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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Support for Relative VTables C++ ABI in Whole Program Devirtualization"
href="https://bugs.llvm.org/show_bug.cgi?id=50048">50048</a>
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<th>Summary</th>
<td>Support for Relative VTables C++ ABI in Whole Program Devirtualization
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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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>Global Analyses
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>leonardchan@google.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Currently, whole program devirtualization doesn't actually take effect on
vtables built under the relative vtables ABI. The `DevirtModule` pass is still
run, but it doesn't detect any vtable call slots because the transformation
attempts to pattern match against instructions that look like a load into the
vtable (ie. bitcasts, loads, GEPs).
More specifically, within `DevirtModule::run`, virtual function call sites are
added through a call to `DevirtModule::scanTypeTestUsers`, which eventually
calls `findLoadCallsAtConstantOffset`. This function attempts to check if calls
made to a vptr are done through bitcasts, loads, or GEPs. Under RV, loads to
the vtable are instead made with a call to the intrinsic `llvm.load.relative`.
Because we don't currently check for this intrinsic, no call sites are
recorded, and the pass doesn't make any transformations. This means that
devirtualization won't actually take place under RV.
A first step would be to add the proper checks for supporting RV in this case.
It's likely that we will run into other build/runtime issues after this though
since there may be other assumptions made about the vtable layout or how it's
accessed within WPD.</pre>
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