The more general fold was not poison-safe, so it was removed:
rG5486e00
...but it is ok to have this transform if analysis can determine
the vector contains no poison. The test shows a simple example
of that: constant integer elements are not poison.
if (isa<UndefValue>(Idx))
return UndefValue::get(Vec->getType());
+ // If the scalar is undef, and there is no risk of propagating poison from the
+ // vector value, simplify to the vector value.
+ if (isa<UndefValue>(Val) && isGuaranteedNotToBeUndefOrPoison(Vec))
+ return Vec;
+
// If we are extracting a value from a vector, then inserting it into the same
// place, that's the input vector:
// insertelt Vec, (extractelt Vec, Idx), Idx --> Vec
ret <4 x i32> %B
}
+; Constant is not poison, so this can simplify.
+
define <2 x i32> @undef_into_constant_vector_with_variable_index(<2 x i32> %A, i32 %Index) {
; CHECK-LABEL: @undef_into_constant_vector_with_variable_index(
-; CHECK-NEXT: [[B:%.*]] = insertelement <2 x i32> <i32 42, i32 -42>, i32 undef, i32 [[INDEX:%.*]]
-; CHECK-NEXT: ret <2 x i32> [[B]]
+; CHECK-NEXT: ret <2 x i32> <i32 42, i32 -42>
;
%B = insertelement <2 x i32> <i32 42, i32 -42>, i32 undef, i32 %Index
ret <2 x i32> %B