/in/foo.cc: In function 'void primes()':
/in/foo.cc:16:9: warning: iteration 999998 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(v[i]==0) 
      ~~~^
/in/foo.cc:14:15: note: within this loop
  for(int i=2;i<=1000000;i++)
              ~^~~~~~~~~