Unexpected Time taken.

Блог пользователя Kr_Shubham

Автор Kr_Shubham, 3 года назад, По-английски

This code is taking around 1300 ms. (size of set shouldn't exceed more than 1 at any time, Since I am inserting same element every time)

Spoiler

#include<bits//stdc++.h>
using namespace std;
int main()
{   
    set<int>s;
   for(int i=1;i<=7000001;i++)
   {    
        s.insert(1);
        s.erase(1);
   }
   
}

But , After commenting the s.erase(), it just took 15 ms.

Spoiler

#include<bits//stdc++.h>
using namespace std;
int main()
{   
    set<int>s;
   for(int i=1;i<=7000001;i++)
   {    
        s.insert(1);
       // s.erase(1);
   }
   
}

Can anyone please explain the reason behind this? Thanks in advance :)

unexpected things

Kr_Shubham
3 года назад
8

Комментарии (4)

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jalsol

3 года назад, # |

my guess is that the compiler sees that the set isn't actually used for anything, so it's optimized out

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Fanarill

3 года назад, # ^ |

Can we somehow avoid these optimizations? It was the only one moment in my life when I thought to use keyword volatile. But after I did that, code refused to compile, and I don't understand why. Would be glad if someone explained why it is happening.

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Fanarill

3 года назад, # ^ |

Actually compiler gives errors on erase and insert functions, so I suspect existing versions of these function doesn't implemented for volatile case. So it's tempting me to think maybe there is no optimization at all?

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tuna_salad

3 года назад, # |

+13

I think it has to do with memory allocation (e.g., malloc(..)), which is quite an expensive operation.

Without erasing, only the first insertion causes allocation (RB-tree nodes and all), and subsequent insertions only require reads (if the operations are not optimized away). On the other hand, with erasing, every inserting invokes memory allocation. And 7 million times is a lot!

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