diff --git a/clang/test/CodeGen/arm-mve-intrinsics/dup.c b/clang/test/CodeGen/arm-mve-intrinsics/dup.c
--- a/clang/test/CodeGen/arm-mve-intrinsics/dup.c
+++ b/clang/test/CodeGen/arm-mve-intrinsics/dup.c
@@ -242,7 +242,8 @@
 // CHECK-NEXT:    [[TMP1:%.*]] = call <8 x i1> @llvm.arm.mve.pred.i2v.v8i1(i32 [[TMP0]])
 // CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <8 x half> undef, half [[A:%.*]], i32 0
 // CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <8 x half> [[DOTSPLATINSERT]], <8 x half> undef, <8 x i32> zeroinitializer
-// CHECK-NEXT:    ret <8 x half> [[DOTSPLAT]]
+// CHECK-NEXT:    [[TMP2:%.*]] = select <8 x i1> [[TMP1]], <8 x half> [[DOTSPLAT]], <8 x half> undef
+// CHECK-NEXT:    ret <8 x half> [[TMP2]]
 //
 float16x8_t test_vdupq_x_n_f16(float16_t a, mve_pred16_t p)
 {
@@ -255,7 +256,8 @@
 // CHECK-NEXT:    [[TMP1:%.*]] = call <4 x i1> @llvm.arm.mve.pred.i2v.v4i1(i32 [[TMP0]])
 // CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <4 x float> undef, float [[A:%.*]], i32 0
 // CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <4 x float> [[DOTSPLATINSERT]], <4 x float> undef, <4 x i32> zeroinitializer
-// CHECK-NEXT:    ret <4 x float> [[DOTSPLAT]]
+// CHECK-NEXT:    [[TMP2:%.*]] = select <4 x i1> [[TMP1]], <4 x float> [[DOTSPLAT]], <4 x float> undef
+// CHECK-NEXT:    ret <4 x float> [[TMP2]]
 //
 float32x4_t test_vdupq_x_n_f32(float32_t a, mve_pred16_t p)
 {
@@ -268,7 +270,8 @@
 // CHECK-NEXT:    [[TMP1:%.*]] = call <16 x i1> @llvm.arm.mve.pred.i2v.v16i1(i32 [[TMP0]])
 // CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <16 x i8> undef, i8 [[A:%.*]], i32 0
 // CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <16 x i8> [[DOTSPLATINSERT]], <16 x i8> undef, <16 x i32> zeroinitializer
-// CHECK-NEXT:    ret <16 x i8> [[DOTSPLAT]]
+// CHECK-NEXT:    [[TMP2:%.*]] = select <16 x i1> [[TMP1]], <16 x i8> [[DOTSPLAT]], <16 x i8> undef
+// CHECK-NEXT:    ret <16 x i8> [[TMP2]]
 //
 int8x16_t test_vdupq_x_n_s8(int8_t a, mve_pred16_t p)
 {
@@ -281,7 +284,8 @@
 // CHECK-NEXT:    [[TMP1:%.*]] = call <8 x i1> @llvm.arm.mve.pred.i2v.v8i1(i32 [[TMP0]])
 // CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <8 x i16> undef, i16 [[A:%.*]], i32 0
 // CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <8 x i16> [[DOTSPLATINSERT]], <8 x i16> undef, <8 x i32> zeroinitializer
-// CHECK-NEXT:    ret <8 x i16> [[DOTSPLAT]]
+// CHECK-NEXT:    [[TMP2:%.*]] = select <8 x i1> [[TMP1]], <8 x i16> [[DOTSPLAT]], <8 x i16> undef
+// CHECK-NEXT:    ret <8 x i16> [[TMP2]]
 //
 int16x8_t test_vdupq_x_n_s16(int16_t a, mve_pred16_t p)
 {
@@ -294,7 +298,8 @@
 // CHECK-NEXT:    [[TMP1:%.*]] = call <4 x i1> @llvm.arm.mve.pred.i2v.v4i1(i32 [[TMP0]])
 // CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <4 x i32> undef, i32 [[A:%.*]], i32 0
 // CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <4 x i32> [[DOTSPLATINSERT]], <4 x i32> undef, <4 x i32> zeroinitializer
-// CHECK-NEXT:    ret <4 x i32> [[DOTSPLAT]]
+// CHECK-NEXT:    [[TMP2:%.*]] = select <4 x i1> [[TMP1]], <4 x i32> [[DOTSPLAT]], <4 x i32> undef
+// CHECK-NEXT:    ret <4 x i32> [[TMP2]]
 //
 int32x4_t test_vdupq_x_n_s32(int32_t a, mve_pred16_t p)
 {
@@ -307,7 +312,8 @@
 // CHECK-NEXT:    [[TMP1:%.*]] = call <16 x i1> @llvm.arm.mve.pred.i2v.v16i1(i32 [[TMP0]])
 // CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <16 x i8> undef, i8 [[A:%.*]], i32 0
 // CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <16 x i8> [[DOTSPLATINSERT]], <16 x i8> undef, <16 x i32> zeroinitializer
-// CHECK-NEXT:    ret <16 x i8> [[DOTSPLAT]]
+// CHECK-NEXT:    [[TMP2:%.*]] = select <16 x i1> [[TMP1]], <16 x i8> [[DOTSPLAT]], <16 x i8> undef
+// CHECK-NEXT:    ret <16 x i8> [[TMP2]]
 //
 uint8x16_t test_vdupq_x_n_u8(uint8_t a, mve_pred16_t p)
 {
@@ -320,7 +326,8 @@
 // CHECK-NEXT:    [[TMP1:%.*]] = call <8 x i1> @llvm.arm.mve.pred.i2v.v8i1(i32 [[TMP0]])
 // CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <8 x i16> undef, i16 [[A:%.*]], i32 0
 // CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <8 x i16> [[DOTSPLATINSERT]], <8 x i16> undef, <8 x i32> zeroinitializer
-// CHECK-NEXT:    ret <8 x i16> [[DOTSPLAT]]
+// CHECK-NEXT:    [[TMP2:%.*]] = select <8 x i1> [[TMP1]], <8 x i16> [[DOTSPLAT]], <8 x i16> undef
+// CHECK-NEXT:    ret <8 x i16> [[TMP2]]
 //
 uint16x8_t test_vdupq_x_n_u16(uint16_t a, mve_pred16_t p)
 {
@@ -333,7 +340,8 @@
 // CHECK-NEXT:    [[TMP1:%.*]] = call <4 x i1> @llvm.arm.mve.pred.i2v.v4i1(i32 [[TMP0]])
 // CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <4 x i32> undef, i32 [[A:%.*]], i32 0
 // CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <4 x i32> [[DOTSPLATINSERT]], <4 x i32> undef, <4 x i32> zeroinitializer
-// CHECK-NEXT:    ret <4 x i32> [[DOTSPLAT]]
+// CHECK-NEXT:    [[TMP2:%.*]] = select <4 x i1> [[TMP1]], <4 x i32> [[DOTSPLAT]], <4 x i32> undef
+// CHECK-NEXT:    ret <4 x i32> [[TMP2]]
 //
 uint32x4_t test_vdupq_x_n_u32(uint32_t a, mve_pred16_t p)
 {
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
@@ -4118,11 +4118,6 @@
   if (TrueVal == FalseVal)
     return TrueVal;
 
-  if (isa<UndefValue>(TrueVal))   // select ?, undef, X -> X
-    return FalseVal;
-  if (isa<UndefValue>(FalseVal))   // select ?, X, undef -> X
-    return TrueVal;
-
   // Deal with partial undef vector constants: select ?, VecC, VecC' --> VecC''
   Constant *TrueC, *FalseC;
   if (TrueVal->getType()->isVectorTy() && match(TrueVal, m_Constant(TrueC)) &&
diff --git a/llvm/test/Transforms/InstCombine/select.ll b/llvm/test/Transforms/InstCombine/select.ll
--- a/llvm/test/Transforms/InstCombine/select.ll
+++ b/llvm/test/Transforms/InstCombine/select.ll
@@ -2273,3 +2273,43 @@
   %sel = select i1 %cond, i32 %phi, i32 %A
   ret i32 %sel
 }
+
+; Negative tests to ensure we don't remove selects with undef true/false values.
+; See https://bugs.llvm.org/show_bug.cgi?id=31633
+; https://lists.llvm.org/pipermail/llvm-dev/2016-October/106182.html
+; https://reviews.llvm.org/D83360
+define i32 @false_undef(i1 %cond, i32 %x) {
+; CHECK-LABEL: @false_undef(
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], i32 [[X:%.*]], i32 undef
+; CHECK-NEXT:    ret i32 [[S]]
+;
+  %s = select i1 %cond, i32 %x, i32 undef
+  ret i32 %s
+}
+
+define i32 @true_undef(i1 %cond, i32 %x) {
+; CHECK-LABEL: @true_undef(
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], i32 undef, i32 [[X:%.*]]
+; CHECK-NEXT:    ret i32 [[S]]
+;
+  %s = select i1 %cond, i32 undef, i32 %x
+  ret i32 %s
+}
+
+define <2 x i32> @false_undef_vec(i1 %cond, <2 x i32> %x) {
+; CHECK-LABEL: @false_undef_vec(
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], <2 x i32> [[X:%.*]], <2 x i32> undef
+; CHECK-NEXT:    ret <2 x i32> [[S]]
+;
+  %s = select i1 %cond, <2 x i32> %x, <2 x i32> undef
+  ret <2 x i32> %s
+}
+
+define <2 x i32> @true_undef_vec(i1 %cond, <2 x i32> %x) {
+; CHECK-LABEL: @true_undef_vec(
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], <2 x i32> undef, <2 x i32> [[X:%.*]]
+; CHECK-NEXT:    ret <2 x i32> [[S]]
+;
+  %s = select i1 %cond, <2 x i32> undef, <2 x i32> %x
+  ret <2 x i32> %s
+}
diff --git a/llvm/test/Transforms/InstSimplify/select.ll b/llvm/test/Transforms/InstSimplify/select.ll
--- a/llvm/test/Transforms/InstSimplify/select.ll
+++ b/llvm/test/Transforms/InstSimplify/select.ll
@@ -750,3 +750,43 @@
   %c3 = select i1 %c1, i1 %c2, i1 false
   ret i1 %c3
 }
+
+; Negative tests to ensure we don't remove selects with undef true/false values.
+; See https://bugs.llvm.org/show_bug.cgi?id=31633
+; https://lists.llvm.org/pipermail/llvm-dev/2016-October/106182.html
+; https://reviews.llvm.org/D83360
+define i32 @false_undef(i1 %cond, i32 %x) {
+; CHECK-LABEL: @false_undef(
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], i32 [[X:%.*]], i32 undef
+; CHECK-NEXT:    ret i32 [[S]]
+;
+  %s = select i1 %cond, i32 %x, i32 undef
+  ret i32 %s
+}
+
+define i32 @true_undef(i1 %cond, i32 %x) {
+; CHECK-LABEL: @true_undef(
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], i32 undef, i32 [[X:%.*]]
+; CHECK-NEXT:    ret i32 [[S]]
+;
+  %s = select i1 %cond, i32 undef, i32 %x
+  ret i32 %s
+}
+
+define <2 x i32> @false_undef_vec(i1 %cond, <2 x i32> %x) {
+; CHECK-LABEL: @false_undef_vec(
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], <2 x i32> [[X:%.*]], <2 x i32> undef
+; CHECK-NEXT:    ret <2 x i32> [[S]]
+;
+  %s = select i1 %cond, <2 x i32> %x, <2 x i32> undef
+  ret <2 x i32> %s
+}
+
+define <2 x i32> @true_undef_vec(i1 %cond, <2 x i32> %x) {
+; CHECK-LABEL: @true_undef_vec(
+; CHECK-NEXT:    [[S:%.*]] = select i1 [[COND:%.*]], <2 x i32> undef, <2 x i32> [[X:%.*]]
+; CHECK-NEXT:    ret <2 x i32> [[S]]
+;
+  %s = select i1 %cond, <2 x i32> undef, <2 x i32> %x
+  ret <2 x i32> %s
+}