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From: "rsandifo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/98302] [11 Regression] Wrong code on aarch64
Date: Wed, 30 Dec 2020 18:23:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98302

rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |rsandifo at gcc dot=
 gnu.org

--- Comment #12 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.o=
rg> ---
This is caused by the overwidening pattern recognisers.
They correctly realise that in:

  unsigned long long x =3D ...;
  char y =3D x << 37;

only the low 8 bits of x << 37 are needed.  But they then
take it too far and try to do an 8-bit shift by 37, which is
undefined in gimple.

The optimal fix would be to get the vectoriser to replace the
shift result with zero instead, but that's a bit awkward to do
and should really happen before vectorisation.  The simplest fix
is to make sure that we don't narrow further than the shift amount
allows.

Testing a patch.=