text: //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This pass transforms loops that contain branches on loop-invariant conditions
  // to have multiple loops.  For example, it turns the left into the right code:
  //
  //  for (...)                  if (lic)
  //    A                          for (...)
  //    if (lic)                     A; B; C
  //      B                      else
  //    C                          for (...)
  //                                 A; C
  //
  // This can increase the size of the code exponentially (doubling it every time
  // a loop is unswitched) so we only unswitch if the resultant code will be
  // smaller than a threshold.
  //
  // This pass expects LICM to be run before it to hoist invariant conditions out
  // of the loop, to make the unswitching opportunity obvious.
  //
  //===----------------------------------------------------------------------===//
  
  #define DEBUG_TYPE "loop-unswitch"
  #include "llvm/Transforms/Scalar.h"
  #include "llvm/Constants.h"
  #include "llvm/DerivedTypes.h"
  #include "llvm/Function.h"
  #include "llvm/Instructions.h"
  #include "llvm/Analysis/CodeMetrics.h"
  #include "llvm/Analysis/InstructionSimplify.h"
  #include "llvm/Analysis/LoopInfo.h"
  #include "llvm/Analysis/LoopPass.h"
  #include "llvm/Analysis/Dominators.h"
  #include "llvm/Analysis/ScalarEvolution.h"
  #include "llvm/Transforms/Utils/Cloning.h"
  #include "llvm/Transforms/Utils/Local.h"
  #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  #include "llvm/ADT/Statistic.h"
  #include "llvm/ADT/SmallPtrSet.h"
  #include "llvm/ADT/STLExtras.h"
  #include "llvm/Support/CommandLine.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/raw_ostream.h"
  #include <algorithm>
  #include <map>
  #include <set>
  using namespace llvm;
  
  STATISTIC(NumBranches, "Number of branches unswitched");
  STATISTIC(NumSwitches, "Number of switches unswitched");
  STATISTIC(NumSelects , "Number of selects unswitched");
  STATISTIC(NumTrivial , "Number of unswitches that are trivial");
  STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
  STATISTIC(TotalInsts,  "Total number of instructions analyzed");
  
  // The specific value of 100 here was chosen based only on intuition and a
  // few specific examples.
  static cl::opt<unsigned>
  Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
            cl::init(100), cl::Hidden);
  
  namespace {
  
    class LUAnalysisCache {
  
      typedef DenseMap<const SwitchInst*, SmallPtrSet<const Value *, 8> >
        UnswitchedValsMap;
  
      typedef UnswitchedValsMap::iterator UnswitchedValsIt;
  
      struct LoopProperties {
        unsigned CanBeUnswitchedCount;
        unsigned SizeEstimation;
        UnswitchedValsMap UnswitchedVals;
      };
  
      // Here we use std::map instead of DenseMap, since we need to keep valid
      // LoopProperties pointer for current loop for better performance.
      typedef std::map<const Loop*, LoopProperties> LoopPropsMap;
      typedef LoopPropsMap::iterator LoopPropsMapIt;
  
      LoopPropsMap LoopsProperties;
      UnswitchedValsMap* CurLoopInstructions;
      LoopProperties* CurrentLoopProperties;
  
      // Max size of code we can produce on remained iterations.
      unsigned MaxSize;
  
      public:
  
        LUAnalysisCache() :
          CurLoopInstructions(NULL), CurrentLoopProperties(NULL),
          MaxSize(Threshold)
        {}
  
        // Analyze loop. Check its size, calculate is it possible to unswitch
        // it. Returns true if we can unswitch this loop.
        bool countLoop(const Loop* L);
  
        // Clean all data related to given loop.
        void forgetLoop(const Loop* L);
  
        // Mark case value as unswitched.
        // Since SI instruction can be partly unswitched, in order to avoid
        // extra unswitching in cloned loops keep track all unswitched values.
        void setUnswitched(const SwitchInst* SI, const Value* V);
  
        // Check was this case value unswitched before or not.
        bool isUnswitched(const SwitchInst* SI, const Value* V);
  
        // Clone all loop-unswitch related loop properties.
        // Redistribute unswitching quotas.
        // Note, that new loop data is stored inside the VMap.
        void cloneData(const Loop* NewLoop, const Loop* OldLoop,
                       const ValueToValueMapTy& VMap);
    };
  
    class LoopUnswitch : public LoopPass {
      LoopInfo *LI;  // Loop information
      LPPassManager *LPM;
  
      // LoopProcessWorklist - Used to check if second loop needs processing
      // after RewriteLoopBodyWithConditionConstant rewrites first loop.
      std::vector<Loop*> LoopProcessWorklist;
  
      LUAnalysisCache BranchesInfo;
  
      bool OptimizeForSize;
      bool redoLoop;
  
      Loop *currentLoop;
      DominatorTree *DT;
      BasicBlock *loopHeader;
      BasicBlock *loopPreheader;
  
      // LoopBlocks contains all of the basic blocks of the loop, including the
      // preheader of the loop, the body of the loop, and the exit blocks of the
      // loop, in that order.
      std::vector<BasicBlock*> LoopBlocks;
      // NewBlocks contained cloned copy of basic blocks from LoopBlocks.
      std::vector<BasicBlock*> NewBlocks;
  
    public:
      static char ID; // Pass ID, replacement for typeid
      explicit LoopUnswitch(bool Os = false) :
        LoopPass(ID), OptimizeForSize(Os), redoLoop(false),
        currentLoop(NULL), DT(NULL), loopHeader(NULL),
        loopPreheader(NULL) {
          initializeLoopUnswitchPass(*PassRegistry::getPassRegistry());
        }
  
      bool runOnLoop(Loop *L, LPPassManager &LPM);
      bool processCurrentLoop();
  
      /// This transformation requires natural loop information & requires that
      /// loop preheaders be inserted into the CFG.
      ///
      virtual void getAnalysisUsage(AnalysisUsage &AU) const {
        AU.addRequiredID(LoopSimplifyID);
        AU.addPreservedID(LoopSimplifyID);
        AU.addRequired<LoopInfo>();
        AU.addPreserved<LoopInfo>();
        AU.addRequiredID(LCSSAID);
        AU.addPreservedID(LCSSAID);
        AU.addPreserved<DominatorTree>();
        AU.addPreserved<ScalarEvolution>();
      }
  
    private:
  
      virtual void releaseMemory() {
        BranchesInfo.forgetLoop(currentLoop);
      }
  
      /// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
      /// remove it.
      void RemoveLoopFromWorklist(Loop *L) {
        std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
                                                   LoopProcessWorklist.end(), L);
        if (I != LoopProcessWorklist.end())
          LoopProcessWorklist.erase(I);
      }
  
      void initLoopData() {
        loopHeader = currentLoop->getHeader();
        loopPreheader = currentLoop->getLoopPreheader();
      }
  
      /// Split all of the edges from inside the loop to their exit blocks.
      /// Update the appropriate Phi nodes as we do so.
      void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks);
  
      bool UnswitchIfProfitable(Value *LoopCond, Constant *Val);
      void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
                                    BasicBlock *ExitBlock);
      void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
  
      void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
                                                Constant *Val, bool isEqual);
  
      void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
                                          BasicBlock *TrueDest,
                                          BasicBlock *FalseDest,
                                          Instruction *InsertPt);
  
      void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
      void RemoveBlockIfDead(BasicBlock *BB,
                             std::vector<Instruction*> &Worklist, Loop *l);
      void RemoveLoopFromHierarchy(Loop *L);
      bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0,
                                      BasicBlock **LoopExit = 0);
  
    };
  }
  
  // Analyze loop. Check its size, calculate is it possible to unswitch
  // it. Returns true if we can unswitch this loop.
  bool LUAnalysisCache::countLoop(const Loop* L) {
  
    std::pair<LoopPropsMapIt, bool> InsertRes =
        LoopsProperties.insert(std::make_pair(L, LoopProperties()));
  
    LoopProperties& Props = InsertRes.first->second;
  
    if (InsertRes.second) {
      // New loop.
  
      // Limit the number of instructions to avoid causing significant code
      // expansion, and the number of basic blocks, to avoid loops with
      // large numbers of branches which cause loop unswitching to go crazy.
      // This is a very ad-hoc heuristic.
  
      // FIXME: This is overly conservative because it does not take into
      // consideration code simplification opportunities and code that can
      // be shared by the resultant unswitched loops.
      CodeMetrics Metrics;
      for (Loop::block_iterator I = L->block_begin(),
             E = L->block_end();
           I != E; ++I)
        Metrics.analyzeBasicBlock(*I);
  
      Props.SizeEstimation = std::min(Metrics.NumInsts, Metrics.NumBlocks * 5);
      Props.CanBeUnswitchedCount = MaxSize / (Props.SizeEstimation);
      MaxSize -= Props.SizeEstimation * Props.CanBeUnswitchedCount;
    }
  
    if (!Props.CanBeUnswitchedCount) {
      DEBUG(dbgs() << "NOT unswitching loop %"
            << L->getHeader()->getName() << ", cost too high: "
            << L->getBlocks().size() << "\n");
  
      return false;
    }
  
    // Be careful. This links are good only before new loop addition.
    CurrentLoopProperties = &Props;
    CurLoopInstructions = &Props.UnswitchedVals;
  
    return true;
  }
  
  // Clean all data related to given loop.
  void LUAnalysisCache::forgetLoop(const Loop* L) {
  
    LoopPropsMapIt LIt = LoopsProperties.find(L);
  
    if (LIt != LoopsProperties.end()) {
      LoopProperties& Props = LIt->second;
      MaxSize += Props.CanBeUnswitchedCount * Props.SizeEstimation;
      LoopsProperties.erase(LIt);
    }
  
    CurrentLoopProperties = NULL;
    CurLoopInstructions = NULL;
  }
  
  // Mark case value as unswitched.
  // Since SI instruction can be partly unswitched, in order to avoid
  // extra unswitching in cloned loops keep track all unswitched values.
  void LUAnalysisCache::setUnswitched(const SwitchInst* SI, const Value* V) {
    (*CurLoopInstructions)[SI].insert(V);
  }
  
  // Check was this case value unswitched before or not.
  bool LUAnalysisCache::isUnswitched(const SwitchInst* SI, const Value* V) {
    return (*CurLoopInstructions)[SI].count(V);
  }
  
  // Clone all loop-unswitch related loop properties.
  // Redistribute unswitching quotas.
  // Note, that new loop data is stored inside the VMap.
  void LUAnalysisCache::cloneData(const Loop* NewLoop, const Loop* OldLoop,
                       const ValueToValueMapTy& VMap) {
  
    LoopProperties& NewLoopProps = LoopsProperties[NewLoop];
    LoopProperties& OldLoopProps = *CurrentLoopProperties;
    UnswitchedValsMap& Insts = OldLoopProps.UnswitchedVals;
  
    // Reallocate "can-be-unswitched quota"
  
    --OldLoopProps.CanBeUnswitchedCount;
    unsigned Quota = OldLoopProps.CanBeUnswitchedCount;
    NewLoopProps.CanBeUnswitchedCount = Quota / 2;
    OldLoopProps.CanBeUnswitchedCount = Quota - Quota / 2;
  
    NewLoopProps.SizeEstimation = OldLoopProps.SizeEstimation;
  
    // Clone unswitched values info:
    // for new loop switches we clone info about values that was
    // already unswitched and has redundant successors.
    for (UnswitchedValsIt I = Insts.begin(); I != Insts.end(); ++I) {
      const SwitchInst* OldInst = I->first;
      Value* NewI = VMap.lookup(OldInst);
      const SwitchInst* NewInst = cast_or_null<SwitchInst>(NewI);
      assert(NewInst && "All instructions that are in SrcBB must be in VMap.");
  
      NewLoopProps.UnswitchedVals[NewInst] = OldLoopProps.UnswitchedVals[OldInst];
    }
  }
  
  char LoopUnswitch::ID = 0;
  INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops",
                        false, false)
  INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
  INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  INITIALIZE_PASS_DEPENDENCY(LCSSA)
  INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops",
                        false, false)
  
  Pass *llvm::createLoopUnswitchPass(bool Os) {
    return new LoopUnswitch(Os);
  }
  
  /// FindLIVLoopCondition - Cond is a condition that occurs in L.  If it is
  /// invariant in the loop, or has an invariant piece, return the invariant.
  /// Otherwise, return null.
  static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
  
    // We started analyze new instruction, increment scanned instructions counter.
    ++TotalInsts;
  
    // We can never unswitch on vector conditions.
    if (Cond->getType()->isVectorTy())
      return 0;
  
    // Constants should be folded, not unswitched on!
    if (isa<Constant>(Cond)) return 0;
  
    // TODO: Handle: br (VARIANT|INVARIANT).
  
    // Hoist simple values out.
    if (L->makeLoopInvariant(Cond, Changed))
      return Cond;
  
    if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
      if (BO->getOpcode() == Instruction::And ||
          BO->getOpcode() == Instruction::Or) {
        // If either the left or right side is invariant, we can unswitch on this,
        // which will cause the branch to go away in one loop and the condition to
        // simplify in the other one.
        if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
          return LHS;
        if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
          return RHS;
      }
  
    return 0;
  }
  
  bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
    LI = &getAnalysis<LoopInfo>();
    LPM = &LPM_Ref;
    DT = getAnalysisIfAvailable<DominatorTree>();
    currentLoop = L;
    Function *F = currentLoop->getHeader()->getParent();
    bool Changed = false;
    do {
      assert(currentLoop->isLCSSAForm(*DT));
      redoLoop = false;
      Changed |= processCurrentLoop();
    } while(redoLoop);
  
    if (Changed) {
      // FIXME: Reconstruct dom info, because it is not preserved properly.
      if (DT)
        DT->runOnFunction(*F);
    }
    return Changed;
  }
  
  /// processCurrentLoop - Do actual work and unswitch loop if possible
  /// and profitable.
  bool LoopUnswitch::processCurrentLoop() {
    bool Changed = false;
  
    initLoopData();
  
    // If LoopSimplify was unable to form a preheader, don't do any unswitching.
    if (!loopPreheader)
      return false;
  
    // Loops with indirectbr cannot be cloned.
    if (!currentLoop->isSafeToClone())
      return false;
  
    // Without dedicated exits, splitting the exit edge may fail.
    if (!currentLoop->hasDedicatedExits())
      return false;
  
    LLVMContext &Context = loopHeader->getContext();
  
    // Probably we reach the quota of branches for this loop. If so
    // stop unswitching.
    if (!BranchesInfo.countLoop(currentLoop))
      return false;
  
    // Loop over all of the basic blocks in the loop.  If we find an interior
    // block that is branching on a loop-invariant condition, we can unswitch this
    // loop.
    for (Loop::block_iterator I = currentLoop->block_begin(),
           E = currentLoop->block_end(); I != E; ++I) {
      TerminatorInst *TI = (*I)->getTerminator();
      if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
        // If this isn't branching on an invariant condition, we can't unswitch
        // it.
        if (BI->isConditional()) {
          // See if this, or some part of it, is loop invariant.  If so, we can
          // unswitch on it if we desire.
          Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
                                                 currentLoop, Changed);
          if (LoopCond && UnswitchIfProfitable(LoopCond,
                                               ConstantInt::getTrue(Context))) {
            ++NumBranches;
            return true;
          }
        }
      } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
        Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
                                               currentLoop, Changed);
        unsigned NumCases = SI->getNumCases();
        if (LoopCond && NumCases) {
          // Find a value to unswitch on:
          // FIXME: this should chose the most expensive case!
          // FIXME: scan for a case with a non-critical edge?
          Constant *UnswitchVal = NULL;
  
          // Do not process same value again and again.
          // At this point we have some cases already unswitched and
          // some not yet unswitched. Let's find the first not yet unswitched one.
          for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
               i != e; ++i) {
            Constant* UnswitchValCandidate = i.getCaseValue();
            if (!BranchesInfo.isUnswitched(SI, UnswitchValCandidate)) {
              UnswitchVal = UnswitchValCandidate;
              break;
            }
          }
  
          if (!UnswitchVal)
            continue;
  
          if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
            ++NumSwitches;
            return true;
          }
        }
      }
  
      // Scan the instructions to check for unswitchable values.
      for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
           BBI != E; ++BBI)
        if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
          Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
                                                 currentLoop, Changed);
          if (LoopCond && UnswitchIfProfitable(LoopCond,
                                               ConstantInt::getTrue(Context))) {
            ++NumSelects;
            return true;
          }
        }
    }
    return Changed;
  }
  
  /// isTrivialLoopExitBlock - Check to see if all paths from BB exit the
  /// loop with no side effects (including infinite loops).
  ///
  /// If true, we return true and set ExitBB to the block we
  /// exit through.
  ///
  static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
                                           BasicBlock *&ExitBB,
                                           std::set<BasicBlock*> &Visited) {
    if (!Visited.insert(BB).second) {
      // Already visited. Without more analysis, this could indicate an infinite
      // loop.
      return false;
    } else if (!L->contains(BB)) {
      // Otherwise, this is a loop exit, this is fine so long as this is the
      // first exit.
      if (ExitBB != 0) return false;
      ExitBB = BB;
      return true;
    }
  
    // Otherwise, this is an unvisited intra-loop node.  Check all successors.
    for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
      // Check to see if the successor is a trivial loop exit.
      if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
        return false;
    }
  
    // Okay, everything after this looks good, check to make sure that this block
    // doesn't include any side effects.
    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
      if (I->mayHaveSideEffects())
        return false;
  
    return true;
  }
  
  /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
  /// leads to an exit from the specified loop, and has no side-effects in the
  /// process.  If so, return the block that is exited to, otherwise return null.
  static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
    std::set<BasicBlock*> Visited;
    Visited.insert(L->getHeader());  // Branches to header make infinite loops.
    BasicBlock *ExitBB = 0;
    if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
      return ExitBB;
    return 0;
  }
  
  /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
  /// trivial: that is, that the condition controls whether or not the loop does
  /// anything at all.  If this is a trivial condition, unswitching produces no
  /// code duplications (equivalently, it produces a simpler loop and a new empty
  /// loop, which gets deleted).
  ///
  /// If this is a trivial condition, return true, otherwise return false.  When
  /// returning true, this sets Cond and Val to the condition that controls the
  /// trivial condition: when Cond dynamically equals Val, the loop is known to
  /// exit.  Finally, this sets LoopExit to the BB that the loop exits to when
  /// Cond == Val.
  ///
  bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
                                         BasicBlock **LoopExit) {
    BasicBlock *Header = currentLoop->getHeader();
    TerminatorInst *HeaderTerm = Header->getTerminator();
    LLVMContext &Context = Header->getContext();
  
    BasicBlock *LoopExitBB = 0;
    if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
      // If the header block doesn't end with a conditional branch on Cond, we
      // can't handle it.
      if (!BI->isConditional() || BI->getCondition() != Cond)
        return false;
  
      // Check to see if a successor of the branch is guaranteed to
      // exit through a unique exit block without having any
      // side-effects.  If so, determine the value of Cond that causes it to do
      // this.
      if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
                                               BI->getSuccessor(0)))) {
        if (Val) *Val = ConstantInt::getTrue(Context);
      } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
                                                      BI->getSuccessor(1)))) {
        if (Val) *Val = ConstantInt::getFalse(Context);
      }
    } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
      // If this isn't a switch on Cond, we can't handle it.
      if (SI->getCondition() != Cond) return false;
  
      // Check to see if a successor of the switch is guaranteed to go to the
      // latch block or exit through a one exit block without having any
      // side-effects.  If so, determine the value of Cond that causes it to do
      // this.
      // Note that we can't trivially unswitch on the default case or
      // on already unswitched cases.
      for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
           i != e; ++i) {
        BasicBlock* LoopExitCandidate;
        if ((LoopExitCandidate = isTrivialLoopExitBlock(currentLoop,
                                                 i.getCaseSuccessor()))) {
          // Okay, we found a trivial case, remember the value that is trivial.
          ConstantInt* CaseVal = i.getCaseValue();
  
          // Check that it was not unswitched before, since already unswitched
          // trivial vals are looks trivial too.
          if (BranchesInfo.isUnswitched(SI, CaseVal))
            continue;
          LoopExitBB = LoopExitCandidate;
          if (Val) *Val = CaseVal;
          break;
        }
      }
    }
  
    // If we didn't find a single unique LoopExit block, or if the loop exit block
    // contains phi nodes, this isn't trivial.
    if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
      return false;   // Can't handle this.
  
    if (LoopExit) *LoopExit = LoopExitBB;
  
    // We already know that nothing uses any scalar values defined inside of this
    // loop.  As such, we just have to check to see if this loop will execute any
    // side-effecting instructions (e.g. stores, calls, volatile loads) in the
    // part of the loop that the code *would* execute.  We already checked the
    // tail, check the header now.
    for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
      if (I->mayHaveSideEffects())
        return false;
    return true;
  }
  
  /// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
  /// LoopCond == Val to simplify the loop.  If we decide that this is profitable,
  /// unswitch the loop, reprocess the pieces, then return true.
  bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val) {
    Function *F = loopHeader->getParent();
    Constant *CondVal = 0;
    BasicBlock *ExitBlock = 0;
  
    if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
      // If the condition is trivial, always unswitch. There is no code growth
      // for this case.
      UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock);
      return true;
    }
  
    // Check to see if it would be profitable to unswitch current loop.
  
    // Do not do non-trivial unswitch while optimizing for size.
    if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize))
      return false;
  
    UnswitchNontrivialCondition(LoopCond, Val, currentLoop);
    return true;
  }
  
  /// CloneLoop - Recursively clone the specified loop and all of its children,
  /// mapping the blocks with the specified map.
  static Loop *CloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
                         LoopInfo *LI, LPPassManager *LPM) {
    Loop *New = new Loop();
    LPM->insertLoop(New, PL);
  
    // 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->getBase());
  
    // Add all of the subloops to the new loop.
    for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
      CloneLoop(*I, New, VM, LI, LPM);
  
    return New;
  }
  
  /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
  /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest.  Insert the
  /// code immediately before InsertPt.
  void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
                                                    BasicBlock *TrueDest,
                                                    BasicBlock *FalseDest,
                                                    Instruction *InsertPt) {
    // Insert a conditional branch on LIC to the two preheaders.  The original
    // code is the true version and the new code is the false version.
    Value *BranchVal = LIC;
    if (!isa<ConstantInt>(Val) ||
        Val->getType() != Type::getInt1Ty(LIC->getContext()))
      BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val);
    else if (Val != ConstantInt::getTrue(Val->getContext()))
      // We want to enter the new loop when the condition is true.
      std::swap(TrueDest, FalseDest);
  
    // Insert the new branch.
    BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt);
  
    // If either edge is critical, split it. This helps preserve LoopSimplify
    // form for enclosing loops.
    SplitCriticalEdge(BI, 0, this, false, false, true);
    SplitCriticalEdge(BI, 1, this, false, false, true);
  }
  
  /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
  /// condition in it (a cond branch from its header block to its latch block,
  /// where the path through the loop that doesn't execute its body has no
  /// side-effects), unswitch it.  This doesn't involve any code duplication, just
  /// moving the conditional branch outside of the loop and updating loop info.
  void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
                                              Constant *Val,
                                              BasicBlock *ExitBlock) {
    DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
          << loopHeader->getName() << " [" << L->getBlocks().size()
          << " blocks] in Function " << L->getHeader()->getParent()->getName()
          << " on cond: " << *Val << " == " << *Cond << "\n");
  
    // First step, split the preheader, so that we know that there is a safe place
    // to insert the conditional branch.  We will change loopPreheader to have a
    // conditional branch on Cond.
    BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this);
  
    // Now that we have a place to insert the conditional branch, create a place
    // to branch to: this is the exit block out of the loop that we should
    // short-circuit to.
  
    // Split this block now, so that the loop maintains its exit block, and so
    // that the jump from the preheader can execute the contents of the exit block
    // without actually branching to it (the exit block should be dominated by the
    // loop header, not the preheader).
    assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
    BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this);
  
    // Okay, now we have a position to branch from and a position to branch to,
    // insert the new conditional branch.
    EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
                                   loopPreheader->getTerminator());
    LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
    loopPreheader->getTerminator()->eraseFromParent();
  
    // We need to reprocess this loop, it could be unswitched again.
    redoLoop = true;
  
    // Now that we know that the loop is never entered when this condition is a
    // particular value, rewrite the loop with this info.  We know that this will
    // at least eliminate the old branch.
    RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
    ++NumTrivial;
  }
  
  /// SplitExitEdges - Split all of the edges from inside the loop to their exit
  /// blocks.  Update the appropriate Phi nodes as we do so.
  void LoopUnswitch::SplitExitEdges(Loop *L,
                                  const SmallVector<BasicBlock *, 8> &ExitBlocks){
  
    for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
      BasicBlock *ExitBlock = ExitBlocks[i];
      SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
                                         pred_end(ExitBlock));
  
      // Although SplitBlockPredecessors doesn't preserve loop-simplify in
      // general, if we call it on all predecessors of all exits then it does.
      if (!ExitBlock->isLandingPad()) {
        SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa", this);
      } else {
        SmallVector<BasicBlock*, 2> NewBBs;
        SplitLandingPadPredecessors(ExitBlock, Preds, ".us-lcssa", ".us-lcssa",
                                    this, NewBBs);
      }
    }
  }
  
  /// UnswitchNontrivialCondition - We determined that the loop is profitable
  /// to unswitch when LIC equal Val.  Split it into loop versions and test the
  /// condition outside of either loop.  Return the loops created as Out1/Out2.
  void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
                                                 Loop *L) {
    Function *F = loopHeader->getParent();
    DEBUG(dbgs() << "loop-unswitch: Unswitching loop %"
          << loopHeader->getName() << " [" << L->getBlocks().size()
          << " blocks] in Function " << F->getName()
          << " when '" << *Val << "' == " << *LIC << "\n");
  
    if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
      SE->forgetLoop(L);
  
    LoopBlocks.clear();
    NewBlocks.clear();
  
    // First step, split the preheader and exit blocks, and add these blocks to
    // the LoopBlocks list.
    BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this);
    LoopBlocks.push_back(NewPreheader);
  
    // We want the loop to come after the preheader, but before the exit blocks.
    LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
  
    SmallVector<BasicBlock*, 8> ExitBlocks;
    L->getUniqueExitBlocks(ExitBlocks);
  
    // Split all of the edges from inside the loop to their exit blocks.  Update
    // the appropriate Phi nodes as we do so.
    SplitExitEdges(L, ExitBlocks);
  
    // The exit blocks may have been changed due to edge splitting, recompute.
    ExitBlocks.clear();
    L->getUniqueExitBlocks(ExitBlocks);
  
    // Add exit blocks to the loop blocks.
    LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
  
    // Next step, clone all of the basic blocks that make up the loop (including
    // the loop preheader and exit blocks), keeping track of the mapping between
    // the instructions and blocks.
    NewBlocks.reserve(LoopBlocks.size());
    ValueToValueMapTy VMap;
    for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
      BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F);
  
      NewBlocks.push_back(NewBB);
      VMap[LoopBlocks[i]] = NewBB;  // Keep the BB mapping.
      LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L);
    }
  
    // Splice the newly inserted blocks into the function right before the
    // original preheader.
    F->getBasicBlockList().splice(NewPreheader, F->getBasicBlockList(),
                                  NewBlocks[0], F->end());
  
    // Now we create the new Loop object for the versioned loop.
    Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM);
  
    // Recalculate unswitching quota, inherit simplified switches info for NewBB,
    // Probably clone more loop-unswitch related loop properties.
    BranchesInfo.cloneData(NewLoop, L, VMap);
  
    Loop *ParentLoop = L->getParentLoop();
    if (ParentLoop) {
      // Make sure to add the cloned preheader and exit blocks to the parent loop
      // as well.
      ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase());
    }
  
    for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
      BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]);
      // The new exit block should be in the same loop as the old one.
      if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
        ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase());
  
      assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
             "Exit block should have been split to have one successor!");
      BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
  
      // If the successor of the exit block had PHI nodes, add an entry for
      // NewExit.
      PHINode *PN;
      for (BasicBlock::iterator I = ExitSucc->begin(); isa<PHINode>(I); ++I) {
        PN = cast<PHINode>(I);
        Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
        ValueToValueMapTy::iterator It = VMap.find(V);
        if (It != VMap.end()) V = It->second;
        PN->addIncoming(V, NewExit);
      }
  
      if (LandingPadInst *LPad = NewExit->getLandingPadInst()) {
        PN = PHINode::Create(LPad->getType(), 0, "",
                             ExitSucc->getFirstInsertionPt());
  
        for (pred_iterator I = pred_begin(ExitSucc), E = pred_end(ExitSucc);
             I != E; ++I) {
          BasicBlock *BB = *I;
          LandingPadInst *LPI = BB->getLandingPadInst();
          LPI->replaceAllUsesWith(PN);
          PN->addIncoming(LPI, BB);
        }
      }
    }
  
    // Rewrite the code to refer to itself.
    for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
      for (BasicBlock::iterator I = NewBlocks[i]->begin(),
             E = NewBlocks[i]->end(); I != E; ++I)
        RemapInstruction(I, VMap,RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
  
    // Rewrite the original preheader to select between versions of the loop.
    BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
    assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
           "Preheader splitting did not work correctly!");
  
    // Emit the new branch that selects between the two versions of this loop.
    EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
    LPM->deleteSimpleAnalysisValue(OldBR, L);
    OldBR->eraseFromParent();
  
    LoopProcessWorklist.push_back(NewLoop);
    redoLoop = true;
  
    // Keep a WeakVH holding onto LIC.  If the first call to RewriteLoopBody
    // deletes the instruction (for example by simplifying a PHI that feeds into
    // the condition that we're unswitching on), we don't rewrite the second
    // iteration.
    WeakVH LICHandle(LIC);
  
    // Now we rewrite the original code to know that the condition is true and the
    // new code to know that the condition is false.
    RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
  
    // It's possible that simplifying one loop could cause the other to be
    // changed to another value or a constant.  If its a constant, don't simplify
    // it.
    if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
        LICHandle && !isa<Constant>(LICHandle))
      RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
  }
  
  /// RemoveFromWorklist - Remove all instances of I from the worklist vector
  /// specified.
  static void RemoveFromWorklist(Instruction *I,
                                 std::vector<Instruction*> &Worklist) {
    std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
                                                       Worklist.end(), I);
    while (WI != Worklist.end()) {
      unsigned Offset = WI-Worklist.begin();
      Worklist.erase(WI);
      WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
    }
  }
  
  /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
  /// program, replacing all uses with V and update the worklist.
  static void ReplaceUsesOfWith(Instruction *I, Value *V,
                                std::vector<Instruction*> &Worklist,
                                Loop *L, LPPassManager *LPM) {
    DEBUG(dbgs() << "Replace with '" << *V << "': " << *I);
  
    // Add uses to the worklist, which may be dead now.
    for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
      if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
        Worklist.push_back(Use);
  
    // Add users to the worklist which may be simplified now.
    for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
         UI != E; ++UI)
      Worklist.push_back(cast<Instruction>(*UI));
    LPM->deleteSimpleAnalysisValue(I, L);
    RemoveFromWorklist(I, Worklist);
    I->replaceAllUsesWith(V);
    I->eraseFromParent();
    ++NumSimplify;
  }
  
  /// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
  /// information, and remove any dead successors it has.
  ///
  void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
                                       std::vector<Instruction*> &Worklist,
                                       Loop *L) {
    if (pred_begin(BB) != pred_end(BB)) {
      // This block isn't dead, since an edge to BB was just removed, see if there
      // are any easy simplifications we can do now.
      if (BasicBlock *Pred = BB->getSinglePredecessor()) {
        // If it has one pred, fold phi nodes in BB.
        while (isa<PHINode>(BB->begin()))
          ReplaceUsesOfWith(BB->begin(),
                            cast<PHINode>(BB->begin())->getIncomingValue(0),
                            Worklist, L, LPM);
  
        // If this is the header of a loop and the only pred is the latch, we now
        // have an unreachable loop.
        if (Loop *L = LI->getLoopFor(BB))
          if (loopHeader == BB && L->contains(Pred)) {
            // Remove the branch from the latch to the header block, this makes
            // the header dead, which will make the latch dead (because the header
            // dominates the latch).
            LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L);
            Pred->getTerminator()->eraseFromParent();
            new UnreachableInst(BB->getContext(), Pred);
  
            // The loop is now broken, remove it from LI.
            RemoveLoopFromHierarchy(L);
  
            // Reprocess the header, which now IS dead.
            RemoveBlockIfDead(BB, Worklist, L);
            return;
          }
  
        // If pred ends in a uncond branch, add uncond branch to worklist so that
        // the two blocks will get merged.
        if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
          if (BI->isUnconditional())
            Worklist.push_back(BI);
      }
      return;
    }
  
    DEBUG(dbgs() << "Nuking dead block: " << *BB);
  
    // Remove the instructions in the basic block from the worklist.
    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
      RemoveFromWorklist(I, Worklist);
  
      // Anything that uses the instructions in this basic block should have their
      // uses replaced with undefs.
      // If I is not void type then replaceAllUsesWith undef.
      // This allows ValueHandlers and custom metadata to adjust itself.
      if (!I->getType()->isVoidTy())
        I->replaceAllUsesWith(UndefValue::get(I->getType()));
    }
  
    // If this is the edge to the header block for a loop, remove the loop and
    // promote all subloops.
    if (Loop *BBLoop = LI->getLoopFor(BB)) {
      if (BBLoop->getLoopLatch() == BB) {
        RemoveLoopFromHierarchy(BBLoop);
        if (currentLoop == BBLoop) {
          currentLoop = 0;
          redoLoop = false;
        }
      }
    }
  
    // Remove the block from the loop info, which removes it from any loops it
    // was in.
    LI->removeBlock(BB);
  
  
    // Remove phi node entries in successors for this block.
    TerminatorInst *TI = BB->getTerminator();
    SmallVector<BasicBlock*, 4> Succs;
    for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
      Succs.push_back(TI->getSuccessor(i));
      TI->getSuccessor(i)->removePredecessor(BB);
    }
  
    // Unique the successors, remove anything with multiple uses.
    array_pod_sort(Succs.begin(), Succs.end());
    Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
  
    // Remove the basic block, including all of the instructions contained in it.
    LPM->deleteSimpleAnalysisValue(BB, L);
    BB->eraseFromParent();
    // Remove successor blocks here that are not dead, so that we know we only
    // have dead blocks in this list.  Nondead blocks have a way of becoming dead,
    // then getting removed before we revisit them, which is badness.
    //
    for (unsigned i = 0; i != Succs.size(); ++i)
      if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
        // One exception is loop headers.  If this block was the preheader for a
        // loop, then we DO want to visit the loop so the loop gets deleted.
        // We know that if the successor is a loop header, that this loop had to
        // be the preheader: the case where this was the latch block was handled
        // above and headers can only have two predecessors.
        if (!LI->isLoopHeader(Succs[i])) {
          Succs.erase(Succs.begin()+i);
          --i;
        }
      }
  
    for (unsigned i = 0, e = Succs.size(); i != e; ++i)
      RemoveBlockIfDead(Succs[i], Worklist, L);
  }
  
  /// RemoveLoopFromHierarchy - We have discovered that the specified loop has
  /// become unwrapped, either because the backedge was deleted, or because the
  /// edge into the header was removed.  If the edge into the header from the
  /// latch block was removed, the loop is unwrapped but subloops are still alive,
  /// so they just reparent loops.  If the loops are actually dead, they will be
  /// removed later.
  void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
    LPM->deleteLoopFromQueue(L);
    RemoveLoopFromWorklist(L);
  }
  
  // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
  // the value specified by Val in the specified loop, or we know it does NOT have
  // that value.  Rewrite any uses of LIC or of properties correlated to it.
  void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
                                                          Constant *Val,
                                                          bool IsEqual) {
    assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
  
    // FIXME: Support correlated properties, like:
    //  for (...)
    //    if (li1 < li2)
    //      ...
    //    if (li1 > li2)
    //      ...
  
    // FOLD boolean conditions (X|LIC), (X&LIC).  Fold conditional branches,
    // selects, switches.
    std::vector<Instruction*> Worklist;
    LLVMContext &Context = Val->getContext();
  
  
    // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
    // in the loop with the appropriate one directly.
    if (IsEqual || (isa<ConstantInt>(Val) &&
        Val->getType()->isIntegerTy(1))) {
      Value *Replacement;
      if (IsEqual)
        Replacement = Val;
      else
        Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
                                       !cast<ConstantInt>(Val)->getZExtValue());
  
      for (Value::use_iterator UI = LIC->use_begin(), E = LIC->use_end();
           UI != E; ++UI) {
        Instruction *U = dyn_cast<Instruction>(*UI);
        if (!U || !L->contains(U))
          continue;
        Worklist.push_back(U);
      }
  
      for (std::vector<Instruction*>::iterator UI = Worklist.begin();
           UI != Worklist.end(); ++UI)
        (*UI)->replaceUsesOfWith(LIC, Replacement);
  
      SimplifyCode(Worklist, L);
      return;
    }
  
    // Otherwise, we don't know the precise value of LIC, but we do know that it
    // is certainly NOT "Val".  As such, simplify any uses in the loop that we
    // can.  This case occurs when we unswitch switch statements.
    for (Value::use_iterator UI = LIC->use_begin(), E = LIC->use_end();
         UI != E; ++UI) {
      Instruction *U = dyn_cast<Instruction>(*UI);
      if (!U || !L->contains(U))
        continue;
  
      Worklist.push_back(U);
  
      // TODO: We could do other simplifications, for example, turning
      // 'icmp eq LIC, Val' -> false.
  
      // If we know that LIC is not Val, use this info to simplify code.
      SwitchInst *SI = dyn_cast<SwitchInst>(U);
      if (SI == 0 || !isa<ConstantInt>(Val)) continue;
  
      SwitchInst::CaseIt DeadCase = SI->findCaseValue(cast<ConstantInt>(Val));
      // Default case is live for multiple values.
      if (DeadCase == SI->case_default()) continue;
  
      // Found a dead case value.  Don't remove PHI nodes in the
      // successor if they become single-entry, those PHI nodes may
      // be in the Users list.
  
      BasicBlock *Switch = SI->getParent();
      BasicBlock *SISucc = DeadCase.getCaseSuccessor();
      BasicBlock *Latch = L->getLoopLatch();
  
      BranchesInfo.setUnswitched(SI, Val);
  
      if (!SI->findCaseDest(SISucc)) continue;  // Edge is critical.
      // If the DeadCase successor dominates the loop latch, then the
      // transformation isn't safe since it will delete the sole predecessor edge
      // to the latch.
      if (Latch && DT->dominates(SISucc, Latch))
        continue;
  
      // FIXME: This is a hack.  We need to keep the successor around
      // and hooked up so as to preserve the loop structure, because
      // trying to update it is complicated.  So instead we preserve the
      // loop structure and put the block on a dead code path.
      SplitEdge(Switch, SISucc, this);
      // Compute the successors instead of relying on the return value
      // of SplitEdge, since it may have split the switch successor
      // after PHI nodes.
      BasicBlock *NewSISucc = DeadCase.getCaseSuccessor();
      BasicBlock *OldSISucc = *succ_begin(NewSISucc);
      // Create an "unreachable" destination.
      BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
                                             Switch->getParent(),
                                             OldSISucc);
      new UnreachableInst(Context, Abort);
      // Force the new case destination to branch to the "unreachable"
      // block while maintaining a (dead) CFG edge to the old block.
      NewSISucc->getTerminator()->eraseFromParent();
      BranchInst::Create(Abort, OldSISucc,
                         ConstantInt::getTrue(Context), NewSISucc);
      // Release the PHI operands for this edge.
      for (BasicBlock::iterator II = NewSISucc->begin();
           PHINode *PN = dyn_cast<PHINode>(II); ++II)
        PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
                             UndefValue::get(PN->getType()));
      // Tell the domtree about the new block. We don't fully update the
      // domtree here -- instead we force it to do a full recomputation
      // after the pass is complete -- but we do need to inform it of
      // new blocks.
      if (DT)
        DT->addNewBlock(Abort, NewSISucc);
    }
  
    SimplifyCode(Worklist, L);
  }
  
  /// SimplifyCode - Okay, now that we have simplified some instructions in the
  /// loop, walk over it and constant prop, dce, and fold control flow where
  /// possible.  Note that this is effectively a very simple loop-structure-aware
  /// optimizer.  During processing of this loop, L could very well be deleted, so
  /// it must not be used.
  ///
  /// FIXME: When the loop optimizer is more mature, separate this out to a new
  /// pass.
  ///
  void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
    while (!Worklist.empty()) {
      Instruction *I = Worklist.back();
      Worklist.pop_back();
  
      // Simple DCE.
      if (isInstructionTriviallyDead(I)) {
        DEBUG(dbgs() << "Remove dead instruction '" << *I);
  
        // Add uses to the worklist, which may be dead now.
        for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
          if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
            Worklist.push_back(Use);
        LPM->deleteSimpleAnalysisValue(I, L);
        RemoveFromWorklist(I, Worklist);
        I->eraseFromParent();
        ++NumSimplify;
        continue;
      }
  
      // See if instruction simplification can hack this up.  This is common for
      // things like "select false, X, Y" after unswitching made the condition be
      // 'false'.
      if (Value *V = SimplifyInstruction(I, 0, 0, DT))
        if (LI->replacementPreservesLCSSAForm(I, V)) {
          ReplaceUsesOfWith(I, V, Worklist, L, LPM);
          continue;
        }
  
      // Special case hacks that appear commonly in unswitched code.
      if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
        if (BI->isUnconditional()) {
          // If BI's parent is the only pred of the successor, fold the two blocks
          // together.
          BasicBlock *Pred = BI->getParent();
          BasicBlock *Succ = BI->getSuccessor(0);
          BasicBlock *SinglePred = Succ->getSinglePredecessor();
          if (!SinglePred) continue;  // Nothing to do.
          assert(SinglePred == Pred && "CFG broken");
  
          DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "
                << Succ->getName() << "\n");
  
          // Resolve any single entry PHI nodes in Succ.
          while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
            ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
  
          // If Succ has any successors with PHI nodes, update them to have
          // entries coming from Pred instead of Succ.
          Succ->replaceAllUsesWith(Pred);
  
          // Move all of the successor contents from Succ to Pred.
          Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
                                     Succ->end());
          LPM->deleteSimpleAnalysisValue(BI, L);
          BI->eraseFromParent();
          RemoveFromWorklist(BI, Worklist);
  
          // Remove Succ from the loop tree.
          LI->removeBlock(Succ);
          LPM->deleteSimpleAnalysisValue(Succ, L);
          Succ->eraseFromParent();
          ++NumSimplify;
          continue;
        }
  
        if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){
          // Conditional branch.  Turn it into an unconditional branch, then
          // remove dead blocks.
          continue;  // FIXME: Enable.
  
          DEBUG(dbgs() << "Folded branch: " << *BI);
          BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue());
          BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue());
          DeadSucc->removePredecessor(BI->getParent(), true);
          Worklist.push_back(BranchInst::Create(LiveSucc, BI));
          LPM->deleteSimpleAnalysisValue(BI, L);
          BI->eraseFromParent();
          RemoveFromWorklist(BI, Worklist);
          ++NumSimplify;
  
          RemoveBlockIfDead(DeadSucc, Worklist, L);
        }
        continue;
      }
    }
  }
  