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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug sanitizer/114217] -fsanitize=alignment false positive with
 intended unaligned struct member access
Date: Mon, 04 Mar 2024 07:54:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114217

--- Comment #10 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
BTW, with compilers from the last decade or so, it would be much better ide=
a to
just use standard memcpy for get_unaligned/put_unaligned rather than messing
around with pointers to packed types.  The compiler should optimize it
correctly to unaligned loads or stores itself when one operand is cast from
pointer to certain integral type and the size is the size of that type.  Su=
re,
you'd need to still use the GNU ({ ... }) extension
#define __get_unaligned_t(type, ptr) ({=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20
\
        type __get_unaligned_t_x;=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20
\
        memcpy(&__get_unaligned_t_x, (ptr), sizeof (__get_unaligned_t_x));=
=20=20=20=20=20
\
        __get_unaligned_t_x;=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20
\
})
For the get_unaligned from structure element you'd then have something like
get_unaligned_member(type, member, ptr) macro where it would use
typeof (((type *)0)->member) and offsetof (type, member) and memcpy.=