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Implement an efficient and fully general SSA update mechanism that

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works on unstructured CFGs.  This implements PR217, our oldest open PR.

llvm-svn: 83705
This commit is contained in:
Chris Lattner 2009-10-10 09:04:27 +00:00
parent 54621efc37
commit 9c5aa00411
3 changed files with 304 additions and 0 deletions
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71
include/llvm/Transforms/Utils/SSAUpdater.h Normal file
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//===-- SSAUpdater.h - Unstructured SSA Update Tool -------------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file declares the SSAUpdater class.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_TRANSFORMS_UTILS_SSAUPDATER_H
#define LLVM_TRANSFORMS_UTILS_SSAUPDATER_H
namespace llvm {
class Value;
class BasicBlock;
class Use;
/// SSAUpdater - This class updates SSA form for a set of values defined in
/// multiple blocks. This is used when code duplication or another unstructured
/// transformation wants to rewrite a set of uses of one value with uses of a
/// set of values.
class SSAUpdater {
/// AvailableVals - This keeps track of which value to use on a per-block
/// basis. When we insert PHI nodes, we keep track of them here. We use
/// WeakVH's for the value of the map because we RAUW PHI nodes when we
/// eliminate them, and want the WeakVH to track this.
//typedef DenseMap<BasicBlock*, TrackingVH<Value> > AvailableValsTy;
void *AV;
/// PrototypeValue is an arbitrary representative value, which we derive names
/// and a type for PHI nodes.
Value *PrototypeValue;
/// IncomingPredInfo - We use this as scratch space when doing our recursive
/// walk. This should only be used in GetValueInBlockInternal, normally it
/// should be empty.
//std::vector<std::pair<BasicBlock*, TrackingVH<Value> > > IncomingPredInfo;
void *IPI;
public:
SSAUpdater();
~SSAUpdater();
/// Initialize - Reset this object to get ready for a new set of SSA
/// updates. ProtoValue is the value used to name PHI nodes.
void Initialize(Value *ProtoValue);
/// AddAvailableValue - Indicate that a rewritten value is available in the
/// specified block with the specified value.
void AddAvailableValue(BasicBlock *BB, Value *V);
/// GetValueInBlock - Construct SSA form, materializing a value in the
/// specified block.
Value *GetValueInBlock(BasicBlock *BB);
/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
/// which use their value in the corresponding predecessor.
void RewriteUse(Use &U);
private:
Value *GetValueInBlockInternal(BasicBlock *BB);
void operator=(const SSAUpdater&); // DO NOT IMPLEMENT
SSAUpdater(const SSAUpdater&); // DO NOT IMPLEMENT
};
} // End llvm namespace
#endif

1
lib/Transforms/Utils/CMakeLists.txt
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@ -19,6 +19,7 @@ add_llvm_library(LLVMTransformUtils
LowerSwitch.cpp LowerSwitch.cpp
Mem2Reg.cpp Mem2Reg.cpp
PromoteMemoryToRegister.cpp PromoteMemoryToRegister.cpp
SSAUpdater.cpp
SSI.cpp SSI.cpp
SimplifyCFG.cpp SimplifyCFG.cpp
UnifyFunctionExitNodes.cpp UnifyFunctionExitNodes.cpp

232
lib/Transforms/Utils/SSAUpdater.cpp Normal file
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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() : AV(0), PrototypeValue(0), IPI(0) {}
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;
}
/// 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;
}
/// GetValueInBlock - Construct SSA form, materializing a value in the
/// specified block.
Value *SSAUpdater::GetValueInBlock(BasicBlock *BB) {
assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
Value *Res = GetValueInBlockInternal(BB);
assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
return Res;
}
/// 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());
BasicBlock *UseBB = User->getParent();
if (PHINode *UserPN = dyn_cast<PHINode>(User))
UseBB = UserPN->getIncomingBlock(U);
U.set(GetValueInBlock(UseBB));
}
/// GetValueInBlock - 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::GetValueInBlockInternal(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, WeakVH()));
// 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 = GetValueInBlockInternal(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 = GetValueInBlockInternal(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;
}
// Drop the entries we added in IncomingPredInfo to restore the stack.
IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
IncomingPredInfo.end());
return InsertedVal;
}
// 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 (std::vector<std::pair<BasicBlock*, TrackingVH<Value> > >::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(errs() << " Inserted PHI: " << *InsertedPHI << "\n");
}
return InsertedVal;
}