Confused about code/comment in tree.c:build2

[Bin.Cheng]

Hi,
In tree.c:build2 there is following code/comment:

  if ((code == MINUS_EXPR || code == PLUS_EXPR || code == MULT_EXPR)
      && arg0 && arg1 && tt && POINTER_TYPE_P (tt)
      /* When sizetype precision doesn't match that of pointers
         we need to be able to build explicit extensions or truncations
         of the offset argument.  */
      && TYPE_PRECISION (sizetype) == TYPE_PRECISION (tt))
    gcc_assert (TREE_CODE (arg0) == INTEGER_CST
                && TREE_CODE (arg1) == INTEGER_CST);

The comment says specific condition needs to be satisfied if sizetype
precision doesn't match that of pointers, while the code is checking
"==" of precisions?  Any explanation?

Thanks,
bin