为什么ñ++比N = N + 1执行速度更快？ [英] Why does n++ execute faster than n=n+1?

问题描述

在C语言中，为什么ñ++ 执行比快N = N + 1 ？

In C language, Why does n++ execute faster than n=n+1?

(int n=...;  n++;)
(int n=...;  n=n+1;)

我们的教练要求在今天的类问题。 （这不是做作业）

Our instructor asked that question in today's class. (this is not homework)

推荐答案

如果您是在的石器时代的编译器...

That would be true if you are working on a "stone-age" compiler...

在石器时代的情况下：结果
++ñ比高N ++ 快于 N = N + 1

机通常有增加x 以及添加一个const，以X

In case of "stone-age":
++n is faster than n++ is faster than n=n+1
Machine usually have increment x as well as add const to x

• 在的情况下，N + ，你将有2个内存访问只（读N，INC N，写N）


• 在的情况下，N = N + 1 ，你将有3内存访问（读取n，读常量，添加n和常量，写N）

• In case of n++, you will have 2 memory access only (read n, inc n, write n )
• In case of n=n+1, you will have 3 memory access (read n, read const, add n and const, write n)

但是，今天的编译器会自动转换 N = N + 1 到 ++ñ，它会做更多比你可以想象！

But today's compiler will automatically convert n=n+1 to ++n, and it will do more than you may imagine!!

同样在今天的乱序处理器-despite的的石器时代的反编译运行时可能不会受到影响的所有在许多情况下的情况下！

Also on today's out-of-order processors -despite the case of "stone-age" compiler- runtime may not be affected at all in many cases!!

<一个href=\"http://stackoverflow.com/questions/2823043/is-it-faster-to-count-down-than-it-is-to-count-up/2823095#2823095\">Related

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