[llvm-bugs] [Bug 47696] New: InstCombine: incorrect select operand simplification with undef

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47696

            Bug ID: 47696
           Summary: InstCombine: incorrect select operand simplification
                    with undef
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Keywords: miscompilation
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: nunoplopes at sapo.pt
                CC: llvm-bugs at lists.llvm.org, nikita.ppv at gmail.com,
                    regehr at cs.utah.edu, spatel+llvm at rotateright.com

Test: Transforms/InstCombine/select-binop-cmp.ll

Seems to be a regression from https://reviews.llvm.org/D87480
The undef operand in the icmp needs to be fixed to 0 for the transformation to
be correct. See here: https://alive2.llvm.org/ce/z/YYhL33


define <2 x i8> @select_xor_icmp_vec_undef(<2 x i8> %x, <2 x i8> %y, <2 x i8>
%z) {
  %A = icmp eq <2 x i8> %x, { 0, undef }
  %B = xor <2 x i8> %x, %z
  %C = select <2 x i1> %A, <2 x i8> %B, <2 x i8> %y
  ret <2 x i8> %C
}
=>
define <2 x i8> @select_xor_icmp_vec_undef(<2 x i8> %x, <2 x i8> %y, <2 x i8>
%z) {
  %A = icmp eq <2 x i8> %x, { 0, undef }
  %C = select <2 x i1> %A, <2 x i8> %z, <2 x i8> %y
  ret <2 x i8> %C
}
Transformation doesn't verify!
ERROR: Target's return value is more undefined

Example:
<2 x i8> %x = < poison, #x01 (1) >
<2 x i8> %y = < poison, #x00 (0) >
<2 x i8> %z = < poison, #x01 (1) >

Source:
<2 x i1> %A = < poison, any >
<2 x i8> %B = < poison, #x00 (0) >
<2 x i8> %C = < poison, #x00 (0) >

Target:
<2 x i1> %A = < poison, #x1 (1) >
<2 x i8> %C = < poison, #x01 (1) >
Source value: < poison, #x00 (0) >
Target value: < poison, #x01 (1) >


https://web.ist.utl.pt/nuno.lopes/alive2/index.php?hash=1546033e6d866cfc&test=Transforms%2FInstCombine%2Fselect-binop-cmp.ll

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