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package org.jruby.ir.representations;

import java.util.ArrayList;
import java.util.BitSet;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.ListIterator;
import java.util.Map;
import java.util.Set;
import java.util.Stack;
import org.jruby.ir.IRClosure;
import org.jruby.ir.IRScope;
import org.jruby.ir.Operation;
import org.jruby.ir.instructions.BranchInstr;
import org.jruby.ir.instructions.CallBase;
import org.jruby.ir.instructions.ExceptionRegionEndMarkerInstr;
import org.jruby.ir.instructions.ExceptionRegionStartMarkerInstr;
import org.jruby.ir.instructions.Instr;
import org.jruby.ir.instructions.JumpIndirectInstr;
import org.jruby.ir.instructions.JumpInstr;
import org.jruby.ir.instructions.LabelInstr;
import org.jruby.ir.instructions.SetReturnAddressInstr;
import org.jruby.ir.instructions.ThrowExceptionInstr;
import org.jruby.ir.operands.Label;
import org.jruby.ir.operands.Operand;
import org.jruby.ir.operands.Variable;
import org.jruby.ir.operands.WrappedIRClosure;
import org.jruby.ir.transformations.inlining.InlinerInfo;
import org.jruby.ir.util.DirectedGraph;
import org.jruby.ir.util.Edge;
import org.jruby.util.log.Logger;
import org.jruby.util.log.LoggerFactory;

/**
 * Represents the base build of a CFG.  All information here is accessed via
 * delegation from the CFG itself so this is meant as an internal
 * organizational structure for a build.
 */
public class CFG {
    public enum EdgeType {
        REGULAR,       // Any non-special edge.  Not really used.
        EXCEPTION,     // Edge to exception handling basic blocks
        FALL_THROUGH,  // Edge which is the natural fall through choice on a branch
        EXIT           // Edge to dummy exit BB
    }

    private static final Logger LOG = LoggerFactory.getLogger("CFG");

    private IRScope scope;
    private Map bbMap;

    // Map of bb -> first bb of the rescue block that initiates exception handling for all exceptions thrown within this bb
    private Map rescuerMap;

    // Map of bb -> first bb of the ensure block that protects this bb
    private Map ensurerMap;

    private List outermostERs;

    /** Entry BB */
    private BasicBlock entryBB;

    /** Exit BB */
    private BasicBlock exitBB;

    /** BB that traps all exception-edges out of the cfg where we could add any cleanup/ensure code (ex: pop frames, etc.) */
    private BasicBlock globalEnsureBB;

    /** The graph itself */
    private DirectedGraph graph;

    private int nextBBId;       // Next available basic block id

    LinkedList postOrderList; // Post order traversal list of the cfg

    public CFG(IRScope scope) {
        this.scope = scope;
        this.graph = new DirectedGraph();
        this.bbMap = new HashMap();
        this.rescuerMap = new HashMap();
        this.ensurerMap = new HashMap();
        this.outermostERs = new ArrayList();
        this.nextBBId = 0;
        this.entryBB = this.exitBB = null;
        this.globalEnsureBB = null;
        this.postOrderList = null;
    }

    public int getNextBBID() {
        nextBBId++;
        return nextBBId;
    }

    public int getMaxNodeID() {
        return nextBBId;
    }

    public boolean bbIsProtected(BasicBlock b) {
        // No need to look in ensurerMap because (_bbEnsurerMap(b) != null) => (_bbResucerMap(b) != null)
        return getRescuerBBFor(b) != null;
    }

    public BasicBlock getBBForLabel(Label label) {
        return bbMap.get(label);
    }

    public BasicBlock getEnsurerBBFor(BasicBlock block) {
        return ensurerMap.get(block);
    }

    public BasicBlock getEntryBB() {
        return entryBB;
    }

    public BasicBlock getExitBB() {
        return exitBB;
    }

    public BasicBlock getGlobalEnsureBB() {
        return globalEnsureBB;
    }

    public List getOutermostExceptionRegions() {
        return outermostERs;
    }

    public LinkedList postOrderList() {
        if (postOrderList == null) postOrderList = buildPostOrderList();
        return postOrderList;
    }

    public ListIterator getPostOrderTraverser() {
        return postOrderList().listIterator();
    }

    public ListIterator getReversePostOrderTraverser() {
        return postOrderList().listIterator(size());
    }

    public void resetState() {
        // SSS FIXME: anything else?
        postOrderList = null;
    }

    public IRScope getScope() {
        return scope;
    }

    public int size() {
        return graph.size();
    }

    public Collection getBasicBlocks() {
        return graph.allData();
    }

    public Collection getSortedBasicBlocks() {
        return graph.getInorderData();
    }

    public void addEdge(BasicBlock source, BasicBlock destination, Object type) {
        graph.vertexFor(source).addEdgeTo(destination, type);
    }

    public int inDegree(BasicBlock b) {
        return graph.findVertexFor(b).inDegree();
    }

    public int outDegree(BasicBlock b) {
        return graph.findVertexFor(b).outDegree();
    }

    public Iterable getIncomingSources(BasicBlock block) {
        return graph.findVertexFor(block).getIncomingSourcesData();
    }

    public Iterable> getIncomingEdges(BasicBlock block) {
        return graph.findVertexFor(block).getIncomingEdges();
    }

    public BasicBlock getIncomingSource(BasicBlock block) {
        return graph.findVertexFor(block).getIncomingSourceData();
    }

    public BasicBlock getIncomingSourceOfType(BasicBlock block, Object type) {
        return graph.findVertexFor(block).getIncomingSourceDataOfType(type);
    }

    public Edge getIncomingEdgeOfType(BasicBlock block, Object type) {
        return graph.findVertexFor(block).getIncomingEdgeOfType(type);
    }

    public Edge getOutgoingEdgeOfType(BasicBlock block, Object type) {
        return graph.findVertexFor(block).getOutgoingEdgeOfType(type);
    }

    public BasicBlock getOutgoingDestination(BasicBlock block) {
        return graph.findVertexFor(block).getOutgoingDestinationData();
    }

    public BasicBlock getOutgoingDestinationOfType(BasicBlock block, Object type) {
        return graph.findVertexFor(block).getOutgoingDestinationDataOfType(type);
    }

    public Iterable getOutgoingDestinations(BasicBlock block) {
        return graph.findVertexFor(block).getOutgoingDestinationsData();
    }

    public Iterable getOutgoingDestinationsOfType(BasicBlock block, Object type) {
        return graph.findVertexFor(block).getOutgoingDestinationsDataOfType(type);
    }

    public Iterable getOutgoingDestinationsNotOfType(BasicBlock block, Object type) {
        return graph.findVertexFor(block).getOutgoingDestinationsDataNotOfType(type);
    }

    public Set> getOutgoingEdges(BasicBlock block) {
        return graph.findVertexFor(block).getOutgoingEdges();
    }

    public Iterable> getOutgoingEdgesNotOfType(BasicBlock block, Object type) {
        return graph.findVertexFor(block).getOutgoingEdgesNotOfType(type);
    }

    public BasicBlock getRescuerBBFor(BasicBlock block) {
        return rescuerMap.get(block);
    }

    /* Add 'b' as a global ensure block that protects all unprotected blocks in this scope */
    public void addGlobalEnsureBB(BasicBlock geb) {
        assert globalEnsureBB == null: "CFG for scope " + getScope() + " already has a global ensure block.";

        globalEnsureBB = geb;

        addEdge(geb, getExitBB(), EdgeType.EXIT);

        for (BasicBlock b: getBasicBlocks()) {
            if (b != geb && !bbIsProtected(b)) {
                addEdge(b, geb, EdgeType.EXCEPTION);
                setRescuerBB(b, geb);
                setEnsurerBB(b, geb);
            }
        }

        // We are not creating a global exception region and adding it to the list of
        // exc. regions because in graph form, we dont know what the "last BB" is.
        // That requires a linearized form of the CFG's bbs.  So, the JIT can add
        // a special case for this global exception block since it has access to the
        // linearized form.
    }

    public void setEnsurerBB(BasicBlock block, BasicBlock ensureBlock) {
        ensurerMap.put(block, ensureBlock);
    }

    public void setRescuerBB(BasicBlock block, BasicBlock rescuerBlock) {
        rescuerMap.put(block, rescuerBlock);
    }

    /**
     *  Build the Control Flow Graph
     */
    public DirectedGraph build(List instrs) {
        // Map of label & basic blocks which are waiting for a bb with that label
        Map> forwardRefs = new HashMap>();

        // Map of return address variable and all possible targets (required to connect up ensure blocks with their targets)
        Map> retAddrMap = new HashMap>();
        Map retAddrTargetMap = new HashMap();

        // List of bbs that have a 'return' instruction
        List returnBBs = new ArrayList();

        // List of bbs that have a 'throw' instruction
        List exceptionBBs = new ArrayList();

        // Stack of nested rescue regions
        Stack nestedExceptionRegions = new Stack();

        // List of all rescued regions
        List allExceptionRegions = new ArrayList();

        // Dummy entry basic block (see note at end to see why)
        entryBB = createBB(nestedExceptionRegions);

        // First real bb
        BasicBlock firstBB = createBB(nestedExceptionRegions);

        // Build the rest!
        BasicBlock currBB = firstBB;
        BasicBlock newBB;
        boolean bbEnded = false;
        boolean nextBBIsFallThrough = true;
        for (Instr i : instrs) {
            Operation iop = i.getOperation();
            if (iop == Operation.LABEL) {
                Label l = ((LabelInstr) i).label;
                newBB = createBB(l, nestedExceptionRegions);
                // Jump instruction bbs dont add an edge to the succeeding bb by default
                if (nextBBIsFallThrough) graph.addEdge(currBB, newBB, EdgeType.FALL_THROUGH);
                currBB = newBB;
                bbEnded = false;
                nextBBIsFallThrough = true;

                // Add forward reference edges
                List frefs = forwardRefs.get(l);
                if (frefs != null) {
                    for (BasicBlock b : frefs) {
                        graph.addEdge(b, newBB, EdgeType.REGULAR);
                    }
                }
            } else if (bbEnded && (iop != Operation.EXC_REGION_END)) {
                newBB = createBB(nestedExceptionRegions);
                // Jump instruction bbs dont add an edge to the succeeding bb by default
                if (nextBBIsFallThrough) graph.addEdge(currBB, newBB, EdgeType.FALL_THROUGH); // currBB cannot be null!
                currBB = newBB;
                bbEnded = false;
                nextBBIsFallThrough = true;
            }

            if (i instanceof ExceptionRegionStartMarkerInstr) {
                // We dont need the instruction anymore -- so it is not added to the CFG.
                ExceptionRegionStartMarkerInstr ersmi = (ExceptionRegionStartMarkerInstr) i;
                ExceptionRegion rr = new ExceptionRegion(ersmi.firstRescueBlockLabel, ersmi.ensureBlockLabel, currBB);
                rr.addBB(currBB);
                allExceptionRegions.add(rr);

                if (nestedExceptionRegions.empty()) {
                    outermostERs.add(rr);
                } else {
                    nestedExceptionRegions.peek().addNestedRegion(rr);
                }

                nestedExceptionRegions.push(rr);
            } else if (i instanceof ExceptionRegionEndMarkerInstr) {
                // We dont need the instruction anymore -- so it is not added to the CFG.
                nestedExceptionRegions.pop().setEndBB(currBB);
            } else if (iop.endsBasicBlock()) {
                bbEnded = true;
                currBB.addInstr(i);
                Label tgt;
                nextBBIsFallThrough = false;
                if (i instanceof BranchInstr) {
                    tgt = ((BranchInstr) i).getJumpTarget();
                    nextBBIsFallThrough = true;
                } else if (i instanceof JumpInstr) {
                    tgt = ((JumpInstr) i).getJumpTarget();
                } else if (iop.isReturn()) { // BREAK, RETURN, CLOSURE_RETURN
                    tgt = null;
                    returnBBs.add(currBB);
                } else if (i instanceof ThrowExceptionInstr) {
                    tgt = null;
                    exceptionBBs.add(currBB);
                } else if (i instanceof JumpIndirectInstr) {
                    tgt = null;
                    Set retAddrs = retAddrMap.get(((JumpIndirectInstr) i).getJumpTarget());
                    for (Label l : retAddrs) {
                        addEdge(currBB, l, forwardRefs);
                    }
                    // Record the target bb for the retaddr var for any set_addr instrs that appear later and use the same retaddr var
                    retAddrTargetMap.put(((JumpIndirectInstr) i).getJumpTarget(), currBB);
                } else {
                    throw new RuntimeException("Unhandled case in CFG builder for basic block ending instr: " + i);
                }

                if (tgt != null) addEdge(currBB, tgt, forwardRefs);
            } else if (iop != Operation.LABEL) {
                currBB.addInstr(i);
            }

            if (i instanceof SetReturnAddressInstr) {
                Variable v = ((SetReturnAddressInstr) i).getResult();
                Label tgtLbl = ((SetReturnAddressInstr) i).getReturnAddr();
                BasicBlock tgtBB = retAddrTargetMap.get(v);
                // If we have the target bb, add the edge
                // If not, record it for fixup later
                if (tgtBB != null) {
                    addEdge(tgtBB, tgtLbl, forwardRefs);
                } else {
                    Set addrs = retAddrMap.get(v);
                    if (addrs == null) {
                        addrs = new HashSet();
                        retAddrMap.put(v, addrs);
                    }
                    addrs.add(tgtLbl);
                }
            } else if (i instanceof CallBase) { // Build CFG for the closure if there exists one
                Operand closureArg = ((CallBase) i).getClosureArg(getScope().getManager().getNil());
                if (closureArg instanceof WrappedIRClosure) {
                    ((WrappedIRClosure) closureArg).getClosure().buildCFG();
                }
            }
        }

        // Process all rescued regions
        for (ExceptionRegion rr : allExceptionRegions) {
            BasicBlock firstRescueBB = bbMap.get(rr.getFirstRescueBlockLabel());

            // Mark the BB as a rescue entry BB
            firstRescueBB.markRescueEntryBB();

            // 1. Tell the region that firstRescueBB is its protector!
            rr.setFirstRescueBB(firstRescueBB);

            // 2. Record a mapping from the region's exclusive basic blocks to the first bb that will start exception handling for all their exceptions.
            // 3. Add an exception edge from every exclusive bb of the region to firstRescueBB
            BasicBlock ensureBlockBB = rr.getEnsureBlockLabel() == null ? null : bbMap.get(rr.getEnsureBlockLabel());
            for (BasicBlock b : rr.getExclusiveBBs()) {
                setRescuerBB(b, firstRescueBB);
                graph.addEdge(b, firstRescueBB, EdgeType.EXCEPTION);
                if (ensureBlockBB != null) {
                    setEnsurerBB(b, ensureBlockBB);
                    if (ensureBlockBB != firstRescueBB) {
                        // SSS FIXME: This is a conservative edge because when a rescue block is present
                        // that catches an exception, control never reaches the ensure block directly.
                        // Only when we get an error or threadkill even, or when breaks propagate upward
                        // do we need to hit an ensure directly.  This edge is present to account for that
                        // control-flow scenario.
                        graph.addEdge(b, ensureBlockBB, EdgeType.EXCEPTION);
                    }
                }
            }
        }

        buildExitBasicBlock(nestedExceptionRegions, firstBB, returnBBs, exceptionBBs, nextBBIsFallThrough, currBB, entryBB);

        optimize(); // remove useless cfg edges & orphaned bbs

        /*
        for (ExceptionRegion er: this.outermostERs) {
            System.out.println(er);
        }
        */

        return graph;
    }

    private void addEdge(BasicBlock src, Label targetLabel, Map> forwardRefs) {
        BasicBlock target = bbMap.get(targetLabel);

        if (target != null) {
            graph.addEdge(src, target, EdgeType.REGULAR);
            return;
        }

        // Add a forward reference from target -> source
        List forwardReferences = forwardRefs.get(targetLabel);

        if (forwardReferences == null) {
            forwardReferences = new ArrayList();
            forwardRefs.put(targetLabel, forwardReferences);
        }

        forwardReferences.add(src);
    }

    /**
     * Create special empty exit BasicBlock that all BasicBlocks will eventually
     * flow into.  All Edges to this 'dummy' BasicBlock will get marked with
     * an edge type of EXIT.
     *
     * Special BasicBlocks worth noting:
     * 1. Exceptions, Returns, Entry(why?) -> ExitBB
     * 2. Returns -> ExitBB
     */
    private BasicBlock buildExitBasicBlock(Stack nestedExceptionRegions, BasicBlock firstBB,
            List returnBBs, List exceptionBBs, boolean nextIsFallThrough, BasicBlock currBB, BasicBlock entryBB) {
        exitBB = createBB(nestedExceptionRegions);

        graph.addEdge(entryBB, exitBB, EdgeType.EXIT);
        graph.addEdge(entryBB, firstBB, EdgeType.FALL_THROUGH);

        for (BasicBlock rb : returnBBs) {
            graph.addEdge(rb, exitBB, EdgeType.EXIT);
        }

        for (BasicBlock rb : exceptionBBs) {
            graph.addEdge(rb, exitBB, EdgeType.EXIT);
        }

        if (nextIsFallThrough) graph.addEdge(currBB, exitBB, EdgeType.EXIT);

        return exitBB;
    }

    private BasicBlock createBB(Label label, Stack nestedExceptionRegions) {
        BasicBlock basicBlock = new BasicBlock(this, label);
        addBasicBlock(basicBlock);

        if (!nestedExceptionRegions.empty()) nestedExceptionRegions.peek().addBB(basicBlock);

        return basicBlock;
    }

    private BasicBlock createBB(Stack nestedExceptionRegions) {
        return createBB(scope.getNewLabel(), nestedExceptionRegions);
    }

   public void addBasicBlock(BasicBlock bb) {
        graph.vertexFor(bb); // adds vertex to graph
        bbMap.put(bb.getLabel(), bb);
   }

    public void removeEdge(Edge edge) {
        graph.removeEdge(edge);
    }

    public void removeAllOutgoingEdgesForBB(BasicBlock b) {
        graph.findVertexFor(b).removeAllOutgoingEdges();
    }

    private void deleteOrphanedBlocks(DirectedGraph graph) {
        // System.out.println("\nGraph:\n" + toStringGraph());
        // System.out.println("\nInstructions:\n" + toStringInstrs());

        // FIXME: Quick and dirty implementation
        while (true) {
            BasicBlock bbToRemove = null;
            for (BasicBlock b : graph.allData()) {
                if (b == entryBB) continue; // Skip entry bb!

                // Every other bb should have at least one incoming edge
                if (graph.findVertexFor(b).getIncomingEdges().isEmpty()) {
                    bbToRemove = b;
                    break;
                }
            }
            if (bbToRemove == null) break;

            removeBB(bbToRemove);
        }
    }

    private boolean mergeBBs(BasicBlock a, BasicBlock b) {
        BasicBlock aR = getRescuerBBFor(a);
        BasicBlock bR = getRescuerBBFor(b);
        // We can merge 'a' and 'b' if one of the following is true:
        // 1. 'a' and 'b' are both not empty
        //    They are protected by the same rescue block.
        //    NOTE: We need not check the ensure block map because all ensure blocks are already
        //    captured in the bb rescue block map.  So, if aR == bR, it is guaranteed that the
        //    ensure blocks for the two are identical.
        // 2. One of 'a' or 'b' is empty.  We dont need to check for rescue block match because
        //    an empty basic block cannot raise an exception, can it?
        if ((aR == bR) || a.isEmpty() || b.isEmpty()) {
            // First, remove straight-line jump, if present
            Instr lastInstr = a.getLastInstr();
            if (lastInstr instanceof JumpInstr) a.removeInstr(lastInstr);

            // Swallow b's instrs.
            a.swallowBB(b);

            // Fixup edges
            removeEdge(a, b);
            for (Edge e : getOutgoingEdges(b)) {
                addEdge(a, e.getDestination().getData(), e.getType());
            }

            // Delete bb
            removeBB(b);

            // Update rescue and ensure maps
            if ((aR == null) && (bR != null)) {
                setRescuerBB(a, bR);
                BasicBlock aE = getEnsurerBBFor(a);
                BasicBlock bE = getEnsurerBBFor(b);
                if ((aE == null) && (bE != null)) {
                    setRescuerBB(a, bE);
                    setEnsurerBB(a, bE);
                }
            }

            // Fix up exception regions
            for (ExceptionRegion er: this.outermostERs) {
                er.mergeBBs(a, b);
            }

            return true;
        } else {
            return false;
        }
    }

    public void removeBB(BasicBlock b) {
        graph.removeVertexFor(b);
        bbMap.remove(b.getLabel());
        rescuerMap.remove(b);
        ensurerMap.remove(b);
        // SSS FIXME: Patch up rescued regions as well??
    }

    public void collapseStraightLineBBs() {
        // Collect cfgs in a list first since the cfg/graph API returns an iterator
        // over live data.  But, basic block merging modifies that live data.
        //
        // SSS FIXME: So, we need a cfg/graph API that returns an iterator over
        // frozen data rather than live data.
        List cfgBBs = new ArrayList();
        for (BasicBlock b: getBasicBlocks()) cfgBBs.add(b);

        Set mergedBBs = new HashSet();
        for (BasicBlock b: cfgBBs) {
            if (!mergedBBs.contains(b) && (outDegree(b) == 1)) {
                for (Edge e : getOutgoingEdges(b)) {
                    BasicBlock outB = e.getDestination().getData();
                    if ((e.getType() != EdgeType.EXCEPTION) && (inDegree(outB) == 1) && (mergeBBs(b, outB) == true)) {
                        mergedBBs.add(outB);
                    }
                }
            }
        }
    }

    private void optimize() {
        // SSS FIXME: Can't we not add some of these exception edges in the first place??
        // Remove exception edges from blocks that couldn't possibly thrown an exception!
        List toRemove = new ArrayList();
        for (BasicBlock b : graph.allData()) {
            boolean noExceptions = true;
            for (Instr i : b.getInstrs()) {
                if (i.canRaiseException()) {
                    noExceptions = false;
                    break;
                }
            }

            if (noExceptions) {
                for (Edge e : graph.findVertexFor(b).getOutgoingEdgesOfType(EdgeType.EXCEPTION)) {
                    BasicBlock source = e.getSource().getData();
                    BasicBlock destination = e.getDestination().getData();
                    toRemove.add(e);

                    if (rescuerMap.get(source) == destination) rescuerMap.remove(source);
                    if (ensurerMap.get(source) == destination) ensurerMap.remove(source);
                }
            }
        }

        if (!toRemove.isEmpty()) {
            for (Edge edge: toRemove) {
                graph.removeEdge(edge);
            }
        }

        deleteOrphanedBlocks(graph);

        collapseStraightLineBBs();
    }

    public String toStringGraph() {
        return graph.toString();
    }

    public String toStringInstrs() {
        StringBuilder buf = new StringBuilder();

        for (BasicBlock b : getSortedBasicBlocks()) {
            buf.append(b.toStringInstrs());
        }
        buf.append("\n\n------ Rescue block map ------\n");
        for (BasicBlock bb : rescuerMap.keySet()) {
            buf.append("BB ").append(bb.getID()).append(" --> BB ").append(rescuerMap.get(bb).getID()).append("\n");
        }
        buf.append("\n\n------ Ensure block map ------\n");
        for (BasicBlock bb : ensurerMap.keySet()) {
            buf.append("BB ").append(bb.getID()).append(" --> BB ").append(ensurerMap.get(bb).getID()).append("\n");
        }

        List closures = scope.getClosures();
        if (!closures.isEmpty()) {
            buf.append("\n\n------ Closures encountered in this scope ------\n");
            for (IRClosure c : closures) {
                buf.append(c.toStringBody());
            }
            buf.append("------------------------------------------------\n");
        }

        return buf.toString();
    }

    public void removeEdge(BasicBlock a, BasicBlock b) {
       graph.removeEdge(a, b);
    }

    private LinkedList buildPostOrderList() {
        BasicBlock             root    = getEntryBB();
        LinkedList list    = new LinkedList();
        Stack      stack   = new Stack();
        BitSet                 visited = new BitSet(1 + getMaxNodeID());

        stack.push(root);
        visited.set(root.getID());

        // Non-recursive post-order traversal (the added flag is required to handle cycles and common ancestors)
        while (!stack.empty()) {
            // Check if all children of the top of the stack have been added
            BasicBlock b = stack.peek();
            boolean allChildrenVisited = true;
            for (BasicBlock dst: getOutgoingDestinations(b)) {
                int dstID = dst.getID();
                if (!visited.get(dstID)) {
                    allChildrenVisited = false;
                    // This ensures that no matter what order we visit children, we process exit nodes before anything.
                    // else.  Alternatively, getOutgoingDestinations(..) would have to return nodes in a specific order
                    // that is dependent on basic block numbering -- which would be fragile.
                    if (graph.findVertexFor(dst).outDegree() == 0) {
                        list.add(dst);
                    } else {
                        stack.push(dst);
                    }
                    visited.set(dstID);
                }
            }

            // If all children have been added previously, we are ready with 'b' in this round!
            if (allChildrenVisited) {
                stack.pop();
                list.add(b);
            }
        }

        // Sanity check!
        for (BasicBlock b : getBasicBlocks()) {
            if (!visited.get(b.getID())) {
                printError("BB " + b.getID() + " missing from po list!");
                break;
            }
        }

        return list;
    }

    public CFG cloneForCloningClosure(IRScope scope, InlinerInfo ii) {
        Map cloneBBMap = new HashMap();
        CFG clone = new CFG(scope);

        // clone bbs
        for (BasicBlock b : getBasicBlocks()) {
            BasicBlock bCloned = new BasicBlock(clone, b.getLabel().clone());
            for (Instr i: b.getInstrs()) {
                Instr clonedInstr = i.cloneForBlockCloning(ii);
                if (clonedInstr != null) bCloned.addInstr(clonedInstr);
            }
            clone.addBasicBlock(bCloned);
            cloneBBMap.put(b, bCloned);
        }

        // clone edges
        for (BasicBlock x : getBasicBlocks()) {
             BasicBlock rx = cloneBBMap.get(x);
             for (Edge e : getOutgoingEdges(x)) {
                 BasicBlock b = e.getDestination().getData();
                 clone.addEdge(rx, cloneBBMap.get(b), e.getType());
             }
        }

        clone.entryBB = cloneBBMap.get(entryBB);
        clone.exitBB  = cloneBBMap.get(exitBB);

        // SSS FIXME: Is this required after cfg is built?
        clone.outermostERs = null;

        // Clone rescuer map
        for (BasicBlock b: rescuerMap.keySet()) {
            clone.setRescuerBB(cloneBBMap.get(b), cloneBBMap.get(rescuerMap.get(b)));
        }

        // Clone ensurer map
        for (BasicBlock b: ensurerMap.keySet()) {
            clone.setEnsurerBB(cloneBBMap.get(b), cloneBBMap.get(ensurerMap.get(b)));
        }

        return clone;
    }

    private void printError(String message) {
        LOG.error(message + "\nGraph:\n" + this + "\nInstructions:\n" + toStringInstrs());
    }
}