比零比任何其他数字快吗？ [英] Is comparing to zero faster than comparing to any other number?

问题描述

是

if(!test)

快于

if(test==-1)

我可以生产装配，但生产的装配太多，我不能找到我之后的细节。我希望有人只知道答案。我猜想他们是一样的，除非大多数CPU架构有某种比较零的捷径。

I can produce assembly but there is too much assembly produced and I can never locate the particulars I'm after. I was hoping someone just knows the answer. I would guess they are the same unless most CPU architectures have some sort of "compare to zero" short cut.

感谢任何帮助。

推荐答案

通常情况下，是的。在典型的处理器测试零，或测试符号（负/正）是简单的条件代码检查。这意味着可以重新排序指令以省略测试指令。在伪汇编中，考虑这样：

Typically, yes. In typical processors testing against zero, or testing sign (negative/positive) are simple condition code checks. This means that instructions can be re-ordered to omit a test instruction. In pseudo assembly, consider this:

Loop:
  LOADCC r1, test // load test into register 1, and set condition codes
  BCZS   Loop     // If zero was set, go to Loop

测试1：

Loop:
  LOAD   r1, test // load test into register 1
  SUBT   r1, 1    // Subtract Test instruction, with destination suppressed
  BCNE   Loop     // If not equal to 1, go to Loop

现在对于通常的优化前免责声明：您的程序太慢了？不进行优化，对其进行配置。

Now for the usual pre-optimization disclaimer: Is your program too slow? Don't optimize, profile it.

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