diff --git a/llvm/include/llvm-c/Core.h b/llvm/include/llvm-c/Core.h
--- a/llvm/include/llvm-c/Core.h
+++ b/llvm/include/llvm-c/Core.h
@@ -4093,8 +4093,8 @@
  * Get the mask value at position Elt in the mask of a ShuffleVector
  * instruction.
  *
- * \Returns the result of \c LLVMGetUndefMaskElem() if the mask value is undef
- * at that position.
+ * \Returns the result of \c LLVMGetUndefMaskElem() if the mask value is
+ * poison at that position.
  */
 int LLVMGetMaskValue(LLVMValueRef ShuffleVectorInst, unsigned Elt);
 
diff --git a/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h b/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
--- a/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
+++ b/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
@@ -1269,7 +1269,7 @@
           APInt DemandedDstElts =
               APInt::getZero(Shuffle->getShuffleMask().size());
           for (auto I : enumerate(Shuffle->getShuffleMask())) {
-            if (I.value() != UndefMaskElem)
+            if (I.value() != PoisonMaskElem)
               DemandedDstElts.setBit(I.index());
           }
           return TargetTTI->getReplicationShuffleCost(
diff --git a/llvm/include/llvm/IR/Instructions.h b/llvm/include/llvm/IR/Instructions.h
--- a/llvm/include/llvm/IR/Instructions.h
+++ b/llvm/include/llvm/IR/Instructions.h
@@ -2002,7 +2002,7 @@
 //                           ShuffleVectorInst Class
 //===----------------------------------------------------------------------===//
 
-constexpr int UndefMaskElem = -1;
+constexpr int PoisonMaskElem = -1;
 
 /// This instruction constructs a fixed permutation of two
 /// input vectors.
@@ -2010,7 +2010,7 @@
 /// For each element of the result vector, the shuffle mask selects an element
 /// from one of the input vectors to copy to the result. Non-negative elements
 /// in the mask represent an index into the concatenated pair of input vectors.
-/// UndefMaskElem (-1) specifies that the result element is undefined.
+/// PoisonMaskElem (-1) specifies that the result element is poison.
 ///
 /// For scalable vectors, all the elements of the mask must be 0 or -1. This
 /// requirement may be relaxed in the future.
@@ -2068,16 +2068,16 @@
   DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Value);
 
   /// Return the shuffle mask value of this instruction for the given element
-  /// index. Return UndefMaskElem if the element is undef.
+  /// index. Return PoisonMaskElem if the element is undef.
   int getMaskValue(unsigned Elt) const { return ShuffleMask[Elt]; }
 
   /// Convert the input shuffle mask operand to a vector of integers. Undefined
-  /// elements of the mask are returned as UndefMaskElem.
+  /// elements of the mask are returned as PoisonMaskElem.
   static void getShuffleMask(const Constant *Mask,
                              SmallVectorImpl<int> &Result);
 
   /// Return the mask for this instruction as a vector of integers. Undefined
-  /// elements of the mask are returned as UndefMaskElem.
+  /// elements of the mask are returned as PoisonMaskElem.
   void getShuffleMask(SmallVectorImpl<int> &Result) const {
     Result.assign(ShuffleMask.begin(), ShuffleMask.end());
   }
diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -5235,7 +5235,7 @@
                                         ArrayRef<int> Mask, Type *RetTy,
                                         const SimplifyQuery &Q,
                                         unsigned MaxRecurse) {
-  if (all_of(Mask, [](int Elem) { return Elem == UndefMaskElem; }))
+  if (all_of(Mask, [](int Elem) { return Elem == PoisonMaskElem; }))
     return UndefValue::get(RetTy);
 
   auto *InVecTy = cast<VectorType>(Op0->getType());
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -6438,7 +6438,7 @@
     ArrayRef<int> Mask = isa<ConstantExpr>(Op)
                              ? cast<ConstantExpr>(Op)->getShuffleMask()
                              : cast<ShuffleVectorInst>(Op)->getShuffleMask();
-    return is_contained(Mask, UndefMaskElem);
+    return is_contained(Mask, PoisonMaskElem);
   }
   case Instruction::FNeg:
   case Instruction::PHI:
diff --git a/llvm/lib/Analysis/VectorUtils.cpp b/llvm/lib/Analysis/VectorUtils.cpp
--- a/llvm/lib/Analysis/VectorUtils.cpp
+++ b/llvm/lib/Analysis/VectorUtils.cpp
@@ -444,13 +444,13 @@
       int Idx = I * SzDest + K;
       if (Idx == Sz)
         break;
-      if (Mask[Idx] >= Sz || Mask[Idx] == UndefMaskElem)
+      if (Mask[Idx] >= Sz || Mask[Idx] == PoisonMaskElem)
         continue;
       int SrcRegIdx = Mask[Idx] / SzSrc;
       // Add a cost of PermuteTwoSrc for each new source register permute,
       // if we have more than one source registers.
       if (RegMasks[SrcRegIdx].empty())
-        RegMasks[SrcRegIdx].assign(SzDest, UndefMaskElem);
+        RegMasks[SrcRegIdx].assign(SzDest, PoisonMaskElem);
       RegMasks[SrcRegIdx][K] = Mask[Idx] % SzSrc;
     }
   }
@@ -482,8 +482,8 @@
       auto &&CombineMasks = [](MutableArrayRef<int> FirstMask,
                                ArrayRef<int> SecondMask) {
         for (int Idx = 0, VF = FirstMask.size(); Idx < VF; ++Idx) {
-          if (SecondMask[Idx] != UndefMaskElem) {
-            assert(FirstMask[Idx] == UndefMaskElem &&
+          if (SecondMask[Idx] != PoisonMaskElem) {
+            assert(FirstMask[Idx] == PoisonMaskElem &&
                    "Expected undefined mask element.");
             FirstMask[Idx] = SecondMask[Idx] + VF;
           }
@@ -491,7 +491,7 @@
       };
       auto &&NormalizeMask = [](MutableArrayRef<int> Mask) {
         for (int Idx = 0, VF = Mask.size(); Idx < VF; ++Idx) {
-          if (Mask[Idx] != UndefMaskElem)
+          if (Mask[Idx] != PoisonMaskElem)
             Mask[Idx] = Idx;
         }
       };
diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
@@ -2383,7 +2383,7 @@
     EVT EltVT = NewVT.getVectorElementType();
     SmallVector<SDValue> Ops(NewElts, DAG.getUNDEF(EltVT));
     for (unsigned I = 0; I < NewElts; ++I) {
-      if (Mask[I] == UndefMaskElem)
+      if (Mask[I] == PoisonMaskElem)
         continue;
       unsigned Idx = Mask[I];
       if (Idx >= NewElts)
@@ -2423,11 +2423,11 @@
       // Use shuffles operands instead of shuffles themselves.
       // 1. Adjust mask.
       for (int &Idx : Mask) {
-        if (Idx == UndefMaskElem)
+        if (Idx == PoisonMaskElem)
           continue;
         unsigned SrcRegIdx = Idx / NewElts;
         if (Inputs[SrcRegIdx].isUndef()) {
-          Idx = UndefMaskElem;
+          Idx = PoisonMaskElem;
           continue;
         }
         auto *Shuffle =
@@ -2435,8 +2435,8 @@
         if (!Shuffle || !is_contained(P.second, SrcRegIdx))
           continue;
         int MaskElt = Shuffle->getMaskElt(Idx % NewElts);
-        if (MaskElt == UndefMaskElem) {
-          Idx = UndefMaskElem;
+        if (MaskElt == PoisonMaskElem) {
+          Idx = PoisonMaskElem;
           continue;
         }
         Idx = MaskElt % NewElts +
@@ -2455,11 +2455,11 @@
     // Check if any concat_vectors can be simplified.
     SmallBitVector UsedSubVector(2 * std::size(Inputs));
     for (int &Idx : Mask) {
-      if (Idx == UndefMaskElem)
+      if (Idx == PoisonMaskElem)
         continue;
       unsigned SrcRegIdx = Idx / NewElts;
       if (Inputs[SrcRegIdx].isUndef()) {
-        Idx = UndefMaskElem;
+        Idx = PoisonMaskElem;
         continue;
       }
       TargetLowering::LegalizeTypeAction TypeAction =
@@ -2489,7 +2489,7 @@
       if (!Pairs.empty() && Pairs.front().size() > 1) {
         // Adjust mask.
         for (int &Idx : Mask) {
-          if (Idx == UndefMaskElem)
+          if (Idx == PoisonMaskElem)
             continue;
           unsigned SrcRegIdx = Idx / NewElts;
           auto *It = find_if(
@@ -2531,14 +2531,14 @@
                    !Shuffle->getOperand(1).isUndef()) {
           // Find the only used operand, if possible.
           for (int &Idx : Mask) {
-            if (Idx == UndefMaskElem)
+            if (Idx == PoisonMaskElem)
               continue;
             unsigned SrcRegIdx = Idx / NewElts;
             if (SrcRegIdx != I)
               continue;
             int MaskElt = Shuffle->getMaskElt(Idx % NewElts);
-            if (MaskElt == UndefMaskElem) {
-              Idx = UndefMaskElem;
+            if (MaskElt == PoisonMaskElem) {
+              Idx = PoisonMaskElem;
               continue;
             }
             int OpIdx = MaskElt / NewElts;
@@ -2564,14 +2564,14 @@
             // Found that operand is used already.
             // 1. Fix the mask for the reused operand.
             for (int &Idx : Mask) {
-              if (Idx == UndefMaskElem)
+              if (Idx == PoisonMaskElem)
                 continue;
               unsigned SrcRegIdx = Idx / NewElts;
               if (SrcRegIdx != I)
                 continue;
               int MaskElt = Shuffle->getMaskElt(Idx % NewElts);
-              if (MaskElt == UndefMaskElem) {
-                Idx = UndefMaskElem;
+              if (MaskElt == PoisonMaskElem) {
+                Idx = PoisonMaskElem;
                 continue;
               }
               int MaskIdx = MaskElt / NewElts;
@@ -2588,7 +2588,7 @@
           Inputs[I] = Shuffle->getOperand(Op);
           // Adjust mask.
           for (int &Idx : Mask) {
-            if (Idx == UndefMaskElem)
+            if (Idx == PoisonMaskElem)
               continue;
             unsigned SrcRegIdx = Idx / NewElts;
             if (SrcRegIdx != I)
@@ -2622,11 +2622,11 @@
       auto &&UniqueConstantVec = UniqueConstantInputs.takeVector();
       unsigned ConstNum = UniqueConstantVec.size();
       for (int &Idx : Mask) {
-        if (Idx == UndefMaskElem)
+        if (Idx == PoisonMaskElem)
           continue;
         unsigned SrcRegIdx = Idx / NewElts;
         if (Inputs[SrcRegIdx].isUndef()) {
-          Idx = UndefMaskElem;
+          Idx = PoisonMaskElem;
           continue;
         }
         const auto It = find(UniqueConstantVec, Inputs[SrcRegIdx]);
@@ -2655,7 +2655,7 @@
     // Build a shuffle mask for the output, discovering on the fly which
     // input vectors to use as shuffle operands.
     unsigned FirstMaskIdx = High * NewElts;
-    SmallVector<int> Mask(NewElts * std::size(Inputs), UndefMaskElem);
+    SmallVector<int> Mask(NewElts * std::size(Inputs), PoisonMaskElem);
     copy(ArrayRef(OrigMask).slice(FirstMaskIdx, NewElts), Mask.begin());
     assert(!Output && "Expected default initialized initial value.");
     TryPeekThroughShufflesInputs(Mask);
diff --git a/llvm/lib/IR/AsmWriter.cpp b/llvm/lib/IR/AsmWriter.cpp
--- a/llvm/lib/IR/AsmWriter.cpp
+++ b/llvm/lib/IR/AsmWriter.cpp
@@ -425,7 +425,7 @@
   bool FirstElt = true;
   if (all_of(Mask, [](int Elt) { return Elt == 0; })) {
     Out << "zeroinitializer";
-  } else if (all_of(Mask, [](int Elt) { return Elt == UndefMaskElem; })) {
+  } else if (all_of(Mask, [](int Elt) { return Elt == PoisonMaskElem; })) {
     Out << "poison";
   } else {
     Out << "<";
@@ -435,7 +435,7 @@
       else
         Out << ", ";
       Out << "i32 ";
-      if (Elt == UndefMaskElem)
+      if (Elt == PoisonMaskElem)
         Out << "poison";
       else
         Out << Elt;
diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -711,7 +711,7 @@
   Type *EltTy = V1VTy->getElementType();
 
   // Undefined shuffle mask -> undefined value.
-  if (all_of(Mask, [](int Elt) { return Elt == UndefMaskElem; })) {
+  if (all_of(Mask, [](int Elt) { return Elt == PoisonMaskElem; })) {
     return UndefValue::get(VectorType::get(EltTy, MaskEltCount));
   }
 
diff --git a/llvm/lib/IR/Core.cpp b/llvm/lib/IR/Core.cpp
--- a/llvm/lib/IR/Core.cpp
+++ b/llvm/lib/IR/Core.cpp
@@ -3949,7 +3949,7 @@
   return I->getMaskValue(Elt);
 }
 
-int LLVMGetUndefMaskElem(void) { return UndefMaskElem; }
+int LLVMGetUndefMaskElem(void) { return PoisonMaskElem; }
 
 LLVMBool LLVMIsAtomicSingleThread(LLVMValueRef AtomicInst) {
   Value *P = unwrap(AtomicInst);
diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -2164,8 +2164,8 @@
   SmallVector<int, 16> NewMask(NumMaskElts);
   for (int i = 0; i != NumMaskElts; ++i) {
     int MaskElt = getMaskValue(i);
-    if (MaskElt == UndefMaskElem) {
-      NewMask[i] = UndefMaskElem;
+    if (MaskElt == PoisonMaskElem) {
+      NewMask[i] = PoisonMaskElem;
       continue;
     }
     assert(MaskElt >= 0 && MaskElt < 2 * NumOpElts && "Out-of-range mask");
@@ -2186,11 +2186,11 @@
   int V1Size =
       cast<VectorType>(V1->getType())->getElementCount().getKnownMinValue();
   for (int Elem : Mask)
-    if (Elem != UndefMaskElem && Elem >= V1Size * 2)
+    if (Elem != PoisonMaskElem && Elem >= V1Size * 2)
       return false;
 
   if (isa<ScalableVectorType>(V1->getType()))
-    if ((Mask[0] != 0 && Mask[0] != UndefMaskElem) || !all_equal(Mask))
+    if ((Mask[0] != 0 && Mask[0] != PoisonMaskElem) || !all_equal(Mask))
       return false;
 
   return true;
@@ -2289,7 +2289,7 @@
   }
   SmallVector<Constant *, 16> MaskConst;
   for (int Elem : Mask) {
-    if (Elem == UndefMaskElem)
+    if (Elem == PoisonMaskElem)
       MaskConst.push_back(UndefValue::get(Int32Ty));
     else
       MaskConst.push_back(ConstantInt::get(Int32Ty, Elem));
@@ -2627,7 +2627,7 @@
            "Run out of mask?");
     Mask = Mask.drop_front(ReplicationFactor);
     if (!all_of(CurrSubMask, [CurrElt](int MaskElt) {
-          return MaskElt == UndefMaskElem || MaskElt == CurrElt;
+          return MaskElt == PoisonMaskElem || MaskElt == CurrElt;
         }))
       return false;
   }
@@ -2639,7 +2639,7 @@
 bool ShuffleVectorInst::isReplicationMask(ArrayRef<int> Mask,
                                           int &ReplicationFactor, int &VF) {
   // undef-less case is trivial.
-  if (!llvm::is_contained(Mask, UndefMaskElem)) {
+  if (!llvm::is_contained(Mask, PoisonMaskElem)) {
     ReplicationFactor =
         Mask.take_while([](int MaskElt) { return MaskElt == 0; }).size();
     if (ReplicationFactor == 0 || Mask.size() % ReplicationFactor != 0)
@@ -2657,7 +2657,7 @@
   // Before doing that, let's perform basic correctness checking first.
   int Largest = -1;
   for (int MaskElt : Mask) {
-    if (MaskElt == UndefMaskElem)
+    if (MaskElt == PoisonMaskElem)
       continue;
     // Elements must be in non-decreasing order.
     if (MaskElt < Largest)
@@ -2703,11 +2703,11 @@
     return false;
   for (unsigned K = 0, Sz = Mask.size(); K < Sz; K += VF) {
     ArrayRef<int> SubMask = Mask.slice(K, VF);
-    if (all_of(SubMask, [](int Idx) { return Idx == UndefMaskElem; }))
+    if (all_of(SubMask, [](int Idx) { return Idx == PoisonMaskElem; }))
       continue;
     SmallBitVector Used(VF, false);
     for_each(SubMask, [&Used, VF](int Idx) {
-      if (Idx != UndefMaskElem && Idx < VF)
+      if (Idx != PoisonMaskElem && Idx < VF)
         Used.set(Idx);
     });
     if (!Used.all())
diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -14744,9 +14744,9 @@
   auto Mask = SVI->getShuffleMask();
 
   // Sanity check if all the indices are NOT in range.
-  // If mask is `undef` or `poison`, `Mask` may be a vector of -1s.
-  // If all of them are `undef`, OOB read will happen later.
-  if (llvm::all_of(Mask, [](int Idx) { return Idx == UndefMaskElem; })) {
+  // If mask is `poison`, `Mask` may be a vector of -1s.
+  // If all of them are `poison`, OOB read will happen later.
+  if (llvm::all_of(Mask, [](int Idx) { return Idx == PoisonMaskElem; })) {
     return false;
   }
   // A 64bit st2 which does not start at element 0 will involved adding extra
diff --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
@@ -3265,9 +3265,9 @@
       unsigned NumSources = 0;
       for (unsigned E = 0; E < LTNumElts; E++) {
         int MaskElt = (N * LTNumElts + E < TpNumElts) ? Mask[N * LTNumElts + E]
-                                                      : UndefMaskElem;
+                                                      : PoisonMaskElem;
         if (MaskElt < 0) {
-          NMask.push_back(UndefMaskElem);
+          NMask.push_back(PoisonMaskElem);
           continue;
         }
 
diff --git a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
--- a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
+++ b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
@@ -1612,7 +1612,7 @@
             LegalVT.getVectorNumElements() * std::max(NumOfSrcs, E);
         unsigned NumOfSrcRegs = NormalizedVF / LegalVT.getVectorNumElements();
         unsigned NumOfDestRegs = NormalizedVF / LegalVT.getVectorNumElements();
-        SmallVector<int> NormalizedMask(NormalizedVF, UndefMaskElem);
+        SmallVector<int> NormalizedMask(NormalizedVF, PoisonMaskElem);
         copy(Mask, NormalizedMask.begin());
         unsigned PrevSrcReg = 0;
         ArrayRef<int> PrevRegMask;
@@ -1634,7 +1634,7 @@
                 return;
               }
               if (SrcReg != DestReg &&
-                  any_of(RegMask, [](int I) { return I != UndefMaskElem; })) {
+                  any_of(RegMask, [](int I) { return I != PoisonMaskElem; })) {
                 // Just a copy of the source register.
                 Cost += TTI::TCC_Basic;
               }
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -2728,7 +2728,7 @@
       for (i = 0; i != SubVecNumElts; ++i)
         WidenMask.push_back(i);
       for (; i != VecNumElts; ++i)
-        WidenMask.push_back(UndefMaskElem);
+        WidenMask.push_back(PoisonMaskElem);
 
       Value *WidenShuffle = Builder.CreateShuffleVector(SubVec, WidenMask);
 
@@ -3023,7 +3023,7 @@
     int Sz = Mask.size();
     SmallBitVector UsedIndices(Sz);
     for (int Idx : Mask) {
-      if (Idx == UndefMaskElem || UsedIndices.test(Idx))
+      if (Idx == PoisonMaskElem || UsedIndices.test(Idx))
         break;
       UsedIndices.set(Idx);
     }
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2496,7 +2496,7 @@
   // in the case of a shuffle with no undefined mask elements.
   ArrayRef<int> Mask;
   if (match(TVal, m_OneUse(m_Shuffle(m_Value(X), m_Value(Y), m_Mask(Mask)))) &&
-      !is_contained(Mask, UndefMaskElem) &&
+      !is_contained(Mask, PoisonMaskElem) &&
       cast<ShuffleVectorInst>(TVal)->isSelect()) {
     if (X == FVal) {
       // select Cond, (shuf_sel X, Y), X --> shuf_sel X, (select Cond, Y, X)
@@ -2510,7 +2510,7 @@
     }
   }
   if (match(FVal, m_OneUse(m_Shuffle(m_Value(X), m_Value(Y), m_Mask(Mask)))) &&
-      !is_contained(Mask, UndefMaskElem) &&
+      !is_contained(Mask, PoisonMaskElem) &&
       cast<ShuffleVectorInst>(FVal)->isSelect()) {
     if (X == TVal) {
       // select Cond, X, (shuf_sel X, Y) --> shuf_sel X, (select Cond, X, Y)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1553,7 +1553,7 @@
       SmallVector<int, 16> Elts;
       for (unsigned i = 0; i < VWidth; ++i) {
         if (UndefElts[i])
-          Elts.push_back(UndefMaskElem);
+          Elts.push_back(PoisonMaskElem);
         else
           Elts.push_back(Shuffle->getMaskValue(i));
       }
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -1344,7 +1344,7 @@
       // (demanded elements analysis may unset it later).
       return nullptr;
     } else {
-      assert(OldMask[i] == UndefMaskElem &&
+      assert(OldMask[i] == PoisonMaskElem &&
              "Unexpected shuffle mask element for identity shuffle");
       NewMask[i] = IdxC;
     }
@@ -2145,7 +2145,7 @@
                                 ConstantExpr::getShuffleVector(IdC, C, Mask);
 
   bool MightCreatePoisonOrUB =
-      is_contained(Mask, UndefMaskElem) &&
+      is_contained(Mask, PoisonMaskElem) &&
       (Instruction::isIntDivRem(BOpcode) || Instruction::isShift(BOpcode));
   if (MightCreatePoisonOrUB)
     NewC = InstCombiner::getSafeVectorConstantForBinop(BOpcode, NewC, true);
@@ -2159,7 +2159,7 @@
   // An undef shuffle mask element may propagate as an undef constant element in
   // the new binop. That would produce poison where the original code might not.
   // If we already made a safe constant, then there's no danger.
-  if (is_contained(Mask, UndefMaskElem) && !MightCreatePoisonOrUB)
+  if (is_contained(Mask, PoisonMaskElem) && !MightCreatePoisonOrUB)
     NewBO->dropPoisonGeneratingFlags();
   return NewBO;
 }
@@ -2194,7 +2194,7 @@
       cast<FixedVectorType>(Shuf.getType())->getNumElements();
   SmallVector<int, 16> NewMask(NumMaskElts, 0);
   for (unsigned i = 0; i != NumMaskElts; ++i)
-    if (Mask[i] == UndefMaskElem)
+    if (Mask[i] == PoisonMaskElem)
       NewMask[i] = Mask[i];
 
   return new ShuffleVectorInst(NewIns, NewMask);
@@ -2279,7 +2279,7 @@
   // mask element, the result is undefined, but it is not poison or undefined
   // behavior. That is not necessarily true for div/rem/shift.
   bool MightCreatePoisonOrUB =
-      is_contained(Mask, UndefMaskElem) &&
+      is_contained(Mask, PoisonMaskElem) &&
       (Instruction::isIntDivRem(BOpc) || Instruction::isShift(BOpc));
   if (MightCreatePoisonOrUB)
     NewC = InstCombiner::getSafeVectorConstantForBinop(BOpc, NewC,
@@ -2330,7 +2330,7 @@
     NewI->andIRFlags(B1);
     if (DropNSW)
       NewI->setHasNoSignedWrap(false);
-    if (is_contained(Mask, UndefMaskElem) && !MightCreatePoisonOrUB)
+    if (is_contained(Mask, PoisonMaskElem) && !MightCreatePoisonOrUB)
       NewI->dropPoisonGeneratingFlags();
   }
   return replaceInstUsesWith(Shuf, NewBO);
@@ -2366,7 +2366,7 @@
       SrcType->getScalarSizeInBits() / DestType->getScalarSizeInBits();
   ArrayRef<int> Mask = Shuf.getShuffleMask();
   for (unsigned i = 0, e = Mask.size(); i != e; ++i) {
-    if (Mask[i] == UndefMaskElem)
+    if (Mask[i] == PoisonMaskElem)
       continue;
     uint64_t LSBIndex = IsBigEndian ? (i + 1) * TruncRatio - 1 : i * TruncRatio;
     assert(LSBIndex <= INT32_MAX && "Overflowed 32-bits");
@@ -2552,7 +2552,7 @@
   for (unsigned i = 0; i != NumElts; ++i) {
     int ExtractMaskElt = Shuf.getMaskValue(i);
     int MaskElt = Mask[i];
-    NewMask[i] = ExtractMaskElt == UndefMaskElem ? ExtractMaskElt : MaskElt;
+    NewMask[i] = ExtractMaskElt == PoisonMaskElem ? ExtractMaskElt : MaskElt;
   }
   return new ShuffleVectorInst(X, Y, NewMask);
 }
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1707,9 +1707,9 @@
     // TODO: Allow arbitrary shuffles by shuffling after binop?
     //       That might be legal, but we have to deal with poison.
     if (LShuf->isSelect() &&
-        !is_contained(LShuf->getShuffleMask(), UndefMaskElem) &&
+        !is_contained(LShuf->getShuffleMask(), PoisonMaskElem) &&
         RShuf->isSelect() &&
-        !is_contained(RShuf->getShuffleMask(), UndefMaskElem)) {
+        !is_contained(RShuf->getShuffleMask(), PoisonMaskElem)) {
       // Example:
       // LHS = shuffle V1, V2, <0, 5, 6, 3>
       // RHS = shuffle V2, V1, <0, 5, 6, 3>
diff --git a/llvm/lib/Transforms/Scalar/ScalarizeMaskedMemIntrin.cpp b/llvm/lib/Transforms/Scalar/ScalarizeMaskedMemIntrin.cpp
--- a/llvm/lib/Transforms/Scalar/ScalarizeMaskedMemIntrin.cpp
+++ b/llvm/lib/Transforms/Scalar/ScalarizeMaskedMemIntrin.cpp
@@ -658,7 +658,7 @@
   if (isConstantIntVector(Mask)) {
     unsigned MemIndex = 0;
     VResult = PoisonValue::get(VecType);
-    SmallVector<int, 16> ShuffleMask(VectorWidth, UndefMaskElem);
+    SmallVector<int, 16> ShuffleMask(VectorWidth, PoisonMaskElem);
     for (unsigned Idx = 0; Idx < VectorWidth; ++Idx) {
       Value *InsertElt;
       if (cast<Constant>(Mask)->getAggregateElement(Idx)->isNullValue()) {
diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -349,7 +349,7 @@
                                    UseMask MaskArg) {
   SmallBitVector UseMask(VF, true);
   for (auto [Idx, Value] : enumerate(Mask)) {
-    if (Value == UndefMaskElem) {
+    if (Value == PoisonMaskElem) {
       if (MaskArg == UseMask::UndefsAsMask)
         UseMask.reset(Idx);
       continue;
@@ -467,7 +467,7 @@
   Value *Vec2 = nullptr;
   enum ShuffleMode { Unknown, Select, Permute };
   ShuffleMode CommonShuffleMode = Unknown;
-  Mask.assign(VL.size(), UndefMaskElem);
+  Mask.assign(VL.size(), PoisonMaskElem);
   for (unsigned I = 0, E = VL.size(); I < E; ++I) {
     // Undef can be represented as an undef element in a vector.
     if (isa<UndefValue>(VL[I]))
@@ -644,7 +644,7 @@
         !isa<ConstantInt, UndefValue>(EI->getIndexOperand()) ||
         is_contained(UndefVectorExtracts, I))
       continue;
-    if (Mask[I] == UndefMaskElem && !isa<PoisonValue>(GatheredExtracts[I]))
+    if (Mask[I] == PoisonMaskElem && !isa<PoisonValue>(GatheredExtracts[I]))
       std::swap(VL[I], GatheredExtracts[I]);
   }
   return Res;
@@ -958,12 +958,12 @@
     Mask.append(SubMask.begin(), SubMask.end());
     return;
   }
-  SmallVector<int> NewMask(SubMask.size(), UndefMaskElem);
+  SmallVector<int> NewMask(SubMask.size(), PoisonMaskElem);
   int TermValue = std::min(Mask.size(), SubMask.size());
   for (int I = 0, E = SubMask.size(); I < E; ++I) {
     if ((!ExtendingManyInputs &&
          (SubMask[I] >= TermValue || Mask[SubMask[I]] >= TermValue)) ||
-        SubMask[I] == UndefMaskElem)
+        SubMask[I] == PoisonMaskElem)
       continue;
     NewMask[I] = Mask[SubMask[I]];
   }
@@ -1010,7 +1010,7 @@
                                SmallVectorImpl<int> &Mask) {
   Mask.clear();
   const unsigned E = Indices.size();
-  Mask.resize(E, UndefMaskElem);
+  Mask.resize(E, PoisonMaskElem);
   for (unsigned I = 0; I < E; ++I)
     Mask[Indices[I]] = I;
 }
@@ -1023,7 +1023,7 @@
                             UndefValue::get(Scalars.front()->getType()));
   Prev.swap(Scalars);
   for (unsigned I = 0, E = Prev.size(); I < E; ++I)
-    if (Mask[I] != UndefMaskElem)
+    if (Mask[I] != PoisonMaskElem)
       Scalars[Mask[I]] = Prev[I];
 }
 
@@ -2538,8 +2538,8 @@
                std::equal(VL.begin(), VL.end(), Mask.begin(),
                           [Scalars](Value *V, int Idx) {
                             return (isa<UndefValue>(V) &&
-                                    Idx == UndefMaskElem) ||
-                                   (Idx != UndefMaskElem && V == Scalars[Idx]);
+                                    Idx == PoisonMaskElem) ||
+                                   (Idx != PoisonMaskElem && V == Scalars[Idx]);
                           });
       };
       if (!ReorderIndices.empty()) {
@@ -3719,7 +3719,7 @@
   SmallVector<int> Prev(Reuses.begin(), Reuses.end());
   Prev.swap(Reuses);
   for (unsigned I = 0, E = Prev.size(); I < E; ++I)
-    if (Mask[I] != UndefMaskElem)
+    if (Mask[I] != PoisonMaskElem)
       Reuses[Mask[I]] = Prev[I];
 }
 
@@ -3743,7 +3743,7 @@
   }
   Order.assign(Mask.size(), Mask.size());
   for (unsigned I = 0, E = Mask.size(); I < E; ++I)
-    if (MaskOrder[I] != UndefMaskElem)
+    if (MaskOrder[I] != PoisonMaskElem)
       Order[MaskOrder[I]] = I;
   fixupOrderingIndices(Order);
 }
@@ -4078,14 +4078,14 @@
           std::optional<unsigned> Idx = getExtractIndex(cast<Instruction>(V));
           return Idx && *Idx < Sz;
         })) {
-      SmallVector<int> ReorderMask(Sz, UndefMaskElem);
+      SmallVector<int> ReorderMask(Sz, PoisonMaskElem);
       if (TE.ReorderIndices.empty())
         std::iota(ReorderMask.begin(), ReorderMask.end(), 0);
       else
         inversePermutation(TE.ReorderIndices, ReorderMask);
       for (unsigned I = 0; I < VF; ++I) {
         int &Idx = ReusedMask[I];
-        if (Idx == UndefMaskElem)
+        if (Idx == PoisonMaskElem)
           continue;
         Value *V = TE.Scalars[ReorderMask[Idx]];
         std::optional<unsigned> EI = getExtractIndex(cast<Instruction>(V));
@@ -4100,7 +4100,7 @@
     for (unsigned K = 0; K < VF; K += Sz) {
       OrdersType CurrentOrder(TE.ReorderIndices);
       SmallVector<int> SubMask{ArrayRef(ReusedMask).slice(K, Sz)};
-      if (SubMask.front() == UndefMaskElem)
+      if (SubMask.front() == PoisonMaskElem)
         std::iota(SubMask.begin(), SubMask.end(), 0);
       reorderOrder(CurrentOrder, SubMask);
       transform(CurrentOrder, It, [K](unsigned Pos) { return Pos + K; });
@@ -4437,7 +4437,7 @@
         unsigned E = Order.size();
         OrdersType CurrentOrder(E, E);
         transform(Mask, CurrentOrder.begin(), [E](int Idx) {
-          return Idx == UndefMaskElem ? E : static_cast<unsigned>(Idx);
+          return Idx == PoisonMaskElem ? E : static_cast<unsigned>(Idx);
         });
         fixupOrderingIndices(CurrentOrder);
         ++OrdersUses.insert(std::make_pair(CurrentOrder, 0)).first->second;
@@ -4462,10 +4462,10 @@
       continue;
     SmallVector<int> Mask;
     inversePermutation(BestOrder, Mask);
-    SmallVector<int> MaskOrder(BestOrder.size(), UndefMaskElem);
+    SmallVector<int> MaskOrder(BestOrder.size(), PoisonMaskElem);
     unsigned E = BestOrder.size();
     transform(BestOrder, MaskOrder.begin(), [E](unsigned I) {
-      return I < E ? static_cast<int>(I) : UndefMaskElem;
+      return I < E ? static_cast<int>(I) : PoisonMaskElem;
     });
     // Do an actual reordering, if profitable.
     for (std::unique_ptr<TreeEntry> &TE : VectorizableTree) {
@@ -4676,7 +4676,7 @@
           unsigned E = Order.size();
           OrdersType CurrentOrder(E, E);
           transform(Mask, CurrentOrder.begin(), [E](int Idx) {
-            return Idx == UndefMaskElem ? E : static_cast<unsigned>(Idx);
+            return Idx == PoisonMaskElem ? E : static_cast<unsigned>(Idx);
           });
           fixupOrderingIndices(CurrentOrder);
           OrdersUses.insert(std::make_pair(CurrentOrder, 0)).first->second +=
@@ -4755,10 +4755,10 @@
       VisitedOps.clear();
       SmallVector<int> Mask;
       inversePermutation(BestOrder, Mask);
-      SmallVector<int> MaskOrder(BestOrder.size(), UndefMaskElem);
+      SmallVector<int> MaskOrder(BestOrder.size(), PoisonMaskElem);
       unsigned E = BestOrder.size();
       transform(BestOrder, MaskOrder.begin(), [E](unsigned I) {
-        return I < E ? static_cast<int>(I) : UndefMaskElem;
+        return I < E ? static_cast<int>(I) : PoisonMaskElem;
       });
       for (const std::pair<unsigned, TreeEntry *> &Op : Data.second) {
         TreeEntry *TE = Op.second;
@@ -5167,7 +5167,7 @@
     for (Value *V : VL) {
       if (isConstant(V)) {
         ReuseShuffleIndicies.emplace_back(
-            isa<UndefValue>(V) ? UndefMaskElem : UniqueValues.size());
+            isa<UndefValue>(V) ? PoisonMaskElem : UniqueValues.size());
         UniqueValues.emplace_back(V);
         continue;
       }
@@ -6298,7 +6298,7 @@
                       SmallVectorImpl<Value *> *OpScalars = nullptr,
                       SmallVectorImpl<Value *> *AltScalars = nullptr) {
   unsigned Sz = VL.size();
-  Mask.assign(Sz, UndefMaskElem);
+  Mask.assign(Sz, PoisonMaskElem);
   SmallVector<int> OrderMask;
   if (!ReorderIndices.empty())
     inversePermutation(ReorderIndices, OrderMask);
@@ -6318,9 +6318,9 @@
     }
   }
   if (!ReusesIndices.empty()) {
-    SmallVector<int> NewMask(ReusesIndices.size(), UndefMaskElem);
+    SmallVector<int> NewMask(ReusesIndices.size(), PoisonMaskElem);
     transform(ReusesIndices, NewMask.begin(), [&Mask](int Idx) {
-      return Idx != UndefMaskElem ? Mask[Idx] : UndefMaskElem;
+      return Idx != PoisonMaskElem ? Mask[Idx] : PoisonMaskElem;
     });
     Mask.swap(NewMask);
   }
@@ -6440,13 +6440,13 @@
   static void combineMasks(unsigned LocalVF, SmallVectorImpl<int> &Mask,
                            ArrayRef<int> ExtMask) {
     unsigned VF = Mask.size();
-    SmallVector<int> NewMask(ExtMask.size(), UndefMaskElem);
+    SmallVector<int> NewMask(ExtMask.size(), PoisonMaskElem);
     for (int I = 0, Sz = ExtMask.size(); I < Sz; ++I) {
-      if (ExtMask[I] == UndefMaskElem)
+      if (ExtMask[I] == PoisonMaskElem)
         continue;
       int MaskedIdx = Mask[ExtMask[I] % VF];
       NewMask[I] =
-          MaskedIdx == UndefMaskElem ? UndefMaskElem : MaskedIdx % LocalVF;
+          MaskedIdx == PoisonMaskElem ? PoisonMaskElem : MaskedIdx % LocalVF;
     }
     Mask.swap(NewMask);
   }
@@ -6533,9 +6533,9 @@
       if (auto *SVOpTy =
               dyn_cast<FixedVectorType>(SV->getOperand(0)->getType()))
         LocalVF = SVOpTy->getNumElements();
-      SmallVector<int> ExtMask(Mask.size(), UndefMaskElem);
+      SmallVector<int> ExtMask(Mask.size(), PoisonMaskElem);
       for (auto [Idx, I] : enumerate(Mask)) {
-        if (I == UndefMaskElem ||
+        if (I == PoisonMaskElem ||
             static_cast<unsigned>(I) >= SV->getShuffleMask().size())
           continue;
         ExtMask[Idx] = SV->getMaskValue(I);
@@ -6551,11 +6551,11 @@
       if (!IsOp1Undef && !IsOp2Undef) {
         // Update mask and mark undef elems.
         for (int &I : Mask) {
-          if (I == UndefMaskElem)
+          if (I == PoisonMaskElem)
             continue;
           if (SV->getMaskValue(I % SV->getShuffleMask().size()) ==
-              UndefMaskElem)
-            I = UndefMaskElem;
+              PoisonMaskElem)
+            I = PoisonMaskElem;
         }
         break;
       }
@@ -6577,8 +6577,8 @@
                "Expected masks of same sizes.");
         // Clear known poison elements.
         for (auto [I, Idx] : enumerate(Mask))
-          if (Idx == UndefMaskElem)
-            IdentityMask[I] = UndefMaskElem;
+          if (Idx == PoisonMaskElem)
+            IdentityMask[I] = PoisonMaskElem;
         Mask.swap(IdentityMask);
         auto *Shuffle = dyn_cast<ShuffleVectorInst>(V);
         return SinglePermute &&
@@ -6614,8 +6614,8 @@
       Value *Op2 = V2;
       int VF =
           cast<VectorType>(V1->getType())->getElementCount().getKnownMinValue();
-      SmallVector<int> CombinedMask1(Mask.size(), UndefMaskElem);
-      SmallVector<int> CombinedMask2(Mask.size(), UndefMaskElem);
+      SmallVector<int> CombinedMask1(Mask.size(), PoisonMaskElem);
+      SmallVector<int> CombinedMask2(Mask.size(), PoisonMaskElem);
       for (int I = 0, E = Mask.size(); I < E; ++I) {
         if (Mask[I] < VF)
           CombinedMask1[I] = Mask[I];
@@ -6633,9 +6633,9 @@
         // again.
         if (auto *SV1 = dyn_cast<ShuffleVectorInst>(Op1))
           if (auto *SV2 = dyn_cast<ShuffleVectorInst>(Op2)) {
-            SmallVector<int> ExtMask1(Mask.size(), UndefMaskElem);
+            SmallVector<int> ExtMask1(Mask.size(), PoisonMaskElem);
             for (auto [Idx, I] : enumerate(CombinedMask1)) {
-                if (I == UndefMaskElem)
+                if (I == PoisonMaskElem)
                 continue;
                 ExtMask1[Idx] = SV1->getMaskValue(I);
             }
@@ -6643,9 +6643,9 @@
                 cast<FixedVectorType>(SV1->getOperand(1)->getType())
                     ->getNumElements(),
                 ExtMask1, UseMask::SecondArg);
-            SmallVector<int> ExtMask2(CombinedMask2.size(), UndefMaskElem);
+            SmallVector<int> ExtMask2(CombinedMask2.size(), PoisonMaskElem);
             for (auto [Idx, I] : enumerate(CombinedMask2)) {
-                if (I == UndefMaskElem)
+                if (I == PoisonMaskElem)
                 continue;
                 ExtMask2[Idx] = SV2->getMaskValue(I);
             }
@@ -6685,8 +6685,8 @@
                         ->getElementCount()
                         .getKnownMinValue());
       for (int I = 0, E = Mask.size(); I < E; ++I) {
-        if (CombinedMask2[I] != UndefMaskElem) {
-          assert(CombinedMask1[I] == UndefMaskElem &&
+        if (CombinedMask2[I] != PoisonMaskElem) {
+          assert(CombinedMask1[I] == PoisonMaskElem &&
                  "Expected undefined mask element");
           CombinedMask1[I] = CombinedMask2[I] + (Op1 == Op2 ? 0 : VF);
         }
@@ -6880,25 +6880,25 @@
     // Process extracts in blocks of EltsPerVector to check if the source vector
     // operand can be re-used directly. If not, add the cost of creating a
     // shuffle to extract the values into a vector register.
-    SmallVector<int> RegMask(EltsPerVector, UndefMaskElem);
+    SmallVector<int> RegMask(EltsPerVector, PoisonMaskElem);
     for (auto *V : VL) {
       ++Idx;
 
       // Reached the start of a new vector registers.
       if (Idx % EltsPerVector == 0) {
-        RegMask.assign(EltsPerVector, UndefMaskElem);
+        RegMask.assign(EltsPerVector, PoisonMaskElem);
         AllConsecutive = true;
         continue;
       }
 
       // Need to exclude undefs from analysis.
-      if (isa<UndefValue>(V) || Mask[Idx] == UndefMaskElem)
+      if (isa<UndefValue>(V) || Mask[Idx] == PoisonMaskElem)
         continue;
 
       // Check all extracts for a vector register on the target directly
       // extract values in order.
       unsigned CurrentIdx = *getExtractIndex(cast<Instruction>(V));
-      if (!isa<UndefValue>(VL[Idx - 1]) && Mask[Idx - 1] != UndefMaskElem) {
+      if (!isa<UndefValue>(VL[Idx - 1]) && Mask[Idx - 1] != PoisonMaskElem) {
         unsigned PrevIdx = *getExtractIndex(cast<Instruction>(VL[Idx - 1]));
         AllConsecutive &= PrevIdx + 1 == CurrentIdx &&
                           CurrentIdx % EltsPerVector == Idx % EltsPerVector;
@@ -6945,7 +6945,7 @@
     DenseMap<Value *, int> ExtractVectorsTys;
     for (auto [I, V] : enumerate(VL)) {
       // Ignore non-extractelement scalars.
-      if (isa<UndefValue>(V) || (!Mask.empty() && Mask[I] == UndefMaskElem))
+      if (isa<UndefValue>(V) || (!Mask.empty() && Mask[I] == PoisonMaskElem))
         continue;
       // If all users of instruction are going to be vectorized and this
       // instruction itself is not going to be vectorized, consider this
@@ -7167,7 +7167,7 @@
       }
       // Remove shuffled elements from list of gathers.
       for (int I = 0, Sz = Mask.size(); I < Sz; ++I) {
-        if (Mask[I] != UndefMaskElem)
+        if (Mask[I] != PoisonMaskElem)
           GatheredScalars[I] = PoisonValue::get(ScalarTy);
       }
       LLVM_DEBUG(dbgs() << "SLP: shuffled " << Entries.size()
@@ -7398,7 +7398,7 @@
 
     unsigned NumOfParts = TTI->getNumberOfParts(SrcVecTy);
 
-    SmallVector<int> InsertMask(NumElts, UndefMaskElem);
+    SmallVector<int> InsertMask(NumElts, PoisonMaskElem);
     unsigned OffsetBeg = *getInsertIndex(VL.front());
     unsigned OffsetEnd = OffsetBeg;
     InsertMask[OffsetBeg] = 0;
@@ -7435,13 +7435,13 @@
     SmallVector<int> Mask;
     if (!E->ReorderIndices.empty()) {
       inversePermutation(E->ReorderIndices, Mask);
-      Mask.append(InsertVecSz - Mask.size(), UndefMaskElem);
+      Mask.append(InsertVecSz - Mask.size(), PoisonMaskElem);
     } else {
-      Mask.assign(VecSz, UndefMaskElem);
+      Mask.assign(VecSz, PoisonMaskElem);
       std::iota(Mask.begin(), std::next(Mask.begin(), InsertVecSz), 0);
     }
     bool IsIdentity = true;
-    SmallVector<int> PrevMask(InsertVecSz, UndefMaskElem);
+    SmallVector<int> PrevMask(InsertVecSz, PoisonMaskElem);
     Mask.swap(PrevMask);
     for (unsigned I = 0; I < NumScalars; ++I) {
       unsigned InsertIdx = *getInsertIndex(VL[PrevMask[I]]);
@@ -7484,14 +7484,14 @@
                                     InsertVecTy);
       } else {
         for (unsigned I = 0, End = OffsetBeg - Offset; I < End; ++I)
-          Mask[I] = InMask.test(I) ? UndefMaskElem : I;
+          Mask[I] = InMask.test(I) ? PoisonMaskElem : I;
         for (unsigned I = OffsetBeg - Offset, End = OffsetEnd - Offset;
              I <= End; ++I)
-          if (Mask[I] != UndefMaskElem)
+          if (Mask[I] != PoisonMaskElem)
             Mask[I] = I + VecSz;
         for (unsigned I = OffsetEnd + 1 - Offset; I < VecSz; ++I)
           Mask[I] =
-              ((I >= InMask.size()) || InMask.test(I)) ? UndefMaskElem : I;
+              ((I >= InMask.size()) || InMask.test(I)) ? PoisonMaskElem : I;
         Cost += TTI->getShuffleCost(TTI::SK_PermuteTwoSrc, InsertVecTy, Mask);
       }
     }
@@ -8133,8 +8133,8 @@
         ResizeAction(ShuffleMask.begin()->first, Mask, /*ForSingleMask=*/false);
     SmallBitVector IsBasePoison = isUndefVector<true>(Base, UseMask);
     for (unsigned Idx = 0, VF = Mask.size(); Idx < VF; ++Idx) {
-      if (Mask[Idx] == UndefMaskElem)
-        Mask[Idx] = IsBasePoison.test(Idx) ? UndefMaskElem : Idx;
+      if (Mask[Idx] == PoisonMaskElem)
+        Mask[Idx] = IsBasePoison.test(Idx) ? PoisonMaskElem : Idx;
       else
         Mask[Idx] = (Res.second ? Idx : Mask[Idx]) + VF;
     }
@@ -8163,8 +8163,8 @@
       // can shuffle them directly.
       ArrayRef<int> SecMask = VMIt->second;
       for (unsigned I = 0, VF = Mask.size(); I < VF; ++I) {
-        if (SecMask[I] != UndefMaskElem) {
-          assert(Mask[I] == UndefMaskElem && "Multiple uses of scalars.");
+        if (SecMask[I] != PoisonMaskElem) {
+          assert(Mask[I] == PoisonMaskElem && "Multiple uses of scalars.");
           Mask[I] = SecMask[I] + Vec1VF;
         }
       }
@@ -8177,12 +8177,12 @@
           ResizeAction(VMIt->first, VMIt->second, /*ForSingleMask=*/false);
       ArrayRef<int> SecMask = VMIt->second;
       for (unsigned I = 0, VF = Mask.size(); I < VF; ++I) {
-        if (Mask[I] != UndefMaskElem) {
-          assert(SecMask[I] == UndefMaskElem && "Multiple uses of scalars.");
+        if (Mask[I] != PoisonMaskElem) {
+          assert(SecMask[I] == PoisonMaskElem && "Multiple uses of scalars.");
           if (Res1.second)
             Mask[I] = I;
-        } else if (SecMask[I] != UndefMaskElem) {
-          assert(Mask[I] == UndefMaskElem && "Multiple uses of scalars.");
+        } else if (SecMask[I] != PoisonMaskElem) {
+          assert(Mask[I] == PoisonMaskElem && "Multiple uses of scalars.");
           Mask[I] = (Res2.second ? I : SecMask[I]) + VF;
         }
       }
@@ -8199,11 +8199,11 @@
         ResizeAction(VMIt->first, VMIt->second, /*ForSingleMask=*/false);
     ArrayRef<int> SecMask = VMIt->second;
     for (unsigned I = 0, VF = Mask.size(); I < VF; ++I) {
-      if (SecMask[I] != UndefMaskElem) {
-        assert((Mask[I] == UndefMaskElem || IsBaseNotUndef) &&
+      if (SecMask[I] != PoisonMaskElem) {
+        assert((Mask[I] == PoisonMaskElem || IsBaseNotUndef) &&
                "Multiple uses of scalars.");
         Mask[I] = (Res.second ? I : SecMask[I]) + VF;
-      } else if (Mask[I] != UndefMaskElem) {
+      } else if (Mask[I] != PoisonMaskElem) {
         Mask[I] = I;
       }
     }
@@ -8298,7 +8298,7 @@
             (void)ShuffleMasks.emplace_back();
             SmallVectorImpl<int> &Mask = ShuffleMasks.back()[ScalarTE];
             if (Mask.empty())
-              Mask.assign(FTy->getNumElements(), UndefMaskElem);
+              Mask.assign(FTy->getNumElements(), PoisonMaskElem);
             // Find the insertvector, vectorized in tree, if any.
             Value *Base = VU;
             while (auto *IEBase = dyn_cast<InsertElementInst>(Base)) {
@@ -8312,7 +8312,7 @@
                 do {
                   IEBase = cast<InsertElementInst>(Base);
                   int Idx = *getInsertIndex(IEBase);
-                  assert(Mask[Idx] == UndefMaskElem &&
+                  assert(Mask[Idx] == PoisonMaskElem &&
                          "InsertElementInstruction used already.");
                   Mask[Idx] = Idx;
                   Base = IEBase->getOperand(0);
@@ -8332,7 +8332,7 @@
           int InIdx = *InsertIdx;
           SmallVectorImpl<int> &Mask = ShuffleMasks[VecId][ScalarTE];
           if (Mask.empty())
-            Mask.assign(FTy->getNumElements(), UndefMaskElem);
+            Mask.assign(FTy->getNumElements(), PoisonMaskElem);
           Mask[InIdx] = EU.Lane;
           DemandedElts[VecId].setBit(InIdx);
           continue;
@@ -8371,7 +8371,7 @@
          (all_of(Mask,
                  [VF](int Idx) { return Idx < 2 * static_cast<int>(VF); }) &&
           !ShuffleVectorInst::isIdentityMask(Mask)))) {
-      SmallVector<int> OrigMask(VecVF, UndefMaskElem);
+      SmallVector<int> OrigMask(VecVF, PoisonMaskElem);
       std::copy(Mask.begin(), std::next(Mask.begin(), std::min(VF, VecVF)),
                 OrigMask.begin());
       C = TTI->getShuffleCost(
@@ -8457,7 +8457,7 @@
   // No need to check for the topmost gather node.
   if (TE == VectorizableTree.front().get())
     return std::nullopt;
-  Mask.assign(VL.size(), UndefMaskElem);
+  Mask.assign(VL.size(), PoisonMaskElem);
   assert(TE->UserTreeIndices.size() == 1 &&
          "Expected only single user of the gather node.");
   // TODO: currently checking only for Scalars in the tree entry, need to count
@@ -8620,7 +8620,7 @@
       // Clear undef scalars.
       for (int I = 0, Sz = VL.size(); I < Sz; ++I)
         if (isa<PoisonValue>(VL[I]))
-          Mask[I] = UndefMaskElem;
+          Mask[I] = PoisonMaskElem;
       return TargetTransformInfo::SK_PermuteSingleSrc;
     }
     // No perfect match, just shuffle, so choose the first tree node from the
@@ -9069,7 +9069,7 @@
       if (isa<PoisonValue>(VL[I]))
         continue;
       if (auto *SV = dyn_cast<ShuffleVectorInst>(Root)) {
-        if (SV->getMaskValue(I) == UndefMaskElem)
+        if (SV->getMaskValue(I) == PoisonMaskElem)
           continue;
       }
     }
@@ -9180,7 +9180,7 @@
       int V2VF = cast<FixedVectorType>(V2->getType())->getNumElements();
       int VF = std::max(V1VF, V2VF);
       int MinVF = std::min(V1VF, V2VF);
-      SmallVector<int> IdentityMask(VF, UndefMaskElem);
+      SmallVector<int> IdentityMask(VF, PoisonMaskElem);
       std::iota(IdentityMask.begin(), std::next(IdentityMask.begin(), MinVF),
                 0);
       Value *&Op = MinVF == V1VF ? V1 : V2;
@@ -9211,7 +9211,7 @@
   static void transformMaskAfterShuffle(MutableArrayRef<int> CommonMask,
                                         ArrayRef<int> Mask) {
     for (unsigned Idx = 0, Sz = CommonMask.size(); Idx < Sz; ++Idx)
-      if (Mask[Idx] != UndefMaskElem)
+      if (Mask[Idx] != PoisonMaskElem)
         CommonMask[Idx] = Idx;
   }
 
@@ -9224,7 +9224,7 @@
     Value *VecBase = nullptr;
     for (int I = 0, Sz = Mask.size(); I < Sz; ++I) {
       int Idx = Mask[I];
-      if (Idx == UndefMaskElem)
+      if (Idx == PoisonMaskElem)
         continue;
       auto *EI = cast<ExtractElementInst>(E->Scalars[I]);
       VecBase = EI->getVectorOperand();
@@ -9272,7 +9272,7 @@
     }
     V1 = createShuffle(V1, V2, Mask);
     for (unsigned Idx = 0, Sz = CommonMask.size(); Idx < Sz; ++Idx)
-      if (Mask[Idx] != UndefMaskElem)
+      if (Mask[Idx] != PoisonMaskElem)
         CommonMask[Idx] = Idx + Sz;
     InVectors.front() = Vec;
     if (InVectors.size() == 2)
@@ -9285,7 +9285,7 @@
     if (InVectors.empty()) {
       if (!isa<FixedVectorType>(V1->getType())) {
         V1 = createShuffle(V1, nullptr, CommonMask);
-        CommonMask.assign(Mask.size(), UndefMaskElem);
+        CommonMask.assign(Mask.size(), PoisonMaskElem);
         transformMaskAfterShuffle(CommonMask, Mask);
       }
       InVectors.push_back(V1);
@@ -9307,7 +9307,7 @@
           transformMaskAfterShuffle(CommonMask, CommonMask);
         }
         for (unsigned Idx = 0, Sz = CommonMask.size(); Idx < Sz; ++Idx)
-          if (CommonMask[Idx] == UndefMaskElem && Mask[Idx] != UndefMaskElem)
+          if (CommonMask[Idx] == PoisonMaskElem && Mask[Idx] != PoisonMaskElem)
             CommonMask[Idx] =
                 V->getType() != V1->getType()
                     ? Idx + Sz
@@ -9325,7 +9325,7 @@
       // Check if second vector is required if the used elements are already
       // used from the first one.
       for (unsigned Idx = 0, Sz = CommonMask.size(); Idx < Sz; ++Idx)
-        if (Mask[Idx] != UndefMaskElem && CommonMask[Idx] == UndefMaskElem) {
+        if (Mask[Idx] != PoisonMaskElem && CommonMask[Idx] == PoisonMaskElem) {
           InVectors.push_back(V1);
           break;
         }
@@ -9334,7 +9334,7 @@
     if (auto *FTy = dyn_cast<FixedVectorType>(V1->getType()))
       VF = FTy->getNumElements();
     for (unsigned Idx = 0, Sz = CommonMask.size(); Idx < Sz; ++Idx)
-      if (Mask[Idx] != UndefMaskElem && CommonMask[Idx] == UndefMaskElem)
+      if (Mask[Idx] != PoisonMaskElem && CommonMask[Idx] == PoisonMaskElem)
         CommonMask[Idx] = Mask[Idx] + (It == InVectors.begin() ? 0 : VF);
   }
   /// Adds another one input vector and the mask for the shuffling.
@@ -9359,13 +9359,13 @@
         Vec = createShuffle(Vec, nullptr, CommonMask);
       }
       for (unsigned Idx = 0, Sz = CommonMask.size(); Idx < Sz; ++Idx)
-        if (CommonMask[Idx] != UndefMaskElem)
+        if (CommonMask[Idx] != PoisonMaskElem)
           CommonMask[Idx] = Idx;
       assert(VF > 0 &&
              "Expected vector length for the final value before action.");
       unsigned VecVF = cast<FixedVectorType>(Vec->getType())->getNumElements();
       if (VecVF < VF) {
-        SmallVector<int> ResizeMask(VF, UndefMaskElem);
+        SmallVector<int> ResizeMask(VF, PoisonMaskElem);
         std::iota(ResizeMask.begin(), std::next(ResizeMask.begin(), VecVF), 0);
         Vec = createShuffle(Vec, nullptr, ResizeMask);
       }
@@ -9376,9 +9376,9 @@
       if (CommonMask.empty()) {
         CommonMask.assign(ExtMask.begin(), ExtMask.end());
       } else {
-        SmallVector<int> NewMask(ExtMask.size(), UndefMaskElem);
+        SmallVector<int> NewMask(ExtMask.size(), PoisonMaskElem);
         for (int I = 0, Sz = ExtMask.size(); I < Sz; ++I) {
-          if (ExtMask[I] == UndefMaskElem)
+          if (ExtMask[I] == PoisonMaskElem)
             continue;
           NewMask[I] = CommonMask[ExtMask[I]];
         }
@@ -9449,18 +9449,18 @@
           // ... (use %2)
           // %shuffle = shuffle <2 x> %2, poison, <2 x> {2, 0}
           // br %block
-          SmallVector<int> UniqueIdxs(VF, UndefMaskElem);
+          SmallVector<int> UniqueIdxs(VF, PoisonMaskElem);
           SmallSet<int, 4> UsedIdxs;
           int Pos = 0;
           for (int Idx : VE->ReuseShuffleIndices) {
-            if (Idx != static_cast<int>(VF) && Idx != UndefMaskElem &&
+            if (Idx != static_cast<int>(VF) && Idx != PoisonMaskElem &&
                 UsedIdxs.insert(Idx).second)
               UniqueIdxs[Idx] = Pos;
             ++Pos;
           }
           assert(VF >= UsedIdxs.size() && "Expected vectorization factor "
                                           "less than original vector size.");
-          UniqueIdxs.append(VF - UsedIdxs.size(), UndefMaskElem);
+          UniqueIdxs.append(VF - UsedIdxs.size(), PoisonMaskElem);
           V = FinalShuffle(V, UniqueIdxs);
         } else {
           assert(VF < cast<FixedVectorType>(V->getType())->getNumElements() &&
@@ -9543,7 +9543,7 @@
     if (It == VectorizableTree.end())
       return false;
     unsigned I =
-        *find_if_not(Mask, [](int Idx) { return Idx == UndefMaskElem; });
+        *find_if_not(Mask, [](int Idx) { return Idx == PoisonMaskElem; });
     int Sz = Mask.size();
     if (all_of(Mask, [Sz](int Idx) { return Idx < 2 * Sz; }) &&
         ShuffleVectorInst::isIdentityMask(Mask))
@@ -9603,10 +9603,10 @@
         // Restore the mask for previous partially matched values.
         for (auto [I, V] : enumerate(E->Scalars)) {
           if (isa<PoisonValue>(V)) {
-            Mask[I] = UndefMaskElem;
+            Mask[I] = PoisonMaskElem;
             continue;
           }
-          if (Mask[I] == UndefMaskElem)
+          if (Mask[I] == PoisonMaskElem)
             Mask[I] = Entries.front()->findLaneForValue(V);
         }
         ShuffleBuilder.add(Entries.front()->VectorizedValue, Mask);
@@ -9622,7 +9622,7 @@
       }
       // Remove shuffled elements from list of gathers.
       for (int I = 0, Sz = Mask.size(); I < Sz; ++I) {
-        if (Mask[I] != UndefMaskElem)
+        if (Mask[I] != PoisonMaskElem)
           GatheredScalars[I] = PoisonValue::get(ScalarTy);
       }
     }
@@ -9669,7 +9669,7 @@
     if (NumNonConsts == 1) {
       // Restore single insert element.
       if (IsSplat) {
-        ReuseMask.assign(VF, UndefMaskElem);
+        ReuseMask.assign(VF, PoisonMaskElem);
         std::swap(Scalars.front(), Scalars[SinglePos]);
         if (!UndefPos.empty() && UndefPos.front() == 0)
           Scalars.front() = UndefValue::get(ScalarTy);
@@ -9708,7 +9708,7 @@
         // Replace undefs by the poisons, emit broadcast and then emit
         // freeze.
         for_each(UndefPos, [&](int I) {
-          ReuseMask[I] = UndefMaskElem;
+          ReuseMask[I] = PoisonMaskElem;
           if (isa<UndefValue>(Scalars[I]))
             Scalars[I] = PoisonValue::get(ScalarTy);
         });
@@ -9726,9 +9726,9 @@
       // Find input vectors.
       Value *Vec2 = nullptr;
       for (unsigned I = 0, Sz = ExtractMask.size(); I < Sz; ++I) {
-        if (ExtractMask[I] == UndefMaskElem ||
-            (!Mask.empty() && Mask[I] != UndefMaskElem)) {
-          ExtractMask[I] = UndefMaskElem;
+        if (ExtractMask[I] == PoisonMaskElem ||
+            (!Mask.empty() && Mask[I] != PoisonMaskElem)) {
+          ExtractMask[I] = PoisonMaskElem;
           continue;
         }
         if (isa<UndefValue>(E->Scalars[I]))
@@ -9815,7 +9815,7 @@
     }
     // Generate constants for final shuffle and build a mask for them.
     if (!all_of(GatheredScalars, PoisonValue::classof)) {
-      SmallVector<int> BVMask(GatheredScalars.size(), UndefMaskElem);
+      SmallVector<int> BVMask(GatheredScalars.size(), PoisonMaskElem);
       TryPackScalars(GatheredScalars, BVMask, /*IsRootPoison=*/true);
       Value *BV = gather(GatheredScalars);
       ShuffleBuilder.add(BV, BVMask);
@@ -9835,14 +9835,14 @@
           });
   } else if (!allConstant(GatheredScalars)) {
     // Gather unique scalars and all constants.
-    SmallVector<int> ReuseMask(GatheredScalars.size(), UndefMaskElem);
+    SmallVector<int> ReuseMask(GatheredScalars.size(), PoisonMaskElem);
     TryPackScalars(GatheredScalars, ReuseMask, /*IsRootPoison=*/true);
     Vec = gather(GatheredScalars);
     ShuffleBuilder.add(Vec, ReuseMask);
     Vec = ShuffleBuilder.finalize(E->ReuseShuffleIndices);
   } else {
     // Gather all constants.
-    SmallVector<int> Mask(E->Scalars.size(), UndefMaskElem);
+    SmallVector<int> Mask(E->Scalars.size(), PoisonMaskElem);
     for (auto [I, V] : enumerate(E->Scalars)) {
       if (!isa<PoisonValue>(V))
         Mask[I] = I;
@@ -9986,14 +9986,14 @@
       SmallVector<int> Mask;
       if (!E->ReorderIndices.empty()) {
         inversePermutation(E->ReorderIndices, Mask);
-        Mask.append(NumElts - NumScalars, UndefMaskElem);
+        Mask.append(NumElts - NumScalars, PoisonMaskElem);
       } else {
-        Mask.assign(NumElts, UndefMaskElem);
+        Mask.assign(NumElts, PoisonMaskElem);
         std::iota(Mask.begin(), std::next(Mask.begin(), NumScalars), 0);
       }
       // Create InsertVector shuffle if necessary
       bool IsIdentity = true;
-      SmallVector<int> PrevMask(NumElts, UndefMaskElem);
+      SmallVector<int> PrevMask(NumElts, PoisonMaskElem);
       Mask.swap(PrevMask);
       for (unsigned I = 0; I < NumScalars; ++I) {
         Value *Scalar = E->Scalars[PrevMask[I]];
@@ -10009,9 +10009,9 @@
         }
       }
 
-      SmallVector<int> InsertMask(NumElts, UndefMaskElem);
+      SmallVector<int> InsertMask(NumElts, PoisonMaskElem);
       for (unsigned I = 0; I < NumElts; I++) {
-        if (Mask[I] != UndefMaskElem)
+        if (Mask[I] != PoisonMaskElem)
           InsertMask[Offset + I] = I;
       }
       SmallBitVector UseMask =
@@ -10026,7 +10026,7 @@
               isUndefVector<true>(FirstInsert->getOperand(0), UseMask);
           if (!IsFirstPoison.all()) {
             for (unsigned I = 0; I < NumElts; I++) {
-              if (InsertMask[I] == UndefMaskElem && !IsFirstPoison.test(I))
+              if (InsertMask[I] == PoisonMaskElem && !IsFirstPoison.test(I))
                 InsertMask[I] = I + NumElts;
             }
             }
@@ -10044,8 +10044,8 @@
           SmallBitVector IsFirstPoison =
               isUndefVector<true>(FirstInsert->getOperand(0), UseMask);
           for (unsigned I = 0; I < NumElts; I++) {
-            if (InsertMask[I] == UndefMaskElem)
-              InsertMask[I] = IsFirstPoison.test(I) ? UndefMaskElem : I;
+            if (InsertMask[I] == PoisonMaskElem)
+              InsertMask[I] = IsFirstPoison.test(I) ? PoisonMaskElem : I;
             else
               InsertMask[I] += NumElts;
           }
@@ -10707,7 +10707,7 @@
                              ShuffledInserts.size() - 1);
               SmallVectorImpl<int> &Mask = It->ValueMasks[Vec];
               if (Mask.empty())
-                Mask.assign(FTy->getNumElements(), UndefMaskElem);
+                Mask.assign(FTy->getNumElements(), PoisonMaskElem);
               // Find the insertvector, vectorized in tree, if any.
               Value *Base = VU;
               while (auto *IEBase = dyn_cast<InsertElementInst>(Base)) {
@@ -10720,7 +10720,7 @@
                   do {
                     IEBase = cast<InsertElementInst>(Base);
                     int IEIdx = *getInsertIndex(IEBase);
-                    assert(Mask[Idx] == UndefMaskElem &&
+                    assert(Mask[Idx] == PoisonMaskElem &&
                            "InsertElementInstruction used already.");
                     Mask[IEIdx] = IEIdx;
                     Base = IEBase->getOperand(0);
@@ -10738,7 +10738,7 @@
             }
             SmallVectorImpl<int> &Mask = It->ValueMasks[Vec];
             if (Mask.empty())
-              Mask.assign(FTy->getNumElements(), UndefMaskElem);
+              Mask.assign(FTy->getNumElements(), PoisonMaskElem);
             Mask[Idx] = ExternalUse.Lane;
             It->InsertElements.push_back(cast<InsertElementInst>(User));
             continue;
@@ -10780,8 +10780,8 @@
   }
 
   auto CreateShuffle = [&](Value *V1, Value *V2, ArrayRef<int> Mask) {
-    SmallVector<int> CombinedMask1(Mask.size(), UndefMaskElem);
-    SmallVector<int> CombinedMask2(Mask.size(), UndefMaskElem);
+    SmallVector<int> CombinedMask1(Mask.size(), PoisonMaskElem);
+    SmallVector<int> CombinedMask2(Mask.size(), PoisonMaskElem);
     int VF = cast<FixedVectorType>(V1->getType())->getNumElements();
     for (int I = 0, E = Mask.size(); I < E; ++I) {
       if (Mask[I] < VF)
@@ -10806,9 +10806,9 @@
         return std::make_pair(Vec, true);
       }
       if (!ForSingleMask) {
-        SmallVector<int> ResizeMask(VF, UndefMaskElem);
+        SmallVector<int> ResizeMask(VF, PoisonMaskElem);
         for (unsigned I = 0; I < VF; ++I) {
-          if (Mask[I] != UndefMaskElem)
+          if (Mask[I] != PoisonMaskElem)
             ResizeMask[Mask[I]] = Mask[I];
         }
         Vec = CreateShuffle(Vec, nullptr, ResizeMask);
@@ -11011,14 +11011,14 @@
     // registers.
     unsigned LastUndefsCnt = 0;
     for (int I = 0, E = NewMask.size(); I < E; ++I) {
-      if (SM1[I] == UndefMaskElem)
+      if (SM1[I] == PoisonMaskElem)
         ++LastUndefsCnt;
       else
         LastUndefsCnt = 0;
-      if (NewMask[I] != UndefMaskElem && SM1[I] != UndefMaskElem &&
+      if (NewMask[I] != PoisonMaskElem && SM1[I] != PoisonMaskElem &&
           NewMask[I] != SM1[I])
         return false;
-      if (NewMask[I] == UndefMaskElem)
+      if (NewMask[I] == PoisonMaskElem)
         NewMask[I] = SM1[I];
     }
     // Check if the last undefs actually change the final number of used vector
@@ -13804,7 +13804,7 @@
       // 7>, if elements 4th and 6th elements have even number of repeats.
       SmallVector<int> Mask(
           cast<FixedVectorType>(VectorizedValue->getType())->getNumElements(),
-          UndefMaskElem);
+          PoisonMaskElem);
       std::iota(Mask.begin(), Mask.end(), 0);
       bool NeedShuffle = false;
       for (unsigned I = 0, VF = VL.size(); I < VF; ++I) {
diff --git a/llvm/lib/Transforms/Vectorize/VectorCombine.cpp b/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
--- a/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
+++ b/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
@@ -244,7 +244,7 @@
   // still need a shuffle to change the vector size.
   auto *Ty = cast<FixedVectorType>(I.getType());
   unsigned OutputNumElts = Ty->getNumElements();
-  SmallVector<int, 16> Mask(OutputNumElts, UndefMaskElem);
+  SmallVector<int, 16> Mask(OutputNumElts, PoisonMaskElem);
   assert(OffsetEltIndex < MinVecNumElts && "Address offset too big");
   Mask[0] = OffsetEltIndex;
   if (OffsetEltIndex)
@@ -480,7 +480,7 @@
   // to the new element index. Example for OldIndex == 2 and NewIndex == 0:
   // ShufMask = { 2, undef, undef, undef }
   auto *VecTy = cast<FixedVectorType>(Vec->getType());
-  SmallVector<int, 32> ShufMask(VecTy->getNumElements(), UndefMaskElem);
+  SmallVector<int, 32> ShufMask(VecTy->getNumElements(), PoisonMaskElem);
   ShufMask[NewIndex] = OldIndex;
   return Builder.CreateShuffleVector(Vec, ShufMask, "shift");
 }
@@ -914,7 +914,7 @@
   auto *CmpTy = cast<FixedVectorType>(CmpInst::makeCmpResultType(X->getType()));
   InstructionCost NewCost = TTI.getCmpSelInstrCost(
       CmpOpcode, X->getType(), CmpInst::makeCmpResultType(X->getType()), Pred);
-  SmallVector<int, 32> ShufMask(VecTy->getNumElements(), UndefMaskElem);
+  SmallVector<int, 32> ShufMask(VecTy->getNumElements(), PoisonMaskElem);
   ShufMask[CheapIndex] = ExpensiveIndex;
   NewCost += TTI.getShuffleCost(TargetTransformInfo::SK_PermuteSingleSrc, CmpTy,
                                 ShufMask);
@@ -1593,12 +1593,12 @@
     V2B.push_back(GetBaseMaskValue(SVI1B, V2[I].first));
   }
   while (V1A.size() < NumElts) {
-    V1A.push_back(UndefMaskElem);
-    V1B.push_back(UndefMaskElem);
+    V1A.push_back(PoisonMaskElem);
+    V1B.push_back(PoisonMaskElem);
   }
   while (V2A.size() < NumElts) {
-    V2A.push_back(UndefMaskElem);
-    V2B.push_back(UndefMaskElem);
+    V2A.push_back(PoisonMaskElem);
+    V2B.push_back(PoisonMaskElem);
   }
 
   auto AddShuffleCost = [&](InstructionCost C, Instruction *I) {