http://llvm.org/bugs/show_bug.cgi?id=20190
Bug ID: 20190
Summary: Feature Request: Spot redundant lambda and
std::function<> construction/destruction in loops
Product: clang
Version: unspecified
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Static Analyzer
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
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
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