In C++, which is better i>

2018-06-10 23:12:47

This question already has an answer here:

Is < faster than <=? 13 answers

x > -1 vs x >= 0, is there a performance difference 10 answers

In assembly language, both are on the same structure:

i > -1

cmp   [register with i value],-1
jg    [somewhere]

i >= 0

cmp   [register with i value],0
jge   [somewhere]

According to used jump flags, the instruction jg make two flags comparaisons (ZF = 0 and SF = OF) but jge does only one (SF = OF).

So I'm tempted to say that both use almost same registers and CPU cycles, with maybe a very little quicker comparaison for i >= 0 .

那么，根据逻辑>操作可能比“=便宜”，但我想你正在编译与优化选项启用，所以编译器可能会做他想优化你的代码，所以我会说这并不重要，即使其中一个速度更快，编译器可能会将其更改为最佳选项

当编写for循环时，将它从for (i = 0; i < 1000; i++)转换for (i = 1000; i > 0; i--)会是有益的，因为在某些体系结构中，编译器可以跳过比较当我达到0时，作为标志的指令将被设置。在现代架构中，我不确定它是否重要。

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