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package mockit.asm.controlFlow;
import javax.annotation.*;
import mockit.asm.util.*;
/**
* 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.).
*/
public final class Label
{
/**
* Constants for the current status of a label.
*/
interface Status {
/**
* 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).
*/
int DEBUG = 1;
/**
* Indicates if the position of this label is known.
*/
int RESOLVED = 2;
/**
* Indicates if this basic block has been pushed in the basic block stack.
*/
int PUSHED = 8;
/**
* Indicates if this label is the target of a jump instruction, or the start of an exception handler.
*/
int TARGET = 16;
/**
* Indicates if a stack map frame must be stored for this label.
*/
int STORE = 32;
/**
* Indicates if this label corresponds to a reachable basic block.
*/
int REACHABLE = 64;
}
/**
* Field used to associate user information to a label.
*/
@Nullable public Object info;
/**
* Flags that indicate the {@link Status Status} of this label.
*/
private int status;
/**
* The line number corresponding to this label, if known.
*/
@Nonnegative public int line;
/**
* The position of this label in the code, if known.
*/
@Nonnegative public int position;
/**
* Number of forward references to this label, times two.
*/
@Nonnegative private int referenceCount;
/**
* Information 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 bit set to store the subroutines to which a basic block belongs.
*/
private int[] srcAndRefPositions;
// Fields for the control flow and data flow graph analysis algorithms (used to compute the maximum stack size or the stack map frames).
// A control flow graph contains one node per "basic block", and one edge per "jump" from one basic block to another.
// Each node (i.e., each basic block) is represented by the Label object that corresponds to the first instruction of this basic block.
// Each node also stores the list of its successors in the graph, as a linked list of Edge objects.
//
// The control flow analysis algorithms used to compute the maximum stack size or the stack map frames are similar and use two steps.
// The first step, during the visit of each instruction, builds information about the state of the local variables and the operand stack
// at the end of each basic block, called the "output frame", relatively to the frame state at the beginning of the basic block,
// which is called the "input frame", and which is unknown during this step.
// The second step, in {@link MethodWriter#visitMaxStack}, is a fix point algorithm that computes information about the input frame of
// each basic block, from the input state of the first basic block (known from the method signature), and by the using the previously
// computed relative output frames.
//
// The algorithm used to compute the maximum stack size only computes the relative output and absolute input stack heights, while the
// algorithm used to compute stack map frames computes relative output frames and absolute input frames.
/**
* 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.
*/
@Nonnegative int inputStackTop;
/**
* Maximum height reached by the output stack, relatively to the top of the input stack. This maximum is always positive or null.
*/
@Nonnegative int outputStackMax;
/**
* Information about the input and output stack map frames of this basic block. This field is only used for classfiles of version 1.7+.
*/
Frame frame;
/**
* The successor of this label, in the order they are visited. This linked list does not include labels used for debug info only.
* If the classfile being read is of version 1.7+ 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).
*/
@Nullable Label successor;
/**
* The successors of this node in the control flow graph.
* These successors are stored in a linked list of {@link Edge Edge} objects, linked to each other by their {@link Edge#next} field.
*/
Edge successors;
/**
* 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.
*/
@Nullable Label next;
/**
* Returns the {@link #frame} this basic block belongs to, if any.
*/
public Frame getFrame() { return frame; }
public boolean isDebug() { return (status & Status.DEBUG) != 0; }
public boolean isResolved() { return (status & Status.RESOLVED) != 0; }
boolean isPushed() { return (status & Status.PUSHED) != 0; }
public boolean isTarget() { return (status & Status.TARGET) != 0; }
public boolean isStoringFrame() { return (status & Status.STORE) != 0; }
public boolean isReachable() { return (status & Status.REACHABLE) != 0; }
public void markAsDebug() { status |= Status.DEBUG; }
private void markAsResolved() { status |= Status.RESOLVED; }
void markAsPushed() { status |= Status.PUSHED; }
public void markAsTarget() { status |= Status.TARGET; }
void markAsStoringFrame() { status |= Status.STORE; }
void markAsReachable() { status |= Status.REACHABLE; }
void markAsTarget(@Nonnull Label target) { status |= target.status & Status.TARGET; }
/**
* 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.
*
* @param out the bytecode of the method
* @param source the position of first byte of the bytecode instruction that contains this label
* @param wideOffset true if the reference must be stored in 4 bytes, or false if it must be stored with 2 bytes
* @throws IllegalArgumentException if this label has not been created by the given code writer
*/
public void put(@Nonnull ByteVector out, @Nonnegative int source, boolean wideOffset) {
if (isResolved()) {
int reference = position - source;
if (wideOffset) {
out.putInt(reference);
}
else {
out.putShort(reference);
}
}
else {
if (wideOffset) {
addReference(-1 - source, out.getLength());
out.putInt(-1);
}
else {
addReference(source, out.getLength());
out.putShort(-1);
}
}
}
/**
* 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.
*
* @param sourcePosition the position of the referencing instruction, which will be used to compute the offset of this forward reference
* @param referencePosition the position where the offset for this forward reference must be stored
*/
private void addReference(@Nonnegative int sourcePosition, @Nonnegative int referencePosition) {
if (srcAndRefPositions == null) {
srcAndRefPositions = new int[6];
}
if (referenceCount >= srcAndRefPositions.length) {
int[] a = new int[srcAndRefPositions.length + 6];
System.arraycopy(srcAndRefPositions, 0, a, 0, srcAndRefPositions.length);
srcAndRefPositions = a;
}
srcAndRefPositions[referenceCount++] = sourcePosition;
srcAndRefPositions[referenceCount++] = referencePosition;
}
/**
* 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.
*
* @param methodBytecode bytecode of the method containing this label
*/
@SuppressWarnings("NumericCastThatLosesPrecision")
void resolve(@Nonnull ByteVector methodBytecode) {
markAsResolved();
byte[] data = methodBytecode.getData();
int pos = methodBytecode.getLength();
position = pos;
int[] srcAndRefPos = srcAndRefPositions;
for (int i = 0, refCount = referenceCount; i < refCount; i += 2) {
int source = srcAndRefPos[i];
int reference = srcAndRefPos[i + 1];
int offset;
if (source >= 0) {
offset = pos - source;
}
else {
offset = pos + source + 1;
data[reference++] = (byte) (offset >>> 24);
data[reference++] = (byte) (offset >>> 16);
}
data[reference++] = (byte) (offset >>> 8);
data[reference] = (byte) offset;
}
}
/**
* 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, returns the label itself.
* For other labels, returns the first label of the series.
*
* @return the first label of the series to which this label belongs
*/
@Nonnull
public Label getFirst() {
return frame == null ? this : frame.owner;
}
@Nonnegative
int decrementInputStackTop() { return --inputStackTop; }
@Nonnegative
void decrementInputStackTop(@Nonnegative int amount) { inputStackTop -= amount; }
/**
* Returns this label's {@link #successor}, if any.
*/
@Nullable
public Label getSuccessor() { return successor; }
@Nullable
public Label setSuccessors(@Nonnull Edge edge) {
edge.setNext(successors);
successors = edge;
return successor;
}
/**
* Updates the maximum height reached by the output stack, if needed.
*/
void updateOutputStackMaxHeight(@Nonnegative int outputStackTop) {
int top = inputStackTop + outputStackTop;
if (top > outputStackMax) {
outputStackMax = top;
}
}
@Override
public String toString() {
return "L" + System.identityHashCode(this);
}
}
