哪个更快?函数调用还是条件 if 语句? [英] Which one is faster ? Function call or Conditional if Statement?

问题描述

在回答这个问题之前，请考虑分支预测.

Please consider the branch prediction too before answering this question.

我有一些场景，我可以在函数指针的帮助下用对函数的调用替换条件语句.类似这样的事情.(对于类似的场景，您可以考虑基于组件的编程而不是继承)

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.

       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();
        } 
     }

如果您考虑上述情况，两者都会达到相同的结果.但是，我正在考虑性能开销.在第二种情况下，我通过调用函数来避免出现分支预测问题.

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.

现在让我知道在这种情况下哪个是更好的实践和更好的优化代码".(顺便说一句，我不喜欢过早的优化是万恶之源"的说法，因为优化有它的好处，所以我确实考虑优化我的代码！)

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:我不介意是否有人详细概述分支预测有多糟糕"，即使在汇编代码中也是如此.

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:在分析之后(类似上面的代码)，
如果 Condition 在这种 senario 中成功.有人可以给出原因吗?由于没有分支代码，因此可以预取函数调用代码，对吗?但在这里它看起来是另一种方式......分支代码获胜！:O在 Intel Mac Osx、GCC O3/Os 优化上进行了分析.

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

您用间接语句替换了 if 语句.

You replaced an if statement with an indirection.

您的 if 语句和间接访问都需要内存访问.

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

但是，if 会导致短跳转 - 这可能不会使管道无效，而间接寻址可能会使管道无效.

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

另一方面，间接是跳转，而if语句是条件跳转.分支预测器可能会丢失.

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

如果不进行测试，很难判断哪个更快.我预测 if 语句会获胜.

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

请分享您的结果！

这篇关于哪个更快?函数调用还是条件 if 语句?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！