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Implement merging of blocks with the same condition if the block has multiple
predecessors. This implements branch-phi-thread.ll::test1 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23395 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -890,6 +890,26 @@ HoistTerminator:
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return true;
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}
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/// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch
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/// across this block.
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static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) {
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BranchInst *BI = cast<BranchInst>(BB->getTerminator());
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Value *Cond = BI->getCondition();
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// If this basic block contains anything other than a PHI (which controls the
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// branch) and branch itself, bail out. FIXME: improve this in the future.
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for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) {
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if (!isa<PHINode>(BBI)) return false;
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if (&*BBI != Cond || !Cond->hasOneUse())
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return false;
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// Looks ok, continue checking.
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}
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return true;
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}
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/// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value
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/// that is defined in the same block as the branch and if any PHI entries are
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/// constants, thread edges corresponding to that entry to be branches to their
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@ -899,7 +919,8 @@ static bool FoldCondBranchOnPHI(BranchInst *BI) {
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PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
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// NOTE: we currently cannot transform this case if the PHI node is used
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// outside of the block.
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if (!PN || PN->getParent() != BB || !PN->hasOneUse()) return false;
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if (!PN || PN->getParent() != BB || !PN->hasOneUse())
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return false;
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// Degenerate case of a single entry PHI.
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if (PN->getNumIncomingValues() == 1) {
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@ -912,12 +933,7 @@ static bool FoldCondBranchOnPHI(BranchInst *BI) {
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}
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// Now we know that this block has multiple preds and two succs.
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// If this basic block contains anything other than the PHI and branch, bail
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// out. FIXME: improve this in the future.
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BasicBlock::iterator BBI = BB->begin();
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if (&*BBI != PN || &*++BBI != BI)
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return false;
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if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false;
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// Okay, this is a simple enough basic block. See if any phi values are
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// constants.
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@ -1272,22 +1288,44 @@ 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 one
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// predecessor, see if the previous block ended with a branch on the same
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// condition, which makes this conditional branch redundant.
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if (BasicBlock *OnlyPred = BB->getSinglePredecessor())
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if (BranchInst *PBI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
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if (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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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) != BB || PBI->getSuccessor(1) != BB)) {
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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. Delete the outgoing CFG edge that is impossible to
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// execute.
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bool CondIsTrue = PBI->getSuccessor(0) == BB;
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BI->getSuccessor(CondIsTrue)->removePredecessor(BB);
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new BranchInst(BI->getSuccessor(!CondIsTrue), BB);
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BB->getInstList().erase(BI);
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return SimplifyCFG(BB) | true;
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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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}
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} else if (isa<UnreachableInst>(BB->getTerminator())) {
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