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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Pointless PHI after loop rotation"
href="https://bugs.llvm.org/show_bug.cgi?id=46583">46583</a>
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<tr>
<th>Summary</th>
<td>Pointless PHI after loop rotation
</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>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre><a href="https://godbolt.org/z/d2JskK">https://godbolt.org/z/d2JskK</a>
Given
%i.0 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%0 = and i32 %i.0, 1
<...>
%inc = add nuw nsw i32 %i.0, 1
loop rotation produces
%0 = phi i32 [ 0, %for.body.lr.ph ], [ %2, %for.body ]
%i.010 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
<...>
%inc = add nuw nsw i32 %i.010, 1
%2 = and i32 %inc, 1
Which seems kinda sub-optimal for obvious reasons - more registers used,
%inc remains anyway, we loose potential to lower it to `test`, etc.
Where would be the best place to undo this this?</pre>
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