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C++ Return value optimization benchmark
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| #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]
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On my MacBook Pro this example works pretty the same with and without RVO optimization.