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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86] cmov conversion hurts binary search performance"
href="https://bugs.llvm.org/show_bug.cgi?id=40027">40027</a>
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<th>Summary</th>
<td>[X86] cmov conversion hurts binary search performance
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>nikita.ppv@gmail.com
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>Originally reported at: <a href="https://github.com/rust-lang/rust/issues/53823">https://github.com/rust-lang/rust/issues/53823</a>
The cmov conversion pass converts a cmov in Rust's binary search implementation
into a branch. As binary search branches are badly predicted, this
significantly hurts performance. For small data sets, which are not dominated
by cache performance, the slowdown is around 2-3x.
<a href="https://gist.github.com/nikic/13e907d7097f74a92d082fcc61bc212c">https://gist.github.com/nikic/13e907d7097f74a92d082fcc61bc212c</a> has the LLVM IR
for the function, as well as generated assembly with cmov conversion enabled
(default) and disabled (-x86-cmov-converter=false).
It would be great if the cmov conversion heuristics could be adjusted in some
way to avoid converting this case.</pre>
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