Evaluates to the same result as its operand, but ensures the operand is evaluated once even if this Expression is evaluated several times. More...

#include <qevaluationcache_p.h>

Inheritance diagram for QPatternist::EvaluationCache< IsForGlobal >:

Public Functions
virtual ExpressionVisitorResult::Ptr	accept (const ExpressionVisitor::Ptr &visitor) const

virtual const SourceLocationReflection *	actualReflection () const
	Returns `this`. More...

virtual Expression::Ptr	compress (const StaticContext::Ptr &context)

virtual Item::Iterator::Ptr	evaluateSequence (const DynamicContext::Ptr &context) const

virtual Item	evaluateSingleton (const DynamicContext::Ptr &context) const

	EvaluationCache (const Expression::Ptr &operand, const VariableDeclaration *varDecl, const VariableSlotID slot)

virtual SequenceType::List	expectedOperandTypes () const

virtual Properties	properties () const

VariableSlotID	slot () const

virtual SequenceType::Ptr	staticType () const

virtual Expression::Ptr	typeCheck (const StaticContext::Ptr &context, const SequenceType::Ptr &reqType)

Public Functions inherited from QPatternist::SingleContainer
virtual bool	compressOperands (const StaticContext::Ptr &)

virtual Expression::List	operands () const

virtual void	setOperands (const Expression::List &operands)

Public Functions inherited from QPatternist::Expression
virtual void	announceFocusType (const ItemType::Ptr &itemType)

Properties	deepProperties () const
	Computes the union of properties for this Expression and all its descending children. More...

virtual Properties	dependencies () const

virtual QString	description () const

virtual bool	evaluateEBV (const DynamicContext::Ptr &context) const

virtual void	evaluateToSequenceReceiver (const DynamicContext::Ptr &context) const

virtual ItemType::Ptr	expectedContextItemType () const

	Expression ()

bool	has (const Property prop) const

bool	hasDependency (const Property prop) const

virtual ID	id () const

bool	is (const ID id) const

bool	isEvaluated () const

virtual ItemType::Ptr	newFocusType () const

virtual QList< QExplicitlySharedDataPointer< OptimizationPass > >	optimizationPasses () const

virtual PatternPriority	patternPriority () const

const Expression::Ptr &	rewrite (const Expression::Ptr &to, const StaticContext::Ptr &context) const
	Rewrites this Expression to `to`, and return `to`. More...

virtual	~Expression ()

Public Functions inherited from QSharedData
	QSharedData ()
	Constructs a QSharedData object with a reference count of 0. More...

	QSharedData (const QSharedData &)
	Constructs a QSharedData object with reference count 0. More...

Public Functions inherited from QPatternist::CppCastingHelper< Expression >
const TCastTarget *	as () const

TCastTarget *	as ()

Public Functions inherited from QPatternist::SourceLocationReflection
virtual QSourceLocation	sourceLocation () const

	SourceLocationReflection ()

virtual	~SourceLocationReflection ()

Static Private Functions
static DynamicContext::Ptr	topFocusContext (const DynamicContext::Ptr &context)

Properties
const VariableDeclaration *	m_declaration

bool	m_declarationUsedByMany

const VariableSlotID	m_varSlot

Additional Inherited Members
Public Types inherited from QPatternist::Expression
typedef QExplicitlySharedDataPointer< const Expression >	ConstPtr
	A smart pointer wrapping `const` Expression instances. More...

enum	ID { IDBooleanValue = 1, IDCountFN, IDEmptyFN, IDExistsFN, IDExpressionSequence, IDGeneralComparison, IDIfThenClause, IDIgnorableExpression, IDIntegerValue, IDPositionFN, IDStringValue, IDValueComparison, IDRangeVariableReference, IDContextItem, IDUserFunctionCallsite, IDExpressionVariableReference, IDAttributeConstructor, IDUpperCaseFN, IDLowerCaseFN, IDFirstItemPredicate, IDEmptySequence, IDReturnOrderBy, IDLetClause, IDForClause, IDPath, IDNamespaceConstructor, IDArgumentReference, IDGenericPredicate, IDAxisStep, IDFloat, IDCombineNodes, IDUnresolvedVariableReference, IDCardinalityVerifier }

typedef QList< Expression::Ptr >	List

typedef QFlags< Property >	Properties

enum	Property { UseContextItem = 1, DisableElimination = 1 << 1, IsEvaluated = 1 << 2, DisableTypingDeduction = 1 << 3, EmptynessFollowsChild = 1 << 4, RewriteToEmptyOnEmpty = 1 << 5, RequiresFocus = 1 << 6, AffectsOrderOnly = 1 << 7, RequiresContextItem = (1 << 8) \| RequiresFocus, CreatesFocusForLast = 1 << 9, LastOperandIsCollation = 1 << 10, DependsOnLocalVariable = (1 << 11) \| DisableElimination, EvaluationCacheRedundant = (1 << 12), IsNodeConstructor = 1 << 13, RequiresCurrentItem = 1 << 14 }

typedef QExplicitlySharedDataPointer< Expression >	Ptr
	A smart pointer wrapping mutable Expression instances. More...

typedef ::QAbstractXmlForwardIterator< Expression::Ptr >	QAbstractXmlForwardIterator

typedef QVector< Expression::Ptr >	Vector

Static Public Functions inherited from QPatternist::Expression
static void	rewrite (Expression::Ptr &old, const Expression::Ptr &New, const StaticContext::Ptr &context)

Public Variables inherited from QSharedData
QAtomicInt	ref

Protected Functions inherited from QPatternist::SingleContainer
	SingleContainer (const Expression::Ptr &operand)

Protected Functions inherited from QPatternist::Expression
void	typeCheckOperands (const StaticContext::Ptr &context)

Protected Functions inherited from QPatternist::CppCastingHelper< Expression >
	CppCastingHelper ()

Protected Variables inherited from QPatternist::SingleContainer
Expression::Ptr	m_operand

Detailed Description

template<bool IsForGlobal>
class QPatternist::EvaluationCache< IsForGlobal >

Evaluates to the same result as its operand, but ensures the operand is evaluated once even if this Expression is evaluated several times.

EvaluationCache does this in a pipelined way, by delivering items from its cache, which is stored in the DynamicContext. If the cache has less items than what the caller requests, EvaluationCache continues to deliver but this time from the source, which it also populates into the cache.

EvaluationCache is used as an optimization in order to avoid running expensive code paths multiple times, but also is sometimes a necessity: for instance, when objects must be unique, such as potentially in the case of node identity.

EvaluationCache is in particular used for variables, whose sole purpose is to store it once(at least conceptually) and then use it in multiple places.

In some cases an EvaluationCache isn't necessary. For instance, when a variable is only referenced once. In those cases EvaluationCache removes itself as an optimization; implemented in compress().

Author: Frans Englich frans.nosp@m..eng.nosp@m.lich@.nosp@m.noki.nosp@m.a.com

Definition at line 96 of file qevaluationcache_p.h.

Constructors and Destructors

◆ EvaluationCache()

template<bool IsForGlobal>

QPatternist::EvaluationCache< IsForGlobal >::EvaluationCache	(	const Expression::Ptr &	operand,
		const VariableDeclaration *	varDecl,
		const VariableSlotID	slot
	)

Definition at line 49 of file qevaluationcache.cpp.

                                                                           : SingleContainer(op)
                                                                           , m_declarationUsedByMany(varDecl->usedByMany())
                                                                           , m_varSlot(aSlot)
 {
     Q_ASSERT(m_varSlot > -1);
 }

Functions

◆ accept()

template<bool IsForGlobal>

ExpressionVisitorResult::Ptr QPatternist::EvaluationCache< IsForGlobal >::accept ( const ExpressionVisitor::Ptr & visitor ) const

virtual

Implements QPatternist::Expression.

Definition at line 263 of file qevaluationcache.cpp.

 {
     return visitor->visit(this);
 }

◆ actualReflection()

template<bool IsForGlobal>

const SourceLocationReflection * QPatternist::EvaluationCache< IsForGlobal >::actualReflection ( ) const

virtual

Returns this.

Reimplemented from QPatternist::Expression.

Definition at line 269 of file qevaluationcache.cpp.

 {
     return m_operand->actualReflection();
 }

◆ compress()

template<bool IsForGlobal>

Expression::Ptr QPatternist::EvaluationCache< IsForGlobal >::compress ( const StaticContext::Ptr & context )

virtual

compress() is the last stage performs in compiling an expression, done after the initial AST build and calling typeCheck(). compress() performs crucial simplifications, either by having drastic performance implications or that some expressions depend on it for proper behavior.

The default implementation performs a sparse conditional constant propagation. In short, a recursive process is performed in the AST which examines if the Expression's operands are constant values, and if so, performs a const fold(AST rewrite) into the result of evaluating the expression in question. This default behavior can be disabled by letting properties() return DisableElimination.

This compress() stage can be relative effective due to the design of XPath, in part because intrinsic functions are heavily used. Many Expressions override compress() and do optimizations specific to what they do. Also, many Expressions performs optimizations in their typeCheck().

Parameters

context the static context. Supplies compile time information, and is the channel for communicating error messages.

See also: Wikipedia, the free encyclopedia, Sparse conditional constant propagation; Wikipedia, the free encyclopedia, Intrinsic function; Wikipedia, the free encyclopedia, Compiler optimization

Reimplemented from QPatternist::Expression.

Definition at line 191 of file qevaluationcache.cpp.

 {
     const Expression::Ptr me(SingleContainer::compress(context));
 
     if(me != this)
         return me;
 
     if(m_operand->is(IDRangeVariableReference))
         return m_operand;
 
     if (m_declarationUsedByMany)
     {
         /* If it's only an atomic value an EvaluationCache is overkill. However,
          * it's still needed for functions like fn:current-time() that must adhere to
          * query stability. */
         const Properties props(m_operand->properties());
 
         if(props.testFlag(EvaluationCacheRedundant) ||
            ((props.testFlag(IsEvaluated)) &&
             !props.testFlag(DisableElimination) &&
             CommonSequenceTypes::ExactlyOneAtomicType->matches(m_operand->staticType())))
         {
             return m_operand;
         }
         else
             return me;
     }
     else
     {
         /* If we're only used once, there's no need for an EvaluationCache. */
         return m_operand;
     }
 }

◆ evaluateSequence()

template<bool IsForGlobal>

Item::Iterator::Ptr QPatternist::EvaluationCache< IsForGlobal >::evaluateSequence ( const DynamicContext::Ptr & context ) const

virtual

Evaluate this Expression by iterating over it. This is a central function for evaluating expressions.

Expressions must always always return a valid QAbstractXmlForwardIterator and may never return 0. If an empty result is of interest to be returned, the EmptyIterator should be returned.

The default implementation returns a SingletonIterator over the item returned from evaluateSingleton().

Note: This function may raise an exception when calling, not only when QAbstractXmlForwardIterator::next() is called on the return value. This is because in some cases evaluateSingleton() is called directly.

We don't use makeListIterator() here because the MIPSPro compiler can't handle it.

Reimplemented from QPatternist::Expression.

Definition at line 108 of file qevaluationcache.cpp.

 {
     ItemSequenceCacheCell::Vector &cells = IsForGlobal ? context->globalItemSequenceCacheCells(m_varSlot) : context->itemSequenceCacheCells(m_varSlot);
     ItemSequenceCacheCell &cell = cells[m_varSlot];
 
 
     if(cell.inUse)
     {
         context->error(QtXmlPatterns::tr("Circularity detected"),
                        ReportContext::XTDE0640, this);
     }
 
     switch(cell.cacheState)
     {
         case ItemSequenceCacheCell::Full:
         {
             return Item::Iterator::Ptr(new ListIterator<Item, Item::List>(cell.cachedItems));
         }
         case ItemSequenceCacheCell::Empty:
         {
             cell.inUse = true;
             cell.sourceIterator = m_operand->evaluateSequence(IsForGlobal ? topFocusContext(context) : context);
             cell.cacheState = ItemSequenceCacheCell::PartiallyPopulated;
             /* Fallthrough. */
         }
         case ItemSequenceCacheCell::PartiallyPopulated:
         {
             cell.inUse = false;
             Q_ASSERT_X(cells.at(m_varSlot).sourceIterator, Q_FUNC_INFO,
                        "This trigger for a cache bug which hasn't yet been analyzed.");
             return Item::Iterator::Ptr(new CachingIterator(cells, m_varSlot, IsForGlobal ? topFocusContext(context) : context));
         }
         default:
         {
             Q_ASSERT_X(false, Q_FUNC_INFO, "This path is not supposed to be run.");
             return Item::Iterator::Ptr();
         }
     }
 }

◆ evaluateSingleton()

template<bool IsForGlobal>

Item QPatternist::EvaluationCache< IsForGlobal >::evaluateSingleton ( const DynamicContext::Ptr & context ) const

virtual

Reimplemented from QPatternist::Expression.

Definition at line 76 of file qevaluationcache.cpp.

 {
     ItemCacheCell &cell = IsForGlobal ? context->globalItemCacheCell(m_varSlot) : context->itemCacheCell(m_varSlot);
 
     if(cell.cacheState == ItemCacheCell::Full)
         return cell.cachedItem;
     else
     {
         Q_ASSERT(cell.cacheState == ItemCacheCell::Empty);
         cell.cachedItem = m_operand->evaluateSingleton(IsForGlobal ? topFocusContext(context) : context);
         cell.cacheState = ItemCacheCell::Full;
         return cell.cachedItem;
     }
 }

◆ expectedOperandTypes()

template<bool IsForGlobal>

SequenceType::List QPatternist::EvaluationCache< IsForGlobal >::expectedOperandTypes ( ) const

virtual

The first operand must be exactly one xs:string.

Implements QPatternist::Expression.

Definition at line 232 of file qevaluationcache.cpp.

 {
     /* Remember that EvaluationCache::typeCheck() will be called from multiple locations,
      * which potentially have different type requirements. For instance, one wants a node,
      * and another requires atomization and casting.
      *
      * Returning ZeroOrMoreItems is safe here because staticType() returns the operand's type
      * and therefore the convertors like Atomizer will be parents to us, and hence only affect
      * the relevant path.
      *
      * ZeroOrMoreItems also make sense logically since we're actually only used where the
      * variable references reference us. */
     SequenceType::List result;
     result.append(CommonSequenceTypes::ZeroOrMoreItems);
 
     return result;
 }

◆ properties()

template<bool IsForGlobal>

Expression::Properties QPatternist::EvaluationCache< IsForGlobal >::properties ( ) const

virtual

Returns: a bitwise OR'd value of properties, describing the characteristics of the expression. These properties affects how this Expression is treated in for example type checking stages.

The default implementation returns 0. Override and let the function return a different value, if that's of interest.

An important decision when re-implementing properties() is whether to OR in the properties() of ones operands. For instance, if an operand has RequiresFocus set, that flag nost likely applies to the apparent as well, since it depends on its operand.

See also: deepProperties()

Returns: Expression::None, meaning no special properties

Reimplemented from QPatternist::Expression.

Definition at line 251 of file qevaluationcache.cpp.

 {
     /* We cannot return the operand's properties unconditionally, because some
      * doesn't hold for this Expression.
      *
      * However, some of the properties must propagate through, which are the ones being OR'd here.
      */
     return m_operand->properties() & (DisableElimination | IsEvaluated | DisableTypingDeduction);
 }

◆ slot()

template<bool IsForGlobal>

VariableSlotID QPatternist::EvaluationCache< IsForGlobal >::slot ( ) const

inline

Definition at line 120 of file qevaluationcache_p.h.

         {
             return m_varSlot;
         }

◆ staticType()

template<bool IsForGlobal>

SequenceType::Ptr QPatternist::EvaluationCache< IsForGlobal >::staticType ( ) const

virtual

Returns: the static type of this Expression. For example, an 'and' expression have as static type xs:boolean

Implements QPatternist::Expression.

Definition at line 226 of file qevaluationcache.cpp.

 {
     return m_operand->staticType();
 }

◆ topFocusContext()

template<bool IsForGlobal>

DynamicContext::Ptr QPatternist::EvaluationCache< IsForGlobal >::topFocusContext ( const DynamicContext::Ptr & context )

staticprivate

Definition at line 59 of file qevaluationcache.cpp.

Referenced by QPatternist::EvaluationCache< IsForGlobal >::evaluateSequence(), QPatternist::EvaluationCache< IsForGlobal >::evaluateSingleton(), and QPatternist::EvaluationCache< IsForGlobal >::slot().

 {
     DynamicContext::Ptr result(context);
 
     while(true)
     {
         DynamicContext::Ptr candidate(result->previousContext());
 
         /* We want the top focus, not GenericDynamicContext. */
         if(candidate && candidate->focusIterator())
             result = candidate;
         else
             return result;
     }
 }

◆ typeCheck()

template<bool IsForGlobal>

Expression::Ptr QPatternist::EvaluationCache< IsForGlobal >::typeCheck	(	const StaticContext::Ptr &	context,
		const SequenceType::Ptr &	reqType
	)

virtual

This implementation guarantees to never rewrite away this Expression, but at most rewrite it as a child of another expression(that presumably have a type checking role). It is therefore always safe to override this function and call this implementation and not worry about that this Expression becomes deleted.

Many Expressions override typeCheck() and performs optimizations, as opposed to doing it in the compress() stage. This is due to that the design of those Expressions often are tied to that certain simplifications are done at the typeCheck() stage of the compilation process or that it in some other way is related to what the typeCheck() do. Also, the earlier the AST can be simplified, the better the chances are for subsequent optimizations.

It is important that the super class's typeCheck() is called before doing any custom type checking, since the call can change the children(notably, the childrens' static types). For example, if the Expression, MyExpression in the example, does not match the required type, typeCheck returns the Expression wrapped in for example ItemVerifier, CardinalityVerifier, or both.

typeCheck() may be called many times. typeCheck() must either raise an error if this Expression is an invalid expression. Thus, it is guaranteed that an Expression is valid after typeCheck() is called.

Parameters

context	supplies information, such as namespace bindings and available function signatures, that can be needed at compilation time. `context` is guaranteed by the caller to never null.
reqType	the static type that this Expression must match when evaluated. `reqType` is guaranteed by the caller to never null.

Returns: an Expression that can be this Expression, or another expression, which somehow is necessary for making this Expression conforming to reqType

Reimplemented from QPatternist::Expression.

Definition at line 152 of file qevaluationcache.cpp.

 {
     /* It's important that we do the typeCheck() before checking for the use of local variables,
      * because ExpressionVariableReference can reference an expression that is a local variable,
      * so it must rewrite itself to it operand before, and it does that in EvaluationCache::typeCheck(). */
     const Expression::Ptr me(SingleContainer::typeCheck(context, reqType));
 
     OperandsIterator it(me, OperandsIterator::ExcludeParent);
     Expression::Ptr next(it.next());
 
     /* If our operand or any sub operand gets its value from a for-loop, we cannot
      * cache it since then our cache would be filled -- but not invalidated -- on the
      * first for-iteration. Consider this query:
      *
      * <tt>for $i in expr
      * let $v := $i/p
      * return ($v, $v)</tt>
      *
      * An evaluation cache is inserted for the two operands in the return clause. However,
      * $i changes for each iteration so the cache can only be active on a per-iteration basis,
      * it it's possible(which it isn't).
      *
      * This means that for some queries we don't cache what we really should, and hence evaluate
      * in a sub-optimal way, since this DependsOnLocalVariable don't communicate whether it references
      * a loop that affects us. The correct fix for this would be to let ForExpression reset the
      * relevant caches only, but we don't know which ones that are. */
     while(next)
     {
         if(next->has(DependsOnLocalVariable))
             return m_operand->typeCheck(context, reqType);
 
         next = it.next();
     }
 
     return me;
 }

Properties

◆ m_declaration

template<bool IsForGlobal>

const VariableDeclaration* QPatternist::EvaluationCache< IsForGlobal >::m_declaration

private

Definition at line 127 of file qevaluationcache_p.h.

◆ m_declarationUsedByMany

template<bool IsForGlobal>

bool QPatternist::EvaluationCache< IsForGlobal >::m_declarationUsedByMany

private

Definition at line 128 of file qevaluationcache_p.h.

Referenced by QPatternist::EvaluationCache< IsForGlobal >::compress().

◆ m_varSlot

template<bool IsForGlobal>

const VariableSlotID QPatternist::EvaluationCache< IsForGlobal >::m_varSlot

private

This variable must not be called m_slot. If it so, a compiler bug on HP-UX-aCC-64 is triggered in the constructor initializor. See the preprocessor output.

Note that this is the cache slot, and is disjoint to any variable's regular slot.

Definition at line 137 of file qevaluationcache_p.h.

Referenced by QPatternist::EvaluationCache< IsForGlobal >::evaluateSequence(), QPatternist::EvaluationCache< IsForGlobal >::evaluateSingleton(), QPatternist::EvaluationCache< IsForGlobal >::EvaluationCache(), and QPatternist::EvaluationCache< IsForGlobal >::slot().

The documentation for this class was generated from the following files:

/src/xmlpatterns/expr/qevaluationcache_p.h
/src/xmlpatterns/expr/qevaluationcache.cpp

Public Functions

Static Private Functions

Properties

Additional Inherited Members

Detailed Description

template<bool IsForGlobal> class QPatternist::EvaluationCache< IsForGlobal >

Constructors and Destructors

◆ EvaluationCache()

Functions

◆ accept()

◆ actualReflection()

◆ compress()

◆ evaluateSequence()

◆ evaluateSingleton()

◆ expectedOperandTypes()

◆ properties()

◆ slot()

◆ staticType()

◆ topFocusContext()

◆ typeCheck()

Properties

◆ m_declaration

◆ m_declarationUsedByMany

◆ m_varSlot

template<bool IsForGlobal>
class QPatternist::EvaluationCache< IsForGlobal >