label.java

开源的java 编辑器源代码

/***
 * ASM: a very small and fast Java bytecode manipulation framework
 * Copyright (C) 2000 INRIA, France Telecom
 * Copyright (C) 2002 France Telecom
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 * Contact: [e-mail]
 *
 * Author: Eric Bruneton
 */

package org.objectweb.asm;

/**
 * A label represents a position in the bytecode of a method. Labels are used
 * for jump, goto, and switch instructions, and for try catch blocks.
 */

public class Label {

  /**
   * The code writer to which this label belongs, or <tt>null</tt> if unknown.
   */

  CodeWriter owner;

  /**
   * Indicates if the position of this label is known.
   */

  boolean resolved;

  /**
   * The position of this label in the code, if known.
   */

  int position;

  /**
   * Number of forward references to this label, times two.
   */

  private int referenceCount;

  /**
   * Informations about forward references. Each forward reference is described
   * by two consecutive integers in this array: the first one is the position
   * of the first byte of the bytecode instruction that contains the forward
   * reference, while the second is the position of the first byte of the
   * forward reference itself. In fact the sign of the first integer indicates
   * if this reference uses 2 or 4 bytes, and its absolute value gives the
   * position of the bytecode instruction.
   */

  private int[] srcAndRefPositions;

  // --------------------------------------------------------------------------
  // Fields for the control flow graph analysis algorithm (used to compute the
  // maximum stack size). 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 by the Label object that
  // corresponds to the first instruction of this basic block. Each node also
  // stores the list of it successors in the graph, as a linked list of Edge
  // objects.
  // --------------------------------------------------------------------------

  /**
   * The stack size at the beginning of this basic block.
   * This size is initially unknown. It is computed by the control flow
   * analysis algorithm (see {@link CodeWriter#visitMaxs visitMaxs}).
   */

  int beginStackSize;

  /**
   * The (relative) maximum stack size corresponding to this basic block. This
   * size is relative to the stack size at the beginning of the basic block,
   * i.e., the true maximum stack size is equal to {@link #beginStackSize
   * beginStackSize} + {@link #maxStackSize maxStackSize}.
   */

  int maxStackSize;

  /**
   * The successors of this node in the control flow graph. These successors
   * are stored in a linked list of {@link Edge Edge} objects, linked to each
   * other by their {@link Edge#next} field.
   */

  Edge successors;

  /**
   * The next basic block in the basic block stack.
   * See {@link CodeWriter#visitMaxs visitMaxs}.
   */

  Label next;

  /**
   * <tt>true</tt> if this basic block has been pushed in the basic block stack.
   * See {@link CodeWriter#visitMaxs visitMaxs}.
   */

  boolean pushed;

  // --------------------------------------------------------------------------
  // Constructor
  // --------------------------------------------------------------------------

  /**
   * Constructs a new label.
   */

  public Label () {
  }

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

  /**
   * Puts a reference to this label in the bytecode of a method. If the position
   * of the label is known, the offset is computed and written directly.
   * Otherwise, a null offset is written and a new forward reference is declared
   * for this label.
   *
   * @param owner the code writer that calls this method.
   * @param out the bytecode of the method.
   * @param source the position of first byte of the bytecode instruction that
   *      contains this label.
   * @param wideOffset <tt>true</tt> if the reference must be stored in 4 bytes,
   *      or <tt>false</tt> if it must be stored with 2 bytes.
   * @throws IllegalArgumentException if this label has not been created by the
   *      given code writer.
   */

  void put (
    final CodeWriter owner,
    final ByteVector out,
    final int source,
    final boolean wideOffset)
  {
    if (CodeWriter.CHECK) {
      if (this.owner == null) {
        this.owner = owner;
      } else if (this.owner != owner) {
        throw new IllegalArgumentException();
      }
    }
    if (resolved) {
      if (wideOffset) {
        out.put4(position - source);
      } else {
        out.put2(position - source);
      }
    } else {
      if (wideOffset) {
        addReference(-1 - source, out.length);
        out.put4(-1);
      } else {
        addReference(source, out.length);
        out.put2(-1);
      }
    }
  }

  /**
   * 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 offset of the reference can be, and must be,
   * computed and stored directly.
   *
   * @param sourcePosition the position of the referencing instruction. This
   *      position will be used to compute the offset of this forward reference.
   * @param referencePosition the position where the offset for this forward
   *      reference must be stored.
   */

  private void addReference (
    final int sourcePosition,
    final int referencePosition)
  {
    if (srcAndRefPositions == null) {
      srcAndRefPositions = new int[6];
    }
    if (referenceCount >= srcAndRefPositions.length) {
      int[] a = new int[srcAndRefPositions.length + 6];
      System.arraycopy(srcAndRefPositions, 0, a, 0, srcAndRefPositions.length);
      srcAndRefPositions = a;
    }
    srcAndRefPositions[referenceCount++] = sourcePosition;
    srcAndRefPositions[referenceCount++] = referencePosition;
  }

  /**
   * Resolves all forward references to this label. This method must be called
   * when this label is added to the bytecode of the method, i.e. when its
   * position becomes known. This method fills in the blanks that where left in
   * the bytecode by each forward reference previously added to this label.
   *
   * @param owner the code writer that calls this method.
   * @param position the position of this label in the bytecode.
   * @param data the bytecode of the method.
   * @return <tt>true</tt> if a blank that was left for this label was to small
   *      to store the offset. In such a case the corresponding jump instruction
   *      is replaced with a pseudo instruction (using unused opcodes) using an
   *      unsigned two bytes offset. These pseudo instructions will need to be
   *      replaced with true instructions with wider offsets (4 bytes instead of
   *      2). This is done in {@link CodeWriter#resizeInstructions}.
   * @throws IllegalArgumentException if this label has already been resolved,
   *      or if it has not been created by the given code writer.
   */

  boolean resolve (
    final CodeWriter owner,
    final int position,
    final byte[] data)
  {
    if (CodeWriter.CHECK) {
      if (this.owner == null) {
        this.owner = owner;
      }
      if (resolved || this.owner != owner) {
        throw new IllegalArgumentException();
      }
    }
    boolean needUpdate = false;
    this.resolved = true;
    this.position = position;
    int i = 0;
    while (i < referenceCount) {
      int source = srcAndRefPositions[i++];
      int reference = srcAndRefPositions[i++];
      int offset;
      if (source >= 0) {
        offset = position - source;
        if (offset < Short.MIN_VALUE || offset > Short.MAX_VALUE) {
          // changes the opcode of the jump instruction, in order to be able to
          // find it later (see resizeInstructions in CodeWriter). These
          // temporary 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 = data[reference - 1] & 0xFF;
          if (opcode <= Constants.JSR) {
            // changes IFEQ ... JSR to opcodes 202 to 217 (inclusive)
            data[reference - 1] = (byte)(opcode + 49);
          } else {
            // changes IFNULL and IFNONNULL to opcodes 218 and 219 (inclusive)
            data[reference - 1] = (byte)(opcode + 20);
          }
          needUpdate = true;
        }
        data[reference++] = (byte)(offset >>> 8);
        data[reference] = (byte)offset;
      } else {
        offset = position + source + 1;
        data[reference++] = (byte)(offset >>> 24);
        data[reference++] = (byte)(offset >>> 16);
        data[reference++] = (byte)(offset >>> 8);
        data[reference] = (byte)offset;
      }
    }
    return needUpdate;
  }
}