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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Incorrect fold of uadd.with.overflow with undef"
href="https://bugs.llvm.org/show_bug.cgi?id=43188">43188</a>
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<th>Summary</th>
<td>Incorrect fold of uadd.with.overflow with undef
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Keywords</th>
<td>miscompilation
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<th>Severity</th>
<td>normal
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Scalar Optimizations
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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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, nikita.ppv@gmail.com, regehr@cs.utah.edu
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<p>
<div>
<pre>Alive2 complains about a transformation in
Transforms/ConstProp/overflow-ops.ll:
define {i8, i1} @uadd_undef() {
%0:
%t = uadd_overflow i8 142, undef
ret {i8, i1} %t
}
=>
define {i8, i1} @uadd_undef() {
%0:
ret {i8, i1} undef
}
Transformation doesn't verify!
ERROR: Value mismatch
Example:
Source:
{i8, i1} %t = { #x8e (142, -114), #x0 (0) } [based on undef value]
Target:
Source value: { #x8e (142, -114), #x0 (0) } [based on undef value]
Target value: { #x00 (0), #x0 (0) }
In summary, there's no value in the source that the undef can take that allows
uadd to return {0, 0}. To return 0, it has to overflow (unsigned), and hence
the 2nd value would be 1.
Two valid return values I can think off: {%a, 0}; {undef, 1}.</pre>
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