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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 --- - nested loops 500x slower to compile at -O2 than -Os"
href="http://llvm.org/bugs/show_bug.cgi?id=16196">16196</a>
</td>
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<th>Summary</th>
<td>nested loops 500x slower to compile at -O2 than -Os
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>dhazeghi@yahoo.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>The following code takes much longer to compile at -O2 than at -Os, using
current clang-trunk on x86_64-linux. This is a regression from clang 3.0.
$ clang-trunk -v
clang version 3.4 (trunk 183083)
Target: x86_64-pc-linux-gnu
Thread model: posix
$ time clang-trunk -Os long.c
real 0m0.047s
user 0m0.025s
sys 0m0.017s
$ time clang-3.0 -O2 long.c
real 0m0.060s
user 0m0.036s
sys 0m0.016s
$ time clang-trunk -O2 long.c
real 0m25.436s
user 0m25.242s
sys 0m0.135s
-----------
int *a;
int b, d, e, f;
void fn1 ()
{
int c;
a = &c;
}
void fn2 ()
{
for (; b; ++b)
{
for (d = 0; d < 9; ++d)
{
for (e = 0; e >= -24; --e)
{
for (f = 0; f <= 31; f = f + 1)
fn1 ();
}
}
}
}
void fn3 ()
{
fn2 ();
}
int main ()
{
fn3 ();
return 0;
}</pre>
</div>
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