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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/106069] [12/13 Regression] wrong code with -O
 -fno-tree-forwprop -maltivec on ppc64le
Date: Wed, 03 Aug 2022 08:24:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106069

--- Comment #22 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 3 Aug 2022, linkw at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106069
>=20
> --- Comment #21 from Kewen Lin <linkw at gcc dot gnu.org> ---
> I didn't look into this in details, but something in the culprit commit c=
aught
> my eyes, take altivec_vmrghh as example:
>=20
> Before the patch, the pattern
>=20
>    [(set (match_operand:V8HI 0 "register_operand" "=3Dv")
>          (vec_select:V8HI
>            (vec_concat:V16HI
>              (match_operand:V8HI 1 "register_operand" "v")
>              (match_operand:V8HI 2 "register_operand" "v"))
>            (parallel [(const_int 0) (const_int 8)
>                       (const_int 1) (const_int 9)
>                       (const_int 2) (const_int 10)
>                       (const_int 3) (const_int 11)])))]
>=20
> can match vmrghh on BE while vmrglh on LE. It indicates this pattern has
> different semantic from underlying instruction perspectives.
>=20
> After the patch, this pattern only matches vmrghh.
>=20
> IMHO, this part has semantic change before and after the patch. The code =
before
> the patch looks more reasonable to me, since the pattern can have differe=
nt
> meanings on BE and LE (underlying behavior).

Ideally we would avoid semantic difference of RTL depending on the target.
If that's not avoidable there should be target macros/hooks that specify
the desired semantics.  I assume the semantic difference is in
vec_concat behavior but that's just documented as

@findex vec_concat
@item (vec_concat:@var{m} @var{x1} @var{x2})
Describes a vector concat operation.  The result is a concatenation of the
vectors or scalars @var{x1} and @var{x2}; its length is the sum of the
lengths of the two inputs.

which is a bit unspecific.  To me it implies that
vec_select of a single lane N of the concat result can be distributed
to the operands of the vec_concat in the obvious way (if N >=3D
GET_MODE_NUNITS (x1) subtract GET_MODE_NUNITS and use x2)=