[PATCH PR100740]Fix overflow check in simplifying exit cond comparing two IVs.

[Richard Biener]

On Tue, Jun 1, 2021 at 4:00 PM bin.cheng via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi,
> As described in patch summary, this fixes the wrong code issue by adding overflow-ness
> check for iv1.step - iv2.step.
>
> Bootstrap and test on x86_64.  Any comments?

+         bool wrap_p = TYPE_OVERFLOW_WRAPS (step_type);
+         if (wrap_p)
+           {
+             tree t = fold_binary_to_constant (GE_EXPR, step_type,
+                                               iv0->step, iv1->step);
+             wrap_p = integer_zerop (t);
+           }

I think we can't use TYPE_OVERFLOW_WRAPS/TYPE_OVERFLOW_UNDEFINED since
that's only relevant for expressions written by the user - we're
computing iv0.step - iv1.step
which can even overflow when TYPE_OVERFLOW_UNDEFINED (in fact we may not
even generate this expression then!).  So I think we have to do sth like

   /* If the iv0->step - iv1->step wraps, fail.  */
   if (!operand_equal_p (iv0->step, iv1->step)
       && (TREE_CODE (iv0->step) != INTEGER_CST || TREE_CODE
(iv1->step) != INTEGER_CST)
       && !wi::gt (wi::to_widest (iv0->step), wi::to_widest (iv1->step))
     return false;

which only handles equality and all integer constant steps. You could
also use ranges
like

 wide_int min0, max0, min1, max1;
  if (!operand_equal_p (iv->step, iv1->step)
      && (determine_value_range (iv0->step, &min0, &max0) != VR_RANGE
             || determine_value_range (iv1->step, &min1, &max1) != VR_RANGE
             || !wi::ge (min0, max1)))
   return false;

Note I'm not sure why

       iv0->step = step;
       if (!POINTER_TYPE_P (type))
        iv0->no_overflow = false;

here the no_overflow reset does not happen for pointer types?  Or
rather why does
it happen at all?  Don't we strictly make the step less in absolute value?

> Thanks,
> bin