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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/110751] RISC-V: Suport undefined value that allows
 VSETVL PASS use TA/MA
Date: Thu, 20 Jul 2023 13:32:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110751

--- Comment #14 from rguenther at suse dot de <rguenther at suse dot de> ---
On Thu, 20 Jul 2023, rsandifo at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110751
>=20
> rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> changed:
>=20
>            What    |Removed                     |Added
> -------------------------------------------------------------------------=
---
>                  CC|                            |rsandifo at gcc dot gnu.=
org
>=20
> --- Comment #13 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu=
.org> ---
> I understand the concern with undefined values, since it then becomes less
> obvious whether e.g.:
>=20
>   a =3D undef
>   b =3D a =3D=3D a
>=20
> is guaranteed to be true, or whether it reduces to:
>=20
>   b =3D undef =3D=3D undef
>=20
> which is presumably undef.
>=20
> But I don't think a null operand really helps.  The same question would a=
pply
> to:
>=20
>   x =3D COND_LEN_ADD (a, b, null, len, bias)
>   y =3D x =3D=3D x
>=20
> vs.
>=20
>   x1 =3D COND_LEN_ADD (a, b, null, len, bias)
>   x2 =3D COND_LEN_ADD (a, b, null, len, bias)
>   y =3D x1 =3D=3D x2
>=20
> Do both of these ys evaluate to true, or is one or both be undefined?
>=20
> So if we're prepared to accept undefinedness, I'd prefer to have a ?prope=
r?
> representation of it.  We could probably adopt LLVM's semantics for undef.
>=20
> (SVE might have some uses for this too.)

Note I simply proposed 'NULL' as the "proper" representation of it.
That doesn't fix any of the semantic issues but I think neither does
any other representation ...=