LoopCacheAnalysis.cpp

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//===- LoopCacheAnalysis.cpp - Loop Cache Analysis -------------------------==//
//
//                     The LLVM Compiler Infrastructure
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file defines the implementation for the loop cache analysis.
/// The implementation is largely based on the following paper:
///
///       Compiler Optimizations for Improving Data Locality
///       By: Steve Carr, Katherine S. McKinley, Chau-Wen Tseng
///       http://www.cs.utexas.edu/users/mckinley/papers/asplos-1994.pdf
///
/// The general approach taken to estimate the number of cache lines used by the
/// memory references in an inner loop is:
///    1. Partition memory references that exhibit temporal or spacial reuse
///       into reference groups.
///    2. For each loop L in the a loop nest LN:
///       a. Compute the cost of the reference group
///       b. Compute the loop cost by summing up the reference groups costs
//===----------------------------------------------------------------------===//
 
#include "llvm/Analysis/LoopCacheAnalysis.h"
#include "llvm/ADT/BreadthFirstIterator.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Delinearization.h"
#include "llvm/Analysis/DependenceAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
 
using namespace llvm;
 
#define DEBUG_TYPE "loop-cache-cost"
 
static cl::opt<unsigned> DefaultTripCount(
    "default-trip-count", cl::init(100), cl::Hidden,
    cl::desc("Use this to specify the default trip count of a loop"));
 
// In this analysis two array references are considered to exhibit temporal
// reuse if they access either the same memory location, or a memory location
// with distance smaller than a configurable threshold.
static cl::opt<unsigned> TemporalReuseThreshold(
    "temporal-reuse-threshold", cl::init(2), cl::Hidden,
    cl::desc("Use this to specify the max. distance between array elements "
             "accessed in a loop so that the elements are classified to have "
             "temporal reuse"));
 
/// Retrieve the innermost loop in the given loop nest \p Loops. It returns a
/// nullptr if any loops in the loop vector supplied has more than one sibling.
/// The loop vector is expected to contain loops collected in breadth-first
/// order.
static Loop *getInnerMostLoop(const LoopVectorTy &Loops) {
  assert(!Loops.empty() && "Expecting a non-empy loop vector");
 
  Loop *LastLoop = Loops.back();
  Loop *ParentLoop = LastLoop->getParentLoop();
 
  if (ParentLoop == nullptr) {
    assert(Loops.size() == 1 && "Expecting a single loop");
    return LastLoop;
  }
 
  return (llvm::is_sorted(Loops,
                          [](const Loop *L1, const Loop *L2) {
                            return L1->getLoopDepth() < L2->getLoopDepth();
                          }))
             ? LastLoop
             : nullptr;
}
 
static bool isOneDimensionalArray(const SCEV &AccessFn, const SCEV &ElemSize,
                                  const Loop &L, ScalarEvolution &SE) {
  const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(&AccessFn);
  if (!AR || !AR->isAffine())
    return false;
 
  assert(AR->getLoop() && "AR should have a loop");
 
  // Check that start and increment are not add recurrences.
  const SCEV *Start = AR->getStart();
  const SCEV *Step = AR->getStepRecurrence(SE);
  if (isa<SCEVAddRecExpr>(Start) || isa<SCEVAddRecExpr>(Step))
    return false;
 
  // Check that start and increment are both invariant in the loop.
  if (!SE.isLoopInvariant(Start, &L) || !SE.isLoopInvariant(Step, &L))
    return false;
 
  const SCEV *StepRec = AR->getStepRecurrence(SE);
  if (StepRec && SE.isKnownNegative(StepRec))
    StepRec = SE.getNegativeSCEV(StepRec);
 
  return StepRec == &ElemSize;
}
 
/// Compute the trip count for the given loop \p L or assume a default value if
/// it is not a compile time constant. Return the SCEV expression for the trip
/// count.
static const SCEV *computeTripCount(const Loop &L, const SCEV &ElemSize,
                                    ScalarEvolution &SE) {
  const SCEV *BackedgeTakenCount = SE.getBackedgeTakenCount(&L);
  const SCEV *TripCount = (!isa<SCEVCouldNotCompute>(BackedgeTakenCount) &&
                           isa<SCEVConstant>(BackedgeTakenCount))
                              ? SE.getTripCountFromExitCount(BackedgeTakenCount)
                              : nullptr;
 
  if (!TripCount) {
    LLVM_DEBUG(dbgs() << "Trip count of loop " << L.getName()
               << " could not be computed, using DefaultTripCount\n");
    TripCount = SE.getConstant(ElemSize.getType(), DefaultTripCount);
  }
 
  return TripCount;
}
 
//===----------------------------------------------------------------------===//
// IndexedReference implementation
//
raw_ostream &llvm::operator<<(raw_ostream &OS, const IndexedReference &R) {
  if (!R.IsValid) {
    OS << R.StoreOrLoadInst;
    OS << ", IsValid=false.";
    return OS;
  }
 
  OS << *R.BasePointer;
  for (const SCEV *Subscript : R.Subscripts)
    OS << "[" << *Subscript << "]";
 
  OS << ", Sizes: ";
  for (const SCEV *Size : R.Sizes)
    OS << "[" << *Size << "]";
 
  return OS;
}
 
IndexedReference::IndexedReference(Instruction &StoreOrLoadInst,
                                   const LoopInfo &LI, ScalarEvolution &SE)
    : StoreOrLoadInst(StoreOrLoadInst), SE(SE) {
  assert((isa<StoreInst>(StoreOrLoadInst) || isa<LoadInst>(StoreOrLoadInst)) &&
         "Expecting a load or store instruction");
 
  IsValid = delinearize(LI);
  if (IsValid)
    LLVM_DEBUG(dbgs().indent(2) << "Succesfully delinearized: " << *this
                                << "\n");
}
 
std::optional<bool>
IndexedReference::hasSpacialReuse(const IndexedReference &Other, unsigned CLS,
                                  AAResults &AA) const {
  assert(IsValid && "Expecting a valid reference");
 
  if (BasePointer != Other.getBasePointer() && !isAliased(Other, AA)) {
    LLVM_DEBUG(dbgs().indent(2)
               << "No spacial reuse: different base pointers\n");
    return false;
  }
 
  unsigned NumSubscripts = getNumSubscripts();
  if (NumSubscripts != Other.getNumSubscripts()) {
    LLVM_DEBUG(dbgs().indent(2)
               << "No spacial reuse: different number of subscripts\n");
    return false;
  }
 
  // all subscripts must be equal, except the leftmost one (the last one).
  for (auto SubNum : seq<unsigned>(0, NumSubscripts - 1)) {
    if (getSubscript(SubNum) != Other.getSubscript(SubNum)) {
      LLVM_DEBUG(dbgs().indent(2) << "No spacial reuse, different subscripts: "
                                  << "\n\t" << *getSubscript(SubNum) << "\n\t"
                                  << *Other.getSubscript(SubNum) << "\n");
      return false;
    }
  }
 
  // the difference between the last subscripts must be less than the cache line
  // size.
  const SCEV *LastSubscript = getLastSubscript();
  const SCEV *OtherLastSubscript = Other.getLastSubscript();
  const SCEVConstant *Diff = dyn_cast<SCEVConstant>(
      SE.getMinusSCEV(LastSubscript, OtherLastSubscript));
 
  if (Diff == nullptr) {
    LLVM_DEBUG(dbgs().indent(2)
               << "No spacial reuse, difference between subscript:\n\t"
               << *LastSubscript << "\n\t" << OtherLastSubscript
               << "\nis not constant.\n");
    return std::nullopt;
  }
 
  bool InSameCacheLine = (Diff->getValue()->getSExtValue() < CLS);
 
  LLVM_DEBUG({
    if (InSameCacheLine)
      dbgs().indent(2) << "Found spacial reuse.\n";
    else
      dbgs().indent(2) << "No spacial reuse.\n";
  });
 
  return InSameCacheLine;
}
 
std::optional<bool>
IndexedReference::hasTemporalReuse(const IndexedReference &Other,
                                   unsigned MaxDistance, const Loop &L,
                                   DependenceInfo &DI, AAResults &AA) const {
  assert(IsValid && "Expecting a valid reference");
 
  if (BasePointer != Other.getBasePointer() && !isAliased(Other, AA)) {
    LLVM_DEBUG(dbgs().indent(2)
               << "No temporal reuse: different base pointer\n");
    return false;
  }
 
  std::unique_ptr<Dependence> D =
      DI.depends(&StoreOrLoadInst, &Other.StoreOrLoadInst);
 
  if (D == nullptr) {
    LLVM_DEBUG(dbgs().indent(2) << "No temporal reuse: no dependence\n");
    return false;
  }
 
  if (D->isLoopIndependent()) {
    LLVM_DEBUG(dbgs().indent(2) << "Found temporal reuse\n");
    return true;
  }
 
  // Check the dependence distance at every loop level. There is temporal reuse
  // if the distance at the given loop's depth is small (|d| <= MaxDistance) and
  // it is zero at every other loop level.
  int LoopDepth = L.getLoopDepth();
  int Levels = D->getLevels();
  for (int Level = 1; Level <= Levels; ++Level) {
    const SCEV *Distance = D->getDistance(Level);
    const SCEVConstant *SCEVConst = dyn_cast_or_null<SCEVConstant>(Distance);
 
    if (SCEVConst == nullptr) {
      LLVM_DEBUG(dbgs().indent(2) << "No temporal reuse: distance unknown\n");
      return std::nullopt;
    }
 
    const ConstantInt &CI = *SCEVConst->getValue();
    if (Level != LoopDepth && !CI.isZero()) {
      LLVM_DEBUG(dbgs().indent(2)
                 << "No temporal reuse: distance is not zero at depth=" << Level
                 << "\n");
      return false;
    } else if (Level == LoopDepth && CI.getSExtValue() > MaxDistance) {
      LLVM_DEBUG(
          dbgs().indent(2)
          << "No temporal reuse: distance is greater than MaxDistance at depth="
          << Level << "\n");
      return false;
    }
  }
 
  LLVM_DEBUG(dbgs().indent(2) << "Found temporal reuse\n");
  return true;
}
 
CacheCostTy IndexedReference::computeRefCost(const Loop &L,
                                             unsigned CLS) const {
  assert(IsValid && "Expecting a valid reference");
  LLVM_DEBUG({
    dbgs().indent(2) << "Computing cache cost for:\n";
    dbgs().indent(4) << *this << "\n";
  });
 
  // If the indexed reference is loop invariant the cost is one.
  if (isLoopInvariant(L)) {
    LLVM_DEBUG(dbgs().indent(4) << "Reference is loop invariant: RefCost=1\n");
    return 1;
  }
 
  const SCEV *TripCount = computeTripCount(L, *Sizes.back(), SE);
  assert(TripCount && "Expecting valid TripCount");
  LLVM_DEBUG(dbgs() << "TripCount=" << *TripCount << "\n");
 
  const SCEV *RefCost = nullptr;
  const SCEV *Stride = nullptr;
  if (isConsecutive(L, Stride, CLS)) {
    // If the indexed reference is 'consecutive' the cost is
    // (TripCount*Stride)/CLS.
    assert(Stride != nullptr &&
           "Stride should not be null for consecutive access!");
    Type *WiderType = SE.getWiderType(Stride->getType(), TripCount->getType());
    const SCEV *CacheLineSize = SE.getConstant(WiderType, CLS);
    Stride = SE.getNoopOrAnyExtend(Stride, WiderType);
    TripCount = SE.getNoopOrZeroExtend(TripCount, WiderType);
    const SCEV *Numerator = SE.getMulExpr(Stride, TripCount);
    // Round the fractional cost up to the nearest integer number.
    // The impact is the most significant when cost is calculated
    // to be a number less than one, because it makes more sense
    // to say one cache line is used rather than zero cache line
    // is used.
    RefCost = SE.getUDivCeilSCEV(Numerator, CacheLineSize);
 
    LLVM_DEBUG(dbgs().indent(4)
               << "Access is consecutive: RefCost=(TripCount*Stride)/CLS="
               << *RefCost << "\n");
  } else {
    // If the indexed reference is not 'consecutive' the cost is proportional to
    // the trip count and the depth of the dimension which the subject loop
    // subscript is accessing. We try to estimate this by multiplying the cost
    // by the trip counts of loops corresponding to the inner dimensions. For
    // example, given the indexed reference 'A[i][j][k]', and assuming the
    // i-loop is in the innermost position, the cost would be equal to the
    // iterations of the i-loop multiplied by iterations of the j-loop.
    RefCost = TripCount;
 
    int Index = getSubscriptIndex(L);
    assert(Index >= 0 && "Could not locate a valid Index");
 
    for (unsigned I = Index + 1; I < getNumSubscripts() - 1; ++I) {
      const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(getSubscript(I));
      assert(AR && AR->getLoop() && "Expecting valid loop");
      const SCEV *TripCount =
          computeTripCount(*AR->getLoop(), *Sizes.back(), SE);
      Type *WiderType = SE.getWiderType(RefCost->getType(), TripCount->getType());
      // For the multiplication result to fit, request a type twice as wide.
      WiderType = WiderType->getExtendedType();
      RefCost = SE.getMulExpr(SE.getNoopOrZeroExtend(RefCost, WiderType),
                              SE.getNoopOrZeroExtend(TripCount, WiderType));
    }
 
    LLVM_DEBUG(dbgs().indent(4)
               << "Access is not consecutive: RefCost=" << *RefCost << "\n");
  }
  assert(RefCost && "Expecting a valid RefCost");
 
  // Attempt to fold RefCost into a constant.
  // CacheCostTy is a signed integer, but the tripcount value can be large
  // and may not fit, so saturate/limit the value to the maximum signed
  // integer value.
  if (auto ConstantCost = dyn_cast<SCEVConstant>(RefCost))
    return ConstantCost->getValue()->getLimitedValue(
        std::numeric_limits<int64_t>::max());
 
  LLVM_DEBUG(dbgs().indent(4)
             << "RefCost is not a constant! Setting to RefCost=InvalidCost "
                "(invalid value).\n");
 
  return CacheCostTy::getInvalid();
}
 
bool IndexedReference::tryDelinearizeFixedSize(
    const SCEV *AccessFn, SmallVectorImpl<const SCEV *> &Subscripts,
    const SCEV *ElementSize) {
  const SCEV *Offset = SE.removePointerBase(AccessFn);
  if (!delinearizeFixedSizeArray(SE, Offset, Subscripts, Sizes, ElementSize)) {
    Sizes.clear();
    return false;
  }
 
  // We expect Sizes and Subscipts have the same number of elements, and the
  // last element of Sizes is ElementSize. It is for ensuring consistency with
  // the load/store instruction being analyzed. It is not needed for further
  // analysis.
  // TODO: Maybe this property should be enforced in delinearizeFixedSizeArray.
#ifndef NDEBUG
  assert(!Sizes.empty() && Subscripts.size() == Sizes.size() &&
         "Inconsistent length of Sizes and Subscripts");
  Type *WideTy =
      SE.getWiderType(ElementSize->getType(), Sizes.back()->getType());
  const SCEV *ElemSizeExt = SE.getNoopOrZeroExtend(ElementSize, WideTy);
  const SCEV *LastSizeExt = SE.getNoopOrZeroExtend(Sizes.back(), WideTy);
  assert(ElemSizeExt == LastSizeExt && "Unexpected last element of Sizes");
#endif
 
  Sizes.pop_back();
  return true;
}
 
bool IndexedReference::delinearize(const LoopInfo &LI) {
  assert(Subscripts.empty() && "Subscripts should be empty");
  assert(Sizes.empty() && "Sizes should be empty");
  assert(!IsValid && "Should be called once from the constructor");
  LLVM_DEBUG(dbgs() << "Delinearizing: " << StoreOrLoadInst << "\n");
 
  const SCEV *ElemSize = SE.getElementSize(&StoreOrLoadInst);
  const BasicBlock *BB = StoreOrLoadInst.getParent();
 
  if (Loop *L = LI.getLoopFor(BB)) {
    const SCEV *AccessFn =
        SE.getSCEVAtScope(getPointerOperand(&StoreOrLoadInst), L);
 
    BasePointer = dyn_cast<SCEVUnknown>(SE.getPointerBase(AccessFn));
    if (BasePointer == nullptr) {
      LLVM_DEBUG(
          dbgs().indent(2)
          << "ERROR: failed to delinearize, can't identify base pointer\n");
      return false;
    }
 
    bool IsFixedSize = false;
    // Try to delinearize fixed-size arrays.
    if (tryDelinearizeFixedSize(AccessFn, Subscripts, ElemSize)) {
      IsFixedSize = true;
      // The last element of Sizes is the element size.
      Sizes.push_back(ElemSize);
      LLVM_DEBUG(dbgs().indent(2) << "In Loop '" << L->getName()
                                  << "', AccessFn: " << *AccessFn << "\n");
    }
 
    AccessFn = SE.getMinusSCEV(AccessFn, BasePointer);
 
    // Try to delinearize parametric-size arrays.
    if (!IsFixedSize) {
      LLVM_DEBUG(dbgs().indent(2) << "In Loop '" << L->getName()
                                  << "', AccessFn: " << *AccessFn << "\n");
      llvm::delinearize(SE, AccessFn, Subscripts, Sizes,
                        SE.getElementSize(&StoreOrLoadInst));
    }
 
    if (Subscripts.empty() || Sizes.empty() ||
        Subscripts.size() != Sizes.size()) {
      // Attempt to determine whether we have a single dimensional array access.
      // before giving up.
      if (!isOneDimensionalArray(*AccessFn, *ElemSize, *L, SE)) {
        LLVM_DEBUG(dbgs().indent(2)
                   << "ERROR: failed to delinearize reference\n");
        Subscripts.clear();
        Sizes.clear();
        return false;
      }
 
      // The array may be accessed in reverse, for example:
      //   for (i = N; i > 0; i--)
      //     A[i] = 0;
      // In this case, reconstruct the access function using the absolute value
      // of the step recurrence.
      const SCEVAddRecExpr *AccessFnAR = dyn_cast<SCEVAddRecExpr>(AccessFn);
      const SCEV *StepRec = AccessFnAR ? AccessFnAR->getStepRecurrence(SE) : nullptr;
 
      if (StepRec && SE.isKnownNegative(StepRec))
        AccessFn = SE.getAddRecExpr(
            AccessFnAR->getStart(), SE.getNegativeSCEV(StepRec),
            AccessFnAR->getLoop(), SCEV::NoWrapFlags::FlagAnyWrap);
      const SCEV *Div = SE.getUDivExactExpr(AccessFn, ElemSize);
      Subscripts.push_back(Div);
      Sizes.push_back(ElemSize);
    }
 
    return all_of(Subscripts, [&](const SCEV *Subscript) {
      return isSimpleAddRecurrence(*Subscript, *L);
    });
  }
 
  return false;
}
 
bool IndexedReference::isLoopInvariant(const Loop &L) const {
  Value *Addr = getPointerOperand(&StoreOrLoadInst);
  assert(Addr != nullptr && "Expecting either a load or a store instruction");
  assert(SE.isSCEVable(Addr->getType()) && "Addr should be SCEVable");
 
  if (SE.isLoopInvariant(SE.getSCEV(Addr), &L))
    return true;
 
  // The indexed reference is loop invariant if none of the coefficients use
  // the loop induction variable.
  bool allCoeffForLoopAreZero = all_of(Subscripts, [&](const SCEV *Subscript) {
    return isCoeffForLoopZeroOrInvariant(*Subscript, L);
  });
 
  return allCoeffForLoopAreZero;
}
 
bool IndexedReference::isConsecutive(const Loop &L, const SCEV *&Stride,
                                     unsigned CLS) const {
  // The indexed reference is 'consecutive' if the only coefficient that uses
  // the loop induction variable is the last one...
  const SCEV *LastSubscript = Subscripts.back();
  for (const SCEV *Subscript : Subscripts) {
    if (Subscript == LastSubscript)
      continue;
    if (!isCoeffForLoopZeroOrInvariant(*Subscript, L))
      return false;
  }
 
  // ...and the access stride is less than the cache line size.
  const SCEV *Coeff = getLastCoefficient();
  const SCEV *ElemSize = Sizes.back();
  Type *WiderType = SE.getWiderType(Coeff->getType(), ElemSize->getType());
  // FIXME: This assumes that all values are signed integers which may
  // be incorrect in unusual codes and incorrectly use sext instead of zext.
  // for (uint32_t i = 0; i < 512; ++i) {
  //   uint8_t trunc = i;
  //   A[trunc] = 42;
  // }
  // This consecutively iterates twice over A. If `trunc` is sign-extended,
  // we would conclude that this may iterate backwards over the array.
  // However, LoopCacheAnalysis is heuristic anyway and transformations must
  // not result in wrong optimizations if the heuristic was incorrect.
  Stride = SE.getMulExpr(SE.getNoopOrSignExtend(Coeff, WiderType),
                         SE.getNoopOrSignExtend(ElemSize, WiderType));
  const SCEV *CacheLineSize = SE.getConstant(Stride->getType(), CLS);
 
  Stride = SE.isKnownNegative(Stride) ? SE.getNegativeSCEV(Stride) : Stride;
  return SE.isKnownPredicate(ICmpInst::ICMP_ULT, Stride, CacheLineSize);
}
 
int IndexedReference::getSubscriptIndex(const Loop &L) const {
  for (auto Idx : seq<int>(0, getNumSubscripts())) {
    const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(getSubscript(Idx));
    if (AR && AR->getLoop() == &L) {
      return Idx;
    }
  }
  return -1;
}
 
const SCEV *IndexedReference::getLastCoefficient() const {
  const SCEV *LastSubscript = getLastSubscript();
  auto *AR = cast<SCEVAddRecExpr>(LastSubscript);
  return AR->getStepRecurrence(SE);
}
 
bool IndexedReference::isCoeffForLoopZeroOrInvariant(const SCEV &Subscript,
                                                     const Loop &L) const {
  const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(&Subscript);
  return (AR != nullptr) ? AR->getLoop() != &L
                         : SE.isLoopInvariant(&Subscript, &L);
}
 
bool IndexedReference::isSimpleAddRecurrence(const SCEV &Subscript,
                                             const Loop &L) const {
  if (!isa<SCEVAddRecExpr>(Subscript))
    return false;
 
  const SCEVAddRecExpr *AR = cast<SCEVAddRecExpr>(&Subscript);
  assert(AR->getLoop() && "AR should have a loop");
 
  if (!AR->isAffine())
    return false;
 
  const SCEV *Start = AR->getStart();
  const SCEV *Step = AR->getStepRecurrence(SE);
 
  if (!SE.isLoopInvariant(Start, &L) || !SE.isLoopInvariant(Step, &L))
    return false;
 
  return true;
}
 
bool IndexedReference::isAliased(const IndexedReference &Other,
                                 AAResults &AA) const {
  const auto &Loc1 = MemoryLocation::get(&StoreOrLoadInst);
  const auto &Loc2 = MemoryLocation::get(&Other.StoreOrLoadInst);
  return AA.isMustAlias(Loc1, Loc2);
}
 
//===----------------------------------------------------------------------===//
// CacheCost implementation
//
raw_ostream &llvm::operator<<(raw_ostream &OS, const CacheCost &CC) {
  for (const auto &LC : CC.LoopCosts) {
    const Loop *L = LC.first;
    OS << "Loop '" << L->getName() << "' has cost = " << LC.second << "\n";
  }
  return OS;
}
 
CacheCost::CacheCost(const LoopVectorTy &Loops, const LoopInfo &LI,
                     ScalarEvolution &SE, TargetTransformInfo &TTI,
                     AAResults &AA, DependenceInfo &DI,
                     std::optional<unsigned> TRT)
    : Loops(Loops), TRT(TRT.value_or(TemporalReuseThreshold)), LI(LI), SE(SE),
      TTI(TTI), AA(AA), DI(DI) {
  assert(!Loops.empty() && "Expecting a non-empty loop vector.");
 
  for (const Loop *L : Loops) {
    unsigned TripCount = SE.getSmallConstantTripCount(L);
    TripCount = (TripCount == 0) ? DefaultTripCount : TripCount;
    TripCounts.push_back({L, TripCount});
  }
 
  calculateCacheFootprint();
}
 
std::unique_ptr<CacheCost>
CacheCost::getCacheCost(Loop &Root, LoopStandardAnalysisResults &AR,
                        DependenceInfo &DI, std::optional<unsigned> TRT) {
  if (!Root.isOutermost()) {
    LLVM_DEBUG(dbgs() << "Expecting the outermost loop in a loop nest\n");
    return nullptr;
  }
 
  LoopVectorTy Loops;
  append_range(Loops, breadth_first(&Root));
 
  if (!getInnerMostLoop(Loops)) {
    LLVM_DEBUG(dbgs() << "Cannot compute cache cost of loop nest with more "
                         "than one innermost loop\n");
    return nullptr;
  }
 
  return std::make_unique<CacheCost>(Loops, AR.LI, AR.SE, AR.TTI, AR.AA, DI, TRT);
}
 
void CacheCost::calculateCacheFootprint() {
  LLVM_DEBUG(dbgs() << "POPULATING REFERENCE GROUPS\n");
  ReferenceGroupsTy RefGroups;
  if (!populateReferenceGroups(RefGroups))
    return;
 
  LLVM_DEBUG(dbgs() << "COMPUTING LOOP CACHE COSTS\n");
  for (const Loop *L : Loops) {
    assert(llvm::none_of(
               LoopCosts,
               [L](const LoopCacheCostTy &LCC) { return LCC.first == L; }) &&
           "Should not add duplicate element");
    CacheCostTy LoopCost = computeLoopCacheCost(*L, RefGroups);
    LoopCosts.push_back(std::make_pair(L, LoopCost));
  }
 
  sortLoopCosts();
  RefGroups.clear();
}
 
bool CacheCost::populateReferenceGroups(ReferenceGroupsTy &RefGroups) const {
  assert(RefGroups.empty() && "Reference groups should be empty");
 
  unsigned CLS = TTI.getCacheLineSize();
  Loop *InnerMostLoop = getInnerMostLoop(Loops);
  assert(InnerMostLoop != nullptr && "Expecting a valid innermost loop");
 
  for (BasicBlock *BB : InnerMostLoop->getBlocks()) {
    for (Instruction &I : *BB) {
      if (!isa<StoreInst>(I) && !isa<LoadInst>(I))
        continue;
 
      std::unique_ptr<IndexedReference> R(new IndexedReference(I, LI, SE));
      if (!R->isValid())
        continue;
 
      bool Added = false;
      for (ReferenceGroupTy &RefGroup : RefGroups) {
        const IndexedReference &Representative = *RefGroup.front();
        LLVM_DEBUG({
          dbgs() << "References:\n";
          dbgs().indent(2) << *R << "\n";
          dbgs().indent(2) << Representative << "\n";
        });
 
 
       // FIXME: Both positive and negative access functions will be placed
       // into the same reference group, resulting in a bi-directional array
       // access such as:
       //   for (i = N; i > 0; i--)
       //     A[i] = A[N - i];
       // having the same cost calculation as a single dimention access pattern
       //   for (i = 0; i < N; i++)
       //     A[i] = A[i];
       // when in actuality, depending on the array size, the first example
       // should have a cost closer to 2x the second due to the two cache
       // access per iteration from opposite ends of the array
        std::optional<bool> HasTemporalReuse =
            R->hasTemporalReuse(Representative, *TRT, *InnerMostLoop, DI, AA);
        std::optional<bool> HasSpacialReuse =
            R->hasSpacialReuse(Representative, CLS, AA);
 
        if ((HasTemporalReuse && *HasTemporalReuse) ||
            (HasSpacialReuse && *HasSpacialReuse)) {
          RefGroup.push_back(std::move(R));
          Added = true;
          break;
        }
      }
 
      if (!Added) {
        ReferenceGroupTy RG;
        RG.push_back(std::move(R));
        RefGroups.push_back(std::move(RG));
      }
    }
  }
 
  if (RefGroups.empty())
    return false;
 
  LLVM_DEBUG({
    dbgs() << "\nIDENTIFIED REFERENCE GROUPS:\n";
    int n = 1;
    for (const ReferenceGroupTy &RG : RefGroups) {
      dbgs().indent(2) << "RefGroup " << n << ":\n";
      for (const auto &IR : RG)
        dbgs().indent(4) << *IR << "\n";
      n++;
    }
    dbgs() << "\n";
  });
 
  return true;
}
 
CacheCostTy
CacheCost::computeLoopCacheCost(const Loop &L,
                                const ReferenceGroupsTy &RefGroups) const {
  if (!L.isLoopSimplifyForm())
    return CacheCostTy::getInvalid();
 
  LLVM_DEBUG(dbgs() << "Considering loop '" << L.getName()
                    << "' as innermost loop.\n");
 
  // Compute the product of the trip counts of each other loop in the nest.
  CacheCostTy TripCountsProduct = 1;
  for (const auto &TC : TripCounts) {
    if (TC.first == &L)
      continue;
    TripCountsProduct *= TC.second;
  }
 
  CacheCostTy LoopCost = 0;
  for (const ReferenceGroupTy &RG : RefGroups) {
    CacheCostTy RefGroupCost = computeRefGroupCacheCost(RG, L);
    LoopCost += RefGroupCost * TripCountsProduct;
  }
 
  LLVM_DEBUG(dbgs().indent(2) << "Loop '" << L.getName()
                              << "' has cost=" << LoopCost << "\n");
 
  return LoopCost;
}
 
CacheCostTy CacheCost::computeRefGroupCacheCost(const ReferenceGroupTy &RG,
                                                const Loop &L) const {
  assert(!RG.empty() && "Reference group should have at least one member.");
 
  const IndexedReference *Representative = RG.front().get();
  return Representative->computeRefCost(L, TTI.getCacheLineSize());
}
 
//===----------------------------------------------------------------------===//
// LoopCachePrinterPass implementation
//
PreservedAnalyses LoopCachePrinterPass::run(Loop &L, LoopAnalysisManager &AM,
                                            LoopStandardAnalysisResults &AR,
                                            LPMUpdater &U) {
  Function *F = L.getHeader()->getParent();
  DependenceInfo DI(F, &AR.AA, &AR.SE, &AR.LI);
 
  if (auto CC = CacheCost::getCacheCost(L, AR, DI))
    OS << *CC;
 
  return PreservedAnalyses::all();
}