<html>
<head>
<base href="http://llvm.org/bugs/" />
</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 --- - Feature Request: Spot redundant lambda and std::function<> construction/destruction in loops"
href="http://llvm.org/bugs/show_bug.cgi?id=20190">20190</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Feature Request: Spot redundant lambda and std::function<> construction/destruction in loops
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Static Analyzer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>kremenek@apple.com
</td>
</tr>
<tr>
<th>Reporter</th>
<td>s_llvm@nedprod.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>An inefficient code pattern I keep seeing looks a bit like this:
std::vector<std::function<void(int)>> vs;
for(...)
{
double &f;
vs.push_back(std::bind([](int, double) {...}, _1, f));
}
This code needlessly constructs a lambda every loop iteration, plus a
std::function every loop iteration. A more efficient implementation might be
this:
std::vector<std::function<void(int)>> vs;
for(...)
{
double &f;
static auto c=[](int, double) {...};
vs.push_back(std::bind(c, _1, f));
}
Which saves on a lambda construction. Or even better again:
std::vector<std::function<void(int)>> vs;
std::function<void(int, double)> c=[](int, double) {...};
for(...)
{
double &f;
vs.push_back(std::bind(c, _1, f));
}
Which also saves on a std::function construction.
This may seem asinine for the clang static analyser to warn about, but many
programmers don't seem to realise that declaring lambdas inside a loop equals a
construction/destruction round. I myself find myself doing it because it's easy
to forget you're in a loop, and then you kick yourself later when you see the
assembler copying memory around needlessly.
A clang static analyser warning here would be useful.
Niall</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>