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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Reassociation messes up SLPVectorizer reduction"
href="https://bugs.llvm.org/show_bug.cgi?id=36481">36481</a>
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<th>Summary</th>
<td>Reassociation messes up SLPVectorizer reduction
</td>
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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>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>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Scalar Optimizations
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>timshen91@gmail.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
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<pre>Created <span class=""><a href="attachment.cgi?id=19931" name="attach_19931" title="Test">attachment 19931</a> <a href="attachment.cgi?id=19931&action=edit" title="Test">[details]</a></span>
Test
Running `opt -S -slp-vectorizer $TEST_FILE`, the function gets SLP-vectorized;
Running `opt -S -reassociate -slp-vectorizer $TEST_FILE`, the function doesn't
get SLP-vectorized.
This is because the reassociation pass re-associates instructions like `acc =
add ith_element, acc` to `acc = add acc, ith_element`, but didn't re-assocate
the first instruction `acc = add first_element, second_element` to `acc = add
second_element, first_element`.
However, as add is associative, a smarter SLP vectorizer could have ignored the
associations and vectorize the unordered adds anyway.
I'm not sure where to put the fix on.</pre>
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