做視頻算法10多年，經常要算絕對值，整數的絕對值有快速算法，但浮點數的絕對值沒看到有快速算法，經常不段發現，得到如下浮點數的快速算法：

快6倍多，

#include <Windows.h>
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>union funion{float fnum;BYTE  bnum[4];
} ;float fabs_11(float x)
{return  (x < 0) ? -x : x;
}
void main(void)
{int iTime;
float ret;
union funion tmp1,tmp2;
float  x =- 2110000.0001;
iTime = GetTickCount();
for(; x < 2210000.987;x+=0.3)
{tmp1.fnum = x;//printf("%f\t%2.2x%2.2x%2.2x%2.2x\n",tmp1.fnum,tmp1.bnum[3],tmp1.bnum[2],tmp1.bnum[1],tmp1.bnum[0]);//  printf("%f\t%2.2x%2.2x%2.2x%2.2x\n",tmp2.fnum,tmp2.bnum[3],tmp2.bnum[2],tmp2.bnum[1],tmp2.bnum[0]);tmp1.bnum[3] = tmp1.bnum[3]&0x7f;//printf("%f\t%2.2x%2.2x%2.2x%2.2x\n",tmp1.fnum,tmp1.bnum[3],tmp1.bnum[2],tmp1.bnum[1],tmp1.bnum[0]);//  printf("%f\t%2.2x%2.2x%2.2x%2.2x\n",tmp2.fnum,tmp2.bnum[3],tmp2.bnum[2],tmp2.bnum[1],tmp2.bnum[0]);
}
printf("gai  time = %d\n",GetTickCount() - iTime);iTime = GetTickCount();
x =- 2110000.0001;
for(; x < 2210000.987;x+=0.3)
{fabs_11(x);
}
printf("old  time = %d\n",GetTickCount() - iTime);
}

如果浮點數是double，將如何生成呢？

請大家先想想，后看答案。

//

#include <Windows.h>
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>union funion{double fnum;BYTE  bnum[8];
} ;double fabs_11(double x)
{return  (x < 0) ? -x : x;
}
void main(void)
{int iTime;
double ret;
union funion tmp1,tmp2;
double  x =- 2110000.0001;
iTime = GetTickCount();
for(; x < 2210000.987;x+=0.3)
{tmp1.fnum = x;printf("%f\t%2.2x%2.2x%2.2x%2.2x\n",tmp1.fnum,tmp1.bnum[3],tmp1.bnum[2],tmp1.bnum[1],tmp1.bnum[0]);//  printf("%f\t%2.2x%2.2x%2.2x%2.2x\n",tmp2.fnum,tmp2.bnum[3],tmp2.bnum[2],tmp2.bnum[1],tmp2.bnum[0]);tmp1.bnum[7] = tmp1.bnum[7]&0x7f;printf("%f\t%2.2x%2.2x%2.2x%2.2x\n",tmp1.fnum,tmp1.bnum[3],tmp1.bnum[2],tmp1.bnum[1],tmp1.bnum[0]);//  printf("%f\t%2.2x%2.2x%2.2x%2.2x\n",tmp2.fnum,tmp2.bnum[3],tmp2.bnum[2],tmp2.bnum[1],tmp2.bnum[0]);
}
printf("gai  time = %d\n",GetTickCount() - iTime);iTime = GetTickCount();
x =- 2110000.0001;
for(; x < 2210000.987;x+=0.3)
{fabs_11(x);
}
printf("old  time = %d\n",GetTickCount() - iTime);
}