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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [SCEV] Inconsistent SCEV formation for zext"
href="https://bugs.llvm.org/show_bug.cgi?id=36738">36738</a>
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<th>Summary</th>
<td>[SCEV] Inconsistent SCEV formation for zext
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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>All
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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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>Global Analyses
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>pankaj.chawla@intel.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
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<pre>Created <span class=""><a href="attachment.cgi?id=20063" name="attach_20063" title="test case">attachment 20063</a> <a href="attachment.cgi?id=20063&action=edit" title="test case">[details]</a></span>
test case
SCEV is behaving inconsistently when forming SCEV for this zext instruction in
the attached test case-
%conv5 = zext i32 %dec to i64
If we request a SCEV for the instruction, it returns-
(zext i32 {{-1,+,1}<nw><%for.body>,+,-1}<nw><%for.body7> to i64)
This can be seen by invoking-
$ opt -analyze -scalar-evolution inconsistent-scev-zext.ll
But when computing the backedge taken count of the containing loop for.body7,
it is able to push zext inside the AddRec and forms the following for the same
instruction-
{(zext i32 {-1,+,1}<%for.body> to i64),+,-1}<nw><%for.body7>
This allows ScalarEvolution to compute a valid backedge taken count for the
loop.
Reference to email discussion regarding the bug-
<a href="http://lists.llvm.org/pipermail/llvm-dev/2018-February/121107.html">http://lists.llvm.org/pipermail/llvm-dev/2018-February/121107.html</a></pre>
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