Package com.saxonica.ee.bytecode

Class ByteCodeCandidate

  • All Implemented Interfaces:
    ExportAgent, TailCallReturner, Locatable, IdentityComparable, Traceable
    public class ByteCodeCandidate
extends UnaryExpression
implements TailCallReturner
    A ByteCodeCandidate is an expression on the expression tree which, if evaluated, returns the value of its single operand. As a side-effect, it counts how many times it has been evaluated, and when a certain threshold is reached, it generates bytecode for its operand. In future evaluations thereafter the expression is evaluated by executing the bytecode rather than by using the interpreter.

    Note, after the operand of the ByteCodeCandidate is compiled, the ByteCodeCandidate remains on the expression tree. We experimented with removing it from the tree, but run-time structural changes to the expression tree cause problems, for example when evaluating a closure which saves a reference to the tree.

    • Field Detail

      • GENERATED_COUNTER

        public static int GENERATED_COUNTER

      • GENERATION_TIMER

        public static long GENERATION_TIMER

      • compiledMode

        public int compiledMode

    • Constructor Detail

      • ByteCodeCandidate

        public ByteCodeCandidate​(ExpressionOwner owner,
                         Expression child,
                         java.lang.String objectName,
                         int requiredEvaluationModes)

    • Method Detail

      • getId

        public int getId()

      • setCountDown

        public void setCountDown​(int val)

      • getCountDown

        public long getCountDown()

      • setByteCodeMonitor

        public void setByteCodeMonitor​(ByteCodeMonitor monitor)

      • optimize

        public Expression optimize​(ExpressionVisitor visitor,
                           ContextItemStaticInfo contextInfo)
                    throws XPathException
        Description copied from class: UnaryExpression
        Perform optimisation of an expression and its subexpressions.

        This method is called after all references to functions and variables have been resolved to the declaration of the function or variable, and after all type checking has been done.

        Overrides:
        optimize in class UnaryExpression
        Parameters:
        visitor - an expression visitor
        contextInfo - the static type of "." at the point where this expression is invoked. The parameter is set to null if it is known statically that the context item will be undefined. If the type of the context item is not known statically, the argument is set to Type.ITEM_TYPE
        Returns:
        the original expression, rewritten if appropriate to optimize execution
        Throws:
        XPathException - if an error is discovered during this phase (typically a type error)

      • export

        public void export​(ExpressionPresenter out)
            throws XPathException
        Description copied from class: UnaryExpression
        Diagnostic print of expression structure. The abstract expression tree is written to the supplied output destination.
        Specified by:
        export in interface ExportAgent
        Overrides:
        export in class UnaryExpression
        Parameters:
        out - the expression presenter used to display the structure
        Throws:
        XPathException - if the export fails, for example if an expression is found that won't work in the target environment.

      • isVacuousExpression

        public boolean isVacuousExpression()
        Determine whether this is a vacuous expression as defined in the XQuery update specification
        Overrides:
        isVacuousExpression in class Expression
        Returns:
        true if this expression is vacuous

      • isUpdatingExpression

        public boolean isUpdatingExpression()
        Description copied from class: Expression
        Determine whether this is an updating expression as defined in the XQuery update specification
        Overrides:
        isUpdatingExpression in class Expression
        Returns:
        true if this is an updating expression

      • evaluatePendingUpdates

        public void evaluatePendingUpdates​(XPathContext context,
                                   PendingUpdateList pul)
                            throws XPathException
        Description copied from class: Expression
        Evaluate an updating expression, adding the results to a Pending Update List. The default implementation of this method, which is used for non-updating expressions, throws an UnsupportedOperationException
        Overrides:
        evaluatePendingUpdates in class Expression
        Parameters:
        context - the XPath dynamic evaluation context
        pul - the pending update list to which the results should be written
        Throws:
        XPathException - if evaluation fails

      • getImplementationMethod

        public int getImplementationMethod()
        Description copied from class: Expression
        An implementation of Expression must provide at least one of the methods evaluateItem(), iterate(), or process(). This method indicates which of these methods is provided directly. The other methods will always be available indirectly, using an implementation that relies on one of the other methods.
        Specified by:
        getImplementationMethod in class Expression
        Returns:
        the implementation method, for example Expression.ITERATE_METHOD or Expression.EVALUATE_METHOD or Expression.PROCESS_METHOD

      • copy

        public Expression copy​(RebindingMap rebindings)
        Description copied from class: Expression
        Copy an expression. This makes a deep copy.
        Specified by:
        copy in class Expression
        Parameters:
        rebindings - a mutable list of (old binding, new binding) pairs that is used to update the bindings held in any local variable references that are copied.
        Returns:
        the copy of the original expression

      • effectiveBooleanValue

        public boolean effectiveBooleanValue​(XPathContext context)
                              throws XPathException
        Description copied from class: Expression
        Get the effective boolean value of the expression. This returns false if the value is the empty sequence, a zero-length string, a number equal to zero, or the boolean false. Otherwise it returns true.
        Overrides:
        effectiveBooleanValue in class Expression
        Parameters:
        context - The context in which the expression is to be evaluated
        Returns:
        the effective boolean value
        Throws:
        XPathException - if any dynamic error occurs evaluating the expression

      • process

        public void process​(Outputter output,
                    XPathContext context)
             throws XPathException
        Description copied from class: Expression
        Process the instruction, without returning any tail calls
        Overrides:
        process in class Expression
        Parameters:
        output - the destination for the result
        context - The dynamic context, giving access to the current node, the current variables, etc.
        Throws:
        XPathException - if a dynamic error occurs

      • processLeavingTail

        public TailCall processLeavingTail​(Outputter output,
                                   XPathContext context)
                            throws XPathException
        Description copied from interface: TailCallReturner
        ProcessLeavingTail: called to do the real work of this instruction. This method must be implemented in each subclass. The results of the instruction are written to the current Receiver, which can be obtained via the Controller.
        Specified by:
        processLeavingTail in interface TailCallReturner
        Parameters:
        output - the destination for the result
        context - The dynamic context of the transformation, giving access to the current node, the current variables, etc.
        Returns:
        null if the instruction has completed execution; or a TailCall indicating a function call or template call that is delegated to the caller, to be made after the stack has been unwound so as to save stack space.
        Throws:
        XPathException - if a dynamic error occurs during the evaluation of the instruction

      • evaluateItem

        public Item evaluateItem​(XPathContext context)
                  throws XPathException
        Evaluate an expression as a single item. This always returns either a single Item or null (denoting the empty sequence). No conversion is done. This method should not be used unless the static type of the expression is a subtype of "item" or "item?": that is, it should not be called if the expression may return a sequence. There is no guarantee that this condition will be detected.
        Overrides:
        evaluateItem in class Expression
        Parameters:
        context - The context in which the expression is to be evaluated
        Returns:
        the node or atomic value that results from evaluating the expression; or null to indicate that the result is an empty sequence
        Throws:
        XPathException - if any dynamic error occurs evaluating the expression

      • iterate

        public SequenceIterator iterate​(XPathContext context)
                         throws XPathException
        Return an Iterator to iterate over the values of a sequence. The value of every expression can be regarded as a sequence, so this method is supported for all expressions. This default implementation handles iteration for expressions that return singleton values: for non-singleton expressions, the subclass must provide its own implementation.
        Overrides:
        iterate in class Expression
        Parameters:
        context - supplies the context for evaluation
        Returns:
        a SequenceIterator that can be used to iterate over the result of the expression
        Throws:
        XPathException - if any dynamic error occurs evaluating the expression

      • getExpressionName

        public java.lang.String getExpressionName()
        Description copied from class: Expression
        Get a name identifying the kind of expression, in terms meaningful to a user.
        Overrides:
        getExpressionName in class Expression
        Returns:
        a name identifying the kind of expression, in terms meaningful to a user. The name will always be in the form of a lexical XML QName, and should match the name used in export() output displaying the expression.