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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 transformation 'x > (x & undef)' -> 'x > undef'"
href="https://bugs.llvm.org/show_bug.cgi?id=44383">44383</a>
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<th>Summary</th>
<td>Instcombine: incorrect transformation 'x > (x & undef)' -> 'x > undef'
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Keywords</th>
<td>miscompilation
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>nunoplopes@sapo.pt
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<th>CC</th>
<td>lebedev.ri@gmail.com, llvm-bugs@lists.llvm.org, regehr@cs.utah.edu, spatel+llvm@rotateright.com
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<pre>The following transformation in
Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-sgt-to-icmp-sgt.ll
is incorrect:
define <3 x i1> @p3_vec_splat_undef() {
%x = call <3 x i8> @gen3x8()
%tmp0 = and <3 x i8> %x, { 3, undef, 3 }
%ret = icmp sgt <3 x i8> %x, %tmp0
ret <3 x i1> %ret
}
=>
define <3 x i1> @p3_vec_splat_undef() {
%x = call <3 x i8> @gen3x8()
%1 = icmp sgt <3 x i8> %x, { 3, undef, 3 }
ret <3 x i1> %1
}
Transformation doesn't verify!
ERROR: Value mismatch
Example:
Source:
<3 x i8> %x = < poison, #x00 (0), poison >
<3 x i8> %tmp0 = < poison, #x00 (0), poison >
<3 x i1> %ret = < poison, #x0 (0), poison >
Target:
<3 x i8> %x = < poison, #x00 (0), poison >
<3 x i1> %1 = < poison, #x1 (1), poison >
Source value: < poison, #x0 (0), poison >
Target value: < poison, #x1 (1), poison >
The transformation does 'x > (x & undef)' -> 'x > undef'.
Take x=0, undef=-1, and we get '0 > 0' -> '0 > -1'.</pre>
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