org.jikesrvm.compilers.opt.controlflow
Class MIRBranchOptimizations

java.lang.Object
  org.jikesrvm.compilers.opt.driver.CompilerPhase
      org.jikesrvm.compilers.opt.controlflow.BranchOptimizationDriver
          org.jikesrvm.compilers.opt.controlflow.MIRBranchOptimizations

public final class MIRBranchOptimizations
extends BranchOptimizationDriver

Perform simple peephole optimizations for MIR branches.

MIRBranchOptimizations

public MIRBranchOptimizations(int level)

Parameters:

level - the minimum optimization level at which the branch optimizations should be performed.

optimizeBranchInstruction

protected boolean optimizeBranchInstruction(IR ir,
                                            Instruction s,
                                            BasicBlock bb)

This method actually does the work of attempting to peephole optimize a branch instruction. See Muchnick ~p.590

Specified by:

optimizeBranchInstruction in class BranchOptimizationDriver

Parameters:

ir - the containing IR

s - the branch instruction to optimize

bb - the containing basic block

Returns:

true if an optimization was applied, false otherwise

processGoto

private boolean processGoto(IR ir,
                            Instruction g,
                            BasicBlock bb)

Perform optimizations for an unconditonal branch.

Patterns:

    1)      GOTO A       replaced by  GOTO B
         A: GOTO B

    2)   GOTO next instruction eliminated
    3)      GOTO A       replaced by  GOTO B
         A: LABEL
            BBEND
         B:
 

 
 Precondition: MIR_Branch.conforms(g)


Parameters:
ir - governing IR
g - the instruction to optimize
bb - the basic block holding g
Returns:
true if made a transformation

processCondBranch

private boolean processCondBranch(IR ir,
                                  Instruction cb,
                                  BasicBlock bb)

Perform optimizations for a conditional branch.

 1)   IF .. GOTO A          replaced by  IF .. GOTO B
      ...
   A: GOTO B
 2)   conditional branch to next instruction eliminated
 3)   IF (condition) GOTO A  replaced by  IF (!condition) GOTO B
      GOTO B                           A: ...
   A: ...
 4)   IF .. GOTO A       replaced by  IF .. GOTO B
   A: LABEL
      BBEND
   B:
 5)  fallthrough to a goto: replicate goto to enable other optimizations.
 

Precondition: MIR_CondBranch.conforms(cb)

Parameters:

ir - the governing IR

cb - the instruction to optimize

bb - the basic block holding if

Returns:

true iff made a transformation

processTwoTargetConditionalBranch

private boolean processTwoTargetConditionalBranch(IR ir,
                                                  Instruction cb,
                                                  BasicBlock bb)

Perform optimizations for a two way conditional branch.

 1)   IF .. GOTO A          replaced by  IF .. GOTO B
      ...
   A: GOTO B
 2)   conditional branch to next instruction eliminated
 3)   IF .. GOTO A       replaced by  IF .. GOTO B
   A: LABEL
      BBEND
   B:
 4)  fallthrough to a goto: replicate goto to enable other optimizations.
 

Precondition: MIR_CondBranch2.conforms(cb)

Parameters:

ir - the governing IR

cb - the instruction to optimize

bb - the basic block holding if

Returns:

true iff made a transformation

isFlipCandidate

private boolean isFlipCandidate(Instruction cb,
                                Instruction target)

Is a conditional branch a candidate to be flipped? See comment 3) of processCondBranch

Precondition: MIR_CondBranch.conforms(cb)

Parameters:

cb - the conditional branch instruction

target - the target instruction (real instruction) of the conditional branch

Returns:

boolean result

flipConditionalBranch

private void flipConditionalBranch(Instruction cb)

Flip a conditional branch and remove the trailing goto. See comment 3) of processCondBranch

Precondition isFlipCandidate(cb)

Parameters:

cb - the conditional branch instruction