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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 - Failure to optimize tautological comparisons of comparisons to a single one"
href="https://bugs.llvm.org/show_bug.cgi?id=46476">46476</a>
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<th>Summary</th>
<td>Failure to optimize tautological comparisons of comparisons to a single one
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<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
</td>
</tr>
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<th>Reporter</th>
<td>gabravier@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>int f(int a, int b)
{
return (((b != 0) & (a == 0)) | ((a != 0) & (b == 0)));
}
This can be optimized to `(a != 0) ^ (b != 0)`. I originally found this while
compiling this code :
inline bool nand(bool a, bool b)
{
return !(a && b);
}
int f(int a, int b)
{
return nand(nand(b, nand(a, a)), nand(a, nand(b, b)));
}
Which GCC compiles to the above example, and that LLVM optimizes with the
transformation I gave (strangely, LLVM does not seem to optimize the example at
the top of this bug report to the transformed version if directly given the top
example, which is why I'm giving these details as I'm thinking there could be
some kind of UB weirdness or something like that with the transformations here)</pre>
</div>
</p>
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