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); }

總結

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