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");
}

On my MacBook Pro this example works pretty the same with and without RVO optimization.

$uname -a
Darwin hackmachine 15.2.0 Darwin Kernel Version 15.2.0: Fri Nov 13 19:56:56 PST 2015; root:xnu-3248.20.55~2/RELEASE_X86_64 x86_64

$g++ --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.3.0 (clang-703.0.31)
Target: x86_64-apple-darwin15.2.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

$g++ -std=c++11 -S -o /dev/stdout main.cpp | grep memcpy | wc -l
       0

$g++ -std=c++11 -fno-elide-constructors -S -o /dev/stdout main.cpp | grep memcpy | wc -l
       4

$ g++ -std=c++11 main.cpp
$ ./a.out
By value (MEDIUM) 123 ms [stub -1593461165]
By value (LARGE) 7741 ms [stub -1299931242]
By ref (MEDIUM) 120 ms [stub -1955762550]
By ref (LARGE) 7835 ms [stub 911248215]
$ ./a.out
By value (MEDIUM) 118 ms [stub 2094615909]
By value (LARGE) 7840 ms [stub -1276864738]
By ref (MEDIUM) 118 ms [stub -223778153]
By ref (LARGE) 7890 ms [stub -381745773]
$ ./a.out
By value (MEDIUM) 120 ms [stub -352419427]
By value (LARGE) 7802 ms [stub 1969353349]
By ref (MEDIUM) 122 ms [stub 41925584]
By ref (LARGE) 7725 ms [stub 364989310]

$ g++ -std=c++11 -fno-elide-constructors main.cpp
$ ./a.out
By value (MEDIUM) 122 ms [stub 1921645226]
By value (LARGE) 8078 ms [stub 1869896539]
By ref (MEDIUM) 123 ms [stub -676968691]
By ref (LARGE) 7855 ms [stub 1621698360]
$ ./a.out
By value (MEDIUM) 119 ms [stub 954834819]
By value (LARGE) 7779 ms [stub 98742842]
By ref (MEDIUM) 118 ms [stub 1498384025]
By ref (LARGE) 7505 ms [stub -118604845]
$ ./a.out
By value (MEDIUM) 126 ms [stub -546810574]
By value (LARGE) 8066 ms [stub 210266301]
By ref (MEDIUM) 123 ms [stub -1941518511]
By ref (LARGE) 7637 ms [stub -2073135477]

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]