2006-09-21 Martin Baulig <martin@ximian.com>

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

//
// flowanalyis.cs: The control flow analysis code
//
// Author:
//   Martin Baulig (martin@ximian.com)
//   Raja R Harinath (rharinath@novell.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
{
        public enum TriState : byte {
                // Never < Sometimes < Always
                Never,
                Sometimes,
                Always
        }

        // <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 : byte {
                        // Normal (conditional or toplevel) block.
                        Block,

                        // Conditional.
                        Conditional,

                        // A loop block.
                        Loop,

                        // The statement embedded inside a loop
                        Embedded,

                        // part of a block headed by a jump target
                        Labeled,

                        // Try/Catch block.
                        Exception,

                        // Switch block.
                        Switch,

                        // Switch section.
                        SwitchSection,

                        // The toplevel block of a function
                        Toplevel
                }

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

                public sealed class Reachability
                {
                        TriState returns, throws, barrier;

                        public TriState Returns {
                                get { return returns; }
                        }
                        public TriState Throws {
                                get { return throws; }
                        }
                        public TriState Barrier {
                                get { return barrier; }
                        }

                        Reachability (TriState returns, TriState throws, TriState barrier)
                        {
                                this.returns = returns;
                                this.throws = throws;
                                this.barrier = barrier;
                        }

                        public Reachability Clone ()
                        {
                                return new Reachability (returns, throws, barrier);
                        }

                        public static TriState TriState_Meet (TriState a, TriState b)
                        {
                                // (1) if both are Never, return Never
                                // (2) if both are Always, return Always
                                // (3) otherwise, return Sometimes
                                // note that (3) => (3') if both are Sometimes, return Sometimes
                                return a == b ? a : TriState.Sometimes;
                        }

                        public static TriState TriState_Max (TriState a, TriState b)
                        {
                                return ((byte) a > (byte) b) ? a : b;
                        }

                        public void Meet (Reachability b)
                        {
                                if ((AlwaysReturns && b.AlwaysHasBarrier) || (AlwaysHasBarrier && b.AlwaysReturns))
                                        returns = TriState.Always;
                                else
                                        returns = TriState_Meet (returns, b.returns);

                                throws = TriState_Meet (throws, b.throws);
                                barrier = TriState_Meet (barrier, b.barrier);
                        }

                        public void Or (Reachability b)
                        {
                                returns = TriState_Max (returns, b.returns);
                                throws = TriState_Max (throws, b.throws);
                                barrier = TriState_Max (barrier, b.barrier);
                        }

                        public static Reachability Always ()
                        {
                                return new Reachability (TriState.Never, TriState.Never, TriState.Never);
                        }

                        TriState Unreachable {
                                get { return TriState_Max (returns, TriState_Max (throws, barrier)); }
                        }

                        TriState Reachable {
                                get {
                                        TriState unreachable = Unreachable;
                                        if (unreachable == TriState.Sometimes)
                                                return TriState.Sometimes;
                                        return unreachable == TriState.Always ? TriState.Never : TriState.Always;
                                }
                        }

                        public bool AlwaysReturns {
                                get { return returns == TriState.Always; }
                        }

                        public bool AlwaysThrows {
                                get { return throws == TriState.Always; }
                        }

                        public bool AlwaysHasBarrier {
                                get { return barrier == TriState.Always; }
                        }

                        public bool IsUnreachable {
                                get { return Unreachable == TriState.Always; }
                        }

                        public void SetReturns ()
                        {
                                returns = TriState.Always;
                        }

                        public void SetThrows ()
                        {
                                throws = TriState.Always;
                        }

                        public void SetBarrier ()
                        {
                                barrier = TriState.Always;
                        }

                        static string ShortName (TriState returns)
                        {
                                switch (returns) {
                                case TriState.Never:
                                        return "N";
                                case TriState.Sometimes:
                                        return "S";
                                default:
                                        return "A";
                                }
                        }

                        public override string ToString ()
                        {
                                return String.Format ("[{0}:{1}:{2}:{3}]",
                                                      ShortName (returns), ShortName (throws), ShortName (barrier),
                                                      ShortName (Reachable));
                        }
                }

                public static FlowBranching CreateBranching (FlowBranching parent, BranchingType type, Block block, Location loc)
                {
                        switch (type) {
                        case BranchingType.Exception:
                        case BranchingType.Labeled:
                        case BranchingType.Toplevel:
                                throw new InvalidOperationException ();

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

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

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

                        case BranchingType.Loop:
                                return new FlowBranchingBreakable (parent, type, SiblingType.Conditional, block, loc);

                        case BranchingType.Embedded:
                                return new FlowBranchingContinuable (parent, type, SiblingType.Conditional, 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;

                protected VariableMap param_map, local_map;

                static int next_id = 0;
                int id;

                // <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 Location Location;

                        // <summary>
                        //   This is only valid for SwitchSection, Try, Catch and Finally.
                        // </summary>
                        public readonly Block Block;

                        // <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;

                        // <summary>
                        //   This is used to construct a list of UsageVector's.
                        // </summary>
                        public UsageVector Next;

                        //
                        // Private.
                        //
                        MyBitVector locals, parameters;
                        Reachability reachability;

                        static int next_id = 0;
                        int id;

                        //
                        // Normally, you should not use any of these constructors.
                        //
                        public UsageVector (SiblingType type, UsageVector parent, Block block, Location loc, int num_params, int num_locals)
                        {
                                this.Type = type;
                                this.Block = block;
                                this.Location = loc;
                                this.InheritsFrom = parent;
                                this.CountParameters = num_params;
                                this.CountLocals = num_locals;

                                locals = num_locals == 0 
                                        ? MyBitVector.Empty
                                        : new MyBitVector (parent == null ? MyBitVector.Empty : parent.locals, num_locals);

                                parameters = num_params == 0
                                        ? MyBitVector.Empty
                                        : new MyBitVector (parent == null ? MyBitVector.Empty : parent.parameters, num_params);

                                reachability = parent == null ? Reachability.Always () : parent.Reachability.Clone ();

                                id = ++next_id;
                        }

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

                        public UsageVector (MyBitVector parameters, MyBitVector locals, Reachability reachability, Block block, Location loc)
                        {
                                this.Type = SiblingType.Block;
                                this.Location = loc;
                                this.Block = block;

                                this.reachability = reachability;
                                this.parameters = parameters;
                                this.locals = locals;

                                id = ++next_id;
                        }

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

                                retval.locals = locals.Clone ();
                                retval.parameters = parameters.Clone ();
                                retval.reachability = reachability.Clone ();

                                return retval;
                        }

                        public bool IsAssigned (VariableInfo var, bool ignoreReachability)
                        {
                                if (!ignoreReachability && !var.IsParameter && Reachability.IsUnreachable)
                                        return true;

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

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

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

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

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

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

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

                        public Reachability Reachability {
                                get { return reachability; }
                        }

                        public void Return ()
                        {
                                if (!reachability.IsUnreachable)
                                        reachability.SetReturns ();
                        }

                        public void Throw ()
                        {
                                if (!reachability.IsUnreachable) {
                                        reachability.SetThrows ();
                                        reachability.SetBarrier ();
                                }
                        }

                        public void Goto ()
                        {
                                if (!reachability.IsUnreachable)
                                        reachability.SetBarrier ();
                        }

                        public static UsageVector MergeSiblings (UsageVector sibling_list, Location loc)
                        {
                                if (sibling_list.Next == null)
                                        return sibling_list;

                                MyBitVector locals = null;
                                MyBitVector parameters = null;
                                Reachability reachability = null;

                                for (UsageVector child = sibling_list; child != null; child = child.Next) {
                                        Report.Debug (2, "    MERGING SIBLING   ", reachability, child);

                                        if (reachability == null)
                                                reachability = child.Reachability.Clone ();
                                        else
                                                reachability.Meet (child.Reachability);

                                        // 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.
                                        // 
                                        bool unreachable = child.Reachability.IsUnreachable;

                                        Report.Debug (2, "    MERGING SIBLING #1", reachability,
                                                      child.Type, child.Reachability.IsUnreachable, unreachable);

                                        if (!unreachable)
                                                locals &= child.locals;

                                        // An `out' parameter must be assigned in all branches which do
                                        // not always throw an exception.
                                        if (!child.Reachability.AlwaysThrows)
                                                parameters &= child.parameters;

                                        Report.Debug (2, "    MERGING SIBLING #2", parameters, locals);
                                }
                                
                                if (reachability == null)
                                        throw new InternalErrorException ("Cannot happen: the loop above runs at least twice");

                                return new UsageVector (parameters, locals, reachability, null, loc);
                        }

                        // <summary>
                        //   Merges a child branching.
                        // </summary>
                        public UsageVector MergeChild (UsageVector child, bool overwrite)
                        {
                                Report.Debug (2, "    MERGING CHILD EFFECTS", this, child, reachability, Type);

                                Reachability new_r = child.Reachability;

                                //
                                // 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).
                                //

                                if ((Type == SiblingType.SwitchSection) && !new_r.IsUnreachable) {
                                        Report.Error (163, Location,
                                                      "Control cannot fall through from one " +
                                                      "case label to another");
                                        return child;
                                }

                                locals |= child.locals;
                                parameters |= child.parameters;

                                if (overwrite)
                                        reachability = new_r.Clone ();
                                else
                                        reachability.Or (new_r);

                                return child;
                        }

                        public void MergeOrigins (UsageVector o_vectors)
                        {
                                Report.Debug (1, "  MERGING BREAK ORIGINS", this);

                                if (o_vectors == null)
                                        return;

                                if (reachability.IsUnreachable) {
                                        if (locals != null)
                                                locals.SetAll (true);
                                        if (parameters != null)
                                                parameters.SetAll (true);
                                }

                                for (UsageVector vector = o_vectors; vector != null; vector = vector.Next) {
                                        Report.Debug (1, "    MERGING BREAK ORIGIN", vector);
                                        if (vector.Reachability.IsUnreachable)
                                                continue;
                                        locals &= vector.locals;
                                        parameters &= vector.parameters;
                                        reachability.Meet (vector.Reachability);
                                }

                                Report.Debug (1, "  MERGING BREAK ORIGINS DONE", this);
                        }

                        //
                        // Debugging stuff.
                        //

                        public override string ToString ()
                        {
                                return String.Format ("Vector ({0},{1},{2}-{3}-{4})", Type, id, reachability, parameters, locals);
                        }
                }

                // <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, Block, loc,
                                        param_map.Length, local_map.Length);
                        } else {
                                param_map = Parent.param_map;
                                local_map = Parent.local_map;
                                vector = new UsageVector (
                                        stype, Parent.CurrentUsageVector, null, loc);
                        }

                        AddSibling (vector);
                }

                public abstract UsageVector CurrentUsageVector {
                        get;
                }                               

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

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

                public void CreateSibling ()
                {
                        CreateSibling (null, SiblingType.Conditional);
                }

                protected abstract void AddSibling (UsageVector uv);

                protected abstract UsageVector Merge ();

                // <summary>
                //   Merge a child branching.
                // </summary>
                public UsageVector MergeChild (FlowBranching child)
                {
                        bool overwrite = child.Type == BranchingType.Labeled ||
                                (child.Type == BranchingType.Block && child.Block.Implicit);
                        Report.Debug (2, "  MERGING CHILD", this, child);
                        UsageVector result = CurrentUsageVector.MergeChild (child.Merge (), overwrite);
                        Report.Debug (2, "  MERGING CHILD DONE", this, result);
                        return result;
                }

                public virtual bool InTryWithCatch ()
                {
                        return Parent.InTryWithCatch ();
                }

                // returns true if we crossed an unwind-protected region (try/catch/finally, lock, using, ...)
                public virtual bool AddBreakOrigin (UsageVector vector, Location loc)
                {
                        return Parent.AddBreakOrigin (vector, loc);
                }

                // returns true if we crossed an unwind-protected region (try/catch/finally, lock, using, ...)
                public virtual bool AddContinueOrigin (UsageVector vector, Location loc)
                {
                        return Parent.AddContinueOrigin (vector, loc);
                }

                // returns true if we crossed an unwind-protected region (try/catch/finally, lock, using, ...)
                public virtual bool AddReturnOrigin (UsageVector vector, Location loc)
                {
                        return Parent.AddReturnOrigin (vector, loc);
                }

                // returns true if we crossed an unwind-protected region (try/catch/finally, lock, using, ...)
                public virtual bool AddGotoOrigin (UsageVector vector, Goto goto_stmt)
                {
                        return Parent.AddGotoOrigin (vector, goto_stmt);
                }

                public virtual void StealFinallyClauses (ref ArrayList list)
                {
                        Parent.StealFinallyClauses (ref list);
                }

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

                public bool IsFieldAssigned (VariableInfo vi, string field_name)
                {
                        return CurrentUsageVector.IsAssigned (vi, false) || 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 string Name {
                        get { return String.Format ("{0} ({1}:{2}:{3})", GetType (), id, Type, Location); }
                }
        }

        public class FlowBranchingBlock : FlowBranching
        {
                UsageVector sibling_list = null;

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

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

                protected override void AddSibling (UsageVector sibling)
                {
                        sibling.Next = sibling_list;
                        sibling_list = sibling;
                }

                public override bool AddGotoOrigin (UsageVector vector, Goto goto_stmt)
                {
                        LabeledStatement stmt = Block == null ? null : Block.LookupLabel (goto_stmt.Target);
                        if (stmt == null)
                                return Parent.AddGotoOrigin (vector, goto_stmt);

                        // forward jump
                        goto_stmt.SetResolvedTarget (stmt);
                        stmt.AddUsageVector (vector);
                        return false;
                }

                protected override UsageVector Merge ()
                {
                        Report.Debug (2, "  MERGING SIBLINGS", Name);
                        UsageVector vector = UsageVector.MergeSiblings (sibling_list, Location);
                        Report.Debug (2, "  MERGING SIBLINGS DONE", Name, vector);
                        return vector;
                }
        }

        public class FlowBranchingBreakable : FlowBranchingBlock
        {
                UsageVector break_origins;

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

                public override bool AddBreakOrigin (UsageVector vector, Location loc)
                {
                        vector = vector.Clone ();
                        vector.Next = break_origins;
                        break_origins = vector;
                        return false;
                }

                protected override UsageVector Merge ()
                {
                        UsageVector vector = base.Merge ();
                        vector.MergeOrigins (break_origins);
                        return vector;
                }
        }

        public class FlowBranchingContinuable : FlowBranchingBlock
        {
                UsageVector continue_origins;

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

                public override bool AddContinueOrigin (UsageVector vector, Location loc)
                {
                        vector = vector.Clone ();
                        vector.Next = continue_origins;
                        continue_origins = vector;
                        return false;
                }

                protected override UsageVector Merge ()
                {
                        UsageVector vector = base.Merge ();
                        vector.MergeOrigins (continue_origins);
                        return vector;
                }
        }

        public class FlowBranchingLabeled : FlowBranchingBlock
        {
                LabeledStatement stmt;
                UsageVector actual;

                public FlowBranchingLabeled (FlowBranching parent, LabeledStatement stmt)
                        : base (parent, BranchingType.Labeled, SiblingType.Conditional, null, stmt.loc)
                {
                        this.stmt = stmt;
                        CurrentUsageVector.MergeOrigins (stmt.JumpOrigins);
                        actual = CurrentUsageVector.Clone ();

                        // stand-in for backward jumps
                        CurrentUsageVector.Reachability.Meet (Reachability.Always ());
                }

                public override bool AddGotoOrigin (UsageVector vector, Goto goto_stmt)
                {
                        if (goto_stmt.Target != stmt.Name)
                                return Parent.AddGotoOrigin (vector, goto_stmt);

                        // backward jump
                        goto_stmt.SetResolvedTarget (stmt);
                        actual.MergeOrigins (vector.Clone ());

                        return false;
                }

                protected override UsageVector Merge ()
                {
                        UsageVector vector = base.Merge ();

                        if (actual.Reachability.IsUnreachable)
                                Report.Warning (162, 2, stmt.loc, "Unreachable code detected");

                        actual.MergeChild (vector, false);
                        return actual;
                }
        }

        public class FlowBranchingToplevel : FlowBranchingBlock
        {
                UsageVector return_origins;

                public FlowBranchingToplevel (FlowBranching parent, ToplevelBlock stmt)
                        : base (parent, BranchingType.Toplevel, SiblingType.Conditional, stmt, stmt.loc)
                {
                }

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

                                if (var == null)
                                        continue;

                                if (vector.IsAssigned (var, false))
                                        continue;

                                Report.Error (177, loc, "The out parameter `{0}' must be assigned to before control leaves the current method",
                                        var.Name);
                        }
                }

                public override bool InTryWithCatch ()
                {
                        return false;
                }

                public override bool AddBreakOrigin (UsageVector vector, Location loc)
                {
                        Report.Error (139, loc, "No enclosing loop out of which to break or continue");
                        return false;
                }

                public override bool AddContinueOrigin (UsageVector vector, Location loc)
                {
                        Report.Error (139, loc, "No enclosing loop out of which to break or continue");
                        return false;
                }

                public override bool AddReturnOrigin (UsageVector vector, Location loc)
                {
                        vector = vector.Clone ();
                        vector.Location = loc;
                        vector.Next = return_origins;
                        return_origins = vector;
                        return false;
                }

                public override void StealFinallyClauses (ref ArrayList list)
                {
                        // nothing to do
                }

                public override bool AddGotoOrigin (UsageVector vector, Goto goto_stmt)
                {
                        string name = goto_stmt.Target;
                        LabeledStatement s = Block.LookupLabel (name);
                        if (s != null)
                                throw new InternalErrorException ("Shouldn't get here");

                        if (Parent == null) {
                                Report.Error (159, goto_stmt.loc, "No such label `{0}' in this scope", name);
                                return false;
                        }

                        int errors = Report.Errors;
                        Parent.AddGotoOrigin (vector, goto_stmt);
                        if (errors == Report.Errors)
                                Report.Error (1632, goto_stmt.loc, "Control cannot leave the body of an anonymous method");
                        return false;
                }

                protected override UsageVector Merge ()
                {
                        for (UsageVector origin = return_origins; origin != null; origin = origin.Next)
                                CheckOutParameters (origin, origin.Location);

                        UsageVector vector = base.Merge ();
                        CheckOutParameters (vector, Block.loc);
                        // Note: we _do_not_ merge in the return origins
                        return vector;
                }

                public Reachability End ()
                {
                        return Merge ().Reachability;
                }
        }

        public class FlowBranchingException : FlowBranching
        {
                ExceptionStatement stmt;
                UsageVector current_vector;
                UsageVector catch_vectors;
                UsageVector finally_vector;

                UsageVector break_origins;
                UsageVector continue_origins;
                UsageVector return_origins;
                GotoOrigin goto_origins;

                class GotoOrigin {
                        public GotoOrigin Next;
                        public Goto GotoStmt;
                        public UsageVector Vector;

                        public GotoOrigin (UsageVector vector, Goto goto_stmt, GotoOrigin next)
                        {
                                Vector = vector;
                                GotoStmt = goto_stmt;
                                Next = next;
                        }
                }

                bool emit_finally;

                public FlowBranchingException (FlowBranching parent,
                                               ExceptionStatement stmt)
                        : base (parent, BranchingType.Exception, SiblingType.Try,
                                null, stmt.loc)
                {
                        this.stmt = stmt;
                        this.emit_finally = true;
                }

                protected override void AddSibling (UsageVector sibling)
                {
                        switch (sibling.Type) {
                        case SiblingType.Try:
                        case SiblingType.Catch:
                                sibling.Next = catch_vectors;
                                catch_vectors = sibling;
                                break;
                        case SiblingType.Finally:
                                finally_vector = sibling;
                                break;
                        default:
                                throw new InvalidOperationException ();
                        }
                        current_vector = sibling;
                }

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

                public override bool InTryWithCatch ()
                {
                        if (finally_vector == null) {
                                Try t = stmt as Try;
                                if (t != null && t.HasCatch)
                                        return true;
                        }

                        return base.InTryWithCatch ();
                }

                public override bool AddBreakOrigin (UsageVector vector, Location loc)
                {
                        vector = vector.Clone ();
                        if (finally_vector != null) {
                                vector.MergeChild (finally_vector, false);
                                int errors = Report.Errors;
                                Parent.AddBreakOrigin (vector, loc);
                                if (errors == Report.Errors)
                                        Report.Error (157, loc, "Control cannot leave the body of a finally clause");
                        } else {
                                vector.Location = loc;
                                vector.Next = break_origins;
                                break_origins = vector;
                        }
                        return true;
                }

                public override bool AddContinueOrigin (UsageVector vector, Location loc)
                {
                        vector = vector.Clone ();
                        if (finally_vector != null) {
                                vector.MergeChild (finally_vector, false);
                                int errors = Report.Errors;
                                Parent.AddContinueOrigin (vector, loc);
                                if (errors == Report.Errors)
                                        Report.Error (157, loc, "Control cannot leave the body of a finally clause");
                        } else {
                                vector.Location = loc;
                                vector.Next = continue_origins;
                                continue_origins = vector;
                        }
                        return true;
                }

                public override bool AddReturnOrigin (UsageVector vector, Location loc)
                {
                        vector = vector.Clone ();
                        if (finally_vector != null) {
                                vector.MergeChild (finally_vector, false);
                                int errors = Report.Errors;
                                Parent.AddReturnOrigin (vector, loc);
                                if (errors == Report.Errors)
                                        Report.Error (157, loc, "Control cannot leave the body of a finally clause");
                        } else {
                                vector.Location = loc;
                                vector.Next = return_origins;
                                return_origins = vector;
                        }
                        return true;
                }

                public override bool AddGotoOrigin (UsageVector vector, Goto goto_stmt)
                {
                        LabeledStatement s = current_vector.Block == null ? null : current_vector.Block.LookupLabel (goto_stmt.Target);
                        if (s != null)
                                throw new InternalErrorException ("Shouldn't get here");

                        vector = vector.Clone ();
                        if (finally_vector != null) {
                                vector.MergeChild (finally_vector, false);
                                int errors = Report.Errors;
                                Parent.AddGotoOrigin (vector, goto_stmt);
                                if (errors == Report.Errors)
                                        Report.Error (157, goto_stmt.loc, "Control cannot leave the body of a finally clause");
                        } else {
                                goto_origins = new GotoOrigin (vector, goto_stmt, goto_origins);
                        }
                        return true;
                }

                public override void StealFinallyClauses (ref ArrayList list)
                {
                        if (list == null)
                                list = new ArrayList ();
                        list.Add (stmt);
                        emit_finally = false;
                        base.StealFinallyClauses (ref list);
                }

                public bool EmitFinally {
                        get { return emit_finally; }
                }

                protected override UsageVector Merge ()
                {
                        Report.Debug (2, "  MERGING TRY/CATCH", Name);
                        UsageVector vector = UsageVector.MergeSiblings (catch_vectors, Location);
                        Report.Debug (2, "  MERGING TRY/CATCH DONE", vector);

                        if (finally_vector != null)
                                vector.MergeChild (finally_vector, false);

                        for (UsageVector origin = break_origins; origin != null; origin = origin.Next) {
                                if (finally_vector != null)
                                        origin.MergeChild (finally_vector, false);
                                Parent.AddBreakOrigin (origin, origin.Location);
                        }

                        for (UsageVector origin = continue_origins; origin != null; origin = origin.Next) {
                                if (finally_vector != null)
                                        origin.MergeChild (finally_vector, false);
                                Parent.AddContinueOrigin (origin, origin.Location);
                        }

                        for (UsageVector origin = return_origins; origin != null; origin = origin.Next) {
                                if (finally_vector != null)
                                        origin.MergeChild (finally_vector, false);
                                Parent.AddReturnOrigin (origin, origin.Location);
                        }

                        for (GotoOrigin origin = goto_origins; origin != null; origin = origin.Next) {
                                if (finally_vector != null)
                                        origin.Vector.MergeChild (finally_vector, false);
                                Parent.AddGotoOrigin (origin.Vector, origin.GotoStmt);
                        }

                        return vector;
                }
        }

        // <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 `{0}' must be fully assigned before control leaves the constructor",
                                                TypeManager.GetFullNameSignature (field));
                                        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 = null;
                                        if (tc != null)
                                                fields = tc.Fields;

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

                                        if (fields != null) {
                                                foreach (FieldMember 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);
#if GMCS_SOURCE
                                } else if (type is GenericTypeParameterBuilder) {
                                        CountPublic = CountNonPublic = Count = 0;

                                        Fields = new FieldInfo [0];
#endif
                                } 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.OmitStructFlowAnalysis && TypeInfo.IsStruct ||
                                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 == null)
                                return true;

                        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.OmitStructFlowAnalysis && TypeInfo.IsStruct ||
                                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;

                VariableInfo[] map;

                public VariableMap (Parameters ip)
                {
                        Count = ip != null ? ip.Count : 0;
                        
                        // Dont bother allocating anything!
                        if (Count == 0)
                                return;
                        
                        Length = 0;

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

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

                                // Dont allocate till we find an out var.
                                if (map == null)
                                        map = new VariableInfo [Count];

                                map [i] = new VariableInfo (ip.ParameterName (i),
                                        TypeManager.GetElementType (ip.ParameterType (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 && parent.map != null) {
                                offset = parent.Length;
                                start = parent.Count;
                        }

                        Count = locals.Length + start;
                        
                        if (Count == 0)
                                return;
                        
                        map = new VariableInfo [Count];
                        Length = offset;

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

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

                                if (li.VariableType == null)
                                        continue;

                                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 {
                                if (map == null)
                                        return null;
                                
                                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 static readonly MyBitVector Empty = new MyBitVector ();

                BitArray vector, shared;

                MyBitVector ()
                {
                        vector = null;
                        shared = new BitArray (0, false);
                        Count = 0;
                }

                public MyBitVector (MyBitVector InheritsFrom, int Count)
                {
                        vector = null;
                        shared = InheritsFrom == null ? null : InheritsFrom.Shared;
                        this.Count = Count;
                }

                BitArray Shared {
                        get {
                                if (shared == null) {
                                        shared = vector;
                                        vector = null;
                                }
                                return shared;
                        }
                }

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

                                if (vector != null)
                                        return vector [index];
                                if (shared == null)
                                        return true;
                                if (index < shared.Count)
                                        return shared [index];
                                return false;
                        }

                        set {
                                // Only copy the vector if we're actually modifying it.
                                if (this [index] != value) {
                                        if (vector == null)
                                                initialize_vector ();
                                        vector [index] = value;
                                }
                        }
                }

                // <summary>
                //   Performs an `or' operation on the bit vector.  The `new_vector' may have a
                //   different size than the current one.
                // </summary>
                private MyBitVector Or (MyBitVector new_vector)
                {
                        int min = new_vector.Count;
                        if (Count < min)
                                min = Count;

                        for (int i = 0; i < min; i++)
                                this [i] |= new_vector [i];

                        return this;
                }

                // <summary>
                //   Perfonrms an `and' operation on the bit vector.  The `new_vector' may have
                //   a different size than the current one.
                // </summary>
                private MyBitVector And (MyBitVector new_vector)
                {
                        int min = new_vector.Count;
                        if (Count < min)
                                min = Count;

                        for (int i = 0; i < min; i++)
                                this [i] &= new_vector [i];

                        for (int i = min; i < Count; i++)
                                this [i] = false;

                        return this;
                }

                public static MyBitVector operator & (MyBitVector a, MyBitVector b)
                {
                        if (a == null && b == null)
                                return null;
                        if (a == null)
                                return b.Clone ();
                        if (b == null)
                                return a.Clone ();
                        if (a.Count > b.Count)
                                return a.Clone ().And (b);
                        else
                                return b.Clone ().And (a);                                      
                }

                public static MyBitVector operator | (MyBitVector a, MyBitVector b)
                {
                        if (a == null && b == null)
                                return null;
                        if (a == null)
                                return new MyBitVector (null, b.Count);
                        if (b == null)
                                return new MyBitVector (null, a.Count);
                        if (a.Count > b.Count)
                                return a.Clone ().Or (b);
                        else
                                return b.Clone ().Or (a);
                }

                public MyBitVector Clone ()
                {
                        return Count == 0 ? Empty : new MyBitVector (this, Count);
                }

                public void SetAll (bool value)
                {
                        // Don't clobber Empty
                        if (Count == 0)
                                return;
                        shared = value ? null : Empty.Shared;
                        vector = null;
                }

                void initialize_vector ()
                {
                        if (shared == null) {
                                vector = new BitArray (Count, true);
                                return;
                        }

                        vector = new BitArray (Count, false);

                        int min = shared.Count;
                        if (min > Count)
                                min = Count;

                        for (int i = 0; i < min; i++)
                                vector [i] = shared [i];

                        shared = null;
                }

                StringBuilder Dump (StringBuilder sb)
                {
                        BitArray dump = vector == null ? shared : vector;
                        if (dump == null)
                                return sb.Append ("/");
                        if (dump == shared)
                                sb.Append ("=");
                        for (int i = 0; i < dump.Count; i++)
                                sb.Append (dump [i] ? "1" : "0");
                        return sb;
                }

                public override string ToString ()
                {
                        return Dump (new StringBuilder ("{")).Append ("}").ToString ();
                }
        }
}