Strange TLE by cin using GNU C++20 (64), and some weird subtle changes to (surprisingly) fix it

Правка en4, от -14, 2024-03-01 02:11:45

When I was doing the problem C. Pokémon Arena in the last Div. 1 round, I submitted my solution and got a TLE on pretest 20.

I was not suspecting anything but my code, I thought there may be some degenerate issue, undefined behavior or constant issue, but nothing really found. After some unsuccessful attempts, I was able to pass the pretest using fast IO. It's then I found something weird: it only takes a few hundred milliseconds, which is contradictory to my intuition: cin with sync_with_stdio(false) is fairly fast and it should not take up so much more (at least 10x) time.

After the contest, I submitted exactly the same code with different language. You know what?

It's not only me, and some other participants also encountered such issue. For example:

But there are also some successful cin submission using GNU C++20 (64).

Here's snippet for the key code:

int n, m;
cin >> n >> m;
vector a(n, vector (m, 0));
vector c(n, 0);
for (int i = 0; i < n; i++) cin >> c[i];
for (int i = 0; i < n; i++) {
	for (int j = 0; j < m; j++) {
		cin >> a[i][j];
	}
}

After some investigation by (including but not limit to) Sugar_fan, Boboge, -skyline-, the key components of TLE is:

  • The language must be GNU C++20 (64).
  • vector must be of int. If the elements are long long, it passed. 249004456
  • The definition of 2D vector must be before reading c. If swap these two lines, it passed. 249004217

So here's the thing. It could not even be interpreted as some branch mispredictions or cache misses, it seems something is completely broken, but we still don't understand what is wrong.

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  Rev. Язык Кто Когда Δ Комментарий
en11 Английский -14 2024-03-01 11:28:51 10
en10 Английский -14 2024-03-01 10:50:50 11
en9 Английский -14 2024-03-01 02:16:56 1 Tiny change: '248983571]\n\nIt's n' -> '248983571].\n\nIt's n'
en8 Английский -14 2024-03-01 02:15:47 2 Tiny change: ' not limit to) [user' -> ' not limited to) [user'
en7 Английский -14 2024-03-01 02:15:26 148
en6 Английский -14 2024-03-01 02:15:00 8
en5 Английский -14 2024-03-01 02:12:23 22 (published)
en4 Английский -14 2024-03-01 02:11:45 2 Tiny change: 'ion of 2D vector must be b' -> 'ion of 2D `vector` must be b'
en3 Английский -14 2024-03-01 02:11:02 125
en2 Английский -14 2024-03-01 02:08:21 78
en1 Английский -14 2024-03-01 02:06:15 2059 Initial revision (saved to drafts)