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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/95396] [8/9/10/11 Regression] GCC produces
 incorrect code with -O3 for loops since r8-6511-g3ae129323d150621
Date: Tue, 07 Jul 2020 08:26:50 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D95396

--- 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
>=20
>                 wide_int var_min, var_max;
>                 value_range_kind vr_type =3D get_range_info (tmp_var, &va=
r_min,
>                                                            &var_max);
>=20
> [241, 255]
>=20
> the add of 15 then overflows for both min and max and thus we happily acc=
ept
> the result but compute the difference signed (in widest_int):
>=20
>                 /* Calculate (ssizetype) OP0 - (ssizetype) TMP_VAR.  */
>                 widest_int diff =3D (widest_int::from (op0_min, sgn)
>                                    - widest_int::from (var_min, sgn));
>                 var0 =3D tmp_var;
>                 *off =3D wide_int_to_tree (ssizetype, diff);
>=20
> which then results in 0-241 =3D=3D -241.
>=20
> To me it looks like we should instead do
>=20
>                 /* Calculate (ssizetype) (OP0 - TMP_VAR).  */
>                 widest_int diff =3D widest_int::from (op0_min - var_min, =
sgn);
>=20
> which fixes this particular testcase.

Or alternatively

                widest_int diff =3D 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.=