C++ Return value optimization benchmark

rvo_bench.cpp

#include <chrono>
#include <iostream>
#include <stdlib.h>
#include <time.h>
#define SIZE_MEDIUM 4096
#define SIZE_LARGE  16*4096
#define ITER_COUNT  100000
using namespace std;
using namespace std::chrono;

struct MediumStruct {
    int data[SIZE_MEDIUM];
};
struct LargeStruct {
    int data[SIZE_LARGE];
};

template<typename T>
T by_value(int size) {
    T rv;
    memset(rv.data, rand(), size*sizeof(rv.data[0]));
    return rv;
}

template<typename T>
void by_ref(T &v, int size) {
    memset(v.data, rand(), size*sizeof(v.data[0]));    
}

template<typename T>
void bench_by_value(int size, const string &suite) {
    high_resolution_clock::time_point t1 = high_resolution_clock::now();

    int counter = 0;
    for (int i = 0; i < ITER_COUNT; i++) {
        T r = by_value<T>(size);
        for (int j = 0; j < size; j++) {
            counter += r.data[j];
        }
    }

    high_resolution_clock::time_point t2 = high_resolution_clock::now();
    auto duration = duration_cast<milliseconds>(t2 - t1).count();
    cout << "By value (" << suite << ") " << duration << " ms " 
         << "[stub " << counter << "]" << endl;
}

template<typename T>
void bench_by_ref(int size, const string &suite) {
    high_resolution_clock::time_point t1 = high_resolution_clock::now();

    int counter = 0;
    for (int i = 0; i < ITER_COUNT; i++) {
        T r;
        by_ref<T>(r, size);
        for (int j = 0; j < size; j++) {
            counter += r.data[j];
        }
    }

    high_resolution_clock::time_point t2 = high_resolution_clock::now();
    auto duration = duration_cast<milliseconds>(t2 - t1).count();
    cout << "By ref (" << suite << ") " << duration << " ms " 
         << "[stub " << counter << "]" << endl;
}

int main() {
    srand(time(NULL));
    bench_by_value<MediumStruct>(SIZE_MEDIUM, "MEDIUM");
    bench_by_value<LargeStruct>(SIZE_LARGE, "LARGE");
    bench_by_ref<MediumStruct>(SIZE_MEDIUM, "MEDIUM");
    bench_by_ref<LargeStruct>(SIZE_LARGE, "LARGE");
}

After reducing amount of rand() calls in the benchmark, difference in the execution time became more visible.

$ g++ -std=c++11 main.cpp
$ ./a.out
By value (MEDIUM) 938 ms [stub 132300800]
By value (LARGE) 16048 ms [stub -1764818944]
By ref (MEDIUM) 914 ms [stub -1316978688]
By ref (LARGE) 16133 ms [stub 580321280]
$ ./a.out
By value (MEDIUM) 924 ms [stub -36802560]
By value (LARGE) 16523 ms [stub 1703346176]
By ref (MEDIUM) 930 ms [stub -87310336]
By ref (LARGE) 16630 ms [stub 594935808]

$ g++ -std=c++11 -fno-elide-constructors main.cpp
$ ./a.out
By value (MEDIUM) 1143 ms [stub -406900736]
By value (LARGE) 19949 ms [stub 1393557504]
By ref (MEDIUM) 909 ms [stub -1232310272]
By ref (LARGE) 16726 ms [stub -2084503552]
$ ./a.out
By value (MEDIUM) 1128 ms [stub -1636438016]
By value (LARGE) 18869 ms [stub 109707264]
By ref (MEDIUM) 996 ms [stub -2006835200]
By ref (LARGE) 17156 ms [stub 1202454528]