split vector stuff out to InstCombineVectorOps.cpp

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92683 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 557 insertions, 0 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
new file mode 100644
index 0000000000..9977b830f3
--- /dev/null
+++ b/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -0,0 +1,557 @@
+//===- InstCombineVectorOps.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 instcombine for ExtractElement, InsertElement and
+// ShuffleVector.
+//
+//===----------------------------------------------------------------------===//
+
+#include "InstCombine.h"
+using namespace llvm;
+
+/// CheapToScalarize - Return true if the value is cheaper to scalarize than it
+/// is to leave as a vector operation.
+static bool CheapToScalarize(Value *V, bool isConstant) {
+  if (isa<ConstantAggregateZero>(V)) 
+    return true;
+  if (ConstantVector *C = dyn_cast<ConstantVector>(V)) {
+    if (isConstant) return true;
+    // If all elts are the same, we can extract.
+    Constant *Op0 = C->getOperand(0);
+    for (unsigned i = 1; i < C->getNumOperands(); ++i)
+      if (C->getOperand(i) != Op0)
+        return false;
+    return true;
+  }
+  Instruction *I = dyn_cast<Instruction>(V);
+  if (!I) return false;
+  
+  // Insert element gets simplified to the inserted element or is deleted if
+  // this is constant idx extract element and its a constant idx insertelt.
+  if (I->getOpcode() == Instruction::InsertElement && isConstant &&
+      isa<ConstantInt>(I->getOperand(2)))
+    return true;
+  if (I->getOpcode() == Instruction::Load && I->hasOneUse())
+    return true;
+  if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I))
+    if (BO->hasOneUse() &&
+        (CheapToScalarize(BO->getOperand(0), isConstant) ||
+         CheapToScalarize(BO->getOperand(1), isConstant)))
+      return true;
+  if (CmpInst *CI = dyn_cast<CmpInst>(I))
+    if (CI->hasOneUse() &&
+        (CheapToScalarize(CI->getOperand(0), isConstant) ||
+         CheapToScalarize(CI->getOperand(1), isConstant)))
+      return true;
+  
+  return false;
+}
+
+/// Read and decode a shufflevector mask.
+///
+/// It turns undef elements into values that are larger than the number of
+/// elements in the input.
+static std::vector<unsigned> getShuffleMask(const ShuffleVectorInst *SVI) {
+  unsigned NElts = SVI->getType()->getNumElements();
+  if (isa<ConstantAggregateZero>(SVI->getOperand(2)))
+    return std::vector<unsigned>(NElts, 0);
+  if (isa<UndefValue>(SVI->getOperand(2)))
+    return std::vector<unsigned>(NElts, 2*NElts);
+  
+  std::vector<unsigned> Result;
+  const ConstantVector *CP = cast<ConstantVector>(SVI->getOperand(2));
+  for (User::const_op_iterator i = CP->op_begin(), e = CP->op_end(); i!=e; ++i)
+    if (isa<UndefValue>(*i))
+      Result.push_back(NElts*2);  // undef -> 8
+    else
+      Result.push_back(cast<ConstantInt>(*i)->getZExtValue());
+  return Result;
+}
+
+/// FindScalarElement - Given a vector and an element number, see if the scalar
+/// value is already around as a register, for example if it were inserted then
+/// extracted from the vector.
+static Value *FindScalarElement(Value *V, unsigned EltNo) {
+  assert(isa<VectorType>(V->getType()) && "Not looking at a vector?");
+  const VectorType *PTy = cast<VectorType>(V->getType());
+  unsigned Width = PTy->getNumElements();
+  if (EltNo >= Width)  // Out of range access.
+    return UndefValue::get(PTy->getElementType());
+  
+  if (isa<UndefValue>(V))
+    return UndefValue::get(PTy->getElementType());
+  else if (isa<ConstantAggregateZero>(V))
+    return Constant::getNullValue(PTy->getElementType());
+  else if (ConstantVector *CP = dyn_cast<ConstantVector>(V))
+    return CP->getOperand(EltNo);
+  else if (InsertElementInst *III = dyn_cast<InsertElementInst>(V)) {
+    // If this is an insert to a variable element, we don't know what it is.
+    if (!isa<ConstantInt>(III->getOperand(2))) 
+      return 0;
+    unsigned IIElt = cast<ConstantInt>(III->getOperand(2))->getZExtValue();
+    
+    // If this is an insert to the element we are looking for, return the
+    // inserted value.
+    if (EltNo == IIElt) 
+      return III->getOperand(1);
+    
+    // Otherwise, the insertelement doesn't modify the value, recurse on its
+    // vector input.
+    return FindScalarElement(III->getOperand(0), EltNo);
+  } else if (ShuffleVectorInst *SVI = dyn_cast<ShuffleVectorInst>(V)) {
+    unsigned LHSWidth =
+    cast<VectorType>(SVI->getOperand(0)->getType())->getNumElements();
+    unsigned InEl = getShuffleMask(SVI)[EltNo];
+    if (InEl < LHSWidth)
+      return FindScalarElement(SVI->getOperand(0), InEl);
+    else if (InEl < LHSWidth*2)
+      return FindScalarElement(SVI->getOperand(1), InEl - LHSWidth);
+    else
+      return UndefValue::get(PTy->getElementType());
+  }
+  
+  // Otherwise, we don't know.
+  return 0;
+}
+
+Instruction *InstCombiner::visitExtractElementInst(ExtractElementInst &EI) {
+  // If vector val is undef, replace extract with scalar undef.
+  if (isa<UndefValue>(EI.getOperand(0)))
+    return ReplaceInstUsesWith(EI, UndefValue::get(EI.getType()));
+  
+  // If vector val is constant 0, replace extract with scalar 0.
+  if (isa<ConstantAggregateZero>(EI.getOperand(0)))
+    return ReplaceInstUsesWith(EI, Constant::getNullValue(EI.getType()));
+  
+  if (ConstantVector *C = dyn_cast<ConstantVector>(EI.getOperand(0))) {
+    // If vector val is constant with all elements the same, replace EI with
+    // that element. When the elements are not identical, we cannot replace yet
+    // (we do that below, but only when the index is constant).
+    Constant *op0 = C->getOperand(0);
+    for (unsigned i = 1; i != C->getNumOperands(); ++i)
+      if (C->getOperand(i) != op0) {
+        op0 = 0; 
+        break;
+      }
+    if (op0)
+      return ReplaceInstUsesWith(EI, op0);
+  }
+  
+  // If extracting a specified index from the vector, see if we can recursively
+  // find a previously computed scalar that was inserted into the vector.
+  if (ConstantInt *IdxC = dyn_cast<ConstantInt>(EI.getOperand(1))) {
+    unsigned IndexVal = IdxC->getZExtValue();
+    unsigned VectorWidth = EI.getVectorOperandType()->getNumElements();
+    
+    // If this is extracting an invalid index, turn this into undef, to avoid
+    // crashing the code below.
+    if (IndexVal >= VectorWidth)
+      return ReplaceInstUsesWith(EI, UndefValue::get(EI.getType()));
+    
+    // This instruction only demands the single element from the input vector.
+    // If the input vector has a single use, simplify it based on this use
+    // property.
+    if (EI.getOperand(0)->hasOneUse() && VectorWidth != 1) {
+      APInt UndefElts(VectorWidth, 0);
+      APInt DemandedMask(VectorWidth, 1 << IndexVal);
+      if (Value *V = SimplifyDemandedVectorElts(EI.getOperand(0),
+                                                DemandedMask, UndefElts)) {
+        EI.setOperand(0, V);
+        return &EI;
+      }
+    }
+    
+    if (Value *Elt = FindScalarElement(EI.getOperand(0), IndexVal))
+      return ReplaceInstUsesWith(EI, Elt);
+    
+    // If the this extractelement is directly using a bitcast from a vector of
+    // the same number of elements, see if we can find the source element from
+    // it.  In this case, we will end up needing to bitcast the scalars.
+    if (BitCastInst *BCI = dyn_cast<BitCastInst>(EI.getOperand(0))) {
+      if (const VectorType *VT = 
+          dyn_cast<VectorType>(BCI->getOperand(0)->getType()))
+        if (VT->getNumElements() == VectorWidth)
+          if (Value *Elt = FindScalarElement(BCI->getOperand(0), IndexVal))
+            return new BitCastInst(Elt, EI.getType());
+    }
+  }
+  
+  if (Instruction *I = dyn_cast<Instruction>(EI.getOperand(0))) {
+    // Push extractelement into predecessor operation if legal and
+    // profitable to do so
+    if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
+      if (I->hasOneUse() &&
+          CheapToScalarize(BO, isa<ConstantInt>(EI.getOperand(1)))) {
+        Value *newEI0 =
+        Builder->CreateExtractElement(BO->getOperand(0), EI.getOperand(1),
+                                      EI.getName()+".lhs");
+        Value *newEI1 =
+        Builder->CreateExtractElement(BO->getOperand(1), EI.getOperand(1),
+                                      EI.getName()+".rhs");
+        return BinaryOperator::Create(BO->getOpcode(), newEI0, newEI1);
+      }
+    } else if (InsertElementInst *IE = dyn_cast<InsertElementInst>(I)) {
+      // Extracting the inserted element?
+      if (IE->getOperand(2) == EI.getOperand(1))
+        return ReplaceInstUsesWith(EI, IE->getOperand(1));
+      // If the inserted and extracted elements are constants, they must not
+      // be the same value, extract from the pre-inserted value instead.
+      if (isa<Constant>(IE->getOperand(2)) && isa<Constant>(EI.getOperand(1))) {
+        Worklist.AddValue(EI.getOperand(0));
+        EI.setOperand(0, IE->getOperand(0));
+        return &EI;
+      }
+    } else if (ShuffleVectorInst *SVI = dyn_cast<ShuffleVectorInst>(I)) {
+      // If this is extracting an element from a shufflevector, figure out where
+      // it came from and extract from the appropriate input element instead.
+      if (ConstantInt *Elt = dyn_cast<ConstantInt>(EI.getOperand(1))) {
+        unsigned SrcIdx = getShuffleMask(SVI)[Elt->getZExtValue()];
+        Value *Src;
+        unsigned LHSWidth =
+        cast<VectorType>(SVI->getOperand(0)->getType())->getNumElements();
+        
+        if (SrcIdx < LHSWidth)
+          Src = SVI->getOperand(0);
+        else if (SrcIdx < LHSWidth*2) {
+          SrcIdx -= LHSWidth;
+          Src = SVI->getOperand(1);
+        } else {
+          return ReplaceInstUsesWith(EI, UndefValue::get(EI.getType()));
+        }
+        return ExtractElementInst::Create(Src,
+                                          ConstantInt::get(Type::getInt32Ty(EI.getContext()),
+                                                           SrcIdx, false));
+      }
+    }
+    // FIXME: Canonicalize extractelement(bitcast) -> bitcast(extractelement)
+  }
+  return 0;
+}
+
+/// CollectSingleShuffleElements - If V is a shuffle of values that ONLY returns
+/// elements from either LHS or RHS, return the shuffle mask and true. 
+/// Otherwise, return false.
+static bool CollectSingleShuffleElements(Value *V, Value *LHS, Value *RHS,
+                                         std::vector<Constant*> &Mask) {
+  assert(V->getType() == LHS->getType() && V->getType() == RHS->getType() &&
+         "Invalid CollectSingleShuffleElements");
+  unsigned NumElts = cast<VectorType>(V->getType())->getNumElements();
+  
+  if (isa<UndefValue>(V)) {
+    Mask.assign(NumElts, UndefValue::get(Type::getInt32Ty(V->getContext())));
+    return true;
+  }
+  
+  if (V == LHS) {
+    for (unsigned i = 0; i != NumElts; ++i)
+      Mask.push_back(ConstantInt::get(Type::getInt32Ty(V->getContext()), i));
+    return true;
+  }
+  
+  if (V == RHS) {
+    for (unsigned i = 0; i != NumElts; ++i)
+      Mask.push_back(ConstantInt::get(Type::getInt32Ty(V->getContext()),
+                                      i+NumElts));
+    return true;
+  }
+  
+  if (InsertElementInst *IEI = dyn_cast<InsertElementInst>(V)) {
+    // If this is an insert of an extract from some other vector, include it.
+    Value *VecOp    = IEI->getOperand(0);
+    Value *ScalarOp = IEI->getOperand(1);
+    Value *IdxOp    = IEI->getOperand(2);
+    
+    if (!isa<ConstantInt>(IdxOp))
+      return false;
+    unsigned InsertedIdx = cast<ConstantInt>(IdxOp)->getZExtValue();
+    
+    if (isa<UndefValue>(ScalarOp)) {  // inserting undef into vector.
+      // Okay, we can handle this if the vector we are insertinting into is
+      // transitively ok.
+      if (CollectSingleShuffleElements(VecOp, LHS, RHS, Mask)) {
+        // If so, update the mask to reflect the inserted undef.
+        Mask[InsertedIdx] = UndefValue::get(Type::getInt32Ty(V->getContext()));
+        return true;
+      }      
+    } else if (ExtractElementInst *EI = dyn_cast<ExtractElementInst>(ScalarOp)){
+      if (isa<ConstantInt>(EI->getOperand(1)) &&
+          EI->getOperand(0)->getType() == V->getType()) {
+        unsigned ExtractedIdx =
+        cast<ConstantInt>(EI->getOperand(1))->getZExtValue();
+        
+        // This must be extracting from either LHS or RHS.
+        if (EI->getOperand(0) == LHS || EI->getOperand(0) == RHS) {
+          // Okay, we can handle this if the vector we are insertinting into is
+          // transitively ok.
+          if (CollectSingleShuffleElements(VecOp, LHS, RHS, Mask)) {
+            // If so, update the mask to reflect the inserted value.
+            if (EI->getOperand(0) == LHS) {
+              Mask[InsertedIdx % NumElts] = 
+              ConstantInt::get(Type::getInt32Ty(V->getContext()),
+                               ExtractedIdx);
+            } else {
+              assert(EI->getOperand(0) == RHS);
+              Mask[InsertedIdx % NumElts] = 
+              ConstantInt::get(Type::getInt32Ty(V->getContext()),
+                               ExtractedIdx+NumElts);
+              
+            }
+            return true;
+          }
+        }
+      }
+    }
+  }
+  // TODO: Handle shufflevector here!
+  
+  return false;
+}
+
+/// CollectShuffleElements - We are building a shuffle of V, using RHS as the
+/// RHS of the shuffle instruction, if it is not null.  Return a shuffle mask
+/// that computes V and the LHS value of the shuffle.
+static Value *CollectShuffleElements(Value *V, std::vector<Constant*> &Mask,
+                                     Value *&RHS) {
+  assert(isa<VectorType>(V->getType()) && 
+         (RHS == 0 || V->getType() == RHS->getType()) &&
+         "Invalid shuffle!");
+  unsigned NumElts = cast<VectorType>(V->getType())->getNumElements();
+  
+  if (isa<UndefValue>(V)) {
+    Mask.assign(NumElts, UndefValue::get(Type::getInt32Ty(V->getContext())));
+    return V;
+  } else if (isa<ConstantAggregateZero>(V)) {
+    Mask.assign(NumElts, ConstantInt::get(Type::getInt32Ty(V->getContext()),0));
+    return V;
+  } else if (InsertElementInst *IEI = dyn_cast<InsertElementInst>(V)) {
+    // If this is an insert of an extract from some other vector, include it.
+    Value *VecOp    = IEI->getOperand(0);
+    Value *ScalarOp = IEI->getOperand(1);
+    Value *IdxOp    = IEI->getOperand(2);
+    
+    if (ExtractElementInst *EI = dyn_cast<ExtractElementInst>(ScalarOp)) {
+      if (isa<ConstantInt>(EI->getOperand(1)) && isa<ConstantInt>(IdxOp) &&
+          EI->getOperand(0)->getType() == V->getType()) {
+        unsigned ExtractedIdx =
+        cast<ConstantInt>(EI->getOperand(1))->getZExtValue();
+        unsigned InsertedIdx = cast<ConstantInt>(IdxOp)->getZExtValue();
+        
+        // Either the extracted from or inserted into vector must be RHSVec,
+        // otherwise we'd end up with a shuffle of three inputs.
+        if (EI->getOperand(0) == RHS || RHS == 0) {
+          RHS = EI->getOperand(0);
+          Value *V = CollectShuffleElements(VecOp, Mask, RHS);
+          Mask[InsertedIdx % NumElts] = 
+          ConstantInt::get(Type::getInt32Ty(V->getContext()),
+                           NumElts+ExtractedIdx);
+          return V;
+        }
+        
+        if (VecOp == RHS) {
+          Value *V = CollectShuffleElements(EI->getOperand(0), Mask, RHS);
+          // Everything but the extracted element is replaced with the RHS.
+          for (unsigned i = 0; i != NumElts; ++i) {
+            if (i != InsertedIdx)
+              Mask[i] = ConstantInt::get(Type::getInt32Ty(V->getContext()),
+                                         NumElts+i);
+          }
+          return V;
+        }
+        
+        // If this insertelement is a chain that comes from exactly these two
+        // vectors, return the vector and the effective shuffle.
+        if (CollectSingleShuffleElements(IEI, EI->getOperand(0), RHS, Mask))
+          return EI->getOperand(0);
+      }
+    }
+  }
+  // TODO: Handle shufflevector here!
+  
+  // Otherwise, can't do anything fancy.  Return an identity vector.
+  for (unsigned i = 0; i != NumElts; ++i)
+    Mask.push_back(ConstantInt::get(Type::getInt32Ty(V->getContext()), i));
+  return V;
+}
+
+Instruction *InstCombiner::visitInsertElementInst(InsertElementInst &IE) {
+  Value *VecOp    = IE.getOperand(0);
+  Value *ScalarOp = IE.getOperand(1);
+  Value *IdxOp    = IE.getOperand(2);
+  
+  // Inserting an undef or into an undefined place, remove this.
+  if (isa<UndefValue>(ScalarOp) || isa<UndefValue>(IdxOp))
+    ReplaceInstUsesWith(IE, VecOp);
+  
+  // If the inserted element was extracted from some other vector, and if the 
+  // indexes are constant, try to turn this into a shufflevector operation.
+  if (ExtractElementInst *EI = dyn_cast<ExtractElementInst>(ScalarOp)) {
+    if (isa<ConstantInt>(EI->getOperand(1)) && isa<ConstantInt>(IdxOp) &&
+        EI->getOperand(0)->getType() == IE.getType()) {
+      unsigned NumVectorElts = IE.getType()->getNumElements();
+      unsigned ExtractedIdx =
+      cast<ConstantInt>(EI->getOperand(1))->getZExtValue();
+      unsigned InsertedIdx = cast<ConstantInt>(IdxOp)->getZExtValue();
+      
+      if (ExtractedIdx >= NumVectorElts) // Out of range extract.
+        return ReplaceInstUsesWith(IE, VecOp);
+      
+      if (InsertedIdx >= NumVectorElts)  // Out of range insert.
+        return ReplaceInstUsesWith(IE, UndefValue::get(IE.getType()));
+      
+      // If we are extracting a value from a vector, then inserting it right
+      // back into the same place, just use the input vector.
+      if (EI->getOperand(0) == VecOp && ExtractedIdx == InsertedIdx)
+        return ReplaceInstUsesWith(IE, VecOp);      
+      
+      // If this insertelement isn't used by some other insertelement, turn it
+      // (and any insertelements it points to), into one big shuffle.
+      if (!IE.hasOneUse() || !isa<InsertElementInst>(IE.use_back())) {
+        std::vector<Constant*> Mask;
+        Value *RHS = 0;
+        Value *LHS = CollectShuffleElements(&IE, Mask, RHS);
+        if (RHS == 0) RHS = UndefValue::get(LHS->getType());
+        // We now have a shuffle of LHS, RHS, Mask.
+        return new ShuffleVectorInst(LHS, RHS,
+                                     ConstantVector::get(Mask));
+      }
+    }
+  }
+  
+  unsigned VWidth = cast<VectorType>(VecOp->getType())->getNumElements();
+  APInt UndefElts(VWidth, 0);
+  APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
+  if (SimplifyDemandedVectorElts(&IE, AllOnesEltMask, UndefElts))
+    return &IE;
+  
+  return 0;
+}
+
+
+Instruction *InstCombiner::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
+  Value *LHS = SVI.getOperand(0);
+  Value *RHS = SVI.getOperand(1);
+  std::vector<unsigned> Mask = getShuffleMask(&SVI);
+  
+  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();
+  
+  if (VWidth != cast<VectorType>(LHS->getType())->getNumElements())
+    return 0;
+  
+  APInt UndefElts(VWidth, 0);
+  APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));
+  if (SimplifyDemandedVectorElts(&SVI, AllOnesEltMask, UndefElts)) {
+    LHS = SVI.getOperand(0);
+    RHS = SVI.getOperand(1);
+    MadeChange = true;
+  }
+  
+  // 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.
+      return ReplaceInstUsesWith(SVI, LHS);
+    }
+    
+    // Remap any references to RHS to use LHS.
+    std::vector<Constant*> Elts;
+    for (unsigned i = 0, e = Mask.size(); i != e; ++i) {
+      if (Mask[i] >= 2*e)
+        Elts.push_back(UndefValue::get(Type::getInt32Ty(SVI.getContext())));
+      else {
+        if ((Mask[i] >= e && isa<UndefValue>(RHS)) ||
+            (Mask[i] <  e && isa<UndefValue>(LHS))) {
+          Mask[i] = 2*e;     // 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;
+  }
+  
+  // 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] >= e*2) continue;  // Ignore undef values.
+    // Is this an identity shuffle of the LHS value?
+    isLHSID &= (Mask[i] == 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 the LHS is a shufflevector itself, see if we can combine it with this
+  // one without producing an unusual shuffle.  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 one of the two 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.
+  if (ShuffleVectorInst *LHSSVI = dyn_cast<ShuffleVectorInst>(LHS)) {
+    if (isa<UndefValue>(RHS)) {
+      std::vector<unsigned> LHSMask = getShuffleMask(LHSSVI);
+      
+      if (LHSMask.size() == Mask.size()) {
+        std::vector<unsigned> NewMask;
+        for (unsigned i = 0, e = Mask.size(); i != e; ++i)
+          if (Mask[i] >= e)
+            NewMask.push_back(2*e);
+          else
+            NewMask.push_back(LHSMask[Mask[i]]);
+        
+        // If the result mask is equal to the src shuffle or this
+        // shuffle mask, do the replacement.
+        if (NewMask == LHSMask || NewMask == Mask) {
+          unsigned LHSInNElts =
+          cast<VectorType>(LHSSVI->getOperand(0)->getType())->
+          getNumElements();
+          std::vector<Constant*> Elts;
+          for (unsigned i = 0, e = NewMask.size(); i != e; ++i) {
+            if (NewMask[i] >= LHSInNElts*2) {
+              Elts.push_back(UndefValue::get(
+                                             Type::getInt32Ty(SVI.getContext())));
+            } else {
+              Elts.push_back(ConstantInt::get(
+                                              Type::getInt32Ty(SVI.getContext()),
+                                              NewMask[i]));
+            }
+          }
+          return new ShuffleVectorInst(LHSSVI->getOperand(0),
+                                       LHSSVI->getOperand(1),
+                                       ConstantVector::get(Elts));
+        }
+      }
+    }
+  }
+  
+  return MadeChange ? &SVI : 0;
+}
+