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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Missed optimization due to incompleteness in GVN"
href="https://llvm.org/bugs/show_bug.cgi?id=31463">31463</a>
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<th>Summary</th>
<td>Missed optimization due to incompleteness in GVN
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<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>
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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>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>dberlin@dberlin.org
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Code that looks like this:
%.0 = phi i32 [ 10, %4 ], [ 5, %2 ]
%11 = add nsw i32 %.0, 5
generates a constant (phi i32 [15, %4], [10, %2])), but neither GVN nor NewGVN
will detect this.
This is due to one of the standard forms of GVN incompleteness:
the missing equivalence of A op Phi (B, C) and Phi(A op B, A op C)
(IE add 5, phi [10], [5] == phi [add 5, 10], [add 5, 5])
It's exponential time/space to do the transforms back and forth in every case
(because the arguments can be N levels deep)
However, for phis of all constants, and operations on them, we may want to do
it.</pre>
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