text: //===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
  //
  // 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 implements some loop unrolling utilities. It does not define any
  // actual pass or policy, but provides a single function to perform loop
  // unrolling.
  //
  // The process of unrolling can produce extraneous basic blocks linked with
  // unconditional branches.  This will be corrected in the future.
  //
  //===----------------------------------------------------------------------===//
  
  #include "llvm/ADT/ArrayRef.h"
  #include "llvm/ADT/DenseMap.h"
  #include "llvm/ADT/Optional.h"
  #include "llvm/ADT/STLExtras.h"
  #include "llvm/ADT/SetVector.h"
  #include "llvm/ADT/SmallVector.h"
  #include "llvm/ADT/Statistic.h"
  #include "llvm/ADT/StringRef.h"
  #include "llvm/ADT/Twine.h"
  #include "llvm/ADT/ilist_iterator.h"
  #include "llvm/ADT/iterator_range.h"
  #include "llvm/Analysis/AssumptionCache.h"
  #include "llvm/Analysis/DomTreeUpdater.h"
  #include "llvm/Analysis/InstructionSimplify.h"
  #include "llvm/Analysis/LoopInfo.h"
  #include "llvm/Analysis/LoopIterator.h"
  #include "llvm/Analysis/OptimizationRemarkEmitter.h"
  #include "llvm/Analysis/ScalarEvolution.h"
  #include "llvm/IR/BasicBlock.h"
  #include "llvm/IR/CFG.h"
  #include "llvm/IR/Constants.h"
  #include "llvm/IR/DebugInfoMetadata.h"
  #include "llvm/IR/DebugLoc.h"
  #include "llvm/IR/DiagnosticInfo.h"
  #include "llvm/IR/Dominators.h"
  #include "llvm/IR/Function.h"
  #include "llvm/IR/Instruction.h"
  #include "llvm/IR/Instructions.h"
  #include "llvm/IR/IntrinsicInst.h"
  #include "llvm/IR/Metadata.h"
  #include "llvm/IR/Module.h"
  #include "llvm/IR/Use.h"
  #include "llvm/IR/User.h"
  #include "llvm/IR/ValueHandle.h"
  #include "llvm/IR/ValueMap.h"
  #include "llvm/Support/Casting.h"
  #include "llvm/Support/CommandLine.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/GenericDomTree.h"
  #include "llvm/Support/MathExtras.h"
  #include "llvm/Support/raw_ostream.h"
  #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  #include "llvm/Transforms/Utils/Cloning.h"
  #include "llvm/Transforms/Utils/Local.h"
  #include "llvm/Transforms/Utils/LoopPeel.h"
  #include "llvm/Transforms/Utils/LoopSimplify.h"
  #include "llvm/Transforms/Utils/LoopUtils.h"
  #include "llvm/Transforms/Utils/SimplifyIndVar.h"
  #include "llvm/Transforms/Utils/UnrollLoop.h"
  #include "llvm/Transforms/Utils/ValueMapper.h"
  #include <algorithm>
  #include <assert.h>
  #include <type_traits>
  #include <vector>
  
  namespace llvm {
  class DataLayout;
  class Value;
  } // namespace llvm
  
  using namespace llvm;
  
  #define DEBUG_TYPE "loop-unroll"
  
  // TODO: Should these be here or in LoopUnroll?
  STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
  STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)");
  STATISTIC(NumUnrolledNotLatch, "Number of loops unrolled without a conditional "
                                 "latch (completely or otherwise)");
  
  static cl::opt<bool>
  UnrollRuntimeEpilog("unroll-runtime-epilog", cl::init(false), cl::Hidden,
                      cl::desc("Allow runtime unrolled loops to be unrolled "
                               "with epilog instead of prolog."));
  
  static cl::opt<bool>
  UnrollVerifyDomtree("unroll-verify-domtree", cl::Hidden,
                      cl::desc("Verify domtree after unrolling"),
  #ifdef EXPENSIVE_CHECKS
      cl::init(true)
  #else
      cl::init(false)
  #endif
                      );
  
  /// Check if unrolling created a situation where we need to insert phi nodes to
  /// preserve LCSSA form.
  /// \param Blocks is a vector of basic blocks representing unrolled loop.
  /// \param L is the outer loop.
  /// It's possible that some of the blocks are in L, and some are not. In this
  /// case, if there is a use is outside L, and definition is inside L, we need to
  /// insert a phi-node, otherwise LCSSA will be broken.
  /// The function is just a helper function for llvm::UnrollLoop that returns
  /// true if this situation occurs, indicating that LCSSA needs to be fixed.
  static bool needToInsertPhisForLCSSA(Loop *L,
                                       const std::vector<BasicBlock *> &Blocks,
                                       LoopInfo *LI) {
    for (BasicBlock *BB : Blocks) {
      if (LI->getLoopFor(BB) == L)
        continue;
      for (Instruction &I : *BB) {
        for (Use &U : I.operands()) {
          if (const auto *Def = dyn_cast<Instruction>(U)) {
            Loop *DefLoop = LI->getLoopFor(Def->getParent());
            if (!DefLoop)
              continue;
            if (DefLoop->contains(L))
              return true;
          }
        }
      }
    }
    return false;
  }
  
  /// Adds ClonedBB to LoopInfo, creates a new loop for ClonedBB if necessary
  /// and adds a mapping from the original loop to the new loop to NewLoops.
  /// Returns nullptr if no new loop was created and a pointer to the
  /// original loop OriginalBB was part of otherwise.
  const Loop* llvm::addClonedBlockToLoopInfo(BasicBlock *OriginalBB,
                                             BasicBlock *ClonedBB, LoopInfo *LI,
                                             NewLoopsMap &NewLoops) {
    // Figure out which loop New is in.
    const Loop *OldLoop = LI->getLoopFor(OriginalBB);
    assert(OldLoop && "Should (at least) be in the loop being unrolled!");
  
    Loop *&NewLoop = NewLoops[OldLoop];
    if (!NewLoop) {
      // Found a new sub-loop.
      assert(OriginalBB == OldLoop->getHeader() &&
             "Header should be first in RPO");
  
      NewLoop = LI->AllocateLoop();
      Loop *NewLoopParent = NewLoops.lookup(OldLoop->getParentLoop());
  
      if (NewLoopParent)
        NewLoopParent->addChildLoop(NewLoop);
      else
        LI->addTopLevelLoop(NewLoop);
  
      NewLoop->addBasicBlockToLoop(ClonedBB, *LI);
      return OldLoop;
    } else {
      NewLoop->addBasicBlockToLoop(ClonedBB, *LI);
      return nullptr;
    }
  }
  
  /// The function chooses which type of unroll (epilog or prolog) is more
  /// profitabale.
  /// Epilog unroll is more profitable when there is PHI that starts from
  /// constant.  In this case epilog will leave PHI start from constant,
  /// but prolog will convert it to non-constant.
  ///
  /// loop:
  ///   PN = PHI [I, Latch], [CI, PreHeader]
  ///   I = foo(PN)
  ///   ...
  ///
  /// Epilog unroll case.
  /// loop:
  ///   PN = PHI [I2, Latch], [CI, PreHeader]
  ///   I1 = foo(PN)
  ///   I2 = foo(I1)
  ///   ...
  /// Prolog unroll case.
  ///   NewPN = PHI [PrologI, Prolog], [CI, PreHeader]
  /// loop:
  ///   PN = PHI [I2, Latch], [NewPN, PreHeader]
  ///   I1 = foo(PN)
  ///   I2 = foo(I1)
  ///   ...
  ///
  static bool isEpilogProfitable(Loop *L) {
    BasicBlock *PreHeader = L->getLoopPreheader();
    BasicBlock *Header = L->getHeader();
    assert(PreHeader && Header);
    for (const PHINode &PN : Header->phis()) {
      if (isa<ConstantInt>(PN.getIncomingValueForBlock(PreHeader)))
        return true;
    }
    return false;
  }
  
  /// Perform some cleanup and simplifications on loops after unrolling. It is
  /// useful to simplify the IV's in the new loop, as well as do a quick
  /// simplify/dce pass of the instructions.
  void llvm::simplifyLoopAfterUnroll(Loop *L, bool SimplifyIVs, LoopInfo *LI,
                                     ScalarEvolution *SE, DominatorTree *DT,
                                     AssumptionCache *AC,
                                     const TargetTransformInfo *TTI) {
    // Simplify any new induction variables in the partially unrolled loop.
    if (SE && SimplifyIVs) {
      SmallVector<WeakTrackingVH, 16> DeadInsts;
      simplifyLoopIVs(L, SE, DT, LI, TTI, DeadInsts);
  
      // Aggressively clean up dead instructions that simplifyLoopIVs already
      // identified. Any remaining should be cleaned up below.
      while (!DeadInsts.empty()) {
        Value *V = DeadInsts.pop_back_val();
        if (Instruction *Inst = dyn_cast_or_null<Instruction>(V))
          RecursivelyDeleteTriviallyDeadInstructions(Inst);
      }
    }
  
    // At this point, the code is well formed.  We now do a quick sweep over the
    // inserted code, doing constant propagation and dead code elimination as we
    // go.
    const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
    for (BasicBlock *BB : L->getBlocks()) {
      for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
        Instruction *Inst = &*I++;
  
        if (Value *V = SimplifyInstruction(Inst, {DL, nullptr, DT, AC}))
          if (LI->replacementPreservesLCSSAForm(Inst, V))
            Inst->replaceAllUsesWith(V);
        if (isInstructionTriviallyDead(Inst))
          BB->getInstList().erase(Inst);
      }
    }
  
    // TODO: after peeling or unrolling, previously loop variant conditions are
    // likely to fold to constants, eagerly propagating those here will require
    // fewer cleanup passes to be run.  Alternatively, a LoopEarlyCSE might be
    // appropriate.
  }
  
  /// Unroll the given loop by Count. The loop must be in LCSSA form.  Unrolling
  /// can only fail when the loop's latch block is not terminated by a conditional
  /// branch instruction. However, if the trip count (and multiple) are not known,
  /// loop unrolling will mostly produce more code that is no faster.
  ///
  /// TripCount is the upper bound of the iteration on which control exits
  /// LatchBlock. Control may exit the loop prior to TripCount iterations either
  /// via an early branch in other loop block or via LatchBlock terminator. This
  /// is relaxed from the general definition of trip count which is the number of
  /// times the loop header executes. Note that UnrollLoop assumes that the loop
  /// counter test is in LatchBlock in order to remove unnecesssary instances of
  /// the test.  If control can exit the loop from the LatchBlock's terminator
  /// prior to TripCount iterations, flag PreserveCondBr needs to be set.
  ///
  /// PreserveCondBr indicates whether the conditional branch of the LatchBlock
  /// needs to be preserved.  It is needed when we use trip count upper bound to
  /// fully unroll the loop. If PreserveOnlyFirst is also set then only the first
  /// conditional branch needs to be preserved.
  ///
  /// Similarly, TripMultiple divides the number of times that the LatchBlock may
  /// execute without exiting the loop.
  ///
  /// If AllowRuntime is true then UnrollLoop will consider unrolling loops that
  /// have a runtime (i.e. not compile time constant) trip count.  Unrolling these
  /// loops require a unroll "prologue" that runs "RuntimeTripCount % Count"
  /// iterations before branching into the unrolled loop.  UnrollLoop will not
  /// runtime-unroll the loop if computing RuntimeTripCount will be expensive and
  /// AllowExpensiveTripCount is false.
  ///
  /// If we want to perform PGO-based loop peeling, PeelCount is set to the
  /// number of iterations we want to peel off.
  ///
  /// The LoopInfo Analysis that is passed will be kept consistent.
  ///
  /// This utility preserves LoopInfo. It will also preserve ScalarEvolution and
  /// DominatorTree if they are non-null.
  ///
  /// If RemainderLoop is non-null, it will receive the remainder loop (if
  /// required and not fully unrolled).
  LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
                                    ScalarEvolution *SE, DominatorTree *DT,
                                    AssumptionCache *AC,
                                    const TargetTransformInfo *TTI,
                                    OptimizationRemarkEmitter *ORE,
                                    bool PreserveLCSSA, Loop **RemainderLoop) {
  
    if (!L->getLoopPreheader()) {
      LLVM_DEBUG(dbgs() << "  Can't unroll; loop preheader-insertion failed.\n");
      return LoopUnrollResult::Unmodified;
    }
  
    if (!L->getLoopLatch()) {
      LLVM_DEBUG(dbgs() << "  Can't unroll; loop exit-block-insertion failed.\n");
      return LoopUnrollResult::Unmodified;
    }
  
    // Loops with indirectbr cannot be cloned.
    if (!L->isSafeToClone()) {
      LLVM_DEBUG(dbgs() << "  Can't unroll; Loop body cannot be cloned.\n");
      return LoopUnrollResult::Unmodified;
    }
  
    if (L->getHeader()->hasAddressTaken()) {
      // The loop-rotate pass can be helpful to avoid this in many cases.
      LLVM_DEBUG(
          dbgs() << "  Won't unroll loop: address of header block is taken.\n");
      return LoopUnrollResult::Unmodified;
    }
  
    if (ULO.TripCount != 0)
      LLVM_DEBUG(dbgs() << "  Trip Count = " << ULO.TripCount << "\n");
    if (ULO.TripMultiple != 1)
      LLVM_DEBUG(dbgs() << "  Trip Multiple = " << ULO.TripMultiple << "\n");
  
    // Effectively "DCE" unrolled iterations that are beyond the tripcount
    // and will never be executed.
    if (ULO.TripCount != 0 && ULO.Count > ULO.TripCount)
      ULO.Count = ULO.TripCount;
  
    // Don't enter the unroll code if there is nothing to do.
    if (ULO.TripCount == 0 && ULO.Count < 2 && ULO.PeelCount == 0) {
      LLVM_DEBUG(dbgs() << "Won't unroll; almost nothing to do\n");
      return LoopUnrollResult::Unmodified;
    }
  
    assert(ULO.Count > 0);
    assert(ULO.TripMultiple > 0);
    assert(ULO.TripCount == 0 || ULO.TripCount % ULO.TripMultiple == 0);
  
    // Are we eliminating the loop control altogether?
    bool CompletelyUnroll = ULO.Count == ULO.TripCount;
  
    // We assume a run-time trip count if the compiler cannot
    // figure out the loop trip count and the unroll-runtime
    // flag is specified.
    bool RuntimeTripCount =
        (ULO.TripCount == 0 && ULO.Count > 0 && ULO.AllowRuntime);
  
    assert((!RuntimeTripCount || !ULO.PeelCount) &&
           "Did not expect runtime trip-count unrolling "
           "and peeling for the same loop");
  
    bool Peeled = false;
    if (ULO.PeelCount) {
      Peeled = peelLoop(L, ULO.PeelCount, LI, SE, DT, AC, PreserveLCSSA);
  
      // Successful peeling may result in a change in the loop preheader/trip
      // counts. If we later unroll the loop, we want these to be updated.
      if (Peeled) {
        // According to our guards and profitability checks the only
        // meaningful exit should be latch block. Other exits go to deopt,
        // so we do not worry about them.
        BasicBlock *ExitingBlock = L->getLoopLatch();
        assert(ExitingBlock && "Loop without exiting block?");
        assert(L->isLoopExiting(ExitingBlock) && "Latch is not exiting?");
        ULO.TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);
        ULO.TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock);
      }
    }
  
    // All these values should be taken only after peeling because they might have
    // changed.
    BasicBlock *Preheader = L->getLoopPreheader();
    BasicBlock *Header = L->getHeader();
    BasicBlock *LatchBlock = L->getLoopLatch();
    SmallVector<BasicBlock *, 4> ExitBlocks;
    L->getExitBlocks(ExitBlocks);
    std::vector<BasicBlock *> OriginalLoopBlocks = L->getBlocks();
  
    // Go through all exits of L and see if there are any phi-nodes there. We just
    // conservatively assume that they're inserted to preserve LCSSA form, which
    // means that complete unrolling might break this form. We need to either fix
    // it in-place after the transformation, or entirely rebuild LCSSA. TODO: For
    // now we just recompute LCSSA for the outer loop, but it should be possible
    // to fix it in-place.
    bool NeedToFixLCSSA =
        PreserveLCSSA && CompletelyUnroll &&
        any_of(ExitBlocks,
               [](const BasicBlock *BB) { return isa<PHINode>(BB->begin()); });
  
    // The current loop unroll pass can unroll loops that have
    // (1) single latch; and
    // (2a) latch is unconditional; or
    // (2b) latch is conditional and is an exiting block
    // FIXME: The implementation can be extended to work with more complicated
    // cases, e.g. loops with multiple latches.
    BranchInst *LatchBI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
  
    // A conditional branch which exits the loop, which can be optimized to an
    // unconditional branch in the unrolled loop in some cases.
    BranchInst *ExitingBI = nullptr;
    bool LatchIsExiting = L->isLoopExiting(LatchBlock);
    if (LatchIsExiting)
      ExitingBI = LatchBI;
    else if (BasicBlock *ExitingBlock = L->getExitingBlock())
      ExitingBI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
    if (!LatchBI || (LatchBI->isConditional() && !LatchIsExiting)) {
      // If the peeling guard is changed this assert may be relaxed or even
      // deleted.
      assert(!Peeled && "Peeling guard changed!");
      LLVM_DEBUG(
          dbgs() << "Can't unroll; a conditional latch must exit the loop");
      return LoopUnrollResult::Unmodified;
    }
    LLVM_DEBUG({
      if (ExitingBI)
        dbgs() << "  Exiting Block = " << ExitingBI->getParent()->getName()
               << "\n";
      else
        dbgs() << "  No single exiting block\n";
    });
  
    // Loops containing convergent instructions must have a count that divides
    // their TripMultiple.
    LLVM_DEBUG(
        {
          bool HasConvergent = false;
          for (auto &BB : L->blocks())
            for (auto &I : *BB)
              if (auto *CB = dyn_cast<CallBase>(&I))
                HasConvergent |= CB->isConvergent();
          assert((!HasConvergent || ULO.TripMultiple % ULO.Count == 0) &&
                 "Unroll count must divide trip multiple if loop contains a "
                 "convergent operation.");
        });
  
    bool EpilogProfitability =
        UnrollRuntimeEpilog.getNumOccurrences() ? UnrollRuntimeEpilog
                                                : isEpilogProfitable(L);
  
    if (RuntimeTripCount && ULO.TripMultiple % ULO.Count != 0 &&
        !UnrollRuntimeLoopRemainder(L, ULO.Count, ULO.AllowExpensiveTripCount,
                                    EpilogProfitability, ULO.UnrollRemainder,
                                    ULO.ForgetAllSCEV, LI, SE, DT, AC, TTI,
                                    PreserveLCSSA, RemainderLoop)) {
      if (ULO.Force)
        RuntimeTripCount = false;
      else {
        LLVM_DEBUG(dbgs() << "Won't unroll; remainder loop could not be "
                             "generated when assuming runtime trip count\n");
        return LoopUnrollResult::Unmodified;
      }
    }
  
    // If we know the trip count, we know the multiple...
    unsigned BreakoutTrip = 0;
    if (ULO.TripCount != 0) {
      BreakoutTrip = ULO.TripCount % ULO.Count;
      ULO.TripMultiple = 0;
    } else {
      // Figure out what multiple to use.
      BreakoutTrip = ULO.TripMultiple =
          (unsigned)GreatestCommonDivisor64(ULO.Count, ULO.TripMultiple);
    }
  
    using namespace ore;
    // Report the unrolling decision.
    if (CompletelyUnroll) {
      LLVM_DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName()
                        << " with trip count " << ULO.TripCount << "!\n");
      if (ORE)
        ORE->emit([&]() {
          return OptimizationRemark(DEBUG_TYPE, "FullyUnrolled", L->getStartLoc(),
                                    L->getHeader())
                 << "completely unrolled loop with "
                 << NV("UnrollCount", ULO.TripCount) << " iterations";
        });
    } else if (ULO.PeelCount) {
      LLVM_DEBUG(dbgs() << "PEELING loop %" << Header->getName()
                        << " with iteration count " << ULO.PeelCount << "!\n");
      if (ORE)
        ORE->emit([&]() {
          return OptimizationRemark(DEBUG_TYPE, "Peeled", L->getStartLoc(),
                                    L->getHeader())
                 << " peeled loop by " << NV("PeelCount", ULO.PeelCount)
                 << " iterations";
        });
    } else {
      auto DiagBuilder = [&]() {
        OptimizationRemark Diag(DEBUG_TYPE, "PartialUnrolled", L->getStartLoc(),
                                L->getHeader());
        return Diag << "unrolled loop by a factor of "
                    << NV("UnrollCount", ULO.Count);
      };
  
      LLVM_DEBUG(dbgs() << "UNROLLING loop %" << Header->getName() << " by "
                        << ULO.Count);
      if (ULO.TripMultiple == 0 || BreakoutTrip != ULO.TripMultiple) {
        LLVM_DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip);
        if (ORE)
          ORE->emit([&]() {
            return DiagBuilder() << " with a breakout at trip "
                                 << NV("BreakoutTrip", BreakoutTrip);
          });
      } else if (ULO.TripMultiple != 1) {
        LLVM_DEBUG(dbgs() << " with " << ULO.TripMultiple << " trips per branch");
        if (ORE)
          ORE->emit([&]() {
            return DiagBuilder()
                   << " with " << NV("TripMultiple", ULO.TripMultiple)
                   << " trips per branch";
          });
      } else if (RuntimeTripCount) {
        LLVM_DEBUG(dbgs() << " with run-time trip count");
        if (ORE)
          ORE->emit(
              [&]() { return DiagBuilder() << " with run-time trip count"; });
      }
      LLVM_DEBUG(dbgs() << "!\n");
    }
  
    // We are going to make changes to this loop. SCEV may be keeping cached info
    // about it, in particular about backedge taken count. The changes we make
    // are guaranteed to invalidate this information for our loop. It is tempting
    // to only invalidate the loop being unrolled, but it is incorrect as long as
    // all exiting branches from all inner loops have impact on the outer loops,
    // and if something changes inside them then any of outer loops may also
    // change. When we forget outermost loop, we also forget all contained loops
    // and this is what we need here.
    if (SE) {
      if (ULO.ForgetAllSCEV)
        SE->forgetAllLoops();
      else
        SE->forgetTopmostLoop(L);
    }
  
    if (!LatchIsExiting)
      ++NumUnrolledNotLatch;
    Optional<bool> ContinueOnTrue = None;
    BasicBlock *LoopExit = nullptr;
    if (ExitingBI) {
      ContinueOnTrue = L->contains(ExitingBI->getSuccessor(0));
      LoopExit = ExitingBI->getSuccessor(*ContinueOnTrue);
    }
  
    // For the first iteration of the loop, we should use the precloned values for
    // PHI nodes.  Insert associations now.
    ValueToValueMapTy LastValueMap;
    std::vector<PHINode*> OrigPHINode;
    for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
      OrigPHINode.push_back(cast<PHINode>(I));
    }
  
    std::vector<BasicBlock *> Headers;
    std::vector<BasicBlock *> ExitingBlocks;
    std::vector<BasicBlock *> ExitingSucc;
    std::vector<BasicBlock *> Latches;
    Headers.push_back(Header);
    Latches.push_back(LatchBlock);
    if (ExitingBI) {
      ExitingBlocks.push_back(ExitingBI->getParent());
      ExitingSucc.push_back(ExitingBI->getSuccessor(!(*ContinueOnTrue)));
    }
  
    // The current on-the-fly SSA update requires blocks to be processed in
    // reverse postorder so that LastValueMap contains the correct value at each
    // exit.
    LoopBlocksDFS DFS(L);
    DFS.perform(LI);
  
    // Stash the DFS iterators before adding blocks to the loop.
    LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO();
    LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO();
  
    std::vector<BasicBlock*> UnrolledLoopBlocks = L->getBlocks();
  
    // Loop Unrolling might create new loops. While we do preserve LoopInfo, we
    // might break loop-simplified form for these loops (as they, e.g., would
    // share the same exit blocks). We'll keep track of loops for which we can
    // break this so that later we can re-simplify them.
    SmallSetVector<Loop *, 4> LoopsToSimplify;
    for (Loop *SubLoop : *L)
      LoopsToSimplify.insert(SubLoop);
  
    if (Header->getParent()->isDebugInfoForProfiling())
      for (BasicBlock *BB : L->getBlocks())
        for (Instruction &I : *BB)
          if (!isa<DbgInfoIntrinsic>(&I))
            if (const DILocation *DIL = I.getDebugLoc()) {
              auto NewDIL = DIL->cloneByMultiplyingDuplicationFactor(ULO.Count);
              if (NewDIL)
                I.setDebugLoc(NewDIL.getValue());
              else
                LLVM_DEBUG(dbgs()
                           << "Failed to create new discriminator: "
                           << DIL->getFilename() << " Line: " << DIL->getLine());
            }
  
    // Identify what noalias metadata is inside the loop: if it is inside the
    // loop, the associated metadata must be cloned for each iteration.
    SmallVector<MDNode *, 6> LoopLocalNoAliasDeclScopes;
    identifyNoAliasScopesToClone(L->getBlocks(), LoopLocalNoAliasDeclScopes);
  
    for (unsigned It = 1; It != ULO.Count; ++It) {
      SmallVector<BasicBlock *, 8> NewBlocks;
      SmallDenseMap<const Loop *, Loop *, 4> NewLoops;
      NewLoops[L] = L;
  
      for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
        ValueToValueMapTy VMap;
        BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It));
        Header->getParent()->getBasicBlockList().push_back(New);
  
        assert((*BB != Header || LI->getLoopFor(*BB) == L) &&
               "Header should not be in a sub-loop");
        // Tell LI about New.
        const Loop *OldLoop = addClonedBlockToLoopInfo(*BB, New, LI, NewLoops);
        if (OldLoop)
          LoopsToSimplify.insert(NewLoops[OldLoop]);
  
        if (*BB == Header)
          // Loop over all of the PHI nodes in the block, changing them to use
          // the incoming values from the previous block.
          for (PHINode *OrigPHI : OrigPHINode) {
            PHINode *NewPHI = cast<PHINode>(VMap[OrigPHI]);
            Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
            if (Instruction *InValI = dyn_cast<Instruction>(InVal))
              if (It > 1 && L->contains(InValI))
                InVal = LastValueMap[InValI];
            VMap[OrigPHI] = InVal;
            New->getInstList().erase(NewPHI);
          }
  
        // Update our running map of newest clones
        LastValueMap[*BB] = New;
        for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
             VI != VE; ++VI)
          LastValueMap[VI->first] = VI->second;
  
        // Add phi entries for newly created values to all exit blocks.
        for (BasicBlock *Succ : successors(*BB)) {
          if (L->contains(Succ))
            continue;
          for (PHINode &PHI : Succ->phis()) {
            Value *Incoming = PHI.getIncomingValueForBlock(*BB);
            ValueToValueMapTy::iterator It = LastValueMap.find(Incoming);
            if (It != LastValueMap.end())
              Incoming = It->second;
            PHI.addIncoming(Incoming, New);
          }
        }
        // Keep track of new headers and latches as we create them, so that
        // we can insert the proper branches later.
        if (*BB == Header)
          Headers.push_back(New);
        if (*BB == LatchBlock)
          Latches.push_back(New);
  
        // Keep track of the exiting block and its successor block contained in
        // the loop for the current iteration.
        if (ExitingBI) {
          if (*BB == ExitingBlocks[0])
            ExitingBlocks.push_back(New);
          if (*BB == ExitingSucc[0])
            ExitingSucc.push_back(New);
        }
  
        NewBlocks.push_back(New);
        UnrolledLoopBlocks.push_back(New);
  
        // Update DomTree: since we just copy the loop body, and each copy has a
        // dedicated entry block (copy of the header block), this header's copy
        // dominates all copied blocks. That means, dominance relations in the
        // copied body are the same as in the original body.
        if (DT) {
          if (*BB == Header)
            DT->addNewBlock(New, Latches[It - 1]);
          else {
            auto BBDomNode = DT->getNode(*BB);
            auto BBIDom = BBDomNode->getIDom();
            BasicBlock *OriginalBBIDom = BBIDom->getBlock();
            DT->addNewBlock(
                New, cast<BasicBlock>(LastValueMap[cast<Value>(OriginalBBIDom)]));
          }
        }
      }
  
      // Remap all instructions in the most recent iteration
      remapInstructionsInBlocks(NewBlocks, LastValueMap);
      for (BasicBlock *NewBlock : NewBlocks) {
        for (Instruction &I : *NewBlock) {
          if (auto *II = dyn_cast<IntrinsicInst>(&I))
            if (II->getIntrinsicID() == Intrinsic::assume)
              AC->registerAssumption(II);
        }
      }
  
      {
        // Identify what other metadata depends on the cloned version. After
        // cloning, replace the metadata with the corrected version for both
        // memory instructions and noalias intrinsics.
        std::string ext = (Twine("It") + Twine(It)).str();
        cloneAndAdaptNoAliasScopes(LoopLocalNoAliasDeclScopes, NewBlocks,
                                   Header->getContext(), ext);
      }
    }
  
    // Loop over the PHI nodes in the original block, setting incoming values.
    for (PHINode *PN : OrigPHINode) {
      if (CompletelyUnroll) {
        PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
        Header->getInstList().erase(PN);
      } else if (ULO.Count > 1) {
        Value *InVal = PN->removeIncomingValue(LatchBlock, false);
        // If this value was defined in the loop, take the value defined by the
        // last iteration of the loop.
        if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
          if (L->contains(InValI))
            InVal = LastValueMap[InVal];
        }
        assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch");
        PN->addIncoming(InVal, Latches.back());
      }
    }
  
    auto setDest = [](BasicBlock *Src, BasicBlock *Dest, BasicBlock *BlockInLoop,
                      bool NeedConditional, Optional<bool> ContinueOnTrue,
                      bool IsDestLoopExit) {
      auto *Term = cast<BranchInst>(Src->getTerminator());
      if (NeedConditional) {
        // Update the conditional branch's successor for the following
        // iteration.
        assert(ContinueOnTrue.hasValue() &&
               "Expecting valid ContinueOnTrue when NeedConditional is true");
        Term->setSuccessor(!(*ContinueOnTrue), Dest);
      } else {
        // Remove phi operands at this loop exit
        if (!IsDestLoopExit) {
          BasicBlock *BB = Src;
          for (BasicBlock *Succ : successors(BB)) {
            // Preserve the incoming value from BB if we are jumping to the block
            // in the current loop.
            if (Succ == BlockInLoop)
              continue;
            for (PHINode &Phi : Succ->phis())
              Phi.removeIncomingValue(BB, false);
          }
        }
        // Replace the conditional branch with an unconditional one.
        BranchInst::Create(Dest, Term);
        Term->eraseFromParent();
      }
    };
  
    // Connect latches of the unrolled iterations to the headers of the next
    // iteration. If the latch is also the exiting block, the conditional branch
    // may have to be preserved.
    for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
      // The branch destination.
      unsigned j = (i + 1) % e;
      BasicBlock *Dest = Headers[j];
      bool NeedConditional = LatchIsExiting;
  
      if (LatchIsExiting) {
        if (RuntimeTripCount && j != 0)
          NeedConditional = false;
  
        // For a complete unroll, make the last iteration end with a branch
        // to the exit block.
        if (CompletelyUnroll) {
          if (j == 0)
            Dest = LoopExit;
          // If using trip count upper bound to completely unroll, we need to
          // keep the conditional branch except the last one because the loop
          // may exit after any iteration.
          assert(NeedConditional &&
                 "NeedCondition cannot be modified by both complete "
                 "unrolling and runtime unrolling");
          NeedConditional =
              (ULO.PreserveCondBr && j && !(ULO.PreserveOnlyFirst && i != 0));
        } else if (j != BreakoutTrip &&
                   (ULO.TripMultiple == 0 || j % ULO.TripMultiple != 0)) {
          // If we know the trip count or a multiple of it, we can safely use an
          // unconditional branch for some iterations.
          NeedConditional = false;
        }
      }
  
      setDest(Latches[i], Dest, Headers[i], NeedConditional, ContinueOnTrue,
              Dest == LoopExit);
    }
  
    if (!LatchIsExiting) {
      // If the latch is not exiting, we may be able to simplify the conditional
      // branches in the unrolled exiting blocks.
      for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
        // The branch destination.
        unsigned j = (i + 1) % e;
        bool NeedConditional = true;
  
        if (RuntimeTripCount && j != 0)
          NeedConditional = false;
  
        if (CompletelyUnroll)
          // We cannot drop the conditional branch for the last condition, as we
          // may have to execute the loop body depending on the condition.
          NeedConditional = j == 0 || ULO.PreserveCondBr;
        else if (j != BreakoutTrip &&
                 (ULO.TripMultiple == 0 || j % ULO.TripMultiple != 0))
          // If we know the trip count or a multiple of it, we can safely use an
          // unconditional branch for some iterations.
          NeedConditional = false;
  
        // Conditional branches from non-latch exiting block have successors
        // either in the same loop iteration or outside the loop. The branches are
        // already correct.
        if (NeedConditional)
          continue;
        setDest(ExitingBlocks[i], ExitingSucc[i], ExitingSucc[i], NeedConditional,
                None, false);
      }
  
      // When completely unrolling, the last latch becomes unreachable.
      if (CompletelyUnroll) {
        BranchInst *Term = cast<BranchInst>(Latches.back()->getTerminator());
        new UnreachableInst(Term->getContext(), Term);
        Term->eraseFromParent();
      }
    }
  
    // Update dominators of blocks we might reach through exits.
    // Immediate dominator of such block might change, because we add more
    // routes which can lead to the exit: we can now reach it from the copied
    // iterations too.
    if (DT && ULO.Count > 1) {
      for (auto *BB : OriginalLoopBlocks) {
        auto *BBDomNode = DT->getNode(BB);
        SmallVector<BasicBlock *, 16> ChildrenToUpdate;
        for (auto *ChildDomNode : BBDomNode->children()) {
          auto *ChildBB = ChildDomNode->getBlock();
          if (!L->contains(ChildBB))
            ChildrenToUpdate.push_back(ChildBB);
        }
        BasicBlock *NewIDom;
        if (ExitingBI && BB == ExitingBlocks[0]) {
          // The latch is special because we emit unconditional branches in
          // some cases where the original loop contained a conditional branch.
          // Since the latch is always at the bottom of the loop, if the latch
          // dominated an exit before unrolling, the new dominator of that exit
          // must also be a latch.  Specifically, the dominator is the first
          // latch which ends in a conditional branch, or the last latch if
          // there is no such latch.
          // For loops exiting from non latch exiting block, we limit the
          // branch simplification to single exiting block loops.
          NewIDom = ExitingBlocks.back();
          for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
            Instruction *Term = ExitingBlocks[i]->getTerminator();
            if (isa<BranchInst>(Term) && cast<BranchInst>(Term)->isConditional()) {
              NewIDom =
                  DT->findNearestCommonDominator(ExitingBlocks[i], Latches[i]);
              break;
            }
          }
        } else {
          // The new idom of the block will be the nearest common dominator
          // of all copies of the previous idom. This is equivalent to the
          // nearest common dominator of the previous idom and the first latch,
          // which dominates all copies of the previous idom.
          NewIDom = DT->findNearestCommonDominator(BB, LatchBlock);
        }
        for (auto *ChildBB : ChildrenToUpdate)
          DT->changeImmediateDominator(ChildBB, NewIDom);
      }
    }
  
    assert(!DT || !UnrollVerifyDomtree ||
           DT->verify(DominatorTree::VerificationLevel::Fast));
  
    DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
    // Merge adjacent basic blocks, if possible.
    for (BasicBlock *Latch : Latches) {
      BranchInst *Term = dyn_cast<BranchInst>(Latch->getTerminator());
      assert((Term ||
              (CompletelyUnroll && !LatchIsExiting && Latch == Latches.back())) &&
             "Need a branch as terminator, except when fully unrolling with "
             "unconditional latch");
      if (Term && Term->isUnconditional()) {
        BasicBlock *Dest = Term->getSuccessor(0);
        BasicBlock *Fold = Dest->getUniquePredecessor();
        if (MergeBlockIntoPredecessor(Dest, &DTU, LI)) {
          // Dest has been folded into Fold. Update our worklists accordingly.
          std::replace(Latches.begin(), Latches.end(), Dest, Fold);
          llvm::erase_value(UnrolledLoopBlocks, Dest);
        }
      }
    }
    // Apply updates to the DomTree.
    DT = &DTU.getDomTree();
  
    // At this point, the code is well formed.  We now simplify the unrolled loop,
    // doing constant propagation and dead code elimination as we go.
    simplifyLoopAfterUnroll(L, !CompletelyUnroll && (ULO.Count > 1 || Peeled), LI,
                            SE, DT, AC, TTI);
  
    NumCompletelyUnrolled += CompletelyUnroll;
    ++NumUnrolled;
  
    Loop *OuterL = L->getParentLoop();
    // Update LoopInfo if the loop is completely removed.
    if (CompletelyUnroll)
      LI->erase(L);
  
    // After complete unrolling most of the blocks should be contained in OuterL.
    // However, some of them might happen to be out of OuterL (e.g. if they
    // precede a loop exit). In this case we might need to insert PHI nodes in
    // order to preserve LCSSA form.
    // We don't need to check this if we already know that we need to fix LCSSA
    // form.
    // TODO: For now we just recompute LCSSA for the outer loop in this case, but
    // it should be possible to fix it in-place.
    if (PreserveLCSSA && OuterL && CompletelyUnroll && !NeedToFixLCSSA)
      NeedToFixLCSSA |= ::needToInsertPhisForLCSSA(OuterL, UnrolledLoopBlocks, LI);
  
    // If we have a pass and a DominatorTree we should re-simplify impacted loops
    // to ensure subsequent analyses can rely on this form. We want to simplify
    // at least one layer outside of the loop that was unrolled so that any
    // changes to the parent loop exposed by the unrolling are considered.
    if (DT) {
      if (OuterL) {
        // OuterL includes all loops for which we can break loop-simplify, so
        // it's sufficient to simplify only it (it'll recursively simplify inner
        // loops too).
        if (NeedToFixLCSSA) {
          // LCSSA must be performed on the outermost affected loop. The unrolled
          // loop's last loop latch is guaranteed to be in the outermost loop
          // after LoopInfo's been updated by LoopInfo::erase.
          Loop *LatchLoop = LI->getLoopFor(Latches.back());
          Loop *FixLCSSALoop = OuterL;
          if (!FixLCSSALoop->contains(LatchLoop))
            while (FixLCSSALoop->getParentLoop() != LatchLoop)
              FixLCSSALoop = FixLCSSALoop->getParentLoop();
  
          formLCSSARecursively(*FixLCSSALoop, *DT, LI, SE);
        } else if (PreserveLCSSA) {
          assert(OuterL->isLCSSAForm(*DT) &&
                 "Loops should be in LCSSA form after loop-unroll.");
        }
  
        // TODO: That potentially might be compile-time expensive. We should try
        // to fix the loop-simplified form incrementally.
        simplifyLoop(OuterL, DT, LI, SE, AC, nullptr, PreserveLCSSA);
      } else {
        // Simplify loops for which we might've broken loop-simplify form.
        for (Loop *SubLoop : LoopsToSimplify)
          simplifyLoop(SubLoop, DT, LI, SE, AC, nullptr, PreserveLCSSA);
      }
    }
  
    return CompletelyUnroll ? LoopUnrollResult::FullyUnrolled
                            : LoopUnrollResult::PartiallyUnrolled;
  }
  
  /// Given an llvm.loop loop id metadata node, 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.
  MDNode *llvm::GetUnrollMetadata(MDNode *LoopID, StringRef Name) {
    // First operand should refer to the loop id itself.
    assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
    assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
  
    for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
      MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
      if (!MD)
        continue;
  
      MDString *S = dyn_cast<MDString>(MD->getOperand(0));
      if (!S)
        continue;
  
      if (Name.equals(S->getString()))
        return MD;
    }
    return nullptr;
  }
  