迴圈開銷(loop overhead)測試程式

同樣的運算次數，for 迴圈的個數不同，開銷也是不同的。

我表示很無知……求解釋~

 

linux下代碼：

#include <iostream><br />#include <sys/time.h><br />#include <unistd.h><br />using namespace std;<br />int main() {<br /> long s;<br /> struct timeval start, end;<br /> for (int k = 0; k < 20; k++){<br />//8<br />gettimeofday(&start, NULL);<br />s = 0;<br />for (int i = 0; i < 10; i ++) {<br />for (int j = 0; j < 10; j ++) {<br />for (int l = 0; l < 10; l ++) {<br />for (int m = 0; m < 10; m ++) {<br />for (int n = 0; n < 10; n ++) {<br />for (int o = 0; o < 10; o ++) {<br />for (int p = 0; p < 10; p ++) {<br />for (int q = 0; q < 10; q ++) {<br />s++;<br />}<br />}<br />}<br />}<br />}<br />}<br />}<br />}<br />gettimeofday(&end, NULL);<br />double span = end.tv_sec - start.tv_sec + (end.tv_usec - start.tv_usec)/1000000.0;<br />cout << span << endl;<br />//4<br />gettimeofday(&start, NULL);<br />s = 0;<br />for (int i = 0; i < 100; i ++) {<br />for (int j = 0; j < 100; j ++) {<br />for (int l = 0; l < 100; l ++) {<br />for (int m = 0; m < 100; m ++) {<br />s++;<br />}<br />}<br />}<br />}<br />gettimeofday(&end, NULL);<br />span = end.tv_sec - start.tv_sec + (end.tv_usec - start.tv_usec)/1000000.0;<br />cout << span << endl;<br /> //2<br /> gettimeofday(&start, NULL);<br /> s = 0;<br /> for (int i = 0; i < 10000; i ++) {<br /> for (int j = 0; j < 10000; j ++) {<br /> s++;<br /> }<br /> }<br /> gettimeofday(&end, NULL);<br /> span = end.tv_sec - start.tv_sec + (end.tv_usec - start.tv_usec)/1000000.0;<br /> cout << span << endl;<br /> //1<br /> gettimeofday(&start, NULL);<br /> s = 0;<br /> for (int i = 0; i < 100000000; i ++) {<br /> s++;<br /> }<br /> gettimeofday(&end, NULL);<br /> span = end.tv_sec - start.tv_sec + (end.tv_usec - start.tv_usec)/1000000.0;<br /> cout << span << endl;<br /> }<br />return 0;<br />}<br /> 

 

 

結果：

分析：同樣的運算量，for迴圈越多，耗時越長。迴圈體為空白，效果更明顯