<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - performance bug: libc++ std::isinf() with integer input is very slow vs libstdc++"
href="https://bugs.llvm.org/show_bug.cgi?id=32554">32554</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>performance bug: libc++ std::isinf() with integer input is very slow vs libstdc++
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>4.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>MacOS X
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>sean@rogue-research.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=18239" name="attach_18239" title="repro case">attachment 18239</a> <a href="attachment.cgi?id=18239&action=edit" title="repro case">[details]</a></span>
repro case
(filing this for a colleague that doesn't have an account here.)
While writing a templated function for both floating point and integral types,
I noticed that std::isinf with integer input is slow. I reduced the code to the
attached benchmark and ran it on macOS 10.11 and Fedora 25 with both clang and
GCC. With clang and libc++, using tag dispatch to first check if the type has
infinity speeds up the calculation by several times. With either compiler and
libstdc++, timings in both cases are fast and nearly identical.
It seems there is a performance bug in libc++'s std::isinf().
macOS 10.11 w/ clang 4.0.0 from homebrew
$ /usr/local/opt/llvm/bin/clang++ -O3 -std=c++1z isinf_libcxx.cpp
$ ./a.out
Using Detail::isinf
Time (s): 0.184412
Standard Deviation: 0.0100509
Using std::isinf
Time (s): 0.579823
Standard Deviation: 0.0205093
macOS 10.11 w/ gcc 6.3.0 from homebrew
$ g++-6 -O3 -std=c++1z isinf_libcxx?.cpp
$ ./a.out
Using Detail::isinf
Time (s): 0.195277
Standard Deviation: 0.016476
Using std::isinf
Time (s): 0.196114
Standard Deviation: 0.0161064
Fedora 25 w/ clang 3.9.1
$ clang++ -O3 -std=c++1z -stdlib=libc++ isinf_libcxx.cpp
$ ./a.out
Using Detail::isinf
Time (s): 0.161073
Standard Deviation: 0.00445347
Using std::isinf
Time (s): 1.40411
Standard Deviation: 0.089235
Fedora 25 w/ clang 3.9.1
$ clang++ -O3 -std=c++1z -stdlib=libstdc++ isinf_libcxx.cpp
$ ./a.out
Using Detail::isinf
Time (s): 0.162251
Standard Deviation: 0.00350547
Using std::isinf
Time (s): 0.160011
Standard Deviation: 0.00220987</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>