
                                        TotalLength += sinfo [i].TotalLength;
                                }
                        }

                        public int this [string name] {
                                get {
                                        if (field_hash.Contains (name))
                                                return (int) field_hash [name];
                                        else
                                                return 0;
                                }
                        }

                        public TypeInfo GetStructField (string name)
                        {
                                return (TypeInfo) struct_field_hash [name];
                        }

                        public static StructInfo GetStructInfo (Type type)
                        {
                                if (!TypeManager.IsValueType (type) || TypeManager.IsEnumType (type) ||
                                    TypeManager.IsBuiltinType (type))
                                        return null;

                                StructInfo info = (StructInfo) field_type_hash [type];
                                if (info != null)
                                        return info;

                                return new StructInfo (type);
                        }

                        public static StructInfo GetStructInfo (TypeContainer tc)
                        {
                                StructInfo info = (StructInfo) field_type_hash [tc.TypeBuilder];
                                if (info != null)
                                        return info;

                                return new StructInfo (tc.TypeBuilder);
                        }
                }
        }

        // <summary>
        //   This is used by the flow analysis code to store information about a single local variable
        //   or parameter.  Depending on the variable's type, we need to allocate one or more elements
        //   in the BitVector - if it's a fundamental or reference type, we just need to know whether
        //   it has been assigned or not, but for structs, we need this information for each of its fields.
        // </summary>
        public class VariableInfo {
                public readonly string Name;
                public readonly TypeInfo TypeInfo;

                // <summary>
                //   The bit offset of this variable in the flow vector.
                // </summary>
                public readonly int Offset;

                // <summary>
                //   The number of bits this variable needs in the flow vector.
                //   The first bit always specifies whether the variable as such has been assigned while
                //   the remaining bits contain this information for each of a struct's fields.
                // </summary>
                public readonly int Length;

                // <summary>
                //   If this is a parameter of local variable.
                // </summary>
                public readonly bool IsParameter;

                public readonly LocalInfo LocalInfo;
                public readonly int ParameterIndex;

                readonly VariableInfo Parent;
                VariableInfo[] sub_info;

                protected VariableInfo (string name, Type type, int offset)
                {
                        this.Name = name;
                        this.Offset = offset;
                        this.TypeInfo = TypeInfo.GetTypeInfo (type);

                        Length = TypeInfo.TotalLength;

                        Initialize ();
                }

                protected VariableInfo (VariableInfo parent, TypeInfo type)
                {
                        this.Name = parent.Name;
                        this.TypeInfo = type;
                        this.Offset = parent.Offset + type.Offset;
                        this.Parent = parent;
                        this.Length = type.TotalLength;

                        this.IsParameter = parent.IsParameter;
                        this.LocalInfo = parent.LocalInfo;
                        this.ParameterIndex = parent.ParameterIndex;

                        Initialize ();
                }

                protected void Initialize ()
                {
                        TypeInfo[] sub_fields = TypeInfo.SubStructInfo;
                        if (sub_fields != null) {
                                sub_info = new VariableInfo [sub_fields.Length];
                                for (int i = 0; i < sub_fields.Length; i++) {
                                        if (sub_fields [i] != null)
                                                sub_info [i] = new VariableInfo (this, sub_fields [i]);
                                }
                        } else
                                sub_info = new VariableInfo [0];
                }

                public VariableInfo (LocalInfo local_info, int offset)
                        : this (local_info.Name, local_info.VariableType, offset)
                {
                        this.LocalInfo = local_info;
                        this.IsParameter = false;
                }

                public VariableInfo (string name, Type type, int param_idx, int offset)
                        : this (name, type, offset)
                {
                        this.ParameterIndex = param_idx;
                        this.IsParameter = true;
                }

                public bool IsAssigned (EmitContext ec)
                {
                        return !ec.DoFlowAnalysis || ec.CurrentBranching.IsAssigned (this);
                }

                public bool IsAssigned (EmitContext ec, Location loc)
                {
                        if (IsAssigned (ec))
                                return true;

                        Report.Error (165, loc,
                                      "Use of unassigned local variable `" + Name + "'");
                        ec.CurrentBranching.SetAssigned (this);
                        return false;
                }

                public bool IsAssigned (MyBitVector vector)
                {
                        if (vector [Offset])
                                return true;

                        for (VariableInfo parent = Parent; parent != null; parent = parent.Parent)
                                if (vector [parent.Offset])
                                        return true;

                        // Return unless this is a struct.
                        if (!TypeInfo.IsStruct)
                                return false;

                        // Ok, so each field must be assigned.
                        for (int i = 0; i < TypeInfo.Length; i++) {
                                if (!vector [Offset + i + 1])
                                        return false;
                        }

                        // Ok, now check all fields which are structs.
                        for (int i = 0; i < sub_info.Length; i++) {
                                VariableInfo sinfo = sub_info [i];
                                if (sinfo == null)
                                        continue;

                                if (!sinfo.IsAssigned (vector))
                                        return false;
                        }

                        vector [Offset] = true;
                        return true;
                }

                public void SetAssigned (EmitContext ec)
                {
                        if (ec.DoFlowAnalysis)
                                ec.CurrentBranching.SetAssigned (this);
                }

                public void SetAssigned (MyBitVector vector)
                {
                        vector [Offset] = true;
                }

                public bool IsFieldAssigned (EmitContext ec, string name, Location loc)
                {
                        if (!ec.DoFlowAnalysis || ec.CurrentBranching.IsFieldAssigned (this, name))
                                return true;

                        Report.Error (170, loc,
                                      "Use of possibly unassigned field `" + name + "'");
                        ec.CurrentBranching.SetFieldAssigned (this, name);
                        return false;
                }

                public bool IsFieldAssigned (MyBitVector vector, string field_name)
                {
                        int field_idx = TypeInfo.GetFieldIndex (field_name);

                        if (field_idx == 0)
                                return true;

                        return vector [Offset + field_idx];
                }

                public void SetFieldAssigned (EmitContext ec, string name)
                {
                        if (ec.DoFlowAnalysis)
                                ec.CurrentBranching.SetFieldAssigned (this, name);
                }

                public void SetFieldAssigned (MyBitVector vector, string field_name)
                {
                        int field_idx = TypeInfo.GetFieldIndex (field_name);

                        if (field_idx == 0)
                                return;

                        vector [Offset + field_idx] = true;
                }

                public VariableInfo GetSubStruct (string name)
                {
                        TypeInfo type = TypeInfo.GetSubStruct (name);

                        if (type == null)
                                return null;

                        return new VariableInfo (this, type);
                }

                public override string ToString ()
                {
                        return String.Format ("VariableInfo ({0}:{1}:{2}:{3}:{4})",
                                              Name, TypeInfo, Offset, Length, IsParameter);
                }
        }

        // <summary>
        //   This is used by the flow code to hold the `layout' of the flow vector for
        //   all locals and all parameters (ie. we create one instance of this class for the
        //   locals and another one for the params).
        // </summary>
        public class VariableMap {
                // <summary>
                //   The number of variables in the map.
                // </summary>
                public readonly int Count;

                // <summary>
                //   Total length of the flow vector for this map.
                // <summary>
                public readonly int Length;

                // <summary>
                //   Type and name of all the variables.
                //   Note that this is null for variables for which we do not need to compute
                //   assignment info.
                // </summary>
                public readonly Type[] VariableTypes;
                public readonly string[] VariableNames;

                VariableInfo[] map;

                public VariableMap (InternalParameters ip)
                {
                        Count = ip != null ? ip.Count : 0;
                        map = new VariableInfo [Count];
                        VariableNames = new string [Count];
                        VariableTypes = new Type [Count];
                        Length = 0;

                        for (int i = 0; i < Count; i++) {
                                Parameter.Modifier mod = ip.ParameterModifier (i);

                                if ((mod & Parameter.Modifier.OUT) == 0)
                                        continue;

                                VariableNames [i] = ip.ParameterName (i);
                                VariableTypes [i] = TypeManager.GetElementType (ip.ParameterType (i));

                                map [i] = new VariableInfo (VariableNames [i], VariableTypes [i], i, Length);
                                Length += map [i].Length;
                        }
                }

                public VariableMap (LocalInfo[] locals)
                        : this (null, locals)
                { }

                public VariableMap (VariableMap parent, LocalInfo[] locals)
                {
                        int offset = 0, start = 0;
                        if (parent != null) {
                                offset = parent.Length;
                                start = parent.Count;
                        }

                        Count = locals.Length + start;
                        map = new VariableInfo [Count];
                        VariableNames = new string [Count];
                        VariableTypes = new Type [Count];
                        Length = offset;

                        if (parent != null) {
                                parent.map.CopyTo (map, 0);
                                parent.VariableNames.CopyTo (VariableNames, 0);
                                parent.VariableTypes.CopyTo (VariableTypes, 0);
                        }

                        for (int i = start; i < Count; i++) {
                                LocalInfo li = locals [i-start];

                                if (li.VariableType == null)
                                        continue;

                                VariableNames [i] = li.Name;
                                VariableTypes [i] = li.VariableType;

                                map [i] = li.VariableInfo = new VariableInfo (li, Length);
                                Length += map [i].Length;
                        }
                }

                // <summary>
                //   Returns the VariableInfo for variable @index or null if we don't need to
                //   compute assignment info for this variable.
                // </summary>
                public VariableInfo this [int index] {
                        get {
                                return map [index];
                        }
                }

                public override string ToString ()
                {
                        return String.Format ("VariableMap ({0}:{1})", Count, Length);
                }
        }

        // <summary>
        //   This is a special bit vector which can inherit from another bit vector doing a
        //   copy-on-write strategy.  The inherited vector may have a smaller size than the
        //   current one.
        // </summary>
        public class MyBitVector {
                public readonly int Count;
                public readonly MyBitVector InheritsFrom;

                bool is_dirty;
                BitArray vector;

                public MyBitVector (int Count)
                        : this (null, Count)
                { }

                public MyBitVector (MyBitVector InheritsFrom, int Count)
                {
                        this.InheritsFrom = InheritsFrom;
                        this.Count = Count;
                }

                // <summary>
                //   Checks whether this bit vector has been modified.  After setting this to true,
                //   we won't use the inherited vector anymore, but our own copy of it.
                // </summary>
                public bool IsDirty {
                        get {
                                return is_dirty;
                        }

                        set {
                                if (!is_dirty)
                                        initialize_vector ();
                        }
                }

                // <summary>
                //   Get/set bit `index' in the bit vector.
                // </summary>
                public bool this [int index]
                {
                        get {
                                if (index > Count)
                                        throw new ArgumentOutOfRangeException ();

                                // We're doing a "copy-on-write" strategy here; as long
                                // as nobody writes to the array, we can use our parent's
                                // copy instead of duplicating the vector.

                                if (vector != null)
                                        return vector [index];
                                else if (InheritsFrom != null) {
                                        BitArray inherited = InheritsFrom.Vector;

                                        if (index < inherited.Count)
                                                return inherited [index];
                                        else
                                                return false;
                                } else
                                        return false;
                        }

                        set {
                                if (index > Count)
                                        throw new ArgumentOutOfRangeException ();

                                // Only copy the vector if we're actually modifying it.

                                if (this [index] != value) {
                                        initialize_vector ();

                                        vector [index] = value;
                                }
                        }
                }

                // <summary>
                //   If you explicitly convert the MyBitVector to a BitArray, you will get a deep
                //   copy of the bit vector.
                // </summary>
                public static explicit operator BitArray (MyBitVector vector)
                {
                        vector.initialize_vector ();
                        return vector.Vector;
                }

                // <summary>
                //   Performs an `or' operation on the bit vector.  The `new_vector' may have a
                //   different size than the current one.
                // </summary>
                public void Or (MyBitVector new_vector)
                {
                        BitArray new_array = new_vector.Vector;

                        initialize_vector ();

                        int upper;
                        if (vector.Count < new_array.Count)
                                upper = vector.Count;
                        else
                                upper = new_array.Count;

                        for (int i = 0; i < upper; i++)
                                vector [i] = vector [i] | new_array [i];
                }

                // <summary>
                //   Perfonrms an `and' operation on the bit vector.  The `new_vector' may have
                //   a different size than the current one.
                // </summary>
                public void And (MyBitVector new_vector)
                {
                        BitArray new_array = new_vector.Vector;

                        initialize_vector ();

                        int lower, upper;
                        if (vector.Count < new_array.Count)
                                lower = upper = vector.Count;
                        else {
                                lower = new_array.Count;
                                upper = vector.Count;
                        }

                        for (int i = 0; i < lower; i++)
                                vector [i] = vector [i] & new_array [i];

                        for (int i = lower; i < upper; i++)
                                vector [i] = false;
                }

                public static void And (ref MyBitVector target, MyBitVector vector)
                {
                        if (target != null)
                                target.And (vector);
                        else
                                target = vector.Clone ();
                }

                public static void Or (ref MyBitVector target, MyBitVector vector)
                {
                        if (target != null)
                                target.Or (vector);
                        else
                                target = vector.Clone ();
                }

                // <summary>
                //   This does a deep copy of the bit vector.
                // </summary>
                public MyBitVector Clone ()
                {
                        MyBitVector retval = new MyBitVector (Count);

                        retval.Vector = Vector;

                        return retval;
                }

                BitArray Vector {
                        get {
                                if (vector != null)
                                        return vector;
                                else if (!is_dirty && (InheritsFrom != null))
                                        return InheritsFrom.Vector;

                                initialize_vector ();

                                return vector;
                        }

                        set {
                                initialize_vector ();

                                for (int i = 0; i < System.Math.Min (vector.Count, value.Count); i++)
                                        vector [i] = value [i];
                        }
                }

                void initialize_vector ()
                {
                        if (vector != null)
                                return;

                        vector = new BitArray (Count, false);
                        if (InheritsFrom != null)
                                Vector = InheritsFrom.Vector;

                        is_dirty = true;
                }

                public override string ToString ()
                {
                        StringBuilder sb = new StringBuilder ("MyBitVector (");

                        BitArray vector = Vector;
                        sb.Append (Count);
                        sb.Append (",");
                        if (!IsDirty)
                                sb.Append ("INHERITED - ");
                        for (int i = 0; i < vector.Count; i++) {
                                if (i > 0)
                                        sb.Append (",");
                                sb.Append (vector [i]);
                        }
                        
                        sb.Append (")");
                        return sb.ToString ();
                }
        }
}