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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LICM sinking a single unused load out of a loop prevents DSE in some other function!?!"
href="http://llvm.org/bugs/show_bug.cgi?id=18773">18773</a>
</td>
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<tr>
<th>Summary</th>
<td>LICM sinking a single unused load out of a loop prevents DSE in some other function!?!
</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>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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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>chandlerc@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</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=12031" name="attach_12031" title="reduced test case">attachment 12031</a> <a href="attachment.cgi?id=12031&action=edit" title="reduced test case">[details]</a></span>
reduced test case
This is crazy weird. The essential problem is that LICM got slightly more
powerful in r200067 and started sinking loads out of the loop body in a corner
case. When it does so, we mysteriously stop being able to DSE stores in *a
separate function*!
The code for this is heavily reduced from Adobe-C++/loop_unroll.cpp. To
reproduce the weird behavior, take an "opt" binary from trunk and "old opt"
from r200066, and compare:
% opt < loop_unroll.reduced.ll -std-link-opts | llc -O3 -o loop_unroll.new.s
% clang++ -lm -o loop_unroll.new loop_unroll.new.s
vs.
% old_opt < loop_unroll.reduced.ll -std-link-opts | llc -O3 -o
loop_unroll.old.s
% clang++ -lm -o loop_unroll.old loop_unroll.old.s
When I benchmark these on my sandybridge machine I get:
% perf stat -r5 ./loop_unroll.new && perf stat -r5 ./loop_unroll.old
Performance counter stats for './loop_unroll.new' (5 runs):
1376.720033 task-clock # 0.998 CPUs utilized
( +- 0.10% )
1 context-switches # 0.001 K/sec
( +- 31.62% )
0 cpu-migrations # 0.000 K/sec
( +- 61.24% )
152 page-faults # 0.111 K/sec
( +- 0.16% )
5,208,493,602 cycles # 3.783 GHz
( +- 0.02% )
2,657,152,524 stalled-cycles-frontend # 51.02% frontend cycles idle
( +- 0.04% )
279,081,881 stalled-cycles-backend # 5.36% backend cycles idle
( +- 0.34% )
8,515,758,351 instructions # 1.63 insns per cycle
# 0.31 stalled cycles per insn
( +- 0.00% )
356,723,894 branches # 259.111 M/sec
( +- 0.00% )
201,373 branch-misses # 0.06% of all branches
( +- 0.01% )
1.378844222 seconds time elapsed
( +- 0.10% )
Performance counter stats for './loop_unroll.old' (5 runs):
877.683533 task-clock # 0.998 CPUs utilized
( +- 0.09% )
1 context-switches # 0.001 K/sec
( +- 40.82% )
0 cpu-migrations # 0.000 K/sec
( +-100.00% )
152 page-faults # 0.174 K/sec
( +- 0.16% )
3,320,502,190 cycles # 3.783 GHz
( +- 0.00% )
11,331,992 stalled-cycles-frontend # 0.34% frontend cycles idle
( +- 2.39% )
35,003,292 stalled-cycles-backend # 1.05% backend cycles idle
( +- 1.14% )
6,978,870,054 instructions # 2.10 insns per cycle
# 0.01 stalled cycles per insn
( +- 0.00% )
356,371,371 branches # 406.036 M/sec
( +- 0.00% )
200,790 branch-misses # 0.06% of all branches
( +- 0.04% )
0.879302485 seconds time elapsed
( +- 0.09% )
Note the 50% stalled cycles on the new one!!!
I'm working on getting two A/B inputs to trunk 'opt' that exhibit the behavior,
lacking any good ideas about why its actually happening.
Note that I've checked -- top of tree and r200067 behave exactly the same. The
change is only in the patch committed with r200067.</pre>
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