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llvm-mirror/lib/Transforms/Utils/SSAUpdater.cpp

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