Extended Description

Test: Transforms/InstCombine/vector-xor.ll
Summary: (undef u>> a) ^ -1 always leaves top bits as 1 when a > 0. However 'undef >> a' can leave top bits as 0 or 1, depending on the sign bit of undef.

define <4 x i32> @​test_v4i32_not_lshr_nonnegative_const_undef(<4 x i32> %a0) {
%1 = lshr <4 x i32> { 3, 5, undef, 9 }, %a0
%2 = xor <4 x i32> { 4294967295, 4294967295, 4294967295, undef }, %1
ret <4 x i32> %2
}
=>
define <4 x i32> @​test_v4i32_not_lshr_nonnegative_const_undef(<4 x i32> %a0) {
%1 = ashr <4 x i32> { 4294967292, 4294967290, undef, 4294967286 }, %a0
ret <4 x i32> %1
}
Transformation doesn't verify!
ERROR: Value mismatch

Example:
<4 x i32> %a0 = < #x00000000 (0), #xfffffffe (4294967294, -2), #x0000001f (31), #xfffffffe (4294967294, -2) >

Source:
<4 x i32> %1 = < #x00000003 (3), poison, #x00000000 (0) [based on undef value], poison >
<4 x i32> %2 = < #xfffffffc (4294967292, -4), poison, #xffffffff (4294967295, -1), poison >

Target:
<4 x i32> %1 = < #xfffffffc (4294967292, -4), poison, #x00000000 (0), poison >
Source value: < #xfffffffc (4294967292, -4), poison, #xffffffff (4294967295, -1), poison >
Target value: < #xfffffffc (4294967292, -4), poison, #x00000000 (0), poison >

https://web.ist.utl.pt/nuno.lopes/alive2/index.php?hash=2009353267698970&test=Transforms%2FInstCombine%2Fvector-xor.ll