com.alibaba.fastjson2.internal.asm

Class Label

java.lang.Object

com.alibaba.fastjson2.internal.asm.Label

public class Label
extends java.lang.Object

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.).

Field Summary

Fields

Modifier and Type	Field and Description
(package private) int	bytecodeOffset The offset of this label in the bytecode of its method, in bytes.
(package private) static Label	EMPTY_LIST A sentinel element used to indicate the end of a list of labels.
(package private) static int	FLAG_DEBUG_ONLY A flag indicating that a label is only used for debug attributes.
(package private) static int	FLAG_JUMP_TARGET A flag indicating that a label is the target of a jump instruction, or the start of an exception handler.
(package private) static int	FLAG_REACHABLE A flag indicating that a label corresponds to a reachable basic block.
(package private) static int	FLAG_RESOLVED A flag indicating that the bytecode offset of a label is known.
(package private) short	flags
(package private) static int	FORWARD_REFERENCE_HANDLE_MASK The bit mask to extract the 'handle' of a forward reference to this label.
(package private) static int	FORWARD_REFERENCE_TYPE_MASK The bit mask to extract the type of a forward reference to this label.
(package private) static int	FORWARD_REFERENCE_TYPE_SHORT The type of forward references stored with two bytes in the bytecode.
(package private) static int	FORWARD_REFERENCE_TYPE_WIDE The type of forward references stored in four bytes in the bytecode.
(package private) static int	FORWARD_REFERENCES_CAPACITY_INCREMENT The number of elements to add to the forwardReferences array when it needs to be resized to store a new forward reference.
private int[]	forwardReferences The forward references to this label.
(package private) Frame	frame
java.lang.Object	info A user managed state associated with this label.
(package private) Label	nextBasicBlock
(package private) Label	nextListElement
(package private) Edge	outgoingEdges
(package private) short	outputStackMax The maximum height reached by the output stack, relatively to the top of the input stack, in the basic block corresponding to this label.

Constructor Summary

Constructors

Constructor and Description
Label() Constructs a new label.

Method Summary

Modifier and Type	Method and Description
private void	addForwardReference(int sourceInsnBytecodeOffset, int referenceType, int referenceHandle) Adds a forward reference to this label.
(package private) Label	getCanonicalInstance()
(package private) void	put(ByteVector code, int sourceInsnBytecodeOffset, boolean wideReference) Puts a reference to this label in the bytecode of a method.
(package private) boolean	resolve(byte[] code, int bytecodeOffset) Sets the bytecode offset of this label to the given value and resolves the forward references to this label, if any.

Methods inherited from class java.lang.Object

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

Field Detail

FLAG_DEBUG_ONLY

static final int FLAG_DEBUG_ONLY

A flag indicating that a 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

FLAG_JUMP_TARGET

static final int FLAG_JUMP_TARGET

A flag indicating that a label is the target of a jump instruction, or the start of an exception handler.

See Also:

Constant Field Values

FLAG_RESOLVED

static final int FLAG_RESOLVED

A flag indicating that the bytecode offset of a label is known.

See Also:

Constant Field Values

FLAG_REACHABLE

static final int FLAG_REACHABLE

A flag indicating that a label corresponds to a reachable basic block.

See Also:

Constant Field Values

FORWARD_REFERENCES_CAPACITY_INCREMENT

static final int FORWARD_REFERENCES_CAPACITY_INCREMENT

The number of elements to add to the forwardReferences array when it needs to be resized to store a new forward reference.

See Also:

Constant Field Values

FORWARD_REFERENCE_TYPE_MASK

static final int FORWARD_REFERENCE_TYPE_MASK

The bit mask to extract the type of a forward reference to this label. The extracted type is either FORWARD_REFERENCE_TYPE_SHORT or FORWARD_REFERENCE_TYPE_WIDE.

See Also:

forwardReferences, Constant Field Values

FORWARD_REFERENCE_TYPE_SHORT

static final int FORWARD_REFERENCE_TYPE_SHORT

The type of forward references stored with two bytes in the bytecode. This is the case, for instance, of a forward reference from an ifnull instruction.

See Also:

Constant Field Values

FORWARD_REFERENCE_TYPE_WIDE

static final int FORWARD_REFERENCE_TYPE_WIDE

The type of forward references stored in four bytes in the bytecode. This is the case, for instance, of a forward reference from a lookupswitch instruction.

See Also:

Constant Field Values

FORWARD_REFERENCE_HANDLE_MASK

static final int FORWARD_REFERENCE_HANDLE_MASK

The bit mask to extract the 'handle' of a forward reference to this label. The extracted handle is the bytecode offset where the forward reference value is stored (using either 2 or 4 bytes, as indicated by the FORWARD_REFERENCE_TYPE_MASK).

See Also:

forwardReferences, Constant Field Values

EMPTY_LIST

static final Label EMPTY_LIST

A sentinel element used to indicate the end of a list of labels.

See Also:

nextListElement

info

public java.lang.Object info

A user managed state associated with this 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 getLabelNode method in MethodNode.

flags

short flags

bytecodeOffset

int bytecodeOffset

The offset of this label in the bytecode of its method, in bytes. This value is set if and only if the FLAG_RESOLVED flag is set.

forwardReferences

private int[] forwardReferences

The forward references to this label. The first element is the number of forward references, times 2 (this corresponds to the index of the last element actually used in this array). Then, each forward reference is described with two consecutive integers noted 'sourceInsnBytecodeOffset' and 'reference':

• 'sourceInsnBytecodeOffset' is the bytecode offset of the instruction that contains the forward reference,
• 'reference' contains the type and the offset in the bytecode where the forward reference value must be stored, which can be extracted with FORWARD_REFERENCE_TYPE_MASK and FORWARD_REFERENCE_HANDLE_MASK.

For instance, for an ifnull instruction at bytecode offset x, 'sourceInsnBytecodeOffset' is equal to x, and 'reference' is of type FORWARD_REFERENCE_TYPE_SHORT with value x + 1 (because the ifnull instruction uses a 2 bytes bytecode offset operand stored one byte after the start of the instruction itself). For the default case of a lookupswitch instruction at bytecode offset x, 'sourceInsnBytecodeOffset' is equal to x, and 'reference' is of type FORWARD_REFERENCE_TYPE_WIDE with value between x + 1 and x + 4 (because the lookupswitch instruction uses a 4 bytes bytecode offset operand stored one to four bytes after the start of the instruction itself).

outputStackMax

short outputStackMax

The maximum height reached by the output stack, relatively to the top of the input stack, in the basic block corresponding to this label. This maximum is always positive or null.

frame

Frame frame

nextBasicBlock

Label nextBasicBlock

outgoingEdges

Edge outgoingEdges

nextListElement

Label nextListElement

Constructor Detail

Label

public Label()

Constructs a new label.

Method Detail

getCanonicalInstance

final Label getCanonicalInstance()

put

final void put(ByteVector code,
               int sourceInsnBytecodeOffset,
               boolean wideReference)

Puts a reference to this label in the bytecode of a method. If the bytecode offset of the label is known, the relative bytecode offset between the label and the instruction referencing it is computed and written directly. Otherwise, a null relative offset is written and a new forward reference is declared for this label.

Parameters:

code - the bytecode of the method. This is where the reference is appended.

sourceInsnBytecodeOffset - the bytecode offset of the instruction that contains the reference to be appended.

wideReference - whether the reference must be stored in 4 bytes (instead of 2 bytes).

addForwardReference

private void addForwardReference(int sourceInsnBytecodeOffset,
                                 int referenceType,
                                 int referenceHandle)

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 relative bytecode offset of the reference can be, and must be, computed and stored directly.

Parameters:

sourceInsnBytecodeOffset - the bytecode offset of the instruction that contains the reference stored at referenceHandle.

referenceType - either FORWARD_REFERENCE_TYPE_SHORT or FORWARD_REFERENCE_TYPE_WIDE.

referenceHandle - the offset in the bytecode where the forward reference value must be stored.

resolve

final boolean resolve(byte[] code,
                      int bytecodeOffset)

Sets the bytecode offset of this label to the given value and resolves the forward references to this label, if any. This method must be called when this label is added to the bytecode of the method, i.e. when its bytecode offset becomes known. This method fills in the blanks that where left in the bytecode by each forward reference previously added to this label.

Parameters:

code - the bytecode of the method.

bytecodeOffset - the bytecode offset of this label.

Returns:

true if a blank that was left for this label was too small to store the offset. In such a case the corresponding jump instruction is replaced with an equivalent ASM specific instruction using an unsigned two bytes offset. These ASM specific instructions are later replaced with standard bytecode instructions with wider offsets (4 bytes instead of 2), in ClassReader.