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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Canonicalization/simplification loop between LoopSimplify and CFGSimplify can cause compilation time slowdown"
href="https://llvm.org/bugs/show_bug.cgi?id=30895">30895</a>
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<th>Summary</th>
<td>Canonicalization/simplification loop between LoopSimplify and CFGSimplify can cause compilation time slowdown
</td>
</tr>
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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>All
</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
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>igor@azulsystems.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>LoopSimplify requires loop preheader to dominate all exit blocks. In order to
achieve that it splits loop exit blocks that have predecessors outside of the
loop. CFGSimplify merges them back. This cycle repeats multiple times during
compilation.
This process causes around 40% compile time slowdown on one of our internal
out-of-tree tests. This test is somewhat absurd since it contains around 1700
loops nested in each other up to the 8th level.
You can observe described behaviour using this simple reproducer:
declare void @some_call()
define i32 @test(i32 %cnt, i32 %cnt-inner) personality i32* ()*
@"personality_function" {
entry:
br label %loop
loop:
%i = phi i32 [0, %entry], [%i.next, %loop-latch]
invoke void @some_call()
to label %inner-loop unwind label %exception1
inner-loop:
%j = phi i32 [0, %loop], [%j.next, %inner-loop-latch]
invoke void @some_call()
to label %inner-loop-latch unwind label %exception1
inner-loop-latch:
%j.next = add i32 %j, 1
%inner-exit-cond = icmp sle i32 %j.next, %cnt-inner
br i1 %inner-exit-cond, label %inner-loop, label %loop-latch
loop-latch:
%i.next = add i32 %i, 1
%exit-cond = icmp sle i32 %i.next, %cnt
br i1 %exit-cond, label %loop, label %loop-normal-exit
loop-normal-exit:
ret i32 0
exception1:
%landing-pad1 = landingpad { i8*, i32 }
cleanup
ret i32 1
exception2:
%landing-pad2 = landingpad { i8*, i32 }
cleanup
ret i32 2
}
declare i32* @"personality_function"()</pre>
</div>
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