sahaun's blog

C++ 64-bit running almost 3s faster!
By sahaun, history, 2 years ago, In English

I submitted exactly the same code for 86D - Мощный массив on different platforms. Here are the results:

There is a small increase in memory when using 64-bit, but the time difference is pretty huge!

Can anybody explain why?

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2 years ago, # |
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long long is a 64-bit data type and obviously works best on a 64-bit system

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    2 years ago, # ^ |
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    Makes sense, thanks.

    Is there a reason to use non-64-bit versions at all?

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      2 years ago, # ^ |
        Vote: I like it +34 Vote: I do not like it

      On the 32-bit version, pointers are 32 bit, while on 64-bit, pointers are 64 bit. So if you for some reason need to store a ton of pointers, then the 32-bit version would use half the memory compared to the 64-bit version. That said, 64-bit is better to use in basically any other scenario.

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2 years ago, # |
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It is actually different in my case.

GNU C++17 (64): 165333338 $$$→$$$ 156 ms

GNU C++17: 165333363 $$$→$$$ 62 ms

Solution also uses long long int. There is no difference in memory usage.

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    2 years ago, # ^ |
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    I think you will see the differences when running a heavy program or using more pointers as pajenegod suggested above.

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    2 years ago, # ^ |
    ← Rev. 2   Vote: I like it +14 Vote: I do not like it

    What you see there is scanf being faster in C++17(32) than C++17(64). Switching to cin/cout makes your code run in 46 ms 165347564 in C++17(64).

    I belive that part of the reason why scanf is faster in C++17(32) than C++17(64) is that Codeforces is using a custom made I/O speed up patch for C++17(32) (link).

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      2 years ago, # ^ |
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      Sorry, shouldn't there be faster instead of slower in first sentence of your comment.

      Thank you, I thought cin is always slower than scanf.

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        2 years ago, # ^ |
        ← Rev. 4   Vote: I like it +1 Vote: I do not like it

        Ah thanks, it should have been faster.

        About cin and scanf. I don't think there is any fundamental reason why one is faster than the other in general.

        I'd personally recommend using cin/cout instead of scanf/printf. Just remember to put something like ios::sync_with_stdio(false); cin.tie(0); in the beginning of main if you are using cin/cout.

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          2 years ago, # ^ |
          ← Rev. 4   Vote: I like it -38 Vote: I do not like it

          Confirmed that I'm wrong on this point, actually cin/cout is safe to use.

          The thing is you should avoid to use cin/cout together with scanf/printf with the stdio sync off, otherwise cin/cout will print everything before scanf/printf.

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            2 years ago, # ^ |
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            I have never experienced any problems with cin/cout. Maybe you are just doing it wrong?

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            2 years ago, # ^ |
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            Could you point me towards such examples, preferably some problem on an online judge, or maybe a custom test generation setup that makes them fail?

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            2 years ago, # ^ |
              Vote: I like it +8 Vote: I do not like it

            I've never ever seen cin "skip some input randomly".

            One thing that is true is that ios::sync_with_stdio(false); is not always faster. For example, on cf the following program takes

            #include <bits/stdc++.h>
using namespace std;
int main() {
    //ios::sync_with_stdio(false);
    for (int i = 0; i < 10000000; ++i)
      cout << i << '\n';
}

            Without ios::sync_with_stdio(false);

            • C++14(32) 1559 ms

            • C++17(32) 1574 ms

            • C++17(64) 1185 ms

            • C++20(64) 1216 ms

            With ios::sync_with_stdio(false);

            • C++14(32) 1575 ms

            • C++17(32) 1559 ms

            • C++17(64) 1372 ms

            • C++20(64) 889 ms

            So C++17(64) cout actually becomes slower when you use ios::sync_with_stdio(false);. That said, cin becomes a lot faster in C++17(64) with ios::sync_with_stdio(false);, so it is still worth to use it.

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