org.objectweb.asm
Class Label

java.lang.Object
  org.objectweb.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.

Author:

Eric Bruneton

Field Summary

(package private) static int	DEBUG Indicates if this label is only used for debug attributes.
(package private) Frame	frame Information about the input and output stack map frames of this basic block.
java.lang.Object	info Field used to associate user information to a label.
(package private) int	inputStackTop Start of the output stack relatively to the input stack.
(package private) static int	JSR Indicates if this basic block ends with a JSR instruction.
(package private) int	line The line number corresponding to this label, if known.
(package private) Label	next The next basic block in the basic block stack.
(package private) int	outputStackMax Maximum height reached by the output stack, relatively to the top of the input stack.
(package private) int	position The position of this label in the code, if known.
(package private) static int	PUSHED Indicates if this basic block has been pushed in the basic block stack.
(package private) static int	REACHABLE Indicates if this label corresponds to a reachable basic block.
private int	referenceCount Number of forward references to this label, times two.
(package private) static int	RESIZED Indicates if this label has been updated, after instruction resizing.
(package private) static int	RESOLVED Indicates if the position of this label is known.
(package private) static int	RET Indicates if this basic block ends with a RET instruction.
private int[]	srcAndRefPositions Informations about forward references.
(package private) int	status Flags that indicate the status of this label.
(package private) static int	STORE Indicates if a stack map frame must be stored for this label.
(package private) Label	successor The successor of this label, in the order they are visited.
(package private) Edge	successors The successors of this node in the control flow graph.
(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()
Constructs a new label.

Label(boolean debug)
Constructs a new label.

Method Summary

private void	addReference(int sourcePosition, int referencePosition) Adds a forward reference to this label.
(package private) Label	getFirst() Returns the first label of the series to which this label belongs.
int	getOffset() Returns the offset corresponding 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.
java.lang.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

DEBUG

static final int DEBUG

Indicates if this 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).

See Also:

Constant Field Values

RESOLVED

static final int RESOLVED

Indicates if the position of this label is known.

See Also:

Constant Field Values

RESIZED

static final int RESIZED

Indicates if this label has been updated, after instruction resizing.

See Also:

Constant Field Values

PUSHED

static final int PUSHED

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

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

STORE

static final int STORE

Indicates if a stack map frame must be stored for this label.

See Also:

Constant Field Values

REACHABLE

static final int REACHABLE

Indicates if this label corresponds to a reachable basic block.

See Also:

Constant Field Values

JSR

static final int JSR

Indicates if this basic block ends with a JSR instruction.

See Also:

Constant Field Values

RET

static final int RET

Indicates if this basic block ends with a RET instruction.

See Also:

Constant Field Values

info

public java.lang.Object info

Field used to associate user information to a label.

status

int status

Flags that indicate the status of this label.

See Also:

DEBUG, RESOLVED, RESIZED, PUSHED, TARGET, STORE, REACHABLE, JSR, RET

line

int line

The line number corresponding to this label, if 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.

inputStackTop

int inputStackTop

Start of the output stack relatively to the input stack. The exact semantics of this field depends on the algorithm that is used. When only the maximum stack size is computed, this field is the number of elements in the input stack. When the stack map frames are completely computed, this field is the offset of the first output stack element relatively to the top of the input stack. This offset is always negative or null. A null offset means that the output stack must be appended to the input stack. A -n offset means that the first n output stack elements must replace the top n input stack elements, and that the other elements must be appended to the input stack.

outputStackMax

int outputStackMax

Maximum height reached by the output stack, relatively to the top of the input stack. This maximum is always positive or null.

frame

Frame frame

Information about the input and output stack map frames of this basic block. This field is only used when ClassWriter.COMPUTE_FRAMES option is used.

successor

Label successor

The successor of this label, in the order they are visited. This linked list does not include labels used for debug info only. If ClassWriter.COMPUTE_FRAMES option is used then, in addition, it does not contain successive labels that denote the same bytecode position (in this case only the first label appears in this list).

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. This stack is used in the main loop of the fix point algorithm used in the second step of the control flow analysis algorithms.

See Also:

MethodWriter.visitMaxs(int, int)

Constructor Detail

Label

public Label()

Constructs a new label.

Label

Label(boolean debug)

Constructs a new label.

Parameters:

debug - if this label is only used for debug attributes.

Method Detail

getOffset

public int getOffset()

Returns the offset corresponding to this label. This offset is computed from the start of the method's bytecode. This method is intended for Attribute sub classes, and is normally not needed by class generators or adapters.

Returns:

the offset corresponding to this label.

Throws:

java.lang.IllegalStateException - if this label is not resolved yet.

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:

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(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 MethodWriter.resizeInstructions().

Throws:

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

getFirst

Label getFirst()

Returns the first label of the series to which this label belongs. For an isolated label or for the first label in a series of successive labels, this method returns the label itself. For other labels it returns the first label of the series.

Returns:

the first label of the series to which this label belongs.

toString

public java.lang.String toString()

Returns a string representation of this label.

Overrides:

toString in class java.lang.Object

Returns:

a string representation of this label.