Index: docs/LangRef.rst
===================================================================
--- docs/LangRef.rst
+++ docs/LangRef.rst
@@ -7071,9 +7071,10 @@
 The elements of the two input vectors are numbered from left to right
 across both of the vectors. The shuffle mask operand specifies, for each
 element of the result vector, which element of the two input vectors the
-result element gets. The element selector may be undef (meaning "don't
-care") and the second operand may be undef if performing a shuffle from
-only one vector.
+result element gets. If the shuffle mask is undef, the result vector is
+undef. If any element of the mask operand is undef, that element of the
+result is undef. If the shuffle mask selects an undef element from one
+of the input vectors, the resulting element is undef.
 
 Example:
 """"""""
Index: lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1053,8 +1053,10 @@
       // If the select condition element is false, choose from the 2nd vector.
       Mask.push_back(ConstantInt::get(Int32Ty, i + NumElts));
     } else if (isa<UndefValue>(Elt)) {
-      // If the select condition element is undef, the shuffle mask is undef.
-      Mask.push_back(UndefValue::get(Int32Ty));
+      // Undef in a select condition (choose one of the operands) does not mean
+      // the same thing as undef in a shuffle mask (any value is acceptable), so
+      // give up.
+      return nullptr;
     } else {
       // Bail out on a constant expression.
       return nullptr;
Index: test/Transforms/InstCombine/select.ll
===================================================================
--- test/Transforms/InstCombine/select.ll
+++ test/Transforms/InstCombine/select.ll
@@ -1298,11 +1298,22 @@
   ret <2 x i32> %x.xor
 }
 
-; Make sure that undef elements of the select condition are translated into undef elements of the shuffle mask.
-
 define <4 x i32> @canonicalize_to_shuffle(<4 x i32> %a, <4 x i32> %b) {
 ; CHECK-LABEL: @canonicalize_to_shuffle(
-; CHECK-NEXT:    [[SEL:%.*]] = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32> <i32 0, i32 undef, i32 6, i32 undef>
+; CHECK-NEXT:    [[SEL:%.*]] = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32> <i32 0, i32 5, i32 6, i32 3>
+; CHECK-NEXT:    ret <4 x i32> [[SEL]]
+;
+  %sel = select <4 x i1> <i1 true, i1 false, i1 false, i1 true>, <4 x i32> %a, <4 x i32> %b
+  ret <4 x i32> %sel
+}
+
+; Undef elements of the select condition may not be translated into undef elements of a shuffle mask
+; because undef in a shuffle mask means we can return anything, not just one of the selected values.
+; https://bugs.llvm.org/show_bug.cgi?id=32486
+
+define <4 x i32> @undef_elts_in_condition(<4 x i32> %a, <4 x i32> %b) {
+; CHECK-LABEL: @undef_elts_in_condition(
+; CHECK-NEXT:    [[SEL:%.*]] = select <4 x i1> <i1 true, i1 undef, i1 false, i1 undef>, <4 x i32> %a, <4 x i32> %b
 ; CHECK-NEXT:    ret <4 x i32> [[SEL]]
 ;
   %sel = select <4 x i1> <i1 true, i1 undef, i1 false, i1 undef>, <4 x i32> %a, <4 x i32> %b