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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Infinite loop in DAGCombiner::Run"
href="https://bugs.llvm.org/show_bug.cgi?id=46877">46877</a>
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<th>Summary</th>
<td>Infinite loop in DAGCombiner::Run
</td>
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<th>Product</th>
<td>new-bugs
</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>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>ruhler@google.com
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, qshanz@cn.ibm.com, ruhler@google.com, spatel+llvm@rotateright.com
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=23783" name="attach_23783" title="The program and DAG that hangs.">attachment 23783</a> <a href="attachment.cgi?id=23783&action=edit" title="The program and DAG that hangs.">[details]</a></span>
The program and DAG that hangs.
For the program hangs.ll in the attached hangs.tar.gz, the following command
hangs:
llc -O3 -mcpu=haswell hangs.ll
Debugging shows it is stuck in an infinite while loop processing the worklist
in the function DAGCombiner::Run in
llvm-project/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp.
Debug logging suggests it is repeatedly trying to combine the same node without
making any progress:
Combining: t2747: f32 = fmul nsz contract reassoc t2742, t2746
Combining: t2604: f32 = fmul nsz contract reassoc t2594, t2636
Combining: t2746: f32 = fmul nsz contract reassoc t2744, t36
Combining: t2747: f32 = fmul nsz contract reassoc t2742, t2746
Combining: t2689: f32 = fmul nsz contract reassoc t52, t2726
Combining: t2746: f32 = fmul nsz contract reassoc t2744, t36
Combining: t2747: f32 = fmul nsz contract reassoc t2742, t2746
...
See dag.txt in the attached hangs.tar.gz for a dump of the DAG in question and
additional debug logs.
The infinite loop reproduces on trunk today (git revision
3fac05e49fe3eb4d6becf1761b4df01dbd871aa4).
git bisect shows the infinite loop first started appearing at:
commit 2b59e9f1bdd8d0f8a315f9ddba2cdc87a3e682fb
Author: QingShan Zhang <<a href="mailto:qshanz@cn.ibm.com">qshanz@cn.ibm.com</a>>
Date: Wed May 20 02:11:16 2020 +0000
[DAGCombine] Remove the getNegatibleCost to avoid the out of sync with
getNegatedExpression</pre>
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</p>
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