Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1417,21 +1417,21 @@
       }
     }
     if (MayChange) {
-      // It's not safe to use a vector with undef elements because the entire
-      // instruction can be folded to undef (for example, div/rem divisors).
-      // Replace undef lanes with the first non-undef element. Vector demanded
-      // elements can change those back to undef values if that is safe.
-      Constant *SafeDummyConstant = nullptr;
-      for (unsigned i = 0; i < VWidth; ++i) {
-        if (!isa<UndefValue>(NewVecC[i])) {
-          SafeDummyConstant = NewVecC[i];
-          break;
-        }
+      // With integer div/rem instructions, it is not safe to use a vector with
+      // undef elements because the entire instruction can be folded to undef.
+      // So replace undef elements with '1' because that can never induce
+      // undefined behavior. All other binop opcodes are always safe to
+      // speculate, and therefore, it is fine to include undef elements for
+      // unused lanes (and using undefs may help optimization).
+      BinaryOperator::BinaryOps Opcode = Inst.getOpcode();
+      if (Opcode == Instruction::UDiv || Opcode == Instruction::URem ||
+          Opcode == Instruction::SDiv || Opcode == Instruction::SRem) {
+        assert(C->getType()->getScalarType()->isIntegerTy() &&
+               "Not expecting FP opcodes/operands/constants here");
+        for (unsigned i = 0; i < VWidth; ++i)
+          if (isa<UndefValue>(NewVecC[i]))
+            NewVecC[i] = ConstantInt::get(NewVecC[i]->getType(), 1);
       }
-      assert(SafeDummyConstant && "Undef constant vector was not simplified?");
-      for (unsigned i = 0; i < VWidth; ++i)
-        if (isa<UndefValue>(NewVecC[i]))
-          NewVecC[i] = SafeDummyConstant;
 
       // Op(shuffle(V1, Mask), C) -> shuffle(Op(V1, NewC), Mask)
       // Op(C, shuffle(V1, Mask)) -> shuffle(Op(NewC, V1), Mask)
Index: test/Transforms/InstCombine/vec_shuffle.ll
===================================================================
--- test/Transforms/InstCombine/vec_shuffle.ll
+++ test/Transforms/InstCombine/vec_shuffle.ll
@@ -452,7 +452,7 @@
 
 define <4 x i32> @lshr_const_half_splat(<4 x i32> %v) {
 ; CHECK-LABEL: @lshr_const_half_splat(
-; CHECK-NEXT:    [[TMP1:%.*]] = lshr <4 x i32> <i32 8, i32 8, i32 9, i32 8>, [[V:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr <4 x i32> <i32 undef, i32 8, i32 9, i32 undef>, [[V:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <4 x i32> [[TMP1]], <4 x i32> undef, <4 x i32> <i32 1, i32 1, i32 2, i32 2>
 ; CHECK-NEXT:    ret <4 x i32> [[R]]
 ;
@@ -643,7 +643,7 @@
 
 define <2 x i32> @shl_splat_constant0(<2 x i32> %x) {
 ; CHECK-LABEL: @shl_splat_constant0(
-; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i32> <i32 5, i32 5>, [[X:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i32> <i32 5, i32 undef>, [[X:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -654,7 +654,7 @@
 
 define <2 x i32> @shl_splat_constant1(<2 x i32> %x) {
 ; CHECK-LABEL: @shl_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i32> [[X:%.*]], <i32 5, i32 5>
+; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i32> [[X:%.*]], <i32 5, i32 undef>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -665,7 +665,7 @@
 
 define <2 x i32> @ashr_splat_constant0(<2 x i32> %x) {
 ; CHECK-LABEL: @ashr_splat_constant0(
-; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i32> <i32 5, i32 5>, [[X:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = ashr <2 x i32> <i32 5, i32 undef>, [[X:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -676,7 +676,7 @@
 
 define <2 x i32> @ashr_splat_constant1(<2 x i32> %x) {
 ; CHECK-LABEL: @ashr_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = ashr <2 x i32> [[X:%.*]], <i32 5, i32 5>
+; CHECK-NEXT:    [[TMP1:%.*]] = ashr <2 x i32> [[X:%.*]], <i32 5, i32 undef>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -687,7 +687,7 @@
 
 define <2 x i32> @lshr_splat_constant0(<2 x i32> %x) {
 ; CHECK-LABEL: @lshr_splat_constant0(
-; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i32> <i32 5, i32 5>, [[X:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i32> <i32 5, i32 undef>, [[X:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -698,7 +698,7 @@
 
 define <2 x i32> @lshr_splat_constant1(<2 x i32> %x) {
 ; CHECK-LABEL: @lshr_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i32> [[X:%.*]], <i32 5, i32 5>
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i32> [[X:%.*]], <i32 5, i32 undef>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -720,7 +720,7 @@
 
 define <2 x i32> @urem_splat_constant1(<2 x i32> %x) {
 ; CHECK-LABEL: @urem_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = urem <2 x i32> [[X:%.*]], <i32 42, i32 42>
+; CHECK-NEXT:    [[TMP1:%.*]] = urem <2 x i32> [[X:%.*]], <i32 42, i32 1>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -742,7 +742,7 @@
 
 define <2 x i32> @srem_splat_constant1(<2 x i32> %x) {
 ; CHECK-LABEL: @srem_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = srem <2 x i32> [[X:%.*]], <i32 42, i32 42>
+; CHECK-NEXT:    [[TMP1:%.*]] = srem <2 x i32> [[X:%.*]], <i32 42, i32 1>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -764,7 +764,7 @@
 
 define <2 x i32> @udiv_splat_constant1(<2 x i32> %x) {
 ; CHECK-LABEL: @udiv_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = udiv <2 x i32> [[X:%.*]], <i32 42, i32 42>
+; CHECK-NEXT:    [[TMP1:%.*]] = udiv <2 x i32> [[X:%.*]], <i32 42, i32 1>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -786,7 +786,7 @@
 
 define <2 x i32> @sdiv_splat_constant1(<2 x i32> %x) {
 ; CHECK-LABEL: @sdiv_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = sdiv <2 x i32> [[X:%.*]], <i32 42, i32 42>
+; CHECK-NEXT:    [[TMP1:%.*]] = sdiv <2 x i32> [[X:%.*]], <i32 42, i32 1>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -830,7 +830,7 @@
 
 define <2 x float> @fadd_splat_constant(<2 x float> %x) {
 ; CHECK-LABEL: @fadd_splat_constant(
-; CHECK-NEXT:    [[TMP1:%.*]] = fadd <2 x float> [[X:%.*]], <float 4.200000e+01, float 4.200000e+01>
+; CHECK-NEXT:    [[TMP1:%.*]] = fadd <2 x float> [[X:%.*]], <float 4.200000e+01, float undef>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x float> [[TMP1]], <2 x float> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;
@@ -841,7 +841,7 @@
 
 define <2 x float> @fsub_splat_constant0(<2 x float> %x) {
 ; CHECK-LABEL: @fsub_splat_constant0(
-; CHECK-NEXT:    [[TMP1:%.*]] = fsub <2 x float> <float 4.200000e+01, float 4.200000e+01>, [[X:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = fsub <2 x float> <float 4.200000e+01, float undef>, [[X:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x float> [[TMP1]], <2 x float> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;
@@ -852,7 +852,7 @@
 
 define <2 x float> @fsub_splat_constant1(<2 x float> %x) {
 ; CHECK-LABEL: @fsub_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = fadd <2 x float> [[X:%.*]], <float -4.200000e+01, float -4.200000e+01>
+; CHECK-NEXT:    [[TMP1:%.*]] = fadd <2 x float> [[X:%.*]], <float -4.200000e+01, float 0x7FF8000000000000>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x float> [[TMP1]], <2 x float> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;
@@ -863,7 +863,7 @@
 
 define <2 x float> @fmul_splat_constant(<2 x float> %x) {
 ; CHECK-LABEL: @fmul_splat_constant(
-; CHECK-NEXT:    [[TMP1:%.*]] = fmul <2 x float> [[X:%.*]], <float 4.200000e+01, float 4.200000e+01>
+; CHECK-NEXT:    [[TMP1:%.*]] = fmul <2 x float> [[X:%.*]], <float 4.200000e+01, float undef>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x float> [[TMP1]], <2 x float> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;
@@ -874,7 +874,7 @@
 
 define <2 x float> @fdiv_splat_constant0(<2 x float> %x) {
 ; CHECK-LABEL: @fdiv_splat_constant0(
-; CHECK-NEXT:    [[TMP1:%.*]] = fdiv <2 x float> <float 4.200000e+01, float 4.200000e+01>, [[X:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = fdiv <2 x float> <float 4.200000e+01, float undef>, [[X:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x float> [[TMP1]], <2 x float> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;
@@ -885,7 +885,7 @@
 
 define <2 x float> @fdiv_splat_constant1(<2 x float> %x) {
 ; CHECK-LABEL: @fdiv_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = fdiv <2 x float> [[X:%.*]], <float 4.200000e+01, float 4.200000e+01>
+; CHECK-NEXT:    [[TMP1:%.*]] = fdiv <2 x float> [[X:%.*]], <float 4.200000e+01, float undef>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x float> [[TMP1]], <2 x float> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;
@@ -896,7 +896,7 @@
 
 define <2 x float> @frem_splat_constant0(<2 x float> %x) {
 ; CHECK-LABEL: @frem_splat_constant0(
-; CHECK-NEXT:    [[TMP1:%.*]] = frem <2 x float> <float 4.200000e+01, float 4.200000e+01>, [[X:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = frem <2 x float> <float 4.200000e+01, float undef>, [[X:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x float> [[TMP1]], <2 x float> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;
@@ -907,7 +907,7 @@
 
 define <2 x float> @frem_splat_constant1(<2 x float> %x) {
 ; CHECK-LABEL: @frem_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = frem <2 x float> [[X:%.*]], <float 4.200000e+01, float 4.200000e+01>
+; CHECK-NEXT:    [[TMP1:%.*]] = frem <2 x float> [[X:%.*]], <float 4.200000e+01, float undef>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x float> [[TMP1]], <2 x float> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;