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From: "linkw at gcc dot gnu.org" <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 08:50:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106069

--- Comment #23 from Kewen Lin <linkw at gcc dot gnu.org> ---
> 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.=20=20

Not sure, IMHO it seems it doesn't depend on the target but on endianness
(BYTES_BIG_ENDIAN)? Segher and Mike may have more insights on this.

> I assume the semantic difference is in
> vec_concat behavior but that's just documented as
>=20
> @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.
>=20
> 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)

Yeah, the documentation isn't clear, neither for vec_select. I guess vec_se=
lect
also matters here, the indexes for vec_select would have the LE ordering li=
ke
subreg byte offset on LE?=