bool InterleavedAccess::lowerInterleavedStore(
    StoreInst *SI, SmallVector<Instruction *, 32> &DeadInsts) {
  if (!SI->isSimple())
    return false;

  ShuffleVectorInst *SVI = dyn_cast<ShuffleVectorInst>(SI->getValueOperand());
  if (!SVI || !SVI->hasOneUse())
    return false;

  // Check if the shufflevector is RE-interleave shuffle.
  unsigned Factor;
  if (!isReInterleaveMask(SVI->getShuffleMask(), Factor))
    return false;

  DEBUG(dbgs() << "IA: Found an interleaved store: " << *SI << "\n");

  // Try to create target specific intrinsics to replace the store and shuffle.
  if (!TLI->lowerInterleavedStore(SI, SVI, Factor))
    return false;

  // Already have a new target specific interleaved store. Erase the old store.
  DeadInsts.push_back(SI);
  DeadInsts.push_back(SVI);
  return true;
}
static bool isByteSwap64(ShuffleVectorInst &SI, SmallVector<int, 16>&RefMasks)
{

    RefMasks.clear();
    unsigned VWidth = cast<VectorType>(SI.getType())->getNumElements();
    VectorType *LHS = cast<VectorType>(SI.getOperand(0)->getType());
    VectorType *RHS = cast<VectorType>(SI.getOperand(1)->getType());

    IntegerType *IT = dyn_cast<IntegerType>(LHS->getElementType());
    //When Element Type is not IntegerType or the Result's element number
    //can't be divided by 8, return false
    //TODO:Need to check all masks are all constants.
    if (IT == nullptr
        || ! IT->isIntegerTy(8)
        || VWidth % 8 != 0) {
        return false;
    }

    SmallVector<int, 16> Masks(SI.getShuffleMask());
    bool isByteSwap = true;

    for (unsigned i = 0; i < VWidth / 8; ++i) {
        unsigned base = Masks[i * 8];
        if (base % 8 != 7) {
            isByteSwap = false;
            break;
        }

        for (unsigned j = 1; j < 8; ++j) {
            if (base - Masks[i * 8 + j] != j) {
                isByteSwap = false;
                break;
            }
        }

        if (isByteSwap) {
            RefMasks.push_back(base / 8);
        } else {
            break;
        }
    }

    if (!isByteSwap) {
        RefMasks.clear();
    }

    return isByteSwap;
}
Beispiel #3
0
static bool matchVectorSplittingReduction(const ExtractElementInst *ReduxRoot,
                                          unsigned &Opcode, Type *&Ty) {
  if (!EnableReduxCost)
    return false;

  // Need to extract the first element.
  ConstantInt *CI = dyn_cast<ConstantInt>(ReduxRoot->getOperand(1));
  unsigned Idx = ~0u;
  if (CI)
    Idx = CI->getZExtValue();
  if (Idx != 0)
    return false;

  BinaryOperator *RdxStart = dyn_cast<BinaryOperator>(ReduxRoot->getOperand(0));
  if (!RdxStart)
    return false;
  unsigned RdxOpcode = RdxStart->getOpcode();

  Type *VecTy = ReduxRoot->getOperand(0)->getType();
  unsigned NumVecElems = VecTy->getVectorNumElements();
  if (!isPowerOf2_32(NumVecElems))
    return false;

  // We look for a sequence of shuffles and adds like the following matching one
  // fadd, shuffle vector pair at a time.
  //
  // %rdx.shuf = shufflevector <4 x float> %rdx, <4 x float> undef,
  //                           <4 x i32> <i32 2, i32 3, i32 undef, i32 undef>
  // %bin.rdx = fadd <4 x float> %rdx, %rdx.shuf
  // %rdx.shuf7 = shufflevector <4 x float> %bin.rdx, <4 x float> undef,
  //                          <4 x i32> <i32 1, i32 undef, i32 undef, i32 undef>
  // %bin.rdx8 = fadd <4 x float> %bin.rdx, %rdx.shuf7
  // %r = extractelement <4 x float> %bin.rdx8, i32 0

  unsigned MaskStart = 1;
  Value *RdxOp = RdxStart;
  SmallVector<int, 32> ShuffleMask(NumVecElems, 0);
  unsigned NumVecElemsRemain = NumVecElems;
  while (NumVecElemsRemain - 1) {
    // Check for the right reduction operation.
    BinaryOperator *BinOp;
    if (!(BinOp = dyn_cast<BinaryOperator>(RdxOp)))
      return false;
    if (BinOp->getOpcode() != RdxOpcode)
      return false;

    Value *NextRdxOp;
    ShuffleVectorInst *Shuffle;
    std::tie(NextRdxOp, Shuffle) = getShuffleAndOtherOprd(BinOp);

    // Check the current reduction operation and the shuffle use the same value.
    if (Shuffle == nullptr)
      return false;
    if (Shuffle->getOperand(0) != NextRdxOp)
      return false;

    // Check that shuffle masks matches.
    for (unsigned j = 0; j != MaskStart; ++j)
      ShuffleMask[j] = MaskStart + j;
    // Fill the rest of the mask with -1 for undef.
    std::fill(&ShuffleMask[MaskStart], ShuffleMask.end(), -1);

    SmallVector<int, 16> Mask = Shuffle->getShuffleMask();
    if (ShuffleMask != Mask)
      return false;

    RdxOp = NextRdxOp;
    NumVecElemsRemain /= 2;
    MaskStart *= 2;
  }

  Opcode = RdxOpcode;
  Ty = VecTy;
  return true;
}
Instruction *InstCombiner::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
  Value *LHS = SVI.getOperand(0);
  Value *RHS = SVI.getOperand(1);
  SmallVector<int, 16> Mask = SVI.getShuffleMask();

  bool MadeChange = false;

  // Undefined shuffle mask -> undefined value.
  if (isa<UndefValue>(SVI.getOperand(2)))
    return ReplaceInstUsesWith(SVI, UndefValue::get(SVI.getType()));

  unsigned VWidth = cast<VectorType>(SVI.getType())->getNumElements();

  APInt UndefElts(VWidth, 0);
  APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
  if (Value *V = SimplifyDemandedVectorElts(&SVI, AllOnesEltMask, UndefElts)) {
    if (V != &SVI)
      return ReplaceInstUsesWith(SVI, V);
    LHS = SVI.getOperand(0);
    RHS = SVI.getOperand(1);
    MadeChange = true;
  }

  unsigned LHSWidth = cast<VectorType>(LHS->getType())->getNumElements();

  // Canonicalize shuffle(x    ,x,mask) -> shuffle(x, undef,mask')
  // Canonicalize shuffle(undef,x,mask) -> shuffle(x, undef,mask').
  if (LHS == RHS || isa<UndefValue>(LHS)) {
    if (isa<UndefValue>(LHS) && LHS == RHS) {
      // shuffle(undef,undef,mask) -> undef.
      Value *Result = (VWidth == LHSWidth)
                      ? LHS : UndefValue::get(SVI.getType());
      return ReplaceInstUsesWith(SVI, Result);
    }

    // Remap any references to RHS to use LHS.
    SmallVector<Constant*, 16> Elts;
    for (unsigned i = 0, e = LHSWidth; i != VWidth; ++i) {
      if (Mask[i] < 0) {
        Elts.push_back(UndefValue::get(Type::getInt32Ty(SVI.getContext())));
        continue;
      }

      if ((Mask[i] >= (int)e && isa<UndefValue>(RHS)) ||
          (Mask[i] <  (int)e && isa<UndefValue>(LHS))) {
        Mask[i] = -1;     // Turn into undef.
        Elts.push_back(UndefValue::get(Type::getInt32Ty(SVI.getContext())));
      } else {
        Mask[i] = Mask[i] % e;  // Force to LHS.
        Elts.push_back(ConstantInt::get(Type::getInt32Ty(SVI.getContext()),
                                        Mask[i]));
      }
    }
    SVI.setOperand(0, SVI.getOperand(1));
    SVI.setOperand(1, UndefValue::get(RHS->getType()));
    SVI.setOperand(2, ConstantVector::get(Elts));
    LHS = SVI.getOperand(0);
    RHS = SVI.getOperand(1);
    MadeChange = true;
  }

  if (VWidth == LHSWidth) {
    // Analyze the shuffle, are the LHS or RHS and identity shuffles?
    bool isLHSID = true, isRHSID = true;

    for (unsigned i = 0, e = Mask.size(); i != e; ++i) {
      if (Mask[i] < 0) continue;  // Ignore undef values.
      // Is this an identity shuffle of the LHS value?
      isLHSID &= (Mask[i] == (int)i);

      // Is this an identity shuffle of the RHS value?
      isRHSID &= (Mask[i]-e == i);
    }

    // Eliminate identity shuffles.
    if (isLHSID) return ReplaceInstUsesWith(SVI, LHS);
    if (isRHSID) return ReplaceInstUsesWith(SVI, RHS);
  }

  if (isa<UndefValue>(RHS) && CanEvaluateShuffled(LHS, Mask)) {
    Value *V = EvaluateInDifferentElementOrder(LHS, Mask);
    return ReplaceInstUsesWith(SVI, V);
  }

  // If the LHS is a shufflevector itself, see if we can combine it with this
  // one without producing an unusual shuffle.
  // Cases that might be simplified:
  // 1.
  // x1=shuffle(v1,v2,mask1)
  //  x=shuffle(x1,undef,mask)
  //        ==>
  //  x=shuffle(v1,undef,newMask)
  // newMask[i] = (mask[i] < x1.size()) ? mask1[mask[i]] : -1
  // 2.
  // x1=shuffle(v1,undef,mask1)
  //  x=shuffle(x1,x2,mask)
  // where v1.size() == mask1.size()
  //        ==>
  //  x=shuffle(v1,x2,newMask)
  // newMask[i] = (mask[i] < x1.size()) ? mask1[mask[i]] : mask[i]
  // 3.
  // x2=shuffle(v2,undef,mask2)
  //  x=shuffle(x1,x2,mask)
  // where v2.size() == mask2.size()
  //        ==>
  //  x=shuffle(x1,v2,newMask)
  // newMask[i] = (mask[i] < x1.size())
  //              ? mask[i] : mask2[mask[i]-x1.size()]+x1.size()
  // 4.
  // x1=shuffle(v1,undef,mask1)
  // x2=shuffle(v2,undef,mask2)
  //  x=shuffle(x1,x2,mask)
  // where v1.size() == v2.size()
  //        ==>
  //  x=shuffle(v1,v2,newMask)
  // newMask[i] = (mask[i] < x1.size())
  //              ? mask1[mask[i]] : mask2[mask[i]-x1.size()]+v1.size()
  //
  // Here we are really conservative:
  // we are absolutely afraid of producing a shuffle mask not in the input
  // program, because the code gen may not be smart enough to turn a merged
  // shuffle into two specific shuffles: it may produce worse code.  As such,
  // we only merge two shuffles if the result is either a splat or one of the
  // input shuffle masks.  In this case, merging the shuffles just removes
  // one instruction, which we know is safe.  This is good for things like
  // turning: (splat(splat)) -> splat, or
  // merge(V[0..n], V[n+1..2n]) -> V[0..2n]
  ShuffleVectorInst* LHSShuffle = dyn_cast<ShuffleVectorInst>(LHS);
  ShuffleVectorInst* RHSShuffle = dyn_cast<ShuffleVectorInst>(RHS);
  if (LHSShuffle)
    if (!isa<UndefValue>(LHSShuffle->getOperand(1)) && !isa<UndefValue>(RHS))
      LHSShuffle = NULL;
  if (RHSShuffle)
    if (!isa<UndefValue>(RHSShuffle->getOperand(1)))
      RHSShuffle = NULL;
  if (!LHSShuffle && !RHSShuffle)
    return MadeChange ? &SVI : 0;

  Value* LHSOp0 = NULL;
  Value* LHSOp1 = NULL;
  Value* RHSOp0 = NULL;
  unsigned LHSOp0Width = 0;
  unsigned RHSOp0Width = 0;
  if (LHSShuffle) {
    LHSOp0 = LHSShuffle->getOperand(0);
    LHSOp1 = LHSShuffle->getOperand(1);
    LHSOp0Width = cast<VectorType>(LHSOp0->getType())->getNumElements();
  }
  if (RHSShuffle) {
    RHSOp0 = RHSShuffle->getOperand(0);
    RHSOp0Width = cast<VectorType>(RHSOp0->getType())->getNumElements();
  }
  Value* newLHS = LHS;
  Value* newRHS = RHS;
  if (LHSShuffle) {
    // case 1
    if (isa<UndefValue>(RHS)) {
      newLHS = LHSOp0;
      newRHS = LHSOp1;
    }
    // case 2 or 4
    else if (LHSOp0Width == LHSWidth) {
      newLHS = LHSOp0;
    }
  }
  // case 3 or 4
  if (RHSShuffle && RHSOp0Width == LHSWidth) {
    newRHS = RHSOp0;
  }
  // case 4
  if (LHSOp0 == RHSOp0) {
    newLHS = LHSOp0;
    newRHS = NULL;
  }

  if (newLHS == LHS && newRHS == RHS)
    return MadeChange ? &SVI : 0;

  SmallVector<int, 16> LHSMask;
  SmallVector<int, 16> RHSMask;
  if (newLHS != LHS)
    LHSMask = LHSShuffle->getShuffleMask();
  if (RHSShuffle && newRHS != RHS)
    RHSMask = RHSShuffle->getShuffleMask();

  unsigned newLHSWidth = (newLHS != LHS) ? LHSOp0Width : LHSWidth;
  SmallVector<int, 16> newMask;
  bool isSplat = true;
  int SplatElt = -1;
  // Create a new mask for the new ShuffleVectorInst so that the new
  // ShuffleVectorInst is equivalent to the original one.
  for (unsigned i = 0; i < VWidth; ++i) {
    int eltMask;
    if (Mask[i] < 0) {
      // This element is an undef value.
      eltMask = -1;
    } else if (Mask[i] < (int)LHSWidth) {
      // This element is from left hand side vector operand.
      //
      // If LHS is going to be replaced (case 1, 2, or 4), calculate the
      // new mask value for the element.
      if (newLHS != LHS) {
        eltMask = LHSMask[Mask[i]];
        // If the value selected is an undef value, explicitly specify it
        // with a -1 mask value.
        if (eltMask >= (int)LHSOp0Width && isa<UndefValue>(LHSOp1))
          eltMask = -1;
      } else
        eltMask = Mask[i];
    } else {
      // This element is from right hand side vector operand
      //
      // If the value selected is an undef value, explicitly specify it
      // with a -1 mask value. (case 1)
      if (isa<UndefValue>(RHS))
        eltMask = -1;
      // If RHS is going to be replaced (case 3 or 4), calculate the
      // new mask value for the element.
      else if (newRHS != RHS) {
        eltMask = RHSMask[Mask[i]-LHSWidth];
        // If the value selected is an undef value, explicitly specify it
        // with a -1 mask value.
        if (eltMask >= (int)RHSOp0Width) {
          assert(isa<UndefValue>(RHSShuffle->getOperand(1))
                 && "should have been check above");
          eltMask = -1;
        }
      } else
        eltMask = Mask[i]-LHSWidth;

      // If LHS's width is changed, shift the mask value accordingly.
      // If newRHS == NULL, i.e. LHSOp0 == RHSOp0, we want to remap any
      // references from RHSOp0 to LHSOp0, so we don't need to shift the mask.
      // If newRHS == newLHS, we want to remap any references from newRHS to
      // newLHS so that we can properly identify splats that may occur due to
      // obfuscation accross the two vectors.
      if (eltMask >= 0 && newRHS != NULL && newLHS != newRHS)
        eltMask += newLHSWidth;
    }

    // Check if this could still be a splat.
    if (eltMask >= 0) {
      if (SplatElt >= 0 && SplatElt != eltMask)
        isSplat = false;
      SplatElt = eltMask;
    }

    newMask.push_back(eltMask);
  }

  // If the result mask is equal to one of the original shuffle masks,
  // or is a splat, do the replacement.
  if (isSplat || newMask == LHSMask || newMask == RHSMask || newMask == Mask) {
    SmallVector<Constant*, 16> Elts;
    Type *Int32Ty = Type::getInt32Ty(SVI.getContext());
    for (unsigned i = 0, e = newMask.size(); i != e; ++i) {
      if (newMask[i] < 0) {
        Elts.push_back(UndefValue::get(Int32Ty));
      } else {
        Elts.push_back(ConstantInt::get(Int32Ty, newMask[i]));
      }
    }
    if (newRHS == NULL)
      newRHS = UndefValue::get(newLHS->getType());
    return new ShuffleVectorInst(newLHS, newRHS, ConstantVector::get(Elts));
  }

  return MadeChange ? &SVI : 0;
}
Beispiel #5
0
Instruction *InstCombiner::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
  Value *LHS = SVI.getOperand(0);
  Value *RHS = SVI.getOperand(1);
  SmallVector<int, 16> Mask = SVI.getShuffleMask();
  Type *Int32Ty = Type::getInt32Ty(SVI.getContext());

  bool MadeChange = false;

  // Undefined shuffle mask -> undefined value.
  if (isa<UndefValue>(SVI.getOperand(2)))
    return replaceInstUsesWith(SVI, UndefValue::get(SVI.getType()));

  unsigned VWidth = cast<VectorType>(SVI.getType())->getNumElements();

  APInt UndefElts(VWidth, 0);
  APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
  if (Value *V = SimplifyDemandedVectorElts(&SVI, AllOnesEltMask, UndefElts)) {
    if (V != &SVI)
      return replaceInstUsesWith(SVI, V);
    LHS = SVI.getOperand(0);
    RHS = SVI.getOperand(1);
    MadeChange = true;
  }

  unsigned LHSWidth = cast<VectorType>(LHS->getType())->getNumElements();

  // Canonicalize shuffle(x    ,x,mask) -> shuffle(x, undef,mask')
  // Canonicalize shuffle(undef,x,mask) -> shuffle(x, undef,mask').
  if (LHS == RHS || isa<UndefValue>(LHS)) {
    if (isa<UndefValue>(LHS) && LHS == RHS) {
      // shuffle(undef,undef,mask) -> undef.
      Value *Result = (VWidth == LHSWidth)
                      ? LHS : UndefValue::get(SVI.getType());
      return replaceInstUsesWith(SVI, Result);
    }

    // Remap any references to RHS to use LHS.
    SmallVector<Constant*, 16> Elts;
    for (unsigned i = 0, e = LHSWidth; i != VWidth; ++i) {
      if (Mask[i] < 0) {
        Elts.push_back(UndefValue::get(Int32Ty));
        continue;
      }

      if ((Mask[i] >= (int)e && isa<UndefValue>(RHS)) ||
          (Mask[i] <  (int)e && isa<UndefValue>(LHS))) {
        Mask[i] = -1;     // Turn into undef.
        Elts.push_back(UndefValue::get(Int32Ty));
      } else {
        Mask[i] = Mask[i] % e;  // Force to LHS.
        Elts.push_back(ConstantInt::get(Int32Ty, Mask[i]));
      }
    }
    SVI.setOperand(0, SVI.getOperand(1));
    SVI.setOperand(1, UndefValue::get(RHS->getType()));
    SVI.setOperand(2, ConstantVector::get(Elts));
    LHS = SVI.getOperand(0);
    RHS = SVI.getOperand(1);
    MadeChange = true;
  }

  if (VWidth == LHSWidth) {
    // Analyze the shuffle, are the LHS or RHS and identity shuffles?
    bool isLHSID, isRHSID;
    recognizeIdentityMask(Mask, isLHSID, isRHSID);

    // Eliminate identity shuffles.
    if (isLHSID) return replaceInstUsesWith(SVI, LHS);
    if (isRHSID) return replaceInstUsesWith(SVI, RHS);
  }

  if (isa<UndefValue>(RHS) && CanEvaluateShuffled(LHS, Mask)) {
    Value *V = EvaluateInDifferentElementOrder(LHS, Mask);
    return replaceInstUsesWith(SVI, V);
  }

  // SROA generates shuffle+bitcast when the extracted sub-vector is bitcast to
  // a non-vector type. We can instead bitcast the original vector followed by
  // an extract of the desired element:
  //
  //   %sroa = shufflevector <16 x i8> %in, <16 x i8> undef,
  //                         <4 x i32> <i32 0, i32 1, i32 2, i32 3>
  //   %1 = bitcast <4 x i8> %sroa to i32
  // Becomes:
  //   %bc = bitcast <16 x i8> %in to <4 x i32>
  //   %ext = extractelement <4 x i32> %bc, i32 0
  //
  // If the shuffle is extracting a contiguous range of values from the input
  // vector then each use which is a bitcast of the extracted size can be
  // replaced. This will work if the vector types are compatible, and the begin
  // index is aligned to a value in the casted vector type. If the begin index
  // isn't aligned then we can shuffle the original vector (keeping the same
  // vector type) before extracting.
  //
  // This code will bail out if the target type is fundamentally incompatible
  // with vectors of the source type.
  //
  // Example of <16 x i8>, target type i32:
  // Index range [4,8):         v-----------v Will work.
  //                +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
  //     <16 x i8>: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
  //     <4 x i32>: |           |           |           |           |
  //                +-----------+-----------+-----------+-----------+
  // Index range [6,10):              ^-----------^ Needs an extra shuffle.
  // Target type i40:           ^--------------^ Won't work, bail.
  if (isShuffleExtractingFromLHS(SVI, Mask)) {
    Value *V = LHS;
    unsigned MaskElems = Mask.size();
    unsigned BegIdx = Mask.front();
    VectorType *SrcTy = cast<VectorType>(V->getType());
    unsigned VecBitWidth = SrcTy->getBitWidth();
    unsigned SrcElemBitWidth = DL.getTypeSizeInBits(SrcTy->getElementType());
    assert(SrcElemBitWidth && "vector elements must have a bitwidth");
    unsigned SrcNumElems = SrcTy->getNumElements();
    SmallVector<BitCastInst *, 8> BCs;
    DenseMap<Type *, Value *> NewBCs;
    for (User *U : SVI.users())
      if (BitCastInst *BC = dyn_cast<BitCastInst>(U))
        if (!BC->use_empty())
          // Only visit bitcasts that weren't previously handled.
          BCs.push_back(BC);
    for (BitCastInst *BC : BCs) {
      Type *TgtTy = BC->getDestTy();
      unsigned TgtElemBitWidth = DL.getTypeSizeInBits(TgtTy);
      if (!TgtElemBitWidth)
        continue;
      unsigned TgtNumElems = VecBitWidth / TgtElemBitWidth;
      bool VecBitWidthsEqual = VecBitWidth == TgtNumElems * TgtElemBitWidth;
      bool BegIsAligned = 0 == ((SrcElemBitWidth * BegIdx) % TgtElemBitWidth);
      if (!VecBitWidthsEqual)
        continue;
      if (!VectorType::isValidElementType(TgtTy))
        continue;
      VectorType *CastSrcTy = VectorType::get(TgtTy, TgtNumElems);
      if (!BegIsAligned) {
        // Shuffle the input so [0,NumElements) contains the output, and
        // [NumElems,SrcNumElems) is undef.
        SmallVector<Constant *, 16> ShuffleMask(SrcNumElems,
                                                UndefValue::get(Int32Ty));
        for (unsigned I = 0, E = MaskElems, Idx = BegIdx; I != E; ++Idx, ++I)
          ShuffleMask[I] = ConstantInt::get(Int32Ty, Idx);
        V = Builder->CreateShuffleVector(V, UndefValue::get(V->getType()),
                                         ConstantVector::get(ShuffleMask),
                                         SVI.getName() + ".extract");
        BegIdx = 0;
      }
      unsigned SrcElemsPerTgtElem = TgtElemBitWidth / SrcElemBitWidth;
      assert(SrcElemsPerTgtElem);
      BegIdx /= SrcElemsPerTgtElem;
      bool BCAlreadyExists = NewBCs.find(CastSrcTy) != NewBCs.end();
      auto *NewBC =
          BCAlreadyExists
              ? NewBCs[CastSrcTy]
              : Builder->CreateBitCast(V, CastSrcTy, SVI.getName() + ".bc");
      if (!BCAlreadyExists)
        NewBCs[CastSrcTy] = NewBC;
      auto *Ext = Builder->CreateExtractElement(
          NewBC, ConstantInt::get(Int32Ty, BegIdx), SVI.getName() + ".extract");
      // The shufflevector isn't being replaced: the bitcast that used it
      // is. InstCombine will visit the newly-created instructions.
      replaceInstUsesWith(*BC, Ext);
      MadeChange = true;
    }
  }

  // If the LHS is a shufflevector itself, see if we can combine it with this
  // one without producing an unusual shuffle.
  // Cases that might be simplified:
  // 1.
  // x1=shuffle(v1,v2,mask1)
  //  x=shuffle(x1,undef,mask)
  //        ==>
  //  x=shuffle(v1,undef,newMask)
  // newMask[i] = (mask[i] < x1.size()) ? mask1[mask[i]] : -1
  // 2.
  // x1=shuffle(v1,undef,mask1)
  //  x=shuffle(x1,x2,mask)
  // where v1.size() == mask1.size()
  //        ==>
  //  x=shuffle(v1,x2,newMask)
  // newMask[i] = (mask[i] < x1.size()) ? mask1[mask[i]] : mask[i]
  // 3.
  // x2=shuffle(v2,undef,mask2)
  //  x=shuffle(x1,x2,mask)
  // where v2.size() == mask2.size()
  //        ==>
  //  x=shuffle(x1,v2,newMask)
  // newMask[i] = (mask[i] < x1.size())
  //              ? mask[i] : mask2[mask[i]-x1.size()]+x1.size()
  // 4.
  // x1=shuffle(v1,undef,mask1)
  // x2=shuffle(v2,undef,mask2)
  //  x=shuffle(x1,x2,mask)
  // where v1.size() == v2.size()
  //        ==>
  //  x=shuffle(v1,v2,newMask)
  // newMask[i] = (mask[i] < x1.size())
  //              ? mask1[mask[i]] : mask2[mask[i]-x1.size()]+v1.size()
  //
  // Here we are really conservative:
  // we are absolutely afraid of producing a shuffle mask not in the input
  // program, because the code gen may not be smart enough to turn a merged
  // shuffle into two specific shuffles: it may produce worse code.  As such,
  // we only merge two shuffles if the result is either a splat or one of the
  // input shuffle masks.  In this case, merging the shuffles just removes
  // one instruction, which we know is safe.  This is good for things like
  // turning: (splat(splat)) -> splat, or
  // merge(V[0..n], V[n+1..2n]) -> V[0..2n]
  ShuffleVectorInst* LHSShuffle = dyn_cast<ShuffleVectorInst>(LHS);
  ShuffleVectorInst* RHSShuffle = dyn_cast<ShuffleVectorInst>(RHS);
  if (LHSShuffle)
    if (!isa<UndefValue>(LHSShuffle->getOperand(1)) && !isa<UndefValue>(RHS))
      LHSShuffle = nullptr;
  if (RHSShuffle)
    if (!isa<UndefValue>(RHSShuffle->getOperand(1)))
      RHSShuffle = nullptr;
  if (!LHSShuffle && !RHSShuffle)
    return MadeChange ? &SVI : nullptr;

  Value* LHSOp0 = nullptr;
  Value* LHSOp1 = nullptr;
  Value* RHSOp0 = nullptr;
  unsigned LHSOp0Width = 0;
  unsigned RHSOp0Width = 0;
  if (LHSShuffle) {
    LHSOp0 = LHSShuffle->getOperand(0);
    LHSOp1 = LHSShuffle->getOperand(1);
    LHSOp0Width = cast<VectorType>(LHSOp0->getType())->getNumElements();
  }
  if (RHSShuffle) {
    RHSOp0 = RHSShuffle->getOperand(0);
    RHSOp0Width = cast<VectorType>(RHSOp0->getType())->getNumElements();
  }
  Value* newLHS = LHS;
  Value* newRHS = RHS;
  if (LHSShuffle) {
    // case 1
    if (isa<UndefValue>(RHS)) {
      newLHS = LHSOp0;
      newRHS = LHSOp1;
    }
    // case 2 or 4
    else if (LHSOp0Width == LHSWidth) {
      newLHS = LHSOp0;
    }
  }
  // case 3 or 4
  if (RHSShuffle && RHSOp0Width == LHSWidth) {
    newRHS = RHSOp0;
  }
  // case 4
  if (LHSOp0 == RHSOp0) {
    newLHS = LHSOp0;
    newRHS = nullptr;
  }

  if (newLHS == LHS && newRHS == RHS)
    return MadeChange ? &SVI : nullptr;

  SmallVector<int, 16> LHSMask;
  SmallVector<int, 16> RHSMask;
  if (newLHS != LHS)
    LHSMask = LHSShuffle->getShuffleMask();
  if (RHSShuffle && newRHS != RHS)
    RHSMask = RHSShuffle->getShuffleMask();

  unsigned newLHSWidth = (newLHS != LHS) ? LHSOp0Width : LHSWidth;
  SmallVector<int, 16> newMask;
  bool isSplat = true;
  int SplatElt = -1;
  // Create a new mask for the new ShuffleVectorInst so that the new
  // ShuffleVectorInst is equivalent to the original one.
  for (unsigned i = 0; i < VWidth; ++i) {
    int eltMask;
    if (Mask[i] < 0) {
      // This element is an undef value.
      eltMask = -1;
    } else if (Mask[i] < (int)LHSWidth) {
      // This element is from left hand side vector operand.
      //
      // If LHS is going to be replaced (case 1, 2, or 4), calculate the
      // new mask value for the element.
      if (newLHS != LHS) {
        eltMask = LHSMask[Mask[i]];
        // If the value selected is an undef value, explicitly specify it
        // with a -1 mask value.
        if (eltMask >= (int)LHSOp0Width && isa<UndefValue>(LHSOp1))
          eltMask = -1;
      } else
        eltMask = Mask[i];
    } else {
      // This element is from right hand side vector operand
      //
      // If the value selected is an undef value, explicitly specify it
      // with a -1 mask value. (case 1)
      if (isa<UndefValue>(RHS))
        eltMask = -1;
      // If RHS is going to be replaced (case 3 or 4), calculate the
      // new mask value for the element.
      else if (newRHS != RHS) {
        eltMask = RHSMask[Mask[i]-LHSWidth];
        // If the value selected is an undef value, explicitly specify it
        // with a -1 mask value.
        if (eltMask >= (int)RHSOp0Width) {
          assert(isa<UndefValue>(RHSShuffle->getOperand(1))
                 && "should have been check above");
          eltMask = -1;
        }
      } else
        eltMask = Mask[i]-LHSWidth;

      // If LHS's width is changed, shift the mask value accordingly.
      // If newRHS == NULL, i.e. LHSOp0 == RHSOp0, we want to remap any
      // references from RHSOp0 to LHSOp0, so we don't need to shift the mask.
      // If newRHS == newLHS, we want to remap any references from newRHS to
      // newLHS so that we can properly identify splats that may occur due to
      // obfuscation across the two vectors.
      if (eltMask >= 0 && newRHS != nullptr && newLHS != newRHS)
        eltMask += newLHSWidth;
    }

    // Check if this could still be a splat.
    if (eltMask >= 0) {
      if (SplatElt >= 0 && SplatElt != eltMask)
        isSplat = false;
      SplatElt = eltMask;
    }

    newMask.push_back(eltMask);
  }

  // If the result mask is equal to one of the original shuffle masks,
  // or is a splat, do the replacement.
  if (isSplat || newMask == LHSMask || newMask == RHSMask || newMask == Mask) {
    SmallVector<Constant*, 16> Elts;
    for (unsigned i = 0, e = newMask.size(); i != e; ++i) {
      if (newMask[i] < 0) {
        Elts.push_back(UndefValue::get(Int32Ty));
      } else {
        Elts.push_back(ConstantInt::get(Int32Ty, newMask[i]));
      }
    }
    if (!newRHS)
      newRHS = UndefValue::get(newLHS->getType());
    return new ShuffleVectorInst(newLHS, newRHS, ConstantVector::get(Elts));
  }

  // If the result mask is an identity, replace uses of this instruction with
  // corresponding argument.
  bool isLHSID, isRHSID;
  recognizeIdentityMask(newMask, isLHSID, isRHSID);
  if (isLHSID && VWidth == LHSOp0Width) return replaceInstUsesWith(SVI, newLHS);
  if (isRHSID && VWidth == RHSOp0Width) return replaceInstUsesWith(SVI, newRHS);

  return MadeChange ? &SVI : nullptr;
}