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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Missed forced vectorization due to llvm.mem.parallel_loop_access getting lost"
href="https://llvm.org/bugs/show_bug.cgi?id=28981">28981</a>
</td>
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<th>Summary</th>
<td>Missed forced vectorization due to llvm.mem.parallel_loop_access getting lost
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>opt
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>dorit.nuzman@intel.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=16953" name="attach_16953" title="Example loop with #pragma clang loop vectorize(assume_safety)">attachment 16953</a> <a href="attachment.cgi?id=16953&action=edit" title="Example loop with #pragma clang loop vectorize(assume_safety)">[details]</a></span>
Example loop with #pragma clang loop vectorize(assume_safety)
The following loop does not get vectorized because we lose some of the
information that is implied by "#pragma clang loop vectorize(assume_safety)":
void test(Complex *out, long size)
{
long m = size / 2;
#pragma clang loop vectorize(assume_safety)
for (long offset = 0; offset < m; ++offset) {
Complex t0 = out[offset];
Complex t1 = out[offset+m];
out[offset] = t0 + t1;
out[offset+m] = t0 - t1;
}
}
clang++ -S force-vect-test1.cpp -g -m64 -Ofast -funroll-loops -static
-mfpmath=sse -march=core-avx2 -Rpass-analysis=loop-vectorize
force-vect-test1.cpp:29:5: remark: loop not vectorized: cannot check memory
dependencies at runtime [-Rpass-analysis]
for (long offset = 0; offset < m; ++offset) {
^
force-vect-test1.cpp:29:5: warning: loop not vectorized: failed explicitly
specified loop vectorization [-Wpass-failed]
By the time this loop reaches the vectorizer, some of the memory operations in
the loop don't have the llvm.mem.parallel_loop_access metadata, because:
1) clang does not annotate the memcpy that it creates in the loop with the
llvm.mem.parallel_loop_access metadata (this is PR28980)
2) Even if we annotate the memcpy manually, InstCombine later replaces it with
a ld/st and does not copy over the llvm.mem.parallel_loop_access metadata (this
is D23499)
3) later SROA does not preserve the llvm.mem.parallel_loop_access metadata
during its transformations (will upload a proposed patch shortly)</pre>
</div>
</p>
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