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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/98167] [x86] Failure to optimize operation on
 indentically shuffled operands into a shuffle of the result of the operation
Date: Fri, 27 Aug 2021 06:48:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98167

--- Comment #17 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Hongtao.liu from comment #16)
> typedef int v4si __attribute__ ((vector_size(16)));
>=20
> v4si f(v4si a, v4si b) {
>     v4si a1 =3D __builtin_shufflevector (a, a, 2, 3 ,1 ,0);
>     v4si b1 =3D __builtin_shufflevector (b, a, 2, 3 ,1 ,0);
>     return a1 * b1;
> }
>=20
> gcc generate=20
>=20
> f:
>         vpshufd xmm1, xmm1, 30
>         vpshufd xmm0, xmm0, 30
>         vpmulld xmm0, xmm0, xmm1
>         ret
>=20
> llvm generate
>=20
> f:                                      # @f
>         vpmulld xmm0, xmm1, xmm0
>         vpshufd xmm0, xmm0, 30                  # xmm0 =3D xmm0[2,3,1,0]
>         ret

For the above, this is safe for -ftrapping-math as all elements are still u=
sed.
 It is when elements that are not used it might not be safe ...=