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
@@ -4750,7 +4750,8 @@
       // Keeps the reordered operands to avoid code duplication.
       SmallVector<ValueList, 2> OperandsVec;
       for (unsigned I = 0, E = PH->getNumIncomingValues(); I < E; ++I) {
-        if (!DT->isReachableFromEntry(PH->getIncomingBlock(I))) {
+        if (!DT->isReachableFromEntry(PH->getIncomingBlock(I)) &&
+            !isa<UndefValue>(PH->getIncomingValue(I))) {
           ValueList Operands(VL.size(), PoisonValue::get(PH->getType()));
           TE->setOperand(I, Operands);
           OperandsVec.push_back(Operands);
diff --git a/llvm/test/Transforms/SLPVectorizer/X86/malformed_phis.ll b/llvm/test/Transforms/SLPVectorizer/X86/malformed_phis.ll
--- a/llvm/test/Transforms/SLPVectorizer/X86/malformed_phis.ll
+++ b/llvm/test/Transforms/SLPVectorizer/X86/malformed_phis.ll
@@ -116,7 +116,7 @@
 ; CHECK:       bb2:
 ; CHECK-NEXT:    br label [[BB3]]
 ; CHECK:       bb3:
-; CHECK-NEXT:    [[TMP0:%.*]] = phi <2 x i32> [ undef, [[BB1]] ], [ poison, [[BB2:%.*]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = phi <2 x i32> [ undef, [[BB1]] ], [ undef, [[BB2:%.*]] ]
 ; CHECK-NEXT:    [[SHUFFLE:%.*]] = shufflevector <2 x i32> [[TMP0]], <2 x i32> poison, <8 x i32> <i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 1>
 ; CHECK-NEXT:    [[TMP1:%.*]] = extractelement <8 x i32> [[SHUFFLE]], i32 0
 ; CHECK-NEXT:    [[TMP2:%.*]] = insertelement <16 x i32> poison, i32 [[TMP1]], i32 0
diff --git a/llvm/test/Transforms/SLPVectorizer/X86/phi-unreachable-input-block.ll b/llvm/test/Transforms/SLPVectorizer/X86/phi-unreachable-input-block.ll
--- a/llvm/test/Transforms/SLPVectorizer/X86/phi-unreachable-input-block.ll
+++ b/llvm/test/Transforms/SLPVectorizer/X86/phi-unreachable-input-block.ll
@@ -15,7 +15,7 @@
 ; CHECK:       bb2:
 ; CHECK-NEXT:    br label [[BB3]]
 ; CHECK:       bb3:
-; CHECK-NEXT:    [[TMP2:%.*]] = phi <2 x i8> [ [[TMP1]], [[BB1:%.*]] ], [ poison, [[BB2:%.*]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = phi <2 x i8> [ [[TMP1]], [[BB1:%.*]] ], [ <i8 undef, i8 0>, [[BB2:%.*]] ]
 ; CHECK-NEXT:    [[TMP3:%.*]] = zext <2 x i8> [[TMP2]] to <2 x i64>
 ; CHECK-NEXT:    [[GEP0:%.*]] = getelementptr i64, i64* [[PTR:%.*]], i64 0
 ; CHECK-NEXT:    [[TMP4:%.*]] = bitcast i64* [[GEP0]] to <2 x i64>*