[TOC] ## 利用代數恒等式 ``` not a and not b not (a or b) ``` 使用第二種表達式,可以避免一次not 操作,提高性能 ## 削弱運算強度 用加法代替乘法。 用乘法代替冪乘。 利用三角恒等式代換等價的三角函數。 用1ong或int來代替1ong1ong整數(但請注意使用機器字長的整數和非 機器字長整數所帶來的差異)。 用定點數或整型數代替浮點數。 用單精度數代替雙精度數。 **用移位操作代替整數乘2或除2** bad ``` value= coefficient(0) For power 1 To order value= value coefficient( power )*x^power Next ``` good:把冪改為乘法 ``` value= coefficient(0) pwoerOfX = x For power 1 To order value= value coefficient( power )*x^power pwoerOfX = pwoerOfX * X Next ``` very good:再次減少乘法 ``` value = 0 For power = order to 1Step -1 value = (value+confficient(pwoer)) *x Next value = value + coefficient(0) ``` ## 編譯初始化 如把log 計算轉為整數 bad ``` unsigned int Log2(unsigned int x){ return (unsigned int) (log(x) / log(2)); } ``` good ``` const double LOG2 = 0.6314718; unsigned int Log2(unsigned int x){ return (unsigned int) (log(x) / LOG2); } ```