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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Vectorizer failure"
href="https://llvm.org/bugs/show_bug.cgi?id=26314">26314</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Vectorizer failure
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>egor.kochetov@intel.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=15718" name="attach_15718" title="The reproducer (20 SLOC, either C or C++, does not matter)">attachment 15718</a> <a href="attachment.cgi?id=15718&action=edit" title="The reproducer (20 SLOC, either C or C++, does not matter)">[details]</a></span>
The reproducer (20 SLOC, either C or C++, does not matter)
While compiling the attached 'main.cpp' I've encountered clang vectorization
problem, the performance difference is 58ms vs 20ms (almost 200%). After the
commit r257134 clang fails to vectorize 'slow_function'.
I compile with:
clang++ -O2 -m64 -march=slm -mfpmath=sse -fPIE -flto -g -fvectorize
Results for -O2 and -Ofast, for -m32 and -m64 are all the same.
Here is what I get from an old (fast) clang:
main.cpp:11:9: remark: the cost-model indicates that interleaving is not
beneficial [-Rpass-analysis=loop-vectorize]
for (int k=0; k<Z; k++) {
Here is what I get from a new (slow) clang:
main.cpp:11:9: remark: loop not vectorized: cannot check memory
dependencies at runtime [-Rpass-analysis=loop-vectorize]
for (int k=0; k<Z; k++) {
The triggering SVN change is:
Index: lib/Analysis/LoopAccessAnalysis.cpp
===================================================================
--- lib/Analysis/LoopAccessAnalysis.cpp (revision 257133)
+++ lib/Analysis/LoopAccessAnalysis.cpp (revision 257134)
@@ -845,6 +845,7 @@
if (Lp != AR->getLoop()) {
DEBUG(dbgs() << "LAA: Bad stride - Not striding over innermost loop "
<<
*Ptr << " SCEV: " << *PtrScev << "\n");
+ return 0;
}
// The address calculation must not wrap. Otherwise, a dependence could
be
For comparison, here is what I get from GCC 6.0 trunk with
'-fopt-info-vec-note':
loop at main.cpp:12: if (ivtmp_126 >= bnd.8_75)
main.cpp:11:24: note: LOOP VECTORIZED
The binary compiled by GCC is slightly faster than that compiled by old clang
(well, if I add fwrite of s to /dev/null at the end of 'main').</pre>
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