c++ - 为什么 C++ lambda 在多次调用时比普通函数慢？

Question

我只是尝试比较 C++11 中 lambda 表达式的性能，所以我做了测试——计算 double 值 vector 中元素的总和。这是实现:

#include <vector>
#include <algorithm>
#include <iostream>
#include <ctime>

#define LOG(x) { std::cout << #x << " = " << (x) << "\n"; }
#define TIME(t) { std::cout << ((double)(clock() - (t)) / CLOCKS_PER_SEC) << " s\n"; }

double sum(const std::vector<double>& v)
{
    double s = 0.0;
    for (auto i = v.cbegin(); i != v.cend(); ++i)
        s += *i;
    return s;
}

int main()
{
    const size_t MAX = 1; // number of tests
    const size_t SIZE = 100000000; // length of the vector

    std::vector<double> v(SIZE, 1.0);
    double out;

    clock_t clk;

    std::cout << "iterator\n";

    clk = clock();
    out = 0.0;
    for (size_t i = 0; i < MAX; ++i)
        out += sum(v);
    TIME(clk)
    LOG(out)

    std::cout << "\nlambda\n";

    clk = clock();
    out = 0.0;
    for (size_t i = 0; i < MAX; ++i)
        std::for_each(v.cbegin(), v.cend(), [&](double d) { out += d; });
    TIME(clk)
    LOG(out)

    return 0;
}

这是这个程序的结果(在VS2010 SP1中编译，在Release模式下):

iterator
0.32 s
out = 1e+008

lambda
0.326 s
out = 1e+008

As one may see, there is practically no difference in performance. However, if I give 10 as the value of MAX (it means summation will be performed 10 times instead of one), results differ:

iterator
0.287 s
out = 1e+009

lambda
2.84 s
out = 1e+009

Test for lambda expression took approximately 10 times more time. Why? I thought it may be caused by the fact, that on every iteration new lambda is created, but whet I tried this:

out = 0.0;
auto f = [&](double d) { out += d; };
for (size_t i = 0; i < MAX; ++i)
    std::for_each(v.cbegin(), v.cend(), f);

结果没有改变。有人可以向我解释这种行为吗？

Answer 1

事实证明，这不是 lambda 表达式的任何问题，只是编译器在第一种情况下通过缓存 sum() 函数的结果优化了外循环。将第一种情况更改为这种形式后:

out = 0.0;
for (size_t i = 0; i < MAX; ++i)
{
    out += sum(v);
    v[i] = 1.0; // this adds O(1) time and prevents caching
}

两种情况的时间大致相等，最喜欢的是 lambda。