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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] Loop vectorizer wrongly propagate 'nuw' flag"
href="https://bugs.llvm.org/show_bug.cgi?id=52111">52111</a>
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<th>Summary</th>
<td>[LV] Loop vectorizer wrongly propagate 'nuw' flag
</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>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>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>diegocaballero@google.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=25337" name="attach_25337" title="Reproducer">attachment 25337</a> <a href="attachment.cgi?id=25337&action=edit" title="Reproducer">[details]</a></span>
Reproducer
Hello,
I think we found a correctness issue in the loop vectorizer related to the
incorrect propagation of the ‘nuw’ flag for instructions guarded by a
condition. The following pseudo-code illustrates the problem:
for (i = 0; i<4; ++i) {
if(i > 0) {
… = a[i-1]
}
}
In this example, the computation of `i-1` could be flagged with ‘nuw’ because
the guarding condition `i > 0` guarantees that the unsigned result will always
be within the range [0 - 2] and, therefore, will never wrap around. However,
when the loop is vectorized and predication flattens the control flow, the
previous property is no longer true:
for (i = 0; i<4; i+=4) {
mask = {1, 1, 1, 0};
… = masked_load(&a[i-1], mask);
}
In this case, the computation of `i-1` is no longer guarded by the original
condition and the unsigned result will wrap-around for i=0. The vectorizer
currently preserves the original ‘nuw’ value for `i-1`, which leads to a poison
value that is later optimized away, resulting in an incorrect vector code.
Please, find attached a test case. The problematic code before vectorization
is:
concat_index_from_operand_id1: ; preds =
%concatenate.pivot.1.4
%20 = add nsw i64 %fusion.indvar.dim.32, -1
%21 = mul nuw nsw i64 %fusion.indvar.dim.24, 3
%22 = add nuw nsw i64 %20, %21
%23 = getelementptr inbounds [1 x [6 x float]], [1 x [6 x float]]* %11, i64
0, i64 0, i64 %22
After vectorization (opt before_lv.ll -scoped-noalias-aa -loop-vectorize
-ipsccp -mcpu=skx -S -o after_lv.ll):
vector.body: ; preds = %vector.ph
%14 = mul nuw nsw i64 %fusion.indvar.dim.24, 3
%15 = add nuw nsw i64 -1, %14 // nuw & -1 -> Poison!
%16 = getelementptr inbounds [1 x [6 x float]], [1 x [6 x float]]* %11, i64
0, i64 0, i64 %15
I haven’t thought too much about the solution. Maybe we could drop the
‘nuw’/’nsw’ flags of uniform operations that are not immediately nested in the
loop body. Perhaps, during predication?
Thanks,
Diego</pre>
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