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//===- SSAUpdater.cpp - Unstructured SSA Update Tool ----------------------===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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// This file implements the SSAUpdater class.
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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#include "llvm/Instructions.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/ValueHandle.h"
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#include "llvm/Support/raw_ostream.h"
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typedef DenseMap<BasicBlock*, TrackingVH<Value> > AvailableValsTy;
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typedef std::vector<std::pair<BasicBlock*, TrackingVH<Value> > >
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static AvailableValsTy &getAvailableVals(void *AV) {
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return *static_cast<AvailableValsTy*>(AV);
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static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) {
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return *static_cast<IncomingPredInfoTy*>(IPI);
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SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI)
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: AV(0), PrototypeValue(0), IPI(0), InsertedPHIs(NewPHI) {}
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SSAUpdater::~SSAUpdater() {
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delete &getAvailableVals(AV);
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delete &getIncomingPredInfo(IPI);
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/// Initialize - Reset this object to get ready for a new set of SSA
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/// updates. ProtoValue is the value used to name PHI nodes.
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void SSAUpdater::Initialize(Value *ProtoValue) {
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AV = new AvailableValsTy();
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getAvailableVals(AV).clear();
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IPI = new IncomingPredInfoTy();
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getIncomingPredInfo(IPI).clear();
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PrototypeValue = ProtoValue;
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/// HasValueForBlock - Return true if the SSAUpdater already has a value for
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/// the specified block.
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bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
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return getAvailableVals(AV).count(BB);
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/// AddAvailableValue - Indicate that a rewritten value is available in the
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/// specified block with the specified value.
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void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
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assert(PrototypeValue != 0 && "Need to initialize SSAUpdater");
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assert(PrototypeValue->getType() == V->getType() &&
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"All rewritten values must have the same type");
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getAvailableVals(AV)[BB] = V;
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/// IsEquivalentPHI - Check if PHI has the same incoming value as specified
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/// in ValueMapping for each predecessor block.
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static bool IsEquivalentPHI(PHINode *PHI,
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DenseMap<BasicBlock*, Value*> &ValueMapping) {
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unsigned PHINumValues = PHI->getNumIncomingValues();
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if (PHINumValues != ValueMapping.size())
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// Scan the phi to see if it matches.
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for (unsigned i = 0, e = PHINumValues; i != e; ++i)
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if (ValueMapping[PHI->getIncomingBlock(i)] !=
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PHI->getIncomingValue(i)) {
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/// GetExistingPHI - Check if BB already contains a phi node that is equivalent
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/// to the specified mapping from predecessor blocks to incoming values.
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static Value *GetExistingPHI(BasicBlock *BB,
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DenseMap<BasicBlock*, Value*> &ValueMapping) {
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for (BasicBlock::iterator It = BB->begin();
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(SomePHI = dyn_cast<PHINode>(It)); ++It) {
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if (IsEquivalentPHI(SomePHI, ValueMapping))
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/// GetExistingPHI - Check if BB already contains an equivalent phi node.
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/// The InputIt type must be an iterator over std::pair<BasicBlock*, Value*>
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/// objects that specify the mapping from predecessor blocks to incoming values.
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template<typename InputIt>
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static Value *GetExistingPHI(BasicBlock *BB, const InputIt &I,
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// Avoid create the mapping if BB has no phi nodes at all.
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if (!isa<PHINode>(BB->begin()))
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DenseMap<BasicBlock*, Value*> ValueMapping(I, E);
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return GetExistingPHI(BB, ValueMapping);
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/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
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/// live at the end of the specified block.
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Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
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assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
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Value *Res = GetValueAtEndOfBlockInternal(BB);
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assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
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/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
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/// is live in the middle of the specified block.
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/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
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/// important case: if there is a definition of the rewritten value after the
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/// 'use' in BB. Consider code like this:
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/// br Cond, SomeBB, OutBB
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/// In this case, there are two values (X1 and X2) added to the AvailableVals
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/// set by the client of the rewriter, and those values are both live out of
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/// their respective blocks. However, the use of X happens in the *middle* of
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/// a block. Because of this, we need to insert a new PHI node in SomeBB to
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/// merge the appropriate values, and this value isn't live out of the block.
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Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
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// If there is no definition of the renamed variable in this block, just use
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// GetValueAtEndOfBlock to do our work.
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if (!getAvailableVals(AV).count(BB))
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return GetValueAtEndOfBlock(BB);
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// Otherwise, we have the hard case. Get the live-in values for each
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SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues;
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Value *SingularValue = 0;
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// We can get our predecessor info by walking the pred_iterator list, but it
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// is relatively slow. If we already have PHI nodes in this block, walk one
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// of them to get the predecessor list instead.
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if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
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for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
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BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
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Value *PredVal = GetValueAtEndOfBlock(PredBB);
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PredValues.push_back(std::make_pair(PredBB, PredVal));
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// Compute SingularValue.
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SingularValue = PredVal;
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else if (PredVal != SingularValue)
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bool isFirstPred = true;
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for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
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BasicBlock *PredBB = *PI;
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Value *PredVal = GetValueAtEndOfBlock(PredBB);
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PredValues.push_back(std::make_pair(PredBB, PredVal));
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// Compute SingularValue.
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SingularValue = PredVal;
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} else if (PredVal != SingularValue)
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// If there are no predecessors, just return undef.
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if (PredValues.empty())
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return UndefValue::get(PrototypeValue->getType());
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// Otherwise, if all the merged values are the same, just use it.
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if (SingularValue != 0)
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return SingularValue;
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// Otherwise, we do need a PHI.
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if (Value *ExistingPHI = GetExistingPHI(BB, PredValues.begin(),
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// Ok, we have no way out, insert a new one now.
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PHINode *InsertedPHI = PHINode::Create(PrototypeValue->getType(),
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PrototypeValue->getName(),
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InsertedPHI->reserveOperandSpace(PredValues.size());
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// Fill in all the predecessors of the PHI.
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for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
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InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first);
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// See if the PHI node can be merged to a single value. This can happen in
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// loop cases when we get a PHI of itself and one other value.
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if (Value *ConstVal = InsertedPHI->hasConstantValue()) {
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InsertedPHI->eraseFromParent();
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// If the client wants to know about all new instructions, tell it.
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if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
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DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n");
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/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
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/// which use their value in the corresponding predecessor.
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void SSAUpdater::RewriteUse(Use &U) {
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Instruction *User = cast<Instruction>(U.getUser());
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if (PHINode *UserPN = dyn_cast<PHINode>(User))
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V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
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V = GetValueInMiddleOfBlock(User->getParent());
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/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
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/// for the specified BB and if so, return it. If not, construct SSA form by
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/// walking predecessors inserting PHI nodes as needed until we get to a block
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/// where the value is available.
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Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
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AvailableValsTy &AvailableVals = getAvailableVals(AV);
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// Query AvailableVals by doing an insertion of null.
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std::pair<AvailableValsTy::iterator, bool> InsertRes =
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AvailableVals.insert(std::make_pair(BB, TrackingVH<Value>()));
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// Handle the case when the insertion fails because we have already seen BB.
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if (!InsertRes.second) {
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// If the insertion failed, there are two cases. The first case is that the
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// value is already available for the specified block. If we get this, just
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if (InsertRes.first->second != 0)
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return InsertRes.first->second;
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// Otherwise, if the value we find is null, then this is the value is not
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// known but it is being computed elsewhere in our recursion. This means
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// that we have a cycle. Handle this by inserting a PHI node and returning
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// it. When we get back to the first instance of the recursion we will fill
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return InsertRes.first->second =
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PHINode::Create(PrototypeValue->getType(), PrototypeValue->getName(),
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// Okay, the value isn't in the map and we just inserted a null in the entry
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// to indicate that we're processing the block. Since we have no idea what
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// value is in this block, we have to recurse through our predecessors.
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// While we're walking our predecessors, we keep track of them in a vector,
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// then insert a PHI node in the end if we actually need one. We could use a
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// smallvector here, but that would take a lot of stack space for every level
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// of the recursion, just use IncomingPredInfo as an explicit stack.
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IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI);
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unsigned FirstPredInfoEntry = IncomingPredInfo.size();
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// As we're walking the predecessors, keep track of whether they are all
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// producing the same value. If so, this value will capture it, if not, it
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// will get reset to null. We distinguish the no-predecessor case explicitly
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TrackingVH<Value> ExistingValue;
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// We can get our predecessor info by walking the pred_iterator list, but it
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// is relatively slow. If we already have PHI nodes in this block, walk one
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// of them to get the predecessor list instead.
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if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
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for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
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BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
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Value *PredVal = GetValueAtEndOfBlockInternal(PredBB);
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IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
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// Set ExistingValue to singular value from all predecessors so far.
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ExistingValue = PredVal;
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else if (PredVal != ExistingValue)
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bool isFirstPred = true;
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for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
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BasicBlock *PredBB = *PI;
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Value *PredVal = GetValueAtEndOfBlockInternal(PredBB);
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IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
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// Set ExistingValue to singular value from all predecessors so far.
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ExistingValue = PredVal;
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} else if (PredVal != ExistingValue)
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// If there are no predecessors, then we must have found an unreachable block
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// just return 'undef'. Since there are no predecessors, InsertRes must not
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if (IncomingPredInfo.size() == FirstPredInfoEntry)
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return InsertRes.first->second = UndefValue::get(PrototypeValue->getType());
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/// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If
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/// this block is involved in a loop, a no-entry PHI node will have been
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/// inserted as InsertedVal. Otherwise, we'll still have the null we inserted
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TrackingVH<Value> &InsertedVal = AvailableVals[BB];
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// If the predecessor values are not all the same, then check to see if there
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// is an existing PHI that can be used.
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ExistingValue = GetExistingPHI(BB,
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IncomingPredInfo.begin()+FirstPredInfoEntry,
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IncomingPredInfo.end());
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// If there is an existing value we can use, then we don't need to insert a
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// PHI. This is the simple and common case.
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// If a PHI node got inserted, replace it with the existing value and delete
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PHINode *OldVal = cast<PHINode>(InsertedVal);
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// Be careful about dead loops. These RAUW's also update InsertedVal.
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if (InsertedVal != ExistingValue)
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OldVal->replaceAllUsesWith(ExistingValue);
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OldVal->replaceAllUsesWith(UndefValue::get(InsertedVal->getType()));
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OldVal->eraseFromParent();
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InsertedVal = ExistingValue;
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// Either path through the 'if' should have set InsertedVal -> ExistingVal.
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assert((InsertedVal == ExistingValue || isa<UndefValue>(InsertedVal)) &&
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"RAUW didn't change InsertedVal to be ExistingValue");
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// Drop the entries we added in IncomingPredInfo to restore the stack.
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IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
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IncomingPredInfo.end());
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return ExistingValue;
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// Otherwise, we do need a PHI: insert one now if we don't already have one.
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if (InsertedVal == 0)
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InsertedVal = PHINode::Create(PrototypeValue->getType(),
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PrototypeValue->getName(), &BB->front());
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PHINode *InsertedPHI = cast<PHINode>(InsertedVal);
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InsertedPHI->reserveOperandSpace(IncomingPredInfo.size()-FirstPredInfoEntry);
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// Fill in all the predecessors of the PHI.
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for (IncomingPredInfoTy::iterator I =
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IncomingPredInfo.begin()+FirstPredInfoEntry,
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E = IncomingPredInfo.end(); I != E; ++I)
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InsertedPHI->addIncoming(I->second, I->first);
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// Drop the entries we added in IncomingPredInfo to restore the stack.
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IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
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IncomingPredInfo.end());
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// See if the PHI node can be merged to a single value. This can happen in
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// loop cases when we get a PHI of itself and one other value.
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if (Value *ConstVal = InsertedPHI->hasConstantValue()) {
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InsertedPHI->replaceAllUsesWith(ConstVal);
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InsertedPHI->eraseFromParent();
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InsertedVal = ConstVal;
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DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n");
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// If the client wants to know about all new instructions, tell it.
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if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);