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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 - UB avoiding bit rotate pattern is not recognized for uint8 and uint16"
href="https://bugs.llvm.org/show_bug.cgi?id=39624">39624</a>
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<th>Summary</th>
<td>UB avoiding bit rotate pattern is not recognized for uint8 and uint16
</td>
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<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>trass3r@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>The following pattern should be used to avoid UB and is also emitted by the
_rotl intrinsic in clang-cl mode but fails for small types:
uint8_t rotl(uint8_t x, unsigned int n)
{
constexpr unsigned width = 8*sizeof(x);
return (x << (n % width)) | (x >> (-n % width));
}
uint16_t rotl(uint16_t x, unsigned int n)
{
constexpr unsigned width = 8*sizeof(x);
return (x << (n % width)) | (x >> (-n % width));
}
uint32_t rotl(uint32_t x, unsigned int n)
{
constexpr unsigned width = 8*sizeof(x);
return (x << (n % width)) | (x >> (-n % width));
}
Inserting casts doesn't help.
<a href="https://godbolt.org/z/VQzPHk">https://godbolt.org/z/VQzPHk</a></pre>
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