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<p class="MsoNormal">Hi Scott,<o:p></o:p></p>
<p class="MsoNormal">I’m actually fixing a correctness problem in that function and the node ordering plays a role in it. I will put more attention to trying to reduce the complexity there.<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:9.0pt;font-family:Consolas">-- </span>
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<p class="MsoNormal"><span style="font-size:8.0pt;font-family:Consolas">Krzysztof Parzyszek
<a href="mailto:kparzysz@quicinc.com">kparzysz@quicinc.com</a> AI tools development<o:p></o:p></span></p>
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<p class="MsoNormal"><b>From:</b> Constable, Scott D <scott.d.constable@intel.com>
<br>
<b>Sent:</b> Thursday, April 30, 2020 3:49 PM<br>
<b>To:</b> llvm-dev@lists.llvm.org; Krzysztof Parzyszek <kparzysz@quicinc.com><br>
<b>Cc:</b> Matthew Riley <mattdr@google.com>; Topper, Craig <craig.topper@intel.com><br>
<b>Subject:</b> [EXT] [RDF] Performance Issues for rdf::Liveness::computePhiInfo()<o:p></o:p></p>
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<p class="MsoNormal">Hi Krzysztof,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I have been working to debug some performance issues in <a href="https://reviews.llvm.org/D75936">
https://reviews.llvm.org/D75936</a>, and I have found that the main issue is rdf::Liveness::computePhiInfo(). Specifically, a single line introduces 20-30% overhead for the entire llc X86 pipeline:<o:p></o:p></p>
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<p class="MsoNormal"><a href="https://github.com/llvm/llvm-project/blob/b5be1c5419e2a38eb60fc7e785567b54b6d9e0e0/llvm/lib/CodeGen/RDFLiveness.cpp#L208">https://github.com/llvm/llvm-project/blob/b5be1c5419e2a38eb60fc7e785567b54b6d9e0e0/llvm/lib/CodeGen/RDFLiveness.cpp#L208</a><o:p></o:p></p>
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<p class="MsoNormal">Linux perf shows that the culprit is MachineDominatorTree::dominates(), which is being invoked on nearly every less-than comparison between nodes. The documentation for dominates() reads “dominates -Returns true iff A dominates B. Note
that this is not a constant time operation!”<o:p></o:p></p>
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<p class="MsoNormal">Do you think that this could be fixed? Maybe some memorization could help?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks and regards,<o:p></o:p></p>
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<p class="MsoNormal">Scott Constable<o:p></o:p></p>
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