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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - InstCombine: incorrect select operand simplification with undef"
href="https://bugs.llvm.org/show_bug.cgi?id=47696">47696</a>
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<th>Summary</th>
<td>InstCombine: incorrect select operand simplification with undef
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Keywords</th>
<td>miscompilation
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nunoplopes@sapo.pt
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, nikita.ppv@gmail.com, regehr@cs.utah.edu, spatel+llvm@rotateright.com
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</tr></table>
<p>
<div>
<pre>Test: Transforms/InstCombine/select-binop-cmp.ll
Seems to be a regression from <a href="https://reviews.llvm.org/D87480">https://reviews.llvm.org/D87480</a>
The undef operand in the icmp needs to be fixed to 0 for the transformation to
be correct. See here: <a href="https://alive2.llvm.org/ce/z/YYhL33">https://alive2.llvm.org/ce/z/YYhL33</a>
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) >
<a href="https://web.ist.utl.pt/nuno.lopes/alive2/index.php?hash=1546033e6d866cfc&test=Transforms%2FInstCombine%2Fselect-binop-cmp.ll">https://web.ist.utl.pt/nuno.lopes/alive2/index.php?hash=1546033e6d866cfc&test=Transforms%2FInstCombine%2Fselect-binop-cmp.ll</a></pre>
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