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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/99207] #pragma pack(1) and __int128 lead to bad optimization
 under O2 and higher optimization
Date: Mon, 22 Feb 2021 17:10:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99207

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
                 CC|                            |jakub at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
That is a user error, don't do that.
The fact that d doesn't point to properly aligned object needs to be visibl=
e on
the pointer dereference location, so you would need to use e.g. aligned(1)
attribute on e.g. a typedef to __int128, e.g.
typedef __int128 T __attribute__((aligned (1)));
T *d =3D &c.b;
Otherwise it is UB and -fsanitize=3Dundefined would have diagnosed it.=