如何将分裂成按位移位两当电源？ [英] How to turn a division into a bitwise shift when power of two?

问题描述

我有以下的部门，我需要经常做：

I have the following division that I need to do often:

int index = pos / 64;

司可能是昂贵的，在CPU的水平。我希望有一种方法可以做到这一点与按位转变。我也想了解，你可以从分工转移，换句话说，我不希望只是记住按位EX pression。

Division can be expensive in the cpu level. I am hoping there is a way to do that with bitwise shift. I would also like to understand how you can go from division to shift, in other words, I don't want to just memorize the bitwise expression.

推荐答案

INT指数= POS＆GT;＆GT; 6 将做到这一点，但这是不必要的。任何合理的编译器会做这样的事情你。当然，太阳/ Oracle的编译器。

int index = pos >> 6 will do it, but this is unnecessary. Any reasonable compiler will do this sort of thing for you. Certainly the Sun/Oracle compiler will.

一般的规则是， I /（2 ^ N）可与实施I＆GT;＆GT; ñ。同样 I *（2 ^ N）是 I＆LT;＆LT; ñ。

The general rule is that i/(2^n) can be implemented with i >> n. Similarly i*(2^n) is i << n.

您需要用负数再presentation担心，如果我签署。例如。二进制补码产生合理的结果（如果右移是算术 - 符号位复制）。签名幅度没有。

You need to be concerned with negative number representation if i is signed. E.g. twos-complement produces reasonable results (if right shift is arithmetic--sign bit copied). Signed magnitude does not.

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