org.objectweb.asm
Class Label

java.lang.Object
  extended by 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. A label designates the instruction 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

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) static int SUBROUTINE
          Indicates if this basic block is the start of a subroutine.
(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.
(package private) static int VISITED
          Indicates if this subroutine basic block has been visited.
 
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 addToSubroutine(long id, int nbSubroutines)
          Marks this basic block as belonging to the given subroutine.
(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)  boolean inSameSubroutine(Label block)
          Returns true if this basic block and the given one belong to a common subroutine.
(package private)  boolean inSubroutine(long id)
          Returns true is this basic block belongs to the given subroutine.
(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.
(package private)  void visitSubroutine(Label JSR, long id, int nbSubroutines)
          Finds the basic blocks that belong to a given subroutine, and marks these blocks as belonging to this subroutine.
 
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

SUBROUTINE

static final int SUBROUTINE
Indicates if this basic block is the start of a subroutine.

See Also:
Constant Field Values

VISITED

static final int VISITED
Indicates if this subroutine basic block has been visited.

See Also:
Constant Field Values

info

public java.lang.Object info
Field used to associate user information to a 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 MethodNode.getLabelNode(org.objectweb.asm.Label) method.


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. This array is also used as a bitset to store the subroutines to which a basic block belongs. This information is needed in , after all forward references have been resolved. Hence the same array can be used for both purposes without problems.


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. It is also used in visitSubroutine(org.objectweb.asm.Label, long, int) to avoid using a recursive method.

See Also:
MethodWriter.visitMaxs(int, int)
Constructor Detail

Label

public Label()
Constructs a new label.

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.

inSubroutine

boolean inSubroutine(long id)
Returns true is this basic block belongs to the given subroutine.

Parameters:
id - a subroutine id.
Returns:
true is this basic block belongs to the given subroutine.

inSameSubroutine

boolean inSameSubroutine(Label block)
Returns true if this basic block and the given one belong to a common subroutine.

Parameters:
block - another basic block.
Returns:
true if this basic block and the given one belong to a common subroutine.

addToSubroutine

void addToSubroutine(long id,
                     int nbSubroutines)
Marks this basic block as belonging to the given subroutine.

Parameters:
id - a subroutine id.
nbSubroutines - the total number of subroutines in the method.

visitSubroutine

void visitSubroutine(Label JSR,
                     long id,
                     int nbSubroutines)
Finds the basic blocks that belong to a given subroutine, 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 block WITHOUT following any JSR target.

Parameters:
JSR - a JSR block that jumps to this subroutine. If this JSR is not null it is added to the successor of the RET blocks found in the subroutine.
id - the id of this subroutine.
nbSubroutines - the total number of subroutines in the method.

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.