c++ - 平方根倒数算法的效率比较的一个问题
问题源于这样一道作业题：
作业中提到的的那个invSqrt的代码如下：
float Q_rsqrt( float number )
{
long i;
float x2, y;
const float threehalfs = 1.5F;
x2 = number * 0.5F;
y = number;
i = * ( long * ) &y; // evil floating point bit level hacking
i = 0x5f3759df - ( i >> 1 ); // what the fuck?
y = * ( float * ) &i;
y = y * ( threehalfs - ( x2 * y * y ) ); // 1st iteration
// y = y * ( threehalfs - ( x2 * y * y ) ); // 2nd iteration, this can be removed
return y;
}
然后就自己写了个比较性能和精度的程序：
#include<stdio.h>
#include<math.h>
#include<time.h>
float Q_rsqrt(float number) //从wiki上获取的源代码
{
long i;
float x2, y;
const float threehalfs = 1.5F;
x2 = number * 0.5F;
y = number;
i = *(long *)&y; // evil floating point bit level hacking
i = 0x5f3759df - (i >> 1); // what the fuck?
y = *(float *)&i;
y = y * (threehalfs - (x2 * y * y)); // 1st iteration
y = y * (threehalfs - (x2 * y * y)); // 2nd iteration, this can be removed
return y;
}
int main()
{
int LoopCount = 1000000; //循环计算次数
float dur = 0; //时间间隔，初始化为0
float x; //待计算数
printf("Please input a number:"); //输入提示
scanf_s("%f", &x, sizeof(float));
float y;
clock_t t1 = clock();
for (int i = 0; i < LoopCount; i++) {
y =(float)( 1 / sqrt(x));
}
t1 = clock() - t1;
printf("1/sqrt(%f)=%f\n", x, y);
printf("%ld\n", t1);
printf("Use Time_1:%.10f s\n", ((float)t1 / CLOCKS_PER_SEC));
float z;
clock_t t2 = clock();
for (int j = 0; j < LoopCount; j++) {
z = Q_rsqrt(x);
}
t2 = clock() - t2;
printf("Q_rsqrt(%f)=%f\n", x, z);
printf("%ld\n", t2);
printf("Use Time_2:%.10f s\n", ((float)t2 / CLOCKS_PER_SEC));
getchar();
getchar();
return 0;
}
之后，出现了一个很奇怪的问题：
是的。。。C中的库函数效率基本上是下面那个算法的4倍！！！
但是，如果用C++中的库函数：
#include<stdio.h>
#include<cmath>
#include<time.h>
float Q_rsqrt(float number) //从wiki上获取的源代码
{
long i;
float x2, y;
const float threehalfs = 1.5F;
x2 = number * 0.5F;
y = number;
i = *(long *)&y; // evil floating point bit level hacking
i = 0x5f3759df - (i >> 1); // what the fuck?
y = *(float *)&i;
y = y * (threehalfs - (x2 * y * y)); // 1st iteration
y = y * (threehalfs - (x2 * y * y)); // 2nd iteration, this can be removed
return y;
}
int main()
{
int LoopCount = 1000000; //循环计算次数
float dur = 0; //时间间隔，初始化为0
float x; //待计算数
printf("Please input a number:"); //输入提示
scanf_s("%f", &x, sizeof(float));
float y;
clock_t t1 = clock();
for (int i = 0; i < LoopCount; i++) {
y =( 1 / sqrt(x));
}
t1 = clock() - t1;
printf("1/sqrt(%f)=%f\n", x, y);
printf("%ld\n", t1);
printf("Use Time_1:%.10f s\n", ((float)t1 / CLOCKS_PER_SEC));
float z;
clock_t t2 = clock();
for (int j = 0; j < LoopCount; j++) {
z = Q_rsqrt(x);
}
t2 = clock() - t2;
printf("Q_rsqrt(%f)=%f\n", x, z);
printf("%ld\n", t2);
printf("Use Time_2:%.10f s\n", ((float)t2 / CLOCKS_PER_SEC));
getchar();
getchar();
return 0;
}
然后跑出来的结果是这样的：
因此。。。似乎C的效率比C++高了很多很多？现在在VS2015中的实现就连那个神奇的快速算法也比不过了？（还是。。。我这样的比较是不对的。。。）
疑惑中。。。
