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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: Mon, 24 Jul 2023 06:20:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110751

--- Comment #21 from rguenther at suse dot de <rguenther at suse dot de> ---
On Fri, 21 Jul 2023, rsandifo at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110751
>=20
> --- Comment #20 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu=
.org> ---
> (In reply to Richard Biener from comment #19)
> > Sure, I can kind of see the usefulness elsewhere.  Just for this partic=
ular
> > issue it doesn't seem necessary to sit down and design this when we can
> > represent it like we do for MASK_LOAD (omit the 'else' value).
> Yeah, that's fair.
>=20
> For the ifn->optab interface, I think it'd be natural to use an actual rtx
> rather than a null pointer, since e.g. predicates are not set up to handle
> nulls.  So perhaps we should start the process there.  We could add an UN=
DEF
> rtl code that is initially only used for the ifn->optab interface, and ex=
pand
> it as we find new use cases.  We can grow the semantics based on those use
> cases and based on LLVM's experience.
>=20
> > In other context we discussed specifying zero for MASK_LOAD masked elem=
ents
> > so we can for example CSE better.  CSE with UNDEF might be possible as =
well,
> > but I'm not sure what LLVM's undef would allow and whether it's defined
> > rigidly enough.
> One of the main optimisations I wanted from that was:
>   a =3D IFN_MASK_LOAD (?, mask)
>   b =3D VEC_COND_EXPR <mask, a, {0,0,?}>
> ?
>   a =3D IFN_MASK_LOAD (?, mask)
>   b =3D a
> which wouldn't be valid for undef.

Right.  It would be valid to do

  b =3D IFN_MASK_LOAD (?, mask, {0,0,?});

if we add ELSE to IFN_MASK_LOAD.=