scale.score.trans
Class SCC

java.lang.Object
  scale.score.trans.Optimization
      scale.score.trans.SCC

public class SCC
extends Optimization

This class performs the sparse conditional constant propagation.

$Id: SCC.java,v 1.111 2007-10-04 19:58:37 burrill Exp $

Copyright 2008 by the Scale Compiler Group,
Department of Computer Science
University of Massachusetts,
Amherst MA. 01003, USA
All Rights Reserved.

See Constant Propagation with Conditional Branches by Mark N. Wegman and F. Kenneth Zadeck in ACM Transactions on Programming Languages and Systems, Vol 13, No. 2, April 1991, Pages 181-210.

This optimization attempts to do two things:

• replace expressions with constants, and
• eliminate CFG edges that will never be executed.

It it can be proved that an expression's value is always the same, then that expression may be replaced by \a literal of that value. If the predicate of a decision node is a constant, then un-selected edges out of that node may be eliminated. If only one edge remains, the the decision node may also be eliminated.

Unlike most optimizations this optimization never increases register pressure. Consequently, there is never a case where a legal transformation is not also beneficial.

Field Summary

static boolean	classTrace True if traces are to be performed.
static boolean	useHeuristics If true, use heuristics that prune the cases where the optimization is applied.

Fields inherited from class scale.score.trans.Optimization

dChanged, fpReorder, hasDummyAliases, IN_SSA, minimumExecutionCost, NA_SSA, NO_SSA, rChanged, scribble, signedIntsWrapOnOverflow, trace, un, unsafe, unsignedIntsWrapOnOverflow, VALID_SSA

Constructor Summary

SCC(Scribble scribble)
Perform sparse conditional constant propagation.

Method Summary

static int	deadCFGNodes() Return the number of dead nodes removed.
static int	newCFGNodes() Return the number of new nodes created.
void	perform() Propagate constants through the Scribble graph.
static int	propagations() Return the number of times copy propagation was performed.
static int	uselessDecisions() Return the number of DecisionChords deleted.

Methods inherited from class scale.score.trans.Optimization

assertTrace, assertTrace, assertTrace, assertTrace, assertTrace, genTemp, insertStores, requiresSSA, setTrace, sort

Methods inherited from class java.lang.Object

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

Field Detail

classTrace

public static boolean classTrace

True if traces are to be performed.

useHeuristics

public static boolean useHeuristics

If true, use heuristics that prune the cases where the optimization is applied.

Constructor Detail

SCC

public SCC(Scribble scribble)

Perform sparse conditional constant propagation.

Parameters:

scribble - the Scribble graph to be transformed

Method Detail

deadCFGNodes

public static int deadCFGNodes()

Return the number of dead nodes removed.

propagations

public static int propagations()

Return the number of times copy propagation was performed.

newCFGNodes

public static int newCFGNodes()

Return the number of new nodes created.

uselessDecisions

public static int uselessDecisions()

Return the number of DecisionChords deleted.

perform

public void perform()

Propagate constants through the Scribble graph. Eliminate any CFG nodes that are not executable because of the constant values deterining the control flow.

Specified by:

perform in class Optimization