关于卡马克算法和系统函数库方法快速开平方根快慢的研究

蔚落 2022-07-14 11:18 120阅读 0赞

class MyMath
    {
    public:
    	float GetSqrtMy(float num);
    	float GetSqrtSys(float num);
    private :
    	long i;
    	float x2, y;
    	const float threehalfs = 1.5F;
    };

#include "MyMath.h"
    #include <iostream>
    #include<cmath>
    #include<cstdlib>
    #include<cstring>
    #include<cfloat>
    
    float MyMath::GetSqrtMy(float number)
    {
    	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));
    	y = y * (threehalfs - (x2 * y * y));
    	y = y * (threehalfs - (x2 * y * y));
    	return y*number;
    }
    float MyMath::GetSqrtSys(float number)
    {
    	float sq = sqrt(number);
    	return sq;
    }

#include "MyMath.h"
    #include <iostream>
    #include <ctime>
    
    int main()
    {
    	clock_t startTime, endTime;
    	MyMath* myMath = new MyMath();
    	int res = 0;
    	long average = 0l;
    	for (int count = 0; count < 10; count++)
    	{
    		startTime = clock();
    		for (int i = 0; i < 100000000; i++)
    		{
    			res = myMath->GetSqrtSys(float(i));
    		}
    		endTime = clock();
    		average += endTime - startTime;
    	}
    	printf("res = %d,start = %ld, end = %ld,sys = %ld\n", res, startTime, endTime, average / 10);
    
    	average = 0l;
    	for (int count = 0; count < 10; count++)
    	{
    		startTime = clock();
    		for (int i = 0; i < 100000000; i++)
    		{
    			res = myMath->GetSqrtMy(float(i));
    		}
    		endTime = clock();
    		average += endTime - startTime;
    	}
    	printf("res = %d,start = %ld, end = %ld, my = %ld\n", res, startTime, endTime, average / 10);
    }

release编译结果（10次求平均值）：

x86

![Center][]

x64  
![Center 1][]

但是下午在公司电脑上测试的，当release时，卡马克的更快，大概快不到一倍，在debug，系统函数要快。但是在我自己电脑上却是上面的结果。很奇怪。

[Center]: /images/20220714/329f312932ce4b53a2b102e423b7cab4.png
[Center 1]: /images/20220714/217077eacccc460e8a369f7093a65f56.png