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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - MergedLoadStoreMotion is not actually preserving MemDep"
href="https://bugs.llvm.org/show_bug.cgi?id=36063">36063</a>
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<th>Summary</th>
<td>MergedLoadStoreMotion is not actually preserving MemDep
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<th>Product</th>
<td>new-bugs
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Linux
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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>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>bsteinbr@gmail.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>we recently made some slight adjustments the pass order for the Rust compiler,
and that exposed what I believe to be a bug in either MergedLoadStoreMotion or
MemDep.
The rust issue this is about is: <a href="https://github.com/rust-lang/rust/issues/47364">https://github.com/rust-lang/rust/issues/47364</a>
What's ultimately causing the failure is a misoptimization happenings in GVN,
the code looks roughly like this:
%x = getelementptr ...
store i64 %val, i64* %x
// ...
%val2 = load i64, i64* %x
// ..
store i64 %val2, i64* %dst
Which GVN replaces with:
store i64 %val, i64* %x
// ...
store i64 undef, i64* %dst
The problem here is that before MLSM ran, the initial store of %val to %x was
in a different BB, which caused MemDep to create a local dep cache entry marked
as non-local.
MLSM then actually turned that into a local dependency, but the cache entry was
never invalidated, because the non-local dependency was never resolved, so
there is no entry in the reserve lookup tables that would have allowed
removeInstruction() to find the cache entry.
So GVN still gets a non-local result, asks MemDep to evaluate it, and gets the
uninitialized alloca instead of the store instruction as a result, causing it
to replace the store with an undef.
Unfortunately I don't have a minimized test case for this yet.</pre>
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