Label.java
// ASM: a very small and fast Java bytecode manipulation framework
// Copyright (c) 2000-2011 INRIA, France Telecom
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//
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package com.alibaba.fastjson2.internal.asm;
/**
* 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 <i>instruction</i> 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
*/
public class Label {
/**
* 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).
*/
static final int FLAG_DEBUG_ONLY = 1;
/**
* A flag indicating that a label is the target of a jump instruction, or the start of an
* exception handler.
*/
static final int FLAG_JUMP_TARGET = 2;
/**
* A flag indicating that the bytecode offset of a label is known.
*/
static final int FLAG_RESOLVED = 4;
/**
* A flag indicating that a label corresponds to a reachable basic block.
*/
static final int FLAG_REACHABLE = 8;
/**
* The number of elements to add to the {@link #forwardReferences} array when it needs to be
* resized to store a new forward reference.
*/
static final int FORWARD_REFERENCES_CAPACITY_INCREMENT = 6;
/**
* The bit mask to extract the type of a forward reference to this label. The extracted type is
* either {@link #FORWARD_REFERENCE_TYPE_SHORT} or {@link #FORWARD_REFERENCE_TYPE_WIDE}.
*
* @see #forwardReferences
*/
static final int FORWARD_REFERENCE_TYPE_MASK = 0xF0000000;
/**
* 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.
*/
static final int FORWARD_REFERENCE_TYPE_SHORT = 0x10000000;
/**
* 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.
*/
static final int FORWARD_REFERENCE_TYPE_WIDE = 0x20000000;
/**
* 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 {@link #FORWARD_REFERENCE_TYPE_MASK}).
*
* @see #forwardReferences
*/
static final int FORWARD_REFERENCE_HANDLE_MASK = 0x0FFFFFFF;
/**
* A sentinel element used to indicate the end of a list of labels.
*
* @see #nextListElement
*/
static final Label EMPTY_LIST = new Label();
/**
* 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.
*/
public Object info;
short flags;
/**
* The offset of this label in the bytecode of its method, in bytes. This value is set if and only
* if the {@link #FLAG_RESOLVED} flag is set.
*/
int bytecodeOffset;
/**
* 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':
*
* <ul>
* <li>'sourceInsnBytecodeOffset' is the bytecode offset of the instruction that contains the
* forward reference,
* <li>'reference' contains the type and the offset in the bytecode where the forward reference
* value must be stored, which can be extracted with {@link #FORWARD_REFERENCE_TYPE_MASK}
* and {@link #FORWARD_REFERENCE_HANDLE_MASK}.
* </ul>
*
* <p>For instance, for an ifnull instruction at bytecode offset x, 'sourceInsnBytecodeOffset' is
* equal to x, and 'reference' is of type {@link #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 {@link
* #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).
*/
private int[] forwardReferences;
/**
* 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 {@literal
* null}.
*/
short outputStackMax;
Frame frame;
Label nextBasicBlock;
Edge outgoingEdges;
Label nextListElement;
// -----------------------------------------------------------------------------------------------
// Constructor and accessors
// -----------------------------------------------------------------------------------------------
/**
* Constructs a new label.
*/
public Label() {
// Nothing to do.
}
final Label getCanonicalInstance() {
return frame == null ? this : frame.owner;
}
// -----------------------------------------------------------------------------------------------
// Methods to compute offsets and to manage forward references
// -----------------------------------------------------------------------------------------------
/**
* 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.
*
* @param code the bytecode of the method. This is where the reference is appended.
* @param sourceInsnBytecodeOffset the bytecode offset of the instruction that contains the
* reference to be appended.
* @param wideReference whether the reference must be stored in 4 bytes (instead of 2 bytes).
*/
final void put(
final ByteVector code, final int sourceInsnBytecodeOffset, final boolean wideReference) {
if ((flags & FLAG_RESOLVED) == 0) {
if (wideReference) {
addForwardReference(sourceInsnBytecodeOffset, FORWARD_REFERENCE_TYPE_WIDE, code.length);
code.putInt(-1);
} else {
addForwardReference(sourceInsnBytecodeOffset, FORWARD_REFERENCE_TYPE_SHORT, code.length);
code.putShort(-1);
}
} else {
if (wideReference) {
code.putInt(bytecodeOffset - sourceInsnBytecodeOffset);
} else {
code.putShort(bytecodeOffset - sourceInsnBytecodeOffset);
}
}
}
/**
* 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.
*
* @param sourceInsnBytecodeOffset the bytecode offset of the instruction that contains the
* reference stored at referenceHandle.
* @param referenceType either {@link #FORWARD_REFERENCE_TYPE_SHORT} or {@link
* #FORWARD_REFERENCE_TYPE_WIDE}.
* @param referenceHandle the offset in the bytecode where the forward reference value must be
* stored.
*/
private void addForwardReference(
final int sourceInsnBytecodeOffset, final int referenceType, final int referenceHandle) {
if (forwardReferences == null) {
forwardReferences = new int[FORWARD_REFERENCES_CAPACITY_INCREMENT];
}
int lastElementIndex = forwardReferences[0];
if (lastElementIndex + 2 >= forwardReferences.length) {
int[] newValues = new int[forwardReferences.length + FORWARD_REFERENCES_CAPACITY_INCREMENT];
System.arraycopy(forwardReferences, 0, newValues, 0, forwardReferences.length);
forwardReferences = newValues;
}
forwardReferences[++lastElementIndex] = sourceInsnBytecodeOffset;
forwardReferences[++lastElementIndex] = referenceType | referenceHandle;
forwardReferences[0] = lastElementIndex;
}
/**
* 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.
*
* @param code the bytecode of the method.
* @param bytecodeOffset the bytecode offset of this label.
* @return {@literal 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.
*/
final boolean resolve(final byte[] code, final int bytecodeOffset) {
this.flags |= FLAG_RESOLVED;
this.bytecodeOffset = bytecodeOffset;
if (forwardReferences == null) {
return false;
}
boolean hasAsmInstructions = false;
for (int i = forwardReferences[0]; i > 0; i -= 2) {
final int sourceInsnBytecodeOffset = forwardReferences[i - 1];
final int reference = forwardReferences[i];
final int relativeOffset = bytecodeOffset - sourceInsnBytecodeOffset;
int handle = reference & FORWARD_REFERENCE_HANDLE_MASK;
if ((reference & FORWARD_REFERENCE_TYPE_MASK) == FORWARD_REFERENCE_TYPE_SHORT) {
if (relativeOffset < Short.MIN_VALUE || relativeOffset > Short.MAX_VALUE) {
// Change the opcode of the jump instruction, in order to be able to find it later in
// ClassReader. These ASM specific opcodes are similar to jump instruction opcodes, except
// that the 2 bytes offset is unsigned (and can therefore represent values from 0 to
// 65535, which is sufficient since the size of a method is limited to 65535 bytes).
int opcode = code[sourceInsnBytecodeOffset] & 0xFF;
if (opcode < Opcodes.IFNULL) {
// Change IFEQ ... JSR to ASM_IFEQ ... ASM_JSR.
code[sourceInsnBytecodeOffset] = (byte) (opcode + Constants.ASM_OPCODE_DELTA);
} else {
// Change IFNULL and IFNONNULL to ASM_IFNULL and ASM_IFNONNULL.
code[sourceInsnBytecodeOffset] = (byte) (opcode + Constants.ASM_IFNULL_OPCODE_DELTA);
}
hasAsmInstructions = true;
}
} else {
code[handle++] = (byte) (relativeOffset >>> 24);
code[handle++] = (byte) (relativeOffset >>> 16);
}
code[handle++] = (byte) (relativeOffset >>> 8);
code[handle] = (byte) relativeOffset;
}
return hasAsmInstructions;
}
}