scale.score.trans
Class DeadVarElimination

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

public class DeadVarElimination
extends Optimization

This class performs dead variable elimination.

$Id: DeadVarElimination.java,v 1.9 2007-02-28 18:00:36 burrill Exp $

Copyright 2007 by the Scale Compiler Group,
Department of Computer Science
University of Massachusetts,
Amherst MA. 01003, USA
All Rights Reserved.

A variable is dead if

• it is not a global variable,
• its address is not taken,
• it is not a COMMON variable,
• it is not a static variable,
• it is not a function parameter,
• the expression ssspecifying its value has side effects, and
• its value is not used.

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

DeadVarElimination(Scribble scribble)

Method Summary

void	perform() Removes statements that set the value of a variable that is not used.
int	requiresSSA() Return whether this optimization requires that the CFG be in SSA form.

Methods inherited from class scale.score.trans.Optimization

assertTrace, assertTrace, assertTrace, assertTrace, assertTrace, genTemp, insertStores, 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

DeadVarElimination

public DeadVarElimination(Scribble scribble)

Method Detail

perform

public void perform()

Removes statements that set the value of a variable that is not used. See Algorithm 19.12 in "Modern Compiler Implementation in Java" by Appel.

Specified by:

perform in class Optimization

See Also:

SSA.coalesceVariables()

requiresSSA

public int requiresSSA()

Return whether this optimization requires that the CFG be in SSA form.

Overrides:

requiresSSA in class Optimization