/**
* This method actually does the work of attempting to peephole optimize a branch instruction. See
* Muchnick ~p.590
*
* @param ir the containing IR
* @param s the branch instruction to optimize
* @param bb the containing basic block
* @return {@code true} if an optimization was applied, {@code false} otherwise
*/
@Override
protected boolean optimizeBranchInstruction(IR ir, Instruction s, BasicBlock bb) {
if (Goto.conforms(s)) {
return processGoto(ir, s, bb);
} else if (IfCmp.conforms(s)) {
return processConditionalBranch(ir, s, bb);
} else if (InlineGuard.conforms(s)) {
return processInlineGuard(ir, s, bb);
} else if (IfCmp2.conforms(s)) {
return processTwoTargetConditionalBranch(ir, s, bb);
} else {
return false;
}
}
/**
* Perform optimizations for a two way conditional branch.
*
* <p>Precondition: IfCmp2.conforms(cb)
*
* @param ir the governing IR
* @param cb the instruction to optimize
* @param bb the basic block holding if
* @return {@code true} iff made a transformation
*/
private boolean processTwoTargetConditionalBranch(IR ir, Instruction cb, BasicBlock bb) {
// First condition/target
Instruction target1Label = IfCmp2.getTarget1(cb).target;
Instruction target1Inst = firstRealInstructionFollowing(target1Label);
Instruction nextLabel = firstLabelFollowing(cb);
boolean endsBlock = cb.nextInstructionInCodeOrder().operator() == BBEND;
if (target1Inst != null && target1Inst != cb) {
if (Goto.conforms(target1Inst)) {
// conditional branch to unconditional branch.
// change conditional branch target to latter's target
IfCmp2.setTarget1(cb, (BranchOperand) Goto.getTarget(target1Inst).copy());
bb.recomputeNormalOut(ir); // fix CFG
return true;
}
BasicBlock target1Block = target1Label.getBasicBlock();
if (target1Block.isEmpty()) {
// branch to an empty block. Change target to the next block.
BasicBlock nextBlock = target1Block.getFallThroughBlock();
IfCmp2.setTarget1(cb, nextBlock.makeJumpTarget());
bb.recomputeNormalOut(ir); // fix the CFG
return true;
}
}
// Second condition/target
Instruction target2Label = IfCmp2.getTarget2(cb).target;
Instruction target2Inst = firstRealInstructionFollowing(target2Label);
if (target2Inst != null && target2Inst != cb) {
if (Goto.conforms(target2Inst)) {
// conditional branch to unconditional branch.
// change conditional branch target to latter's target
IfCmp2.setTarget2(cb, (BranchOperand) Goto.getTarget(target2Inst).copy());
bb.recomputeNormalOut(ir); // fix CFG
return true;
}
if ((target2Label == nextLabel) && endsBlock) {
// found a conditional branch to the next instruction. Reduce to IfCmp
if (VM.VerifyAssertions) VM._assert(cb.operator() == INT_IFCMP2);
IfCmp.mutate(
cb,
INT_IFCMP,
IfCmp2.getGuardResult(cb),
IfCmp2.getVal1(cb),
IfCmp2.getVal2(cb),
IfCmp2.getCond1(cb),
IfCmp2.getTarget1(cb),
IfCmp2.getBranchProfile1(cb));
return true;
}
BasicBlock target2Block = target2Label.getBasicBlock();
if (target2Block.isEmpty()) {
// branch to an empty block. Change target to the next block.
BasicBlock nextBlock = target2Block.getFallThroughBlock();
IfCmp2.setTarget2(cb, nextBlock.makeJumpTarget());
bb.recomputeNormalOut(ir); // fix the CFG
return true;
}
}
// if fall through to a goto; replicate the goto
if (endsBlock) {
Instruction nextI = firstRealInstructionFollowing(nextLabel);
if (nextI != null && Goto.conforms(nextI)) {
// replicate Goto
cb.insertAfter(nextI.copyWithoutLinks());
bb.recomputeNormalOut(ir); // fix the CFG
return true;
}
}
return false;
}