register variable???

[Sungkee Lee]

#include <stdio.h>
  int  list[5] = {0, 1, 2, 3, 4};

main()
{
  register int  i;

  printf("list = %d %d %d %d %d\n",
	 list[0], list[1], list[2], list[3], list[4]);
  i = 0;
  while (i < 4)  list[i] = list[++i];
  printf("list = %d %d %d %d %d\n",
	 list[0], list[1], list[2], list[3], list[4]);
}
  
Above program gave me the result,

list = 0 1 2 3 4
list = 0 1 2 3 4.

However, if I define "int i" instead of "register int i",
the result is

list = 0 1 2 3 4
list = 1 2 3 4 4.

Is this the way register variable is designed?
Or, is this compiler error? I ran this program on Sun 3/60?