llvm-project/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

//===- InstCombineCasts.cpp -----------------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the visit functions for cast operations.
//
//===----------------------------------------------------------------------===//

#include "InstCombine.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include "llvm/Support/PatternMatch.h"
using namespace llvm;
using namespace PatternMatch;

/// DecomposeSimpleLinearExpr - Analyze 'Val', seeing if it is a simple linear
/// expression.  If so, decompose it, returning some value X, such that Val is
/// X*Scale+Offset.
///
static Value *DecomposeSimpleLinearExpr(Value *Val, unsigned &Scale,
                                        uint64_t &Offset) {
  if (ConstantInt *CI = dyn_cast<ConstantInt>(Val)) {
    Offset = CI->getZExtValue();
    Scale  = 0;
    return ConstantInt::get(Val->getType(), 0);
  }
  
  if (BinaryOperator *I = dyn_cast<BinaryOperator>(Val)) {
    // Cannot look past anything that might overflow.
    OverflowingBinaryOperator *OBI = dyn_cast<OverflowingBinaryOperator>(Val);
    if (OBI && !OBI->hasNoUnsignedWrap() && !OBI->hasNoSignedWrap()) {
      Scale = 1;
      Offset = 0;
      return Val;
    }

    if (ConstantInt *RHS = dyn_cast<ConstantInt>(I->getOperand(1))) {
      if (I->getOpcode() == Instruction::Shl) {
        // This is a value scaled by '1 << the shift amt'.
        Scale = UINT64_C(1) << RHS->getZExtValue();
        Offset = 0;
        return I->getOperand(0);
      }
      
      if (I->getOpcode() == Instruction::Mul) {
        // This value is scaled by 'RHS'.
        Scale = RHS->getZExtValue();
        Offset = 0;
        return I->getOperand(0);
      }
      
      if (I->getOpcode() == Instruction::Add) {
        // We have X+C.  Check to see if we really have (X*C2)+C1, 
        // where C1 is divisible by C2.
        unsigned SubScale;
        Value *SubVal = 
          DecomposeSimpleLinearExpr(I->getOperand(0), SubScale, Offset);
        Offset += RHS->getZExtValue();
        Scale = SubScale;
        return SubVal;
      }
    }
  }

  // Otherwise, we can't look past this.
  Scale = 1;
  Offset = 0;
  return Val;
}

/// PromoteCastOfAllocation - If we find a cast of an allocation instruction,
/// try to eliminate the cast by moving the type information into the alloc.
Instruction *InstCombiner::PromoteCastOfAllocation(BitCastInst &CI,
                                                   AllocaInst &AI) {
  // This requires TargetData to get the alloca alignment and size information.
  if (!TD) return 0;

  PointerType *PTy = cast<PointerType>(CI.getType());
  
  BuilderTy AllocaBuilder(*Builder);
  AllocaBuilder.SetInsertPoint(AI.getParent(), &AI);

  // Get the type really allocated and the type casted to.
  Type *AllocElTy = AI.getAllocatedType();
  Type *CastElTy = PTy->getElementType();
  if (!AllocElTy->isSized() || !CastElTy->isSized()) return 0;

  unsigned AllocElTyAlign = TD->getABITypeAlignment(AllocElTy);
  unsigned CastElTyAlign = TD->getABITypeAlignment(CastElTy);
  if (CastElTyAlign < AllocElTyAlign) return 0;

  // If the allocation has multiple uses, only promote it if we are strictly
  // increasing the alignment of the resultant allocation.  If we keep it the
  // same, we open the door to infinite loops of various kinds.
  if (!AI.hasOneUse() && CastElTyAlign == AllocElTyAlign) return 0;

  uint64_t AllocElTySize = TD->getTypeAllocSize(AllocElTy);
  uint64_t CastElTySize = TD->getTypeAllocSize(CastElTy);
  if (CastElTySize == 0 || AllocElTySize == 0) return 0;

  // See if we can satisfy the modulus by pulling a scale out of the array
  // size argument.
  unsigned ArraySizeScale;
  uint64_t ArrayOffset;
  Value *NumElements = // See if the array size is a decomposable linear expr.
    DecomposeSimpleLinearExpr(AI.getOperand(0), ArraySizeScale, ArrayOffset);
 
  // If we can now satisfy the modulus, by using a non-1 scale, we really can
  // do the xform.
  if ((AllocElTySize*ArraySizeScale) % CastElTySize != 0 ||
      (AllocElTySize*ArrayOffset   ) % CastElTySize != 0) return 0;

  unsigned Scale = (AllocElTySize*ArraySizeScale)/CastElTySize;
  Value *Amt = 0;
  if (Scale == 1) {
    Amt = NumElements;
  } else {
    Amt = ConstantInt::get(AI.getArraySize()->getType(), Scale);
    // Insert before the alloca, not before the cast.
    Amt = AllocaBuilder.CreateMul(Amt, NumElements);
  }
  
  if (uint64_t Offset = (AllocElTySize*ArrayOffset)/CastElTySize) {
    Value *Off = ConstantInt::get(AI.getArraySize()->getType(),
                                  Offset, true);
    Amt = AllocaBuilder.CreateAdd(Amt, Off);
  }
  
  AllocaInst *New = AllocaBuilder.CreateAlloca(CastElTy, Amt);
  New->setAlignment(AI.getAlignment());
  New->takeName(&AI);
  
  // If the allocation has multiple real uses, insert a cast and change all
  // things that used it to use the new cast.  This will also hack on CI, but it
  // will die soon.
  if (!AI.hasOneUse()) {
    // New is the allocation instruction, pointer typed. AI is the original
    // allocation instruction, also pointer typed. Thus, cast to use is BitCast.
    Value *NewCast = AllocaBuilder.CreateBitCast(New, AI.getType(), "tmpcast");
    ReplaceInstUsesWith(AI, NewCast);
  }
  return ReplaceInstUsesWith(CI, New);
}

/// EvaluateInDifferentType - Given an expression that 
/// CanEvaluateTruncated or CanEvaluateSExtd returns true for, actually
/// insert the code to evaluate the expression.
Value *InstCombiner::EvaluateInDifferentType(Value *V, Type *Ty, 
                                             bool isSigned) {
  if (Constant *C = dyn_cast<Constant>(V)) {
    C = ConstantExpr::getIntegerCast(C, Ty, isSigned /*Sext or ZExt*/);
    // If we got a constantexpr back, try to simplify it with TD info.
    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C))
      C = ConstantFoldConstantExpression(CE, TD, TLI);
    return C;
  }

  // Otherwise, it must be an instruction.
  Instruction *I = cast<Instruction>(V);
  Instruction *Res = 0;
  unsigned Opc = I->getOpcode();
  switch (Opc) {
  case Instruction::Add:
  case Instruction::Sub:
  case Instruction::Mul:
  case Instruction::And:
  case Instruction::Or:
  case Instruction::Xor:
  case Instruction::AShr:
  case Instruction::LShr:
  case Instruction::Shl:
  case Instruction::UDiv:
  case Instruction::URem: {
    Value *LHS = EvaluateInDifferentType(I->getOperand(0), Ty, isSigned);
    Value *RHS = EvaluateInDifferentType(I->getOperand(1), Ty, isSigned);
    Res = BinaryOperator::Create((Instruction::BinaryOps)Opc, LHS, RHS);
    break;
  }    
  case Instruction::Trunc:
  case Instruction::ZExt:
  case Instruction::SExt:
    // If the source type of the cast is the type we're trying for then we can
    // just return the source.  There's no need to insert it because it is not
    // new.
    if (I->getOperand(0)->getType() == Ty)
      return I->getOperand(0);
    
    // Otherwise, must be the same type of cast, so just reinsert a new one.
    // This also handles the case of zext(trunc(x)) -> zext(x).
    Res = CastInst::CreateIntegerCast(I->getOperand(0), Ty,
                                      Opc == Instruction::SExt);
    break;
  case Instruction::Select: {
    Value *True = EvaluateInDifferentType(I->getOperand(1), Ty, isSigned);
    Value *False = EvaluateInDifferentType(I->getOperand(2), Ty, isSigned);
    Res = SelectInst::Create(I->getOperand(0), True, False);
    break;
  }
  case Instruction::PHI: {
    PHINode *OPN = cast<PHINode>(I);
    PHINode *NPN = PHINode::Create(Ty, OPN->getNumIncomingValues());
    for (unsigned i = 0, e = OPN->getNumIncomingValues(); i != e; ++i) {
      Value *V =EvaluateInDifferentType(OPN->getIncomingValue(i), Ty, isSigned);
      NPN->addIncoming(V, OPN->getIncomingBlock(i));
    }
    Res = NPN;
    break;
  }
  default: 
    // TODO: Can handle more cases here.
    llvm_unreachable("Unreachable!");
  }
  
  Res->takeName(I);
  return InsertNewInstWith(Res, *I);
}


/// This function is a wrapper around CastInst::isEliminableCastPair. It
/// simply extracts arguments and returns what that function returns.
static Instruction::CastOps 
isEliminableCastPair(
  const CastInst *CI, ///< The first cast instruction
  unsigned opcode,       ///< The opcode of the second cast instruction
  Type *DstTy,     ///< The target type for the second cast instruction
  TargetData *TD         ///< The target data for pointer size
) {

  Type *SrcTy = CI->getOperand(0)->getType();   // A from above
  Type *MidTy = CI->getType();                  // B from above

  // Get the opcodes of the two Cast instructions
  Instruction::CastOps firstOp = Instruction::CastOps(CI->getOpcode());
  Instruction::CastOps secondOp = Instruction::CastOps(opcode);

  unsigned Res = CastInst::isEliminableCastPair(firstOp, secondOp, SrcTy, MidTy,
                                                DstTy,
                                  TD ? TD->getIntPtrType(CI->getContext()) : 0);
  
  // We don't want to form an inttoptr or ptrtoint that converts to an integer
  // type that differs from the pointer size.
  if ((Res == Instruction::IntToPtr &&
          (!TD || SrcTy != TD->getIntPtrType(CI->getContext()))) ||
      (Res == Instruction::PtrToInt &&
          (!TD || DstTy != TD->getIntPtrType(CI->getContext()))))
    Res = 0;
  
  return Instruction::CastOps(Res);
}

/// ShouldOptimizeCast - Return true if the cast from "V to Ty" actually
/// results in any code being generated and is interesting to optimize out. If
/// the cast can be eliminated by some other simple transformation, we prefer
/// to do the simplification first.
bool InstCombiner::ShouldOptimizeCast(Instruction::CastOps opc, const Value *V,
                                      Type *Ty) {
  // Noop casts and casts of constants should be eliminated trivially.
  if (V->getType() == Ty || isa<Constant>(V)) return false;
  
  // If this is another cast that can be eliminated, we prefer to have it
  // eliminated.
  if (const CastInst *CI = dyn_cast<CastInst>(V))
    if (isEliminableCastPair(CI, opc, Ty, TD))
      return false;
  
  // If this is a vector sext from a compare, then we don't want to break the
  // idiom where each element of the extended vector is either zero or all ones.
  if (opc == Instruction::SExt && isa<CmpInst>(V) && Ty->isVectorTy())
    return false;
  
  return true;
}


/// @brief Implement the transforms common to all CastInst visitors.
Instruction *InstCombiner::commonCastTransforms(CastInst &CI) {
  Value *Src = CI.getOperand(0);

  // Many cases of "cast of a cast" are eliminable. If it's eliminable we just
  // eliminate it now.
  if (CastInst *CSrc = dyn_cast<CastInst>(Src)) {   // A->B->C cast
    if (Instruction::CastOps opc = 
        isEliminableCastPair(CSrc, CI.getOpcode(), CI.getType(), TD)) {
      // The first cast (CSrc) is eliminable so we need to fix up or replace
      // the second cast (CI). CSrc will then have a good chance of being dead.
      return CastInst::Create(opc, CSrc->getOperand(0), CI.getType());
    }
  }

  // If we are casting a select then fold the cast into the select
  if (SelectInst *SI = dyn_cast<SelectInst>(Src))
    if (Instruction *NV = FoldOpIntoSelect(CI, SI))
      return NV;

  // If we are casting a PHI then fold the cast into the PHI
  if (isa<PHINode>(Src)) {
    // We don't do this if this would create a PHI node with an illegal type if
    // it is currently legal.
    if (!Src->getType()->isIntegerTy() ||
        !CI.getType()->isIntegerTy() ||
        ShouldChangeType(CI.getType(), Src->getType()))
      if (Instruction *NV = FoldOpIntoPhi(CI))
        return NV;
  }
  
  return 0;
}

/// CanEvaluateTruncated - Return true if we can evaluate the specified
/// expression tree as type Ty instead of its larger type, and arrive with the
/// same value.  This is used by code that tries to eliminate truncates.
///
/// Ty will always be a type smaller than V.  We should return true if trunc(V)
/// can be computed by computing V in the smaller type.  If V is an instruction,
/// then trunc(inst(x,y)) can be computed as inst(trunc(x),trunc(y)), which only
/// makes sense if x and y can be efficiently truncated.
///
/// This function works on both vectors and scalars.
///
static bool CanEvaluateTruncated(Value *V, Type *Ty) {
  // We can always evaluate constants in another type.
  if (isa<Constant>(V))
    return true;
  
  Instruction *I = dyn_cast<Instruction>(V);
  if (!I) return false;
  
  Type *OrigTy = V->getType();
  
  // If this is an extension from the dest type, we can eliminate it, even if it
  // has multiple uses.
  if ((isa<ZExtInst>(I) || isa<SExtInst>(I)) && 
      I->getOperand(0)->getType() == Ty)
    return true;

  // We can't extend or shrink something that has multiple uses: doing so would
  // require duplicating the instruction in general, which isn't profitable.
  if (!I->hasOneUse()) return false;

  unsigned Opc = I->getOpcode();
  switch (Opc) {
  case Instruction::Add:
  case Instruction::Sub:
  case Instruction::Mul:
  case Instruction::And:
  case Instruction::Or:
  case Instruction::Xor:
    // These operators can all arbitrarily be extended or truncated.
    return CanEvaluateTruncated(I->getOperand(0), Ty) &&
           CanEvaluateTruncated(I->getOperand(1), Ty);

  case Instruction::UDiv:
  case Instruction::URem: {
    // UDiv and URem can be truncated if all the truncated bits are zero.
    uint32_t OrigBitWidth = OrigTy->getScalarSizeInBits();
    uint32_t BitWidth = Ty->getScalarSizeInBits();
    if (BitWidth < OrigBitWidth) {
      APInt Mask = APInt::getHighBitsSet(OrigBitWidth, OrigBitWidth-BitWidth);
      if (MaskedValueIsZero(I->getOperand(0), Mask) &&
          MaskedValueIsZero(I->getOperand(1), Mask)) {
        return CanEvaluateTruncated(I->getOperand(0), Ty) &&
               CanEvaluateTruncated(I->getOperand(1), Ty);
      }
    }
    break;
  }
  case Instruction::Shl:
    // If we are truncating the result of this SHL, and if it's a shift of a
    // constant amount, we can always perform a SHL in a smaller type.
    if (ConstantInt *CI = dyn_cast<ConstantInt>(I->getOperand(1))) {
      uint32_t BitWidth = Ty->getScalarSizeInBits();
      if (CI->getLimitedValue(BitWidth) < BitWidth)
        return CanEvaluateTruncated(I->getOperand(0), Ty);
    }
    break;
  case Instruction::LShr:
    // If this is a truncate of a logical shr, we can truncate it to a smaller
    // lshr iff we know that the bits we would otherwise be shifting in are
    // already zeros.
    if (ConstantInt *CI = dyn_cast<ConstantInt>(I->getOperand(1))) {
      uint32_t OrigBitWidth = OrigTy->getScalarSizeInBits();
      uint32_t BitWidth = Ty->getScalarSizeInBits();
      if (MaskedValueIsZero(I->getOperand(0),
            APInt::getHighBitsSet(OrigBitWidth, OrigBitWidth-BitWidth)) &&
          CI->getLimitedValue(BitWidth) < BitWidth) {
        return CanEvaluateTruncated(I->getOperand(0), Ty);
      }
    }
    break;
  case Instruction::Trunc:
    // trunc(trunc(x)) -> trunc(x)
    return true;
  case Instruction::ZExt:
  case Instruction::SExt:
    // trunc(ext(x)) -> ext(x) if the source type is smaller than the new dest
    // trunc(ext(x)) -> trunc(x) if the source type is larger than the new dest
    return true;
  case Instruction::Select: {
    SelectInst *SI = cast<SelectInst>(I);
    return CanEvaluateTruncated(SI->getTrueValue(), Ty) &&
           CanEvaluateTruncated(SI->getFalseValue(), Ty);
  }
  case Instruction::PHI: {
    // We can change a phi if we can change all operands.  Note that we never
    // get into trouble with cyclic PHIs here because we only consider
    // instructions with a single use.
    PHINode *PN = cast<PHINode>(I);
    for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
      if (!CanEvaluateTruncated(PN->getIncomingValue(i), Ty))
        return false;
    return true;
  }
  default:
    // TODO: Can handle more cases here.
    break;
  }
  
  return false;
}

Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
  if (Instruction *Result = commonCastTransforms(CI))
    return Result;
  
  // See if we can simplify any instructions used by the input whose sole 
  // purpose is to compute bits we don't care about.
  if (SimplifyDemandedInstructionBits(CI))
    return &CI;
  
  Value *Src = CI.getOperand(0);
  Type *DestTy = CI.getType(), *SrcTy = Src->getType();
  
  // Attempt to truncate the entire input expression tree to the destination
  // type.   Only do this if the dest type is a simple type, don't convert the
  // expression tree to something weird like i93 unless the source is also
  // strange.
  if ((DestTy->isVectorTy() || ShouldChangeType(SrcTy, DestTy)) &&
      CanEvaluateTruncated(Src, DestTy)) {
      
    // If this cast is a truncate, evaluting in a different type always
    // eliminates the cast, so it is always a win.
    DEBUG(dbgs() << "ICE: EvaluateInDifferentType converting expression type"
          " to avoid cast: " << CI << '\n');
    Value *Res = EvaluateInDifferentType(Src, DestTy, false);
    assert(Res->getType() == DestTy);
    return ReplaceInstUsesWith(CI, Res);
  }

  // Canonicalize trunc x to i1 -> (icmp ne (and x, 1), 0), likewise for vector.
  if (DestTy->getScalarSizeInBits() == 1) {
    Constant *One = ConstantInt::get(Src->getType(), 1);
    Src = Builder->CreateAnd(Src, One);
    Value *Zero = Constant::getNullValue(Src->getType());
    return new ICmpInst(ICmpInst::ICMP_NE, Src, Zero);
  }
  
  // Transform trunc(lshr (zext A), Cst) to eliminate one type conversion.
  Value *A = 0; ConstantInt *Cst = 0;
  if (Src->hasOneUse() &&
      match(Src, m_LShr(m_ZExt(m_Value(A)), m_ConstantInt(Cst)))) {
    // We have three types to worry about here, the type of A, the source of
    // the truncate (MidSize), and the destination of the truncate. We know that
    // ASize < MidSize   and MidSize > ResultSize, but don't know the relation
    // between ASize and ResultSize.
    unsigned ASize = A->getType()->getPrimitiveSizeInBits();
    
    // If the shift amount is larger than the size of A, then the result is
    // known to be zero because all the input bits got shifted out.
    if (Cst->getZExtValue() >= ASize)
      return ReplaceInstUsesWith(CI, Constant::getNullValue(CI.getType()));

    // Since we're doing an lshr and a zero extend, and know that the shift
    // amount is smaller than ASize, it is always safe to do the shift in A's
    // type, then zero extend or truncate to the result.
    Value *Shift = Builder->CreateLShr(A, Cst->getZExtValue());
    Shift->takeName(Src);
    return CastInst::CreateIntegerCast(Shift, CI.getType(), false);
  }
  
  // Transform "trunc (and X, cst)" -> "and (trunc X), cst" so long as the dest
  // type isn't non-native.
  if (Src->hasOneUse() && isa<IntegerType>(Src->getType()) &&
      ShouldChangeType(Src->getType(), CI.getType()) &&
      match(Src, m_And(m_Value(A), m_ConstantInt(Cst)))) {
    Value *NewTrunc = Builder->CreateTrunc(A, CI.getType(), A->getName()+".tr");
    return BinaryOperator::CreateAnd(NewTrunc,
                                     ConstantExpr::getTrunc(Cst, CI.getType()));
  }

  return 0;
}

/// transformZExtICmp - Transform (zext icmp) to bitwise / integer operations
/// in order to eliminate the icmp.
Instruction *InstCombiner::transformZExtICmp(ICmpInst *ICI, Instruction &CI,
                                             bool DoXform) {
  // If we are just checking for a icmp eq of a single bit and zext'ing it
  // to an integer, then shift the bit to the appropriate place and then
  // cast to integer to avoid the comparison.
  if (ConstantInt *Op1C = dyn_cast<ConstantInt>(ICI->getOperand(1))) {
    const APInt &Op1CV = Op1C->getValue();
      
    // zext (x <s  0) to i32 --> x>>u31      true if signbit set.
    // zext (x >s -1) to i32 --> (x>>u31)^1  true if signbit clear.
    if ((ICI->getPredicate() == ICmpInst::ICMP_SLT && Op1CV == 0) ||
        (ICI->getPredicate() == ICmpInst::ICMP_SGT &&Op1CV.isAllOnesValue())) {
      if (!DoXform) return ICI;

      Value *In = ICI->getOperand(0);
      Value *Sh = ConstantInt::get(In->getType(),
                                   In->getType()->getScalarSizeInBits()-1);
      In = Builder->CreateLShr(In, Sh, In->getName()+".lobit");
      if (In->getType() != CI.getType())
        In = Builder->CreateIntCast(In, CI.getType(), false/*ZExt*/);

      if (ICI->getPredicate() == ICmpInst::ICMP_SGT) {
        Constant *One = ConstantInt::get(In->getType(), 1);
        In = Builder->CreateXor(In, One, In->getName()+".not");
      }

      return ReplaceInstUsesWith(CI, In);
    }

    // zext (X == 0) to i32 --> X^1      iff X has only the low bit set.
    // zext (X == 0) to i32 --> (X>>1)^1 iff X has only the 2nd bit set.
    // zext (X == 1) to i32 --> X        iff X has only the low bit set.
    // zext (X == 2) to i32 --> X>>1     iff X has only the 2nd bit set.
    // zext (X != 0) to i32 --> X        iff X has only the low bit set.
    // zext (X != 0) to i32 --> X>>1     iff X has only the 2nd bit set.
    // zext (X != 1) to i32 --> X^1      iff X has only the low bit set.
    // zext (X != 2) to i32 --> (X>>1)^1 iff X has only the 2nd bit set.
    if ((Op1CV == 0 || Op1CV.isPowerOf2()) && 
        // This only works for EQ and NE
        ICI->isEquality()) {
      // If Op1C some other power of two, convert:
      uint32_t BitWidth = Op1C->getType()->getBitWidth();
      APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
      ComputeMaskedBits(ICI->getOperand(0), KnownZero, KnownOne);
        
      APInt KnownZeroMask(~KnownZero);
      if (KnownZeroMask.isPowerOf2()) { // Exactly 1 possible 1?
        if (!DoXform) return ICI;

        bool isNE = ICI->getPredicate() == ICmpInst::ICMP_NE;
        if (Op1CV != 0 && (Op1CV != KnownZeroMask)) {
          // (X&4) == 2 --> false
          // (X&4) != 2 --> true
          Constant *Res = ConstantInt::get(Type::getInt1Ty(CI.getContext()),
                                           isNE);
          Res = ConstantExpr::getZExt(Res, CI.getType());
          return ReplaceInstUsesWith(CI, Res);
        }
          
        uint32_t ShiftAmt = KnownZeroMask.logBase2();
        Value *In = ICI->getOperand(0);
        if (ShiftAmt) {
          // Perform a logical shr by shiftamt.
          // Insert the shift to put the result in the low bit.
          In = Builder->CreateLShr(In, ConstantInt::get(In->getType(),ShiftAmt),
                                   In->getName()+".lobit");
        }
          
        if ((Op1CV != 0) == isNE) { // Toggle the low bit.
          Constant *One = ConstantInt::get(In->getType(), 1);
          In = Builder->CreateXor(In, One);
        }
          
        if (CI.getType() == In->getType())
          return ReplaceInstUsesWith(CI, In);
        return CastInst::CreateIntegerCast(In, CI.getType(), false/*ZExt*/);
      }
    }
  }

  // icmp ne A, B is equal to xor A, B when A and B only really have one bit.
  // It is also profitable to transform icmp eq into not(xor(A, B)) because that
  // may lead to additional simplifications.
  if (ICI->isEquality() && CI.getType() == ICI->getOperand(0)->getType()) {
    if (IntegerType *ITy = dyn_cast<IntegerType>(CI.getType())) {
      uint32_t BitWidth = ITy->getBitWidth();
      Value *LHS = ICI->getOperand(0);
      Value *RHS = ICI->getOperand(1);

      APInt KnownZeroLHS(BitWidth, 0), KnownOneLHS(BitWidth, 0);
      APInt KnownZeroRHS(BitWidth, 0), KnownOneRHS(BitWidth, 0);
      ComputeMaskedBits(LHS, KnownZeroLHS, KnownOneLHS);
      ComputeMaskedBits(RHS, KnownZeroRHS, KnownOneRHS);

      if (KnownZeroLHS == KnownZeroRHS && KnownOneLHS == KnownOneRHS) {
        APInt KnownBits = KnownZeroLHS | KnownOneLHS;
        APInt UnknownBit = ~KnownBits;
        if (UnknownBit.countPopulation() == 1) {
          if (!DoXform) return ICI;

          Value *Result = Builder->CreateXor(LHS, RHS);

          // Mask off any bits that are set and won't be shifted away.
          if (KnownOneLHS.uge(UnknownBit))
            Result = Builder->CreateAnd(Result,
                                        ConstantInt::get(ITy, UnknownBit));

          // Shift the bit we're testing down to the lsb.
          Result = Builder->CreateLShr(
               Result, ConstantInt::get(ITy, UnknownBit.countTrailingZeros()));

          if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
            Result = Builder->CreateXor(Result, ConstantInt::get(ITy, 1));
          Result->takeName(ICI);
          return ReplaceInstUsesWith(CI, Result);
        }
      }
    }
  }

  return 0;
}

/// CanEvaluateZExtd - Determine if the specified value can be computed in the
/// specified wider type and produce the same low bits.  If not, return false.
///
/// If this function returns true, it can also return a non-zero number of bits
/// (in BitsToClear) which indicates that the value it computes is correct for
/// the zero extend, but that the additional BitsToClear bits need to be zero'd
/// out.  For example, to promote something like:
///
///   %B = trunc i64 %A to i32
///   %C = lshr i32 %B, 8
///   %E = zext i32 %C to i64
///
/// CanEvaluateZExtd for the 'lshr' will return true, and BitsToClear will be
/// set to 8 to indicate that the promoted value needs to have bits 24-31
/// cleared in addition to bits 32-63.  Since an 'and' will be generated to
/// clear the top bits anyway, doing this has no extra cost.
///
/// This function works on both vectors and scalars.
static bool CanEvaluateZExtd(Value *V, Type *Ty, unsigned &BitsToClear) {
  BitsToClear = 0;
  if (isa<Constant>(V))
    return true;
  
  Instruction *I = dyn_cast<Instruction>(V);
  if (!I) return false;
  
  // If the input is a truncate from the destination type, we can trivially
  // eliminate it, even if it has multiple uses.
  // FIXME: This is currently disabled until codegen can handle this without
  // pessimizing code, PR5997.
  if (0 && isa<TruncInst>(I) && I->getOperand(0)->getType() == Ty)
    return true;
  
  // We can't extend or shrink something that has multiple uses: doing so would
  // require duplicating the instruction in general, which isn't profitable.
  if (!I->hasOneUse()) return false;
  
  unsigned Opc = I->getOpcode(), Tmp;
  switch (Opc) {
  case Instruction::ZExt:  // zext(zext(x)) -> zext(x).
  case Instruction::SExt:  // zext(sext(x)) -> sext(x).
  case Instruction::Trunc: // zext(trunc(x)) -> trunc(x) or zext(x)
    return true;
  case Instruction::And:
  case Instruction::Or:
  case Instruction::Xor:
  case Instruction::Add:
  case Instruction::Sub:
  case Instruction::Mul:
  case Instruction::Shl:
    if (!CanEvaluateZExtd(I->getOperand(0), Ty, BitsToClear) ||
        !CanEvaluateZExtd(I->getOperand(1), Ty, Tmp))
      return false;
    // These can all be promoted if neither operand has 'bits to clear'.
    if (BitsToClear == 0 && Tmp == 0)
      return true;
      
    // If the operation is an AND/OR/XOR and the bits to clear are zero in the
    // other side, BitsToClear is ok.
    if (Tmp == 0 &&
        (Opc == Instruction::And || Opc == Instruction::Or ||
         Opc == Instruction::Xor)) {
      // We use MaskedValueIsZero here for generality, but the case we care
      // about the most is constant RHS.
      unsigned VSize = V->getType()->getScalarSizeInBits();
      if (MaskedValueIsZero(I->getOperand(1),
                            APInt::getHighBitsSet(VSize, BitsToClear)))
        return true;
    }
      
    // Otherwise, we don't know how to analyze this BitsToClear case yet.
    return false;
      
  case Instruction::LShr:
    // We can promote lshr(x, cst) if we can promote x.  This requires the
    // ultimate 'and' to clear out the high zero bits we're clearing out though.
    if (ConstantInt *Amt = dyn_cast<ConstantInt>(I->getOperand(1))) {
      if (!CanEvaluateZExtd(I->getOperand(0), Ty, BitsToClear))
        return false;
      BitsToClear += Amt->getZExtValue();
      if (BitsToClear > V->getType()->getScalarSizeInBits())
        BitsToClear = V->getType()->getScalarSizeInBits();
      return true;
    }
    // Cannot promote variable LSHR.
    return false;
  case Instruction::Select:
    if (!CanEvaluateZExtd(I->getOperand(1), Ty, Tmp) ||
        !CanEvaluateZExtd(I->getOperand(2), Ty, BitsToClear) ||
        // TODO: If important, we could handle the case when the BitsToClear are
        // known zero in the disagreeing side.
        Tmp != BitsToClear)
      return false;
    return true;
      
  case Instruction::PHI: {
    // We can change a phi if we can change all operands.  Note that we never
    // get into trouble with cyclic PHIs here because we only consider
    // instructions with a single use.
    PHINode *PN = cast<PHINode>(I);
    if (!CanEvaluateZExtd(PN->getIncomingValue(0), Ty, BitsToClear))
      return false;
    for (unsigned i = 1, e = PN->getNumIncomingValues(); i != e; ++i)
      if (!CanEvaluateZExtd(PN->getIncomingValue(i), Ty, Tmp) ||
          // TODO: If important, we could handle the case when the BitsToClear
          // are known zero in the disagreeing input.
          Tmp != BitsToClear)
        return false;
    return true;
  }
  default:
    // TODO: Can handle more cases here.
    return false;
  }
}

Instruction *InstCombiner::visitZExt(ZExtInst &CI) {
  // If this zero extend is only used by a truncate, let the truncate by
  // eliminated before we try to optimize this zext.
  if (CI.hasOneUse() && isa<TruncInst>(CI.use_back()))
    return 0;
  
  // If one of the common conversion will work, do it.
  if (Instruction *Result = commonCastTransforms(CI))
    return Result;

  // See if we can simplify any instructions used by the input whose sole 
  // purpose is to compute bits we don't care about.
  if (SimplifyDemandedInstructionBits(CI))
    return &CI;
  
  Value *Src = CI.getOperand(0);
  Type *SrcTy = Src->getType(), *DestTy = CI.getType();
  
  // Attempt to extend the entire input expression tree to the destination
  // type.   Only do this if the dest type is a simple type, don't convert the
  // expression tree to something weird like i93 unless the source is also
  // strange.
  unsigned BitsToClear;
  if ((DestTy->isVectorTy() || ShouldChangeType(SrcTy, DestTy)) &&
      CanEvaluateZExtd(Src, DestTy, BitsToClear)) { 
    assert(BitsToClear < SrcTy->getScalarSizeInBits() &&
           "Unreasonable BitsToClear");
    
    // Okay, we can transform this!  Insert the new expression now.
    DEBUG(dbgs() << "ICE: EvaluateInDifferentType converting expression type"
          " to avoid zero extend: " << CI);
    Value *Res = EvaluateInDifferentType(Src, DestTy, false);
    assert(Res->getType() == DestTy);
    
    uint32_t SrcBitsKept = SrcTy->getScalarSizeInBits()-BitsToClear;
    uint32_t DestBitSize = DestTy->getScalarSizeInBits();
    
    // If the high bits are already filled with zeros, just replace this
    // cast with the result.
    if (MaskedValueIsZero(Res, APInt::getHighBitsSet(DestBitSize,
                                                     DestBitSize-SrcBitsKept)))
      return ReplaceInstUsesWith(CI, Res);
    
    // We need to emit an AND to clear the high bits.
    Constant *C = ConstantInt::get(Res->getType(),
                               APInt::getLowBitsSet(DestBitSize, SrcBitsKept));
    return BinaryOperator::CreateAnd(Res, C);
  }

  // If this is a TRUNC followed by a ZEXT then we are dealing with integral
  // types and if the sizes are just right we can convert this into a logical
  // 'and' which will be much cheaper than the pair of casts.
  if (TruncInst *CSrc = dyn_cast<TruncInst>(Src)) {   // A->B->C cast
    // TODO: Subsume this into EvaluateInDifferentType.
    
    // Get the sizes of the types involved.  We know that the intermediate type
    // will be smaller than A or C, but don't know the relation between A and C.
    Value *A = CSrc->getOperand(0);
    unsigned SrcSize = A->getType()->getScalarSizeInBits();
    unsigned MidSize = CSrc->getType()->getScalarSizeInBits();
    unsigned DstSize = CI.getType()->getScalarSizeInBits();
    // If we're actually extending zero bits, then if
    // SrcSize <  DstSize: zext(a & mask)
    // SrcSize == DstSize: a & mask
    // SrcSize  > DstSize: trunc(a) & mask
    if (SrcSize < DstSize) {
      APInt AndValue(APInt::getLowBitsSet(SrcSize, MidSize));
      Constant *AndConst = ConstantInt::get(A->getType(), AndValue);
      Value *And = Builder->CreateAnd(A, AndConst, CSrc->getName()+".mask");
      return new ZExtInst(And, CI.getType());
    }
    
    if (SrcSize == DstSize) {
      APInt AndValue(APInt::getLowBitsSet(SrcSize, MidSize));
      return BinaryOperator::CreateAnd(A, ConstantInt::get(A->getType(),
                                                           AndValue));
    }
    if (SrcSize > DstSize) {
      Value *Trunc = Builder->CreateTrunc(A, CI.getType());
      APInt AndValue(APInt::getLowBitsSet(DstSize, MidSize));
      return BinaryOperator::CreateAnd(Trunc, 
                                       ConstantInt::get(Trunc->getType(),
                                                        AndValue));
    }
  }

  if (ICmpInst *ICI = dyn_cast<ICmpInst>(Src))
    return transformZExtICmp(ICI, CI);

  BinaryOperator *SrcI = dyn_cast<BinaryOperator>(Src);
  if (SrcI && SrcI->getOpcode() == Instruction::Or) {
    // zext (or icmp, icmp) --> or (zext icmp), (zext icmp) if at least one
    // of the (zext icmp) will be transformed.
    ICmpInst *LHS = dyn_cast<ICmpInst>(SrcI->getOperand(0));
    ICmpInst *RHS = dyn_cast<ICmpInst>(SrcI->getOperand(1));
    if (LHS && RHS && LHS->hasOneUse() && RHS->hasOneUse() &&
        (transformZExtICmp(LHS, CI, false) ||
         transformZExtICmp(RHS, CI, false))) {
      Value *LCast = Builder->CreateZExt(LHS, CI.getType(), LHS->getName());
      Value *RCast = Builder->CreateZExt(RHS, CI.getType(), RHS->getName());
      return BinaryOperator::Create(Instruction::Or, LCast, RCast);
    }
  }

  // zext(trunc(t) & C) -> (t & zext(C)).
  if (SrcI && SrcI->getOpcode() == Instruction::And && SrcI->hasOneUse())
    if (ConstantInt *C = dyn_cast<ConstantInt>(SrcI->getOperand(1)))
      if (TruncInst *TI = dyn_cast<TruncInst>(SrcI->getOperand(0))) {
        Value *TI0 = TI->getOperand(0);
        if (TI0->getType() == CI.getType())
          return
            BinaryOperator::CreateAnd(TI0,
                                ConstantExpr::getZExt(C, CI.getType()));
      }

  // zext((trunc(t) & C) ^ C) -> ((t & zext(C)) ^ zext(C)).
  if (SrcI && SrcI->getOpcode() == Instruction::Xor && SrcI->hasOneUse())
    if (ConstantInt *C = dyn_cast<ConstantInt>(SrcI->getOperand(1)))
      if (BinaryOperator *And = dyn_cast<BinaryOperator>(SrcI->getOperand(0)))
        if (And->getOpcode() == Instruction::And && And->hasOneUse() &&
            And->getOperand(1) == C)
          if (TruncInst *TI = dyn_cast<TruncInst>(And->getOperand(0))) {
            Value *TI0 = TI->getOperand(0);
            if (TI0->getType() == CI.getType()) {
              Constant *ZC = ConstantExpr::getZExt(C, CI.getType());
              Value *NewAnd = Builder->CreateAnd(TI0, ZC);
              return BinaryOperator::CreateXor(NewAnd, ZC);
            }
          }

  // zext (xor i1 X, true) to i32  --> xor (zext i1 X to i32), 1
  Value *X;
  if (SrcI && SrcI->hasOneUse() && SrcI->getType()->isIntegerTy(1) &&
      match(SrcI, m_Not(m_Value(X))) &&
      (!X->hasOneUse() || !isa<CmpInst>(X))) {
    Value *New = Builder->CreateZExt(X, CI.getType());
    return BinaryOperator::CreateXor(New, ConstantInt::get(CI.getType(), 1));
  }
  
  return 0;
}

/// transformSExtICmp - Transform (sext icmp) to bitwise / integer operations
/// in order to eliminate the icmp.
Instruction *InstCombiner::transformSExtICmp(ICmpInst *ICI, Instruction &CI) {
  Value *Op0 = ICI->getOperand(0), *Op1 = ICI->getOperand(1);
  ICmpInst::Predicate Pred = ICI->getPredicate();

  if (ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
    // (x <s  0) ? -1 : 0 -> ashr x, 31        -> all ones if negative
    // (x >s -1) ? -1 : 0 -> not (ashr x, 31)  -> all ones if positive
    if ((Pred == ICmpInst::ICMP_SLT && Op1C->isZero()) ||
        (Pred == ICmpInst::ICMP_SGT && Op1C->isAllOnesValue())) {

      Value *Sh = ConstantInt::get(Op0->getType(),
                                   Op0->getType()->getScalarSizeInBits()-1);
      Value *In = Builder->CreateAShr(Op0, Sh, Op0->getName()+".lobit");
      if (In->getType() != CI.getType())
        In = Builder->CreateIntCast(In, CI.getType(), true/*SExt*/);

      if (Pred == ICmpInst::ICMP_SGT)
        In = Builder->CreateNot(In, In->getName()+".not");
      return ReplaceInstUsesWith(CI, In);
    }

    // If we know that only one bit of the LHS of the icmp can be set and we
    // have an equality comparison with zero or a power of 2, we can transform
    // the icmp and sext into bitwise/integer operations.
    if (ICI->hasOneUse() &&
        ICI->isEquality() && (Op1C->isZero() || Op1C->getValue().isPowerOf2())){
      unsigned BitWidth = Op1C->getType()->getBitWidth();
      APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
      ComputeMaskedBits(Op0, KnownZero, KnownOne);

      APInt KnownZeroMask(~KnownZero);
      if (KnownZeroMask.isPowerOf2()) {
        Value *In = ICI->getOperand(0);

        // If the icmp tests for a known zero bit we can constant fold it.
        if (!Op1C->isZero() && Op1C->getValue() != KnownZeroMask) {
          Value *V = Pred == ICmpInst::ICMP_NE ?
                       ConstantInt::getAllOnesValue(CI.getType()) :
                       ConstantInt::getNullValue(CI.getType());
          return ReplaceInstUsesWith(CI, V);
        }

        if (!Op1C->isZero() == (Pred == ICmpInst::ICMP_NE)) {
          // sext ((x & 2^n) == 0)   -> (x >> n) - 1
          // sext ((x & 2^n) != 2^n) -> (x >> n) - 1
          unsigned ShiftAmt = KnownZeroMask.countTrailingZeros();
          // Perform a right shift to place the desired bit in the LSB.
          if (ShiftAmt)
            In = Builder->CreateLShr(In,
                                     ConstantInt::get(In->getType(), ShiftAmt));

          // At this point "In" is either 1 or 0. Subtract 1 to turn
          // {1, 0} -> {0, -1}.
          In = Builder->CreateAdd(In,
                                  ConstantInt::getAllOnesValue(In->getType()),
                                  "sext");
        } else {
          // sext ((x & 2^n) != 0)   -> (x << bitwidth-n) a>> bitwidth-1
          // sext ((x & 2^n) == 2^n) -> (x << bitwidth-n) a>> bitwidth-1
          unsigned ShiftAmt = KnownZeroMask.countLeadingZeros();
          // Perform a left shift to place the desired bit in the MSB.
          if (ShiftAmt)
            In = Builder->CreateShl(In,
                                    ConstantInt::get(In->getType(), ShiftAmt));

          // Distribute the bit over the whole bit width.
          In = Builder->CreateAShr(In, ConstantInt::get(In->getType(),
                                                        BitWidth - 1), "sext");
        }

        if (CI.getType() == In->getType())
          return ReplaceInstUsesWith(CI, In);
        return CastInst::CreateIntegerCast(In, CI.getType(), true/*SExt*/);
      }
    }
  }

  // vector (x <s 0) ? -1 : 0 -> ashr x, 31   -> all ones if signed.
  if (VectorType *VTy = dyn_cast<VectorType>(CI.getType())) {
    if (Pred == ICmpInst::ICMP_SLT && match(Op1, m_Zero()) &&
        Op0->getType() == CI.getType()) {
      Type *EltTy = VTy->getElementType();

      // splat the shift constant to a constant vector.
      Constant *VSh = ConstantInt::get(VTy, EltTy->getScalarSizeInBits()-1);
      Value *In = Builder->CreateAShr(Op0, VSh, Op0->getName()+".lobit");
      return ReplaceInstUsesWith(CI, In);
    }
  }

  return 0;
}

/// CanEvaluateSExtd - Return true if we can take the specified value
/// and return it as type Ty without inserting any new casts and without
/// changing the value of the common low bits.  This is used by code that tries
/// to promote integer operations to a wider types will allow us to eliminate
/// the extension.
///
/// This function works on both vectors and scalars.
///
static bool CanEvaluateSExtd(Value *V, Type *Ty) {
  assert(V->getType()->getScalarSizeInBits() < Ty->getScalarSizeInBits() &&
         "Can't sign extend type to a smaller type");
  // If this is a constant, it can be trivially promoted.
  if (isa<Constant>(V))
    return true;
  
  Instruction *I = dyn_cast<Instruction>(V);
  if (!I) return false;
  
  // If this is a truncate from the dest type, we can trivially eliminate it,
  // even if it has multiple uses.
  // FIXME: This is currently disabled until codegen can handle this without
  // pessimizing code, PR5997.
  if (0 && isa<TruncInst>(I) && I->getOperand(0)->getType() == Ty)
    return true;
  
  // We can't extend or shrink something that has multiple uses: doing so would
  // require duplicating the instruction in general, which isn't profitable.
  if (!I->hasOneUse()) return false;

  switch (I->getOpcode()) {
  case Instruction::SExt:  // sext(sext(x)) -> sext(x)
  case Instruction::ZExt:  // sext(zext(x)) -> zext(x)
  case Instruction::Trunc: // sext(trunc(x)) -> trunc(x) or sext(x)
    return true;
  case Instruction::And:
  case Instruction::Or:
  case Instruction::Xor:
  case Instruction::Add:
  case Instruction::Sub:
  case Instruction::Mul:
    // These operators can all arbitrarily be extended if their inputs can.
    return CanEvaluateSExtd(I->getOperand(0), Ty) &&
           CanEvaluateSExtd(I->getOperand(1), Ty);
      
  //case Instruction::Shl:   TODO
  //case Instruction::LShr:  TODO
      
  case Instruction::Select:
    return CanEvaluateSExtd(I->getOperand(1), Ty) &&
           CanEvaluateSExtd(I->getOperand(2), Ty);
      
  case Instruction::PHI: {
    // We can change a phi if we can change all operands.  Note that we never
    // get into trouble with cyclic PHIs here because we only consider
    // instructions with a single use.
    PHINode *PN = cast<PHINode>(I);
    for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
      if (!CanEvaluateSExtd(PN->getIncomingValue(i), Ty)) return false;
    return true;
  }
  default:
    // TODO: Can handle more cases here.
    break;
  }
  
  return false;
}

Instruction *InstCombiner::visitSExt(SExtInst &CI) {
  // If this sign extend is only used by a truncate, let the truncate by
  // eliminated before we try to optimize this zext.
  if (CI.hasOneUse() && isa<TruncInst>(CI.use_back()))
    return 0;
  
  if (Instruction *I = commonCastTransforms(CI))
    return I;
  
  // See if we can simplify any instructions used by the input whose sole 
  // purpose is to compute bits we don't care about.
  if (SimplifyDemandedInstructionBits(CI))
    return &CI;
  
  Value *Src = CI.getOperand(0);
  Type *SrcTy = Src->getType(), *DestTy = CI.getType();

  // Attempt to extend the entire input expression tree to the destination
  // type.   Only do this if the dest type is a simple type, don't convert the
  // expression tree to something weird like i93 unless the source is also
  // strange.
  if ((DestTy->isVectorTy() || ShouldChangeType(SrcTy, DestTy)) &&
      CanEvaluateSExtd(Src, DestTy)) {
    // Okay, we can transform this!  Insert the new expression now.
    DEBUG(dbgs() << "ICE: EvaluateInDifferentType converting expression type"
          " to avoid sign extend: " << CI);
    Value *Res = EvaluateInDifferentType(Src, DestTy, true);
    assert(Res->getType() == DestTy);

    uint32_t SrcBitSize = SrcTy->getScalarSizeInBits();
    uint32_t DestBitSize = DestTy->getScalarSizeInBits();

    // If the high bits are already filled with sign bit, just replace this
    // cast with the result.
    if (ComputeNumSignBits(Res) > DestBitSize - SrcBitSize)
      return ReplaceInstUsesWith(CI, Res);
    
    // We need to emit a shl + ashr to do the sign extend.
    Value *ShAmt = ConstantInt::get(DestTy, DestBitSize-SrcBitSize);
    return BinaryOperator::CreateAShr(Builder->CreateShl(Res, ShAmt, "sext"),
                                      ShAmt);
  }

  // If this input is a trunc from our destination, then turn sext(trunc(x))
  // into shifts.
  if (TruncInst *TI = dyn_cast<TruncInst>(Src))
    if (TI->hasOneUse() && TI->getOperand(0)->getType() == DestTy) {
      uint32_t SrcBitSize = SrcTy->getScalarSizeInBits();
      uint32_t DestBitSize = DestTy->getScalarSizeInBits();
      
      // We need to emit a shl + ashr to do the sign extend.
      Value *ShAmt = ConstantInt::get(DestTy, DestBitSize-SrcBitSize);
      Value *Res = Builder->CreateShl(TI->getOperand(0), ShAmt, "sext");
      return BinaryOperator::CreateAShr(Res, ShAmt);
    }

  if (ICmpInst *ICI = dyn_cast<ICmpInst>(Src))
    return transformSExtICmp(ICI, CI);

  // If the input is a shl/ashr pair of a same constant, then this is a sign
  // extension from a smaller value.  If we could trust arbitrary bitwidth
  // integers, we could turn this into a truncate to the smaller bit and then
  // use a sext for the whole extension.  Since we don't, look deeper and check
  // for a truncate.  If the source and dest are the same type, eliminate the
  // trunc and extend and just do shifts.  For example, turn:
  //   %a = trunc i32 %i to i8
  //   %b = shl i8 %a, 6
  //   %c = ashr i8 %b, 6
  //   %d = sext i8 %c to i32
  // into:
  //   %a = shl i32 %i, 30
  //   %d = ashr i32 %a, 30
  Value *A = 0;
  // TODO: Eventually this could be subsumed by EvaluateInDifferentType.
  ConstantInt *BA = 0, *CA = 0;
  if (match(Src, m_AShr(m_Shl(m_Trunc(m_Value(A)), m_ConstantInt(BA)),
                        m_ConstantInt(CA))) &&
      BA == CA && A->getType() == CI.getType()) {
    unsigned MidSize = Src->getType()->getScalarSizeInBits();
    unsigned SrcDstSize = CI.getType()->getScalarSizeInBits();
    unsigned ShAmt = CA->getZExtValue()+SrcDstSize-MidSize;
    Constant *ShAmtV = ConstantInt::get(CI.getType(), ShAmt);
    A = Builder->CreateShl(A, ShAmtV, CI.getName());
    return BinaryOperator::CreateAShr(A, ShAmtV);
  }
  
  return 0;
}


/// FitsInFPType - Return a Constant* for the specified FP constant if it fits
/// in the specified FP type without changing its value.
static Constant *FitsInFPType(ConstantFP *CFP, const fltSemantics &Sem) {
  bool losesInfo;
  APFloat F = CFP->getValueAPF();
  (void)F.convert(Sem, APFloat::rmNearestTiesToEven, &losesInfo);
  if (!losesInfo)
    return ConstantFP::get(CFP->getContext(), F);
  return 0;
}

/// LookThroughFPExtensions - If this is an fp extension instruction, look
/// through it until we get the source value.
static Value *LookThroughFPExtensions(Value *V) {
  if (Instruction *I = dyn_cast<Instruction>(V))
    if (I->getOpcode() == Instruction::FPExt)
      return LookThroughFPExtensions(I->getOperand(0));
  
  // If this value is a constant, return the constant in the smallest FP type
  // that can accurately represent it.  This allows us to turn
  // (float)((double)X+2.0) into x+2.0f.
  if (ConstantFP *CFP = dyn_cast<ConstantFP>(V)) {
    if (CFP->getType() == Type::getPPC_FP128Ty(V->getContext()))
      return V;  // No constant folding of this.
    // See if the value can be truncated to half and then reextended.
    if (Value *V = FitsInFPType(CFP, APFloat::IEEEhalf))
      return V;
    // See if the value can be truncated to float and then reextended.
    if (Value *V = FitsInFPType(CFP, APFloat::IEEEsingle))
      return V;
    if (CFP->getType()->isDoubleTy())
      return V;  // Won't shrink.
    if (Value *V = FitsInFPType(CFP, APFloat::IEEEdouble))
      return V;
    // Don't try to shrink to various long double types.
  }
  
  return V;
}

Instruction *InstCombiner::visitFPTrunc(FPTruncInst &CI) {
  if (Instruction *I = commonCastTransforms(CI))
    return I;
  
  // If we have fptrunc(fadd (fpextend x), (fpextend y)), where x and y are
  // smaller than the destination type, we can eliminate the truncate by doing
  // the add as the smaller type.  This applies to fadd/fsub/fmul/fdiv as well
  // as many builtins (sqrt, etc).
  BinaryOperator *OpI = dyn_cast<BinaryOperator>(CI.getOperand(0));
  if (OpI && OpI->hasOneUse()) {
    switch (OpI->getOpcode()) {
    default: break;
    case Instruction::FAdd:
    case Instruction::FSub:
    case Instruction::FMul:
    case Instruction::FDiv:
    case Instruction::FRem:
      Type *SrcTy = OpI->getType();
      Value *LHSTrunc = LookThroughFPExtensions(OpI->getOperand(0));
      Value *RHSTrunc = LookThroughFPExtensions(OpI->getOperand(1));
      if (LHSTrunc->getType() != SrcTy && 
          RHSTrunc->getType() != SrcTy) {
        unsigned DstSize = CI.getType()->getScalarSizeInBits();
        // If the source types were both smaller than the destination type of
        // the cast, do this xform.
        if (LHSTrunc->getType()->getScalarSizeInBits() <= DstSize &&
            RHSTrunc->getType()->getScalarSizeInBits() <= DstSize) {
          LHSTrunc = Builder->CreateFPExt(LHSTrunc, CI.getType());
          RHSTrunc = Builder->CreateFPExt(RHSTrunc, CI.getType());
          return BinaryOperator::Create(OpI->getOpcode(), LHSTrunc, RHSTrunc);
        }
      }
      break;  
    }
  }
  
  // Fold (fptrunc (sqrt (fpext x))) -> (sqrtf x)
  CallInst *Call = dyn_cast<CallInst>(CI.getOperand(0));
  if (Call && Call->getCalledFunction() && TLI->has(LibFunc::sqrtf) &&
      Call->getCalledFunction()->getName() == TLI->getName(LibFunc::sqrt) &&
      Call->getNumArgOperands() == 1 &&
      Call->hasOneUse()) {
    CastInst *Arg = dyn_cast<CastInst>(Call->getArgOperand(0));
    if (Arg && Arg->getOpcode() == Instruction::FPExt &&
        CI.getType()->isFloatTy() &&
        Call->getType()->isDoubleTy() &&
        Arg->getType()->isDoubleTy() &&
        Arg->getOperand(0)->getType()->isFloatTy()) {
      Function *Callee = Call->getCalledFunction();
      Module *M = CI.getParent()->getParent()->getParent();
      Constant *SqrtfFunc = M->getOrInsertFunction("sqrtf", 
                                                   Callee->getAttributes(),
                                                   Builder->getFloatTy(),
                                                   Builder->getFloatTy(),
                                                   NULL);
      CallInst *ret = CallInst::Create(SqrtfFunc, Arg->getOperand(0),
                                       "sqrtfcall");
      ret->setAttributes(Callee->getAttributes());
      
      
      // Remove the old Call.  With -fmath-errno, it won't get marked readnone.
      ReplaceInstUsesWith(*Call, UndefValue::get(Call->getType()));
      EraseInstFromFunction(*Call);
      return ret;
    }
  }
  
  return 0;
}

Instruction *InstCombiner::visitFPExt(CastInst &CI) {
  return commonCastTransforms(CI);
}

Instruction *InstCombiner::visitFPToUI(FPToUIInst &FI) {
  Instruction *OpI = dyn_cast<Instruction>(FI.getOperand(0));
  if (OpI == 0)
    return commonCastTransforms(FI);

  // fptoui(uitofp(X)) --> X
  // fptoui(sitofp(X)) --> X
  // This is safe if the intermediate type has enough bits in its mantissa to
  // accurately represent all values of X.  For example, do not do this with
  // i64->float->i64.  This is also safe for sitofp case, because any negative
  // 'X' value would cause an undefined result for the fptoui. 
  if ((isa<UIToFPInst>(OpI) || isa<SIToFPInst>(OpI)) &&
      OpI->getOperand(0)->getType() == FI.getType() &&
      (int)FI.getType()->getScalarSizeInBits() < /*extra bit for sign */
                    OpI->getType()->getFPMantissaWidth())
    return ReplaceInstUsesWith(FI, OpI->getOperand(0));

  return commonCastTransforms(FI);
}

Instruction *InstCombiner::visitFPToSI(FPToSIInst &FI) {
  Instruction *OpI = dyn_cast<Instruction>(FI.getOperand(0));
  if (OpI == 0)
    return commonCastTransforms(FI);
  
  // fptosi(sitofp(X)) --> X
  // fptosi(uitofp(X)) --> X
  // This is safe if the intermediate type has enough bits in its mantissa to
  // accurately represent all values of X.  For example, do not do this with
  // i64->float->i64.  This is also safe for sitofp case, because any negative
  // 'X' value would cause an undefined result for the fptoui. 
  if ((isa<UIToFPInst>(OpI) || isa<SIToFPInst>(OpI)) &&
      OpI->getOperand(0)->getType() == FI.getType() &&
      (int)FI.getType()->getScalarSizeInBits() <=
                    OpI->getType()->getFPMantissaWidth())
    return ReplaceInstUsesWith(FI, OpI->getOperand(0));
  
  return commonCastTransforms(FI);
}

Instruction *InstCombiner::visitUIToFP(CastInst &CI) {
  return commonCastTransforms(CI);
}

Instruction *InstCombiner::visitSIToFP(CastInst &CI) {
  return commonCastTransforms(CI);
}

Instruction *InstCombiner::visitIntToPtr(IntToPtrInst &CI) {
  // If the source integer type is not the intptr_t type for this target, do a
  // trunc or zext to the intptr_t type, then inttoptr of it.  This allows the
  // cast to be exposed to other transforms.
  if (TD) {
    if (CI.getOperand(0)->getType()->getScalarSizeInBits() >
        TD->getPointerSizeInBits()) {
      Value *P = Builder->CreateTrunc(CI.getOperand(0),
                                      TD->getIntPtrType(CI.getContext()));
      return new IntToPtrInst(P, CI.getType());
    }
    if (CI.getOperand(0)->getType()->getScalarSizeInBits() <
        TD->getPointerSizeInBits()) {
      Value *P = Builder->CreateZExt(CI.getOperand(0),
                                     TD->getIntPtrType(CI.getContext()));
      return new IntToPtrInst(P, CI.getType());
    }
  }
  
  if (Instruction *I = commonCastTransforms(CI))
    return I;

  return 0;
}

/// @brief Implement the transforms for cast of pointer (bitcast/ptrtoint)
Instruction *InstCombiner::commonPointerCastTransforms(CastInst &CI) {
  Value *Src = CI.getOperand(0);
  
  if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Src)) {
    // If casting the result of a getelementptr instruction with no offset, turn
    // this into a cast of the original pointer!
    if (GEP->hasAllZeroIndices()) {
      // Changing the cast operand is usually not a good idea but it is safe
      // here because the pointer operand is being replaced with another 
      // pointer operand so the opcode doesn't need to change.
      Worklist.Add(GEP);
      CI.setOperand(0, GEP->getOperand(0));
      return &CI;
    }
    
    // If the GEP has a single use, and the base pointer is a bitcast, and the
    // GEP computes a constant offset, see if we can convert these three
    // instructions into fewer.  This typically happens with unions and other
    // non-type-safe code.
    if (TD && GEP->hasOneUse() && isa<BitCastInst>(GEP->getOperand(0)) &&
        GEP->hasAllConstantIndices()) {
      SmallVector<Value*, 8> Ops(GEP->idx_begin(), GEP->idx_end());
      int64_t Offset = TD->getIndexedOffset(GEP->getPointerOperandType(), Ops);

      // Get the base pointer input of the bitcast, and the type it points to.
      Value *OrigBase = cast<BitCastInst>(GEP->getOperand(0))->getOperand(0);
      Type *GEPIdxTy =
      cast<PointerType>(OrigBase->getType())->getElementType();
      SmallVector<Value*, 8> NewIndices;
      if (FindElementAtOffset(GEPIdxTy, Offset, NewIndices)) {
        // If we were able to index down into an element, create the GEP
        // and bitcast the result.  This eliminates one bitcast, potentially
        // two.
        Value *NGEP = cast<GEPOperator>(GEP)->isInBounds() ?
        Builder->CreateInBoundsGEP(OrigBase, NewIndices) :
        Builder->CreateGEP(OrigBase, NewIndices);
        NGEP->takeName(GEP);
        
        if (isa<BitCastInst>(CI))
          return new BitCastInst(NGEP, CI.getType());
        assert(isa<PtrToIntInst>(CI));
        return new PtrToIntInst(NGEP, CI.getType());
      }      
    }
  }
  
  return commonCastTransforms(CI);
}

Instruction *InstCombiner::visitPtrToInt(PtrToIntInst &CI) {
  // If the destination integer type is not the intptr_t type for this target,
  // do a ptrtoint to intptr_t then do a trunc or zext.  This allows the cast
  // to be exposed to other transforms.
  if (TD) {
    if (CI.getType()->getScalarSizeInBits() < TD->getPointerSizeInBits()) {
      Value *P = Builder->CreatePtrToInt(CI.getOperand(0),
                                         TD->getIntPtrType(CI.getContext()));
      return new TruncInst(P, CI.getType());
    }
    if (CI.getType()->getScalarSizeInBits() > TD->getPointerSizeInBits()) {
      Value *P = Builder->CreatePtrToInt(CI.getOperand(0),
                                         TD->getIntPtrType(CI.getContext()));
      return new ZExtInst(P, CI.getType());
    }
  }
  
  return commonPointerCastTransforms(CI);
}

/// OptimizeVectorResize - This input value (which is known to have vector type)
/// is being zero extended or truncated to the specified vector type.  Try to
/// replace it with a shuffle (and vector/vector bitcast) if possible.
///
/// The source and destination vector types may have different element types.
static Instruction *OptimizeVectorResize(Value *InVal, VectorType *DestTy,
                                         InstCombiner &IC) {
  // We can only do this optimization if the output is a multiple of the input
  // element size, or the input is a multiple of the output element size.
  // Convert the input type to have the same element type as the output.
  VectorType *SrcTy = cast<VectorType>(InVal->getType());
  
  if (SrcTy->getElementType() != DestTy->getElementType()) {
    // The input types don't need to be identical, but for now they must be the
    // same size.  There is no specific reason we couldn't handle things like
    // <4 x i16> -> <4 x i32> by bitcasting to <2 x i32> but haven't gotten
    // there yet. 
    if (SrcTy->getElementType()->getPrimitiveSizeInBits() !=
        DestTy->getElementType()->getPrimitiveSizeInBits())
      return 0;
    
    SrcTy = VectorType::get(DestTy->getElementType(), SrcTy->getNumElements());
    InVal = IC.Builder->CreateBitCast(InVal, SrcTy);
  }
  
  // Now that the element types match, get the shuffle mask and RHS of the
  // shuffle to use, which depends on whether we're increasing or decreasing the
  // size of the input.
  SmallVector<uint32_t, 16> ShuffleMask;
  Value *V2;
  
  if (SrcTy->getNumElements() > DestTy->getNumElements()) {
    // If we're shrinking the number of elements, just shuffle in the low
    // elements from the input and use undef as the second shuffle input.
    V2 = UndefValue::get(SrcTy);
    for (unsigned i = 0, e = DestTy->getNumElements(); i != e; ++i)
      ShuffleMask.push_back(i);
    
  } else {
    // If we're increasing the number of elements, shuffle in all of the
    // elements from InVal and fill the rest of the result elements with zeros
    // from a constant zero.
    V2 = Constant::getNullValue(SrcTy);
    unsigned SrcElts = SrcTy->getNumElements();
    for (unsigned i = 0, e = SrcElts; i != e; ++i)
      ShuffleMask.push_back(i);

    // The excess elements reference the first element of the zero input.
    for (unsigned i = 0, e = DestTy->getNumElements()-SrcElts; i != e; ++i)
      ShuffleMask.push_back(SrcElts);
  }
  
  return new ShuffleVectorInst(InVal, V2,
                               ConstantDataVector::get(V2->getContext(),
                                                       ShuffleMask));
}

static bool isMultipleOfTypeSize(unsigned Value, Type *Ty) {
  return Value % Ty->getPrimitiveSizeInBits() == 0;
}

static unsigned getTypeSizeIndex(unsigned Value, Type *Ty) {
  return Value / Ty->getPrimitiveSizeInBits();
}

/// CollectInsertionElements - V is a value which is inserted into a vector of
/// VecEltTy.  Look through the value to see if we can decompose it into
/// insertions into the vector.  See the example in the comment for
/// OptimizeIntegerToVectorInsertions for the pattern this handles.
/// The type of V is always a non-zero multiple of VecEltTy's size.
///
/// This returns false if the pattern can't be matched or true if it can,
/// filling in Elements with the elements found here.
static bool CollectInsertionElements(Value *V, unsigned ElementIndex,
                                     SmallVectorImpl<Value*> &Elements,
                                     Type *VecEltTy) {
  // Undef values never contribute useful bits to the result.
  if (isa<UndefValue>(V)) return true;
  
  // If we got down to a value of the right type, we win, try inserting into the
  // right element.
  if (V->getType() == VecEltTy) {
    // Inserting null doesn't actually insert any elements.
    if (Constant *C = dyn_cast<Constant>(V))
      if (C->isNullValue())
        return true;
    
    // Fail if multiple elements are inserted into this slot.
    if (ElementIndex >= Elements.size() || Elements[ElementIndex] != 0)
      return false;
    
    Elements[ElementIndex] = V;
    return true;
  }
  
  if (Constant *C = dyn_cast<Constant>(V)) {
    // Figure out the # elements this provides, and bitcast it or slice it up
    // as required.
    unsigned NumElts = getTypeSizeIndex(C->getType()->getPrimitiveSizeInBits(),
                                        VecEltTy);
    // If the constant is the size of a vector element, we just need to bitcast
    // it to the right type so it gets properly inserted.
    if (NumElts == 1)
      return CollectInsertionElements(ConstantExpr::getBitCast(C, VecEltTy),
                                      ElementIndex, Elements, VecEltTy);
    
    // Okay, this is a constant that covers multiple elements.  Slice it up into
    // pieces and insert each element-sized piece into the vector.
    if (!isa<IntegerType>(C->getType()))
      C = ConstantExpr::getBitCast(C, IntegerType::get(V->getContext(),
                                       C->getType()->getPrimitiveSizeInBits()));
    unsigned ElementSize = VecEltTy->getPrimitiveSizeInBits();
    Type *ElementIntTy = IntegerType::get(C->getContext(), ElementSize);
    
    for (unsigned i = 0; i != NumElts; ++i) {
      Constant *Piece = ConstantExpr::getLShr(C, ConstantInt::get(C->getType(),
                                                               i*ElementSize));
      Piece = ConstantExpr::getTrunc(Piece, ElementIntTy);
      if (!CollectInsertionElements(Piece, ElementIndex+i, Elements, VecEltTy))
        return false;
    }
    return true;
  }
  
  if (!V->hasOneUse()) return false;
  
  Instruction *I = dyn_cast<Instruction>(V);
  if (I == 0) return false;
  switch (I->getOpcode()) {
  default: return false; // Unhandled case.
  case Instruction::BitCast:
    return CollectInsertionElements(I->getOperand(0), ElementIndex,
                                    Elements, VecEltTy);  
  case Instruction::ZExt:
    if (!isMultipleOfTypeSize(
                          I->getOperand(0)->getType()->getPrimitiveSizeInBits(),
                              VecEltTy))
      return false;
    return CollectInsertionElements(I->getOperand(0), ElementIndex,
                                    Elements, VecEltTy);  
  case Instruction::Or:
    return CollectInsertionElements(I->getOperand(0), ElementIndex,
                                    Elements, VecEltTy) &&
           CollectInsertionElements(I->getOperand(1), ElementIndex,
                                    Elements, VecEltTy);
  case Instruction::Shl: {
    // Must be shifting by a constant that is a multiple of the element size.
    ConstantInt *CI = dyn_cast<ConstantInt>(I->getOperand(1));
    if (CI == 0) return false;
    if (!isMultipleOfTypeSize(CI->getZExtValue(), VecEltTy)) return false;
    unsigned IndexShift = getTypeSizeIndex(CI->getZExtValue(), VecEltTy);
    
    return CollectInsertionElements(I->getOperand(0), ElementIndex+IndexShift,
                                    Elements, VecEltTy);
  }
      
  }
}


/// OptimizeIntegerToVectorInsertions - If the input is an 'or' instruction, we
/// may be doing shifts and ors to assemble the elements of the vector manually.
/// Try to rip the code out and replace it with insertelements.  This is to
/// optimize code like this:
///
///    %tmp37 = bitcast float %inc to i32
///    %tmp38 = zext i32 %tmp37 to i64
///    %tmp31 = bitcast float %inc5 to i32
///    %tmp32 = zext i32 %tmp31 to i64
///    %tmp33 = shl i64 %tmp32, 32
///    %ins35 = or i64 %tmp33, %tmp38
///    %tmp43 = bitcast i64 %ins35 to <2 x float>
///
/// Into two insertelements that do "buildvector{%inc, %inc5}".
static Value *OptimizeIntegerToVectorInsertions(BitCastInst &CI,
                                                InstCombiner &IC) {
  VectorType *DestVecTy = cast<VectorType>(CI.getType());
  Value *IntInput = CI.getOperand(0);

  SmallVector<Value*, 8> Elements(DestVecTy->getNumElements());
  if (!CollectInsertionElements(IntInput, 0, Elements,
                                DestVecTy->getElementType()))
    return 0;

  // If we succeeded, we know that all of the element are specified by Elements
  // or are zero if Elements has a null entry.  Recast this as a set of
  // insertions.
  Value *Result = Constant::getNullValue(CI.getType());
  for (unsigned i = 0, e = Elements.size(); i != e; ++i) {
    if (Elements[i] == 0) continue;  // Unset element.
    
    Result = IC.Builder->CreateInsertElement(Result, Elements[i],
                                             IC.Builder->getInt32(i));
  }
  
  return Result;
}


/// OptimizeIntToFloatBitCast - See if we can optimize an integer->float/double
/// bitcast.  The various long double bitcasts can't get in here.
static Instruction *OptimizeIntToFloatBitCast(BitCastInst &CI,InstCombiner &IC){
  Value *Src = CI.getOperand(0);
  Type *DestTy = CI.getType();

  // If this is a bitcast from int to float, check to see if the int is an
  // extraction from a vector.
  Value *VecInput = 0;
  // bitcast(trunc(bitcast(somevector)))
  if (match(Src, m_Trunc(m_BitCast(m_Value(VecInput)))) &&
      isa<VectorType>(VecInput->getType())) {
    VectorType *VecTy = cast<VectorType>(VecInput->getType());
    unsigned DestWidth = DestTy->getPrimitiveSizeInBits();

    if (VecTy->getPrimitiveSizeInBits() % DestWidth == 0) {
      // If the element type of the vector doesn't match the result type,
      // bitcast it to be a vector type we can extract from.
      if (VecTy->getElementType() != DestTy) {
        VecTy = VectorType::get(DestTy,
                                VecTy->getPrimitiveSizeInBits() / DestWidth);
        VecInput = IC.Builder->CreateBitCast(VecInput, VecTy);
      }
    
      return ExtractElementInst::Create(VecInput, IC.Builder->getInt32(0));
    }
  }
  
  // bitcast(trunc(lshr(bitcast(somevector), cst))
  ConstantInt *ShAmt = 0;
  if (match(Src, m_Trunc(m_LShr(m_BitCast(m_Value(VecInput)),
                                m_ConstantInt(ShAmt)))) &&
      isa<VectorType>(VecInput->getType())) {
    VectorType *VecTy = cast<VectorType>(VecInput->getType());
    unsigned DestWidth = DestTy->getPrimitiveSizeInBits();
    if (VecTy->getPrimitiveSizeInBits() % DestWidth == 0 &&
        ShAmt->getZExtValue() % DestWidth == 0) {
      // If the element type of the vector doesn't match the result type,
      // bitcast it to be a vector type we can extract from.
      if (VecTy->getElementType() != DestTy) {
        VecTy = VectorType::get(DestTy,
                                VecTy->getPrimitiveSizeInBits() / DestWidth);
        VecInput = IC.Builder->CreateBitCast(VecInput, VecTy);
      }
      
      unsigned Elt = ShAmt->getZExtValue() / DestWidth;
      return ExtractElementInst::Create(VecInput, IC.Builder->getInt32(Elt));
    }
  }
  return 0;
}

Instruction *InstCombiner::visitBitCast(BitCastInst &CI) {
  // If the operands are integer typed then apply the integer transforms,
  // otherwise just apply the common ones.
  Value *Src = CI.getOperand(0);
  Type *SrcTy = Src->getType();
  Type *DestTy = CI.getType();

  // Get rid of casts from one type to the same type. These are useless and can
  // be replaced by the operand.
  if (DestTy == Src->getType())
    return ReplaceInstUsesWith(CI, Src);

  if (PointerType *DstPTy = dyn_cast<PointerType>(DestTy)) {
    PointerType *SrcPTy = cast<PointerType>(SrcTy);
    Type *DstElTy = DstPTy->getElementType();
    Type *SrcElTy = SrcPTy->getElementType();
    
    // If the address spaces don't match, don't eliminate the bitcast, which is
    // required for changing types.
    if (SrcPTy->getAddressSpace() != DstPTy->getAddressSpace())
      return 0;
    
    // If we are casting a alloca to a pointer to a type of the same
    // size, rewrite the allocation instruction to allocate the "right" type.
    // There is no need to modify malloc calls because it is their bitcast that
    // needs to be cleaned up.
    if (AllocaInst *AI = dyn_cast<AllocaInst>(Src))
      if (Instruction *V = PromoteCastOfAllocation(CI, *AI))
        return V;
    
    // If the source and destination are pointers, and this cast is equivalent
    // to a getelementptr X, 0, 0, 0...  turn it into the appropriate gep.
    // This can enhance SROA and other transforms that want type-safe pointers.
    Constant *ZeroUInt =
      Constant::getNullValue(Type::getInt32Ty(CI.getContext()));
    unsigned NumZeros = 0;
    while (SrcElTy != DstElTy && 
           isa<CompositeType>(SrcElTy) && !SrcElTy->isPointerTy() &&
           SrcElTy->getNumContainedTypes() /* not "{}" */) {
      SrcElTy = cast<CompositeType>(SrcElTy)->getTypeAtIndex(ZeroUInt);
      ++NumZeros;
    }

    // If we found a path from the src to dest, create the getelementptr now.
    if (SrcElTy == DstElTy) {
      SmallVector<Value*, 8> Idxs(NumZeros+1, ZeroUInt);
      return GetElementPtrInst::CreateInBounds(Src, Idxs);
    }
  }
  
  // Try to optimize int -> float bitcasts.
  if ((DestTy->isFloatTy() || DestTy->isDoubleTy()) && isa<IntegerType>(SrcTy))
    if (Instruction *I = OptimizeIntToFloatBitCast(CI, *this))
      return I;

  if (VectorType *DestVTy = dyn_cast<VectorType>(DestTy)) {
    if (DestVTy->getNumElements() == 1 && !SrcTy->isVectorTy()) {
      Value *Elem = Builder->CreateBitCast(Src, DestVTy->getElementType());
      return InsertElementInst::Create(UndefValue::get(DestTy), Elem,
                     Constant::getNullValue(Type::getInt32Ty(CI.getContext())));
      // FIXME: Canonicalize bitcast(insertelement) -> insertelement(bitcast)
    }
    
    if (isa<IntegerType>(SrcTy)) {
      // If this is a cast from an integer to vector, check to see if the input
      // is a trunc or zext of a bitcast from vector.  If so, we can replace all
      // the casts with a shuffle and (potentially) a bitcast.
      if (isa<TruncInst>(Src) || isa<ZExtInst>(Src)) {
        CastInst *SrcCast = cast<CastInst>(Src);
        if (BitCastInst *BCIn = dyn_cast<BitCastInst>(SrcCast->getOperand(0)))
          if (isa<VectorType>(BCIn->getOperand(0)->getType()))
            if (Instruction *I = OptimizeVectorResize(BCIn->getOperand(0),
                                               cast<VectorType>(DestTy), *this))
              return I;
      }
      
      // If the input is an 'or' instruction, we may be doing shifts and ors to
      // assemble the elements of the vector manually.  Try to rip the code out
      // and replace it with insertelements.
      if (Value *V = OptimizeIntegerToVectorInsertions(CI, *this))
        return ReplaceInstUsesWith(CI, V);
    }
  }

  if (VectorType *SrcVTy = dyn_cast<VectorType>(SrcTy)) {
    if (SrcVTy->getNumElements() == 1 && !DestTy->isVectorTy()) {
      Value *Elem = 
        Builder->CreateExtractElement(Src,
                   Constant::getNullValue(Type::getInt32Ty(CI.getContext())));
      return CastInst::Create(Instruction::BitCast, Elem, DestTy);
    }
  }

  if (ShuffleVectorInst *SVI = dyn_cast<ShuffleVectorInst>(Src)) {
    // Okay, we have (bitcast (shuffle ..)).  Check to see if this is
    // a bitcast to a vector with the same # elts.
    if (SVI->hasOneUse() && DestTy->isVectorTy() && 
        cast<VectorType>(DestTy)->getNumElements() ==
              SVI->getType()->getNumElements() &&
        SVI->getType()->getNumElements() ==
          cast<VectorType>(SVI->getOperand(0)->getType())->getNumElements()) {
      BitCastInst *Tmp;
      // If either of the operands is a cast from CI.getType(), then
      // evaluating the shuffle in the casted destination's type will allow
      // us to eliminate at least one cast.
      if (((Tmp = dyn_cast<BitCastInst>(SVI->getOperand(0))) && 
           Tmp->getOperand(0)->getType() == DestTy) ||
          ((Tmp = dyn_cast<BitCastInst>(SVI->getOperand(1))) && 
           Tmp->getOperand(0)->getType() == DestTy)) {
        Value *LHS = Builder->CreateBitCast(SVI->getOperand(0), DestTy);
        Value *RHS = Builder->CreateBitCast(SVI->getOperand(1), DestTy);
        // Return a new shuffle vector.  Use the same element ID's, as we
        // know the vector types match #elts.
        return new ShuffleVectorInst(LHS, RHS, SVI->getOperand(2));
      }
    }
  }
  
  if (SrcTy->isPointerTy())
    return commonPointerCastTransforms(CI);
  return commonCastTransforms(CI);
}