// 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
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// THE POSSIBILITY OF SUCH DAMAGE.
package org.objectweb.asm;

/**
 * A position in the bytecode of a method. Labels are used for jump, goto, and switch instructions,
 * and for try catch blocks. A label designates the <i>instruction</i> that is just after. Note
 * however that there can be other elements between a label and the instruction it designates (such
 * as other labels, stack map frames, line numbers, etc.).
 *
 * @author Eric Bruneton
 */
public class Label {

  /**
   * A flag indicating that a label is only used for debug attributes. Such a label is not the start
   * of a basic block, the target of a jump instruction, or an exception handler. It can be safely
   * ignored in control flow graph analysis algorithms (for optimization purposes).
   */
  static final int FLAG_DEBUG_ONLY = 1;

  /**
   * A flag indicating that a label is the target of a jump instruction, or the start of an
   * exception handler.
   */
  static final int FLAG_JUMP_TARGET = 2;

  /** A flag indicating that the bytecode offset of a label is known. */
  static final int FLAG_RESOLVED = 4;

  /** A flag indicating that a label corresponds to a reachable basic block. */
  static final int FLAG_REACHABLE = 8;

  /**
   * A flag indicating that the basic block corresponding to a label ends with a subroutine call. By
   * construction in {@link MethodWriter#visitJumpInsn}, labels with this flag set have at least two
   * outgoing edges:
   *
   * <ul>
   *   <li>the first one corresponds to the instruction that follows the jsr instruction in the
   *       bytecode, i.e. where execution continues when it returns from the jsr call. This is a
   *       virtual control flow edge, since execution never goes directly from the jsr to the next
   *       instruction. Instead, it goes to the subroutine and eventually returns to the instruction
   *       following the jsr. This virtual edge is used to compute the real outgoing edges of the
   *       basic blocks ending with a ret instruction, in {@link #addSubroutineRetSuccessors}.
   *   <li>the second one corresponds to the target of the jsr instruction,
   * </ul>
   */
  static final int FLAG_SUBROUTINE_CALLER = 16;

  /**
   * A flag indicating that the basic block corresponding to a label is the start of a subroutine.
   */
  static final int FLAG_SUBROUTINE_START = 32;

  /** A flag indicating that the basic block corresponding to a label is the end of a subroutine. */
  static final int FLAG_SUBROUTINE_END = 64;

  /** A flag indicating that this label has at least one associated line number. */
  static final int FLAG_LINE_NUMBER = 128;

  /**
   * The number of elements to add to the {@link #otherLineNumbers} array when it needs to be
   * resized to store a new source line number.
   */
  static final int LINE_NUMBERS_CAPACITY_INCREMENT = 4;

  /**
   * The number of elements to add to the {@link #forwardReferences} array when it needs to be
   * resized to store a new forward reference.
   */
  static final int FORWARD_REFERENCES_CAPACITY_INCREMENT = 6;

  /**
   * The bit mask to extract the type of a forward reference to this label. The extracted type is
   * either {@link #FORWARD_REFERENCE_TYPE_SHORT} or {@link #FORWARD_REFERENCE_TYPE_WIDE}.
   *
   * @see #forwardReferences
   */
  static final int FORWARD_REFERENCE_TYPE_MASK = 0xF0000000;

  /**
   * The type of forward references stored with two bytes in the bytecode. This is the case, for
   * instance, of a forward reference from an ifnull instruction.
   */
  static final int FORWARD_REFERENCE_TYPE_SHORT = 0x10000000;

  /**
   * The type of forward references stored in four bytes in the bytecode. This is the case, for
   * instance, of a forward reference from a lookupswitch instruction.
   */
  static final int FORWARD_REFERENCE_TYPE_WIDE = 0x20000000;

  /**
   * The type of forward references stored in two bytes in the <i>stack map table</i>. This is the
   * case of the labels of {@link Frame#ITEM_UNINITIALIZED} stack map frame elements, when the NEW
   * instruction is after the &lt;init&gt; constructor call (in bytecode offset order).
   */
  static final int FORWARD_REFERENCE_TYPE_STACK_MAP = 0x30000000;

  /**
   * The bit mask to extract the 'handle' of a forward reference to this label. The extracted handle
   * is the bytecode offset where the forward reference value is stored (using either 2 or 4 bytes,
   * as indicated by the {@link #FORWARD_REFERENCE_TYPE_MASK}).
   *
   * @see #forwardReferences
   */
  static final int FORWARD_REFERENCE_HANDLE_MASK = 0x0FFFFFFF;

  /**
   * A sentinel element used to indicate the end of a list of labels.
   *
   * @see #nextListElement
   */
  static final Label EMPTY_LIST = new Label();

  /**
   * A user managed state associated with this label. Warning: this field is used by the ASM tree
   * package. In order to use it with the ASM tree package you must override the getLabelNode method
   * in MethodNode.
   */
  public Object info;

  /**
   * The type and status of this label or its corresponding basic block. Must be zero or more of
   * {@link #FLAG_DEBUG_ONLY}, {@link #FLAG_JUMP_TARGET}, {@link #FLAG_RESOLVED}, {@link
   * #FLAG_REACHABLE}, {@link #FLAG_SUBROUTINE_CALLER}, {@link #FLAG_SUBROUTINE_START}, {@link
   * #FLAG_SUBROUTINE_END}.
   */
  short flags;

  /**
   * The source line number corresponding to this label, if {@link #FLAG_LINE_NUMBER} is set. If
   * there are several source line numbers corresponding to this label, the first one is stored in
   * this field, and the remaining ones are stored in {@link #otherLineNumbers}.
   */
  private short lineNumber;

  /**
   * The source line numbers corresponding to this label, in addition to {@link #lineNumber}, or
   * null. The first element of this array is the number n of source line numbers it contains, which
   * are stored between indices 1 and n (inclusive).
   */
  private int[] otherLineNumbers;

  /**
   * The offset of this label in the bytecode of its method, in bytes. This value is set if and only
   * if the {@link #FLAG_RESOLVED} flag is set.
   */
  int bytecodeOffset;

  /**
   * The forward references to this label. The first element is the number of forward references,
   * times 2 (this corresponds to the index of the last element actually used in this array). Then,
   * each forward reference is described with two consecutive integers noted
   * 'sourceInsnBytecodeOffset' and 'reference':
   *
   * <ul>
   *   <li>'sourceInsnBytecodeOffset' is the bytecode offset of the instruction that contains the
   *       forward reference,
   *   <li>'reference' contains the type and the offset in the bytecode where the forward reference
   *       value must be stored, which can be extracted with {@link #FORWARD_REFERENCE_TYPE_MASK}
   *       and {@link #FORWARD_REFERENCE_HANDLE_MASK}.
   * </ul>
   *
   * <p>For instance, for an ifnull instruction at bytecode offset x, 'sourceInsnBytecodeOffset' is
   * equal to x, and 'reference' is of type {@link #FORWARD_REFERENCE_TYPE_SHORT} with value x + 1
   * (because the ifnull instruction uses a 2 bytes bytecode offset operand stored one byte after
   * the start of the instruction itself). For the default case of a lookupswitch instruction at
   * bytecode offset x, 'sourceInsnBytecodeOffset' is equal to x, and 'reference' is of type {@link
   * #FORWARD_REFERENCE_TYPE_WIDE} with value between x + 1 and x + 4 (because the lookupswitch
   * instruction uses a 4 bytes bytecode offset operand stored one to four bytes after the start of
   * the instruction itself).
   */
  private int[] forwardReferences;

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

  // Fields for the control flow and data flow graph analysis algorithms (used to compute the
  // maximum stack size or the stack map frames). A control flow graph contains one node per "basic
  // block", and one edge per "jump" from one basic block to another. Each node (i.e., each basic
  // block) is represented with the Label object that corresponds to the first instruction of this
  // basic block. Each node also stores the list of its successors in the graph, as a linked list of
  // Edge objects.
  //
  // The control flow analysis algorithms used to compute the maximum stack size or the stack map
  // frames are similar and use two steps. The first step, during the visit of each instruction,
  // builds information about the state of the local variables and the operand stack at the end of
  // each basic block, called the "output frame", <i>relatively</i> to the frame state at the
  // beginning of the basic block, which is called the "input frame", and which is <i>unknown</i>
  // during this step. The second step, in {@link MethodWriter#computeAllFrames} and {@link
  // MethodWriter#computeMaxStackAndLocal}, is a fix point algorithm
  // that computes information about the input frame of each basic block, from the input state of
  // the first basic block (known from the method signature), and by the using the previously
  // computed relative output frames.
  //
  // The algorithm used to compute the maximum stack size only computes the relative output and
  // absolute input stack heights, while the algorithm used to compute stack map frames computes
  // relative output frames and absolute input frames.

  /**
   * The number of elements in the input stack of the basic block corresponding to this label. This
   * field is computed in {@link MethodWriter#computeMaxStackAndLocal}.
   */
  short inputStackSize;

  /**
   * The number of elements in the output stack, at the end of the basic block corresponding to this
   * label. This field is only computed for basic blocks that end with a RET instruction.
   */
  short outputStackSize;

  /**
   * The maximum height reached by the output stack, relatively to the top of the input stack, in
   * the basic block corresponding to this label. This maximum is always positive or {@literal
   * null}.
   */
  short outputStackMax;

  /**
   * The id of the subroutine to which this basic block belongs, or 0. If the basic block belongs to
   * several subroutines, this is the id of the "oldest" subroutine that contains it (with the
   * convention that a subroutine calling another one is "older" than the callee). This field is
   * computed in {@link MethodWriter#computeMaxStackAndLocal}, if the method contains JSR
   * instructions.
   */
  short subroutineId;

  /**
   * The input and output stack map frames of the basic block corresponding to this label. This
   * field is only used when the {@link MethodWriter#COMPUTE_ALL_FRAMES} or {@link
   * MethodWriter#COMPUTE_INSERTED_FRAMES} option is used.
   */
  Frame frame;

  /**
   * The successor of this label, in the order they are visited in {@link MethodVisitor#visitLabel}.
   * This linked list does not include labels used for debug info only. If the {@link
   * MethodWriter#COMPUTE_ALL_FRAMES} or {@link MethodWriter#COMPUTE_INSERTED_FRAMES} option is used
   * then it does not contain either successive labels that denote the same bytecode offset (in this
   * case only the first label appears in this list).
   */
  Label nextBasicBlock;

  /**
   * The outgoing edges of the basic block corresponding to this label, in the control flow graph of
   * its method. These edges are stored in a linked list of {@link Edge} objects, linked to each
   * other by their {@link Edge#nextEdge} field.
   */
  Edge outgoingEdges;

  /**
   * The next element in the list of labels to which this label belongs, or {@literal null} if it
   * does not belong to any list. All lists of labels must end with the {@link #EMPTY_LIST}
   * sentinel, in order to ensure that this field is null if and only if this label does not belong
   * to a list of labels. Note that there can be several lists of labels at the same time, but that
   * a label can belong to at most one list at a time (unless some lists share a common tail, but
   * this is not used in practice).
   *
   * <p>List of labels are used in {@link MethodWriter#computeAllFrames} and {@link
   * MethodWriter#computeMaxStackAndLocal} to compute stack map frames and the maximum stack size,
   * respectively, as well as in {@link #markSubroutine} and {@link #addSubroutineRetSuccessors} to
   * compute the basic blocks belonging to subroutines and their outgoing edges. Outside of these
   * methods, this field should be null (this property is a precondition and a postcondition of
   * these methods).
   */
  Label nextListElement;

  // -----------------------------------------------------------------------------------------------
  // Constructor and accessors
  // -----------------------------------------------------------------------------------------------

  /** Constructs a new label. */
  public Label() {
    // Nothing to do.
  }

  /**
   * Returns the bytecode offset corresponding to this label. This offset is computed from the start
   * of the method's bytecode. <i>This method is intended for {@link Attribute} sub classes, and is
   * normally not needed by class generators or adapters.</i>
   *
   * @return the bytecode offset corresponding to this label.
   * @throws IllegalStateException if this label is not resolved yet.
   */
  public int getOffset() {
    if ((flags & FLAG_RESOLVED) == 0) {
      throw new IllegalStateException("Label offset position has not been resolved yet");
    }
    return bytecodeOffset;
  }

  /**
   * Returns the "canonical" {@link Label} instance corresponding to this label's bytecode offset,
   * if known, otherwise the label itself. The canonical instance is the first label (in the order
   * of their visit by {@link MethodVisitor#visitLabel}) corresponding to this bytecode offset. It
   * cannot be known for labels which have not been visited yet.
   *
   * <p><i>This method should only be used when the {@link MethodWriter#COMPUTE_ALL_FRAMES} option
   * is used.</i>
   *
   * @return the label itself if {@link #frame} is null, otherwise the Label's frame owner. This
   *     corresponds to the "canonical" label instance described above thanks to the way the label
   *     frame is set in {@link MethodWriter#visitLabel}.
   */
  final Label getCanonicalInstance() {
    return frame == null ? this : frame.owner;
  }

  // -----------------------------------------------------------------------------------------------
  // Methods to manage line numbers
  // -----------------------------------------------------------------------------------------------

  /**
   * Adds a source line number corresponding to this label.
   *
   * @param lineNumber a source line number (which should be strictly positive).
   */
  final void addLineNumber(final int lineNumber) {
    if ((flags & FLAG_LINE_NUMBER) == 0) {
      flags |= FLAG_LINE_NUMBER;
      this.lineNumber = (short) lineNumber;
    } else {
      if (otherLineNumbers == null) {
        otherLineNumbers = new int[LINE_NUMBERS_CAPACITY_INCREMENT];
      }
      int otherLineNumberIndex = ++otherLineNumbers[0];
      if (otherLineNumberIndex >= otherLineNumbers.length) {
        int[] newLineNumbers = new int[otherLineNumbers.length + LINE_NUMBERS_CAPACITY_INCREMENT];
        System.arraycopy(otherLineNumbers, 0, newLineNumbers, 0, otherLineNumbers.length);
        otherLineNumbers = newLineNumbers;
      }
      otherLineNumbers[otherLineNumberIndex] = lineNumber;
    }
  }

  /**
   * Makes the given visitor visit this label and its source line numbers, if applicable.
   *
   * @param methodVisitor a method visitor.
   * @param visitLineNumbers whether to visit of the label's source line numbers, if any.
   */
  final void accept(final MethodVisitor methodVisitor, final boolean visitLineNumbers) {
    methodVisitor.visitLabel(this);
    if (visitLineNumbers && (flags & FLAG_LINE_NUMBER) != 0) {
      methodVisitor.visitLineNumber(lineNumber & 0xFFFF, this);
      if (otherLineNumbers != null) {
        for (int i = 1; i <= otherLineNumbers[0]; ++i) {
          methodVisitor.visitLineNumber(otherLineNumbers[i], this);
        }
      }
    }
  }

  // -----------------------------------------------------------------------------------------------
  // Methods to compute offsets and to manage forward references
  // -----------------------------------------------------------------------------------------------

  /**
   * Puts a reference to this label in the bytecode of a method. If the bytecode offset of the label
   * is known, the relative bytecode offset between the label and the instruction referencing it is
   * computed and written directly. Otherwise, a null relative offset is written and a new forward
   * reference is declared for this label.
   *
   * @param code the bytecode of the method. This is where the reference is appended.
   * @param sourceInsnBytecodeOffset the bytecode offset of the instruction that contains the
   *     reference to be appended.
   * @param wideReference whether the reference must be stored in 4 bytes (instead of 2 bytes).
   */
  final void put(
      final ByteVector code, final int sourceInsnBytecodeOffset, final boolean wideReference) {
    if ((flags & FLAG_RESOLVED) == 0) {
      if (wideReference) {
        addForwardReference(sourceInsnBytecodeOffset, FORWARD_REFERENCE_TYPE_WIDE, code.length);
        code.putInt(-1);
      } else {
        addForwardReference(sourceInsnBytecodeOffset, FORWARD_REFERENCE_TYPE_SHORT, code.length);
        code.putShort(-1);
      }
    } else {
      if (wideReference) {
        code.putInt(bytecodeOffset - sourceInsnBytecodeOffset);
      } else {
        code.putShort(bytecodeOffset - sourceInsnBytecodeOffset);
      }
    }
  }

  /**
   * Puts a reference to this label in the <i>stack map table</i> of a method. If the bytecode
   * offset of the label is known, it is written directly. Otherwise, a null relative offset is
   * written and a new forward reference is declared for this label.
   *
   * @param stackMapTableEntries the stack map table where the label offset must be added.
   */
  final void put(final ByteVector stackMapTableEntries) {
    if ((flags & FLAG_RESOLVED) == 0) {
      addForwardReference(0, FORWARD_REFERENCE_TYPE_STACK_MAP, stackMapTableEntries.length);
    }
    stackMapTableEntries.putShort(bytecodeOffset);
  }

  /**
   * Adds a forward reference to this label. This method must be called only for a true forward
   * reference, i.e. only if this label is not resolved yet. For backward references, the relative
   * bytecode offset of the reference can be, and must be, computed and stored directly.
   *
   * @param sourceInsnBytecodeOffset the bytecode offset of the instruction that contains the
   *     reference stored at referenceHandle.
   * @param referenceType either {@link #FORWARD_REFERENCE_TYPE_SHORT} or {@link
   *     #FORWARD_REFERENCE_TYPE_WIDE}.
   * @param referenceHandle the offset in the bytecode where the forward reference value must be
   *     stored.
   */
  private void addForwardReference(
      final int sourceInsnBytecodeOffset, final int referenceType, final int referenceHandle) {
    if (forwardReferences == null) {
      forwardReferences = new int[FORWARD_REFERENCES_CAPACITY_INCREMENT];
    }
    int lastElementIndex = forwardReferences[0];
    if (lastElementIndex + 2 >= forwardReferences.length) {
      int[] newValues = new int[forwardReferences.length + FORWARD_REFERENCES_CAPACITY_INCREMENT];
      System.arraycopy(forwardReferences, 0, newValues, 0, forwardReferences.length);
      forwardReferences = newValues;
    }
    forwardReferences[++lastElementIndex] = sourceInsnBytecodeOffset;
    forwardReferences[++lastElementIndex] = referenceType | referenceHandle;
    forwardReferences[0] = lastElementIndex;
  }

  /**
   * Sets the bytecode offset of this label to the given value and resolves the forward references
   * to this label, if any. This method must be called when this label is added to the bytecode of
   * the method, i.e. when its bytecode offset becomes known. This method fills in the blanks that
   * where left in the bytecode (and optionally in the stack map table) by each forward reference
   * previously added to this label.
   *
   * @param code the bytecode of the method.
   * @param stackMapTableEntries the 'entries' array of the StackMapTable code attribute of the
   *     method. Maybe {@literal null}.
   * @param bytecodeOffset the bytecode offset of this label.
   * @return {@literal true} if a blank that was left for this label was too small to store the
   *     offset. In such a case the corresponding jump instruction is replaced with an equivalent
   *     ASM specific instruction using an unsigned two bytes offset. These ASM specific
   *     instructions are later replaced with standard bytecode instructions with wider offsets (4
   *     bytes instead of 2), in ClassReader.
   */
  final boolean resolve(
      final byte[] code, final ByteVector stackMapTableEntries, final int bytecodeOffset) {
    this.flags |= FLAG_RESOLVED;
    this.bytecodeOffset = bytecodeOffset;
    if (forwardReferences == null) {
      return false;
    }
    boolean hasAsmInstructions = false;
    for (int i = forwardReferences[0]; i > 0; i -= 2) {
      final int sourceInsnBytecodeOffset = forwardReferences[i - 1];
      final int reference = forwardReferences[i];
      final int relativeOffset = bytecodeOffset - sourceInsnBytecodeOffset;
      int handle = reference & FORWARD_REFERENCE_HANDLE_MASK;
      if ((reference & FORWARD_REFERENCE_TYPE_MASK) == FORWARD_REFERENCE_TYPE_SHORT) {
        if (relativeOffset < Short.MIN_VALUE || relativeOffset > Short.MAX_VALUE) {
          // Change the opcode of the jump instruction, in order to be able to find it later in
          // ClassReader. These ASM specific opcodes are similar to jump instruction opcodes, except
          // that the 2 bytes offset is unsigned (and can therefore represent values from 0 to
          // 65535, which is sufficient since the size of a method is limited to 65535 bytes).
          int opcode = code[sourceInsnBytecodeOffset] & 0xFF;
          if (opcode < Opcodes.IFNULL) {
            // Change IFEQ ... JSR to ASM_IFEQ ... ASM_JSR.
            code[sourceInsnBytecodeOffset] = (byte) (opcode + Constants.ASM_OPCODE_DELTA);
          } else {
            // Change IFNULL and IFNONNULL to ASM_IFNULL and ASM_IFNONNULL.
            code[sourceInsnBytecodeOffset] = (byte) (opcode + Constants.ASM_IFNULL_OPCODE_DELTA);
          }
          hasAsmInstructions = true;
        }
        code[handle++] = (byte) (relativeOffset >>> 8);
        code[handle] = (byte) relativeOffset;
      } else if ((reference & FORWARD_REFERENCE_TYPE_MASK) == FORWARD_REFERENCE_TYPE_WIDE) {
        code[handle++] = (byte) (relativeOffset >>> 24);
        code[handle++] = (byte) (relativeOffset >>> 16);
        code[handle++] = (byte) (relativeOffset >>> 8);
        code[handle] = (byte) relativeOffset;
      } else {
        stackMapTableEntries.data[handle++] = (byte) (bytecodeOffset >>> 8);
        stackMapTableEntries.data[handle] = (byte) bytecodeOffset;
      }
    }
    return hasAsmInstructions;
  }

  // -----------------------------------------------------------------------------------------------
  // Methods related to subroutines
  // -----------------------------------------------------------------------------------------------

  /**
   * Finds the basic blocks that belong to the subroutine starting with the basic block
   * corresponding to this label, and marks these blocks as belonging to this subroutine. This
   * method follows the control flow graph to find all the blocks that are reachable from the
   * current basic block WITHOUT following any jsr target.
   *
   * <p>Note: a precondition and postcondition of this method is that all labels must have a null
   * {@link #nextListElement}.
   *
   * @param subroutineId the id of the subroutine starting with the basic block corresponding to
   *     this label.
   */
  final void markSubroutine(final short subroutineId) {
    // Data flow algorithm: put this basic block in a list of blocks to process (which are blocks
    // belonging to subroutine subroutineId) and, while there are blocks to process, remove one from
    // the list, mark it as belonging to the subroutine, and add its successor basic blocks in the
    // control flow graph to the list of blocks to process (if not already done).
    Label listOfBlocksToProcess = this;
    listOfBlocksToProcess.nextListElement = EMPTY_LIST;
    while (listOfBlocksToProcess != EMPTY_LIST) {
      // Remove a basic block from the list of blocks to process.
      Label basicBlock = listOfBlocksToProcess;
      listOfBlocksToProcess = listOfBlocksToProcess.nextListElement;
      basicBlock.nextListElement = null;

      // If it is not already marked as belonging to a subroutine, mark it as belonging to
      // subroutineId and add its successors to the list of blocks to process (unless already done).
      if (basicBlock.subroutineId == 0) {
        basicBlock.subroutineId = subroutineId;
        listOfBlocksToProcess = basicBlock.pushSuccessors(listOfBlocksToProcess);
      }
    }
  }

  /**
   * Finds the basic blocks that end a subroutine starting with the basic block corresponding to
   * this label and, for each one of them, adds an outgoing edge to the basic block following the
   * given subroutine call. In other words, completes the control flow graph by adding the edges
   * corresponding to the return from this subroutine, when called from the given caller basic
   * block.
   *
   * <p>Note: a precondition and postcondition of this method is that all labels must have a null
   * {@link #nextListElement}.
   *
   * @param subroutineCaller a basic block that ends with a jsr to the basic block corresponding to
   *     this label. This label is supposed to correspond to the start of a subroutine.
   */
  final void addSubroutineRetSuccessors(final Label subroutineCaller) {
    // Data flow algorithm: put this basic block in a list blocks to process (which are blocks
    // belonging to a subroutine starting with this label) and, while there are blocks to process,
    // remove one from the list, put it in a list of blocks that have been processed, add a return
    // edge to the successor of subroutineCaller if applicable, and add its successor basic blocks
    // in the control flow graph to the list of blocks to process (if not already done).
    Label listOfProcessedBlocks = EMPTY_LIST;
    Label listOfBlocksToProcess = this;
    listOfBlocksToProcess.nextListElement = EMPTY_LIST;
    while (listOfBlocksToProcess != EMPTY_LIST) {
      // Move a basic block from the list of blocks to process to the list of processed blocks.
      Label basicBlock = listOfBlocksToProcess;
      listOfBlocksToProcess = basicBlock.nextListElement;
      basicBlock.nextListElement = listOfProcessedBlocks;
      listOfProcessedBlocks = basicBlock;

      // Add an edge from this block to the successor of the caller basic block, if this block is
      // the end of a subroutine and if this block and subroutineCaller do not belong to the same
      // subroutine.
      if ((basicBlock.flags & FLAG_SUBROUTINE_END) != 0
          && basicBlock.subroutineId != subroutineCaller.subroutineId) {
        basicBlock.outgoingEdges =
            new Edge(
                basicBlock.outputStackSize,
                // By construction, the first outgoing edge of a basic block that ends with a jsr
                // instruction leads to the jsr continuation block, i.e. where execution continues
                // when ret is called (see {@link #FLAG_SUBROUTINE_CALLER}).
                subroutineCaller.outgoingEdges.successor,
                basicBlock.outgoingEdges);
      }
      // Add its successors to the list of blocks to process. Note that {@link #pushSuccessors} does
      // not push basic blocks which are already in a list. Here this means either in the list of
      // blocks to process, or in the list of already processed blocks. This second list is
      // important to make sure we don't reprocess an already processed block.
      listOfBlocksToProcess = basicBlock.pushSuccessors(listOfBlocksToProcess);
    }
    // Reset the {@link #nextListElement} of all the basic blocks that have been processed to null,
    // so that this method can be called again with a different subroutine or subroutine caller.
    while (listOfProcessedBlocks != EMPTY_LIST) {
      Label newListOfProcessedBlocks = listOfProcessedBlocks.nextListElement;
      listOfProcessedBlocks.nextListElement = null;
      listOfProcessedBlocks = newListOfProcessedBlocks;
    }
  }

  /**
   * Adds the successors of this label in the method's control flow graph (except those
   * corresponding to a jsr target, and those already in a list of labels) to the given list of
   * blocks to process, and returns the new list.
   *
   * @param listOfLabelsToProcess a list of basic blocks to process, linked together with their
   *     {@link #nextListElement} field.
   * @return the new list of blocks to process.
   */
  private Label pushSuccessors(final Label listOfLabelsToProcess) {
    Label newListOfLabelsToProcess = listOfLabelsToProcess;
    Edge outgoingEdge = outgoingEdges;
    while (outgoingEdge != null) {
      // By construction, the second outgoing edge of a basic block that ends with a jsr instruction
      // leads to the jsr target (see {@link #FLAG_SUBROUTINE_CALLER}).
      boolean isJsrTarget =
          (flags & Label.FLAG_SUBROUTINE_CALLER) != 0 && outgoingEdge == outgoingEdges.nextEdge;
      if (!isJsrTarget && outgoingEdge.successor.nextListElement == null) {
        // Add this successor to the list of blocks to process, if it does not already belong to a
        // list of labels.
        outgoingEdge.successor.nextListElement = newListOfLabelsToProcess;
        newListOfLabelsToProcess = outgoingEdge.successor;
      }
      outgoingEdge = outgoingEdge.nextEdge;
    }
    return newListOfLabelsToProcess;
  }

  // -----------------------------------------------------------------------------------------------
  // Overridden Object methods
  // -----------------------------------------------------------------------------------------------

  /**
   * Returns a string representation of this label.
   *
   * @return a string representation of this label.
   */
  @Override
  public String toString() {
    return "L" + System.identityHashCode(this);
  }
}
