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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 - simple-loop-unswitch is too aggressive in unswitching leading to compile-time issues"
href="https://bugs.llvm.org/show_bug.cgi?id=39544">39544</a>
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<th>Summary</th>
<td>simple-loop-unswitch is too aggressive in unswitching leading to compile-time issues
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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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>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>fedor.v.sergeev@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=21075" name="attach_21075" title="real testcase leading to excessive unswitch">attachment 21075</a> <a href="attachment.cgi?id=21075&action=edit" title="real testcase leading to excessive unswitch">[details]</a></span>
real testcase leading to excessive unswitch
On a relatively simple testcase simple-loop-unswitch goes unswitching too many
times, generating too many copies of loops and forcing compile-time to become
abnormally high.
] time ~/work/llvm-upstream/builds/linux-release/bin/opt
-enable-nontrivial-unswitch -licm -simple-loop-unswitch
after-reassociate-upstream.ll -disable-output --stats
real 5m17.711s
user 4m33.670s
sys 0m0.188s
]
This testcase is a slightly simplified version of IR we generate on real java
code, so it is a very real practical issue for us.
Here is its loop structure (nothing really weird):
] ~/work/llvm-upstream/builds/linux-release/bin/opt -passes="print<loops>"
unswitch-compile-time.ll -disable-output 2>&1 | sed -n
's/\(Loop.at.depth.[0-9]*\).*/\1/p'
Loop at depth 1
Loop at depth 1
Loop at depth 1
Loop at depth 1
Loop at depth 1
Loop at depth 1
Loop at depth 1
Loop at depth 2
Loop at depth 2
Loop at depth 3
Loop at depth 4
Loop at depth 5
Loop at depth 6
Loop at depth 7
Loop at depth 8
Loop at depth 9
Loop at depth 10
Loop at depth 11
Loop at depth 12
Loop at depth 13
Loop at depth 14
Loop at depth 14
Loop at depth 14
Loop at depth 14
]
It has ~2K lines and ~200 conditions:
] wc -l unswitch-compile-time.ll
2317 unswitch-compile-time.ll
] grep 'br i1 ' -c unswitch-compile-time.ll
197
]
And it goes pretty fast with legacy unswitch:
] time ~/work/llvm-upstream/builds/linux-release/bin/opt
-enable-nontrivial-unswitch -licm -loop-unswitch unswitch-compile-time.ll
-disable-output
real 0m0.018s
user 0m0.018s
sys 0m0.000s
]</pre>
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