/// ProcessBlock - If there are any predecessors whose control can be threaded
/// through to a successor, transform them now.
bool JumpThreading::ProcessBlock(BasicBlock *BB) {
+ // If the block is trivially dead, just return and let the caller nuke it.
+ // This simplifies other transformations.
+ if (pred_begin(BB) == pred_end(BB) &&
+ BB != &BB->getParent()->getEntryBlock())
+ return false;
+
// If this block has a single predecessor, and if that pred has a single
// successor, merge the blocks. This encourages recursive jump threading
// because now the condition in this block can be threaded through
isa<ConstantInt>(BO->getOperand(1)))
return false;
+ // If the first instruction in BB isn't a phi, we won't be able to infer
+ // anything special about any particular predecessor.
+ if (!isa<PHINode>(BB->front()))
+ return false;
+
// If we have a xor as the branch input to this block, and we know that the
// LHS or RHS of the xor in any predecessor is true/false, then we can clone
// the condition into the predecessor and fix that value to true, saving some
BlocksToFoldInto.push_back(XorOpValues[i].second);
}
+ // If we inferred a value for all of the predecessors, then duplication won't
+ // help us. However, we can just replace the LHS or RHS with the constant.
+ if (BlocksToFoldInto.size() ==
+ cast<PHINode>(BB->front()).getNumIncomingValues()) {
+ if (SplitVal == 0) {
+ // If all preds provide undef, just nuke the xor, because it is undef too.
+ BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
+ BO->eraseFromParent();
+ } else if (SplitVal->isZero()) {
+ // If all preds provide 0, replace the xor with the other input.
+ BO->replaceAllUsesWith(BO->getOperand(isLHS));
+ BO->eraseFromParent();
+ } else {
+ // If all preds provide 1, set the computed value to 1.
+ BO->setOperand(!isLHS, SplitVal);
+ }
+
+ return true;
+ }
+
// Try to duplicate BB into PredBB.
return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
}
// Unless PredBB ends with an unconditional branch, split the edge so that we
// can just clone the bits from BB into the end of the new PredBB.
- BranchInst *OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
+ BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
- if (!OldPredBranch->isUnconditional()) {
+ if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) {
PredBB = SplitEdge(PredBB, BB, this);
OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
}