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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Suboptimal instruction schedule leads to spilling and poor performance"
href="https://bugs.llvm.org/show_bug.cgi?id=46934">46934</a>
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<th>Summary</th>
<td>Suboptimal instruction schedule leads to spilling and poor performance
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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>PC
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<th>OS</th>
<td>All
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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>Register Allocator
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>benny.kra@gmail.com
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<th>CC</th>
<td>echristo@gmail.com, llvm-bugs@lists.llvm.org, ntv@google.com, quentin.colombet@gmail.com
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=23801" name="attach_23801" title="test case">attachment 23801</a> <a href="attachment.cgi?id=23801&action=edit" title="test case">[details]</a></span>
test case
When looking at a matmul benchmark we're seeing steep performance cliffs after
tiny changes to the IR. This seems to be due to excessive spilling.
$ llc -mcpu=skx < matmul.ll | grep -c Spill
13
Picking a different scheduler can make this go away. I think it's just getting
lucky though:
$ llc -mcpu=skx < matmul.ll -pre-RA-sched=list-ilp | grep -c Spill
1
The input contains a loop with lots of loads, followed by arithmetic. It is
possible to move some of the loads down into the arithmetic to reduce register
pressure.</pre>
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