Index: lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1260,8 +1260,19 @@
     break;
   }
   case Instruction::Select: {
+    // If this is a vector select, try to transform the select condition based
+    // on the current demanded elements.
+    if (I->getOperand(0)->getType()->isVectorTy()) {
+      if (Value *V = SimplifyDemandedVectorElts(I->getOperand(0), DemandedElts,
+                                                UndefElts, Depth + 1)) {
+        I->setOperand(0, V);
+        MadeChange = true;
+      }
+    }
+
+    // Next, see if we can transform the arms of the select.
     APInt LeftDemanded(DemandedElts), RightDemanded(DemandedElts);
-    if (ConstantVector* CV = dyn_cast<ConstantVector>(I->getOperand(0))) {
+    if (auto *CV = dyn_cast<ConstantVector>(I->getOperand(0))) {
       for (unsigned i = 0; i < VWidth; i++) {
         Constant *CElt = CV->getAggregateElement(i);
         // Method isNullValue always returns false when called on a
Index: test/Transforms/InstCombine/shuffle-select-narrow.ll
===================================================================
--- test/Transforms/InstCombine/shuffle-select-narrow.ll
+++ test/Transforms/InstCombine/shuffle-select-narrow.ll
@@ -16,11 +16,11 @@
   ret <2 x i8> %r
 }
 
-; TODO: The 1st shuffle is not extending with undefs, but demanded elements should correct that.
+; TODO: The 1st shuffle is not extending with undefs, but demanded elements corrects that.
 
 define <2 x i8> @narrow_shuffle_of_select_overspecified_extend(<2 x i1> %cmp, <4 x i8> %x, <4 x i8> %y) {
 ; CHECK-LABEL: @narrow_shuffle_of_select_overspecified_extend(
-; CHECK-NEXT:    [[WIDECMP:%.*]] = shufflevector <2 x i1> [[CMP:%.*]], <2 x i1> undef, <4 x i32> <i32 0, i32 1, i32 0, i32 1>
+; CHECK-NEXT:    [[WIDECMP:%.*]] = shufflevector <2 x i1> [[CMP:%.*]], <2 x i1> undef, <4 x i32> <i32 0, i32 1, i32 undef, i32 undef>
 ; CHECK-NEXT:    [[WIDESEL:%.*]] = select <4 x i1> [[WIDECMP]], <4 x i8> [[X:%.*]], <4 x i8> [[Y:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <4 x i8> [[WIDESEL]], <4 x i8> undef, <2 x i32> <i32 0, i32 1>
 ; CHECK-NEXT:    ret <2 x i8> [[R]]
@@ -35,7 +35,7 @@
 
 define <3 x float> @narrow_shuffle_of_select_undefs(<3 x i1> %cmp, <4 x float> %x, <4 x float> %y) {
 ; CHECK-LABEL: @narrow_shuffle_of_select_undefs(
-; CHECK-NEXT:    [[WIDECMP:%.*]] = shufflevector <3 x i1> [[CMP:%.*]], <3 x i1> undef, <4 x i32> <i32 undef, i32 1, i32 2, i32 undef>
+; CHECK-NEXT:    [[WIDECMP:%.*]] = shufflevector <3 x i1> [[CMP:%.*]], <3 x i1> undef, <4 x i32> <i32 undef, i32 1, i32 undef, i32 undef>
 ; CHECK-NEXT:    [[WIDESEL:%.*]] = select <4 x i1> [[WIDECMP]], <4 x float> [[X:%.*]], <4 x float> [[Y:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <4 x float> [[WIDESEL]], <4 x float> undef, <3 x i32> <i32 0, i32 1, i32 undef>
 ; CHECK-NEXT:    ret <3 x float> [[R]]
@@ -94,7 +94,7 @@
 
 define <3 x i8> @narrow_shuffle_of_select_mismatch_types2(<4 x i1> %cmp, <6 x i8> %x, <6 x i8> %y) {
 ; CHECK-LABEL: @narrow_shuffle_of_select_mismatch_types2(
-; CHECK-NEXT:    [[WIDECMP:%.*]] = shufflevector <4 x i1> [[CMP:%.*]], <4 x i1> undef, <6 x i32> <i32 0, i32 1, i32 2, i32 3, i32 undef, i32 undef>
+; CHECK-NEXT:    [[WIDECMP:%.*]] = shufflevector <4 x i1> [[CMP:%.*]], <4 x i1> undef, <6 x i32> <i32 0, i32 1, i32 2, i32 undef, i32 undef, i32 undef>
 ; CHECK-NEXT:    [[WIDESEL:%.*]] = select <6 x i1> [[WIDECMP]], <6 x i8> [[X:%.*]], <6 x i8> [[Y:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <6 x i8> [[WIDESEL]], <6 x i8> undef, <3 x i32> <i32 0, i32 1, i32 2>
 ; CHECK-NEXT:    ret <3 x i8> [[R]]