/in/foo.cc: In function 'void init()':
/in/foo.cc:11:9: warning: iteration 5000008 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(a[i]==0) prime[++cnt]=i;
      ~~~^
/in/foo.cc:9:15: note: within this loop
  for(int i=2;i<=maxn;i++)
              ~^~~~~~