[Bug tree-optimization/95396] [8/9/10/11 Regression] GCC produces incorrect code with -O3 for loops since r8-6511-g3ae129323d150621

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95396

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
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.