org.curjent.impl.asm
Class Label

java.lang.Object
  extended by org.curjent.impl.asm.Label

public final class Label
extends Object

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.

Author:
Eric Bruneton

Field Summary
(package private)  int position
          The position of this label in the code, if known.
private  int referenceCount
          Number of forward references to this label, times two.
(package private) static int RESOLVED
          Indicates if the position of this label is known.
private  int[] srcAndRefPositions
          Informations about forward references.
(package private)  int status
          Flags that indicate the status of this label.
(package private) static int TARGET
          Indicates if this label is the target of a jump instruction, or the start of an exception handler.
 
Constructor Summary
Label()
           
 
Method Summary
private  void addReference(int sourcePosition, int referencePosition)
          Adds a forward reference to this label.
(package private)  void put(MethodWriter owner, ByteVector out, int source, boolean wideOffset)
          Puts a reference to this label in the bytecode of a method.
(package private)  boolean resolve(MethodWriter owner, int position, byte[] data)
          Resolves all forward references to this label.
 String toString()
          Returns a string representation of this label.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Field Detail

RESOLVED

static final int RESOLVED
Indicates if the position of this label is known.

See Also:
Constant Field Values

TARGET

static final int TARGET
Indicates if this label is the target of a jump instruction, or the start of an exception handler.

See Also:
Constant Field Values

status

int status
Flags that indicate the status of this label.

see #DEBUG

see #RESOLVED

see #RESIZED

see #PUSHED

see #TARGET

see #STORE

see #REACHABLE

see #JSR

see #RET


position

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


referenceCount

private int referenceCount
Number of forward references to this label, times two.


srcAndRefPositions

private int[] srcAndRefPositions
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. This array is also used as a bitset to store the subroutines to which a basic block belongs. This information is needed in { @link MethodWriter#visitMaxs}, after all forward references have been resolved. Hence the same array can be used for both purposes without problems.

Constructor Detail

Label

public Label()
Method Detail

put

void put(MethodWriter owner,
         ByteVector out,
         int source,
         boolean wideOffset)
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.

Parameters:
owner - the code writer that calls this method.
out - the bytecode of the method.
source - the position of first byte of the bytecode instruction that contains this label.
wideOffset - true if the reference must be stored in 4 bytes, or false if it must be stored with 2 bytes.
Throws:
IllegalArgumentException - if this label has not been created by the given code writer.

addReference

private void addReference(int sourcePosition,
                          int referencePosition)
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.

Parameters:
sourcePosition - the position of the referencing instruction. This position will be used to compute the offset of this forward reference.
referencePosition - the position where the offset for this forward reference must be stored.

resolve

boolean resolve(MethodWriter owner,
                int position,
                byte[] data)
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.

Parameters:
owner - the code writer that calls this method.
position - the position of this label in the bytecode.
data - the bytecode of the method.
Returns:
true 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 MethodWriter#resizeInstructions}.
Throws:
IllegalArgumentException - if this label has already been resolved, or if it has not been created by the given code writer.

toString

public String toString()
Returns a string representation of this label.

Overrides:
toString in class Object
Returns:
a string representation of this label.


Copyright 2009-2011 Tom Landon
Apache License 2.0