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.

• GNU C++20 (64), TLE 3000ms 248951083.

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?

• GNU C++17, AC 280ms 248983571

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

• GNU C++20 (64), TLE 248946386 by fallleaves01.

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 simply interpreted as some branch mispredictions or cache misses, it seems something is completely broken, and we still don't understand what is actually wrong.