/in/foo.cc: In function 'void eratos()':
/in/foo.cc:22:10: warning: iteration 46338 invokes undefined behavior [-Waggressive-loop-optimizations]
  if(!pd[i]) prime[++cnt]=i; 
      ~~~~^
/in/foo.cc:21:15: note: within this loop
  for(int i=2;i<=MaxN;i++)
              ~^~~~~~