com.saxonica.ee.bytecode.util

Class OnEmpty.UnwindAndJump

java.lang.Object

com.saxonica.ee.bytecode.util.OnEmpty

com.saxonica.ee.bytecode.util.OnEmpty.UnwindAndJump

Enclosing class:

OnEmpty

public static class OnEmpty.UnwindAndJump
extends OnEmpty

The UnwindAndJump subclass causes the code generator, when an expression evaluates to the empty sequence, to generate a "goto" instruction to a defined label; but before executing the jump, the stack will be unwound to a known position.

Nested Class Summary

Nested classes/interfaces inherited from class com.saxonica.ee.bytecode.util.OnEmpty

OnEmpty.ReturnEmptyString, OnEmpty.ReturnNull, OnEmpty.UnwindAndJump

Field Summary

Fields inherited from class com.saxonica.ee.bytecode.util.OnEmpty

RETURN_EMPTY_STRING, RETURN_NULL

Constructor Summary

Constructors

Constructor and Description
UnwindAndJump(LabelInfo label) Generate an UnwindAndStack action using an empty stack (no unwinding needed)
UnwindAndJump(java.util.Stack<java.lang.Class> unwindStack, LabelInfo label)

Method Summary

Modifier and Type	Method and Description
void	generate(Generator ga) Generate code.
LabelInfo	getLabel() Get the label acting as the jump destination
java.util.Stack<java.lang.Class>	getUnwindStack() Get information about the stack that needs to be unwound before making the jump.

Methods inherited from class java.lang.Object

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

Constructor Detail

UnwindAndJump

public UnwindAndJump(java.util.Stack<java.lang.Class> unwindStack,
                     LabelInfo label)

Parameters:

unwindStack - Information about the classes of object that will be on the bytecode stack and must be popped off the stack before jumping to the onEmpty label: the generated code must pop the items described on this stack before jumping to the label.

label - a label to which control will be transferred (with nothing on the stack) if the result of the evaluation is an empty sequence. This mechanism supports the design pattern whereby many expressions return empty if one of there arguments is empty; rather than repeatedly checking for empty after evaluating each subexpression, there is a "longjump" treating this as an exception case.

UnwindAndJump

public UnwindAndJump(LabelInfo label)

Generate an UnwindAndStack action using an empty stack (no unwinding needed)

Parameters:

label - the label to jump to

Method Detail

generate

public void generate(Generator ga)

Generate code. (This method is not extensively used)

Specified by:

generate in class OnEmpty

Parameters:

ga - the code generator

getLabel

public LabelInfo getLabel()

Get the label acting as the jump destination

Returns:

the label

getUnwindStack

public java.util.Stack<java.lang.Class> getUnwindStack()

Get information about the stack that needs to be unwound before making the jump. This contains information about the class of object at each position, because doubles and longs occupy two words and need a pop2 instruction to remove them

Returns:

details of the stack to be unwound