Package org.eclipse.sisu.space.asm
Class Label
- java.lang.Object
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- org.eclipse.sisu.space.asm.Label
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public final 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.).
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Field Summary
Fields Modifier and Type Field Description (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 by a visitSubroutine(null, ...) call.(package private) static int
VISITED2
Indicates if this subroutine basic block has been visited by a visitSubroutine(!null, ...) call.
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Constructor Summary
Constructors Constructor Description Label()
Constructs a new label.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description 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.
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Field Detail
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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
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RESOLVED
static final int RESOLVED
Indicates if the position of this label is known.- See Also:
- Constant Field Values
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RESIZED
static final int RESIZED
Indicates if this label has been updated, after instruction resizing.- See Also:
- Constant Field Values
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PUSHED
static final int PUSHED
Indicates if this basic block has been pushed in the basic block stack. SeevisitMaxs
.- See Also:
- Constant Field Values
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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
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STORE
static final int STORE
Indicates if a stack map frame must be stored for this label.- See Also:
- Constant Field Values
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REACHABLE
static final int REACHABLE
Indicates if this label corresponds to a reachable basic block.- See Also:
- Constant Field Values
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JSR
static final int JSR
Indicates if this basic block ends with a JSR instruction.- See Also:
- Constant Field Values
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RET
static final int RET
Indicates if this basic block ends with a RET instruction.- See Also:
- Constant Field Values
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SUBROUTINE
static final int SUBROUTINE
Indicates if this basic block is the start of a subroutine.- See Also:
- Constant Field Values
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VISITED
static final int VISITED
Indicates if this subroutine basic block has been visited by a visitSubroutine(null, ...) call.- See Also:
- Constant Field Values
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VISITED2
static final int VISITED2
Indicates if this subroutine basic block has been visited by a visitSubroutine(!null, ...) call.- See Also:
- Constant Field Values
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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 theorg.objectweb.asm.tree.MethodNode#getLabelNode
method.
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status
int status
Flags that indicate the status of this label.
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line
int line
The line number corresponding to this label, if known.
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position
int position
The position of this label in the code, if known.
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referenceCount
private int referenceCount
Number of forward references to this label, times two.
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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.
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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.
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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.
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frame
Frame frame
Information about the input and output stack map frames of this basic block. This field is only used whenClassWriter.COMPUTE_FRAMES
option is used.
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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. IfClassWriter.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).
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successors
Edge successors
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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 invisitSubroutine(org.eclipse.sisu.space.asm.Label, long, int)
to avoid using a recursive method.- See Also:
MethodWriter.visitMaxs(int, int)
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Method Detail
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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 forAttribute
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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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toString
public java.lang.String toString()
Returns a string representation of this label.- Overrides:
toString
in classjava.lang.Object
- Returns:
- a string representation of this label.
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