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<p class="MsoNormal">1%+ overhead is indeed interesting. If you use lld as linker (together with new pass manager), you should be able to have a good profile guided function level layout so dead functions are moved out of the hot pages.
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<p class="MsoNormal">This may also be related to subtle pass ordering issue. Pre-inline counts may not be super accurate, but we can’t use post-inline counts either given CGSCC inline is half way through the opt pipeline. Looking at the patch, it seems the
decision is made at PGO annotation time which is between pre-instrumentation inline and CGSCC inline. <o:p></o:p></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Modi Mo via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Reply-To: </b>Modi Mo <modimo@fb.com><br>
<b>Date: </b>Wednesday, September 9, 2020 at 6:18 PM<br>
<b>To: </b>Min-Yih Hsu <minyihh@uci.edu>, llvm-dev <llvm-dev@lists.llvm.org>, "cfe-dev (cfe-dev@lists.llvm.org)" <cfe-dev@lists.llvm.org>, Hongtao Yu <hoy@fb.com><br>
<b>Subject: </b>Re: [llvm-dev] [RFC] New Feature Proposal: De-Optimizing Cold Functions using PGO Info<o:p></o:p></span></p>
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<p class="MsoNormal">The 1.29% is pretty considerable on functions that should never be hit according to profile information. This can indicate that there might be something amiss with the profile quality and that certain hot functions are not getting caught.
Alternatively, given the ~5% code size increase you mention in the other thread the cold code may not be being moved out to a cold page so i-cache pollution ends up being a factor. I think it would be worthwhile to dig deeper into why there’s any performance
degradation on functions that should never be called.<o:p></o:p></p>
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<p class="MsoNormal">Also if you’re curious on how to build clang itself with PGO the documentation is here: https://llvm.org/docs/HowToBuildWithPGO.html<o:p></o:p></p>
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<p class="MsoNormal">On 9/8/20, 5:21 PM, "llvm-dev on behalf of Min-Yih Hsu via llvm-dev" <<a href="mailto:llvm-dev-bounces@lists.llvm.org">llvm-dev-bounces@lists.llvm.org</a> on behalf of
<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br>
<br>
We also did evaluations on FullLTO, here are the numbers:<br>
<br>
Experiment Name Link Time Speedup Target Overhead<br>
DeOpt Cold Zero Count 10.87% 1.29%<br>
DeOpt Cold 25% 18.76% 1.50%<br>
DeOpt Cold 50% 30.16% 3.94%<br>
DeOpt Cold 75% 38.71% 8.97%<br>
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