哪一个是更快？函数调用或有条件的if语句？ [英] Which one is faster ? Function call or Conditional if Statement?

问题描述

请过回答这个问题之前，需要考虑的分支prediction。

Please consider the branch prediction too before answering this question.

我有一些场景在那里我可以用功能pointer.Some这样的事帮助一个函数的调用替换条件语句。 （你可以认为基于组件的编程在继承相似类型的塞纳里奥）

I have some scenarios where i can replace a conditional statement with a call to a function with the help of function pointer.Some thing like this. (you can think of component based programming over inheritance for a similar type of senario)

     class Shape
     {
        float Area()
        {
            if(type == SQUARE)
             {
                return length*length;
             }
            else if(type == RECTANGLE)
            {
             return length*breadth;
            }
        } 
     }

同一个类可以这样写。

The same class can be written like this.

       class Shape
     {
        void SetAreaFunction(void *funcptr)//this function is used to set the current AreaFunc
        {
             CurrentAreaFunc = funcptr ;//this holds the pointer to current area func
        }
        float SqauareArea();//this will return square area
        float RectangleArea();//this will return rectangle area 
        float Area()
        {
            currentAreaFunc();
        } 
     }

如果你考虑到上述情况，既达到相同results.But，我在想overhead.In我避免了有一个函数调用有分支prediction问题第二种情况下的性能。

IF you consider the above cases, both achieves same results.But, I'm thinking about the performance overhead.In the second case I'm avoiding having branch prediction problem by having a function call.

现在让我知道这是更好的做法，并在这种senarios的更好的优化code'。（顺便说一句，我不喜欢的声明pre-成熟的优化是万恶之源作为，优化有它的好处，所以我认为我的优化code！）

Now let me know which is the better practice and 'better optimized code' in this kind of senarios.(btw, I don't like the statement "Pre-mature optimization is root of all evil" as, optimization has its benefits so i do consider optimizing my code!)

P.S：如果任何人提供有关怎么不好分支prediction可以，即使在组装code的详细介绍，我不介意

P.S: I don't mind if any one give a detailed overview about 'how bad branch prediction can be" even in assembly code.

更新：分析（一种类似上述code的），结果后
     如果条件成功地将这种senario.Can中的任何一个给出这方面的原因？
功能调用code可能是prefetched因为没有分支code吧？但在这里它看起来其他way..branching code胜！ ：o
异形英特尔MAC OSX，GCC O3 / O的优化。

Update: After profiling (similar kind of above code),
If Condition succeeded in this kind of senario.Can any one give a reason for this? Functional call code can be prefetched as there is no Branching code right? But here its looks the other way..branching code wins! :O Profiled on Intel Mac Osx,GCC O3/Os optimisation.

推荐答案

您更换if语句与间接。

You replaced an if statement with an indirection.

您都if语句，而需要间接内存访问。

Both your if statement, and the indirection requires memory access.

然而，如果将导致短跳转 - 这将可能不会失效管道，而间接可能违反管道

However, the if will result in a short jump - which will probably won't invalidate the pipeline, while the indirection may invalidate the pipeline.

在另一方面，在间接是跳跃，而如果语句是一个条件跳转。分支predictor可能会错过。

On the other hand, the indirection is a jump, while the if statement is a conditional jump. The branch predictor may miss.

这是很难说这是没有测试它更快。我predict的if语句将获胜。

It is hard to tell which is faster without testing it. I predict that the if statement will win.

请分享你的成果！

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