Index: clang/test/CodeGen/arm-mve-intrinsics/dup.c
===================================================================
--- clang/test/CodeGen/arm-mve-intrinsics/dup.c
+++ 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)
 {
Index: llvm/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/lib/Analysis/InstructionSimplify.cpp
+++ llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4076,9 +4076,15 @@
   if (TrueVal == FalseVal)
     return TrueVal;
 
-  if (Q.CanUseUndef && isa<UndefValue>(TrueVal)) // select ?, undef, X -> X
+  // If the true or false value is undef, we can fold to the other value as
+  // long as the other value isn't poison.
+  // select ?, undef, X -> X
+  if (Q.CanUseUndef && isa<UndefValue>(TrueVal) &&
+      isGuaranteedNotToBeUndefOrPoison(FalseVal, Q.CxtI, Q.DT))
     return FalseVal;
-  if (Q.CanUseUndef && isa<UndefValue>(FalseVal)) // select ?, X, undef -> X
+  // select ?, X, undef -> X
+  if (Q.CanUseUndef && isa<UndefValue>(FalseVal) &&
+      isGuaranteedNotToBeUndefOrPoison(TrueVal, Q.CxtI, Q.DT))
     return TrueVal;
 
   // Deal with partial undef vector constants: select ?, VecC, VecC' --> VecC''
@@ -4099,9 +4105,11 @@
       // one element is undef, choose the defined element as the safe result.
       if (TEltC == FEltC)
         NewC.push_back(TEltC);
-      else if (Q.CanUseUndef && isa<UndefValue>(TEltC))
+      else if (Q.CanUseUndef && isa<UndefValue>(TEltC) &&
+               isGuaranteedNotToBeUndefOrPoison(FEltC))
         NewC.push_back(FEltC);
-      else if (Q.CanUseUndef && isa<UndefValue>(FEltC))
+      else if (Q.CanUseUndef && isa<UndefValue>(FEltC) &&
+               isGuaranteedNotToBeUndefOrPoison(TEltC))
         NewC.push_back(TEltC);
       else
         break;
Index: llvm/lib/IR/ConstantFold.cpp
===================================================================
--- llvm/lib/IR/ConstantFold.cpp
+++ llvm/lib/IR/ConstantFold.cpp
@@ -779,10 +779,30 @@
     if (isa<UndefValue>(V1)) return V1;
     return V2;
   }
-  if (isa<UndefValue>(V1)) return V2;
-  if (isa<UndefValue>(V2)) return V1;
+
   if (V1 == V2) return V1;
 
+  // If the true or false value is undef, we can fold to the other value as
+  // long as the other value isn't poison.
+  auto NotPoison = [](Constant *C) {
+    // TODO: We can analyze ConstExpr by opcode to determine if there is any
+    //       possibility of poison.
+    if (isa<ConstantExpr>(C))
+      return false;
+
+    if (isa<ConstantInt>(C) || isa<GlobalVariable>(C) || isa<ConstantFP>(C) ||
+        isa<ConstantPointerNull>(C) || isa<Function>(C))
+      return true;
+
+    if (C->getType()->isVectorTy())
+      return !C->containsUndefElement() && !C->containsConstantExpression();
+
+    // TODO: Recursively analyze aggregates or other constants.
+    return false;
+  };
+  if (isa<UndefValue>(V1) && NotPoison(V2)) return V2;
+  if (isa<UndefValue>(V2) && NotPoison(V1)) return V1;
+
   if (ConstantExpr *TrueVal = dyn_cast<ConstantExpr>(V1)) {
     if (TrueVal->getOpcode() == Instruction::Select)
       if (TrueVal->getOperand(0) == Cond)
Index: llvm/test/Transforms/InferAddressSpaces/AMDGPU/select.ll
===================================================================
--- llvm/test/Transforms/InferAddressSpaces/AMDGPU/select.ll
+++ llvm/test/Transforms/InferAddressSpaces/AMDGPU/select.ll
@@ -221,7 +221,7 @@
 }
 
 ; CHECK-LABEL: @store_select_group_global_mismatch_undef_undef_constexpr(
-; CHECK: store i32 7, i32 addrspace(3)* null
+; CHECK: store i32 7, i32* select (i1 icmp eq (i32 ptrtoint (i32 addrspace(3)* @lds1 to i32), i32 4), i32* addrspacecast (i32 addrspace(3)* null to i32*), i32* undef), align 4
 define amdgpu_kernel void @store_select_group_global_mismatch_undef_undef_constexpr() #0 {
   store i32 7, i32* select (i1 icmp eq (i32 ptrtoint (i32 addrspace(3)* @lds1 to i32), i32 4), i32* addrspacecast (i32 addrspace(3)* null to i32*), i32* addrspacecast (i32 addrspace(1)* undef to i32*)), align 4
   ret void
Index: llvm/test/Transforms/InstCombine/select.ll
===================================================================
--- llvm/test/Transforms/InstCombine/select.ll
+++ llvm/test/Transforms/InstCombine/select.ll
@@ -2452,13 +2452,14 @@
   ret i32 %sel
 }
 
-; FIXME: We shouldn't remove selects with undef true/false values.
+; 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:    ret i32 [[X:%.*]]
+; 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
@@ -2466,7 +2467,8 @@
 
 define i32 @true_undef(i1 %cond, i32 %x) {
 ; CHECK-LABEL: @true_undef(
-; CHECK-NEXT:    ret i32 [[X:%.*]]
+; 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
@@ -2474,7 +2476,8 @@
 
 define <2 x i32> @false_undef_vec(i1 %cond, <2 x i32> %x) {
 ; CHECK-LABEL: @false_undef_vec(
-; CHECK-NEXT:    ret <2 x i32> [[X:%.*]]
+; 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
@@ -2482,7 +2485,8 @@
 
 define <2 x i32> @true_undef_vec(i1 %cond, <2 x i32> %x) {
 ; CHECK-LABEL: @true_undef_vec(
-; CHECK-NEXT:    ret <2 x i32> [[X:%.*]]
+; 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
Index: llvm/test/Transforms/InstSimplify/select.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/select.ll
+++ llvm/test/Transforms/InstSimplify/select.ll
@@ -751,13 +751,14 @@
   ret i1 %c3
 }
 
-; FIXME: We shouldn't remove selects with undef true/false values.
+; 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:    ret i32 [[X:%.*]]
+; 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
@@ -765,7 +766,8 @@
 
 define i32 @true_undef(i1 %cond, i32 %x) {
 ; CHECK-LABEL: @true_undef(
-; CHECK-NEXT:    ret i32 [[X:%.*]]
+; 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
@@ -773,7 +775,8 @@
 
 define <2 x i32> @false_undef_vec(i1 %cond, <2 x i32> %x) {
 ; CHECK-LABEL: @false_undef_vec(
-; CHECK-NEXT:    ret <2 x i32> [[X:%.*]]
+; 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
@@ -781,7 +784,8 @@
 
 define <2 x i32> @true_undef_vec(i1 %cond, <2 x i32> %x) {
 ; CHECK-LABEL: @true_undef_vec(
-; CHECK-NEXT:    ret <2 x i32> [[X:%.*]]
+; 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
@@ -843,9 +847,6 @@
 
 @g = external global i32, align 1
 
-; FIXME: We shouldn't fold partial undef vectors when constexprs are involved.
-; We would need to prove the constexpr doesn't result in poison which we aren't
-; equiped to do yet.
 define <2 x i32> @false_undef_true_constextpr_vec(i1 %cond) {
 ; CHECK-LABEL: @false_undef_true_constextpr_vec(
 ; CHECK-NEXT:    ret <2 x i32> <i32 ptrtoint (i32* @g to i32), i32 ptrtoint (i32* @g to i32)>
@@ -886,10 +887,11 @@
   ret <2 x float> %s
 }
 
-; FIXME: We shouldn't fold if the non-undef operand is a constexpr.
+; Negative tests. Don't fold if the non-undef operand is a constexpr.
 define i32 @all_constant_false_undef_true_constexpr() {
 ; CHECK-LABEL: @all_constant_false_undef_true_constexpr(
-; CHECK-NEXT:    ret i32 ptrtoint (i32 ()* @all_constant_false_undef_true_constexpr to i32)
+; CHECK-NEXT:    [[S:%.*]] = select i1 ptrtoint (i32 ()* @all_constant_false_undef_true_constexpr to i1), i32 ptrtoint (i32 ()* @all_constant_false_undef_true_constexpr to i32), i32 undef
+; CHECK-NEXT:    ret i32 [[S]]
 ;
   %s = select i1 ptrtoint (i32 ()* @all_constant_false_undef_true_constexpr to i1), i32 ptrtoint (i32 ()* @all_constant_false_undef_true_constexpr to i32), i32 undef
   ret i32 %s
@@ -897,16 +899,18 @@
 
 define i32 @all_constant_true_undef_false_constexpr() {
 ; CHECK-LABEL: @all_constant_true_undef_false_constexpr(
-; CHECK-NEXT:    ret i32 ptrtoint (i32 ()* @all_constant_true_undef_false_constexpr to i32)
+; CHECK-NEXT:    [[S:%.*]] = select i1 ptrtoint (i32 ()* @all_constant_true_undef_false_constexpr to i1), i32 undef, i32 ptrtoint (i32 ()* @all_constant_true_undef_false_constexpr to i32)
+; CHECK-NEXT:    ret i32 [[S]]
 ;
   %s = select i1 ptrtoint (i32 ()* @all_constant_true_undef_false_constexpr to i1), i32 undef, i32 ptrtoint (i32 ()* @all_constant_true_undef_false_constexpr to i32)
   ret i32 %s
 }
 
-; FIXME: We shouldn't fold if the non-undef operand is a vector containing a constexpr.
+; Negative tests. Don't fold if the non-undef operand is a vector containing a constexpr.
 define <2 x i32> @all_constant_false_undef_true_constexpr_vec() {
 ; CHECK-LABEL: @all_constant_false_undef_true_constexpr_vec(
-; CHECK-NEXT:    ret <2 x i32> <i32 ptrtoint (<2 x i32> ()* @all_constant_false_undef_true_constexpr_vec to i32), i32 -1>
+; CHECK-NEXT:    [[S:%.*]] = select i1 ptrtoint (<2 x i32> ()* @all_constant_false_undef_true_constexpr_vec to i1), <2 x i32> <i32 ptrtoint (<2 x i32> ()* @all_constant_false_undef_true_constexpr_vec to i32), i32 -1>, <2 x i32> undef
+; CHECK-NEXT:    ret <2 x i32> [[S]]
 ;
   %s = select i1 ptrtoint (<2 x i32> ()* @all_constant_false_undef_true_constexpr_vec to i1), <2 x i32> <i32 ptrtoint (<2 x i32> ()* @all_constant_false_undef_true_constexpr_vec to i32), i32 -1>, <2 x i32> undef
   ret <2 x i32> %s
@@ -914,7 +918,8 @@
 
 define <2 x i32> @all_constant_true_undef_false_constexpr_vec() {
 ; CHECK-LABEL: @all_constant_true_undef_false_constexpr_vec(
-; CHECK-NEXT:    ret <2 x i32> <i32 -1, i32 ptrtoint (<2 x i32> ()* @all_constant_true_undef_false_constexpr_vec to i32)>
+; CHECK-NEXT:    [[S:%.*]] = select i1 ptrtoint (<2 x i32> ()* @all_constant_true_undef_false_constexpr_vec to i1), <2 x i32> undef, <2 x i32> <i32 -1, i32 ptrtoint (<2 x i32> ()* @all_constant_true_undef_false_constexpr_vec to i32)>
+; CHECK-NEXT:    ret <2 x i32> [[S]]
 ;
   %s = select i1 ptrtoint (<2 x i32> ()* @all_constant_true_undef_false_constexpr_vec to i1), <2 x i32> undef, <2 x i32><i32 -1, i32 ptrtoint (<2 x i32> ()* @all_constant_true_undef_false_constexpr_vec to i32)>
   ret <2 x i32> %s
@@ -922,8 +927,12 @@
 
 define i1 @expand_binop_undef(i32 %x, i32 %y) {
 ; CHECK-LABEL: @expand_binop_undef(
-; CHECK-NEXT:    [[CMP15:%.*]] = icmp slt i32 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    ret i1 [[CMP15]]
+; CHECK-NEXT:    [[CMP9_NOT_1:%.*]] = icmp eq i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[CMP15:%.*]] = icmp slt i32 [[X]], [[Y]]
+; CHECK-NEXT:    [[SPEC_SELECT39:%.*]] = select i1 [[CMP9_NOT_1]], i1 undef, i1 [[CMP15]]
+; CHECK-NEXT:    [[SPEC_SELECT40:%.*]] = xor i1 [[CMP9_NOT_1]], true
+; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = and i1 [[SPEC_SELECT39]], [[SPEC_SELECT40]]
+; CHECK-NEXT:    ret i1 [[SPEC_SELECT]]
 ;
   %cmp9.not.1 = icmp eq i32 %x, %y
   %cmp15 = icmp slt i32 %x, %y