[llvm-bugs] [Bug 45447] New: Incorrect transformation: (undef u>> a) ^ -1 -> undef >> a, when a != 0

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45447

            Bug ID: 45447
           Summary: Incorrect transformation: (undef u>> a) ^ -1  ->
                    undef >> a, when a != 0
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Keywords: miscompilation
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: nunoplopes at sapo.pt
                CC: juneyoung.lee at sf.snu.ac.kr, lebedev.ri at gmail.com,
                    llvm-bugs at lists.llvm.org, regehr at cs.utah.edu,
                    spatel+llvm at rotateright.com

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

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