llvm/lib/Transforms/Utils/SimplifyCFG.cpp

//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
// 
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
// 
//===----------------------------------------------------------------------===//
//
// Peephole optimize the CFG.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Type.h"
#include "llvm/Support/CFG.h"
#include <algorithm>
#include <functional>
#include <set>
using namespace llvm;

// PropagatePredecessorsForPHIs - This gets "Succ" ready to have the
// predecessors from "BB".  This is a little tricky because "Succ" has PHI
// nodes, which need to have extra slots added to them to hold the merge edges
// from BB's predecessors, and BB itself might have had PHI nodes in it.  This
// function returns true (failure) if the Succ BB already has a predecessor that
// is a predecessor of BB and incoming PHI arguments would not be discernible.
//
// Assumption: Succ is the single successor for BB.
//
static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
  assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");

  if (!isa<PHINode>(Succ->front()))
    return false;  // We can make the transformation, no problem.

  // If there is more than one predecessor, and there are PHI nodes in
  // the successor, then we need to add incoming edges for the PHI nodes
  //
  const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));

  // Check to see if one of the predecessors of BB is already a predecessor of
  // Succ.  If so, we cannot do the transformation if there are any PHI nodes
  // with incompatible values coming in from the two edges!
  //
  for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
    if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
      // Loop over all of the PHI nodes checking to see if there are
      // incompatible values coming in.
      for (BasicBlock::iterator I = Succ->begin();
           PHINode *PN = dyn_cast<PHINode>(I); ++I) {
        // Loop up the entries in the PHI node for BB and for *PI if the values
        // coming in are non-equal, we cannot merge these two blocks (instead we
        // should insert a conditional move or something, then merge the
        // blocks).
        int Idx1 = PN->getBasicBlockIndex(BB);
        int Idx2 = PN->getBasicBlockIndex(*PI);
        assert(Idx1 != -1 && Idx2 != -1 &&
               "Didn't have entries for my predecessors??");
        if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
          return true;  // Values are not equal...
      }
    }

  // Loop over all of the PHI nodes in the successor BB
  for (BasicBlock::iterator I = Succ->begin();
       PHINode *PN = dyn_cast<PHINode>(I); ++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...
    if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
      PHINode *OldValPN = cast<PHINode>(OldVal);
      for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(), 
             End = BBPreds.end(); PredI != End; ++PredI) {
        PN->addIncoming(OldValPN->getIncomingValueForBlock(*PredI), *PredI);
      }
    } else {
      for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(), 
             End = BBPreds.end(); PredI != End; ++PredI) {
        // Add an incoming value for each of the new incoming values...
        PN->addIncoming(OldVal, *PredI);
      }
    }
  }
  return false;
}

/// GetIfCondition - Given a basic block (BB) with two predecessors (and
/// presumably PHI nodes in it), check to see if the merge at this block is due
/// to an "if condition".  If so, return the boolean condition that determines
/// which entry into BB will be taken.  Also, return by references the block
/// that will be entered from if the condition is true, and the block that will
/// be entered if the condition is false.
/// 
///
static Value *GetIfCondition(BasicBlock *BB,
                             BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
  assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
         "Function can only handle blocks with 2 predecessors!");
  BasicBlock *Pred1 = *pred_begin(BB);
  BasicBlock *Pred2 = *++pred_begin(BB);

  // We can only handle branches.  Other control flow will be lowered to
  // branches if possible anyway.
  if (!isa<BranchInst>(Pred1->getTerminator()) ||
      !isa<BranchInst>(Pred2->getTerminator()))
    return 0;
  BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
  BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());

  // Eliminate code duplication by ensuring that Pred1Br is conditional if
  // either are.
  if (Pred2Br->isConditional()) {
    // If both branches are conditional, we don't have an "if statement".  In
    // reality, we could transform this case, but since the condition will be
    // required anyway, we stand no chance of eliminating it, so the xform is
    // probably not profitable.
    if (Pred1Br->isConditional())
      return 0;

    std::swap(Pred1, Pred2);
    std::swap(Pred1Br, Pred2Br);
  }

  if (Pred1Br->isConditional()) {
    // If we found a conditional branch predecessor, make sure that it branches
    // to BB and Pred2Br.  If it doesn't, this isn't an "if statement".
    if (Pred1Br->getSuccessor(0) == BB &&
        Pred1Br->getSuccessor(1) == Pred2) {
      IfTrue = Pred1;
      IfFalse = Pred2;
    } else if (Pred1Br->getSuccessor(0) == Pred2 &&
               Pred1Br->getSuccessor(1) == BB) {
      IfTrue = Pred2;
      IfFalse = Pred1;
    } else {
      // We know that one arm of the conditional goes to BB, so the other must
      // go somewhere unrelated, and this must not be an "if statement".
      return 0;
    }

    // The only thing we have to watch out for here is to make sure that Pred2
    // doesn't have incoming edges from other blocks.  If it does, the condition
    // doesn't dominate BB.
    if (++pred_begin(Pred2) != pred_end(Pred2))
      return 0;

    return Pred1Br->getCondition();
  }

  // Ok, if we got here, both predecessors end with an unconditional branch to
  // BB.  Don't panic!  If both blocks only have a single (identical)
  // predecessor, and THAT is a conditional branch, then we're all ok!
  if (pred_begin(Pred1) == pred_end(Pred1) ||
      ++pred_begin(Pred1) != pred_end(Pred1) ||
      pred_begin(Pred2) == pred_end(Pred2) ||
      ++pred_begin(Pred2) != pred_end(Pred2) ||
      *pred_begin(Pred1) != *pred_begin(Pred2))
    return 0;

  // Otherwise, if this is a conditional branch, then we can use it!
  BasicBlock *CommonPred = *pred_begin(Pred1);
  if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
    assert(BI->isConditional() && "Two successors but not conditional?");
    if (BI->getSuccessor(0) == Pred1) {
      IfTrue = Pred1;
      IfFalse = Pred2;
    } else {
      IfTrue = Pred2;
      IfFalse = Pred1;
    }
    return BI->getCondition();
  }
  return 0;
}


// If we have a merge point of an "if condition" as accepted above, return true
// if the specified value dominates the block.  We don't handle the true
// generality of domination here, just a special case which works well enough
// for us.
static bool DominatesMergePoint(Value *V, BasicBlock *BB, bool AllowAggressive){
  Instruction *I = dyn_cast<Instruction>(V);
  if (!I) return true;    // Non-instructions all dominate instructions.
  BasicBlock *PBB = I->getParent();

  // We don't want to allow wierd loops that might have the "if condition" in
  // the bottom of this block.
  if (PBB == BB) return false;

  // If this instruction is defined in a block that contains an unconditional
  // branch to BB, then it must be in the 'conditional' part of the "if
  // statement".
  if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
    if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
      if (!AllowAggressive) return false;
      // Okay, it looks like the instruction IS in the "condition".  Check to
      // see if its a cheap instruction to unconditionally compute, and if it
      // only uses stuff defined outside of the condition.  If so, hoist it out.
      switch (I->getOpcode()) {
      default: return false;  // Cannot hoist this out safely.
      case Instruction::Load:
        // We can hoist loads that are non-volatile and obviously cannot trap.
        if (cast<LoadInst>(I)->isVolatile())
          return false;
        if (!isa<AllocaInst>(I->getOperand(0)) &&
            !isa<Constant>(I->getOperand(0)) &&
            !isa<GlobalValue>(I->getOperand(0)))
          return false;

        // Finally, we have to check to make sure there are no instructions
        // before the load in its basic block, as we are going to hoist the loop
        // out to its predecessor.
        if (PBB->begin() != BasicBlock::iterator(I))
          return false;
        break;
      case Instruction::Add:
      case Instruction::Sub:
      case Instruction::And:
      case Instruction::Or:
      case Instruction::Xor:
      case Instruction::Shl:
      case Instruction::Shr:
        break;   // These are all cheap and non-trapping instructions.
      }
      
      // Okay, we can only really hoist these out if their operands are not
      // defined in the conditional region.
      for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
        if (!DominatesMergePoint(I->getOperand(i), BB, false))
          return false;
      // Okay, it's safe to do this!
    }

  return true;
}

// GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq
// instructions that compare a value against a constant, return the value being
// compared, and stick the constant into the Values vector.
static Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values){
  if (Instruction *Inst = dyn_cast<Instruction>(V))
    if (Inst->getOpcode() == Instruction::SetEQ) {
      if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
        Values.push_back(C);
        return Inst->getOperand(0);
      } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
        Values.push_back(C);
        return Inst->getOperand(1);
      }
    } else if (Inst->getOpcode() == Instruction::Or) {
      if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
        if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
          if (LHS == RHS)
            return LHS;
    }
  return 0;
}

// GatherConstantSetNEs - Given a potentially 'and'd together collection of
// setne instructions that compare a value against a constant, return the value
// being compared, and stick the constant into the Values vector.
static Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values){
  if (Instruction *Inst = dyn_cast<Instruction>(V))
    if (Inst->getOpcode() == Instruction::SetNE) {
      if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) {
        Values.push_back(C);
        return Inst->getOperand(0);
      } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) {
        Values.push_back(C);
        return Inst->getOperand(1);
      }
    } else if (Inst->getOpcode() == Instruction::Cast) {
      // Cast of X to bool is really a comparison against zero.
      assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!");
      Values.push_back(ConstantInt::get(Inst->getOperand(0)->getType(), 0));
      return Inst->getOperand(0);
    } else if (Inst->getOpcode() == Instruction::And) {
      if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
        if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
          if (LHS == RHS)
            return LHS;
    }
  return 0;
}



/// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
/// bunch of comparisons of one value against constants, return the value and
/// the constants being compared.
static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
                                   std::vector<ConstantInt*> &Values) {
  if (Cond->getOpcode() == Instruction::Or) {
    CompVal = GatherConstantSetEQs(Cond, Values);

    // Return true to indicate that the condition is true if the CompVal is
    // equal to one of the constants.
    return true;
  } else if (Cond->getOpcode() == Instruction::And) {
    CompVal = GatherConstantSetNEs(Cond, Values);
        
    // Return false to indicate that the condition is false if the CompVal is
    // equal to one of the constants.
    return false;
  }
  return false;
}

/// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
/// has no side effects, nuke it.  If it uses any instructions that become dead
/// because the instruction is now gone, nuke them too.
static void ErasePossiblyDeadInstructionTree(Instruction *I) {
  if (isInstructionTriviallyDead(I)) {
    std::vector<Value*> Operands(I->op_begin(), I->op_end());
    I->getParent()->getInstList().erase(I);
    for (unsigned i = 0, e = Operands.size(); i != e; ++i)
      if (Instruction *OpI = dyn_cast<Instruction>(Operands[i]))
        ErasePossiblyDeadInstructionTree(OpI);
  }
}

/// SafeToMergeTerminators - Return true if it is safe to merge these two
/// terminator instructions together.
///
static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
  if (SI1 == SI2) return false;  // Can't merge with self!

  // It is not safe to merge these two switch instructions if they have a common
  // successor, and if that successor has a PHI node, and if that PHI node has
  // conflicting incoming values from the two switch blocks.
  BasicBlock *SI1BB = SI1->getParent();
  BasicBlock *SI2BB = SI2->getParent();
  std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));

  for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
    if (SI1Succs.count(*I))
      for (BasicBlock::iterator BBI = (*I)->begin();
           PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI)
        if (PN->getIncomingValueForBlock(SI1BB) !=
            PN->getIncomingValueForBlock(SI2BB))
          return false;
        
  return true;
}

/// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
/// now be entries in it from the 'NewPred' block.  The values that will be
/// flowing into the PHI nodes will be the same as those coming in from
/// ExistPred, and existing predecessor of Succ.
static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
                                  BasicBlock *ExistPred) {
  assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
         succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
  if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do

  for (BasicBlock::iterator I = Succ->begin();
       PHINode *PN = dyn_cast<PHINode>(I); ++I) {
    Value *V = PN->getIncomingValueForBlock(ExistPred);
    PN->addIncoming(V, NewPred);
  }
}

// isValueEqualityComparison - Return true if the specified terminator checks to
// see if a value is equal to constant integer value.
static Value *isValueEqualityComparison(TerminatorInst *TI) {
  if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
    // Do not permit merging of large switch instructions into their
    // predecessors unless there is only one predecessor.
    if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()),
                                               pred_end(SI->getParent())) > 128)
      return 0;

    return SI->getCondition();
  }
  if (BranchInst *BI = dyn_cast<BranchInst>(TI))
    if (BI->isConditional() && BI->getCondition()->hasOneUse())
      if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition()))
        if ((SCI->getOpcode() == Instruction::SetEQ ||
             SCI->getOpcode() == Instruction::SetNE) && 
            isa<ConstantInt>(SCI->getOperand(1)))
          return SCI->getOperand(0);
  return 0;
}

// Given a value comparison instruction, decode all of the 'cases' that it
// represents and return the 'default' block.
static BasicBlock *
GetValueEqualityComparisonCases(TerminatorInst *TI, 
                                std::vector<std::pair<ConstantInt*,
                                                      BasicBlock*> > &Cases) {
  if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
    Cases.reserve(SI->getNumCases());
    for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
      Cases.push_back(std::make_pair(cast<ConstantInt>(SI->getCaseValue(i)),
                                     SI->getSuccessor(i)));
    return SI->getDefaultDest();
  }

  BranchInst *BI = cast<BranchInst>(TI);
  SetCondInst *SCI = cast<SetCondInst>(BI->getCondition());
  Cases.push_back(std::make_pair(cast<ConstantInt>(SCI->getOperand(1)),
                                 BI->getSuccessor(SCI->getOpcode() ==
                                                        Instruction::SetNE)));
  return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ);
}


// FoldValueComparisonIntoPredecessors - The specified terminator is a value
// equality comparison instruction (either a switch or a branch on "X == c").
// See if any of the predecessors of the terminator block are value comparisons
// on the same value.  If so, and if safe to do so, fold them together.
static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
  BasicBlock *BB = TI->getParent();
  Value *CV = isValueEqualityComparison(TI);  // CondVal
  assert(CV && "Not a comparison?");
  bool Changed = false;

  std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
  while (!Preds.empty()) {
    BasicBlock *Pred = Preds.back();
    Preds.pop_back();
    
    // See if the predecessor is a comparison with the same value.
    TerminatorInst *PTI = Pred->getTerminator();
    Value *PCV = isValueEqualityComparison(PTI);  // PredCondVal

    if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
      // Figure out which 'cases' to copy from SI to PSI.
      std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
      BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);

      std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
      BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);

      // Based on whether the default edge from PTI goes to BB or not, fill in
      // PredCases and PredDefault with the new switch cases we would like to
      // build.
      std::vector<BasicBlock*> NewSuccessors;

      if (PredDefault == BB) {
        // If this is the default destination from PTI, only the edges in TI
        // that don't occur in PTI, or that branch to BB will be activated.
        std::set<ConstantInt*> PTIHandled;
        for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
          if (PredCases[i].second != BB)
            PTIHandled.insert(PredCases[i].first);
          else {
            // The default destination is BB, we don't need explicit targets.
            std::swap(PredCases[i], PredCases.back());
            PredCases.pop_back();
            --i; --e;
          }

        // Reconstruct the new switch statement we will be building.
        if (PredDefault != BBDefault) {
          PredDefault->removePredecessor(Pred);
          PredDefault = BBDefault;
          NewSuccessors.push_back(BBDefault);
        }
        for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
          if (!PTIHandled.count(BBCases[i].first) &&
              BBCases[i].second != BBDefault) {
            PredCases.push_back(BBCases[i]);
            NewSuccessors.push_back(BBCases[i].second);
          }

      } else {
        // If this is not the default destination from PSI, only the edges
        // in SI that occur in PSI with a destination of BB will be
        // activated.
        std::set<ConstantInt*> PTIHandled;
        for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
          if (PredCases[i].second == BB) {
            PTIHandled.insert(PredCases[i].first);
            std::swap(PredCases[i], PredCases.back());
            PredCases.pop_back();
            --i; --e;
          }

        // Okay, now we know which constants were sent to BB from the
        // predecessor.  Figure out where they will all go now.
        for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
          if (PTIHandled.count(BBCases[i].first)) {
            // If this is one we are capable of getting...
            PredCases.push_back(BBCases[i]);
            NewSuccessors.push_back(BBCases[i].second);
            PTIHandled.erase(BBCases[i].first);// This constant is taken care of
          }

        // If there are any constants vectored to BB that TI doesn't handle,
        // they must go to the default destination of TI.
        for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(),
               E = PTIHandled.end(); I != E; ++I) {
          PredCases.push_back(std::make_pair(*I, BBDefault));
          NewSuccessors.push_back(BBDefault);
        }
      }

      // Okay, at this point, we know which new successor Pred will get.  Make
      // sure we update the number of entries in the PHI nodes for these
      // successors.
      for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
        AddPredecessorToBlock(NewSuccessors[i], Pred, BB);

      // Now that the successors are updated, create the new Switch instruction.
      SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PTI);
      for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
        NewSI->addCase(PredCases[i].first, PredCases[i].second);
      Pred->getInstList().erase(PTI);

      // Okay, last check.  If BB is still a successor of PSI, then we must
      // have an infinite loop case.  If so, add an infinitely looping block
      // to handle the case to preserve the behavior of the code.
      BasicBlock *InfLoopBlock = 0;
      for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
        if (NewSI->getSuccessor(i) == BB) {
          if (InfLoopBlock == 0) {
            // Insert it at the end of the loop, because it's either code,
            // or it won't matter if it's hot. :)
            InfLoopBlock = new BasicBlock("infloop", BB->getParent());
            new BranchInst(InfLoopBlock, InfLoopBlock);
          }
          NewSI->setSuccessor(i, InfLoopBlock);
        }
          
      Changed = true;
    }
  }
  return Changed;
}

namespace {
  /// ConstantIntOrdering - This class implements a stable ordering of constant
  /// integers that does not depend on their address.  This is important for
  /// applications that sort ConstantInt's to ensure uniqueness.
  struct ConstantIntOrdering {
    bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const {
      return LHS->getRawValue() < RHS->getRawValue();
    }
  };
}


// SimplifyCFG - This function is used to do simplification of a CFG.  For
// example, it adjusts branches to branches to eliminate the extra hop, it
// eliminates unreachable basic blocks, and does other "peephole" optimization
// of the CFG.  It returns true if a modification was made.
//
// WARNING:  The entry node of a function may not be simplified.
//
bool llvm::SimplifyCFG(BasicBlock *BB) {
  bool Changed = false;
  Function *M = BB->getParent();

  assert(BB && BB->getParent() && "Block not embedded in function!");
  assert(BB->getTerminator() && "Degenerate basic block encountered!");
  assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");

  // Remove basic blocks that have no predecessors... which are unreachable.
  if (pred_begin(BB) == pred_end(BB) ||
      *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
    //cerr << "Removing BB: \n" << BB;

    // Loop through all of our successors and make sure they know that one
    // of their predecessors is going away.
    for_each(succ_begin(BB), succ_end(BB),
	     std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));

    while (!BB->empty()) {
      Instruction &I = BB->back();
      // If this instruction is used, replace uses with an arbitrary
      // constant value.  Because control flow can't get here, we don't care
      // what we replace the value with.  Note that since this block is 
      // unreachable, and all values contained within it must dominate their
      // uses, that all uses will eventually be removed.
      if (!I.use_empty()) 
        // Make all users of this instruction reference the constant instead
        I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
      
      // Remove the instruction from the basic block
      BB->getInstList().pop_back();
    }
    M->getBasicBlockList().erase(BB);
    return true;
  }

  // Check to see if we can constant propagate this terminator instruction
  // away...
  Changed |= ConstantFoldTerminator(BB);

  // Check to see if this block has no non-phi instructions and only a single
  // successor.  If so, replace references to this basic block with references
  // to the successor.
  succ_iterator SI(succ_begin(BB));
  if (SI != succ_end(BB) && ++SI == succ_end(BB)) {  // One succ?

    BasicBlock::iterator BBI = BB->begin();  // Skip over phi nodes...
    while (isa<PHINode>(*BBI)) ++BBI;

    if (BBI->isTerminator()) {   // Terminator is the only non-phi instruction!
      BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
     
      if (Succ != BB) {   // Arg, don't hurt infinite loops!
        // If our successor has PHI nodes, then we need to update them to
        // include entries for BB's predecessors, not for BB itself.
        // Be careful though, if this transformation fails (returns true) then
        // we cannot do this transformation!
        //
	if (!PropagatePredecessorsForPHIs(BB, Succ)) {
          //cerr << "Killing Trivial BB: \n" << BB;
          std::string OldName = BB->getName();

          std::vector<BasicBlock*>
            OldSuccPreds(pred_begin(Succ), pred_end(Succ));

          // Move all PHI nodes in BB to Succ if they are alive, otherwise
          // delete them.
          while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
            if (PN->use_empty())
              BB->getInstList().erase(BB->begin());  // Nuke instruction...
            else {
              // The instruction is alive, so this means that Succ must have
              // *ONLY* had BB as a predecessor, and the PHI node is still valid
              // now.  Simply move it into Succ, because we know that BB
              // strictly dominated Succ.
              BB->getInstList().remove(BB->begin());
              Succ->getInstList().push_front(PN);

              // We need to add new entries for the PHI node to account for
              // predecessors of Succ that the PHI node does not take into
              // account.  At this point, since we know that BB dominated succ,
              // this means that we should any newly added incoming edges should
              // use the PHI node as the value for these edges, because they are
              // loop back edges.
              
              for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
                if (OldSuccPreds[i] != BB)
                  PN->addIncoming(PN, OldSuccPreds[i]);
            }

          // Everything that jumped to BB now goes to Succ...
          BB->replaceAllUsesWith(Succ);

          // Delete the old basic block...
          M->getBasicBlockList().erase(BB);
	
          if (!OldName.empty() && !Succ->hasName())  // Transfer name if we can
            Succ->setName(OldName);
          
          //cerr << "Function after removal: \n" << M;
          return true;
	}
      }
    }
  }

  // If this is a returning block with only PHI nodes in it, fold the return
  // instruction into any unconditional branch predecessors.
  //
  // If any predecessor is a conditional branch that just selects among
  // different return values, fold the replace the branch/return with a select
  // and return.
  if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
    BasicBlock::iterator BBI = BB->getTerminator();
    if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
      // Find predecessors that end with branches.
      std::vector<BasicBlock*> UncondBranchPreds;
      std::vector<BranchInst*> CondBranchPreds;
      for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
        TerminatorInst *PTI = (*PI)->getTerminator();
        if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
          if (BI->isUnconditional())
            UncondBranchPreds.push_back(*PI);
          else
            CondBranchPreds.push_back(BI);
      }
      
      // If we found some, do the transformation!
      if (!UncondBranchPreds.empty()) {
        while (!UncondBranchPreds.empty()) {
          BasicBlock *Pred = UncondBranchPreds.back();
          UncondBranchPreds.pop_back();
          Instruction *UncondBranch = Pred->getTerminator();
          // Clone the return and add it to the end of the predecessor.
          Instruction *NewRet = RI->clone();
          Pred->getInstList().push_back(NewRet);

          // If the return instruction returns a value, and if the value was a
          // PHI node in "BB", propagate the right value into the return.
          if (NewRet->getNumOperands() == 1)
            if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
              if (PN->getParent() == BB)
                NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
          // Update any PHI nodes in the returning block to realize that we no
          // longer branch to them.
          BB->removePredecessor(Pred);
          Pred->getInstList().erase(UncondBranch);
        }

        // If we eliminated all predecessors of the block, delete the block now.
        if (pred_begin(BB) == pred_end(BB))
          // We know there are no successors, so just nuke the block.
          M->getBasicBlockList().erase(BB);

        return true;
      }

      // Check out all of the conditional branches going to this return
      // instruction.  If any of them just select between returns, change the
      // branch itself into a select/return pair.
      while (!CondBranchPreds.empty()) {
        BranchInst *BI = CondBranchPreds.back();
        CondBranchPreds.pop_back();
        BasicBlock *TrueSucc = BI->getSuccessor(0);
        BasicBlock *FalseSucc = BI->getSuccessor(1);
        BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;

        // Check to see if the non-BB successor is also a return block.
        if (isa<ReturnInst>(OtherSucc->getTerminator())) {
          // Check to see if there are only PHI instructions in this block.
          BasicBlock::iterator OSI = OtherSucc->getTerminator();
          if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
            // Okay, we found a branch that is going to two return nodes.  If
            // there is no return value for this function, just change the
            // branch into a return.
            if (RI->getNumOperands() == 0) {
              TrueSucc->removePredecessor(BI->getParent());
              FalseSucc->removePredecessor(BI->getParent());
              new ReturnInst(0, BI);
              BI->getParent()->getInstList().erase(BI);
              return true;
            }

            // Otherwise, figure out what the true and false return values are
            // so we can insert a new select instruction.
            Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
            Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);

            // Unwrap any PHI nodes in the return blocks.
            if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
              if (TVPN->getParent() == TrueSucc)
                TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
            if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
              if (FVPN->getParent() == FalseSucc)
                FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());

            TrueSucc->removePredecessor(BI->getParent());
            FalseSucc->removePredecessor(BI->getParent());

            // Insert a new select instruction.
            Value *NewRetVal = new SelectInst(BI->getCondition(), TrueValue,
                                              FalseValue, "retval", BI);
            new ReturnInst(NewRetVal, BI);
            BI->getParent()->getInstList().erase(BI);
            return true;
          }
        }
      }
    }
  } else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) {
    // Check to see if the first instruction in this block is just an unwind.
    // If so, replace any invoke instructions which use this as an exception
    // destination with call instructions.
    //
    std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
    while (!Preds.empty()) {
      BasicBlock *Pred = Preds.back();
      if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
        if (II->getUnwindDest() == BB) {
          // Insert a new branch instruction before the invoke, because this
          // is now a fall through...
          BranchInst *BI = new BranchInst(II->getNormalDest(), II);
          Pred->getInstList().remove(II);   // Take out of symbol table
          
          // Insert the call now...
          std::vector<Value*> Args(II->op_begin()+3, II->op_end());
          CallInst *CI = new CallInst(II->getCalledValue(), Args,
                                      II->getName(), BI);
          // If the invoke produced a value, the Call now does instead
          II->replaceAllUsesWith(CI);
          delete II;
          Changed = true;
        }
      
      Preds.pop_back();
    }

    // If this block is now dead, remove it.
    if (pred_begin(BB) == pred_end(BB)) {
      // We know there are no successors, so just nuke the block.
      M->getBasicBlockList().erase(BB);
      return true;
    }

  } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->begin())) {
    if (isValueEqualityComparison(SI))
      if (FoldValueComparisonIntoPredecessors(SI))
        return SimplifyCFG(BB) || 1;
  } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
    if (BI->isConditional()) {
      if (Value *CompVal = isValueEqualityComparison(BI)) {
        // This block must be empty, except for the setcond inst, if it exists.
        BasicBlock::iterator I = BB->begin();
        if (&*I == BI ||
            (&*I == cast<Instruction>(BI->getCondition()) &&
             &*++I == BI))
          if (FoldValueComparisonIntoPredecessors(BI))
            return SimplifyCFG(BB) | true;
      }

      // If this basic block is ONLY a setcc and a branch, and if a predecessor
      // branches to us and one of our successors, fold the setcc into the
      // predecessor and use logical operations to pick the right destination.
      BasicBlock *TrueDest  = BI->getSuccessor(0);
      BasicBlock *FalseDest = BI->getSuccessor(1);
      if (BinaryOperator *Cond = dyn_cast<BinaryOperator>(BI->getCondition()))
        if (Cond->getParent() == BB && &BB->front() == Cond &&
            Cond->getNext() == BI && Cond->hasOneUse() &&
            TrueDest != BB && FalseDest != BB)
          for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI)
            if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
              if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) {
                if (PBI->getSuccessor(0) == FalseDest ||
                    PBI->getSuccessor(1) == TrueDest) {
                  // Invert the predecessors condition test (xor it with true),
                  // which allows us to write this code once.
                  Value *NewCond =
                    BinaryOperator::createNot(PBI->getCondition(),
                                    PBI->getCondition()->getName()+".not", PBI);
                  PBI->setCondition(NewCond);
                  BasicBlock *OldTrue = PBI->getSuccessor(0);
                  BasicBlock *OldFalse = PBI->getSuccessor(1);
                  PBI->setSuccessor(0, OldFalse);
                  PBI->setSuccessor(1, OldTrue);
                }

                if (PBI->getSuccessor(0) == TrueDest ||
                    PBI->getSuccessor(1) == FalseDest) {
                  // Clone Cond into the predecessor basic block, and and the
                  // two conditions together.
                  Instruction *New = Cond->clone();
                  New->setName(Cond->getName());
                  Cond->setName(Cond->getName()+".old");
                  (*PI)->getInstList().insert(PBI, New);
                  Instruction::BinaryOps Opcode =
                    PBI->getSuccessor(0) == TrueDest ?
                    Instruction::Or : Instruction::And;
                  Value *NewCond = 
                    BinaryOperator::create(Opcode, PBI->getCondition(),
                                           New, "bothcond", PBI);
                  PBI->setCondition(NewCond);
                  if (PBI->getSuccessor(0) == BB) {
                    AddPredecessorToBlock(TrueDest, *PI, BB);
                    PBI->setSuccessor(0, TrueDest);
                  }
                  if (PBI->getSuccessor(1) == BB) {
                    AddPredecessorToBlock(FalseDest, *PI, BB);
                    PBI->setSuccessor(1, FalseDest);
                  }
                  return SimplifyCFG(BB) | 1;
                }
              }

      // If this block ends with a branch instruction, and if there is one
      // predecessor, see if the previous block ended with a branch on the same
      // condition, which makes this conditional branch redundant.
      pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
      BasicBlock *OnlyPred = *PI++;
      for (; PI != PE; ++PI)// Search all predecessors, see if they are all same
        if (*PI != OnlyPred) {
          OnlyPred = 0;       // There are multiple different predecessors...
          break;
        }
      
      if (OnlyPred)
        if (BranchInst *PBI = dyn_cast<BranchInst>(OnlyPred->getTerminator()))
          if (PBI->isConditional() &&
              PBI->getCondition() == BI->getCondition() &&
              (PBI->getSuccessor(0) != BB || PBI->getSuccessor(1) != BB)) {
            // Okay, the outcome of this conditional branch is statically
            // knowable.  Delete the outgoing CFG edge that is impossible to
            // execute.
            bool CondIsTrue = PBI->getSuccessor(0) == BB;
            BI->getSuccessor(CondIsTrue)->removePredecessor(BB);
            new BranchInst(BI->getSuccessor(!CondIsTrue), BB);
            BB->getInstList().erase(BI);
            return SimplifyCFG(BB) | true;
          }
    }
  }

  // Merge basic blocks into their predecessor if there is only one distinct
  // pred, and if there is only one distinct successor of the predecessor, and
  // if there are no PHI nodes.
  //
  pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
  BasicBlock *OnlyPred = *PI++;
  for (; PI != PE; ++PI)  // Search all predecessors, see if they are all same
    if (*PI != OnlyPred) {
      OnlyPred = 0;       // There are multiple different predecessors...
      break;
    }

  BasicBlock *OnlySucc = 0;
  if (OnlyPred && OnlyPred != BB &&    // Don't break self loops
      OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
    // Check to see if there is only one distinct successor...
    succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
    OnlySucc = BB;
    for (; SI != SE; ++SI)
      if (*SI != OnlySucc) {
        OnlySucc = 0;     // There are multiple distinct successors!
        break;
      }
  }

  if (OnlySucc) {
    //cerr << "Merging: " << BB << "into: " << OnlyPred;
    TerminatorInst *Term = OnlyPred->getTerminator();

    // Resolve any PHI nodes at the start of the block.  They are all
    // guaranteed to have exactly one entry if they exist, unless there are
    // multiple duplicate (but guaranteed to be equal) entries for the
    // incoming edges.  This occurs when there are multiple edges from
    // OnlyPred to OnlySucc.
    //
    while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
      PN->replaceAllUsesWith(PN->getIncomingValue(0));
      BB->getInstList().pop_front();  // Delete the phi node...
    }

    // Delete the unconditional branch from the predecessor...
    OnlyPred->getInstList().pop_back();
      
    // Move all definitions in the successor to the predecessor...
    OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
                                     
    // Make all PHI nodes that referred to BB now refer to Pred as their
    // source...
    BB->replaceAllUsesWith(OnlyPred);

    std::string OldName = BB->getName();

    // Erase basic block from the function... 
    M->getBasicBlockList().erase(BB);

    // Inherit predecessors name if it exists...
    if (!OldName.empty() && !OnlyPred->hasName())
      OnlyPred->setName(OldName);
      
    return true;
  }

  for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
    if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
      // Change br (X == 0 | X == 1), T, F into a switch instruction.
      if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
        Instruction *Cond = cast<Instruction>(BI->getCondition());
        // If this is a bunch of seteq's or'd together, or if it's a bunch of
        // 'setne's and'ed together, collect them.
        Value *CompVal = 0;
        std::vector<ConstantInt*> Values;
        bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
        if (CompVal && CompVal->getType()->isInteger()) {
          // There might be duplicate constants in the list, which the switch
          // instruction can't handle, remove them now.
          std::sort(Values.begin(), Values.end(), ConstantIntOrdering());
          Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
          
          // Figure out which block is which destination.
          BasicBlock *DefaultBB = BI->getSuccessor(1);
          BasicBlock *EdgeBB    = BI->getSuccessor(0);
          if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
          
          // Create the new switch instruction now.
          SwitchInst *New = new SwitchInst(CompVal, DefaultBB, BI);
          
          // Add all of the 'cases' to the switch instruction.
          for (unsigned i = 0, e = Values.size(); i != e; ++i)
            New->addCase(Values[i], EdgeBB);
          
          // We added edges from PI to the EdgeBB.  As such, if there were any
          // PHI nodes in EdgeBB, they need entries to be added corresponding to
          // the number of edges added.
          for (BasicBlock::iterator BBI = EdgeBB->begin();
               PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) {
            Value *InVal = PN->getIncomingValueForBlock(*PI);
            for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
              PN->addIncoming(InVal, *PI);
          }

          // Erase the old branch instruction.
          (*PI)->getInstList().erase(BI);

          // Erase the potentially condition tree that was used to computed the
          // branch condition.
          ErasePossiblyDeadInstructionTree(Cond);
          return true;
        }
      }

  // If there is a trivial two-entry PHI node in this basic block, and we can
  // eliminate it, do so now.
  if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
    if (PN->getNumIncomingValues() == 2) {
      // Ok, this is a two entry PHI node.  Check to see if this is a simple "if
      // statement", which has a very simple dominance structure.  Basically, we
      // are trying to find the condition that is being branched on, which
      // subsequently causes this merge to happen.  We really want control
      // dependence information for this check, but simplifycfg can't keep it up
      // to date, and this catches most of the cases we care about anyway.
      //
      BasicBlock *IfTrue, *IfFalse;
      if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) {
        //std::cerr << "FOUND IF CONDITION!  " << *IfCond << "  T: "
        //       << IfTrue->getName() << "  F: " << IfFalse->getName() << "\n";

        // Figure out where to insert instructions as necessary.
        BasicBlock::iterator AfterPHIIt = BB->begin();
        while (isa<PHINode>(AfterPHIIt)) ++AfterPHIIt;

        BasicBlock::iterator I = BB->begin();
        while (PHINode *PN = dyn_cast<PHINode>(I)) {
          ++I;

          // If we can eliminate this PHI by directly computing it based on the
          // condition, do so now.  We can't eliminate PHI nodes where the
          // incoming values are defined in the conditional parts of the branch,
          // so check for this.
          //
          if (DominatesMergePoint(PN->getIncomingValue(0), BB, true) &&
              DominatesMergePoint(PN->getIncomingValue(1), BB, true)) {
            Value *TrueVal =
              PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
            Value *FalseVal =
              PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);

            // If one of the incoming values is defined in the conditional
            // region, move it into it's predecessor block, which we know is
            // safe.
            if (!DominatesMergePoint(TrueVal, BB, false)) {
              Instruction *TrueI = cast<Instruction>(TrueVal);
              BasicBlock *OldBB = TrueI->getParent();
              OldBB->getInstList().remove(TrueI);
              BasicBlock *NewBB = *pred_begin(OldBB);
              NewBB->getInstList().insert(NewBB->getTerminator(), TrueI);
            }
            if (!DominatesMergePoint(FalseVal, BB, false)) {
              Instruction *FalseI = cast<Instruction>(FalseVal);
              BasicBlock *OldBB = FalseI->getParent();
              OldBB->getInstList().remove(FalseI);
              BasicBlock *NewBB = *pred_begin(OldBB);
              NewBB->getInstList().insert(NewBB->getTerminator(), FalseI);
            }

            // Change the PHI node into a select instruction.
            BasicBlock::iterator InsertPos = PN;
            while (isa<PHINode>(InsertPos)) ++InsertPos;

            std::string Name = PN->getName(); PN->setName("");
            PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal,
                                                  Name, InsertPos));
            BB->getInstList().erase(PN);
            Changed = true;
          }
        }
      }
    }
  
  return Changed;
}