xref: /llvm-project-15.0.7/llvm/lib/Transforms/Utils/LoopUtils.cpp (revision 86c8a785)

//===-- LoopUtils.cpp - Loop Utility functions -------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file defines common loop utility functions.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/PriorityWorklist.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopAccessAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/MustExecute.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/KnownBits.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"

using namespace llvm;
using namespace llvm::PatternMatch;

#define DEBUG_TYPE "loop-utils"

static const char *LLVMLoopDisableNonforced = "llvm.loop.disable_nonforced";
static const char *LLVMLoopDisableLICM = "llvm.licm.disable";
static const char *LLVMLoopMustProgress = "llvm.loop.mustprogress";

bool llvm::formDedicatedExitBlocks(Loop *L, DominatorTree *DT, LoopInfo *LI,
                                   MemorySSAUpdater *MSSAU,
                                   bool PreserveLCSSA) {
  bool Changed = false;

  // We re-use a vector for the in-loop predecesosrs.
  SmallVector<BasicBlock *, 4> InLoopPredecessors;

  auto RewriteExit = [&](BasicBlock *BB) {
    assert(InLoopPredecessors.empty() &&
           "Must start with an empty predecessors list!");
    auto Cleanup = make_scope_exit([&] { InLoopPredecessors.clear(); });

    // See if there are any non-loop predecessors of this exit block and
    // keep track of the in-loop predecessors.
    bool IsDedicatedExit = true;
    for (auto *PredBB : predecessors(BB))
      if (L->contains(PredBB)) {
        if (isa<IndirectBrInst>(PredBB->getTerminator()))
          // We cannot rewrite exiting edges from an indirectbr.
          return false;
        if (isa<CallBrInst>(PredBB->getTerminator()))
          // We cannot rewrite exiting edges from a callbr.
          return false;

        InLoopPredecessors.push_back(PredBB);
      } else {
        IsDedicatedExit = false;
      }

    assert(!InLoopPredecessors.empty() && "Must have *some* loop predecessor!");

    // Nothing to do if this is already a dedicated exit.
    if (IsDedicatedExit)
      return false;

    auto *NewExitBB = SplitBlockPredecessors(
        BB, InLoopPredecessors, ".loopexit", DT, LI, MSSAU, PreserveLCSSA);

    if (!NewExitBB)
      LLVM_DEBUG(
          dbgs() << "WARNING: Can't create a dedicated exit block for loop: "
                 << *L << "\n");
    else
      LLVM_DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
                        << NewExitBB->getName() << "\n");
    return true;
  };

  // Walk the exit blocks directly rather than building up a data structure for
  // them, but only visit each one once.
  SmallPtrSet<BasicBlock *, 4> Visited;
  for (auto *BB : L->blocks())
    for (auto *SuccBB : successors(BB)) {
      // We're looking for exit blocks so skip in-loop successors.
      if (L->contains(SuccBB))
        continue;

      // Visit each exit block exactly once.
      if (!Visited.insert(SuccBB).second)
        continue;

      Changed |= RewriteExit(SuccBB);
    }

  return Changed;
}

/// Returns the instructions that use values defined in the loop.
SmallVector<Instruction *, 8> llvm::findDefsUsedOutsideOfLoop(Loop *L) {
  SmallVector<Instruction *, 8> UsedOutside;

  for (auto *Block : L->getBlocks())
    // FIXME: I believe that this could use copy_if if the Inst reference could
    // be adapted into a pointer.
    for (auto &Inst : *Block) {
      auto Users = Inst.users();
      if (any_of(Users, [&](User *U) {
            auto *Use = cast<Instruction>(U);
            return !L->contains(Use->getParent());
          }))
        UsedOutside.push_back(&Inst);
    }

  return UsedOutside;
}

void llvm::getLoopAnalysisUsage(AnalysisUsage &AU) {
  // By definition, all loop passes need the LoopInfo analysis and the
  // Dominator tree it depends on. Because they all participate in the loop
  // pass manager, they must also preserve these.
  AU.addRequired<DominatorTreeWrapperPass>();
  AU.addPreserved<DominatorTreeWrapperPass>();
  AU.addRequired<LoopInfoWrapperPass>();
  AU.addPreserved<LoopInfoWrapperPass>();

  // We must also preserve LoopSimplify and LCSSA. We locally access their IDs
  // here because users shouldn't directly get them from this header.
  extern char &LoopSimplifyID;
  extern char &LCSSAID;
  AU.addRequiredID(LoopSimplifyID);
  AU.addPreservedID(LoopSimplifyID);
  AU.addRequiredID(LCSSAID);
  AU.addPreservedID(LCSSAID);
  // This is used in the LPPassManager to perform LCSSA verification on passes
  // which preserve lcssa form
  AU.addRequired<LCSSAVerificationPass>();
  AU.addPreserved<LCSSAVerificationPass>();

  // Loop passes are designed to run inside of a loop pass manager which means
  // that any function analyses they require must be required by the first loop
  // pass in the manager (so that it is computed before the loop pass manager
  // runs) and preserved by all loop pasess in the manager. To make this
  // reasonably robust, the set needed for most loop passes is maintained here.
  // If your loop pass requires an analysis not listed here, you will need to
  // carefully audit the loop pass manager nesting structure that results.
  AU.addRequired<AAResultsWrapperPass>();
  AU.addPreserved<AAResultsWrapperPass>();
  AU.addPreserved<BasicAAWrapperPass>();
  AU.addPreserved<GlobalsAAWrapperPass>();
  AU.addPreserved<SCEVAAWrapperPass>();
  AU.addRequired<ScalarEvolutionWrapperPass>();
  AU.addPreserved<ScalarEvolutionWrapperPass>();
  // FIXME: When all loop passes preserve MemorySSA, it can be required and
  // preserved here instead of the individual handling in each pass.
}

/// Manually defined generic "LoopPass" dependency initialization. This is used
/// to initialize the exact set of passes from above in \c
/// getLoopAnalysisUsage. It can be used within a loop pass's initialization
/// with:
///
///   INITIALIZE_PASS_DEPENDENCY(LoopPass)
///
/// As-if "LoopPass" were a pass.
void llvm::initializeLoopPassPass(PassRegistry &Registry) {
  INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
  INITIALIZE_PASS_DEPENDENCY(LCSSAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(BasicAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(SCEVAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
}

/// Create MDNode for input string.
static MDNode *createStringMetadata(Loop *TheLoop, StringRef Name, unsigned V) {
  LLVMContext &Context = TheLoop->getHeader()->getContext();
  Metadata *MDs[] = {
      MDString::get(Context, Name),
      ConstantAsMetadata::get(ConstantInt::get(Type::getInt32Ty(Context), V))};
  return MDNode::get(Context, MDs);
}

/// Set input string into loop metadata by keeping other values intact.
/// If the string is already in loop metadata update value if it is
/// different.
void llvm::addStringMetadataToLoop(Loop *TheLoop, const char *StringMD,
                                   unsigned V) {
  SmallVector<Metadata *, 4> MDs(1);
  // If the loop already has metadata, retain it.
  MDNode *LoopID = TheLoop->getLoopID();
  if (LoopID) {
    for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
      MDNode *Node = cast<MDNode>(LoopID->getOperand(i));
      // If it is of form key = value, try to parse it.
      if (Node->getNumOperands() == 2) {
        MDString *S = dyn_cast<MDString>(Node->getOperand(0));
        if (S && S->getString().equals(StringMD)) {
          ConstantInt *IntMD =
              mdconst::extract_or_null<ConstantInt>(Node->getOperand(1));
          if (IntMD && IntMD->getSExtValue() == V)
            // It is already in place. Do nothing.
            return;
          // We need to update the value, so just skip it here and it will
          // be added after copying other existed nodes.
          continue;
        }
      }
      MDs.push_back(Node);
    }
  }
  // Add new metadata.
  MDs.push_back(createStringMetadata(TheLoop, StringMD, V));
  // Replace current metadata node with new one.
  LLVMContext &Context = TheLoop->getHeader()->getContext();
  MDNode *NewLoopID = MDNode::get(Context, MDs);
  // Set operand 0 to refer to the loop id itself.
  NewLoopID->replaceOperandWith(0, NewLoopID);
  TheLoop->setLoopID(NewLoopID);
}

/// Find string metadata for loop
///
/// If it has a value (e.g. {"llvm.distribute", 1} return the value as an
/// operand or null otherwise.  If the string metadata is not found return
/// Optional's not-a-value.
Optional<const MDOperand *> llvm::findStringMetadataForLoop(const Loop *TheLoop,
                                                            StringRef Name) {
  MDNode *MD = findOptionMDForLoop(TheLoop, Name);
  if (!MD)
    return None;
  switch (MD->getNumOperands()) {
  case 1:
    return nullptr;
  case 2:
    return &MD->getOperand(1);
  default:
    llvm_unreachable("loop metadata has 0 or 1 operand");
  }
}

static Optional<bool> getOptionalBoolLoopAttribute(const Loop *TheLoop,
                                                   StringRef Name) {
  MDNode *MD = findOptionMDForLoop(TheLoop, Name);
  if (!MD)
    return None;
  switch (MD->getNumOperands()) {
  case 1:
    // When the value is absent it is interpreted as 'attribute set'.
    return true;
  case 2:
    if (ConstantInt *IntMD =
            mdconst::extract_or_null<ConstantInt>(MD->getOperand(1).get()))
      return IntMD->getZExtValue();
    return true;
  }
  llvm_unreachable("unexpected number of options");
}

bool llvm::getBooleanLoopAttribute(const Loop *TheLoop, StringRef Name) {
  return getOptionalBoolLoopAttribute(TheLoop, Name).getValueOr(false);
}

Optional<ElementCount>
llvm::getOptionalElementCountLoopAttribute(Loop *TheLoop) {
  Optional<int> Width =
      getOptionalIntLoopAttribute(TheLoop, "llvm.loop.vectorize.width");

  if (Width.hasValue()) {
    Optional<int> IsScalable = getOptionalIntLoopAttribute(
        TheLoop, "llvm.loop.vectorize.scalable.enable");
    return ElementCount::get(*Width, IsScalable.getValueOr(false));
  }

  return None;
}

llvm::Optional<int> llvm::getOptionalIntLoopAttribute(Loop *TheLoop,
                                                      StringRef Name) {
  const MDOperand *AttrMD =
      findStringMetadataForLoop(TheLoop, Name).getValueOr(nullptr);
  if (!AttrMD)
    return None;

  ConstantInt *IntMD = mdconst::extract_or_null<ConstantInt>(AttrMD->get());
  if (!IntMD)
    return None;

  return IntMD->getSExtValue();
}

Optional<MDNode *> llvm::makeFollowupLoopID(
    MDNode *OrigLoopID, ArrayRef<StringRef> FollowupOptions,
    const char *InheritOptionsExceptPrefix, bool AlwaysNew) {
  if (!OrigLoopID) {
    if (AlwaysNew)
      return nullptr;
    return None;
  }

  assert(OrigLoopID->getOperand(0) == OrigLoopID);

  bool InheritAllAttrs = !InheritOptionsExceptPrefix;
  bool InheritSomeAttrs =
      InheritOptionsExceptPrefix && InheritOptionsExceptPrefix[0] != '\0';
  SmallVector<Metadata *, 8> MDs;
  MDs.push_back(nullptr);

  bool Changed = false;
  if (InheritAllAttrs || InheritSomeAttrs) {
    for (const MDOperand &Existing : drop_begin(OrigLoopID->operands())) {
      MDNode *Op = cast<MDNode>(Existing.get());

      auto InheritThisAttribute = [InheritSomeAttrs,
                                   InheritOptionsExceptPrefix](MDNode *Op) {
        if (!InheritSomeAttrs)
          return false;

        // Skip malformatted attribute metadata nodes.
        if (Op->getNumOperands() == 0)
          return true;
        Metadata *NameMD = Op->getOperand(0).get();
        if (!isa<MDString>(NameMD))
          return true;
        StringRef AttrName = cast<MDString>(NameMD)->getString();

        // Do not inherit excluded attributes.
        return !AttrName.startswith(InheritOptionsExceptPrefix);
      };

      if (InheritThisAttribute(Op))
        MDs.push_back(Op);
      else
        Changed = true;
    }
  } else {
    // Modified if we dropped at least one attribute.
    Changed = OrigLoopID->getNumOperands() > 1;
  }

  bool HasAnyFollowup = false;
  for (StringRef OptionName : FollowupOptions) {
    MDNode *FollowupNode = findOptionMDForLoopID(OrigLoopID, OptionName);
    if (!FollowupNode)
      continue;

    HasAnyFollowup = true;
    for (const MDOperand &Option : drop_begin(FollowupNode->operands())) {
      MDs.push_back(Option.get());
      Changed = true;
    }
  }

  // Attributes of the followup loop not specified explicity, so signal to the
  // transformation pass to add suitable attributes.
  if (!AlwaysNew && !HasAnyFollowup)
    return None;

  // If no attributes were added or remove, the previous loop Id can be reused.
  if (!AlwaysNew && !Changed)
    return OrigLoopID;

  // No attributes is equivalent to having no !llvm.loop metadata at all.
  if (MDs.size() == 1)
    return nullptr;

  // Build the new loop ID.
  MDTuple *FollowupLoopID = MDNode::get(OrigLoopID->getContext(), MDs);
  FollowupLoopID->replaceOperandWith(0, FollowupLoopID);
  return FollowupLoopID;
}

bool llvm::hasDisableAllTransformsHint(const Loop *L) {
  return getBooleanLoopAttribute(L, LLVMLoopDisableNonforced);
}

bool llvm::hasDisableLICMTransformsHint(const Loop *L) {
  return getBooleanLoopAttribute(L, LLVMLoopDisableLICM);
}

bool llvm::hasMustProgress(const Loop *L) {
  return getBooleanLoopAttribute(L, LLVMLoopMustProgress);
}

TransformationMode llvm::hasUnrollTransformation(Loop *L) {
  if (getBooleanLoopAttribute(L, "llvm.loop.unroll.disable"))
    return TM_SuppressedByUser;

  Optional<int> Count =
      getOptionalIntLoopAttribute(L, "llvm.loop.unroll.count");
  if (Count.hasValue())
    return Count.getValue() == 1 ? TM_SuppressedByUser : TM_ForcedByUser;

  if (getBooleanLoopAttribute(L, "llvm.loop.unroll.enable"))
    return TM_ForcedByUser;

  if (getBooleanLoopAttribute(L, "llvm.loop.unroll.full"))
    return TM_ForcedByUser;

  if (hasDisableAllTransformsHint(L))
    return TM_Disable;

  return TM_Unspecified;
}

TransformationMode llvm::hasUnrollAndJamTransformation(Loop *L) {
  if (getBooleanLoopAttribute(L, "llvm.loop.unroll_and_jam.disable"))
    return TM_SuppressedByUser;

  Optional<int> Count =
      getOptionalIntLoopAttribute(L, "llvm.loop.unroll_and_jam.count");
  if (Count.hasValue())
    return Count.getValue() == 1 ? TM_SuppressedByUser : TM_ForcedByUser;

  if (getBooleanLoopAttribute(L, "llvm.loop.unroll_and_jam.enable"))
    return TM_ForcedByUser;

  if (hasDisableAllTransformsHint(L))
    return TM_Disable;

  return TM_Unspecified;
}

TransformationMode llvm::hasVectorizeTransformation(Loop *L) {
  Optional<bool> Enable =
      getOptionalBoolLoopAttribute(L, "llvm.loop.vectorize.enable");

  if (Enable == false)
    return TM_SuppressedByUser;

  Optional<ElementCount> VectorizeWidth =
      getOptionalElementCountLoopAttribute(L);
  Optional<int> InterleaveCount =
      getOptionalIntLoopAttribute(L, "llvm.loop.interleave.count");

  // 'Forcing' vector width and interleave count to one effectively disables
  // this tranformation.
  if (Enable == true && VectorizeWidth && VectorizeWidth->isScalar() &&
      InterleaveCount == 1)
    return TM_SuppressedByUser;

  if (getBooleanLoopAttribute(L, "llvm.loop.isvectorized"))
    return TM_Disable;

  if (Enable == true)
    return TM_ForcedByUser;

  if ((VectorizeWidth && VectorizeWidth->isScalar()) && InterleaveCount == 1)
    return TM_Disable;

  if ((VectorizeWidth && VectorizeWidth->isVector()) || InterleaveCount > 1)
    return TM_Enable;

  if (hasDisableAllTransformsHint(L))
    return TM_Disable;

  return TM_Unspecified;
}

TransformationMode llvm::hasDistributeTransformation(Loop *L) {
  if (getBooleanLoopAttribute(L, "llvm.loop.distribute.enable"))
    return TM_ForcedByUser;

  if (hasDisableAllTransformsHint(L))
    return TM_Disable;

  return TM_Unspecified;
}

TransformationMode llvm::hasLICMVersioningTransformation(Loop *L) {
  if (getBooleanLoopAttribute(L, "llvm.loop.licm_versioning.disable"))
    return TM_SuppressedByUser;

  if (hasDisableAllTransformsHint(L))
    return TM_Disable;

  return TM_Unspecified;
}

/// Does a BFS from a given node to all of its children inside a given loop.
/// The returned vector of nodes includes the starting point.
SmallVector<DomTreeNode *, 16>
llvm::collectChildrenInLoop(DomTreeNode *N, const Loop *CurLoop) {
  SmallVector<DomTreeNode *, 16> Worklist;
  auto AddRegionToWorklist = [&](DomTreeNode *DTN) {
    // Only include subregions in the top level loop.
    BasicBlock *BB = DTN->getBlock();
    if (CurLoop->contains(BB))
      Worklist.push_back(DTN);
  };

  AddRegionToWorklist(N);

  for (size_t I = 0; I < Worklist.size(); I++) {
    for (DomTreeNode *Child : Worklist[I]->children())
      AddRegionToWorklist(Child);
  }

  return Worklist;
}

void llvm::deleteDeadLoop(Loop *L, DominatorTree *DT, ScalarEvolution *SE,
                          LoopInfo *LI, MemorySSA *MSSA) {
  assert((!DT || L->isLCSSAForm(*DT)) && "Expected LCSSA!");
  auto *Preheader = L->getLoopPreheader();
  assert(Preheader && "Preheader should exist!");

  std::unique_ptr<MemorySSAUpdater> MSSAU;
  if (MSSA)
    MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);

  // Now that we know the removal is safe, remove the loop by changing the
  // branch from the preheader to go to the single exit block.
  //
  // Because we're deleting a large chunk of code at once, the sequence in which
  // we remove things is very important to avoid invalidation issues.

  // Tell ScalarEvolution that the loop is deleted. Do this before
  // deleting the loop so that ScalarEvolution can look at the loop
  // to determine what it needs to clean up.
  if (SE)
    SE->forgetLoop(L);

  auto *OldBr = dyn_cast<BranchInst>(Preheader->getTerminator());
  assert(OldBr && "Preheader must end with a branch");
  assert(OldBr->isUnconditional() && "Preheader must have a single successor");
  // Connect the preheader to the exit block. Keep the old edge to the header
  // around to perform the dominator tree update in two separate steps
  // -- #1 insertion of the edge preheader -> exit and #2 deletion of the edge
  // preheader -> header.
  //
  //
  // 0.  Preheader          1.  Preheader           2.  Preheader
  //        |                    |   |                   |
  //        V                    |   V                   |
  //      Header <--\            | Header <--\           | Header <--\
  //       |  |     |            |  |  |     |           |  |  |     |
  //       |  V     |            |  |  V     |           |  |  V     |
  //       | Body --/            |  | Body --/           |  | Body --/
  //       V                     V  V                    V  V
  //      Exit                   Exit                    Exit
  //
  // By doing this is two separate steps we can perform the dominator tree
  // update without using the batch update API.
  //
  // Even when the loop is never executed, we cannot remove the edge from the
  // source block to the exit block. Consider the case where the unexecuted loop
  // branches back to an outer loop. If we deleted the loop and removed the edge
  // coming to this inner loop, this will break the outer loop structure (by
  // deleting the backedge of the outer loop). If the outer loop is indeed a
  // non-loop, it will be deleted in a future iteration of loop deletion pass.
  IRBuilder<> Builder(OldBr);

  auto *ExitBlock = L->getUniqueExitBlock();
  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
  if (ExitBlock) {
    assert(ExitBlock && "Should have a unique exit block!");
    assert(L->hasDedicatedExits() && "Loop should have dedicated exits!");

    Builder.CreateCondBr(Builder.getFalse(), L->getHeader(), ExitBlock);
    // Remove the old branch. The conditional branch becomes a new terminator.
    OldBr->eraseFromParent();

    // Rewrite phis in the exit block to get their inputs from the Preheader
    // instead of the exiting block.
    for (PHINode &P : ExitBlock->phis()) {
      // Set the zero'th element of Phi to be from the preheader and remove all
      // other incoming values. Given the loop has dedicated exits, all other
      // incoming values must be from the exiting blocks.
      int PredIndex = 0;
      P.setIncomingBlock(PredIndex, Preheader);
      // Removes all incoming values from all other exiting blocks (including
      // duplicate values from an exiting block).
      // Nuke all entries except the zero'th entry which is the preheader entry.
      // NOTE! We need to remove Incoming Values in the reverse order as done
      // below, to keep the indices valid for deletion (removeIncomingValues
      // updates getNumIncomingValues and shifts all values down into the
      // operand being deleted).
      for (unsigned i = 0, e = P.getNumIncomingValues() - 1; i != e; ++i)
        P.removeIncomingValue(e - i, false);

      assert((P.getNumIncomingValues() == 1 &&
              P.getIncomingBlock(PredIndex) == Preheader) &&
             "Should have exactly one value and that's from the preheader!");
    }

    if (DT) {
      DTU.applyUpdates({{DominatorTree::Insert, Preheader, ExitBlock}});
      if (MSSA) {
        MSSAU->applyUpdates({{DominatorTree::Insert, Preheader, ExitBlock}},
                            *DT);
        if (VerifyMemorySSA)
          MSSA->verifyMemorySSA();
      }
    }

    // Disconnect the loop body by branching directly to its exit.
    Builder.SetInsertPoint(Preheader->getTerminator());
    Builder.CreateBr(ExitBlock);
    // Remove the old branch.
    Preheader->getTerminator()->eraseFromParent();
  } else {
    assert(L->hasNoExitBlocks() &&
           "Loop should have either zero or one exit blocks.");

    Builder.SetInsertPoint(OldBr);
    Builder.CreateUnreachable();
    Preheader->getTerminator()->eraseFromParent();
  }

  if (DT) {
    DTU.applyUpdates({{DominatorTree::Delete, Preheader, L->getHeader()}});
    if (MSSA) {
      MSSAU->applyUpdates({{DominatorTree::Delete, Preheader, L->getHeader()}},
                          *DT);
      SmallSetVector<BasicBlock *, 8> DeadBlockSet(L->block_begin(),
                                                   L->block_end());
      MSSAU->removeBlocks(DeadBlockSet);
      if (VerifyMemorySSA)
        MSSA->verifyMemorySSA();
    }
  }

  // Use a map to unique and a vector to guarantee deterministic ordering.
  llvm::SmallDenseSet<std::pair<DIVariable *, DIExpression *>, 4> DeadDebugSet;
  llvm::SmallVector<DbgVariableIntrinsic *, 4> DeadDebugInst;

  if (ExitBlock) {
    // Given LCSSA form is satisfied, we should not have users of instructions
    // within the dead loop outside of the loop. However, LCSSA doesn't take
    // unreachable uses into account. We handle them here.
    // We could do it after drop all references (in this case all users in the
    // loop will be already eliminated and we have less work to do but according
    // to API doc of User::dropAllReferences only valid operation after dropping
    // references, is deletion. So let's substitute all usages of
    // instruction from the loop with undef value of corresponding type first.
    for (auto *Block : L->blocks())
      for (Instruction &I : *Block) {
        auto *Undef = UndefValue::get(I.getType());
        for (Value::use_iterator UI = I.use_begin(), E = I.use_end();
             UI != E;) {
          Use &U = *UI;
          ++UI;
          if (auto *Usr = dyn_cast<Instruction>(U.getUser()))
            if (L->contains(Usr->getParent()))
              continue;
          // If we have a DT then we can check that uses outside a loop only in
          // unreachable block.
          if (DT)
            assert(!DT->isReachableFromEntry(U) &&
                   "Unexpected user in reachable block");
          U.set(Undef);
        }
        auto *DVI = dyn_cast<DbgVariableIntrinsic>(&I);
        if (!DVI)
          continue;
        auto Key =
            DeadDebugSet.find({DVI->getVariable(), DVI->getExpression()});
        if (Key != DeadDebugSet.end())
          continue;
        DeadDebugSet.insert({DVI->getVariable(), DVI->getExpression()});
        DeadDebugInst.push_back(DVI);
      }

    // After the loop has been deleted all the values defined and modified
    // inside the loop are going to be unavailable.
    // Since debug values in the loop have been deleted, inserting an undef
    // dbg.value truncates the range of any dbg.value before the loop where the
    // loop used to be. This is particularly important for constant values.
    DIBuilder DIB(*ExitBlock->getModule());
    Instruction *InsertDbgValueBefore = ExitBlock->getFirstNonPHI();
    assert(InsertDbgValueBefore &&
           "There should be a non-PHI instruction in exit block, else these "
           "instructions will have no parent.");
    for (auto *DVI : DeadDebugInst)
      DIB.insertDbgValueIntrinsic(UndefValue::get(Builder.getInt32Ty()),
                                  DVI->getVariable(), DVI->getExpression(),
                                  DVI->getDebugLoc(), InsertDbgValueBefore);
  }

  // Remove the block from the reference counting scheme, so that we can
  // delete it freely later.
  for (auto *Block : L->blocks())
    Block->dropAllReferences();

  if (MSSA && VerifyMemorySSA)
    MSSA->verifyMemorySSA();

  if (LI) {
    // Erase the instructions and the blocks without having to worry
    // about ordering because we already dropped the references.
    // NOTE: This iteration is safe because erasing the block does not remove
    // its entry from the loop's block list.  We do that in the next section.
    for (Loop::block_iterator LpI = L->block_begin(), LpE = L->block_end();
         LpI != LpE; ++LpI)
      (*LpI)->eraseFromParent();

    // Finally, the blocks from loopinfo.  This has to happen late because
    // otherwise our loop iterators won't work.

    SmallPtrSet<BasicBlock *, 8> blocks;
    blocks.insert(L->block_begin(), L->block_end());
    for (BasicBlock *BB : blocks)
      LI->removeBlock(BB);

    // The last step is to update LoopInfo now that we've eliminated this loop.
    // Note: LoopInfo::erase remove the given loop and relink its subloops with
    // its parent. While removeLoop/removeChildLoop remove the given loop but
    // not relink its subloops, which is what we want.
    if (Loop *ParentLoop = L->getParentLoop()) {
      Loop::iterator I = find(*ParentLoop, L);
      assert(I != ParentLoop->end() && "Couldn't find loop");
      ParentLoop->removeChildLoop(I);
    } else {
      Loop::iterator I = find(*LI, L);
      assert(I != LI->end() && "Couldn't find loop");
      LI->removeLoop(I);
    }
    LI->destroy(L);
  }
}

static Loop *getOutermostLoop(Loop *L) {
  while (Loop *Parent = L->getParentLoop())
    L = Parent;
  return L;
}

void llvm::breakLoopBackedge(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
                             LoopInfo &LI, MemorySSA *MSSA) {
  auto *Latch = L->getLoopLatch();
  assert(Latch && "multiple latches not yet supported");
  auto *Header = L->getHeader();
  Loop *OutermostLoop = getOutermostLoop(L);

  SE.forgetLoop(L);

  // Note: By splitting the backedge, and then explicitly making it unreachable
  // we gracefully handle corner cases such as non-bottom tested loops and the
  // like.  We also have the benefit of being able to reuse existing well tested
  // code.  It might be worth special casing the common bottom tested case at
  // some point to avoid code churn.

  std::unique_ptr<MemorySSAUpdater> MSSAU;
  if (MSSA)
    MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);

  auto *BackedgeBB = SplitEdge(Latch, Header, &DT, &LI, MSSAU.get());

  DomTreeUpdater DTU(&DT, DomTreeUpdater::UpdateStrategy::Eager);
  (void)changeToUnreachable(BackedgeBB->getTerminator(), /*UseTrap*/false,
                            /*PreserveLCSSA*/true, &DTU, MSSAU.get());

  // Erase (and destroy) this loop instance.  Handles relinking sub-loops
  // and blocks within the loop as needed.
  LI.erase(L);

  // If the loop we broke had a parent, then changeToUnreachable might have
  // caused a block to be removed from the parent loop (see loop_nest_lcssa
  // test case in zero-btc.ll for an example), thus changing the parent's
  // exit blocks.  If that happened, we need to rebuild LCSSA on the outermost
  // loop which might have a had a block removed.
  if (OutermostLoop != L)
    formLCSSARecursively(*OutermostLoop, DT, &LI, &SE);
}


/// Checks if \p L has single exit through latch block except possibly
/// "deoptimizing" exits. Returns branch instruction terminating the loop
/// latch if above check is successful, nullptr otherwise.
static BranchInst *getExpectedExitLoopLatchBranch(Loop *L) {
  BasicBlock *Latch = L->getLoopLatch();
  if (!Latch)
    return nullptr;

  BranchInst *LatchBR = dyn_cast<BranchInst>(Latch->getTerminator());
  if (!LatchBR || LatchBR->getNumSuccessors() != 2 || !L->isLoopExiting(Latch))
    return nullptr;

  assert((LatchBR->getSuccessor(0) == L->getHeader() ||
          LatchBR->getSuccessor(1) == L->getHeader()) &&
         "At least one edge out of the latch must go to the header");

  SmallVector<BasicBlock *, 4> ExitBlocks;
  L->getUniqueNonLatchExitBlocks(ExitBlocks);
  if (any_of(ExitBlocks, [](const BasicBlock *EB) {
        return !EB->getTerminatingDeoptimizeCall();
      }))
    return nullptr;

  return LatchBR;
}

Optional<unsigned>
llvm::getLoopEstimatedTripCount(Loop *L,
                                unsigned *EstimatedLoopInvocationWeight) {
  // Support loops with an exiting latch and other existing exists only
  // deoptimize.
  BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L);
  if (!LatchBranch)
    return None;

  // To estimate the number of times the loop body was executed, we want to
  // know the number of times the backedge was taken, vs. the number of times
  // we exited the loop.
  uint64_t BackedgeTakenWeight, LatchExitWeight;
  if (!LatchBranch->extractProfMetadata(BackedgeTakenWeight, LatchExitWeight))
    return None;

  if (LatchBranch->getSuccessor(0) != L->getHeader())
    std::swap(BackedgeTakenWeight, LatchExitWeight);

  if (!LatchExitWeight)
    return None;

  if (EstimatedLoopInvocationWeight)
    *EstimatedLoopInvocationWeight = LatchExitWeight;

  // Estimated backedge taken count is a ratio of the backedge taken weight by
  // the weight of the edge exiting the loop, rounded to nearest.
  uint64_t BackedgeTakenCount =
      llvm::divideNearest(BackedgeTakenWeight, LatchExitWeight);
  // Estimated trip count is one plus estimated backedge taken count.
  return BackedgeTakenCount + 1;
}

bool llvm::setLoopEstimatedTripCount(Loop *L, unsigned EstimatedTripCount,
                                     unsigned EstimatedloopInvocationWeight) {
  // Support loops with an exiting latch and other existing exists only
  // deoptimize.
  BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L);
  if (!LatchBranch)
    return false;

  // Calculate taken and exit weights.
  unsigned LatchExitWeight = 0;
  unsigned BackedgeTakenWeight = 0;

  if (EstimatedTripCount > 0) {
    LatchExitWeight = EstimatedloopInvocationWeight;
    BackedgeTakenWeight = (EstimatedTripCount - 1) * LatchExitWeight;
  }

  // Make a swap if back edge is taken when condition is "false".
  if (LatchBranch->getSuccessor(0) != L->getHeader())
    std::swap(BackedgeTakenWeight, LatchExitWeight);

  MDBuilder MDB(LatchBranch->getContext());

  // Set/Update profile metadata.
  LatchBranch->setMetadata(
      LLVMContext::MD_prof,
      MDB.createBranchWeights(BackedgeTakenWeight, LatchExitWeight));

  return true;
}

bool llvm::hasIterationCountInvariantInParent(Loop *InnerLoop,
                                              ScalarEvolution &SE) {
  Loop *OuterL = InnerLoop->getParentLoop();
  if (!OuterL)
    return true;

  // Get the backedge taken count for the inner loop
  BasicBlock *InnerLoopLatch = InnerLoop->getLoopLatch();
  const SCEV *InnerLoopBECountSC = SE.getExitCount(InnerLoop, InnerLoopLatch);
  if (isa<SCEVCouldNotCompute>(InnerLoopBECountSC) ||
      !InnerLoopBECountSC->getType()->isIntegerTy())
    return false;

  // Get whether count is invariant to the outer loop
  ScalarEvolution::LoopDisposition LD =
      SE.getLoopDisposition(InnerLoopBECountSC, OuterL);
  if (LD != ScalarEvolution::LoopInvariant)
    return false;

  return true;
}

Value *llvm::createMinMaxOp(IRBuilderBase &Builder, RecurKind RK, Value *Left,
                            Value *Right) {
  CmpInst::Predicate Pred;
  switch (RK) {
  default:
    llvm_unreachable("Unknown min/max recurrence kind");
  case RecurKind::UMin:
    Pred = CmpInst::ICMP_ULT;
    break;
  case RecurKind::UMax:
    Pred = CmpInst::ICMP_UGT;
    break;
  case RecurKind::SMin:
    Pred = CmpInst::ICMP_SLT;
    break;
  case RecurKind::SMax:
    Pred = CmpInst::ICMP_SGT;
    break;
  case RecurKind::FMin:
    Pred = CmpInst::FCMP_OLT;
    break;
  case RecurKind::FMax:
    Pred = CmpInst::FCMP_OGT;
    break;
  }

  Value *Cmp = Builder.CreateCmp(Pred, Left, Right, "rdx.minmax.cmp");
  Value *Select = Builder.CreateSelect(Cmp, Left, Right, "rdx.minmax.select");
  return Select;
}

// Helper to generate an ordered reduction.
Value *llvm::getOrderedReduction(IRBuilderBase &Builder, Value *Acc, Value *Src,
                                 unsigned Op, RecurKind RdxKind,
                                 ArrayRef<Value *> RedOps) {
  unsigned VF = cast<FixedVectorType>(Src->getType())->getNumElements();

  // Extract and apply reduction ops in ascending order:
  // e.g. ((((Acc + Scl[0]) + Scl[1]) + Scl[2]) + ) ... + Scl[VF-1]
  Value *Result = Acc;
  for (unsigned ExtractIdx = 0; ExtractIdx != VF; ++ExtractIdx) {
    Value *Ext =
        Builder.CreateExtractElement(Src, Builder.getInt32(ExtractIdx));

    if (Op != Instruction::ICmp && Op != Instruction::FCmp) {
      Result = Builder.CreateBinOp((Instruction::BinaryOps)Op, Result, Ext,
                                   "bin.rdx");
    } else {
      assert(RecurrenceDescriptor::isMinMaxRecurrenceKind(RdxKind) &&
             "Invalid min/max");
      Result = createMinMaxOp(Builder, RdxKind, Result, Ext);
    }

    if (!RedOps.empty())
      propagateIRFlags(Result, RedOps);
  }

  return Result;
}

// Helper to generate a log2 shuffle reduction.
Value *llvm::getShuffleReduction(IRBuilderBase &Builder, Value *Src,
                                 unsigned Op, RecurKind RdxKind,
                                 ArrayRef<Value *> RedOps) {
  unsigned VF = cast<FixedVectorType>(Src->getType())->getNumElements();
  // VF is a power of 2 so we can emit the reduction using log2(VF) shuffles
  // and vector ops, reducing the set of values being computed by half each
  // round.
  assert(isPowerOf2_32(VF) &&
         "Reduction emission only supported for pow2 vectors!");
  Value *TmpVec = Src;
  SmallVector<int, 32> ShuffleMask(VF);
  for (unsigned i = VF; i != 1; i >>= 1) {
    // Move the upper half of the vector to the lower half.
    for (unsigned j = 0; j != i / 2; ++j)
      ShuffleMask[j] = i / 2 + j;

    // Fill the rest of the mask with undef.
    std::fill(&ShuffleMask[i / 2], ShuffleMask.end(), -1);

    Value *Shuf = Builder.CreateShuffleVector(TmpVec, ShuffleMask, "rdx.shuf");

    if (Op != Instruction::ICmp && Op != Instruction::FCmp) {
      // The builder propagates its fast-math-flags setting.
      TmpVec = Builder.CreateBinOp((Instruction::BinaryOps)Op, TmpVec, Shuf,
                                   "bin.rdx");
    } else {
      assert(RecurrenceDescriptor::isMinMaxRecurrenceKind(RdxKind) &&
             "Invalid min/max");
      TmpVec = createMinMaxOp(Builder, RdxKind, TmpVec, Shuf);
    }
    if (!RedOps.empty())
      propagateIRFlags(TmpVec, RedOps);

    // We may compute the reassociated scalar ops in a way that does not
    // preserve nsw/nuw etc. Conservatively, drop those flags.
    if (auto *ReductionInst = dyn_cast<Instruction>(TmpVec))
      ReductionInst->dropPoisonGeneratingFlags();
  }
  // The result is in the first element of the vector.
  return Builder.CreateExtractElement(TmpVec, Builder.getInt32(0));
}

Value *llvm::createSimpleTargetReduction(IRBuilderBase &Builder,
                                         const TargetTransformInfo *TTI,
                                         Value *Src, RecurKind RdxKind,
                                         ArrayRef<Value *> RedOps) {
  TargetTransformInfo::ReductionFlags RdxFlags;
  RdxFlags.IsMaxOp = RdxKind == RecurKind::SMax || RdxKind == RecurKind::UMax ||
                     RdxKind == RecurKind::FMax;
  RdxFlags.IsSigned = RdxKind == RecurKind::SMax || RdxKind == RecurKind::SMin;

  auto *SrcVecEltTy = cast<VectorType>(Src->getType())->getElementType();
  switch (RdxKind) {
  case RecurKind::Add:
    return Builder.CreateAddReduce(Src);
  case RecurKind::Mul:
    return Builder.CreateMulReduce(Src);
  case RecurKind::And:
    return Builder.CreateAndReduce(Src);
  case RecurKind::Or:
    return Builder.CreateOrReduce(Src);
  case RecurKind::Xor:
    return Builder.CreateXorReduce(Src);
  case RecurKind::FAdd:
    return Builder.CreateFAddReduce(ConstantFP::getNegativeZero(SrcVecEltTy),
                                    Src);
  case RecurKind::FMul:
    return Builder.CreateFMulReduce(ConstantFP::get(SrcVecEltTy, 1.0), Src);
  case RecurKind::SMax:
    return Builder.CreateIntMaxReduce(Src, true);
  case RecurKind::SMin:
    return Builder.CreateIntMinReduce(Src, true);
  case RecurKind::UMax:
    return Builder.CreateIntMaxReduce(Src, false);
  case RecurKind::UMin:
    return Builder.CreateIntMinReduce(Src, false);
  case RecurKind::FMax:
    return Builder.CreateFPMaxReduce(Src);
  case RecurKind::FMin:
    return Builder.CreateFPMinReduce(Src);
  default:
    llvm_unreachable("Unhandled opcode");
  }
}

Value *llvm::createTargetReduction(IRBuilderBase &B,
                                   const TargetTransformInfo *TTI,
                                   RecurrenceDescriptor &Desc, Value *Src) {
  // TODO: Support in-order reductions based on the recurrence descriptor.
  // All ops in the reduction inherit fast-math-flags from the recurrence
  // descriptor.
  IRBuilderBase::FastMathFlagGuard FMFGuard(B);
  B.setFastMathFlags(Desc.getFastMathFlags());
  return createSimpleTargetReduction(B, TTI, Src, Desc.getRecurrenceKind());
}

void llvm::propagateIRFlags(Value *I, ArrayRef<Value *> VL, Value *OpValue) {
  auto *VecOp = dyn_cast<Instruction>(I);
  if (!VecOp)
    return;
  auto *Intersection = (OpValue == nullptr) ? dyn_cast<Instruction>(VL[0])
                                            : dyn_cast<Instruction>(OpValue);
  if (!Intersection)
    return;
  const unsigned Opcode = Intersection->getOpcode();
  VecOp->copyIRFlags(Intersection);
  for (auto *V : VL) {
    auto *Instr = dyn_cast<Instruction>(V);
    if (!Instr)
      continue;
    if (OpValue == nullptr || Opcode == Instr->getOpcode())
      VecOp->andIRFlags(V);
  }
}

bool llvm::isKnownNegativeInLoop(const SCEV *S, const Loop *L,
                                 ScalarEvolution &SE) {
  const SCEV *Zero = SE.getZero(S->getType());
  return SE.isAvailableAtLoopEntry(S, L) &&
         SE.isLoopEntryGuardedByCond(L, ICmpInst::ICMP_SLT, S, Zero);
}

bool llvm::isKnownNonNegativeInLoop(const SCEV *S, const Loop *L,
                                    ScalarEvolution &SE) {
  const SCEV *Zero = SE.getZero(S->getType());
  return SE.isAvailableAtLoopEntry(S, L) &&
         SE.isLoopEntryGuardedByCond(L, ICmpInst::ICMP_SGE, S, Zero);
}

bool llvm::cannotBeMinInLoop(const SCEV *S, const Loop *L, ScalarEvolution &SE,
                             bool Signed) {
  unsigned BitWidth = cast<IntegerType>(S->getType())->getBitWidth();
  APInt Min = Signed ? APInt::getSignedMinValue(BitWidth) :
    APInt::getMinValue(BitWidth);
  auto Predicate = Signed ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT;
  return SE.isAvailableAtLoopEntry(S, L) &&
         SE.isLoopEntryGuardedByCond(L, Predicate, S,
                                     SE.getConstant(Min));
}

bool llvm::cannotBeMaxInLoop(const SCEV *S, const Loop *L, ScalarEvolution &SE,
                             bool Signed) {
  unsigned BitWidth = cast<IntegerType>(S->getType())->getBitWidth();
  APInt Max = Signed ? APInt::getSignedMaxValue(BitWidth) :
    APInt::getMaxValue(BitWidth);
  auto Predicate = Signed ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT;
  return SE.isAvailableAtLoopEntry(S, L) &&
         SE.isLoopEntryGuardedByCond(L, Predicate, S,
                                     SE.getConstant(Max));
}

//===----------------------------------------------------------------------===//
// rewriteLoopExitValues - Optimize IV users outside the loop.
// As a side effect, reduces the amount of IV processing within the loop.
//===----------------------------------------------------------------------===//

// Return true if the SCEV expansion generated by the rewriter can replace the
// original value. SCEV guarantees that it produces the same value, but the way
// it is produced may be illegal IR.  Ideally, this function will only be
// called for verification.
static bool isValidRewrite(ScalarEvolution *SE, Value *FromVal, Value *ToVal) {
  // If an SCEV expression subsumed multiple pointers, its expansion could
  // reassociate the GEP changing the base pointer. This is illegal because the
  // final address produced by a GEP chain must be inbounds relative to its
  // underlying object. Otherwise basic alias analysis, among other things,
  // could fail in a dangerous way. Ultimately, SCEV will be improved to avoid
  // producing an expression involving multiple pointers. Until then, we must
  // bail out here.
  //
  // Retrieve the pointer operand of the GEP. Don't use getUnderlyingObject
  // because it understands lcssa phis while SCEV does not.
  Value *FromPtr = FromVal;
  Value *ToPtr = ToVal;
  if (auto *GEP = dyn_cast<GEPOperator>(FromVal))
    FromPtr = GEP->getPointerOperand();

  if (auto *GEP = dyn_cast<GEPOperator>(ToVal))
    ToPtr = GEP->getPointerOperand();

  if (FromPtr != FromVal || ToPtr != ToVal) {
    // Quickly check the common case
    if (FromPtr == ToPtr)
      return true;

    // SCEV may have rewritten an expression that produces the GEP's pointer
    // operand. That's ok as long as the pointer operand has the same base
    // pointer. Unlike getUnderlyingObject(), getPointerBase() will find the
    // base of a recurrence. This handles the case in which SCEV expansion
    // converts a pointer type recurrence into a nonrecurrent pointer base
    // indexed by an integer recurrence.

    // If the GEP base pointer is a vector of pointers, abort.
    if (!FromPtr->getType()->isPointerTy() || !ToPtr->getType()->isPointerTy())
      return false;

    const SCEV *FromBase = SE->getPointerBase(SE->getSCEV(FromPtr));
    const SCEV *ToBase = SE->getPointerBase(SE->getSCEV(ToPtr));
    if (FromBase == ToBase)
      return true;

    LLVM_DEBUG(dbgs() << "rewriteLoopExitValues: GEP rewrite bail out "
                      << *FromBase << " != " << *ToBase << "\n");

    return false;
  }
  return true;
}

static bool hasHardUserWithinLoop(const Loop *L, const Instruction *I) {
  SmallPtrSet<const Instruction *, 8> Visited;
  SmallVector<const Instruction *, 8> WorkList;
  Visited.insert(I);
  WorkList.push_back(I);
  while (!WorkList.empty()) {
    const Instruction *Curr = WorkList.pop_back_val();
    // This use is outside the loop, nothing to do.
    if (!L->contains(Curr))
      continue;
    // Do we assume it is a "hard" use which will not be eliminated easily?
    if (Curr->mayHaveSideEffects())
      return true;
    // Otherwise, add all its users to worklist.
    for (auto U : Curr->users()) {
      auto *UI = cast<Instruction>(U);
      if (Visited.insert(UI).second)
        WorkList.push_back(UI);
    }
  }
  return false;
}

// Collect information about PHI nodes which can be transformed in
// rewriteLoopExitValues.
struct RewritePhi {
  PHINode *PN;               // For which PHI node is this replacement?
  unsigned Ith;              // For which incoming value?
  const SCEV *ExpansionSCEV; // The SCEV of the incoming value we are rewriting.
  Instruction *ExpansionPoint; // Where we'd like to expand that SCEV?
  bool HighCost;               // Is this expansion a high-cost?

  Value *Expansion = nullptr;
  bool ValidRewrite = false;

  RewritePhi(PHINode *P, unsigned I, const SCEV *Val, Instruction *ExpansionPt,
             bool H)
      : PN(P), Ith(I), ExpansionSCEV(Val), ExpansionPoint(ExpansionPt),
        HighCost(H) {}
};

// Check whether it is possible to delete the loop after rewriting exit
// value. If it is possible, ignore ReplaceExitValue and do rewriting
// aggressively.
static bool canLoopBeDeleted(Loop *L, SmallVector<RewritePhi, 8> &RewritePhiSet) {
  BasicBlock *Preheader = L->getLoopPreheader();
  // If there is no preheader, the loop will not be deleted.
  if (!Preheader)
    return false;

  // In LoopDeletion pass Loop can be deleted when ExitingBlocks.size() > 1.
  // We obviate multiple ExitingBlocks case for simplicity.
  // TODO: If we see testcase with multiple ExitingBlocks can be deleted
  // after exit value rewriting, we can enhance the logic here.
  SmallVector<BasicBlock *, 4> ExitingBlocks;
  L->getExitingBlocks(ExitingBlocks);
  SmallVector<BasicBlock *, 8> ExitBlocks;
  L->getUniqueExitBlocks(ExitBlocks);
  if (ExitBlocks.size() != 1 || ExitingBlocks.size() != 1)
    return false;

  BasicBlock *ExitBlock = ExitBlocks[0];
  BasicBlock::iterator BI = ExitBlock->begin();
  while (PHINode *P = dyn_cast<PHINode>(BI)) {
    Value *Incoming = P->getIncomingValueForBlock(ExitingBlocks[0]);

    // If the Incoming value of P is found in RewritePhiSet, we know it
    // could be rewritten to use a loop invariant value in transformation
    // phase later. Skip it in the loop invariant check below.
    bool found = false;
    for (const RewritePhi &Phi : RewritePhiSet) {
      if (!Phi.ValidRewrite)
        continue;
      unsigned i = Phi.Ith;
      if (Phi.PN == P && (Phi.PN)->getIncomingValue(i) == Incoming) {
        found = true;
        break;
      }
    }

    Instruction *I;
    if (!found && (I = dyn_cast<Instruction>(Incoming)))
      if (!L->hasLoopInvariantOperands(I))
        return false;

    ++BI;
  }

  for (auto *BB : L->blocks())
    if (llvm::any_of(*BB, [](Instruction &I) {
          return I.mayHaveSideEffects();
        }))
      return false;

  return true;
}

int llvm::rewriteLoopExitValues(Loop *L, LoopInfo *LI, TargetLibraryInfo *TLI,
                                ScalarEvolution *SE,
                                const TargetTransformInfo *TTI,
                                SCEVExpander &Rewriter, DominatorTree *DT,
                                ReplaceExitVal ReplaceExitValue,
                                SmallVector<WeakTrackingVH, 16> &DeadInsts) {
  // Check a pre-condition.
  assert(L->isRecursivelyLCSSAForm(*DT, *LI) &&
         "Indvars did not preserve LCSSA!");

  SmallVector<BasicBlock*, 8> ExitBlocks;
  L->getUniqueExitBlocks(ExitBlocks);

  SmallVector<RewritePhi, 8> RewritePhiSet;
  // Find all values that are computed inside the loop, but used outside of it.
  // Because of LCSSA, these values will only occur in LCSSA PHI Nodes.  Scan
  // the exit blocks of the loop to find them.
  for (BasicBlock *ExitBB : ExitBlocks) {
    // If there are no PHI nodes in this exit block, then no values defined
    // inside the loop are used on this path, skip it.
    PHINode *PN = dyn_cast<PHINode>(ExitBB->begin());
    if (!PN) continue;

    unsigned NumPreds = PN->getNumIncomingValues();

    // Iterate over all of the PHI nodes.
    BasicBlock::iterator BBI = ExitBB->begin();
    while ((PN = dyn_cast<PHINode>(BBI++))) {
      if (PN->use_empty())
        continue; // dead use, don't replace it

      if (!SE->isSCEVable(PN->getType()))
        continue;

      // It's necessary to tell ScalarEvolution about this explicitly so that
      // it can walk the def-use list and forget all SCEVs, as it may not be
      // watching the PHI itself. Once the new exit value is in place, there
      // may not be a def-use connection between the loop and every instruction
      // which got a SCEVAddRecExpr for that loop.
      SE->forgetValue(PN);

      // Iterate over all of the values in all the PHI nodes.
      for (unsigned i = 0; i != NumPreds; ++i) {
        // If the value being merged in is not integer or is not defined
        // in the loop, skip it.
        Value *InVal = PN->getIncomingValue(i);
        if (!isa<Instruction>(InVal))
          continue;

        // If this pred is for a subloop, not L itself, skip it.
        if (LI->getLoopFor(PN->getIncomingBlock(i)) != L)
          continue; // The Block is in a subloop, skip it.

        // Check that InVal is defined in the loop.
        Instruction *Inst = cast<Instruction>(InVal);
        if (!L->contains(Inst))
          continue;

        // Okay, this instruction has a user outside of the current loop
        // and varies predictably *inside* the loop.  Evaluate the value it
        // contains when the loop exits, if possible.  We prefer to start with
        // expressions which are true for all exits (so as to maximize
        // expression reuse by the SCEVExpander), but resort to per-exit
        // evaluation if that fails.
        const SCEV *ExitValue = SE->getSCEVAtScope(Inst, L->getParentLoop());
        if (isa<SCEVCouldNotCompute>(ExitValue) ||
            !SE->isLoopInvariant(ExitValue, L) ||
            !isSafeToExpand(ExitValue, *SE)) {
          // TODO: This should probably be sunk into SCEV in some way; maybe a
          // getSCEVForExit(SCEV*, L, ExitingBB)?  It can be generalized for
          // most SCEV expressions and other recurrence types (e.g. shift
          // recurrences).  Is there existing code we can reuse?
          const SCEV *ExitCount = SE->getExitCount(L, PN->getIncomingBlock(i));
          if (isa<SCEVCouldNotCompute>(ExitCount))
            continue;
          if (auto *AddRec = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(Inst)))
            if (AddRec->getLoop() == L)
              ExitValue = AddRec->evaluateAtIteration(ExitCount, *SE);
          if (isa<SCEVCouldNotCompute>(ExitValue) ||
              !SE->isLoopInvariant(ExitValue, L) ||
              !isSafeToExpand(ExitValue, *SE))
            continue;
        }

        // Computing the value outside of the loop brings no benefit if it is
        // definitely used inside the loop in a way which can not be optimized
        // away. Avoid doing so unless we know we have a value which computes
        // the ExitValue already. TODO: This should be merged into SCEV
        // expander to leverage its knowledge of existing expressions.
        if (ReplaceExitValue != AlwaysRepl && !isa<SCEVConstant>(ExitValue) &&
            !isa<SCEVUnknown>(ExitValue) && hasHardUserWithinLoop(L, Inst))
          continue;

        // Check if expansions of this SCEV would count as being high cost.
        bool HighCost = Rewriter.isHighCostExpansion(
            ExitValue, L, SCEVCheapExpansionBudget, TTI, Inst);

        // Note that we must not perform expansions until after
        // we query *all* the costs, because if we perform temporary expansion
        // inbetween, one that we might not intend to keep, said expansion
        // *may* affect cost calculation of the the next SCEV's we'll query,
        // and next SCEV may errneously get smaller cost.

        // Collect all the candidate PHINodes to be rewritten.
        RewritePhiSet.emplace_back(PN, i, ExitValue, Inst, HighCost);
      }
    }
  }

  // Now that we've done preliminary filtering and billed all the SCEV's,
  // we can perform the last sanity check - the expansion must be valid.
  for (RewritePhi &Phi : RewritePhiSet) {
    Phi.Expansion = Rewriter.expandCodeFor(Phi.ExpansionSCEV, Phi.PN->getType(),
                                           Phi.ExpansionPoint);

    LLVM_DEBUG(dbgs() << "rewriteLoopExitValues: AfterLoopVal = "
                      << *(Phi.Expansion) << '\n'
                      << "  LoopVal = " << *(Phi.ExpansionPoint) << "\n");

    // FIXME: isValidRewrite() is a hack. it should be an assert, eventually.
    Phi.ValidRewrite = isValidRewrite(SE, Phi.ExpansionPoint, Phi.Expansion);
    if (!Phi.ValidRewrite) {
      DeadInsts.push_back(Phi.Expansion);
      continue;
    }

#ifndef NDEBUG
    // If we reuse an instruction from a loop which is neither L nor one of
    // its containing loops, we end up breaking LCSSA form for this loop by
    // creating a new use of its instruction.
    if (auto *ExitInsn = dyn_cast<Instruction>(Phi.Expansion))
      if (auto *EVL = LI->getLoopFor(ExitInsn->getParent()))
        if (EVL != L)
          assert(EVL->contains(L) && "LCSSA breach detected!");
#endif
  }

  // TODO: after isValidRewrite() is an assertion, evaluate whether
  // it is beneficial to change how we calculate high-cost:
  // if we have SCEV 'A' which we know we will expand, should we calculate
  // the cost of other SCEV's after expanding SCEV 'A',
  // thus potentially giving cost bonus to those other SCEV's?

  bool LoopCanBeDel = canLoopBeDeleted(L, RewritePhiSet);
  int NumReplaced = 0;

  // Transformation.
  for (const RewritePhi &Phi : RewritePhiSet) {
    if (!Phi.ValidRewrite)
      continue;

    PHINode *PN = Phi.PN;
    Value *ExitVal = Phi.Expansion;

    // Only do the rewrite when the ExitValue can be expanded cheaply.
    // If LoopCanBeDel is true, rewrite exit value aggressively.
    if (ReplaceExitValue == OnlyCheapRepl && !LoopCanBeDel && Phi.HighCost) {
      DeadInsts.push_back(ExitVal);
      continue;
    }

    NumReplaced++;
    Instruction *Inst = cast<Instruction>(PN->getIncomingValue(Phi.Ith));
    PN->setIncomingValue(Phi.Ith, ExitVal);

    // If this instruction is dead now, delete it. Don't do it now to avoid
    // invalidating iterators.
    if (isInstructionTriviallyDead(Inst, TLI))
      DeadInsts.push_back(Inst);

    // Replace PN with ExitVal if that is legal and does not break LCSSA.
    if (PN->getNumIncomingValues() == 1 &&
        LI->replacementPreservesLCSSAForm(PN, ExitVal)) {
      PN->replaceAllUsesWith(ExitVal);
      PN->eraseFromParent();
    }
  }

  // The insertion point instruction may have been deleted; clear it out
  // so that the rewriter doesn't trip over it later.
  Rewriter.clearInsertPoint();
  return NumReplaced;
}

/// Set weights for \p UnrolledLoop and \p RemainderLoop based on weights for
/// \p OrigLoop.
void llvm::setProfileInfoAfterUnrolling(Loop *OrigLoop, Loop *UnrolledLoop,
                                        Loop *RemainderLoop, uint64_t UF) {
  assert(UF > 0 && "Zero unrolled factor is not supported");
  assert(UnrolledLoop != RemainderLoop &&
         "Unrolled and Remainder loops are expected to distinct");

  // Get number of iterations in the original scalar loop.
  unsigned OrigLoopInvocationWeight = 0;
  Optional<unsigned> OrigAverageTripCount =
      getLoopEstimatedTripCount(OrigLoop, &OrigLoopInvocationWeight);
  if (!OrigAverageTripCount)
    return;

  // Calculate number of iterations in unrolled loop.
  unsigned UnrolledAverageTripCount = *OrigAverageTripCount / UF;
  // Calculate number of iterations for remainder loop.
  unsigned RemainderAverageTripCount = *OrigAverageTripCount % UF;

  setLoopEstimatedTripCount(UnrolledLoop, UnrolledAverageTripCount,
                            OrigLoopInvocationWeight);
  setLoopEstimatedTripCount(RemainderLoop, RemainderAverageTripCount,
                            OrigLoopInvocationWeight);
}

/// Utility that implements appending of loops onto a worklist.
/// Loops are added in preorder (analogous for reverse postorder for trees),
/// and the worklist is processed LIFO.
template <typename RangeT>
void llvm::appendReversedLoopsToWorklist(
    RangeT &&Loops, SmallPriorityWorklist<Loop *, 4> &Worklist) {
  // We use an internal worklist to build up the preorder traversal without
  // recursion.
  SmallVector<Loop *, 4> PreOrderLoops, PreOrderWorklist;

  // We walk the initial sequence of loops in reverse because we generally want
  // to visit defs before uses and the worklist is LIFO.
  for (Loop *RootL : Loops) {
    assert(PreOrderLoops.empty() && "Must start with an empty preorder walk.");
    assert(PreOrderWorklist.empty() &&
           "Must start with an empty preorder walk worklist.");
    PreOrderWorklist.push_back(RootL);
    do {
      Loop *L = PreOrderWorklist.pop_back_val();
      PreOrderWorklist.append(L->begin(), L->end());
      PreOrderLoops.push_back(L);
    } while (!PreOrderWorklist.empty());

    Worklist.insert(std::move(PreOrderLoops));
    PreOrderLoops.clear();
  }
}

template <typename RangeT>
void llvm::appendLoopsToWorklist(RangeT &&Loops,
                                 SmallPriorityWorklist<Loop *, 4> &Worklist) {
  appendReversedLoopsToWorklist(reverse(Loops), Worklist);
}

template void llvm::appendLoopsToWorklist<ArrayRef<Loop *> &>(
    ArrayRef<Loop *> &Loops, SmallPriorityWorklist<Loop *, 4> &Worklist);

template void
llvm::appendLoopsToWorklist<Loop &>(Loop &L,
                                    SmallPriorityWorklist<Loop *, 4> &Worklist);

void llvm::appendLoopsToWorklist(LoopInfo &LI,
                                 SmallPriorityWorklist<Loop *, 4> &Worklist) {
  appendReversedLoopsToWorklist(LI, Worklist);
}

Loop *llvm::cloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
                      LoopInfo *LI, LPPassManager *LPM) {
  Loop &New = *LI->AllocateLoop();
  if (PL)
    PL->addChildLoop(&New);
  else
    LI->addTopLevelLoop(&New);

  if (LPM)
    LPM->addLoop(New);

  // Add all of the blocks in L to the new loop.
  for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
       I != E; ++I)
    if (LI->getLoopFor(*I) == L)
      New.addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);

  // Add all of the subloops to the new loop.
  for (Loop *I : *L)
    cloneLoop(I, &New, VM, LI, LPM);

  return &New;
}

/// IR Values for the lower and upper bounds of a pointer evolution.  We
/// need to use value-handles because SCEV expansion can invalidate previously
/// expanded values.  Thus expansion of a pointer can invalidate the bounds for
/// a previous one.
struct PointerBounds {
  TrackingVH<Value> Start;
  TrackingVH<Value> End;
};

/// Expand code for the lower and upper bound of the pointer group \p CG
/// in \p TheLoop.  \return the values for the bounds.
static PointerBounds expandBounds(const RuntimeCheckingPtrGroup *CG,
                                  Loop *TheLoop, Instruction *Loc,
                                  SCEVExpander &Exp) {
  ScalarEvolution *SE = Exp.getSE();
  // TODO: Add helper to retrieve pointers to CG.
  Value *Ptr = CG->RtCheck.Pointers[CG->Members[0]].PointerValue;
  const SCEV *Sc = SE->getSCEV(Ptr);

  unsigned AS = Ptr->getType()->getPointerAddressSpace();
  LLVMContext &Ctx = Loc->getContext();

  // Use this type for pointer arithmetic.
  Type *PtrArithTy = Type::getInt8PtrTy(Ctx, AS);

  if (SE->isLoopInvariant(Sc, TheLoop)) {
    LLVM_DEBUG(dbgs() << "LAA: Adding RT check for a loop invariant ptr:"
                      << *Ptr << "\n");
    // Ptr could be in the loop body. If so, expand a new one at the correct
    // location.
    Instruction *Inst = dyn_cast<Instruction>(Ptr);
    Value *NewPtr = (Inst && TheLoop->contains(Inst))
                        ? Exp.expandCodeFor(Sc, PtrArithTy, Loc)
                        : Ptr;
    // We must return a half-open range, which means incrementing Sc.
    const SCEV *ScPlusOne = SE->getAddExpr(Sc, SE->getOne(PtrArithTy));
    Value *NewPtrPlusOne = Exp.expandCodeFor(ScPlusOne, PtrArithTy, Loc);
    return {NewPtr, NewPtrPlusOne};
  } else {
    Value *Start = nullptr, *End = nullptr;
    LLVM_DEBUG(dbgs() << "LAA: Adding RT check for range:\n");
    Start = Exp.expandCodeFor(CG->Low, PtrArithTy, Loc);
    End = Exp.expandCodeFor(CG->High, PtrArithTy, Loc);
    LLVM_DEBUG(dbgs() << "Start: " << *CG->Low << " End: " << *CG->High
                      << "\n");
    return {Start, End};
  }
}

/// Turns a collection of checks into a collection of expanded upper and
/// lower bounds for both pointers in the check.
static SmallVector<std::pair<PointerBounds, PointerBounds>, 4>
expandBounds(const SmallVectorImpl<RuntimePointerCheck> &PointerChecks, Loop *L,
             Instruction *Loc, SCEVExpander &Exp) {
  SmallVector<std::pair<PointerBounds, PointerBounds>, 4> ChecksWithBounds;

  // Here we're relying on the SCEV Expander's cache to only emit code for the
  // same bounds once.
  transform(PointerChecks, std::back_inserter(ChecksWithBounds),
            [&](const RuntimePointerCheck &Check) {
              PointerBounds First = expandBounds(Check.first, L, Loc, Exp),
                            Second = expandBounds(Check.second, L, Loc, Exp);
              return std::make_pair(First, Second);
            });

  return ChecksWithBounds;
}

std::pair<Instruction *, Instruction *> llvm::addRuntimeChecks(
    Instruction *Loc, Loop *TheLoop,
    const SmallVectorImpl<RuntimePointerCheck> &PointerChecks,
    SCEVExpander &Exp) {
  // TODO: Move noalias annotation code from LoopVersioning here and share with LV if possible.
  // TODO: Pass  RtPtrChecking instead of PointerChecks and SE separately, if possible
  auto ExpandedChecks = expandBounds(PointerChecks, TheLoop, Loc, Exp);

  LLVMContext &Ctx = Loc->getContext();
  Instruction *FirstInst = nullptr;
  IRBuilder<> ChkBuilder(Loc);
  // Our instructions might fold to a constant.
  Value *MemoryRuntimeCheck = nullptr;

  // FIXME: this helper is currently a duplicate of the one in
  // LoopVectorize.cpp.
  auto GetFirstInst = [](Instruction *FirstInst, Value *V,
                         Instruction *Loc) -> Instruction * {
    if (FirstInst)
      return FirstInst;
    if (Instruction *I = dyn_cast<Instruction>(V))
      return I->getParent() == Loc->getParent() ? I : nullptr;
    return nullptr;
  };

  for (const auto &Check : ExpandedChecks) {
    const PointerBounds &A = Check.first, &B = Check.second;
    // Check if two pointers (A and B) conflict where conflict is computed as:
    // start(A) <= end(B) && start(B) <= end(A)
    unsigned AS0 = A.Start->getType()->getPointerAddressSpace();
    unsigned AS1 = B.Start->getType()->getPointerAddressSpace();

    assert((AS0 == B.End->getType()->getPointerAddressSpace()) &&
           (AS1 == A.End->getType()->getPointerAddressSpace()) &&
           "Trying to bounds check pointers with different address spaces");

    Type *PtrArithTy0 = Type::getInt8PtrTy(Ctx, AS0);
    Type *PtrArithTy1 = Type::getInt8PtrTy(Ctx, AS1);

    Value *Start0 = ChkBuilder.CreateBitCast(A.Start, PtrArithTy0, "bc");
    Value *Start1 = ChkBuilder.CreateBitCast(B.Start, PtrArithTy1, "bc");
    Value *End0 = ChkBuilder.CreateBitCast(A.End, PtrArithTy1, "bc");
    Value *End1 = ChkBuilder.CreateBitCast(B.End, PtrArithTy0, "bc");

    // [A|B].Start points to the first accessed byte under base [A|B].
    // [A|B].End points to the last accessed byte, plus one.
    // There is no conflict when the intervals are disjoint:
    // NoConflict = (B.Start >= A.End) || (A.Start >= B.End)
    //
    // bound0 = (B.Start < A.End)
    // bound1 = (A.Start < B.End)
    //  IsConflict = bound0 & bound1
    Value *Cmp0 = ChkBuilder.CreateICmpULT(Start0, End1, "bound0");
    FirstInst = GetFirstInst(FirstInst, Cmp0, Loc);
    Value *Cmp1 = ChkBuilder.CreateICmpULT(Start1, End0, "bound1");
    FirstInst = GetFirstInst(FirstInst, Cmp1, Loc);
    Value *IsConflict = ChkBuilder.CreateAnd(Cmp0, Cmp1, "found.conflict");
    FirstInst = GetFirstInst(FirstInst, IsConflict, Loc);
    if (MemoryRuntimeCheck) {
      IsConflict =
          ChkBuilder.CreateOr(MemoryRuntimeCheck, IsConflict, "conflict.rdx");
      FirstInst = GetFirstInst(FirstInst, IsConflict, Loc);
    }
    MemoryRuntimeCheck = IsConflict;
  }

  if (!MemoryRuntimeCheck)
    return std::make_pair(nullptr, nullptr);

  // We have to do this trickery because the IRBuilder might fold the check to a
  // constant expression in which case there is no Instruction anchored in a
  // the block.
  Instruction *Check =
      BinaryOperator::CreateAnd(MemoryRuntimeCheck, ConstantInt::getTrue(Ctx));
  ChkBuilder.Insert(Check, "memcheck.conflict");
  FirstInst = GetFirstInst(FirstInst, Check, Loc);
  return std::make_pair(FirstInst, Check);
}