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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Non-deterministic result due to use of SmallPtrSet in llvm::removeUnreachableBlocks"
href="https://bugs.llvm.org/show_bug.cgi?id=42574">42574</a>
</td>
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<th>Summary</th>
<td>Non-deterministic result due to use of SmallPtrSet in llvm::removeUnreachableBlocks
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</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>Transformation Utilities
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mikael.holmen@ericsson.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<pre>Created <span class=""><a href="attachment.cgi?id=22222" name="attach_22222" title="reproducer">attachment 22222</a> <a href="attachment.cgi?id=22222&action=edit" title="reproducer">[details]</a></span>
reproducer
Running
opt -simplifycfg -o foo.ll in3.ll -S --preserve-ll-uselistorder
a number of times with a clang 8 built opt we see that the produced output
differs now and then like:
< uselistorder i16 0, { 2, 3, 4, 1, 5, 0, 6 }
---
> uselistorder i16 0, { 3, 2, 4, 1, 5, 0, 6 }
I think it's the DeadBlockSet that is the cause:
- // Loop over all of the basic blocks that are not reachable, dropping all of
- // their internal references. Update DTU and LVI if available.
- std::vector <DominatorTree::UpdateType> Updates;
+ SmallPtrSet<BasicBlock *, 16> DeadBlockSet;
As far as I can see, as long as a SmallPtrSet is "small", the iteration order
is deterministic (== insertion order), but I strongly suspect that when the set
grows to being "big", then it's not deterministic anymore.
DeadBlockSet as above was introduced in r339951:
Update MemorySSA in Local utils removing blocks.
Summary: Extend Local utils to update MemorySSA.
Subscribers: sanjoy, jlebar, Prazek, george.burgess.iv, llvm-commits
Differential Revision: <a href="https://reviews.llvm.org/D48790">https://reviews.llvm.org/D48790</a>
llvm-svn: 339951
Later this caused SCEV and LSR in llc to draw different conclusions and
produce different code.</pre>
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