Transforms/Scalar/LoopUnrollPass.cpp

//===-- LoopUnroll.cpp - Loop unroller pass -------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass implements a simple loop unroller.  It works best when loops have
// been canonicalized by the -indvars pass, allowing it to determine the trip
// counts of loops easily.
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/InstVisitor.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Metadata.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/UnrollLoop.h"
#include <climits>

using namespace llvm;

#define DEBUG_TYPE "loop-unroll"

static cl::opt<unsigned>
    UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
                    cl::desc("The baseline cost threshold for loop unrolling"));

static cl::opt<unsigned> UnrollPercentDynamicCostSavedThreshold(
    "unroll-percent-dynamic-cost-saved-threshold", cl::init(20), cl::Hidden,
    cl::desc("The percentage of estimated dynamic cost which must be saved by "
             "unrolling to allow unrolling up to the max threshold."));

static cl::opt<unsigned> UnrollDynamicCostSavingsDiscount(
    "unroll-dynamic-cost-savings-discount", cl::init(2000), cl::Hidden,
    cl::desc("This is the amount discounted from the total unroll cost when "
             "the unrolled form has a high dynamic cost savings (triggered by "
             "the '-unroll-perecent-dynamic-cost-saved-threshold' flag)."));

static cl::opt<unsigned> UnrollMaxIterationsCountToAnalyze(
    "unroll-max-iteration-count-to-analyze", cl::init(0), cl::Hidden,
    cl::desc("Don't allow loop unrolling to simulate more than this number of"
             "iterations when checking full unroll profitability"));

static cl::opt<unsigned>
UnrollCount("unroll-count", cl::init(0), cl::Hidden,
  cl::desc("Use this unroll count for all loops including those with "
           "unroll_count pragma values, for testing purposes"));

static cl::opt<bool>
UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
  cl::desc("Allows loops to be partially unrolled until "
           "-unroll-threshold loop size is reached."));

static cl::opt<bool>
UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
  cl::desc("Unroll loops with run-time trip counts"));

static cl::opt<unsigned>
PragmaUnrollThreshold("pragma-unroll-threshold", cl::init(16 * 1024), cl::Hidden,
  cl::desc("Unrolled size limit for loops with an unroll(full) or "
           "unroll_count pragma."));

namespace {
  class LoopUnroll : public LoopPass {
  public:
    static char ID; // Pass ID, replacement for typeid
    LoopUnroll(int T = -1, int C = -1, int P = -1, int R = -1) : LoopPass(ID) {
      CurrentThreshold = (T == -1) ? UnrollThreshold : unsigned(T);
      CurrentPercentDynamicCostSavedThreshold =
          UnrollPercentDynamicCostSavedThreshold;
      CurrentDynamicCostSavingsDiscount = UnrollDynamicCostSavingsDiscount;
      CurrentCount = (C == -1) ? UnrollCount : unsigned(C);
      CurrentAllowPartial = (P == -1) ? UnrollAllowPartial : (bool)P;
      CurrentRuntime = (R == -1) ? UnrollRuntime : (bool)R;

      UserThreshold = (T != -1) || (UnrollThreshold.getNumOccurrences() > 0);
      UserPercentDynamicCostSavedThreshold =
          (UnrollPercentDynamicCostSavedThreshold.getNumOccurrences() > 0);
      UserDynamicCostSavingsDiscount =
          (UnrollDynamicCostSavingsDiscount.getNumOccurrences() > 0);
      UserAllowPartial = (P != -1) ||
                         (UnrollAllowPartial.getNumOccurrences() > 0);
      UserRuntime = (R != -1) || (UnrollRuntime.getNumOccurrences() > 0);
      UserCount = (C != -1) || (UnrollCount.getNumOccurrences() > 0);

      initializeLoopUnrollPass(*PassRegistry::getPassRegistry());
    }

    /// A magic value for use with the Threshold parameter to indicate
    /// that the loop unroll should be performed regardless of how much
    /// code expansion would result.
    static const unsigned NoThreshold = UINT_MAX;

    // Threshold to use when optsize is specified (and there is no
    // explicit -unroll-threshold).
    static const unsigned OptSizeUnrollThreshold = 50;

    // Default unroll count for loops with run-time trip count if
    // -unroll-count is not set
    static const unsigned UnrollRuntimeCount = 8;

    unsigned CurrentCount;
    unsigned CurrentThreshold;
    unsigned CurrentPercentDynamicCostSavedThreshold;
    unsigned CurrentDynamicCostSavingsDiscount;
    bool CurrentAllowPartial;
    bool CurrentRuntime;

    // Flags for whether the 'current' settings are user-specified.
    bool UserCount;
    bool UserThreshold;
    bool UserPercentDynamicCostSavedThreshold;
    bool UserDynamicCostSavingsDiscount;
    bool UserAllowPartial;
    bool UserRuntime;

    bool runOnLoop(Loop *L, LPPassManager &LPM) override;

    /// This transformation requires natural loop information & requires that
    /// loop preheaders be inserted into the CFG...
    ///
    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.addRequired<AssumptionCacheTracker>();
      AU.addRequired<LoopInfoWrapperPass>();
      AU.addPreserved<LoopInfoWrapperPass>();
      AU.addRequiredID(LoopSimplifyID);
      AU.addPreservedID(LoopSimplifyID);
      AU.addRequiredID(LCSSAID);
      AU.addPreservedID(LCSSAID);
      AU.addRequired<ScalarEvolution>();
      AU.addPreserved<ScalarEvolution>();
      AU.addRequired<TargetTransformInfoWrapperPass>();
      // FIXME: Loop unroll requires LCSSA. And LCSSA requires dom info.
      // If loop unroll does not preserve dom info then LCSSA pass on next
      // loop will receive invalid dom info.
      // For now, recreate dom info, if loop is unrolled.
      AU.addPreserved<DominatorTreeWrapperPass>();
    }

    // Fill in the UnrollingPreferences parameter with values from the
    // TargetTransformationInfo.
    void getUnrollingPreferences(Loop *L, const TargetTransformInfo &TTI,
                                 TargetTransformInfo::UnrollingPreferences &UP) {
      UP.Threshold = CurrentThreshold;
      UP.PercentDynamicCostSavedThreshold =
          CurrentPercentDynamicCostSavedThreshold;
      UP.DynamicCostSavingsDiscount = CurrentDynamicCostSavingsDiscount;
      UP.OptSizeThreshold = OptSizeUnrollThreshold;
      UP.PartialThreshold = CurrentThreshold;
      UP.PartialOptSizeThreshold = OptSizeUnrollThreshold;
      UP.Count = CurrentCount;
      UP.MaxCount = UINT_MAX;
      UP.Partial = CurrentAllowPartial;
      UP.Runtime = CurrentRuntime;
      UP.AllowExpensiveTripCount = false;
      TTI.getUnrollingPreferences(L, UP);
    }

    // Select and return an unroll count based on parameters from
    // user, unroll preferences, unroll pragmas, or a heuristic.
    // SetExplicitly is set to true if the unroll count is is set by
    // the user or a pragma rather than selected heuristically.
    unsigned
    selectUnrollCount(const Loop *L, unsigned TripCount, bool PragmaFullUnroll,
                      unsigned PragmaCount,
                      const TargetTransformInfo::UnrollingPreferences &UP,
                      bool &SetExplicitly);

    // Select threshold values used to limit unrolling based on a
    // total unrolled size.  Parameters Threshold and PartialThreshold
    // are set to the maximum unrolled size for fully and partially
    // unrolled loops respectively.
    void selectThresholds(const Loop *L, bool HasPragma,
                          const TargetTransformInfo::UnrollingPreferences &UP,
                          unsigned &Threshold, unsigned &PartialThreshold,
                          unsigned &PercentDynamicCostSavedThreshold,
                          unsigned &DynamicCostSavingsDiscount) {
      // Determine the current unrolling threshold.  While this is
      // normally set from UnrollThreshold, it is overridden to a
      // smaller value if the current function is marked as
      // optimize-for-size, and the unroll threshold was not user
      // specified.
      Threshold = UserThreshold ? CurrentThreshold : UP.Threshold;
      PartialThreshold = UserThreshold ? CurrentThreshold : UP.PartialThreshold;
      PercentDynamicCostSavedThreshold =
          UserPercentDynamicCostSavedThreshold
              ? CurrentPercentDynamicCostSavedThreshold
              : UP.PercentDynamicCostSavedThreshold;
      DynamicCostSavingsDiscount = UserDynamicCostSavingsDiscount
                                       ? CurrentDynamicCostSavingsDiscount
                                       : UP.DynamicCostSavingsDiscount;

      if (!UserThreshold &&
          L->getHeader()->getParent()->hasFnAttribute(
              Attribute::OptimizeForSize)) {
        Threshold = UP.OptSizeThreshold;
        PartialThreshold = UP.PartialOptSizeThreshold;
      }
      if (HasPragma) {
        // If the loop has an unrolling pragma, we want to be more
        // aggressive with unrolling limits.  Set thresholds to at
        // least the PragmaTheshold value which is larger than the
        // default limits.
        if (Threshold != NoThreshold)
          Threshold = std::max<unsigned>(Threshold, PragmaUnrollThreshold);
        if (PartialThreshold != NoThreshold)
          PartialThreshold =
              std::max<unsigned>(PartialThreshold, PragmaUnrollThreshold);
      }
    }
    bool canUnrollCompletely(Loop *L, unsigned Threshold,
                             unsigned PercentDynamicCostSavedThreshold,
                             unsigned DynamicCostSavingsDiscount,
                             uint64_t UnrolledCost, uint64_t RolledDynamicCost);
  };
}

char LoopUnroll::ID = 0;
INITIALIZE_PASS_BEGIN(LoopUnroll, "loop-unroll", "Unroll loops", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
INITIALIZE_PASS_DEPENDENCY(LCSSA)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_PASS_END(LoopUnroll, "loop-unroll", "Unroll loops", false, false)

Pass *llvm::createLoopUnrollPass(int Threshold, int Count, int AllowPartial,
                                 int Runtime) {
  return new LoopUnroll(Threshold, Count, AllowPartial, Runtime);
}

Pass *llvm::createSimpleLoopUnrollPass() {
  return llvm::createLoopUnrollPass(-1, -1, 0, 0);
}

namespace {
// This class is used to get an estimate of the optimization effects that we
// could get from complete loop unrolling. It comes from the fact that some
// loads might be replaced with concrete constant values and that could trigger
// a chain of instruction simplifications.
//
// E.g. we might have:
//   int a[] = {0, 1, 0};
//   v = 0;
//   for (i = 0; i < 3; i ++)
//     v += b[i]*a[i];
// If we completely unroll the loop, we would get:
//   v = b[0]*a[0] + b[1]*a[1] + b[2]*a[2]
// Which then will be simplified to:
//   v = b[0]* 0 + b[1]* 1 + b[2]* 0
// And finally:
//   v = b[1]
class UnrolledInstAnalyzer : private InstVisitor<UnrolledInstAnalyzer, bool> {
  typedef InstVisitor<UnrolledInstAnalyzer, bool> Base;
  friend class InstVisitor<UnrolledInstAnalyzer, bool>;
  struct SimplifiedAddress {
    Value *Base = nullptr;
    ConstantInt *Offset = nullptr;
  };

public:
  UnrolledInstAnalyzer(unsigned Iteration,
                       DenseMap<Value *, Constant *> &SimplifiedValues,
                       const Loop *L, ScalarEvolution &SE)
      : Iteration(Iteration), SimplifiedValues(SimplifiedValues), L(L), SE(SE) {
      IterationNumber = SE.getConstant(APInt(64, Iteration));
  }

  // Allow access to the initial visit method.
  using Base::visit;

private:
  /// \brief A cache of pointer bases and constant-folded offsets corresponding
  /// to GEP (or derived from GEP) instructions.
  ///
  /// In order to find the base pointer one needs to perform non-trivial
  /// traversal of the corresponding SCEV expression, so it's good to have the
  /// results saved.
  DenseMap<Value *, SimplifiedAddress> SimplifiedAddresses;

  /// \brief Number of currently simulated iteration.
  ///
  /// If an expression is ConstAddress+Constant, then the Constant is
  /// Start + Iteration*Step, where Start and Step could be obtained from
  /// SCEVGEPCache.
  unsigned Iteration;

  /// \brief SCEV expression corresponding to number of currently simulated
  /// iteration.
  const SCEV *IterationNumber;

  /// \brief A Value->Constant map for keeping values that we managed to
  /// constant-fold on the given iteration.
  ///
  /// While we walk the loop instructions, we build up and maintain a mapping
  /// of simplified values specific to this iteration.  The idea is to propagate
  /// any special information we have about loads that can be replaced with
  /// constants after complete unrolling, and account for likely simplifications
  /// post-unrolling.
  DenseMap<Value *, Constant *> &SimplifiedValues;

  const Loop *L;
  ScalarEvolution &SE;

  /// \brief Try to simplify instruction \param I using its SCEV expression.
  ///
  /// The idea is that some AddRec expressions become constants, which then
  /// could trigger folding of other instructions. However, that only happens
  /// for expressions whose start value is also constant, which isn't always the
  /// case. In another common and important case the start value is just some
  /// address (i.e. SCEVUnknown) - in this case we compute the offset and save
  /// it along with the base address instead.
  bool simplifyInstWithSCEV(Instruction *I) {
    if (!SE.isSCEVable(I->getType()))
      return false;

    const SCEV *S = SE.getSCEV(I);
    if (auto *SC = dyn_cast<SCEVConstant>(S)) {
      SimplifiedValues[I] = SC->getValue();
      return true;
    }

    auto *AR = dyn_cast<SCEVAddRecExpr>(S);
    if (!AR)
      return false;

    const SCEV *ValueAtIteration = AR->evaluateAtIteration(IterationNumber, SE);
    // Check if the AddRec expression becomes a constant.
    if (auto *SC = dyn_cast<SCEVConstant>(ValueAtIteration)) {
      SimplifiedValues[I] = SC->getValue();
      return true;
    }

    // Check if the offset from the base address becomes a constant.
    auto *Base = dyn_cast<SCEVUnknown>(SE.getPointerBase(S));
    if (!Base)
      return false;
    auto *Offset =
        dyn_cast<SCEVConstant>(SE.getMinusSCEV(ValueAtIteration, Base));
    if (!Offset)
      return false;
    SimplifiedAddress Address;
    Address.Base = Base->getValue();
    Address.Offset = Offset->getValue();
    SimplifiedAddresses[I] = Address;
    return true;
  }

  /// Base case for the instruction visitor.
  bool visitInstruction(Instruction &I) {
    return simplifyInstWithSCEV(&I);
  }

  /// Try to simplify binary operator I.
  ///
  /// TODO: Probaly it's worth to hoist the code for estimating the
  /// simplifications effects to a separate class, since we have a very similar
  /// code in InlineCost already.
  bool visitBinaryOperator(BinaryOperator &I) {
    Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
    if (!isa<Constant>(LHS))
      if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
        LHS = SimpleLHS;
    if (!isa<Constant>(RHS))
      if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
        RHS = SimpleRHS;

    Value *SimpleV = nullptr;
    const DataLayout &DL = I.getModule()->getDataLayout();
    if (auto FI = dyn_cast<FPMathOperator>(&I))
      SimpleV =
          SimplifyFPBinOp(I.getOpcode(), LHS, RHS, FI->getFastMathFlags(), DL);
    else
      SimpleV = SimplifyBinOp(I.getOpcode(), LHS, RHS, DL);

    if (Constant *C = dyn_cast_or_null<Constant>(SimpleV))
      SimplifiedValues[&I] = C;

    if (SimpleV)
      return true;
    return Base::visitBinaryOperator(I);
  }

  /// Try to fold load I.
  bool visitLoad(LoadInst &I) {
    Value *AddrOp = I.getPointerOperand();

    auto AddressIt = SimplifiedAddresses.find(AddrOp);
    if (AddressIt == SimplifiedAddresses.end())
      return false;
    ConstantInt *SimplifiedAddrOp = AddressIt->second.Offset;

    auto *GV = dyn_cast<GlobalVariable>(AddressIt->second.Base);
    // We're only interested in loads that can be completely folded to a
    // constant.
    if (!GV || !GV->hasInitializer())
      return false;

    ConstantDataSequential *CDS =
        dyn_cast<ConstantDataSequential>(GV->getInitializer());
    if (!CDS)
      return false;

    int ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
    assert(SimplifiedAddrOp->getValue().getActiveBits() < 64 &&
           "Unexpectedly large index value.");
    int64_t Index = SimplifiedAddrOp->getSExtValue() / ElemSize;
    if (Index >= CDS->getNumElements()) {
      // FIXME: For now we conservatively ignore out of bound accesses, but
      // we're allowed to perform the optimization in this case.
      return false;
    }

    Constant *CV = CDS->getElementAsConstant(Index);
    assert(CV && "Constant expected.");
    SimplifiedValues[&I] = CV;

    return true;
  }

  bool visitCastInst(CastInst &I) {
    // Propagate constants through casts.
    Constant *COp = dyn_cast<Constant>(I.getOperand(0));
    if (!COp)
      COp = SimplifiedValues.lookup(I.getOperand(0));
    if (COp)
      if (Constant *C =
              ConstantExpr::getCast(I.getOpcode(), COp, I.getType())) {
        SimplifiedValues[&I] = C;
        return true;
      }

    return Base::visitCastInst(I);
  }

  bool visitCmpInst(CmpInst &I) {
    Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);

    // First try to handle simplified comparisons.
    if (!isa<Constant>(LHS))
      if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
        LHS = SimpleLHS;
    if (!isa<Constant>(RHS))
      if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
        RHS = SimpleRHS;

    if (!isa<Constant>(LHS) && !isa<Constant>(RHS)) {
      auto SimplifiedLHS = SimplifiedAddresses.find(LHS);
      if (SimplifiedLHS != SimplifiedAddresses.end()) {
        auto SimplifiedRHS = SimplifiedAddresses.find(RHS);
        if (SimplifiedRHS != SimplifiedAddresses.end()) {
          SimplifiedAddress &LHSAddr = SimplifiedLHS->second;
          SimplifiedAddress &RHSAddr = SimplifiedRHS->second;
          if (LHSAddr.Base == RHSAddr.Base) {
            LHS = LHSAddr.Offset;
            RHS = RHSAddr.Offset;
          }
        }
      }
    }

    if (Constant *CLHS = dyn_cast<Constant>(LHS)) {
      if (Constant *CRHS = dyn_cast<Constant>(RHS)) {
        if (Constant *C = ConstantExpr::getCompare(I.getPredicate(), CLHS, CRHS)) {
          SimplifiedValues[&I] = C;
          return true;
        }
      }
    }

    return Base::visitCmpInst(I);
  }
};
} // namespace


namespace {
struct EstimatedUnrollCost {
  /// \brief The estimated cost after unrolling.
  unsigned UnrolledCost;

  /// \brief The estimated dynamic cost of executing the instructions in the
  /// rolled form.
  unsigned RolledDynamicCost;
};
}

/// \brief Figure out if the loop is worth full unrolling.
///
/// Complete loop unrolling can make some loads constant, and we need to know
/// if that would expose any further optimization opportunities.  This routine
/// estimates this optimization.  It computes cost of unrolled loop
/// (UnrolledCost) and dynamic cost of the original loop (RolledDynamicCost). By
/// dynamic cost we mean that we won't count costs of blocks that are known not
/// to be executed (i.e. if we have a branch in the loop and we know that at the
/// given iteration its condition would be resolved to true, we won't add up the
/// cost of the 'false'-block).
/// \returns Optional value, holding the RolledDynamicCost and UnrolledCost. If
/// the analysis failed (no benefits expected from the unrolling, or the loop is
/// too big to analyze), the returned value is None.
Optional<EstimatedUnrollCost>
analyzeLoopUnrollCost(const Loop *L, unsigned TripCount, ScalarEvolution &SE,
                      const TargetTransformInfo &TTI,
                      unsigned MaxUnrolledLoopSize) {
  // We want to be able to scale offsets by the trip count and add more offsets
  // to them without checking for overflows, and we already don't want to
  // analyze *massive* trip counts, so we force the max to be reasonably small.
  assert(UnrollMaxIterationsCountToAnalyze < (INT_MAX / 2) &&
         "The unroll iterations max is too large!");

  // Don't simulate loops with a big or unknown tripcount
  if (!UnrollMaxIterationsCountToAnalyze || !TripCount ||
      TripCount > UnrollMaxIterationsCountToAnalyze)
    return None;

  SmallSetVector<BasicBlock *, 16> BBWorklist;
  DenseMap<Value *, Constant *> SimplifiedValues;

  // The estimated cost of the unrolled form of the loop. We try to estimate
  // this by simplifying as much as we can while computing the estimate.
  unsigned UnrolledCost = 0;
  // We also track the estimated dynamic (that is, actually executed) cost in
  // the rolled form. This helps identify cases when the savings from unrolling
  // aren't just exposing dead control flows, but actual reduced dynamic
  // instructions due to the simplifications which we expect to occur after
  // unrolling.
  unsigned RolledDynamicCost = 0;

  DEBUG(dbgs() << "Starting LoopUnroll profitability analysis...\n");

  // Simulate execution of each iteration of the loop counting instructions,
  // which would be simplified.
  // Since the same load will take different values on different iterations,
  // we literally have to go through all loop's iterations.
  for (unsigned Iteration = 0; Iteration < TripCount; ++Iteration) {
    DEBUG(dbgs() << " Analyzing iteration " << Iteration << "\n");
    SimplifiedValues.clear();
    UnrolledInstAnalyzer Analyzer(Iteration, SimplifiedValues, L, SE);

    BBWorklist.clear();
    BBWorklist.insert(L->getHeader());
    // Note that we *must not* cache the size, this loop grows the worklist.
    for (unsigned Idx = 0; Idx != BBWorklist.size(); ++Idx) {
      BasicBlock *BB = BBWorklist[Idx];

      // Visit all instructions in the given basic block and try to simplify
      // it.  We don't change the actual IR, just count optimization
      // opportunities.
      for (Instruction &I : *BB) {
        unsigned InstCost = TTI.getUserCost(&I);

        // Visit the instruction to analyze its loop cost after unrolling,
        // and if the visitor returns false, include this instruction in the
        // unrolled cost.
        if (!Analyzer.visit(I))
          UnrolledCost += InstCost;
        else {
          DEBUG(dbgs() << "  " << I
                       << " would be simplified if loop is unrolled.\n");
          (void)0;
        }

        // Also track this instructions expected cost when executing the rolled
        // loop form.
        RolledDynamicCost += InstCost;

        // If unrolled body turns out to be too big, bail out.
        if (UnrolledCost > MaxUnrolledLoopSize) {
          DEBUG(dbgs() << "  Exceeded threshold.. exiting.\n"
                       << "  UnrolledCost: " << UnrolledCost
                       << ", MaxUnrolledLoopSize: " << MaxUnrolledLoopSize
                       << "\n");
          return None;
        }
      }

      TerminatorInst *TI = BB->getTerminator();

      // Add in the live successors by first checking whether we have terminator
      // that may be simplified based on the values simplified by this call.
      if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
        if (BI->isConditional()) {
          if (Constant *SimpleCond =
                  SimplifiedValues.lookup(BI->getCondition())) {
            BasicBlock *Succ = nullptr;
            // Just take the first successor if condition is undef
            if (isa<UndefValue>(SimpleCond))
              Succ = BI->getSuccessor(0);
            else
              Succ = BI->getSuccessor(
                  cast<ConstantInt>(SimpleCond)->isZero() ? 1 : 0);
            if (L->contains(Succ))
              BBWorklist.insert(Succ);
            continue;
          }
        }
      } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
        if (Constant *SimpleCond =
                SimplifiedValues.lookup(SI->getCondition())) {
          BasicBlock *Succ = nullptr;
          // Just take the first successor if condition is undef
          if (isa<UndefValue>(SimpleCond))
            Succ = SI->getSuccessor(0);
          else
            Succ =
                SI->getSuccessor(cast<ConstantInt>(SimpleCond)->getSExtValue());
          if (L->contains(Succ))
            BBWorklist.insert(Succ);
          continue;
        }
      }

      // Add BB's successors to the worklist.
      for (BasicBlock *Succ : successors(BB))
        if (L->contains(Succ))
          BBWorklist.insert(Succ);
    }

    // If we found no optimization opportunities on the first iteration, we
    // won't find them on later ones too.
    if (UnrolledCost == RolledDynamicCost) {
      DEBUG(dbgs() << "  No opportunities found.. exiting.\n"
                   << "  UnrolledCost: " << UnrolledCost << "\n");
      return None;
    }
  }
  DEBUG(dbgs() << "Analysis finished:\n"
               << "UnrolledCost: " << UnrolledCost << ", "
               << "RolledDynamicCost: " << RolledDynamicCost << "\n");
  return {{UnrolledCost, RolledDynamicCost}};
}

/// ApproximateLoopSize - Approximate the size of the loop.
static unsigned ApproximateLoopSize(const Loop *L, unsigned &NumCalls,
                                    bool &NotDuplicatable,
                                    const TargetTransformInfo &TTI,
                                    AssumptionCache *AC) {
  SmallPtrSet<const Value *, 32> EphValues;
  CodeMetrics::collectEphemeralValues(L, AC, EphValues);

  CodeMetrics Metrics;
  for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
       I != E; ++I)
    Metrics.analyzeBasicBlock(*I, TTI, EphValues);
  NumCalls = Metrics.NumInlineCandidates;
  NotDuplicatable = Metrics.notDuplicatable;

  unsigned LoopSize = Metrics.NumInsts;

  // Don't allow an estimate of size zero.  This would allows unrolling of loops
  // with huge iteration counts, which is a compile time problem even if it's
  // not a problem for code quality. Also, the code using this size may assume
  // that each loop has at least three instructions (likely a conditional
  // branch, a comparison feeding that branch, and some kind of loop increment
  // feeding that comparison instruction).
  LoopSize = std::max(LoopSize, 3u);

  return LoopSize;
}

// Returns the loop hint metadata node with the given name (for example,
// "llvm.loop.unroll.count").  If no such metadata node exists, then nullptr is
// returned.
static MDNode *GetUnrollMetadataForLoop(const Loop *L, StringRef Name) {
  if (MDNode *LoopID = L->getLoopID())
    return GetUnrollMetadata(LoopID, Name);
  return nullptr;
}

// Returns true if the loop has an unroll(full) pragma.
static bool HasUnrollFullPragma(const Loop *L) {
  return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.full");
}

// Returns true if the loop has an unroll(disable) pragma.
static bool HasUnrollDisablePragma(const Loop *L) {
  return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.disable");
}

// Returns true if the loop has an runtime unroll(disable) pragma.
static bool HasRuntimeUnrollDisablePragma(const Loop *L) {
  return GetUnrollMetadataForLoop(L, "llvm.loop.unroll.runtime.disable");
}

// If loop has an unroll_count pragma return the (necessarily
// positive) value from the pragma.  Otherwise return 0.
static unsigned UnrollCountPragmaValue(const Loop *L) {
  MDNode *MD = GetUnrollMetadataForLoop(L, "llvm.loop.unroll.count");
  if (MD) {
    assert(MD->getNumOperands() == 2 &&
           "Unroll count hint metadata should have two operands.");
    unsigned Count =
        mdconst::extract<ConstantInt>(MD->getOperand(1))->getZExtValue();
    assert(Count >= 1 && "Unroll count must be positive.");
    return Count;
  }
  return 0;
}

// Remove existing unroll metadata and add unroll disable metadata to
// indicate the loop has already been unrolled.  This prevents a loop
// from being unrolled more than is directed by a pragma if the loop
// unrolling pass is run more than once (which it generally is).
static void SetLoopAlreadyUnrolled(Loop *L) {
  MDNode *LoopID = L->getLoopID();
  if (!LoopID) return;

  // First remove any existing loop unrolling metadata.
  SmallVector<Metadata *, 4> MDs;
  // Reserve first location for self reference to the LoopID metadata node.
  MDs.push_back(nullptr);
  for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
    bool IsUnrollMetadata = false;
    MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
    if (MD) {
      const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
      IsUnrollMetadata = S && S->getString().startswith("llvm.loop.unroll.");
    }
    if (!IsUnrollMetadata)
      MDs.push_back(LoopID->getOperand(i));
  }

  // Add unroll(disable) metadata to disable future unrolling.
  LLVMContext &Context = L->getHeader()->getContext();
  SmallVector<Metadata *, 1> DisableOperands;
  DisableOperands.push_back(MDString::get(Context, "llvm.loop.unroll.disable"));
  MDNode *DisableNode = MDNode::get(Context, DisableOperands);
  MDs.push_back(DisableNode);

  MDNode *NewLoopID = MDNode::get(Context, MDs);
  // Set operand 0 to refer to the loop id itself.
  NewLoopID->replaceOperandWith(0, NewLoopID);
  L->setLoopID(NewLoopID);
}

bool LoopUnroll::canUnrollCompletely(Loop *L, unsigned Threshold,
                                     unsigned PercentDynamicCostSavedThreshold,
                                     unsigned DynamicCostSavingsDiscount,
                                     uint64_t UnrolledCost,
                                     uint64_t RolledDynamicCost) {

  if (Threshold == NoThreshold) {
    DEBUG(dbgs() << "  Can fully unroll, because no threshold is set.\n");
    return true;
  }

  if (UnrolledCost <= Threshold) {
    DEBUG(dbgs() << "  Can fully unroll, because unrolled cost: "
                 << UnrolledCost << "<" << Threshold << "\n");
    return true;
  }

  assert(UnrolledCost && "UnrolledCost can't be 0 at this point.");
  assert(RolledDynamicCost >= UnrolledCost &&
         "Cannot have a higher unrolled cost than a rolled cost!");

  // Compute the percentage of the dynamic cost in the rolled form that is
  // saved when unrolled. If unrolling dramatically reduces the estimated
  // dynamic cost of the loop, we use a higher threshold to allow more
  // unrolling.
  unsigned PercentDynamicCostSaved =
      (uint64_t)(RolledDynamicCost - UnrolledCost) * 100ull / RolledDynamicCost;

  if (PercentDynamicCostSaved >= PercentDynamicCostSavedThreshold &&
      (int64_t)UnrolledCost - (int64_t)DynamicCostSavingsDiscount <=
          (int64_t)Threshold) {
    DEBUG(dbgs() << "  Can fully unroll, because unrolling will reduce the "
                    "expected dynamic cost by " << PercentDynamicCostSaved
                 << "% (threshold: " << PercentDynamicCostSavedThreshold
                 << "%)\n"
                 << "  and the unrolled cost (" << UnrolledCost
                 << ") is less than the max threshold ("
                 << DynamicCostSavingsDiscount << ").\n");
    return true;
  }

  DEBUG(dbgs() << "  Too large to fully unroll:\n");
  DEBUG(dbgs() << "    Threshold: " << Threshold << "\n");
  DEBUG(dbgs() << "    Max threshold: " << DynamicCostSavingsDiscount << "\n");
  DEBUG(dbgs() << "    Percent cost saved threshold: "
               << PercentDynamicCostSavedThreshold << "%\n");
  DEBUG(dbgs() << "    Unrolled cost: " << UnrolledCost << "\n");
  DEBUG(dbgs() << "    Rolled dynamic cost: " << RolledDynamicCost << "\n");
  DEBUG(dbgs() << "    Percent cost saved: " << PercentDynamicCostSaved
               << "\n");
  return false;
}

unsigned LoopUnroll::selectUnrollCount(
    const Loop *L, unsigned TripCount, bool PragmaFullUnroll,
    unsigned PragmaCount, const TargetTransformInfo::UnrollingPreferences &UP,
    bool &SetExplicitly) {
  SetExplicitly = true;

  // User-specified count (either as a command-line option or
  // constructor parameter) has highest precedence.
  unsigned Count = UserCount ? CurrentCount : 0;

  // If there is no user-specified count, unroll pragmas have the next
  // highest precendence.
  if (Count == 0) {
    if (PragmaCount) {
      Count = PragmaCount;
    } else if (PragmaFullUnroll) {
      Count = TripCount;
    }
  }

  if (Count == 0)
    Count = UP.Count;

  if (Count == 0) {
    SetExplicitly = false;
    if (TripCount == 0)
      // Runtime trip count.
      Count = UnrollRuntimeCount;
    else
      // Conservative heuristic: if we know the trip count, see if we can
      // completely unroll (subject to the threshold, checked below); otherwise
      // try to find greatest modulo of the trip count which is still under
      // threshold value.
      Count = TripCount;
  }
  if (TripCount && Count > TripCount)
    return TripCount;
  return Count;
}

bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
  if (skipOptnoneFunction(L))
    return false;

  Function &F = *L->getHeader()->getParent();

  LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
  ScalarEvolution *SE = &getAnalysis<ScalarEvolution>();
  const TargetTransformInfo &TTI =
      getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
  auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);

  BasicBlock *Header = L->getHeader();
  DEBUG(dbgs() << "Loop Unroll: F[" << Header->getParent()->getName()
        << "] Loop %" << Header->getName() << "\n");

  if (HasUnrollDisablePragma(L)) {
    return false;
  }
  bool PragmaFullUnroll = HasUnrollFullPragma(L);
  unsigned PragmaCount = UnrollCountPragmaValue(L);
  bool HasPragma = PragmaFullUnroll || PragmaCount > 0;

  TargetTransformInfo::UnrollingPreferences UP;
  getUnrollingPreferences(L, TTI, UP);

  // Find trip count and trip multiple if count is not available
  unsigned TripCount = 0;
  unsigned TripMultiple = 1;
  // If there are multiple exiting blocks but one of them is the latch, use the
  // latch for the trip count estimation. Otherwise insist on a single exiting
  // block for the trip count estimation.
  BasicBlock *ExitingBlock = L->getLoopLatch();
  if (!ExitingBlock || !L->isLoopExiting(ExitingBlock))
    ExitingBlock = L->getExitingBlock();
  if (ExitingBlock) {
    TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);
    TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock);
  }

  // Select an initial unroll count.  This may be reduced later based
  // on size thresholds.
  bool CountSetExplicitly;
  unsigned Count = selectUnrollCount(L, TripCount, PragmaFullUnroll,
                                     PragmaCount, UP, CountSetExplicitly);

  unsigned NumInlineCandidates;
  bool notDuplicatable;
  unsigned LoopSize =
      ApproximateLoopSize(L, NumInlineCandidates, notDuplicatable, TTI, &AC);
  DEBUG(dbgs() << "  Loop Size = " << LoopSize << "\n");

  // When computing the unrolled size, note that the conditional branch on the
  // backedge and the comparison feeding it are not replicated like the rest of
  // the loop body (which is why 2 is subtracted).
  uint64_t UnrolledSize = (uint64_t)(LoopSize-2) * Count + 2;
  if (notDuplicatable) {
    DEBUG(dbgs() << "  Not unrolling loop which contains non-duplicatable"
                 << " instructions.\n");
    return false;
  }
  if (NumInlineCandidates != 0) {
    DEBUG(dbgs() << "  Not unrolling loop with inlinable calls.\n");
    return false;
  }

  unsigned Threshold, PartialThreshold;
  unsigned PercentDynamicCostSavedThreshold;
  unsigned DynamicCostSavingsDiscount;
  selectThresholds(L, HasPragma, UP, Threshold, PartialThreshold,
                   PercentDynamicCostSavedThreshold,
                   DynamicCostSavingsDiscount);

  // Given Count, TripCount and thresholds determine the type of
  // unrolling which is to be performed.
  enum { Full = 0, Partial = 1, Runtime = 2 };
  int Unrolling;
  if (TripCount && Count == TripCount) {
    Unrolling = Partial;
    // If the loop is really small, we don't need to run an expensive analysis.
    if (canUnrollCompletely(L, Threshold, 100, DynamicCostSavingsDiscount,
                            UnrolledSize, UnrolledSize)) {
      Unrolling = Full;
    } else {
      // The loop isn't that small, but we still can fully unroll it if that
      // helps to remove a significant number of instructions.
      // To check that, run additional analysis on the loop.
      if (Optional<EstimatedUnrollCost> Cost = analyzeLoopUnrollCost(
              L, TripCount, *SE, TTI, Threshold + DynamicCostSavingsDiscount))
        if (canUnrollCompletely(L, Threshold, PercentDynamicCostSavedThreshold,
                                DynamicCostSavingsDiscount, Cost->UnrolledCost,
                                Cost->RolledDynamicCost)) {
          Unrolling = Full;
        }
    }
  } else if (TripCount && Count < TripCount) {
    Unrolling = Partial;
  } else {
    Unrolling = Runtime;
  }

  // Reduce count based on the type of unrolling and the threshold values.
  unsigned OriginalCount = Count;
  bool AllowRuntime =
      (PragmaCount > 0) || (UserRuntime ? CurrentRuntime : UP.Runtime);
  // Don't unroll a runtime trip count loop with unroll full pragma.
  if (HasRuntimeUnrollDisablePragma(L) || PragmaFullUnroll) {
    AllowRuntime = false;
  }
  if (Unrolling == Partial) {
    bool AllowPartial = UserAllowPartial ? CurrentAllowPartial : UP.Partial;
    if (!AllowPartial && !CountSetExplicitly) {
      DEBUG(dbgs() << "  will not try to unroll partially because "
                   << "-unroll-allow-partial not given\n");
      return false;
    }
    if (PartialThreshold != NoThreshold && UnrolledSize > PartialThreshold) {
      // Reduce unroll count to be modulo of TripCount for partial unrolling.
      Count = (std::max(PartialThreshold, 3u)-2) / (LoopSize-2);
      while (Count != 0 && TripCount % Count != 0)
        Count--;
    }
  } else if (Unrolling == Runtime) {
    if (!AllowRuntime && !CountSetExplicitly) {
      DEBUG(dbgs() << "  will not try to unroll loop with runtime trip count "
                   << "-unroll-runtime not given\n");
      return false;
    }
    // Reduce unroll count to be the largest power-of-two factor of
    // the original count which satisfies the threshold limit.
    while (Count != 0 && UnrolledSize > PartialThreshold) {
      Count >>= 1;
      UnrolledSize = (LoopSize-2) * Count + 2;
    }
    if (Count > UP.MaxCount)
      Count = UP.MaxCount;
    DEBUG(dbgs() << "  partially unrolling with count: " << Count << "\n");
  }

  if (HasPragma) {
    if (PragmaCount != 0)
      // If loop has an unroll count pragma mark loop as unrolled to prevent
      // unrolling beyond that requested by the pragma.
      SetLoopAlreadyUnrolled(L);

    // Emit optimization remarks if we are unable to unroll the loop
    // as directed by a pragma.
    DebugLoc LoopLoc = L->getStartLoc();
    Function *F = Header->getParent();
    LLVMContext &Ctx = F->getContext();
    if (PragmaFullUnroll && PragmaCount == 0) {
      if (TripCount && Count != TripCount) {
        emitOptimizationRemarkMissed(
            Ctx, DEBUG_TYPE, *F, LoopLoc,
            "Unable to fully unroll loop as directed by unroll(full) pragma "
            "because unrolled size is too large.");
      } else if (!TripCount) {
        emitOptimizationRemarkMissed(
            Ctx, DEBUG_TYPE, *F, LoopLoc,
            "Unable to fully unroll loop as directed by unroll(full) pragma "
            "because loop has a runtime trip count.");
      }
    } else if (PragmaCount > 0 && Count != OriginalCount) {
      emitOptimizationRemarkMissed(
          Ctx, DEBUG_TYPE, *F, LoopLoc,
          "Unable to unroll loop the number of times directed by "
          "unroll_count pragma because unrolled size is too large.");
    }
  }

  if (Unrolling != Full && Count < 2) {
    // Partial unrolling by 1 is a nop.  For full unrolling, a factor
    // of 1 makes sense because loop control can be eliminated.
    return false;
  }

  // Unroll the loop.
  if (!UnrollLoop(L, Count, TripCount, AllowRuntime, UP.AllowExpensiveTripCount,
                  TripMultiple, LI, this, &LPM, &AC))
    return false;

  return true;
}