/in/foo.cc: In function 'int main()':
/in/foo.cc:11:13: warning: iteration 89999998 invokes undefined behavior [-Waggressive-loop-optimizations]
   if(!mark[i]){
       ~~~~~~^
/in/foo.cc:10:24: note: within this loop
  for(register int i=2;i<=size;i++){
                        ^