| //===- LoopDependenceAnalysis.cpp - LDA Implementation ----------*- C++ -*-===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This is the (beginning) of an implementation of a loop dependence analysis |
| // framework, which is used to detect dependences in memory accesses in loops. |
| // |
| // Please note that this is work in progress and the interface is subject to |
| // change. |
| // |
| // TODO: adapt as implementation progresses. |
| // |
| // TODO: document lingo (pair, subscript, index) |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "lda" |
| #include "llvm/ADT/DenseSet.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/LoopDependenceAnalysis.h" |
| #include "llvm/Analysis/LoopPass.h" |
| #include "llvm/Analysis/ScalarEvolution.h" |
| #include "llvm/Analysis/ScalarEvolutionExpressions.h" |
| #include "llvm/Analysis/ValueTracking.h" |
| #include "llvm/Assembly/Writer.h" |
| #include "llvm/Instructions.h" |
| #include "llvm/Operator.h" |
| #include "llvm/Support/Allocator.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/ErrorHandling.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include "llvm/Target/TargetData.h" |
| using namespace llvm; |
| |
| STATISTIC(NumAnswered, "Number of dependence queries answered"); |
| STATISTIC(NumAnalysed, "Number of distinct dependence pairs analysed"); |
| STATISTIC(NumDependent, "Number of pairs with dependent accesses"); |
| STATISTIC(NumIndependent, "Number of pairs with independent accesses"); |
| STATISTIC(NumUnknown, "Number of pairs with unknown accesses"); |
| |
| LoopPass *llvm::createLoopDependenceAnalysisPass() { |
| return new LoopDependenceAnalysis(); |
| } |
| |
| INITIALIZE_PASS_BEGIN(LoopDependenceAnalysis, "lda", |
| "Loop Dependence Analysis", false, true) |
| INITIALIZE_PASS_DEPENDENCY(ScalarEvolution) |
| INITIALIZE_AG_DEPENDENCY(AliasAnalysis) |
| INITIALIZE_PASS_END(LoopDependenceAnalysis, "lda", |
| "Loop Dependence Analysis", false, true) |
| char LoopDependenceAnalysis::ID = 0; |
| |
| //===----------------------------------------------------------------------===// |
| // Utility Functions |
| //===----------------------------------------------------------------------===// |
| |
| static inline bool IsMemRefInstr(const Value *V) { |
| const Instruction *I = dyn_cast<const Instruction>(V); |
| return I && (I->mayReadFromMemory() || I->mayWriteToMemory()); |
| } |
| |
| static void GetMemRefInstrs(const Loop *L, |
| SmallVectorImpl<Instruction*> &Memrefs) { |
| for (Loop::block_iterator b = L->block_begin(), be = L->block_end(); |
| b != be; ++b) |
| for (BasicBlock::iterator i = (*b)->begin(), ie = (*b)->end(); |
| i != ie; ++i) |
| if (IsMemRefInstr(i)) |
| Memrefs.push_back(i); |
| } |
| |
| static bool IsLoadOrStoreInst(Value *I) { |
| // Returns true if the load or store can be analyzed. Atomic and volatile |
| // operations have properties which this analysis does not understand. |
| if (LoadInst *LI = dyn_cast<LoadInst>(I)) |
| return LI->isUnordered(); |
| else if (StoreInst *SI = dyn_cast<StoreInst>(I)) |
| return SI->isUnordered(); |
| return false; |
| } |
| |
| static Value *GetPointerOperand(Value *I) { |
| if (LoadInst *i = dyn_cast<LoadInst>(I)) |
| return i->getPointerOperand(); |
| if (StoreInst *i = dyn_cast<StoreInst>(I)) |
| return i->getPointerOperand(); |
| llvm_unreachable("Value is no load or store instruction!"); |
| // Never reached. |
| return 0; |
| } |
| |
| static AliasAnalysis::AliasResult UnderlyingObjectsAlias(AliasAnalysis *AA, |
| const Value *A, |
| const Value *B) { |
| const Value *aObj = GetUnderlyingObject(A); |
| const Value *bObj = GetUnderlyingObject(B); |
| return AA->alias(aObj, AA->getTypeStoreSize(aObj->getType()), |
| bObj, AA->getTypeStoreSize(bObj->getType())); |
| } |
| |
| static inline const SCEV *GetZeroSCEV(ScalarEvolution *SE) { |
| return SE->getConstant(Type::getInt32Ty(SE->getContext()), 0L); |
| } |
| |
| //===----------------------------------------------------------------------===// |
| // Dependence Testing |
| //===----------------------------------------------------------------------===// |
| |
| bool LoopDependenceAnalysis::isDependencePair(const Value *A, |
| const Value *B) const { |
| return IsMemRefInstr(A) && |
| IsMemRefInstr(B) && |
| (cast<const Instruction>(A)->mayWriteToMemory() || |
| cast<const Instruction>(B)->mayWriteToMemory()); |
| } |
| |
| bool LoopDependenceAnalysis::findOrInsertDependencePair(Value *A, |
| Value *B, |
| DependencePair *&P) { |
| void *insertPos = 0; |
| FoldingSetNodeID id; |
| id.AddPointer(A); |
| id.AddPointer(B); |
| |
| P = Pairs.FindNodeOrInsertPos(id, insertPos); |
| if (P) return true; |
| |
| P = new (PairAllocator) DependencePair(id, A, B); |
| Pairs.InsertNode(P, insertPos); |
| return false; |
| } |
| |
| void LoopDependenceAnalysis::getLoops(const SCEV *S, |
| DenseSet<const Loop*>* Loops) const { |
| // Refactor this into an SCEVVisitor, if efficiency becomes a concern. |
| for (const Loop *L = this->L; L != 0; L = L->getParentLoop()) |
| if (!SE->isLoopInvariant(S, L)) |
| Loops->insert(L); |
| } |
| |
| bool LoopDependenceAnalysis::isLoopInvariant(const SCEV *S) const { |
| DenseSet<const Loop*> loops; |
| getLoops(S, &loops); |
| return loops.empty(); |
| } |
| |
| bool LoopDependenceAnalysis::isAffine(const SCEV *S) const { |
| const SCEVAddRecExpr *rec = dyn_cast<SCEVAddRecExpr>(S); |
| return isLoopInvariant(S) || (rec && rec->isAffine()); |
| } |
| |
| bool LoopDependenceAnalysis::isZIVPair(const SCEV *A, const SCEV *B) const { |
| return isLoopInvariant(A) && isLoopInvariant(B); |
| } |
| |
| bool LoopDependenceAnalysis::isSIVPair(const SCEV *A, const SCEV *B) const { |
| DenseSet<const Loop*> loops; |
| getLoops(A, &loops); |
| getLoops(B, &loops); |
| return loops.size() == 1; |
| } |
| |
| LoopDependenceAnalysis::DependenceResult |
| LoopDependenceAnalysis::analyseZIV(const SCEV *A, |
| const SCEV *B, |
| Subscript *S) const { |
| assert(isZIVPair(A, B) && "Attempted to ZIV-test non-ZIV SCEVs!"); |
| return A == B ? Dependent : Independent; |
| } |
| |
| LoopDependenceAnalysis::DependenceResult |
| LoopDependenceAnalysis::analyseSIV(const SCEV *A, |
| const SCEV *B, |
| Subscript *S) const { |
| return Unknown; // TODO: Implement. |
| } |
| |
| LoopDependenceAnalysis::DependenceResult |
| LoopDependenceAnalysis::analyseMIV(const SCEV *A, |
| const SCEV *B, |
| Subscript *S) const { |
| return Unknown; // TODO: Implement. |
| } |
| |
| LoopDependenceAnalysis::DependenceResult |
| LoopDependenceAnalysis::analyseSubscript(const SCEV *A, |
| const SCEV *B, |
| Subscript *S) const { |
| DEBUG(dbgs() << " Testing subscript: " << *A << ", " << *B << "\n"); |
| |
| if (A == B) { |
| DEBUG(dbgs() << " -> [D] same SCEV\n"); |
| return Dependent; |
| } |
| |
| if (!isAffine(A) || !isAffine(B)) { |
| DEBUG(dbgs() << " -> [?] not affine\n"); |
| return Unknown; |
| } |
| |
| if (isZIVPair(A, B)) |
| return analyseZIV(A, B, S); |
| |
| if (isSIVPair(A, B)) |
| return analyseSIV(A, B, S); |
| |
| return analyseMIV(A, B, S); |
| } |
| |
| LoopDependenceAnalysis::DependenceResult |
| LoopDependenceAnalysis::analysePair(DependencePair *P) const { |
| DEBUG(dbgs() << "Analysing:\n" << *P->A << "\n" << *P->B << "\n"); |
| |
| // We only analyse loads and stores but no possible memory accesses by e.g. |
| // free, call, or invoke instructions. |
| if (!IsLoadOrStoreInst(P->A) || !IsLoadOrStoreInst(P->B)) { |
| DEBUG(dbgs() << "--> [?] no load/store\n"); |
| return Unknown; |
| } |
| |
| Value *aPtr = GetPointerOperand(P->A); |
| Value *bPtr = GetPointerOperand(P->B); |
| |
| switch (UnderlyingObjectsAlias(AA, aPtr, bPtr)) { |
| case AliasAnalysis::MayAlias: |
| case AliasAnalysis::PartialAlias: |
| // We can not analyse objects if we do not know about their aliasing. |
| DEBUG(dbgs() << "---> [?] may alias\n"); |
| return Unknown; |
| |
| case AliasAnalysis::NoAlias: |
| // If the objects noalias, they are distinct, accesses are independent. |
| DEBUG(dbgs() << "---> [I] no alias\n"); |
| return Independent; |
| |
| case AliasAnalysis::MustAlias: |
| break; // The underlying objects alias, test accesses for dependence. |
| } |
| |
| const GEPOperator *aGEP = dyn_cast<GEPOperator>(aPtr); |
| const GEPOperator *bGEP = dyn_cast<GEPOperator>(bPtr); |
| |
| if (!aGEP || !bGEP) |
| return Unknown; |
| |
| // FIXME: Is filtering coupled subscripts necessary? |
| |
| // Collect GEP operand pairs (FIXME: use GetGEPOperands from BasicAA), adding |
| // trailing zeroes to the smaller GEP, if needed. |
| typedef SmallVector<std::pair<const SCEV*, const SCEV*>, 4> GEPOpdPairsTy; |
| GEPOpdPairsTy opds; |
| for(GEPOperator::const_op_iterator aIdx = aGEP->idx_begin(), |
| aEnd = aGEP->idx_end(), |
| bIdx = bGEP->idx_begin(), |
| bEnd = bGEP->idx_end(); |
| aIdx != aEnd && bIdx != bEnd; |
| aIdx += (aIdx != aEnd), bIdx += (bIdx != bEnd)) { |
| const SCEV* aSCEV = (aIdx != aEnd) ? SE->getSCEV(*aIdx) : GetZeroSCEV(SE); |
| const SCEV* bSCEV = (bIdx != bEnd) ? SE->getSCEV(*bIdx) : GetZeroSCEV(SE); |
| opds.push_back(std::make_pair(aSCEV, bSCEV)); |
| } |
| |
| if (!opds.empty() && opds[0].first != opds[0].second) { |
| // We cannot (yet) handle arbitrary GEP pointer offsets. By limiting |
| // |
| // TODO: this could be relaxed by adding the size of the underlying object |
| // to the first subscript. If we have e.g. (GEP x,0,i; GEP x,2,-i) and we |
| // know that x is a [100 x i8]*, we could modify the first subscript to be |
| // (i, 200-i) instead of (i, -i). |
| return Unknown; |
| } |
| |
| // Now analyse the collected operand pairs (skipping the GEP ptr offsets). |
| for (GEPOpdPairsTy::const_iterator i = opds.begin() + 1, end = opds.end(); |
| i != end; ++i) { |
| Subscript subscript; |
| DependenceResult result = analyseSubscript(i->first, i->second, &subscript); |
| if (result != Dependent) { |
| // We either proved independence or failed to analyse this subscript. |
| // Further subscripts will not improve the situation, so abort early. |
| return result; |
| } |
| P->Subscripts.push_back(subscript); |
| } |
| // We successfully analysed all subscripts but failed to prove independence. |
| return Dependent; |
| } |
| |
| bool LoopDependenceAnalysis::depends(Value *A, Value *B) { |
| assert(isDependencePair(A, B) && "Values form no dependence pair!"); |
| ++NumAnswered; |
| |
| DependencePair *p; |
| if (!findOrInsertDependencePair(A, B, p)) { |
| // The pair is not cached, so analyse it. |
| ++NumAnalysed; |
| switch (p->Result = analysePair(p)) { |
| case Dependent: ++NumDependent; break; |
| case Independent: ++NumIndependent; break; |
| case Unknown: ++NumUnknown; break; |
| } |
| } |
| return p->Result != Independent; |
| } |
| |
| //===----------------------------------------------------------------------===// |
| // LoopDependenceAnalysis Implementation |
| //===----------------------------------------------------------------------===// |
| |
| bool LoopDependenceAnalysis::runOnLoop(Loop *L, LPPassManager &) { |
| this->L = L; |
| AA = &getAnalysis<AliasAnalysis>(); |
| SE = &getAnalysis<ScalarEvolution>(); |
| return false; |
| } |
| |
| void LoopDependenceAnalysis::releaseMemory() { |
| Pairs.clear(); |
| PairAllocator.Reset(); |
| } |
| |
| void LoopDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.setPreservesAll(); |
| AU.addRequiredTransitive<AliasAnalysis>(); |
| AU.addRequiredTransitive<ScalarEvolution>(); |
| } |
| |
| static void PrintLoopInfo(raw_ostream &OS, |
| LoopDependenceAnalysis *LDA, const Loop *L) { |
| if (!L->empty()) return; // ignore non-innermost loops |
| |
| SmallVector<Instruction*, 8> memrefs; |
| GetMemRefInstrs(L, memrefs); |
| |
| OS << "Loop at depth " << L->getLoopDepth() << ", header block: "; |
| WriteAsOperand(OS, L->getHeader(), false); |
| OS << "\n"; |
| |
| OS << " Load/store instructions: " << memrefs.size() << "\n"; |
| for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(), |
| end = memrefs.end(); x != end; ++x) |
| OS << "\t" << (x - memrefs.begin()) << ": " << **x << "\n"; |
| |
| OS << " Pairwise dependence results:\n"; |
| for (SmallVector<Instruction*, 8>::const_iterator x = memrefs.begin(), |
| end = memrefs.end(); x != end; ++x) |
| for (SmallVector<Instruction*, 8>::const_iterator y = x + 1; |
| y != end; ++y) |
| if (LDA->isDependencePair(*x, *y)) |
| OS << "\t" << (x - memrefs.begin()) << "," << (y - memrefs.begin()) |
| << ": " << (LDA->depends(*x, *y) ? "dependent" : "independent") |
| << "\n"; |
| } |
| |
| void LoopDependenceAnalysis::print(raw_ostream &OS, const Module*) const { |
| // TODO: doc why const_cast is safe |
| PrintLoopInfo(OS, const_cast<LoopDependenceAnalysis*>(this), this->L); |
| } |