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From: "clyon at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/114801] [14/15 Regression] arm: ICE in
 find_cached_value, at rtx-vector-builder.cc:100 with MVE intrinsics
Date: Fri, 26 Apr 2024 13:59:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114801

--- Comment #7 from Christophe Lyon <clyon at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #4)
> (In reply to Christophe Lyon from comment #3)
> > lowpart_subreg does not work either.
> >=20
> > If we put the predicates in a variable in the testcase, then in
> > function_expander::add_input_operand() x is already a (subreg/s/v:HI (r=
eg:SI
> > 116 [ _3 ]) 0) so taking gen_lowpart of that is OK (we just want HImode=
 to
> > get the 16 bits of predicates).
>=20
> If x could be e.g. (subreg:SI (reg:DI ...) ...), then one needs to use for
> GET_CODE (x) =3D=3D SUBREG && GET_MODE (x) !=3D mode do a force_reg first.
>=20

No sure I got this right:
if (GET_CODE (x) =3D=3D SUBREG && GET_MODE (x) !=3D mode)
  x =3D force_reg (mode, x);
breaks the assert in emit_move_insn because mode is V4BImode and 'y' is HIm=
ode.

> > But when predicates are passed as a constant as in the testcase, we have
> > x =3D (const_int -13108 [0xffffffffffffcccc])
> > and gen_lowpart (HImode, x) fails on that.
>=20
> Why doesn't lowpart_subreg (mode, x, GET_MODE (x) =3D=3D VOIDmode ? DImod=
e :
> GET_MODE (x));
> work?
> I.e. for CONST_INTs assume it is some constant in a mode equal or wider t=
han
> mode.
> Unless mode can be TImode or x can be __int128 constants, that is.
>
This fails because down the call chain from lowpart_subreg, we reach
gcc_unreachable in rtx_vector_builder::find_cached_value because m_mode =3D=
=3D
V4BImode (so is MODE_VECTOR_BOOL), but is not a valid expected boolean cons=
tant
vector.=