哪个更快:！=或< [英] Which is faster: != or <

问题描述

以下哪个更快?

for (int i = 0; i < n; ++i)

或

for (int i = 0; i != n; ++i)

我想知道是因为我不了解硬件，因此也不了解计算机如何看待<与!=.

I'm wondering because I don't understand hardware and thus don't understand how a computer looks at < versus !=.

推荐答案

对于基本类型，这两个操作都花费完全相同的时间，因为无论您要求哪个，它们实际上都是确定的.

For primitive types, both operations take the exact same amount of time since both are actually determined regardless of which you ask for.

简而言之，每当进行基本比较(< <= > >= ==或!=)时，运算符的一侧都会被减去.然后，减法的结果用于设置许多标志，其中最重要的标志是Z(零)，N(负)和O(溢出).根据名称，您应该能够弄清楚每个标志代表什么.例如:如果减法的结果为零，则将设置Z标志.因此，无论您要的是<=还是!=，处理器所要做的就是检查由于初始减法而已正确设置的所有标志.

In short, whenever you make a basic comparison, < <= > >= == or !=, one side of the operator is subtracted from the other. The result of the subtraction is then used to set a number of flags, the most important of which are Z (zero), N (negative), and O (overflow). Based on the names, you should be able to figure out what each flag represents. Ex: if the result of subtraction is zero, than the Z flag is set. Thus, whether you ask for <= or !=, all the processor is doing is checking the flags which have all been set appropriately as a result of the initial subtraction.

理论上，由于必须检查两个标志(Z和N)而不是一个标志(=只是关心Z)，因此<=花费的时间 会更长一些.但这发生的程度很低，即使是微秒级的结果也很可能忽略不计.

Theoretically, <= should take slightly longer since two flags (Z and N) must be checked instead of one (= just cares about Z). But this happens on such a low level that the results are most likely negligible even on a microsecond scale.

如果您真的有兴趣，请阅读处理器状态寄存器.

If you're really interested, read up on processor status registers.

对于非原始类型(即类)，它取决于关系运算符的实现.

For non-primitive types, i.e. classes, it depends on the implementation of the relational operators.

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