From a0c6057061be055faa542d05b2213f2bd779e160 Mon Sep 17 00:00:00 2001 From: Bob Wilson Date: Wed, 31 Mar 2010 20:51:00 +0000 Subject: [PATCH] Rewrite part of the SSAUpdater to be more careful about inserting redundant PHIs. The previous algorithm was unable to reliably detect when existing PHIs in a cycle can be reused. I'm still working on reducing a testcase. Radar 7711900. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@100047 91177308-0d34-0410-b5e6-96231b3b80d8 --- include/llvm/Transforms/Utils/SSAUpdater.h | 32 +- lib/Transforms/Utils/SSAUpdater.cpp | 423 +++++++++++++-------- 2 files changed, 291 insertions(+), 164 deletions(-) diff --git a/include/llvm/Transforms/Utils/SSAUpdater.h b/include/llvm/Transforms/Utils/SSAUpdater.h index 927e156abfb..748ded30e7b 100644 --- a/include/llvm/Transforms/Utils/SSAUpdater.h +++ b/include/llvm/Transforms/Utils/SSAUpdater.h @@ -27,22 +27,28 @@ namespace llvm { /// transformation wants to rewrite a set of uses of one value with uses of a /// set of values. class SSAUpdater { +public: + class BBInfo; + +private: /// 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 - /// TrackingVH's for the value of the map because we RAUW PHI nodes when we - /// eliminate them, and want the TrackingVH's to track this. - //typedef DenseMap > AvailableValsTy; + /// basis. When we insert PHI nodes, we keep track of them here. + //typedef DenseMap 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 > > IncomingPredInfo; - void *IPI; + /// BBMap - The GetValueAtEndOfBlock method maintains this mapping from + /// basic blocks to BBInfo structures. + /// typedef DenseMap BBMapTy; + void *BM; + + /// Allocator - The GetValueAtEndOfBlock method uses this BumpPtrAllocator to + /// hold its internal data. The allocator and its storage is created and + /// discarded for each invocation of GetValueAtEndOfBlock. + void *BPA; /// InsertedPHIs - If this is non-null, the SSAUpdater adds all PHI nodes that /// it creates to the vector. @@ -99,6 +105,14 @@ public: private: Value *GetValueAtEndOfBlockInternal(BasicBlock *BB); + void FindPHIPlacement(BasicBlock *BB, BBInfo *Info, bool &Changed, + unsigned Counter); + void FindAvailableVal(BasicBlock *BB, BBInfo *Info, unsigned Counter); + void FindExistingPHI(BasicBlock *BB, BBInfo *Info); + bool CheckIfPHIMatches(BasicBlock *BB, BBInfo *Info, Value *Val); + void RecordMatchingPHI(BasicBlock *BB, BBInfo *Info, PHINode *PHI); + void ClearPHITags(BasicBlock *BB, BBInfo *Info, PHINode *PHI); + void operator=(const SSAUpdater&); // DO NOT IMPLEMENT SSAUpdater(const SSAUpdater&); // DO NOT IMPLEMENT }; diff --git a/lib/Transforms/Utils/SSAUpdater.cpp b/lib/Transforms/Utils/SSAUpdater.cpp index a31235a1f5c..20a92d690c2 100644 --- a/lib/Transforms/Utils/SSAUpdater.cpp +++ b/lib/Transforms/Utils/SSAUpdater.cpp @@ -14,31 +14,82 @@ #include "llvm/Transforms/Utils/SSAUpdater.h" #include "llvm/Instructions.h" #include "llvm/ADT/DenseMap.h" +#include "llvm/Support/AlignOf.h" +#include "llvm/Support/Allocator.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 > AvailableValsTy; -typedef std::vector > > - IncomingPredInfoTy; +/// BBInfo - Per-basic block information used internally by SSAUpdater. +/// The predecessors of each block are cached here since pred_iterator is +/// slow and we need to iterate over the blocks at least a few times. +class SSAUpdater::BBInfo { +public: + Value *AvailableVal; // Value to use in this block. + BasicBlock *DefBB; // Block that defines the available value. + unsigned NumPreds; // Number of predecessor blocks. + BasicBlock **Preds; // Array[NumPreds] of predecessor blocks. + unsigned Counter; // Marker to identify blocks already visited. + PHINode *PHITag; // Marker for existing PHIs that match. + BBInfo(BasicBlock *BB, Value *V, BumpPtrAllocator *Allocator); +}; +typedef DenseMap BBMapTy; + +SSAUpdater::BBInfo::BBInfo(BasicBlock *BB, Value *V, + BumpPtrAllocator *Allocator) + : AvailableVal(V), DefBB(0), NumPreds(0), Preds(0), Counter(0), PHITag(0) { + // If this block has a known value, don't bother finding its predecessors. + if (V) { + DefBB = BB; + return; + } + + // 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(BB->begin())) { + NumPreds = SomePhi->getNumIncomingValues(); + Preds = static_cast + (Allocator->Allocate(NumPreds * sizeof(BasicBlock*), + AlignOf::Alignment)); + for (unsigned pi = 0; pi != NumPreds; ++pi) + Preds[pi] = SomePhi->getIncomingBlock(pi); + return; + } + + // Stash the predecessors in a temporary vector until we know how much space + // to allocate for them. + SmallVector TmpPreds; + for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { + TmpPreds.push_back(*PI); + ++NumPreds; + } + Preds = static_cast + (Allocator->Allocate(NumPreds * sizeof(BasicBlock*), + AlignOf::Alignment)); + memcpy(Preds, TmpPreds.data(), NumPreds * sizeof(BasicBlock*)); +} + +typedef DenseMap AvailableValsTy; static AvailableValsTy &getAvailableVals(void *AV) { return *static_cast(AV); } -static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) { - return *static_cast(IPI); +static BBMapTy *getBBMap(void *BM) { + return static_cast(BM); } +static BumpPtrAllocator *getAllocator(void *BPA) { + return static_cast(BPA); +} SSAUpdater::SSAUpdater(SmallVectorImpl *NewPHI) - : AV(0), PrototypeValue(0), IPI(0), InsertedPHIs(NewPHI) {} + : AV(0), PrototypeValue(0), BM(0), BPA(0), InsertedPHIs(NewPHI) {} SSAUpdater::~SSAUpdater() { delete &getAvailableVals(AV); - delete &getIncomingPredInfo(IPI); } /// Initialize - Reset this object to get ready for a new set of SSA @@ -48,11 +99,6 @@ void SSAUpdater::Initialize(Value *ProtoValue) { AV = new AvailableValsTy(); else getAvailableVals(AV).clear(); - - if (IPI == 0) - IPI = new IncomingPredInfoTy(); - else - getIncomingPredInfo(IPI).clear(); PrototypeValue = ProtoValue; } @@ -118,9 +164,9 @@ static Value *GetExistingPHI(BasicBlock *BB, const InputIt &I, /// 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"); + assert(BM == 0 && BPA == 0 && "Unexpected Internal State"); Value *Res = GetValueAtEndOfBlockInternal(BB); - assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State"); + assert(BM == 0 && BPA == 0 && "Unexpected Internal State"); return Res; } @@ -146,7 +192,7 @@ Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) { 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)) + if (!HasValueForBlock(BB)) return GetValueAtEndOfBlock(BB); // Otherwise, we have the hard case. Get the live-in values for each @@ -236,161 +282,228 @@ void SSAUpdater::RewriteUse(Use &U) { 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. -/// +/// first calculating the required placement of PHIs and then inserting new +/// PHIs where needed. Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) { AvailableValsTy &AvailableVals = getAvailableVals(AV); + if (Value *V = AvailableVals[BB]) + return V; - // Query AvailableVals by doing an insertion of null. - std::pair InsertRes = - AvailableVals.insert(std::make_pair(BB, TrackingVH())); + // Pool allocation used internally by GetValueAtEndOfBlock. + BumpPtrAllocator AllocatorObj; + BBMapTy BBMapObj; + BPA = &AllocatorObj; + BM = &BBMapObj; - // 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; + BBInfo *Info = new (AllocatorObj) BBInfo(BB, 0, &AllocatorObj); + BBMapObj[BB] = Info; - // 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()); + bool Changed; + unsigned Counter = 1; + do { + Changed = false; + FindPHIPlacement(BB, Info, Changed, Counter); + ++Counter; + } while (Changed); + + FindAvailableVal(BB, Info, Counter); + + BPA = 0; + BM = 0; + return Info->AvailableVal; +} + +/// FindPHIPlacement - Recursively visit the predecessors of a block to find +/// the reaching definition for each predecessor and then determine whether +/// a PHI is needed in this block. +void SSAUpdater::FindPHIPlacement(BasicBlock *BB, BBInfo *Info, bool &Changed, + unsigned Counter) { + AvailableValsTy &AvailableVals = getAvailableVals(AV); + BBMapTy *BBMap = getBBMap(BM); + BumpPtrAllocator *Allocator = getAllocator(BPA); + bool BBNeedsPHI = false; + BasicBlock *SamePredDefBB = 0; + + // If there are no predecessors, then we must have found an unreachable + // block. Treat it as a definition with 'undef'. + if (Info->NumPreds == 0) { + Info->AvailableVal = UndefValue::get(PrototypeValue->getType()); + Info->DefBB = BB; + return; } - // 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 ExistingValue; - - // 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(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)); - - // Set ExistingValue to singular value from all predecessors so far. - if (i == 0) - ExistingValue = PredVal; - else if (PredVal != ExistingValue) - ExistingValue = 0; + Info->Counter = Counter; + for (unsigned pi = 0; pi != Info->NumPreds; ++pi) { + BasicBlock *Pred = Info->Preds[pi]; + BBMapTy::value_type &BBMapBucket = BBMap->FindAndConstruct(Pred); + if (!BBMapBucket.second) { + Value *PredVal = AvailableVals.lookup(Pred); + BBMapBucket.second = new (*Allocator) BBInfo(Pred, PredVal, Allocator); } - } 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)); + BBInfo *PredInfo = BBMapBucket.second; + BasicBlock *DefBB = 0; + if (!PredInfo->AvailableVal) { + if (PredInfo->Counter != Counter) + FindPHIPlacement(Pred, PredInfo, Changed, Counter); - // Set ExistingValue to singular value from all predecessors so far. - if (isFirstPred) { - ExistingValue = PredVal; - isFirstPred = false; - } else if (PredVal != ExistingValue) - ExistingValue = 0; + // Ignore back edges where the value is not yet known. + if (!PredInfo->DefBB) + continue; + } + DefBB = PredInfo->DefBB; + + if (!SamePredDefBB) + SamePredDefBB = DefBB; + else if (DefBB != SamePredDefBB) + BBNeedsPHI = true; + } + + BasicBlock *NewDefBB = (BBNeedsPHI ? BB : SamePredDefBB); + if (Info->DefBB != NewDefBB) { + Changed = true; + Info->DefBB = NewDefBB; + } +} + +/// FindAvailableVal - If this block requires a PHI, first check if an existing +/// PHI matches the PHI placement and reaching definitions computed earlier, +/// and if not, create a new PHI. Visit all the block's predecessors to +/// calculate the available value for each one and fill in the incoming values +/// for a new PHI. +void SSAUpdater::FindAvailableVal(BasicBlock *BB, BBInfo *Info, + unsigned Counter) { + if (Info->AvailableVal || Info->Counter == Counter) + return; + + AvailableValsTy &AvailableVals = getAvailableVals(AV); + BBMapTy *BBMap = getBBMap(BM); + + // Check if there needs to be a PHI in BB. + PHINode *NewPHI = 0; + if (Info->DefBB == BB) { + // Look for an existing PHI. + FindExistingPHI(BB, Info); + if (!Info->AvailableVal) { + NewPHI = PHINode::Create(PrototypeValue->getType(), + PrototypeValue->getName(), &BB->front()); + NewPHI->reserveOperandSpace(Info->NumPreds); + Info->AvailableVal = NewPHI; + AvailableVals[BB] = NewPHI; } } - // 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 &InsertedVal = AvailableVals[BB]; - - // If the predecessor values are not all the same, then check to see if there - // is an existing PHI that can be used. - if (!ExistingValue) - ExistingValue = GetExistingPHI(BB, - IncomingPredInfo.begin()+FirstPredInfoEntry, - IncomingPredInfo.end()); - - // If there is an existing value we can use, then we don't need to insert a - // PHI. This is the simple and common case. - if (ExistingValue) { - // If a PHI node got inserted, replace it with the existing value and delete - // it. - if (InsertedVal) { - PHINode *OldVal = cast(InsertedVal); - // Be careful about dead loops. These RAUW's also update InsertedVal. - if (InsertedVal != ExistingValue) - OldVal->replaceAllUsesWith(ExistingValue); - else - OldVal->replaceAllUsesWith(UndefValue::get(InsertedVal->getType())); - OldVal->eraseFromParent(); - } else { - InsertedVal = ExistingValue; - } - - // Either path through the 'if' should have set InsertedVal -> ExistingVal. - assert((InsertedVal == ExistingValue || isa(InsertedVal)) && - "RAUW didn't change InsertedVal to be ExistingValue"); - - // Drop the entries we added in IncomingPredInfo to restore the stack. - IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry, - IncomingPredInfo.end()); - return ExistingValue; + // Iterate through the block's predecessors. + Info->Counter = Counter; + for (unsigned pi = 0; pi != Info->NumPreds; ++pi) { + BasicBlock *Pred = Info->Preds[pi]; + BBInfo *PredInfo = (*BBMap)[Pred]; + FindAvailableVal(Pred, PredInfo, Counter); + if (NewPHI) { + // Skip to the nearest preceding definition. + if (PredInfo->DefBB != Pred) + PredInfo = (*BBMap)[PredInfo->DefBB]; + NewPHI->addIncoming(PredInfo->AvailableVal, Pred); + } else if (!Info->AvailableVal) + Info->AvailableVal = PredInfo->AvailableVal; } - - // 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(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 (NewPHI) { + DEBUG(dbgs() << " Inserted PHI: " << *NewPHI << "\n"); // If the client wants to know about all new instructions, tell it. - if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI); + if (InsertedPHIs) InsertedPHIs->push_back(NewPHI); + } +} + +/// FindExistingPHI - Look through the PHI nodes in a block to see if any of +/// them match what is needed. +void SSAUpdater::FindExistingPHI(BasicBlock *BB, BBInfo *Info) { + PHINode *SomePHI; + for (BasicBlock::iterator It = BB->begin(); + (SomePHI = dyn_cast(It)); ++It) { + if (CheckIfPHIMatches(BB, Info, SomePHI)) { + RecordMatchingPHI(BB, Info, SomePHI); + break; + } + ClearPHITags(BB, Info, SomePHI); + } +} + +/// CheckIfPHIMatches - Check if Val is a PHI node in block BB that matches +/// the placement and values in the BBMap. +bool SSAUpdater::CheckIfPHIMatches(BasicBlock *BB, BBInfo *Info, Value *Val) { + if (Info->AvailableVal) + return Val == Info->AvailableVal; + + // Check if Val is a PHI in this block. + PHINode *PHI = dyn_cast(Val); + if (!PHI || PHI->getParent() != BB) + return false; + + // If this block has already been visited, check if this PHI matches. + if (Info->PHITag) + return PHI == Info->PHITag; + Info->PHITag = PHI; + bool IsMatch = true; + + // Iterate through the predecessors. + BBMapTy *BBMap = getBBMap(BM); + for (unsigned i = 0, e = PHI->getNumIncomingValues(); i != e; ++i) { + BasicBlock *Pred = PHI->getIncomingBlock(i); + Value *IncomingVal = PHI->getIncomingValue(i); + BBInfo *PredInfo = (*BBMap)[Pred]; + // Skip to the nearest preceding definition. + if (PredInfo->DefBB != Pred) { + Pred = PredInfo->DefBB; + PredInfo = (*BBMap)[Pred]; + } + if (!CheckIfPHIMatches(Pred, PredInfo, IncomingVal)) { + IsMatch = false; + break; + } + } + return IsMatch; +} + +/// RecordMatchingPHI - For a PHI node that matches, record it in both the +/// BBMap and the AvailableVals mapping. Recursively record its input PHIs +/// as well. +void SSAUpdater::RecordMatchingPHI(BasicBlock *BB, BBInfo *Info, PHINode *PHI) { + if (!Info || Info->AvailableVal) + return; + + // Record the PHI. + AvailableValsTy &AvailableVals = getAvailableVals(AV); + AvailableVals[BB] = PHI; + Info->AvailableVal = PHI; + + // Iterate through the predecessors. + BBMapTy *BBMap = getBBMap(BM); + for (unsigned i = 0, e = PHI->getNumIncomingValues(); i != e; ++i) { + PHINode *PHIVal = dyn_cast(PHI->getIncomingValue(i)); + if (!PHIVal) continue; + BasicBlock *Pred = PHIVal->getParent(); + RecordMatchingPHI(Pred, (*BBMap)[Pred], PHIVal); + } +} + +/// ClearPHITags - When one of the existing PHI nodes fails to match, clear +/// the PHITag values stored in the BBMap while checking to see if it matched. +void SSAUpdater::ClearPHITags(BasicBlock *BB, BBInfo *Info, PHINode *PHI) { + if (!Info || Info->AvailableVal || !Info->PHITag) + return; + + // Clear the tag. + Info->PHITag = 0; + + // Iterate through the predecessors. + BBMapTy *BBMap = getBBMap(BM); + for (unsigned i = 0, e = PHI->getNumIncomingValues(); i != e; ++i) { + PHINode *PHIVal = dyn_cast(PHI->getIncomingValue(i)); + if (!PHIVal) continue; + BasicBlock *Pred = PHIVal->getParent(); + ClearPHITags(Pred, (*BBMap)[Pred], PHIVal); } - - return InsertedVal; }