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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [AArch64] Second LoopUnrollPass may cause compile-time regression with chained stores"
href="http://llvm.org/bugs/show_bug.cgi?id=21759">21759</a>
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<th>Summary</th>
<td>[AArch64] Second LoopUnrollPass may cause compile-time regression with chained stores
</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>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: AArch64
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>mcrosier@codeaurora.org
</td>
</tr>
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<th>CC</th>
<td>apazos@codeaurora.org, chandlerc@gmail.com, james.molloy@arm.com, liujiangning1@gmail.com, llvmbugs@cs.uiuc.edu, t.p.northover@gmail.com
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=13437" name="attach_13437" title="Bad IR">attachment 13437</a> <a href="attachment.cgi?id=13437&action=edit" title="Bad IR">[details]</a></span>
Bad IR
Unfortunately, I can only reproduce this on our internal branch, but the
problem exists on mainline as well.
Given the following loop:
void foo (int a[])
{
int *ptr = a;
for (int i = 0; i <= 127; i++)
*ptr++ = 0xff;
}
Our internal branch will completely unroll the loop during the second call to
the LoopUnrollPass. The resulting IR (see attached bad.ll) causes a severe
compile-time regression when instruction selection begins recursively calling
WalkChainUsers in SelectionDAGISel.cpp.
If unrolling occurs during the first call to the LoopUnrollPass a later call to
InstCombine removes the chain by having all the gep use base + offset (see
attached bad.instcombine.ll).
I imagine the simplest solution would be to run InstCombine after the second
loop unrolling pass, but I'd like to get feedback before moving forward.</pre>
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