--- /dev/null
+//===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
+//
+// This pass performs a limited form of tail duplication, intended to simplify
+// CFGs by removing some unconditional branches. This pass is necessary to
+// straighten out loops created by the C front-end, but also is capable of
+// making other code nicer. After this pass is run, the CFG simplify pass
+// should be run to clean up the mess.
+//
+// This pass could be enhanced in the future to use profile information to be
+// more aggressive.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Function.h"
+#include "llvm/iPHINode.h"
+#include "llvm/iTerminators.h"
+#include "llvm/Pass.h"
+#include "llvm/Type.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "Support/Statistic.h"
+
+namespace {
+ Statistic<> NumEliminated("tailduplicate",
+ "Number of unconditional branches eliminated");
+ Statistic<> NumPHINodes("tailduplicate", "Number of phi nodes inserted");
+
+ class TailDup : public FunctionPass {
+ bool runOnFunction(Function &F);
+ private:
+ inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
+ inline void eliminateUnconditionalBranch(BranchInst *BI);
+ inline void InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
+ BasicBlock *NewBlock);
+ inline Value *GetValueInBlock(BasicBlock *BB, Value *OrigVal,
+ std::map<BasicBlock*, Value*> &ValueMap,
+ std::map<BasicBlock*, Value*> &OutValueMap);
+ inline Value *GetValueOutBlock(BasicBlock *BB, Value *OrigVal,
+ std::map<BasicBlock*, Value*> &ValueMap,
+ std::map<BasicBlock*, Value*> &OutValueMap);
+ };
+ RegisterOpt<TailDup> X("tailduplicate", "Tail Duplication");
+}
+
+Pass *createTailDuplicationPass() { return new TailDup(); }
+
+/// runOnFunction - Top level algorithm - Loop over each unconditional branch in
+/// the function, eliminating it if it looks attractive enough.
+///
+bool TailDup::runOnFunction(Function &F) {
+ bool Changed = false;
+ for (Function::iterator I = F.begin(), E = F.end(); I != E; )
+ if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
+ eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
+ Changed = true;
+ } else {
+ ++I;
+ }
+ return Changed;
+}
+
+/// shouldEliminateUnconditionalBranch - Return true if this branch looks
+/// attractive to eliminate. We eliminate the branch if the destination basic
+/// block has <= 5 instructions in it, not counting PHI nodes. In practice,
+/// since one of these is a terminator instruction, this means that we will add
+/// up to 4 instructions to the new block.
+///
+/// We don't count PHI nodes in the count since they will be removed when the
+/// contents of the block are copied over.
+///
+bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
+ BranchInst *BI = dyn_cast<BranchInst>(TI);
+ if (!BI || !BI->isUnconditional()) return false; // Not an uncond branch!
+
+ BasicBlock *Dest = BI->getSuccessor(0);
+ if (Dest == BI->getParent()) return false; // Do not loop infinitely!
+
+ // Do not bother working on dead blocks...
+ pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
+ if (PI == PE && Dest != Dest->getParent()->begin())
+ return false; // It's just a dead block, ignore it...
+
+ // Also, do not bother with blocks with only a single predecessor: simplify
+ // CFG will fold these two blocks together!
+ ++PI;
+ if (PI == PE) return false; // Exactly one predecessor!
+
+ BasicBlock::iterator I = Dest->begin();
+ while (isa<PHINode>(*I)) ++I;
+
+ for (unsigned Size = 0; I != Dest->end(); ++Size, ++I)
+ if (Size == 6) return false; // The block is too large...
+ return true;
+}
+
+
+/// eliminateUnconditionalBranch - Clone the instructions from the destination
+/// block into the source block, eliminating the specified unconditional branch.
+/// If the destination block defines values used by successors of the dest
+/// block, we may need to insert PHI nodes.
+///
+void TailDup::eliminateUnconditionalBranch(BranchInst *Branch) {
+ BasicBlock *SourceBlock = Branch->getParent();
+ BasicBlock *DestBlock = Branch->getSuccessor(0);
+ assert(SourceBlock != DestBlock && "Our predicate is broken!");
+
+ DEBUG(std::cerr << "TailDuplication[" << SourceBlock->getParent()->getName()
+ << "]: Eliminating branch: " << *Branch);
+
+ // We are going to have to map operands from the original block B to the new
+ // copy of the block B'. If there are PHI nodes in the DestBlock, these PHI
+ // nodes also define part of this mapping. Loop over these PHI nodes, adding
+ // them to our mapping.
+ std::map<Value*, Value*> ValueMapping;
+
+ BasicBlock::iterator BI = DestBlock->begin();
+ bool HadPHINodes = isa<PHINode>(BI);
+ for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
+ ValueMapping[PN] = PN->getIncomingValueForBlock(SourceBlock);
+
+ // Clone the non-phi instructions of the dest block into the source block,
+ // keeping track of the mapping...
+ //
+ for (; BI != DestBlock->end(); ++BI) {
+ Instruction *New = BI->clone();
+ New->setName(BI->getName());
+ SourceBlock->getInstList().push_back(New);
+ ValueMapping[BI] = New;
+ }
+
+ // Now that we have built the mapping information and cloned all of the
+ // instructions (giving us a new terminator, among other things), walk the new
+ // instructions, rewriting references of old instructions to use new
+ // instructions.
+ //
+ BI = Branch; ++BI; // Get an iterator to the first new instruction
+ for (; BI != SourceBlock->end(); ++BI)
+ for (unsigned i = 0, e = BI->getNumOperands(); i != e; ++i)
+ if (Value *Remapped = ValueMapping[BI->getOperand(i)])
+ BI->setOperand(i, Remapped);
+
+ // Next we check to see if any of the successors of DestBlock had PHI nodes.
+ // If so, we need to add entries to the PHI nodes for SourceBlock now.
+ for (succ_iterator SI = succ_begin(DestBlock), SE = succ_end(DestBlock);
+ SI != SE; ++SI) {
+ BasicBlock *Succ = *SI;
+ for (BasicBlock::iterator PNI = Succ->begin();
+ PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
+ // Ok, we have a PHI node. Figure out what the incoming value was for the
+ // DestBlock.
+ Value *IV = PN->getIncomingValueForBlock(DestBlock);
+
+ // Remap the value if necessary...
+ if (Value *MappedIV = ValueMapping[IV])
+ IV = MappedIV;
+ PN->addIncoming(IV, SourceBlock);
+ }
+ }
+
+ // Now that all of the instructions are correctly copied into the SourceBlock,
+ // we have one more minor problem: the successors of the original DestBB may
+ // use the values computed in DestBB either directly (if DestBB dominated the
+ // block), or through a PHI node. In either case, we need to insert PHI nodes
+ // into any successors of DestBB (which are now our successors) for each value
+ // that is computed in DestBB, but is used outside of it. All of these uses
+ // we have to rewrite with the new PHI node.
+ //
+ if (succ_begin(SourceBlock) != succ_end(SourceBlock)) // Avoid wasting time...
+ for (BI = DestBlock->begin(); BI != DestBlock->end(); ++BI)
+ if (BI->getType() != Type::VoidTy)
+ InsertPHINodesIfNecessary(BI, ValueMapping[BI], SourceBlock);
+
+ // Final step: now that we have finished everything up, walk the cloned
+ // instructions one last time, constant propagating and DCE'ing them, because
+ // they may not be needed anymore.
+ //
+ BI = Branch; ++BI; // Get an iterator to the first new instruction
+ if (HadPHINodes)
+ while (BI != SourceBlock->end())
+ if (!dceInstruction(BI) && !doConstantPropagation(BI))
+ ++BI;
+
+ DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes...
+ SourceBlock->getInstList().erase(Branch); // Destroy the uncond branch...
+
+ ++NumEliminated; // We just killed a branch!
+}
+
+/// InsertPHINodesIfNecessary - So at this point, we cloned the OrigInst
+/// instruction into the NewBlock with the value of NewInst. If OrigInst was
+/// used outside of its defining basic block, we need to insert a PHI nodes into
+/// the successors.
+///
+void TailDup::InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
+ BasicBlock *NewBlock) {
+ // Loop over all of the uses of OrigInst, rewriting them to be newly inserted
+ // PHI nodes, unless they are in the same basic block as OrigInst.
+ BasicBlock *OrigBlock = OrigInst->getParent();
+ std::vector<Instruction*> Users;
+ Users.reserve(OrigInst->use_size());
+ for (Value::use_iterator I = OrigInst->use_begin(), E = OrigInst->use_end();
+ I != E; ++I) {
+ Instruction *In = cast<Instruction>(*I);
+ if (In->getParent() != OrigBlock) // Don't modify uses in the orig block!
+ Users.push_back(In);
+ }
+
+ // The common case is that the instruction is only used within the block that
+ // defines it. If we have this case, quick exit.
+ //
+ if (Users.empty()) return;
+
+ // Otherwise, we have a more complex case, handle it now. This requires the
+ // construction of a mapping between a basic block and the value to use when
+ // in the scope of that basic block. This map will map to the original and
+ // new values when in the original or new block, but will map to inserted PHI
+ // nodes when in other blocks.
+ //
+ std::map<BasicBlock*, Value*> ValueMap;
+ std::map<BasicBlock*, Value*> OutValueMap; // The outgoing value map
+ OutValueMap[OrigBlock] = OrigInst;
+ OutValueMap[NewBlock ] = NewInst; // Seed the initial values...
+
+ DEBUG(std::cerr << " ** Inserting PHI nodes for " << OrigInst);
+ while (!Users.empty()) {
+ Instruction *User = Users.back(); Users.pop_back();
+
+ if (PHINode *PN = dyn_cast<PHINode>(User)) {
+ // PHI nodes must be handled specially here, because their operands are
+ // actually defined in predecessor basic blocks, NOT in the block that the
+ // PHI node lives in. Note that we have already added entries to PHI nods
+ // which are in blocks that are immediate successors of OrigBlock, so
+ // don't modify them again.
+ for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+ if (PN->getIncomingValue(i) == OrigInst &&
+ PN->getIncomingBlock(i) != OrigBlock) {
+ Value *V = GetValueOutBlock(PN->getIncomingBlock(i), OrigInst,
+ ValueMap, OutValueMap);
+ PN->setIncomingValue(i, V);
+ }
+
+ } else {
+ // Any other user of the instruction can just replace any uses with the
+ // new value defined in the block it resides in.
+ Value *V = GetValueInBlock(User->getParent(), OrigInst, ValueMap,
+ OutValueMap);
+ User->replaceUsesOfWith(OrigInst, V);
+ }
+ }
+}
+
+/// GetValueInBlock - This is a recursive method which inserts PHI nodes into
+/// the function until there is a value available in basic block BB.
+///
+Value *TailDup::GetValueInBlock(BasicBlock *BB, Value *OrigVal,
+ std::map<BasicBlock*, Value*> &ValueMap,
+ std::map<BasicBlock*, Value*> &OutValueMap) {
+ Value*& BBVal = ValueMap[BB];
+ if (BBVal) return BBVal; // Value already computed for this block?
+
+ assert(pred_begin(BB) != pred_end(BB) &&
+ "Propagating PHI nodes to unreachable blocks?");
+
+ // If there is no value already available in this basic block, we need to
+ // either reuse a value from an incoming, dominating, basic block, or we need
+ // to create a new PHI node to merge in different incoming values. Because we
+ // don't know if we're part of a loop at this point or not, we create a PHI
+ // node, even if we will ultimately eliminate it.
+ PHINode *PN = new PHINode(OrigVal->getType(), OrigVal->getName()+".pn",
+ BB->begin());
+ BBVal = PN; // Insert this into the BBVal slot in case of cycles...
+
+ Value*& BBOutVal = OutValueMap[BB];
+ if (BBOutVal == 0) BBOutVal = PN;
+
+ // Now that we have created the PHI node, loop over all of the predecessors of
+ // this block, computing an incoming value for the predecessor.
+ std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
+ for (unsigned i = 0, e = Preds.size(); i != e; ++i)
+ PN->addIncoming(GetValueOutBlock(Preds[i], OrigVal, ValueMap, OutValueMap),
+ Preds[i]);
+
+ // The PHI node is complete. In many cases, however the PHI node was
+ // ultimately unnecessary: we could have just reused a dominating incoming
+ // value. If this is the case, nuke the PHI node and replace the map entry
+ // with the dominating value.
+ //
+ assert(PN->getNumIncomingValues() > 0 && "No predecessors?");
+
+ // Check to see if all of the elements in the PHI node are either the PHI node
+ // itself or ONE particular value.
+ unsigned i = 0;
+ Value *ReplVal = PN->getIncomingValue(i);
+ for (; ReplVal == PN && i != PN->getNumIncomingValues(); ++i)
+ ReplVal = PN->getIncomingValue(i); // Skip values equal to the PN
+
+ for (; i != PN->getNumIncomingValues(); ++i)
+ if (PN->getIncomingValue(i) != PN && PN->getIncomingValue(i) != ReplVal) {
+ ReplVal = 0;
+ break;
+ }
+
+ // Found a value to replace the PHI node with?
+ if (ReplVal) {
+ PN->replaceAllUsesWith(ReplVal);
+ BBVal = ReplVal;
+ if (BBOutVal == PN) BBOutVal = ReplVal;
+ BB->getInstList().erase(PN); // Erase the PHI node...
+ } else {
+ ++NumPHINodes;
+ }
+
+ return BBVal;
+}
+
+Value *TailDup::GetValueOutBlock(BasicBlock *BB, Value *OrigVal,
+ std::map<BasicBlock*, Value*> &ValueMap,
+ std::map<BasicBlock*, Value*> &OutValueMap) {
+ Value*& BBVal = OutValueMap[BB];
+ if (BBVal) return BBVal; // Value already computed for this block?
+
+ return BBVal = GetValueInBlock(BB, OrigVal, ValueMap, OutValueMap);
+}
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