Алексей 〒.

Нужно бежать со всех ног, чтобы только оставаться на месте, а чтобы куда-то попасть, надо бежать как минимум вдвое быстрее!

for (int dx = -1; dx <= 1; ++dx) {
  for (int dy = -1; dy <= 1; ++dy) {
    if (dx == 0 && dy == 0) continue;
    int nx = (i + dx + height) % height;
    int ny = (j + dy + SIZE) % SIZE;
    neighbours += proc_states[iter][nx * SIZE + ny];
  }  
}

const int dx[8] = {0, 1, 1, 1, 0, -1, -1, -1};
const int dy[8] = { 1, 1, 0, -1, -1, -1, 0, 1};
...
for (int k = 0; k < 8; ++k) {
  int nx = i + dx[k];
  int ny = j + dy[k];
  ...
}

Default initialization is performed in three situations:
1) when a variable with automatic, static, or thread-local storage duration is declared with no initializer;

The effects of default initialization are:
......
otherwise, no initialization is performed: the objects with automatic storage duration (and their subobjects) contain indeterminate values.

Use of an indeterminate value obtained by default-initializing a non-class variable of any type is undefined behavior (in particular, it may be a trap representation), except in the following cases:
...

Compilers are not required to diagnose undefined behavior (although many simple situations are diagnosed), and the compiled program is not required to do anything meaningful.

If an initializer is not provided:
- objects with automatic storage duration are initialized to indeterminate values (which may be trap representations)
...

If an indeterminate value is used as an argument to any standard library call, the behavior is undefined. Otherwise, the result of any expression involving indeterminate values is an indeterminate value (e.g. int n;, n may not compare equal to itself and it may appear to change its value on subsequent reads)