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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - LoopVectorizer produces wrong code with UserVF not dividing small TripCount"
href="https://bugs.llvm.org/show_bug.cgi?id=45679">45679</a>
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<th>Summary</th>
<td>LoopVectorizer produces wrong code with UserVF not dividing small TripCount
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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>Loop Optimizer
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>ayal.zaks@intel.com
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<th>CC</th>
<td>dorit.nuzman@intel.com, florian_hahn@apple.com, gil.rapaport@intel.com, llvm-bugs@lists.llvm.org
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<pre>Created <span class=""><a href="attachment.cgi?id=23402" name="attach_23402" title="Reproducer">attachment 23402</a> <a href="attachment.cgi?id=23402&action=edit" title="Reproducer">[details]</a></span>
Reproducer
Compiling the attached with
opt -loop-vectorize -force-vector-width=4 -S
vectorizes a loop with trip-count=14 using VF=4 w/o a tail, explaining that:
LV: Found trip count: 14
LV: Not allowing scalar epilogue due to low trip count.
LV: Performing code size checks.
LV: The Smallest and Widest types: 32 / 32 bits.
LV: The Widest register safe to use is: 32 bits.
LV: No tail will remain for any chosen VF. <== but LV doesn't get to chose VF
LV: Using user VF 4.
Culprit is computeMaxVF() who checks if TC % MaxVF == 0 where MaxVF =
computeFeasibleMaxVF(TC) (=1 in this case), even if a UserVF is given and
ultimately used.</pre>
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