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Fold two consequtive branches that share a common destination between them.
This implements SimplifyCFG/branch-fold.ll, and is useful on ?:/min/max heavy code git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23410 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner
parent
9a82a3ccac
commit
263d1e469d
@ -1207,6 +1207,8 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
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if (!UncondBranchPreds.empty()) {
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while (!UncondBranchPreds.empty()) {
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BasicBlock *Pred = UncondBranchPreds.back();
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DEBUG(std::cerr << "FOLDING: " << *BB
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<< "INTO UNCOND BRANCH PRED: " << *Pred);
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UncondBranchPreds.pop_back();
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Instruction *UncondBranch = Pred->getTerminator();
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// Clone the return and add it to the end of the predecessor.
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@ -1386,7 +1388,6 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
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if (PN->getParent() == BI->getParent())
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if (FoldCondBranchOnPHI(BI))
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return SimplifyCFG(BB) | true;
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// If this basic block is ONLY a setcc and a branch, and if a predecessor
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// branches to us and one of our successors, fold the setcc into the
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@ -1442,43 +1443,128 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
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}
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}
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// If this block ends with a branch instruction, and if there is a
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// predecessor that ends on a branch of the same condition, make this
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// conditional branch redundant.
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// Scan predessor blocks for conditional branchs.
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for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
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if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
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if (PBI != BI && PBI->isConditional() &&
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PBI->getCondition() == BI->getCondition() &&
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PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
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// Okay, the outcome of this conditional branch is statically
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// knowable. If this block had a single pred, handle specially.
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if (BB->getSinglePredecessor()) {
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// Turn this into a branch on constant.
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bool CondIsTrue = PBI->getSuccessor(0) == BB;
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BI->setCondition(ConstantBool::get(CondIsTrue));
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return SimplifyCFG(BB); // Nuke the branch on constant.
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if (PBI != BI && PBI->isConditional()) {
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// If this block ends with a branch instruction, and if there is a
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// predecessor that ends on a branch of the same condition, make this
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// conditional branch redundant.
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if (PBI->getCondition() == BI->getCondition() &&
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PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
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// Okay, the outcome of this conditional branch is statically
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// knowable. If this block had a single pred, handle specially.
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if (BB->getSinglePredecessor()) {
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// Turn this into a branch on constant.
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bool CondIsTrue = PBI->getSuccessor(0) == BB;
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BI->setCondition(ConstantBool::get(CondIsTrue));
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return SimplifyCFG(BB); // Nuke the branch on constant.
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}
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// Otherwise, if there are multiple predecessors, insert a PHI that
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// merges in the constant and simplify the block result.
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if (BlockIsSimpleEnoughToThreadThrough(BB)) {
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PHINode *NewPN = new PHINode(Type::BoolTy,
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BI->getCondition()->getName()+".pr",
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BB->begin());
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for (PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
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if ((PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) &&
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PBI != BI && PBI->isConditional() &&
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PBI->getCondition() == BI->getCondition() &&
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PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
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bool CondIsTrue = PBI->getSuccessor(0) == BB;
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NewPN->addIncoming(ConstantBool::get(CondIsTrue), *PI);
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} else {
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NewPN->addIncoming(BI->getCondition(), *PI);
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}
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BI->setCondition(NewPN);
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// This will thread the branch.
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return SimplifyCFG(BB) | true;
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}
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}
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// Otherwise, if there are multiple predecessors, insert a PHI that
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// merges in the constant and simplify the block result.
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if (BlockIsSimpleEnoughToThreadThrough(BB)) {
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PHINode *NewPN = new PHINode(Type::BoolTy,
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BI->getCondition()->getName()+".pr",
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BB->begin());
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for (PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
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if ((PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) &&
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PBI != BI && PBI->isConditional() &&
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PBI->getCondition() == BI->getCondition() &&
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PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
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bool CondIsTrue = PBI->getSuccessor(0) == BB;
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NewPN->addIncoming(ConstantBool::get(CondIsTrue), *PI);
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} else {
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NewPN->addIncoming(BI->getCondition(), *PI);
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}
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// If this is a conditional branch in an empty block, and if any
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// predecessors is a conditional branch to one of our destinations,
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// fold the conditions into logical ops and one cond br.
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if (&BB->front() == BI) {
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int PBIOp, BIOp;
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if (PBI->getSuccessor(0) == BI->getSuccessor(0)) {
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PBIOp = BIOp = 0;
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} else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) {
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PBIOp = 0; BIOp = 1;
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} else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) {
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PBIOp = 1; BIOp = 0;
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} else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) {
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PBIOp = BIOp = 1;
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} else {
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PBIOp = BIOp = -1;
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}
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BI->setCondition(NewPN);
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// This will thread the branch.
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return SimplifyCFG(BB) | true;
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// Finally, if everything is ok, fold the branches to logical ops.
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if (PBIOp != -1) {
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BasicBlock *CommonDest = PBI->getSuccessor(PBIOp);
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BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1);
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DEBUG(std::cerr << "FOLDING BRs:" << *PBI->getParent()
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<< "AND: " << *BI->getParent());
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// BI may have other predecessors. Because of this, we leave
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// it alone, but modify PBI.
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// Make sure we get to CommonDest on True&True directions.
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Value *PBICond = PBI->getCondition();
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if (PBIOp)
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PBICond = BinaryOperator::createNot(PBICond,
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PBICond->getName()+".not",
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PBI);
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Value *BICond = BI->getCondition();
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if (BIOp)
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BICond = BinaryOperator::createNot(BICond,
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BICond->getName()+".not",
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PBI);
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// Merge the conditions.
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Value *Cond =
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BinaryOperator::createOr(PBICond, BICond, "brmerge", PBI);
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// Modify PBI to branch on the new condition to the new dests.
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PBI->setCondition(Cond);
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PBI->setSuccessor(0, CommonDest);
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PBI->setSuccessor(1, OtherDest);
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// OtherDest may have phi nodes. If so, add an entry from PBI's
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// block that are identical to the entries for BI's block.
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PHINode *PN;
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for (BasicBlock::iterator II = OtherDest->begin();
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(PN = dyn_cast<PHINode>(II)); ++II) {
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Value *V = PN->getIncomingValueForBlock(BB);
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PN->addIncoming(V, PBI->getParent());
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}
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// We know that the CommonDest already had an edge from PBI to
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// it. If it has PHIs though, the PHIs may have different
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// entries for BB and PBI's BB. If so, insert a select to make
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// them agree.
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for (BasicBlock::iterator II = CommonDest->begin();
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(PN = dyn_cast<PHINode>(II)); ++II) {
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Value * BIV = PN->getIncomingValueForBlock(BB);
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unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent());
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Value *PBIV = PN->getIncomingValue(PBBIdx);
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if (BIV != PBIV) {
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// Insert a select in PBI to pick the right value.
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Value *NV = new SelectInst(PBICond, PBIV, BIV,
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PBIV->getName()+".mux", PBI);
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PN->setIncomingValue(PBBIdx, NV);
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}
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}
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DEBUG(std::cerr << "INTO: " << *PBI->getParent());
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// This basic block is probably dead. We know it has at least
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// one fewer predecessor.
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return SimplifyCFG(BB) | true;
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}
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}
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}
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}
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