http://asm.objectweb.org/asm-tree/: A very small and fast Java bytecode manipulation framework (ObjectWeb)
  • Eric Bruneton
  • Eugene Kuleshov
  • Remi Forax 
ASM: a very small and fast Java bytecode manipulation framework
Copyright (c) 2000-2011 INRIA, France Telecom
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.
3. Neither the name of the copyright holders nor the names of its
   contributors may be used to endorse or promote products derived from
   this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
THE POSSIBILITY OF SUCH DAMAGE.

/***
 * ASM: a very small and fast Java bytecode manipulation framework
 * Copyright (c) 2000-2011 INRIA, France Telecom
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the copyright holders nor the names of its
 *    contributors may be used to endorse or promote products derived from
 *    this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
 * THE POSSIBILITY OF SUCH DAMAGE.
 */
package org.objectweb.asm.tree.analysis;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.objectweb.asm.Opcodes;
import org.objectweb.asm.Type;
import org.objectweb.asm.tree.AbstractInsnNode;
import org.objectweb.asm.tree.IincInsnNode;
import org.objectweb.asm.tree.InsnList;
import org.objectweb.asm.tree.JumpInsnNode;
import org.objectweb.asm.tree.LabelNode;
import org.objectweb.asm.tree.LookupSwitchInsnNode;
import org.objectweb.asm.tree.MethodNode;
import org.objectweb.asm.tree.TableSwitchInsnNode;
import org.objectweb.asm.tree.TryCatchBlockNode;
import org.objectweb.asm.tree.VarInsnNode;


A semantic bytecode analyzer. This class does not fully check that JSR and
RET instructions are valid.

Author:Eric Bruneton
Type parameters:
  • <V> – 
           type of the Value used for the analysis.
/**
 * A semantic bytecode analyzer. <i>This class does not fully check that JSR and
 * RET instructions are valid.</i>
 * 
 * @param <V>
 *            type of the Value used for the analysis.
 * 
 * @author Eric Bruneton
 */
public class Analyzer<V extends Value> implements Opcodes {

    private final Interpreter<V> interpreter;

    private int n;

    private InsnList insns;

    private List<TryCatchBlockNode>[] handlers;

    private Frame<V>[] frames;

    private Subroutine[] subroutines;

    private boolean[] queued;

    private int[] queue;

    private int top;

    
Constructs a new Analyzer. 
Params:
  • interpreter – 
           the interpreter to be used to symbolically interpret the
           bytecode instructions.
/**
     * Constructs a new {@link Analyzer}.
     * 
     * @param interpreter
     *            the interpreter to be used to symbolically interpret the
     *            bytecode instructions.
     */
    public Analyzer(final Interpreter<V> interpreter) {
        this.interpreter = interpreter;
    }

    
Analyzes the given method.

Params:
  • owner – 
           the internal name of the class to which the method belongs.
  • m – 
           the method to be analyzed.
Throws:
Returns:the symbolic state of the execution stack frame at each bytecode
        instruction of the method. The size of the returned array is
        equal to the number of instructions (and labels) of the method. A
        given frame is null if and only if the corresponding
        instruction cannot be reached (dead code).
/**
     * Analyzes the given method.
     * 
     * @param owner
     *            the internal name of the class to which the method belongs.
     * @param m
     *            the method to be analyzed.
     * @return the symbolic state of the execution stack frame at each bytecode
     *         instruction of the method. The size of the returned array is
     *         equal to the number of instructions (and labels) of the method. A
     *         given frame is <tt>null</tt> if and only if the corresponding
     *         instruction cannot be reached (dead code).
     * @throws AnalyzerException
     *             if a problem occurs during the analysis.
     */
    @SuppressWarnings("unchecked")
    public Frame<V>[] analyze(final String owner, final MethodNode m)
            throws AnalyzerException {
        if ((m.access & (ACC_ABSTRACT | ACC_NATIVE)) != 0) {
            frames = (Frame<V>[]) new Frame<?>[0];
            return frames;
        }
        n = m.instructions.size();
        insns = m.instructions;
        handlers = (List<TryCatchBlockNode>[]) new List<?>[n];
        frames = (Frame<V>[]) new Frame<?>[n];
        subroutines = new Subroutine[n];
        queued = new boolean[n];
        queue = new int[n];
        top = 0;

        // computes exception handlers for each instruction
        for (int i = 0; i < m.tryCatchBlocks.size(); ++i) {
            TryCatchBlockNode tcb = m.tryCatchBlocks.get(i);
            int begin = insns.indexOf(tcb.start);
            int end = insns.indexOf(tcb.end);
            for (int j = begin; j < end; ++j) {
                List<TryCatchBlockNode> insnHandlers = handlers[j];
                if (insnHandlers == null) {
                    insnHandlers = new ArrayList<TryCatchBlockNode>();
                    handlers[j] = insnHandlers;
                }
                insnHandlers.add(tcb);
            }
        }

        // computes the subroutine for each instruction:
        Subroutine main = new Subroutine(null, m.maxLocals, null);
        List<AbstractInsnNode> subroutineCalls = new ArrayList<AbstractInsnNode>();
        Map<LabelNode, Subroutine> subroutineHeads = new HashMap<LabelNode, Subroutine>();
        findSubroutine(0, main, subroutineCalls);
        while (!subroutineCalls.isEmpty()) {
            JumpInsnNode jsr = (JumpInsnNode) subroutineCalls.remove(0);
            Subroutine sub = subroutineHeads.get(jsr.label);
            if (sub == null) {
                sub = new Subroutine(jsr.label, m.maxLocals, jsr);
                subroutineHeads.put(jsr.label, sub);
                findSubroutine(insns.indexOf(jsr.label), sub, subroutineCalls);
            } else {
                sub.callers.add(jsr);
            }
        }
        for (int i = 0; i < n; ++i) {
            if (subroutines[i] != null && subroutines[i].start == null) {
                subroutines[i] = null;
            }
        }

        // initializes the data structures for the control flow analysis
        Frame<V> current = newFrame(m.maxLocals, m.maxStack);
        Frame<V> handler = newFrame(m.maxLocals, m.maxStack);
        current.setReturn(interpreter.newValue(Type.getReturnType(m.desc)));
        Type[] args = Type.getArgumentTypes(m.desc);
        int local = 0;
        if ((m.access & ACC_STATIC) == 0) {
            Type ctype = Type.getObjectType(owner);
            current.setLocal(local++, interpreter.newValue(ctype));
        }
        for (int i = 0; i < args.length; ++i) {
            current.setLocal(local++, interpreter.newValue(args[i]));
            if (args[i].getSize() == 2) {
                current.setLocal(local++, interpreter.newValue(null));
            }
        }
        while (local < m.maxLocals) {
            current.setLocal(local++, interpreter.newValue(null));
        }
        merge(0, current, null);

        init(owner, m);

        // control flow analysis
        while (top > 0) {
            int insn = queue[--top];
            Frame<V> f = frames[insn];
            Subroutine subroutine = subroutines[insn];
            queued[insn] = false;

            AbstractInsnNode insnNode = null;
            try {
                insnNode = m.instructions.get(insn);
                int insnOpcode = insnNode.getOpcode();
                int insnType = insnNode.getType();

                if (insnType == AbstractInsnNode.LABEL
                        || insnType == AbstractInsnNode.LINE
                        || insnType == AbstractInsnNode.FRAME) {
                    merge(insn + 1, f, subroutine);
                    newControlFlowEdge(insn, insn + 1);
                } else {
                    current.init(f).execute(insnNode, interpreter);
                    subroutine = subroutine == null ? null : subroutine.copy();

                    if (insnNode instanceof JumpInsnNode) {
                        JumpInsnNode j = (JumpInsnNode) insnNode;
                        if (insnOpcode != GOTO && insnOpcode != JSR) {
                            merge(insn + 1, current, subroutine);
                            newControlFlowEdge(insn, insn + 1);
                        }
                        int jump = insns.indexOf(j.label);
                        if (insnOpcode == JSR) {
                            merge(jump, current, new Subroutine(j.label,
                                    m.maxLocals, j));
                        } else {
                            merge(jump, current, subroutine);
                        }
                        newControlFlowEdge(insn, jump);
                    } else if (insnNode instanceof LookupSwitchInsnNode) {
                        LookupSwitchInsnNode lsi = (LookupSwitchInsnNode) insnNode;
                        int jump = insns.indexOf(lsi.dflt);
                        merge(jump, current, subroutine);
                        newControlFlowEdge(insn, jump);
                        for (int j = 0; j < lsi.labels.size(); ++j) {
                            LabelNode label = lsi.labels.get(j);
                            jump = insns.indexOf(label);
                            merge(jump, current, subroutine);
                            newControlFlowEdge(insn, jump);
                        }
                    } else if (insnNode instanceof TableSwitchInsnNode) {
                        TableSwitchInsnNode tsi = (TableSwitchInsnNode) insnNode;
                        int jump = insns.indexOf(tsi.dflt);
                        merge(jump, current, subroutine);
                        newControlFlowEdge(insn, jump);
                        for (int j = 0; j < tsi.labels.size(); ++j) {
                            LabelNode label = tsi.labels.get(j);
                            jump = insns.indexOf(label);
                            merge(jump, current, subroutine);
                            newControlFlowEdge(insn, jump);
                        }
                    } else if (insnOpcode == RET) {
                        if (subroutine == null) {
                            throw new AnalyzerException(insnNode,
                                    "RET instruction outside of a sub routine");
                        }
                        for (int i = 0; i < subroutine.callers.size(); ++i) {
                            JumpInsnNode caller = subroutine.callers.get(i);
                            int call = insns.indexOf(caller);
                            if (frames[call] != null) {
                                merge(call + 1, frames[call], current,
                                        subroutines[call], subroutine.access);
                                newControlFlowEdge(insn, call + 1);
                            }
                        }
                    } else if (insnOpcode != ATHROW
                            && (insnOpcode < IRETURN || insnOpcode > RETURN)) {
                        if (subroutine != null) {
                            if (insnNode instanceof VarInsnNode) {
                                int var = ((VarInsnNode) insnNode).var;
                                subroutine.access[var] = true;
                                if (insnOpcode == LLOAD || insnOpcode == DLOAD
                                        || insnOpcode == LSTORE
                                        || insnOpcode == DSTORE) {
                                    subroutine.access[var + 1] = true;
                                }
                            } else if (insnNode instanceof IincInsnNode) {
                                int var = ((IincInsnNode) insnNode).var;
                                subroutine.access[var] = true;
                            }
                        }
                        merge(insn + 1, current, subroutine);
                        newControlFlowEdge(insn, insn + 1);
                    }
                }

                List<TryCatchBlockNode> insnHandlers = handlers[insn];
                if (insnHandlers != null) {
                    for (int i = 0; i < insnHandlers.size(); ++i) {
                        TryCatchBlockNode tcb = insnHandlers.get(i);
                        Type type;
                        if (tcb.type == null) {
                            type = Type.getObjectType("java/lang/Throwable");
                        } else {
                            type = Type.getObjectType(tcb.type);
                        }
                        int jump = insns.indexOf(tcb.handler);
                        if (newControlFlowExceptionEdge(insn, tcb)) {
                            handler.init(f);
                            handler.clearStack();
                            handler.push(interpreter.newValue(type));
                            merge(jump, handler, subroutine);
                        }
                    }
                }
            } catch (AnalyzerException e) {
                throw new AnalyzerException(e.node, "Error at instruction "
                        + insn + ": " + e.getMessage(), e);
            } catch (Exception e) {
                throw new AnalyzerException(insnNode, "Error at instruction "
                        + insn + ": " + e.getMessage(), e);
            }
        }

        return frames;
    }

    private void findSubroutine(int insn, final Subroutine sub,
            final List<AbstractInsnNode> calls) throws AnalyzerException {
        while (true) {
            if (insn < 0 || insn >= n) {
                throw new AnalyzerException(null,
                        "Execution can fall off end of the code");
            }
            if (subroutines[insn] != null) {
                return;
            }
            subroutines[insn] = sub.copy();
            AbstractInsnNode node = insns.get(insn);

            // calls findSubroutine recursively on normal successors
            if (node instanceof JumpInsnNode) {
                if (node.getOpcode() == JSR) {
                    // do not follow a JSR, it leads to another subroutine!
                    calls.add(node);
                } else {
                    JumpInsnNode jnode = (JumpInsnNode) node;
                    findSubroutine(insns.indexOf(jnode.label), sub, calls);
                }
            } else if (node instanceof TableSwitchInsnNode) {
                TableSwitchInsnNode tsnode = (TableSwitchInsnNode) node;
                findSubroutine(insns.indexOf(tsnode.dflt), sub, calls);
                for (int i = tsnode.labels.size() - 1; i >= 0; --i) {
                    LabelNode l = tsnode.labels.get(i);
                    findSubroutine(insns.indexOf(l), sub, calls);
                }
            } else if (node instanceof LookupSwitchInsnNode) {
                LookupSwitchInsnNode lsnode = (LookupSwitchInsnNode) node;
                findSubroutine(insns.indexOf(lsnode.dflt), sub, calls);
                for (int i = lsnode.labels.size() - 1; i >= 0; --i) {
                    LabelNode l = lsnode.labels.get(i);
                    findSubroutine(insns.indexOf(l), sub, calls);
                }
            }

            // calls findSubroutine recursively on exception handler successors
            List<TryCatchBlockNode> insnHandlers = handlers[insn];
            if (insnHandlers != null) {
                for (int i = 0; i < insnHandlers.size(); ++i) {
                    TryCatchBlockNode tcb = insnHandlers.get(i);
                    findSubroutine(insns.indexOf(tcb.handler), sub, calls);
                }
            }

            // if insn does not falls through to the next instruction, return.
            switch (node.getOpcode()) {
            case GOTO:
            case RET:
            case TABLESWITCH:
            case LOOKUPSWITCH:
            case IRETURN:
            case LRETURN:
            case FRETURN:
            case DRETURN:
            case ARETURN:
            case RETURN:
            case ATHROW:
                return;
            }
            insn++;
        }
    }

    
Returns the symbolic stack frame for each instruction of the last
recently analyzed method.

Returns:the symbolic state of the execution stack frame at each bytecode
        instruction of the method. The size of the returned array is
        equal to the number of instructions (and labels) of the method. A
        given frame is null if the corresponding instruction
        cannot be reached, or if an error occured during the analysis of
        the method.
/**
     * Returns the symbolic stack frame for each instruction of the last
     * recently analyzed method.
     * 
     * @return the symbolic state of the execution stack frame at each bytecode
     *         instruction of the method. The size of the returned array is
     *         equal to the number of instructions (and labels) of the method. A
     *         given frame is <tt>null</tt> if the corresponding instruction
     *         cannot be reached, or if an error occured during the analysis of
     *         the method.
     */
    public Frame<V>[] getFrames() {
        return frames;
    }

    
Returns the exception handlers for the given instruction.

Params:
  • insn – 
           the index of an instruction of the last recently analyzed
           method.
Returns:a list of TryCatchBlockNode objects.
/**
     * Returns the exception handlers for the given instruction.
     * 
     * @param insn
     *            the index of an instruction of the last recently analyzed
     *            method.
     * @return a list of {@link TryCatchBlockNode} objects.
     */
    public List<TryCatchBlockNode> getHandlers(final int insn) {
        return handlers[insn];
    }

    
Initializes this analyzer. This method is called just before the
execution of control flow analysis loop in #analyze. The default
implementation of this method does nothing.

Params:
  • owner – 
           the internal name of the class to which the method belongs.
  • m – 
           the method to be analyzed.
Throws:
/**
     * Initializes this analyzer. This method is called just before the
     * execution of control flow analysis loop in #analyze. The default
     * implementation of this method does nothing.
     * 
     * @param owner
     *            the internal name of the class to which the method belongs.
     * @param m
     *            the method to be analyzed.
     * @throws AnalyzerException
     *             if a problem occurs.
     */
    protected void init(String owner, MethodNode m) throws AnalyzerException {
    }

    
Constructs a new frame with the given size.

Params:
  • nLocals – 
           the maximum number of local variables of the frame.
  • nStack – 
           the maximum stack size of the frame.
Returns:the created frame.
/**
     * Constructs a new frame with the given size.
     * 
     * @param nLocals
     *            the maximum number of local variables of the frame.
     * @param nStack
     *            the maximum stack size of the frame.
     * @return the created frame.
     */
    protected Frame<V> newFrame(final int nLocals, final int nStack) {
        return new Frame<V>(nLocals, nStack);
    }

    
Constructs a new frame that is identical to the given frame.

Params:
  • src – 
           a frame.
Returns:the created frame.
/**
     * Constructs a new frame that is identical to the given frame.
     * 
     * @param src
     *            a frame.
     * @return the created frame.
     */
    protected Frame<V> newFrame(final Frame<? extends V> src) {
        return new Frame<V>(src);
    }

    
Creates a control flow graph edge. The default implementation of this method does nothing. It can be overriden in order to construct the control flow graph of a method (this method is called by the analyze method during its visit of the method's code). 
Params:
  • insn – 
           an instruction index.
  • successor – 
           index of a successor instruction.
/**
     * Creates a control flow graph edge. The default implementation of this
     * method does nothing. It can be overriden in order to construct the
     * control flow graph of a method (this method is called by the
     * {@link #analyze analyze} method during its visit of the method's code).
     * 
     * @param insn
     *            an instruction index.
     * @param successor
     *            index of a successor instruction.
     */
    protected void newControlFlowEdge(final int insn, final int successor) {
    }

    
Creates a control flow graph edge corresponding to an exception handler. The default implementation of this method does nothing. It can be overridden in order to construct the control flow graph of a method (this method is called by the analyze method during its visit of the method's code). 
Params:
  • insn – 
           an instruction index.
  • successor – 
           index of a successor instruction.
Returns:true if this edge must be considered in the data flow analysis
        performed by this analyzer, or false otherwise. The default
        implementation of this method always returns true.
/**
     * Creates a control flow graph edge corresponding to an exception handler.
     * The default implementation of this method does nothing. It can be
     * overridden in order to construct the control flow graph of a method (this
     * method is called by the {@link #analyze analyze} method during its visit
     * of the method's code).
     * 
     * @param insn
     *            an instruction index.
     * @param successor
     *            index of a successor instruction.
     * @return true if this edge must be considered in the data flow analysis
     *         performed by this analyzer, or false otherwise. The default
     *         implementation of this method always returns true.
     */
    protected boolean newControlFlowExceptionEdge(final int insn,
            final int successor) {
        return true;
    }

    
Creates a control flow graph edge corresponding to an exception handler. The default implementation of this method delegates to 
newControlFlowExceptionEdge(int, int). It can be overridden in order to construct the control flow graph of a method (this method is called by the analyze method during its visit of the method's code). 
Params:
  • insn – 
           an instruction index.
  • tcb – 
           TryCatchBlockNode corresponding to this edge.
Returns:true if this edge must be considered in the data flow analysis performed by this analyzer, or false otherwise. The default implementation of this method delegates to 
        newControlFlowExceptionEdge(int, int).
/**
     * Creates a control flow graph edge corresponding to an exception handler.
     * The default implementation of this method delegates to
     * {@link #newControlFlowExceptionEdge(int, int)
     * newControlFlowExceptionEdge(int, int)}. It can be overridden in order to
     * construct the control flow graph of a method (this method is called by
     * the {@link #analyze analyze} method during its visit of the method's
     * code).
     * 
     * @param insn
     *            an instruction index.
     * @param tcb
     *            TryCatchBlockNode corresponding to this edge.
     * @return true if this edge must be considered in the data flow analysis
     *         performed by this analyzer, or false otherwise. The default
     *         implementation of this method delegates to
     *         {@link #newControlFlowExceptionEdge(int, int)
     *         newControlFlowExceptionEdge(int, int)}.
     */
    protected boolean newControlFlowExceptionEdge(final int insn,
            final TryCatchBlockNode tcb) {
        return newControlFlowExceptionEdge(insn, insns.indexOf(tcb.handler));
    }

    // -------------------------------------------------------------------------

    private void merge(final int insn, final Frame<V> frame,
            final Subroutine subroutine) throws AnalyzerException {
        Frame<V> oldFrame = frames[insn];
        Subroutine oldSubroutine = subroutines[insn];
        boolean changes;

        if (oldFrame == null) {
            frames[insn] = newFrame(frame);
            changes = true;
        } else {
            changes = oldFrame.merge(frame, interpreter);
        }

        if (oldSubroutine == null) {
            if (subroutine != null) {
                subroutines[insn] = subroutine.copy();
                changes = true;
            }
        } else {
            if (subroutine != null) {
                changes |= oldSubroutine.merge(subroutine);
            }
        }
        if (changes && !queued[insn]) {
            queued[insn] = true;
            queue[top++] = insn;
        }
    }

    private void merge(final int insn, final Frame<V> beforeJSR,
            final Frame<V> afterRET, final Subroutine subroutineBeforeJSR,
            final boolean[] access) throws AnalyzerException {
        Frame<V> oldFrame = frames[insn];
        Subroutine oldSubroutine = subroutines[insn];
        boolean changes;

        afterRET.merge(beforeJSR, access);

        if (oldFrame == null) {
            frames[insn] = newFrame(afterRET);
            changes = true;
        } else {
            changes = oldFrame.merge(afterRET, interpreter);
        }

        if (oldSubroutine != null && subroutineBeforeJSR != null) {
            changes |= oldSubroutine.merge(subroutineBeforeJSR);
        }
        if (changes && !queued[insn]) {
            queued[insn] = true;
            queue[top++] = insn;
        }
    }
}