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
@@ -7556,6 +7556,8 @@
       if (UniqueValues.empty()) {
         assert(all_of(VL, UndefValue::classof) && "Expected list of undefs.");
         NumValues = VF;
+      } else if (UniqueValues.size() == 1 && NumValues == 1) {
+        NumValues = VF;
       }
       ReuseShuffleIndicies.clear();
       UniqueValues.clear();
diff --git a/llvm/test/Transforms/SLPVectorizer/X86/phi-undef-input.ll b/llvm/test/Transforms/SLPVectorizer/X86/phi-undef-input.ll
--- a/llvm/test/Transforms/SLPVectorizer/X86/phi-undef-input.ll
+++ b/llvm/test/Transforms/SLPVectorizer/X86/phi-undef-input.ll
@@ -14,7 +14,7 @@
 ; CHECK-NEXT:    [[TMP3:%.*]] = insertelement <4 x i8> [[TMP2]], i8 [[ARG3:%.*]], i32 3
 ; CHECK-NEXT:    br label [[BB3]]
 ; CHECK:       bb3:
-; CHECK-NEXT:    [[TMP4:%.*]] = phi <4 x i8> [ [[TMP3]], [[BB2]] ], [ <i8 0, i8 poison, i8 poison, i8 poison>, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[TMP4:%.*]] = phi <4 x i8> [ [[TMP3]], [[BB2]] ], [ <i8 0, i8 undef, i8 undef, i8 undef>, [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    [[TMP5:%.*]] = zext <4 x i8> [[TMP4]] to <4 x i32>
 ; CHECK-NEXT:    [[TMP6:%.*]] = call i32 @llvm.vector.reduce.or.v4i32(<4 x i32> [[TMP5]])
 ; CHECK-NEXT:    ret i32 [[TMP6]]