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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - InstCombine can't fold "select %c, undef, %foo" to %foo (miscompile)"
href="https://llvm.org/bugs/show_bug.cgi?id=31633">31633</a>
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<th>Summary</th>
<td>InstCombine can't fold "select %c, undef, %foo" to %foo (miscompile)
</td>
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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
</td>
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<th>OS</th>
<td>All
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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
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<th>Component</th>
<td>Scalar Optimizations
</td>
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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>david.majnemer@gmail.com, gil.hur@sf.snu.ac.kr, juneyoung.lee@sf.snu.ac.kr, llvm-bugs@lists.llvm.org, regehr@cs.utah.edu, sanjoy@playingwithpointers.com, spatel+llvm@rotateright.com
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>InstCombine currently folds "select %c, undef, %foo" into %foo, because it
assumes that undef can take any value that %foo may take.
This is not correct since %foo may be poison.
This problem has long been know, but I'm adding now an end-to-end
miscompilation example triggered by this bug.
$ cat select-undef.ll
define i1 @f(i1 %c, i32 %y) {
%y2 = add nsw i32 %y, 1
%s = select i1 %c, i32 undef, i32 %y2
%r = icmp sgt i32 %s, %y
ret i1 %r
}
$ opt -S select-undef.ll -instcombine
define i1 @f(i1 %c, i32 %y) {
ret i1 true
}
Which is wrong for the case %y=0x7FFFFFFF and %c=true. %y2 overflows and
becomes poison, but the select should return undef only, not poison.
Alive report: <a href="http://rise4fun.com/Alive/XGW">http://rise4fun.com/Alive/XGW</a>
Related with PR31632.</pre>
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