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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - LTO uses front-end PGO even when IR-level was requested"
href="https://bugs.llvm.org/show_bug.cgi?id=45050">45050</a>
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<th>Summary</th>
<td>LTO uses front-end PGO even when IR-level was requested
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
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<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>dmajor@mozilla.com
</td>
</tr>
<tr>
<th>CC</th>
<td>davidxl@google.com, htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, tejohnson@google.com
</td>
</tr></table>
<p>
<div>
<pre>Observed on Linux with both clang 9.0.0 and a 10.0.0 release candidate.
Normally, the choice of flag should determine the PGO type.
`-fprofile-generate` uses IR-level, and `-fprofile-instr-generate` uses
frontend-level. But when LTO is enabled, the PGO is always frontend-level.
$ cat a.cpp
int main(int argc, char**) { return argc; }
$ clang++ -fprofile-generate -O2 a.cpp -o NoLTO
$ LLVM_PROFILE_FILE=NoLTO.profraw ./NoLTO
$ llvm-profdata show NoLTO.profraw | grep level
Instrumentation level: IR
$ clang++ -fprofile-generate -O2 a.cpp -o ThinLTO -flto=thin
$ LLVM_PROFILE_FILE=ThinLTO.profraw ./ThinLTO
$ llvm-profdata show ThinLTO.profraw | grep level
Instrumentation level: Front-end
$ clang++ -fprofile-generate -O2 a.cpp -o FullLTO -flto
$ LLVM_PROFILE_FILE=FullLTO.profraw ./FullLTO
$ llvm-profdata show FullLTO.profraw | grep level
Instrumentation level: Front-end</pre>
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