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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/98674] [10/11 Regression] vectorizer failed
 for compilation time alias
Date: Thu, 14 Jan 2021 13:26:32 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98674

Richard Biener <rguenth at gcc dot gnu.org> changed:

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

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> While GCC 9 vectorizes this case (and thus the vectorization failure is a
> regression) dependence analysis isn't presented with the problematical ac=
cess
> but instead we see
>=20
>   _1 =3D *p_22;
>   _5 =3D (unsigned short) _1;
>   bswapdst_10 =3D _5 r>> 8;
>   _8 =3D (short int) bswapdst_10;
>   *p_22 =3D _8;
>=20
> where the problematical access is created by the bswap pass which
> detects
>=20
> 16 bit bswap implementation found at: _8 =3D _4 | _7;
>=20
> creates the replacement load but then fails half-way, not emitting a
> bswap!?  That's a bug worth fixing IMHO (either do all or none of the
> transform).

Oh, so it produces

  load_dst_11 =3D MEM[(short int *)p_22];
  bswapdst_10 =3D load_dst_11 r>> 8;
  _8 =3D (short int) bswapdst_10;
  *p_22 =3D _8;

it's just pattern recog that turns the rotate back to a shift sequence.  So
indeed dependence analysis is what should be fixed.=