text: //===-- Local.cpp - Functions to perform local transformations ------------===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This family of functions perform various local transformations to the
  // program.
  //
  //===----------------------------------------------------------------------===//
  
  #include "llvm/Transforms/Utils/Local.h"
  #include "llvm/Constants.h"
  #include "llvm/DIBuilder.h"
  #include "llvm/DebugInfo.h"
  #include "llvm/DerivedTypes.h"
  #include "llvm/GlobalAlias.h"
  #include "llvm/GlobalVariable.h"
  #include "llvm/IRBuilder.h"
  #include "llvm/Instructions.h"
  #include "llvm/IntrinsicInst.h"
  #include "llvm/Intrinsics.h"
  #include "llvm/MDBuilder.h"
  #include "llvm/Metadata.h"
  #include "llvm/Operator.h"
  #include "llvm/ADT/DenseMap.h"
  #include "llvm/ADT/SmallPtrSet.h"
  #include "llvm/Analysis/Dominators.h"
  #include "llvm/Analysis/InstructionSimplify.h"
  #include "llvm/Analysis/MemoryBuiltins.h"
  #include "llvm/Analysis/ProfileInfo.h"
  #include "llvm/Analysis/ValueTracking.h"
  #include "llvm/Support/CFG.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/GetElementPtrTypeIterator.h"
  #include "llvm/Support/MathExtras.h"
  #include "llvm/Support/ValueHandle.h"
  #include "llvm/Support/raw_ostream.h"
  #include "llvm/Target/TargetData.h"
  using namespace llvm;
  
  //===----------------------------------------------------------------------===//
  //  Local constant propagation.
  //
  
  /// ConstantFoldTerminator - If a terminator instruction is predicated on a
  /// constant value, convert it into an unconditional branch to the constant
  /// destination.  This is a nontrivial operation because the successors of this
  /// basic block must have their PHI nodes updated.
  /// Also calls RecursivelyDeleteTriviallyDeadInstructions() on any branch/switch
  /// conditions and indirectbr addresses this might make dead if
  /// DeleteDeadConditions is true.
  bool llvm::ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions,
                                    const TargetLibraryInfo *TLI) {
    TerminatorInst *T = BB->getTerminator();
    IRBuilder<> Builder(T);
  
    // Branch - See if we are conditional jumping on constant
    if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
      if (BI->isUnconditional()) return false;  // Can't optimize uncond branch
      BasicBlock *Dest1 = BI->getSuccessor(0);
      BasicBlock *Dest2 = BI->getSuccessor(1);
  
      if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
        // Are we branching on constant?
        // YES.  Change to unconditional branch...
        BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
        BasicBlock *OldDest     = Cond->getZExtValue() ? Dest2 : Dest1;
  
        //cerr << "Function: " << T->getParent()->getParent()
        //     << "\nRemoving branch from " << T->getParent()
        //     << "\n\nTo: " << OldDest << endl;
  
        // Let the basic block know that we are letting go of it.  Based on this,
        // it will adjust it's PHI nodes.
        OldDest->removePredecessor(BB);
  
        // Replace the conditional branch with an unconditional one.
        Builder.CreateBr(Destination);
        BI->eraseFromParent();
        return true;
      }
      
      if (Dest2 == Dest1) {       // Conditional branch to same location?
        // This branch matches something like this:
        //     br bool %cond, label %Dest, label %Dest
        // and changes it into:  br label %Dest
  
        // Let the basic block know that we are letting go of one copy of it.
        assert(BI->getParent() && "Terminator not inserted in block!");
        Dest1->removePredecessor(BI->getParent());
  
        // Replace the conditional branch with an unconditional one.
        Builder.CreateBr(Dest1);
        Value *Cond = BI->getCondition();
        BI->eraseFromParent();
        if (DeleteDeadConditions)
          RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
        return true;
      }
      return false;
    }
    
    if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
      // If we are switching on a constant, we can convert the switch into a
      // single branch instruction!
      ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
      BasicBlock *TheOnlyDest = SI->getDefaultDest();
      BasicBlock *DefaultDest = TheOnlyDest;
  
      // Figure out which case it goes to.
      for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
           i != e; ++i) {
        // Found case matching a constant operand?
        if (i.getCaseValue() == CI) {
          TheOnlyDest = i.getCaseSuccessor();
          break;
        }
  
        // Check to see if this branch is going to the same place as the default
        // dest.  If so, eliminate it as an explicit compare.
        if (i.getCaseSuccessor() == DefaultDest) {
          MDNode* MD = SI->getMetadata(LLVMContext::MD_prof);
          // MD should have 2 + NumCases operands.
          if (MD && MD->getNumOperands() == 2 + SI->getNumCases()) {
            // Collect branch weights into a vector.
            SmallVector<uint32_t, 8> Weights;
            for (unsigned MD_i = 1, MD_e = MD->getNumOperands(); MD_i < MD_e;
                 ++MD_i) {
              ConstantInt* CI = dyn_cast<ConstantInt>(MD->getOperand(MD_i));
              assert(CI);
              Weights.push_back(CI->getValue().getZExtValue());
            }
            // Merge weight of this case to the default weight.
            unsigned idx = i.getCaseIndex();
            Weights[0] += Weights[idx+1];
            // Remove weight for this case.
            std::swap(Weights[idx+1], Weights.back());
            Weights.pop_back();
            SI->setMetadata(LLVMContext::MD_prof,
                            MDBuilder(BB->getContext()).
                            createBranchWeights(Weights));
          }
          // Remove this entry.
          DefaultDest->removePredecessor(SI->getParent());
          SI->removeCase(i);
          --i; --e;
          continue;
        }
  
        // Otherwise, check to see if the switch only branches to one destination.
        // We do this by reseting "TheOnlyDest" to null when we find two non-equal
        // destinations.
        if (i.getCaseSuccessor() != TheOnlyDest) TheOnlyDest = 0;
      }
  
      if (CI && !TheOnlyDest) {
        // Branching on a constant, but not any of the cases, go to the default
        // successor.
        TheOnlyDest = SI->getDefaultDest();
      }
  
      // If we found a single destination that we can fold the switch into, do so
      // now.
      if (TheOnlyDest) {
        // Insert the new branch.
        Builder.CreateBr(TheOnlyDest);
        BasicBlock *BB = SI->getParent();
  
        // Remove entries from PHI nodes which we no longer branch to...
        for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
          // Found case matching a constant operand?
          BasicBlock *Succ = SI->getSuccessor(i);
          if (Succ == TheOnlyDest)
            TheOnlyDest = 0;  // Don't modify the first branch to TheOnlyDest
          else
            Succ->removePredecessor(BB);
        }
  
        // Delete the old switch.
        Value *Cond = SI->getCondition();
        SI->eraseFromParent();
        if (DeleteDeadConditions)
          RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
        return true;
      }
      
      if (SI->getNumCases() == 1) {
        // Otherwise, we can fold this switch into a conditional branch
        // instruction if it has only one non-default destination.
        SwitchInst::CaseIt FirstCase = SI->case_begin();
        IntegersSubset& Case = FirstCase.getCaseValueEx();
        if (Case.isSingleNumber()) {
          // FIXME: Currently work with ConstantInt based numbers.
          Value *Cond = Builder.CreateICmpEQ(SI->getCondition(),
               Case.getSingleNumber(0).toConstantInt(),
              "cond");
  
          // Insert the new branch.
          BranchInst *NewBr = Builder.CreateCondBr(Cond,
                                  FirstCase.getCaseSuccessor(),
                                  SI->getDefaultDest());
          MDNode* MD = SI->getMetadata(LLVMContext::MD_prof);
          if (MD && MD->getNumOperands() == 3) {
            ConstantInt *SICase = dyn_cast<ConstantInt>(MD->getOperand(2));
            ConstantInt *SIDef = dyn_cast<ConstantInt>(MD->getOperand(1));
            assert(SICase && SIDef);
            // The TrueWeight should be the weight for the single case of SI.
            NewBr->setMetadata(LLVMContext::MD_prof,
                   MDBuilder(BB->getContext()).
                   createBranchWeights(SICase->getValue().getZExtValue(),
                                       SIDef->getValue().getZExtValue()));
          }
  
          // Delete the old switch.
          SI->eraseFromParent();
          return true;
        }
      }
      return false;
    }
  
    if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(T)) {
      // indirectbr blockaddress(@F, @BB) -> br label @BB
      if (BlockAddress *BA =
            dyn_cast<BlockAddress>(IBI->getAddress()->stripPointerCasts())) {
        BasicBlock *TheOnlyDest = BA->getBasicBlock();
        // Insert the new branch.
        Builder.CreateBr(TheOnlyDest);
        
        for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
          if (IBI->getDestination(i) == TheOnlyDest)
            TheOnlyDest = 0;
          else
            IBI->getDestination(i)->removePredecessor(IBI->getParent());
        }
        Value *Address = IBI->getAddress();
        IBI->eraseFromParent();
        if (DeleteDeadConditions)
          RecursivelyDeleteTriviallyDeadInstructions(Address, TLI);
        
        // If we didn't find our destination in the IBI successor list, then we
        // have undefined behavior.  Replace the unconditional branch with an
        // 'unreachable' instruction.
        if (TheOnlyDest) {
          BB->getTerminator()->eraseFromParent();
          new UnreachableInst(BB->getContext(), BB);
        }
        
        return true;
      }
    }
    
    return false;
  }
  
  
  //===----------------------------------------------------------------------===//
  //  Local dead code elimination.
  //
  
  /// isInstructionTriviallyDead - Return true if the result produced by the
  /// instruction is not used, and the instruction has no side effects.
  ///
  bool llvm::isInstructionTriviallyDead(Instruction *I,
                                        const TargetLibraryInfo *TLI) {
    if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
  
    // We don't want the landingpad instruction removed by anything this general.
    if (isa<LandingPadInst>(I))
      return false;
  
    // We don't want debug info removed by anything this general, unless
    // debug info is empty.
    if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(I)) {
      if (DDI->getAddress())
        return false;
      return true;
    }
    if (DbgValueInst *DVI = dyn_cast<DbgValueInst>(I)) {
      if (DVI->getValue())
        return false;
      return true;
    }
  
    if (!I->mayHaveSideEffects()) return true;
  
    // Special case intrinsics that "may have side effects" but can be deleted
    // when dead.
    if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
      // Safe to delete llvm.stacksave if dead.
      if (II->getIntrinsicID() == Intrinsic::stacksave)
        return true;
  
      // Lifetime intrinsics are dead when their right-hand is undef.
      if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
          II->getIntrinsicID() == Intrinsic::lifetime_end)
        return isa<UndefValue>(II->getArgOperand(1));
    }
  
    if (isAllocLikeFn(I, TLI)) return true;
  
    if (CallInst *CI = isFreeCall(I, TLI))
      if (Constant *C = dyn_cast<Constant>(CI->getArgOperand(0)))
        return C->isNullValue() || isa<UndefValue>(C);
  
    return false;
  }
  
  /// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a
  /// trivially dead instruction, delete it.  If that makes any of its operands
  /// trivially dead, delete them too, recursively.  Return true if any
  /// instructions were deleted.
  bool
  llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V,
                                                   const TargetLibraryInfo *TLI) {
    Instruction *I = dyn_cast<Instruction>(V);
    if (!I || !I->use_empty() || !isInstructionTriviallyDead(I, TLI))
      return false;
    
    SmallVector<Instruction*, 16> DeadInsts;
    DeadInsts.push_back(I);
    
    do {
      I = DeadInsts.pop_back_val();
  
      // Null out all of the instruction's operands to see if any operand becomes
      // dead as we go.
      for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
        Value *OpV = I->getOperand(i);
        I->setOperand(i, 0);
        
        if (!OpV->use_empty()) continue;
      
        // If the operand is an instruction that became dead as we nulled out the
        // operand, and if it is 'trivially' dead, delete it in a future loop
        // iteration.
        if (Instruction *OpI = dyn_cast<Instruction>(OpV))
          if (isInstructionTriviallyDead(OpI, TLI))
            DeadInsts.push_back(OpI);
      }
      
      I->eraseFromParent();
    } while (!DeadInsts.empty());
  
    return true;
  }
  
  /// areAllUsesEqual - Check whether the uses of a value are all the same.
  /// This is similar to Instruction::hasOneUse() except this will also return
  /// true when there are no uses or multiple uses that all refer to the same
  /// value.
  static bool areAllUsesEqual(Instruction *I) {
    Value::use_iterator UI = I->use_begin();
    Value::use_iterator UE = I->use_end();
    if (UI == UE)
      return true;
  
    User *TheUse = *UI;
    for (++UI; UI != UE; ++UI) {
      if (*UI != TheUse)
        return false;
    }
    return true;
  }
  
  /// RecursivelyDeleteDeadPHINode - If the specified value is an effectively
  /// dead PHI node, due to being a def-use chain of single-use nodes that
  /// either forms a cycle or is terminated by a trivially dead instruction,
  /// delete it.  If that makes any of its operands trivially dead, delete them
  /// too, recursively.  Return true if a change was made.
  bool llvm::RecursivelyDeleteDeadPHINode(PHINode *PN,
                                          const TargetLibraryInfo *TLI) {
    SmallPtrSet<Instruction*, 4> Visited;
    for (Instruction *I = PN; areAllUsesEqual(I) && !I->mayHaveSideEffects();
         I = cast<Instruction>(*I->use_begin())) {
      if (I->use_empty())
        return RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
  
      // If we find an instruction more than once, we're on a cycle that
      // won't prove fruitful.
      if (!Visited.insert(I)) {
        // Break the cycle and delete the instruction and its operands.
        I->replaceAllUsesWith(UndefValue::get(I->getType()));
        (void)RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
        return true;
      }
    }
    return false;
  }
  
  /// SimplifyInstructionsInBlock - Scan the specified basic block and try to
  /// simplify any instructions in it and recursively delete dead instructions.
  ///
  /// This returns true if it changed the code, note that it can delete
  /// instructions in other blocks as well in this block.
  bool llvm::SimplifyInstructionsInBlock(BasicBlock *BB, const TargetData *TD,
                                         const TargetLibraryInfo *TLI) {
    bool MadeChange = false;
  
  #ifndef NDEBUG
    // In debug builds, ensure that the terminator of the block is never replaced
    // or deleted by these simplifications. The idea of simplification is that it
    // cannot introduce new instructions, and there is no way to replace the
    // terminator of a block without introducing a new instruction.
    AssertingVH<Instruction> TerminatorVH(--BB->end());
  #endif
  
    for (BasicBlock::iterator BI = BB->begin(), E = --BB->end(); BI != E; ) {
      assert(!BI->isTerminator());
      Instruction *Inst = BI++;
  
      WeakVH BIHandle(BI);
      if (recursivelySimplifyInstruction(Inst, TD)) {
        MadeChange = true;
        if (BIHandle != BI)
          BI = BB->begin();
        continue;
      }
  
      MadeChange |= RecursivelyDeleteTriviallyDeadInstructions(Inst, TLI);
      if (BIHandle != BI)
        BI = BB->begin();
    }
    return MadeChange;
  }
  
  //===----------------------------------------------------------------------===//
  //  Control Flow Graph Restructuring.
  //
  
  
  /// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this
  /// method is called when we're about to delete Pred as a predecessor of BB.  If
  /// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred.
  ///
  /// Unlike the removePredecessor method, this attempts to simplify uses of PHI
  /// nodes that collapse into identity values.  For example, if we have:
  ///   x = phi(1, 0, 0, 0)
  ///   y = and x, z
  ///
  /// .. and delete the predecessor corresponding to the '1', this will attempt to
  /// recursively fold the and to 0.
  void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred,
                                          TargetData *TD) {
    // This only adjusts blocks with PHI nodes.
    if (!isa<PHINode>(BB->begin()))
      return;
    
    // Remove the entries for Pred from the PHI nodes in BB, but do not simplify
    // them down.  This will leave us with single entry phi nodes and other phis
    // that can be removed.
    BB->removePredecessor(Pred, true);
    
    WeakVH PhiIt = &BB->front();
    while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) {
      PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt));
      Value *OldPhiIt = PhiIt;
  
      if (!recursivelySimplifyInstruction(PN, TD))
        continue;
  
      // If recursive simplification ended up deleting the next PHI node we would
      // iterate to, then our iterator is invalid, restart scanning from the top
      // of the block.
      if (PhiIt != OldPhiIt) PhiIt = &BB->front();
    }
  }
  
  
  /// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
  /// predecessor is known to have one successor (DestBB!).  Eliminate the edge
  /// between them, moving the instructions in the predecessor into DestBB and
  /// deleting the predecessor block.
  ///
  void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) {
    // If BB has single-entry PHI nodes, fold them.
    while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
      Value *NewVal = PN->getIncomingValue(0);
      // Replace self referencing PHI with undef, it must be dead.
      if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
      PN->replaceAllUsesWith(NewVal);
      PN->eraseFromParent();
    }
    
    BasicBlock *PredBB = DestBB->getSinglePredecessor();
    assert(PredBB && "Block doesn't have a single predecessor!");
    
    // Zap anything that took the address of DestBB.  Not doing this will give the
    // address an invalid value.
    if (DestBB->hasAddressTaken()) {
      BlockAddress *BA = BlockAddress::get(DestBB);
      Constant *Replacement =
        ConstantInt::get(llvm::Type::getInt32Ty(BA->getContext()), 1);
      BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(Replacement,
                                                       BA->getType()));
      BA->destroyConstant();
    }
    
    // Anything that branched to PredBB now branches to DestBB.
    PredBB->replaceAllUsesWith(DestBB);
    
    // Splice all the instructions from PredBB to DestBB.
    PredBB->getTerminator()->eraseFromParent();
    DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
  
    if (P) {
      DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>();
      if (DT) {
        BasicBlock *PredBBIDom = DT->getNode(PredBB)->getIDom()->getBlock();
        DT->changeImmediateDominator(DestBB, PredBBIDom);
        DT->eraseNode(PredBB);
      }
      ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>();
      if (PI) {
        PI->replaceAllUses(PredBB, DestBB);
        PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB));
      }
    }
    // Nuke BB.
    PredBB->eraseFromParent();
  }
  
  /// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
  /// almost-empty BB ending in an unconditional branch to Succ, into succ.
  ///
  /// Assumption: Succ is the single successor for BB.
  ///
  static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
    assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
  
    DEBUG(dbgs() << "Looking to fold " << BB->getName() << " into " 
          << Succ->getName() << "\n");
    // Shortcut, if there is only a single predecessor it must be BB and merging
    // is always safe
    if (Succ->getSinglePredecessor()) return true;
  
    // Make a list of the predecessors of BB
    SmallPtrSet<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
  
    // Look at all the phi nodes in Succ, to see if they present a conflict when
    // merging these blocks
    for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
      PHINode *PN = cast<PHINode>(I);
  
      // If the incoming value from BB is again a PHINode in
      // BB which has the same incoming value for *PI as PN does, we can
      // merge the phi nodes and then the blocks can still be merged
      PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB));
      if (BBPN && BBPN->getParent() == BB) {
        for (unsigned PI = 0, PE = PN->getNumIncomingValues(); PI != PE; ++PI) {
          BasicBlock *IBB = PN->getIncomingBlock(PI);
          if (BBPreds.count(IBB) &&
              BBPN->getIncomingValueForBlock(IBB) != PN->getIncomingValue(PI)) {
            DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in " 
                  << Succ->getName() << " is conflicting with " 
                  << BBPN->getName() << " with regard to common predecessor "
                  << IBB->getName() << "\n");
            return false;
          }
        }
      } else {
        Value* Val = PN->getIncomingValueForBlock(BB);
        for (unsigned PI = 0, PE = PN->getNumIncomingValues(); PI != PE; ++PI) {
          // See if the incoming value for the common predecessor is equal to the
          // one for BB, in which case this phi node will not prevent the merging
          // of the block.
          BasicBlock *IBB = PN->getIncomingBlock(PI);
          if (BBPreds.count(IBB) && Val != PN->getIncomingValue(PI)) {
            DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in " 
                  << Succ->getName() << " is conflicting with regard to common "
                  << "predecessor " << IBB->getName() << "\n");
            return false;
          }
        }
      }
    }
  
    return true;
  }
  
  /// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an
  /// unconditional branch, and contains no instructions other than PHI nodes,
  /// potential side-effect free intrinsics and the branch.  If possible,
  /// eliminate BB by rewriting all the predecessors to branch to the successor
  /// block and return true.  If we can't transform, return false.
  bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) {
    assert(BB != &BB->getParent()->getEntryBlock() &&
           "TryToSimplifyUncondBranchFromEmptyBlock called on entry block!");
  
    // We can't eliminate infinite loops.
    BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0);
    if (BB == Succ) return false;
    
    // Check to see if merging these blocks would cause conflicts for any of the
    // phi nodes in BB or Succ. If not, we can safely merge.
    if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
  
    // Check for cases where Succ has multiple predecessors and a PHI node in BB
    // has uses which will not disappear when the PHI nodes are merged.  It is
    // possible to handle such cases, but difficult: it requires checking whether
    // BB dominates Succ, which is non-trivial to calculate in the case where
    // Succ has multiple predecessors.  Also, it requires checking whether
    // constructing the necessary self-referential PHI node doesn't intoduce any
    // conflicts; this isn't too difficult, but the previous code for doing this
    // was incorrect.
    //
    // Note that if this check finds a live use, BB dominates Succ, so BB is
    // something like a loop pre-header (or rarely, a part of an irreducible CFG);
    // folding the branch isn't profitable in that case anyway.
    if (!Succ->getSinglePredecessor()) {
      BasicBlock::iterator BBI = BB->begin();
      while (isa<PHINode>(*BBI)) {
        for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
             UI != E; ++UI) {
          if (PHINode* PN = dyn_cast<PHINode>(*UI)) {
            if (PN->getIncomingBlock(UI) != BB)
              return false;
          } else {
            return false;
          }
        }
        ++BBI;
      }
    }
  
    DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB);
    
    if (isa<PHINode>(Succ->begin())) {
      // If there is more than one pred of succ, and there are PHI nodes in
      // the successor, then we need to add incoming edges for the PHI nodes
      //
      const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
      
      // Loop over all of the PHI nodes in the successor of BB.
      for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
        PHINode *PN = cast<PHINode>(I);
        Value *OldVal = PN->removeIncomingValue(BB, false);
        assert(OldVal && "No entry in PHI for Pred BB!");
        
        // If this incoming value is one of the PHI nodes in BB, the new entries
        // in the PHI node are the entries from the old PHI.
        if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
          PHINode *OldValPN = cast<PHINode>(OldVal);
          for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
            // Note that, since we are merging phi nodes and BB and Succ might
            // have common predecessors, we could end up with a phi node with
            // identical incoming branches. This will be cleaned up later (and
            // will trigger asserts if we try to clean it up now, without also
            // simplifying the corresponding conditional branch).
            PN->addIncoming(OldValPN->getIncomingValue(i),
                            OldValPN->getIncomingBlock(i));
        } else {
          // Add an incoming value for each of the new incoming values.
          for (unsigned i = 0, e = BBPreds.size(); i != e; ++i)
            PN->addIncoming(OldVal, BBPreds[i]);
        }
      }
    }
    
    if (Succ->getSinglePredecessor()) {
      // BB is the only predecessor of Succ, so Succ will end up with exactly
      // the same predecessors BB had.
  
      // Copy over any phi, debug or lifetime instruction.
      BB->getTerminator()->eraseFromParent();
      Succ->getInstList().splice(Succ->getFirstNonPHI(), BB->getInstList());
    } else {
      while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
        // We explicitly check for such uses in CanPropagatePredecessorsForPHIs.
        assert(PN->use_empty() && "There shouldn't be any uses here!");
        PN->eraseFromParent();
      }
    }
      
    // Everything that jumped to BB now goes to Succ.
    BB->replaceAllUsesWith(Succ);
    if (!Succ->hasName()) Succ->takeName(BB);
    BB->eraseFromParent();              // Delete the old basic block.
    return true;
  }
  
  /// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI
  /// nodes in this block. This doesn't try to be clever about PHI nodes
  /// which differ only in the order of the incoming values, but instcombine
  /// orders them so it usually won't matter.
  ///
  bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
    bool Changed = false;
  
    // This implementation doesn't currently consider undef operands
    // specially. Theoretically, two phis which are identical except for
    // one having an undef where the other doesn't could be collapsed.
  
    // Map from PHI hash values to PHI nodes. If multiple PHIs have
    // the same hash value, the element is the first PHI in the
    // linked list in CollisionMap.
    DenseMap<uintptr_t, PHINode *> HashMap;
  
    // Maintain linked lists of PHI nodes with common hash values.
    DenseMap<PHINode *, PHINode *> CollisionMap;
  
    // Examine each PHI.
    for (BasicBlock::iterator I = BB->begin();
         PHINode *PN = dyn_cast<PHINode>(I++); ) {
      // Compute a hash value on the operands. Instcombine will likely have sorted
      // them, which helps expose duplicates, but we have to check all the
      // operands to be safe in case instcombine hasn't run.
      uintptr_t Hash = 0;
      // This hash algorithm is quite weak as hash functions go, but it seems
      // to do a good enough job for this particular purpose, and is very quick.
      for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) {
        Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I));
        Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7));
      }
      for (PHINode::block_iterator I = PN->block_begin(), E = PN->block_end();
           I != E; ++I) {
        Hash ^= reinterpret_cast<uintptr_t>(static_cast<BasicBlock *>(*I));
        Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7));
      }
      // Avoid colliding with the DenseMap sentinels ~0 and ~0-1.
      Hash >>= 1;
      // If we've never seen this hash value before, it's a unique PHI.
      std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair =
        HashMap.insert(std::make_pair(Hash, PN));
      if (Pair.second) continue;
      // Otherwise it's either a duplicate or a hash collision.
      for (PHINode *OtherPN = Pair.first->second; ; ) {
        if (OtherPN->isIdenticalTo(PN)) {
          // A duplicate. Replace this PHI with its duplicate.
          PN->replaceAllUsesWith(OtherPN);
          PN->eraseFromParent();
          Changed = true;
          break;
        }
        // A non-duplicate hash collision.
        DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN);
        if (I == CollisionMap.end()) {
          // Set this PHI to be the head of the linked list of colliding PHIs.
          PHINode *Old = Pair.first->second;
          Pair.first->second = PN;
          CollisionMap[PN] = Old;
          break;
        }
        // Proceed to the next PHI in the list.
        OtherPN = I->second;
      }
    }
  
    return Changed;
  }
  
  /// enforceKnownAlignment - If the specified pointer points to an object that
  /// we control, modify the object's alignment to PrefAlign. This isn't
  /// often possible though. If alignment is important, a more reliable approach
  /// is to simply align all global variables and allocation instructions to
  /// their preferred alignment from the beginning.
  ///
  static unsigned enforceKnownAlignment(Value *V, unsigned Align,
                                        unsigned PrefAlign, const TargetData *TD) {
    V = V->stripPointerCasts();
  
    if (AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
      // If the preferred alignment is greater than the natural stack alignment
      // then don't round up. This avoids dynamic stack realignment.
      if (TD && TD->exceedsNaturalStackAlignment(PrefAlign))
        return Align;
      // If there is a requested alignment and if this is an alloca, round up.
      if (AI->getAlignment() >= PrefAlign)
        return AI->getAlignment();
      AI->setAlignment(PrefAlign);
      return PrefAlign;
    }
  
    if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
      // If there is a large requested alignment and we can, bump up the alignment
      // of the global.
      if (GV->isDeclaration()) return Align;
      // If the memory we set aside for the global may not be the memory used by
      // the final program then it is impossible for us to reliably enforce the
      // preferred alignment.
      if (GV->isWeakForLinker()) return Align;
      
      if (GV->getAlignment() >= PrefAlign)
        return GV->getAlignment();
      // We can only increase the alignment of the global if it has no alignment
      // specified or if it is not assigned a section.  If it is assigned a
      // section, the global could be densely packed with other objects in the
      // section, increasing the alignment could cause padding issues.
      if (!GV->hasSection() || GV->getAlignment() == 0)
        GV->setAlignment(PrefAlign);
      return GV->getAlignment();
    }
  
    return Align;
  }
  
  /// getOrEnforceKnownAlignment - If the specified pointer has an alignment that
  /// we can determine, return it, otherwise return 0.  If PrefAlign is specified,
  /// and it is more than the alignment of the ultimate object, see if we can
  /// increase the alignment of the ultimate object, making this check succeed.
  unsigned llvm::getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign,
                                            const TargetData *TD) {
    assert(V->getType()->isPointerTy() &&
           "getOrEnforceKnownAlignment expects a pointer!");
    unsigned BitWidth = TD ? TD->getPointerSizeInBits() : 64;
    APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
    ComputeMaskedBits(V, KnownZero, KnownOne, TD);
    unsigned TrailZ = KnownZero.countTrailingOnes();
    
    // Avoid trouble with rediculously large TrailZ values, such as
    // those computed from a null pointer.
    TrailZ = std::min(TrailZ, unsigned(sizeof(unsigned) * CHAR_BIT - 1));
    
    unsigned Align = 1u << std::min(BitWidth - 1, TrailZ);
    
    // LLVM doesn't support alignments larger than this currently.
    Align = std::min(Align, +Value::MaximumAlignment);
    
    if (PrefAlign > Align)
      Align = enforceKnownAlignment(V, Align, PrefAlign, TD);
      
    // We don't need to make any adjustment.
    return Align;
  }
  
  ///===---------------------------------------------------------------------===//
  ///  Dbg Intrinsic utilities
  ///
  
  /// Inserts a llvm.dbg.value instrinsic before the stores to an alloca'd value
  /// that has an associated llvm.dbg.decl intrinsic.
  bool llvm::ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI,
                                             StoreInst *SI, DIBuilder &Builder) {
    DIVariable DIVar(DDI->getVariable());
    if (!DIVar.Verify())
      return false;
  
    Instruction *DbgVal = NULL;
    // If an argument is zero extended then use argument directly. The ZExt
    // may be zapped by an optimization pass in future.
    Argument *ExtendedArg = NULL;
    if (ZExtInst *ZExt = dyn_cast<ZExtInst>(SI->getOperand(0)))
      ExtendedArg = dyn_cast<Argument>(ZExt->getOperand(0));
    if (SExtInst *SExt = dyn_cast<SExtInst>(SI->getOperand(0)))
      ExtendedArg = dyn_cast<Argument>(SExt->getOperand(0));
    if (ExtendedArg)
      DbgVal = Builder.insertDbgValueIntrinsic(ExtendedArg, 0, DIVar, SI);
    else
      DbgVal = Builder.insertDbgValueIntrinsic(SI->getOperand(0), 0, DIVar, SI);
  
    // Propagate any debug metadata from the store onto the dbg.value.
    DebugLoc SIDL = SI->getDebugLoc();
    if (!SIDL.isUnknown())
      DbgVal->setDebugLoc(SIDL);
    // Otherwise propagate debug metadata from dbg.declare.
    else
      DbgVal->setDebugLoc(DDI->getDebugLoc());
    return true;
  }
  
  /// Inserts a llvm.dbg.value instrinsic before the stores to an alloca'd value
  /// that has an associated llvm.dbg.decl intrinsic.
  bool llvm::ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI,
                                             LoadInst *LI, DIBuilder &Builder) {
    DIVariable DIVar(DDI->getVariable());
    if (!DIVar.Verify())
      return false;
  
    Instruction *DbgVal = 
      Builder.insertDbgValueIntrinsic(LI->getOperand(0), 0,
                                      DIVar, LI);
    
    // Propagate any debug metadata from the store onto the dbg.value.
    DebugLoc LIDL = LI->getDebugLoc();
    if (!LIDL.isUnknown())
      DbgVal->setDebugLoc(LIDL);
    // Otherwise propagate debug metadata from dbg.declare.
    else
      DbgVal->setDebugLoc(DDI->getDebugLoc());
    return true;
  }
  
  /// LowerDbgDeclare - Lowers llvm.dbg.declare intrinsics into appropriate set
  /// of llvm.dbg.value intrinsics.
  bool llvm::LowerDbgDeclare(Function &F) {
    DIBuilder DIB(*F.getParent());
    SmallVector<DbgDeclareInst *, 4> Dbgs;
    for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
      for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ++BI) {
        if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(BI))
          Dbgs.push_back(DDI);
      }
    if (Dbgs.empty())
      return false;
  
    for (SmallVector<DbgDeclareInst *, 4>::iterator I = Dbgs.begin(),
           E = Dbgs.end(); I != E; ++I) {
      DbgDeclareInst *DDI = *I;
      if (AllocaInst *AI = dyn_cast_or_null<AllocaInst>(DDI->getAddress())) {
        bool RemoveDDI = true;
        for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end();
             UI != E; ++UI)
          if (StoreInst *SI = dyn_cast<StoreInst>(*UI))
            ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
          else if (LoadInst *LI = dyn_cast<LoadInst>(*UI))
            ConvertDebugDeclareToDebugValue(DDI, LI, DIB);
          else
            RemoveDDI = false;
        if (RemoveDDI)
          DDI->eraseFromParent();
      }
    }
    return true;
  }
  
  /// FindAllocaDbgDeclare - Finds the llvm.dbg.declare intrinsic describing the
  /// alloca 'V', if any.
  DbgDeclareInst *llvm::FindAllocaDbgDeclare(Value *V) {
    if (MDNode *DebugNode = MDNode::getIfExists(V->getContext(), V))
      for (Value::use_iterator UI = DebugNode->use_begin(),
           E = DebugNode->use_end(); UI != E; ++UI)
        if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(*UI))
          return DDI;
  
    return 0;
  }
  