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From: "rsandifo at gcc dot gnu.org" <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 13:29:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110751

rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rsandifo at gcc dot gnu.org

--- Comment #13 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.o=
rg> ---
I understand the concern with undefined values, since it then becomes less
obvious whether e.g.:

  a =3D undef
  b =3D a =3D=3D a

is guaranteed to be true, or whether it reduces to:

  b =3D undef =3D=3D undef

which is presumably undef.

But I don't think a null operand really helps.  The same question would app=
ly
to:

  x =3D COND_LEN_ADD (a, b, null, len, bias)
  y =3D x =3D=3D x

vs.

  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

Do both of these ys evaluate to true, or is one or both be undefined?

So if we're prepared to accept undefinedness, I'd prefer to have a =E2=80=
=9Cproper=E2=80=9D
representation of it.  We could probably adopt LLVM's semantics for undef.

(SVE might have some uses for this too.)=