Why calling via weak_ptr is so slow? - c++

Question

I have read the question What's the performance penalty of weak_ptr? but my own tests show different results.
I'm making delegates with smart pointers. The simple code below shows reproduces the performance issues with weak_ptr. Can anybody tell me why?
#include <chrono>
#include <functional>
#include <iostream>
#include <memory>
#include <stdint.h>
#include <string>
#include <utility>
struct Foo
{
Foo() : counter(0) { incrStep = 1;}
void bar()
{
counter += incrStep;
}
virtual ~Foo()
{
std::cout << "End " << counter << std::endl;
}
private:
uint64_t counter;
uint64_t incrStep;
};
void pf(const std::string &md, const std::function<void()> &g)
{
const auto st = std::chrono::high_resolution_clock::now();
g();
const auto ft = std::chrono::high_resolution_clock::now();
const auto del = std::chrono::duration_cast<std::chrono::milliseconds>(ft - st);
std::cout << md << " \t: \t" << del.count() << std::endl;
}
And the test:
int main(int , char** )
{
volatile size_t l = 1000000000ULL;
size_t maxCounter = l;
auto a = std::make_shared<Foo>();
std::weak_ptr<Foo> wp = a;
pf("call via raw ptr ", [=](){
for (size_t i = 0; i < maxCounter; ++i)
{
auto p = a.get();
if (p)
{
p->bar();
}
}
});
pf("call via shared_ptr ", [=](){
for (size_t i = 0; i < maxCounter; ++i)
{
if (a)
{
a->bar();
}
}
});
pf("call via weak_ptr ", [=](){
std::shared_ptr<Foo> p;
for (size_t i = 0; i < maxCounter; ++i)
{
p = wp.lock();
if (p)
{
p->bar();
}
}
});
pf("call via shared_ptr copy", [=](){
volatile std::shared_ptr<Foo> p1 = a;
std::shared_ptr<Foo> p;
for (size_t i = 0; i < maxCounter; ++i)
{
p = const_cast<std::shared_ptr<Foo>& >(p1);
if (p)
{
p->bar();
}
}
});
pf("call via mem_fn ", [=](){
auto fff = std::mem_fn(&Foo::bar);
for (size_t i = 0; i < maxCounter; ++i)
{
fff(a.get());
}
});
return 0;
}
Results:
$ ./test
call via raw ptr : 369
call via shared_ptr : 302
call via weak_ptr : 22663
call via shared_ptr copy : 2171
call via mem_fn : 2124
End 5000000000
As you can see, weak_ptr is 10 times slower than shared_ptr with copying and std::mem_fn and 60 times slower than using raw ptr or shared_ptr.get()

Answer 1

In trying to reproduce your test I realised that the optimizer might be eliminating more than it should. What I did was to utilize random numbers to defeat over-optimization and these results seem realistic with std::weak_ptr being nearly three times slower than the std::shared_ptr or its raw pointer.
I calculate a checksum in each test to ensure they are all doing the same work:
#include <chrono>
#include <memory>
#include <random>
#include <vector>
#include <iomanip>
#include <iostream>
#define OUT(m) do{std::cout << m << '\n';}while(0)
class Timer
{
using clock = std::chrono::steady_clock;
using microseconds = std::chrono::microseconds;
clock::time_point tsb;
clock::time_point tse;
public:
void start() { tsb = clock::now(); }
void stop() { tse = clock::now(); }
void clear() { tsb = tse; }
friend std::ostream& operator<<(std::ostream& o, const Timer& timer)
{
return o << timer.secs();
}
// return time difference in seconds
double secs() const
{
if(tse <= tsb)
return 0.0;
auto d = std::chrono::duration_cast<microseconds>(tse - tsb);
return double(d.count()) / 1000000.0;
}
};
constexpr auto N = 100000000U;
int main()
{
std::mt19937 rnd{std::random_device{}()};
std::uniform_int_distribution<int> pick{0, 100};
std::vector<int> random_ints;
for(auto i = 0U; i < 1024; ++i)
random_ints.push_back(pick(rnd));
std::shared_ptr<int> sptr = std::make_shared<int>(std::rand() % 100);
int* rptr = sptr.get();
std::weak_ptr<int> wptr = sptr;
Timer timer;
unsigned sum = 0;
sum = 0;
timer.start();
for(auto i = 0U; i < N; ++i)
{
sum += random_ints[i % random_ints.size()] * *sptr;
}
timer.stop();
OUT("sptr: " << sum << " " << timer);
sum = 0;
timer.start();
for(auto i = 0U; i < N; ++i)
{
sum += random_ints[i % random_ints.size()] * *rptr;
}
timer.stop();
OUT("rptr: " << sum << " " << timer);
sum = 0;
timer.start();
for(auto i = 0U; i < N; ++i)
{
sum += random_ints[i % random_ints.size()] * *wptr.lock();
}
timer.stop();
OUT("wptr: " << sum << " " << timer);
}
Compiler flags:
g++ -std=c++14 -O3 -g0 -D NDEBUG -o bin/timecpp src/timecpp.cpp
Example Output:
sptr: 1367265700 1.26869 // shared pointer
rptr: 1367265700 1.26435 // raw pointer
wptr: 1367265700 2.99008 // weak pointer