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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [LV] predicated strided loads vectorized as interleave-group without masking"
href="https://bugs.llvm.org/show_bug.cgi?id=39099">39099</a>
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<th>Summary</th>
<td>[LV] predicated strided loads vectorized as interleave-group without masking
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>All
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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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>dorit.nuzman@intel.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<pre>Created <span class=""><a href="attachment.cgi?id=20933" name="attach_20933" title="testcase">attachment 20933</a> <a href="attachment.cgi?id=20933&action=edit" title="testcase">[details]</a></span>
testcase
There’s a bug in how we currently handle predicated strided loads -- we end up
vectorizing them without a mask... :
When strided loads are predicated, each of them will form an interleaved-group
(with gaps). However, subsequent stages of vectorization (planning and
transformation) assume that if a load is part of an Interleave-Group it is not
predicated, resulting in wrong code - unmasked wide loads are created.
Compiling the attached testcase with:
opt -loop-vectorize -enable-interleaved-mem-accesses -force-vector-width=8
-march=skx -S predicated-interleaved-bug.ll
you'll see we happily generate unmasked wide-loads and shuffles:
vector.body:
...
%wide.vec = load <16 x i8>, <16 x i8>* %5, align 1
%strided.vec = shufflevector <16 x i8> %wide.vec, <16 x i8> undef, <8 x i32>
<i32 0, i32 2, i32 4, i32 6, i32 8, i32 10, i32 12, i32 14>
%6 = or <8 x i32> %1, <i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32
1>
...
%wide.vec1 = load <16 x i8>, <16 x i8>* %10, align 1
%strided.vec2 = shufflevector <16 x i8> %wide.vec1, <16 x i8> undef, <8 x
i32> <i32 0, i32 2, i32 4, i32 6, i32 8, i32 10, i32 12, i32 14>
...
The fix is simple - don’t allow analyzeInterleaving() open an interleave group
if the load/store is predicated. I'll upload the fix shortly.</pre>
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