org.logicalcobwebs.asm
Class Label

java.lang.Object
  org.logicalcobwebs.asm.Label

public class Label
extends java.lang.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.

Field Summary

(package private) int	beginStackSize The stack size at the beginning of this basic block.
(package private) int	maxStackSize The (relative) maximum stack size corresponding to this basic block.
(package private) Label	next The next basic block in the basic block stack.
(package private) CodeWriter	owner The code writer to which this label belongs, or null if unknown.
(package private) int	position The position of this label in the code, if known.
(package private) boolean	pushed true if this basic block has been pushed in the basic block stack.
private int	referenceCount Number of forward references to this label, times two.
(package private) boolean	resolved Indicates if the position of this label is known.
private int[]	srcAndRefPositions Informations about forward references.
(package private) Edge	successors The successors of this node in the control flow graph.

Constructor Summary

Label()
Constructs a new label.

Method Summary

private void	addReference(int sourcePosition, int referencePosition) Adds a forward reference to this label.
(package private) void	put(CodeWriter owner, ByteVector out, int source, boolean wideOffset) Puts a reference to this label in the bytecode of a method.
(package private) boolean	resolve(CodeWriter owner, int position, byte[] data) Resolves all forward references to this label.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

owner

CodeWriter owner

The code writer to which this label belongs, or null if unknown.

resolved

boolean resolved

Indicates if the position of this label is known.

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.

beginStackSize

int beginStackSize

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 visitMaxs).

maxStackSize

int maxStackSize

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 beginStackSize + maxStackSize.

successors

Edge successors

The successors of this node in the control flow graph. These successors are stored in a linked list of Edge objects, linked to each other by their Edge.next field.

next

Label next

The next basic block in the basic block stack. See visitMaxs.

pushed

boolean pushed

true if this basic block has been pushed in the basic block stack. See visitMaxs.

Constructor Detail

Label

public Label()

Constructs a new label.

Method Detail

put

void put(CodeWriter 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:

java.lang.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(CodeWriter 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 CodeWriter.resizeInstructions(int[], int[], int).

Throws:

java.lang.IllegalArgumentException - if this label has already been resolved, or if it has not been created by the given code writer.