org.jikesrvm.compilers.opt.ssa

Class RedundantBranchElimination

java.lang.Object

org.jikesrvm.compilers.opt.driver.OptimizationPlanElement

org.jikesrvm.compilers.opt.driver.OptimizationPlanCompositeElement

org.jikesrvm.compilers.opt.ssa.RedundantBranchElimination

public final class RedundantBranchElimination
extends OptimizationPlanCompositeElement

Redundant branch elimination based on SSA form, global value numbers, and dominance relationships. The following are sufficient conditions for a conditional branch cb1 to be eliminated as redundant

• It is equivalent (has the same value number) as another conditional branch cb2
• Either (a) the target of the taken branch of cb2 dominates cb1 and said target block has exactly one in edge or (b) the not-taken continuation of cb2 dominates cb1 and said continuation block has exactly one in edge.

NOTE: the check for exactly one in edge is used to rule out situations like the following:

      if (C) goto L2              // cb2
      x = x + 1;
  L2: x = x + 1;
      if (C) goto L3.            // cb1
 

Consider redundant branch elimination for cb1. Here L2 (the target of cb2) dominates cb1, but it is not correct to eliminate cb1 because it is also reachable (but not dominated) from the continuation block of cb2!

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
private static class	RedundantBranchElimination.EnsureSSA
private static class	RedundantBranchElimination.RBE

Constructor Summary

Constructors

Constructor and Description
RedundantBranchElimination() Create this phase element as a composite of other elements

Method Summary

Modifier and Type	Method and Description
boolean	shouldPerform(OptOptions options) Determine, possibly by consulting the passed options object, if this optimization plan element should be performed.

Methods inherited from class org.jikesrvm.compilers.opt.driver.OptimizationPlanCompositeElement

compose, elapsedTime, getName, initializeForMeasureCompilation, perform, printingEnabled, reportStats

Methods inherited from class org.jikesrvm.compilers.opt.driver.OptimizationPlanElement

prettyPrintTime

Methods inherited from class java.lang.Object

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

Constructor Detail

RedundantBranchElimination

public RedundantBranchElimination()

Create this phase element as a composite of other elements

Method Detail

shouldPerform

public boolean shouldPerform(OptOptions options)

Description copied from class: OptimizationPlanElement

Determine, possibly by consulting the passed options object, if this optimization plan element should be performed.

Overrides:

shouldPerform in class OptimizationPlanCompositeElement

Parameters:

options - The Options object for the current compilation.

Returns:

true if the plan element should be performed.