Index: llvm/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/lib/Analysis/InstructionSimplify.cpp
+++ llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4014,6 +4014,39 @@
   if (isa<UndefValue>(FalseVal))   // select ?, X, undef -> X
     return TrueVal;
 
+  // Deal with partial undef vector constants: select ?, VecC, VecC' --> VecC''
+  Constant *TrueC, *FalseC;
+  if (match(TrueVal, m_Constant(TrueC)) &&
+      match(FalseVal, m_Constant(FalseC)) &&
+      TrueC->isElementWiseEqual(FalseC)) {
+    assert(TrueC->getType()->isVectorTy() && FalseC->getType()->isVectorTy() &&
+           "Expected vector constants");
+
+    unsigned NumElts = TrueC->getType()->getVectorNumElements();
+    SmallVector<Constant *, 16> NewC;
+    for (unsigned i = 0; i != NumElts; ++i) {
+      // Bail out on incomplete vector constants.
+      Constant *TEltC = TrueC->getAggregateElement(i);
+      Constant *FEltC = FalseC->getAggregateElement(i);
+      if (!TEltC || !FEltC)
+        break;
+
+      // If the elements match (undef or not), that value is the result. If only
+      // one element is undef, choose the defined element as the safe result.
+      if (TEltC == FEltC) {
+        NewC.push_back(TEltC);
+      } else if (isa<UndefValue>(TEltC)) {
+        assert(!isa<UndefValue>(FEltC) && "Expected non-undef constant");
+        NewC.push_back(FEltC);
+      } else {
+        assert(!isa<UndefValue>(TEltC) && "Expected non-undef constant");
+        NewC.push_back(TEltC);
+      }
+    }
+    if (NewC.size() == NumElts)
+      return ConstantVector::get(NewC);
+  }
+
   if (Value *V =
           simplifySelectWithICmpCond(Cond, TrueVal, FalseVal, Q, MaxRecurse))
     return V;
Index: llvm/test/Transforms/InstSimplify/select.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/select.ll
+++ llvm/test/Transforms/InstSimplify/select.ll
@@ -59,8 +59,7 @@
 
 define <2 x i32> @equal_arms_vec_undef(<2 x i1> %cond) {
 ; CHECK-LABEL: @equal_arms_vec_undef(
-; CHECK-NEXT:    [[V:%.*]] = select <2 x i1> [[COND:%.*]], <2 x i32> <i32 42, i32 undef>, <2 x i32> <i32 undef, i32 42>
-; CHECK-NEXT:    ret <2 x i32> [[V]]
+; CHECK-NEXT:    ret <2 x i32> <i32 42, i32 42>
 ;
   %V = select <2 x i1> %cond, <2 x i32> <i32 42, i32 undef>, <2 x i32> <i32 undef, i32 42>
   ret <2 x i32> %V
@@ -68,8 +67,7 @@
 
 define <3 x float> @equal_arms_vec_less_undef(<3 x i1> %cond) {
 ; CHECK-LABEL: @equal_arms_vec_less_undef(
-; CHECK-NEXT:    [[V:%.*]] = select <3 x i1> [[COND:%.*]], <3 x float> <float 4.200000e+01, float undef, float 4.300000e+01>, <3 x float> <float 4.200000e+01, float 4.200000e+01, float 4.300000e+01>
-; CHECK-NEXT:    ret <3 x float> [[V]]
+; CHECK-NEXT:    ret <3 x float> <float 4.200000e+01, float 4.200000e+01, float 4.300000e+01>
 ;
   %V = select <3 x i1> %cond, <3 x float> <float 42.0, float undef, float 43.0>, <3 x float> <float 42.0, float 42.0, float 43.0>
   ret <3 x float> %V
@@ -77,8 +75,7 @@
 
 define <3 x float> @equal_arms_vec_more_undef(<3 x i1> %cond) {
 ; CHECK-LABEL: @equal_arms_vec_more_undef(
-; CHECK-NEXT:    [[V:%.*]] = select <3 x i1> [[COND:%.*]], <3 x float> <float 4.200000e+01, float undef, float undef>, <3 x float> <float undef, float undef, float 4.300000e+01>
-; CHECK-NEXT:    ret <3 x float> [[V]]
+; CHECK-NEXT:    ret <3 x float> <float 4.200000e+01, float undef, float 4.300000e+01>
 ;
   %V = select <3 x i1> %cond, <3 x float> <float 42.0, float undef, float undef>, <3 x float> <float undef, float undef, float 43.0>
   ret <3 x float> %V