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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [BranchFolding][AArch64] AArch64LoadStoreOpt uses MMOs which are dropped during tail merge"
href="https://llvm.org/bugs/show_bug.cgi?id=25199">25199</a>
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<th>Summary</th>
<td>[BranchFolding][AArch64] AArch64LoadStoreOpt uses MMOs which are dropped during tail merge
</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>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: AArch64
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>mcrosier@codeaurora.org
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>In r231799, MMOs were removed during tail merge to preserve dependencies.
>From the commit message:
"A more robust solution would be to add multiple MMOs from the duplicate MIs to
the new MI. Currently ScheduleDAGInstrs.cpp ignores all MMOs on instructions
with multiple MMOs, so this solution is equivalent for the time being."
The final comment isn't quite true. This change doesn't effect
ScheduleDAGInstrs, but it does prevent the load/store optimization pass from
generating load/store pairs. Because the MMOs are dropped the pass
conservatively assumes the memory instructions are volatile.
>From MachineInstr::hasOrderedMemoryRef()
// Otherwise, if the instruction has no memory reference information,
// conservatively assume it wasn't preserved.
if (memoperands_empty())
return true;
This check causes the pass to bailout.</pre>
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