text: //===- 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/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(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) {
    return isa<LoadInst>(I) || isa<StoreInst>(I);
  }
  
  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 = A->getUnderlyingObject();
    const Value *bObj = B->getUnderlyingObject();
    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 (!S->isLoopInvariant(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:
      // 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);
  }
  