https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95396
--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #6)
> One odd thing is that for unsigned char _3 we get via
>
> wide_int var_min, var_max;
> value_range_kind vr_type = get_range_info (tmp_var, &var_min,
> &var_max);
>
> [241, 255]
>
> the add of 15 then overflows for both min and max and thus we happily accept
> the result but compute the difference signed (in widest_int):
>
> /* Calculate (ssizetype) OP0 - (ssizetype) TMP_VAR. */
> widest_int diff = (widest_int::from (op0_min, sgn)
> - widest_int::from (var_min, sgn));
> var0 = tmp_var;
> *off = wide_int_to_tree (ssizetype, diff);
>
> which then results in 0-241 == -241.
>
> To me it looks like we should instead do
>
> /* Calculate (ssizetype) (OP0 - TMP_VAR). */
> widest_int diff = widest_int::from (op0_min - var_min, sgn);
>
> which fixes this particular testcase.
Or alternatively
widest_int diff = wi::ext (widest_int::from (op0_min, sgn)
- widest_int::from (var_min, sgn),
TYPE_PRECISION (type),
TYPE_SIGN (type));
which maybe more highlights the underlying issue (failing zero-extension
for the sign-changing conversion)?
I don't expect any fallout of either patch possibly due to bad test
coverage so extra thoughts welcome.
