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From: "liuhongt at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/114396] [13/14 Regression] Vector: Runtime
 mismatch at -O2 with -fwrapv since r13-7988-g82919cf4cb2321
Date: Wed, 20 Mar 2024 12:36:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114396

--- Comment #15 from Hongtao Liu <liuhongt at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #9)
> (In reply to Robin Dapp from comment #8)
> > No fallout on x86 or aarch64.
> >=20
> > Of course using false instead of TYPE_SIGN (utype) is also possible and
> > maybe clearer?
>=20
> Well, wi::from_mpz doesn't take a sign argument.  It's comment says
>=20
> /* Returns X converted to TYPE.  If WRAP is true, then out-of-range
>    values of VAL will be wrapped; otherwise, they will be set to the
>    appropriate minimum or maximum TYPE bound.  */
> wide_int
> wi::from_mpz (const_tree type, mpz_t x, bool wrap)
>=20
> I'm not sure if we really want saturating behavior here, so 'true' is
> more correct?  Note if we want an unsigned result we should pass utype he=
re,
> that might be the bug?  So
>=20
>         begin =3D wi::from_mpz (utype, res, true);
>=20
> ?
Yes, it should be.
>=20
> The to_mpz args look like they could be mixing signs as well:
>=20
>     case vect_step_op_mul:
>       {
>         tree utype =3D unsigned_type_for (type);
>         init_expr =3D gimple_convert (stmts, utype, init_expr);
>         wide_int skipn =3D wi::to_wide (skip_niters);
>         wide_int begin =3D wi::to_wide (step_expr);
>         auto_mpz base, exp, mod, res;
>         wi::to_mpz (begin, base, TYPE_SIGN (type));
>=20
> TYPE_SIGN (step_expr)?
step_expr should have same type as init_expr.
>=20
>         wi::to_mpz (skipn, exp, UNSIGNED);
>=20
> TYPE_SIGN (skip_niters) (which should be UNSIGNED I guess)?
skipn must be a postive value, so I assume UNSIGNED/SIGNED doesn't make any
difference here.=