/in/foo.cc: In function 'int main()':
/in/foo.cc:84:37: warning: iteration 1000510 invokes undefined behavior [-Waggressive-loop-optimizations]
  for (int i = 1; i <= M; ++i) id[i] = i;
                               ~~~~~~^~~
/in/foo.cc:84:20: note: within this loop
  for (int i = 1; i <= M; ++i) id[i] = i;
                  ~~^~~~