什么是最好的方法（性能方面）来测试一个值是否落在阈值内？ [英] What is the best way (performance-wise) to test whether a value falls within a threshold?

问题描述

也就是说，测试最快的方法是什么。

  if（a> =（b- c> ;0≤b-c：0）& 
a< =（b + c< 255b + c：255））
 
 ... 
  

如果a，b和c均为 unsigned char aka BYTE 。我试图优化图像扫描过程来找到一个子图像，这样的比较，每次扫描大约300万次，所以即使轻微的优化也是有帮助的。

不知道，但也许是某种按位操作？也许添加1到c和测试小于和大于没有or-equal-to部分？我不知道！

解决方案

好吧，首先让我们看看你正在试图检查什么，下溢检查：

  a> = b  -  c 
a< = b + c 
来自两个：
a  -  b> = -c 
a  -  b <= c 
  

现在等于

  abs（a  -  b）< = c 
  
 $> 
 
 $ b a> b？ab：ba）<= c 
  

现在，不包含（或需要）复杂的下溢/溢出检查。

---

我已经剖析了我和6502的代码1000000000并且正式地没有什么区别。我建议选择最优雅的解决方案（这是IMO矿山，但意见不同），因为性能不是一个争论。

---

但是，我和asker的代码之间有一个显着的区别。这是我使用的分析代码：

  #include< iostream> 
 
 int main（int argc，char * argv []）{
 bool prevent_opti; 
 for（int ai = 0; ai <256; ++ ai）{
 for（int bi = 0; bi <256; ++ bi）{
 for = 0; ci <256; ++ ci）{
 unsigned char a = ai; 
 unsigned char b = bi; 
 unsigned char c = ci; 
 if（（a> b？a-b：b-a）<= c）prevent_opti = true; 
} 
} 
} 
 
 std :: cout< prevent_opti<< \\\
; 
 
 return 0; 
} 
 

对于我的if语句，这平均需要120ms，asker的if语句需要135ms平均。

That is, what is the fastest way to do the test

if( a >= ( b - c > 0 ? b - c : 0 ) &&
    a <= ( b + c < 255 ? b + c : 255 ) )

    ...

if a, b, and c are all unsigned char aka BYTE. I am trying to optimize an image scanning process to find a sub-image, and a comparison such as this is done about 3 million times per scan, so even minor optimizations could be helpful.

Not sure, but maybe some sort of bitwise operation? Maybe adding 1 to c and testing for less-than and greater-than without the or-equal-to part? I don't know!

解决方案

Well, first of all let's see what you are trying to check without all kinds of over/underflow checks:

a >= b - c
a <= b + c
subtract b from both:
a - b >= -c
a - b <= c

Now that is equal to

abs(a - b) <= c

And in code:

(a>b ? a-b : b-a) <= c

Now, this code is a tad faster and doesn't contain (or need) complicated underflow/overflow checks.

---

I've profiled mine and 6502's code with 1000000000 repitions and there officially was no difference whatsoever. I would suggest to pick the most elegant solution (which is IMO mine, but opinions differ), since performance is not an argument.

---

However, there was a notable difference between my and the asker's code. This is the profiling code I used:

#include <iostream>

int main(int argc, char *argv[]) {  
    bool prevent_opti;
    for (int ai = 0; ai < 256; ++ai) {
        for (int bi = 0; bi < 256; ++bi) {
            for (int ci = 0; ci < 256; ++ci) {
                unsigned char a = ai;
                unsigned char b = bi;
                unsigned char c = ci;
                if ((a>b ? a-b : b-a) <= c) prevent_opti = true;
            }
        }
    }

    std::cout << prevent_opti << "\n";

    return 0;
}

With my if statement this took 120ms on average and the asker's if statement took 135ms on average.

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