/in/foo.cc: In function 'int main()':
/in/foo.cc:27:12: warning: iteration 44998 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(!map[i])prime[++tail]=i;
       ~~~~~^
/in/foo.cc:25:11: note: within this loop
  for(i=2;i<=ac;i++)
          ~^~~~